2016 Inorganic perovskite photocatalysts for solar energy utilization
Cite this:Chem.Soc.Rev.,2016,45,5951
Inorganic perovskite photocatalysts for solar energy utilization
Guan Zhang,ab Gang Liu,*c Lianzhou Wang*d and John T.S.Irvine*b
The development and utilization of solar energy in environmental remediation and water splitting is being intensively studied worldwide.During the past few decades,tremendous e?orts have been devoted to developing non-toxic,low-cost,e?cient and stable photocatalysts for water splitting and environmental remediation.To date,several hundreds of photocatalysts mainly based on metal oxides,sulfides and (oxy)nitrides with di?erent structures and compositions have been reported.Among them,perovskite oxides and their derivatives (layered perovskite oxides)comprise a large family of semiconductor photocatalysts because of their structural simplicity and flexibility.This review specifically
focuses on the general background of perovskite and its related materials,summarizes the recent development of perovskite photocatalysts and their applications in water splitting and environmental remediation,discusses the theoretical modelling and calculation of perovskite photocatalysts and presents the key challenges and perspectives on the research of perovskite photocatalysts.
1.Introduction
Solar energy is the ultimate source of renewable energy for addressing the energy crisis and global warming challenge.The utilization of solar energy in environmental remediation and solar chemical conversion is being intensively studied worldwide.1,2Among a variety of solar conversion technologies,semiconductor photocatalysis driven water splitting and CO 2reduction (or artificial photosynthesis)have been
demonstrated
a
School of Civil and Environmental Engineering,Harbin Institute of Technology (Shenzhen),Shenzhen 518055,China b
School of Chemistry,University of St Andrews,St Andrews,KY169ST,UK.E-mail:jtsi@https://www.sodocs.net/doc/499996297.html, c
Institute of Metal Research,Chinese Academy of Sciences,Shenyang 110016,China.E-mail:gangliu@https://www.sodocs.net/doc/499996297.html, d
School of Chemical Engineering,The University of Queensland,St Lucia,Brisbane,QLD 4072,Australia.E-mail:
l.wang@https://www.sodocs.net/doc/499996297.html,.au
Guan Zhang
Guan Zhang received his BS and MS degrees from Liaoning Univer-sity,and a PhD degree (advisor:Prof.Wonyong Choi)from the School of Environmental Science and Engineering at Pohang Univer-sity of Science and Technology (POSTECH)in 2012.He was awarded the ‘‘Outstanding Self-Financed Students Abroad (Korea)’’in 2011by the Chinese government.After one year of postdoctoral work at POSTECH,he worked at the University of St Andrews (UK)as a
research associate from 2014to 2015.From 2016,he was appointed as an associate professor in School of Civil and Environmental Engineering at Harbin Institute of Technology (Shenzhen).His research interests include semiconductor photocatalysis,advanced oxidation processes for water treatment and biomass
conversion.
Gang Liu
Gang Liu received his BS degree in Materials Physics from Jilin University in 2003and a PhD degree in Materials Science at the Institute of Metal Research (IMR),Chinese Academy of Sciences in 2009.During his PhD study,he worked at The University of Queensland for 1.5years in Australia.He was the recipient
of the T.S.Ke
?RESEARCH FELLOWSHIP from 2009to 2012founded by Shenyang National Laboratory for Materials Science.
Now he is a professor of IMR CAS.His current main research interest is to develop semiconductor materials for solar fuels.He is an author of over 100peer-reviewed papers in international journals.
Received 14th October 2015DOI:10.1039/c5cs00769k
https://www.sodocs.net/doc/499996297.html,/chemsocrev
Chem Soc Rev
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as promising ways for converting solar energy into chemical fuels.3–5Using the same concept,semiconductor photocatalysis has also been extensively investigated for potential applications in environmental remediation including the degradation and removal of organic pollutants in aqueous/air phase,6,7reduction of heavy metal ions,8–10bacterial inactivation,11–13etc.During the past few decades,tremendous efforts have been devoted to developing efficient,low-cost and stable photocatalysts,especially those that can work under visible light such as In1àx Ni x TaO4,14 (Ga1àx Zn x)(N1àx O x),15CaBi2O4,16Ag/AgCl,17AgPO4,18hydroge-nated TiO2,19and metal-free photocatalysts including graphitic carbon nitride(C3N4),20boron carbide,21elemental a-sulfur,22 rhombohedral boron,23P-doped graphene,24nanoporous carbon,25carbon dot embedded C3N426and organic polymer photocatalysts.27–29Although water splitting in suspension systems is considered as not practically feasible as that in photoelectrochemical systems in terms of the separation of generated H2and O2for inhibition of back recombination,30–33 this drawback could be overcome by constructing a Z-scheme system34or selectively loading H2and O2evolution co-catalysts on different sites in a single particulate catalyst.35In addition,a particulate semiconductor photocatalyst has the following interesting features that distinguish itself from a photoelectro-chemical system:(1)electrolytes are not needed;(2)each photocatalyst works as a microphotoelectrode;(3)it possesses large surface areas(104to106cm2gà1),and the flux of the photo-generated carriers per unit surface is very small,compared with that of the bulk electrode;(4)it is much simpler for large-scale applications,which do not require applying an external bias and manufacturing nanostructured photoelectrodes.To date,a few hundred photocatalysts mainly including metal oxides,metal sulfides and metal(oxy)nitrides have been reported.However, the reported external quantum yield at a given visible light wavelength for pure water splitting in a particulate system is still far below the solar to hydrogen conversion efficiency goal of10% for practical applications.36Considering the huge potential of ‘‘solar+water-clean fuel’’,photocatalysis has been considered as one of the Holy Grails of chemistry and material fields,and it is highly desirable to develop new photocatalysts to boost the solar conversion efficiency.This has also led to decades-long intensive research efforts on the search for new photocatalysts, especially for those that can harvest the full range of visible light photons.
Among a large library of photocatalyst materials,perovskite oxides and their derivatives(layered perovskite oxides)comprise a large family of promising semiconductor photocatalysts because of their structural simplicity and flexibility,good stability and e?cient photocatalytic performance.The ideal perovskite has a cubic structure with a general formula of ABO3. The A and B sites can accommodate most of the metal elements in the periodic table into the crystal structure,which thus extends the family of perovskite oxides by rationally combining di?erent metal ions at A and B sites.37Apart from the ideal cubic perovskite,structural distortion can be induced by multi-ple metal cation substitutions.Such structural distortion can inevitably change the physical,electronic and photocatalytic properties of pristine materials.In addition,a series of layered perovskites consist of infinite2D slabs of the ABO3structure which are separated by embedded blocks(metal oxides).The possibility of preparing multicomponent perovskites by either partial substitution of metal cations in A or B sites or inserting metal oxides into a layered structure allows researchers to explore and modulate the crystal structures and the related physico-chemical and catalytic properties of the perovskite oxides.To date,more than two hundred perovskites or perovskite-related photocatalysts have been reported,and more importantly some perovskite-based materials have been recorded with‘‘benchmark’’performance for photocatalytic applications.Thus,
perovskite Lianzhou Wang
Lianzhou Wang is currently a
Professor in the School of
Chemical Engineering and Director
of Nanomaterials Centre,the
University of Queensland.Since
joining UQ in2004,he has
worked as Australian Research
Council Queen Elizabeth II
Fellow,Senior Lecturer,Associate
Professor,and is now Professor
and ARC Future Fellow in School
of Chemical Engineering.Lianzhou
has contributed more than200
journal publications,11patents
and delivered over50keynote/invited talks.Lianzhou has won some
prestigious Fellowships/Awards including the ARC QEII Fellowship,
the UQ Research Excellence Award of2008,the Scopus Young
Researcher Award of2011(Australian Universities),and the ARC
Future Fellow of
2012.
John T.S.Irvine
John Irvine is Professor of
Chemistry at the University of St
Andrews and currently holds a
Royal Society Wolfson Merit
Award.In2005he was elected
as a Fellow of the Royal Society
of Edinburgh.In2008he received
the Royal Society of Chemistry
Materials Chemistry Award and
the Royal Society of Edinburgh
Sustainable Energy Award in
2015.He has over400publi-
cations in refereed journals
including Nature and Nature
Materials.His research interests include solid state ionics,new
materials,ceramic processing,electrochemistry,fuel cell
technology,hydrogen,photoelectrochemistry,electrochemical
conversion and heterogeneous catalysis.
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materials have shown great potential for future applications on the basis of devoting more e?orts to them.
Although a number of excellent review articles targeting semiconductor photocatalysts have been published recently,38–41only a few of them paid attention to inorganic perovskite (mainly ABO 3-based)photocatalysts.42–44A comprehensive classification and full coverage of this material family,for instance,layered perovskite photocatalysts is still lacking.The aim of this review is to summarize the recent development of perovskite photocatalysts for water splitting and environmental remediation,discuss recent findings and advances on perovskite photocatalysts and give a perspective on the future research of perovskites.After a brief introduction on the general structure of perovskite materials,the reported perovskite photocatalysts are summarized,classified and discussed based on photocatalyst preparation,optical properties including the band-gap and band edge position,morphologies as well as the photocatalytic activities of the materials.In order to develop more efficient perovskite photocatalysts,theoretical modelling is a powerful tool to give a comprehensive understanding of the band structure configuration of semiconductors and to predict new semiconductor photocatalysts with promising perfor-mance.Some typical examples on the theoretical calculation of perovskite photocatalysts will be illustrated.Finally,a summary will be given to comment on the recent progress and development of perovskite photocatalysts for solar energy utilization.The potential applications,current challenges and the research direction in future will be addressed in the last part.However,the fundamentals on semiconductor photocatalysis and processes in photocatalytic water splitting and environmental remediation will not be covered in this review,since these aspects are already well addressed in the literature.38–41
2.Fundamentals of perovskite materials
2.1
Perovskite structures
The general formula of perovskite-type oxides can be described as ABO 3,where the A cation (normally much larger than B)is 12-fold coordinated and the B cation is 6-fold coordinated to the oxygen anions.Fig.1a depicts the representative coordi-nated skeleton of the ABO 3structure,which is composed of a three-dimensional framework of corner sharing BO 6octahedra and an A cation at the centre.In the ABO 3structure,the A cation is normally an alkali or alkaline earth metal or a rare earth element,while the B cation is typically a metallic transi-tion metal element.An ideal ABO 3perovskite has a cubic crystal structure with a tolerance factor (t )=1,which is defined as t =(r A +r O )/O 2(r B +r O ),where r A ,r B and r O are the ionic radii of A,B and oxygen elements,respectively.37For composing a stable perovskite,it is normally accepted that the t value should lie between 0.75–1.0.A lower t value (o 1)produces a slightly distorted perovskite structure with orthorhombic (Fig.1b)or rhombohedral symmetry.The ideal cubic perovskite structure only exists in limited cases where t is very close to 1and often at high temperatures.Although the t value,determined by the
ionic size,is an important index for the stability of perovskite structures,the octahedral factor (u )u =r B /r O and the contribu-tion of the chemical nature of A and B atoms,such as the coordinating number of the constituent elements,need also to be considered.45Taking into account those influencing factors and the electroneutrality,the ABO 3perovskite can accommo-date a wide range of pairs of A and B with the same or different valences and ionic radii.Furthermore,either A or B cations can be partially substituted by the other dopants,to extend the ABO 3perovskite into a broad family of A x A 1àx 0B y B 1ày 0O 3?d .The substitution of multiple cations into the A-or B-sites can alter the symmetry of the pristine structure and hence the physico-chemical and catalytic properties.In particular,the change in electronic and optical properties has a great influence on the photocatalytic process.2.2
Layered perovskite related structures
In addition to the general ABO 3structure,other typical poly-morphs of the perovskite structure are Brownmillerite (A 2B 2O 5)and K 2NiF 4structures.Brownmillerite is a kind of oxygen deficient perovskite,in which the unit cell is composed of ordered BO 6and BO 4units.Due to the oxygen deficiency,the coordination number of A-site cations decreases to eight.The structure of oxygen deficient perovskite will be discussed in the next paragraph.The K 2NiF 4structure is a combination of oxygen defects and ordered B sites,which is well studied as a superconducting material.The K 2NiF 4structure consists of the KNiF 3perovskite unit and the KF rock salt unit.Because the rock salt unit is embedded in the c -axis direction,K 2NiF 4materials exhibit layered properties.Based on the intergrowth of di?erent numbers of KNiF 3and KF units,there are many structures that can be classified as {100},{110}and {111}layered perovskites according to the layered orientation relative to the principal axis of an ideal cubic perovskite.The general formula for the most well-known layered perovskite oxides
is
Fig.1Crystal structures of perovskites and layered perovskite compounds (red spheres:oxygen;dark blue spheres:B-site element;green and light blue spheres:A-site element).
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described as (Bi 2O 2)(A n à1B n O 3n +1)(Aurivillius phase),A n +1B n O 3n +1or A 20A n à1B n O 3n +1(Ruddlesden–Popper phase)and A 0[A n à1B n O 3n +1](Dion–Jacobson phase)for {100}series,(A n B n O 3n +2)for {110}series and (A n +1B n O 3n +3)for {111}series.In these structures,n represents the number of BO 6octahedra that span a layer,which defines the thickness of the layer.Representative examples of these layered structures are shown in Fig.1c–g.For Aurivillius phases,their structures are built by alternating layers of [Bi 2O 2]2+and pseudo-perovskite blocks.Bi 2WO 6and BiMoO 6(n =1),found as the first ferroelectric Aurivillius compounds,recently have been intensively studied as visible light photocatalysts.For Ruddlesden–Popper phases,their structures result from the intergrowth of perovskite ABO 3and A 0O as the intermediate spacing layer.These com-pounds possess interesting properties of magnetoresistance,superconductivity,ferroelectricity and catalytic activity.Sr 2SnO 4and Li 2CaTa 2O 7are the examples of simple Ruddlesden–Popper type photocatalysts.The Dion–Jacobson phases with a general formula of A 0[A n à1B n O 3n +1](n 41),where A 0separates the perovskite-like blocks and is typically a univalent alkali cation.The representative Dion–Jacobson type photocatalysts are RbLnTa 2O 7(n =2)and KCa 2Nb 3O 10(n =3).Members of the A n B n O 3n +2and A n +1B n O 3n +3structural series with different layered orientations have also been identified in a number of photocatalysts such as Sr 2Ta 2O 7and Sr 5Ta 4O 15(n =4).2.3
Defects in perovskites
Perovskite (ABO 3)materials have three di?erent ionic species,making for diverse and potentially useful defect chemistry.In addition to the partial substitution of A and B ions allowed while preserving the perovskite structure,deficiencies of cations at the A-site or of oxygen anions are frequent.By substitution of parent cations with similar-sized cations of di?erent valence,defects can be introduced into the structure.The defect concen-trations of perovskites can be controlled and tailored by doping.Oxygen ion vacancies or interstitials can be generated by the substitution of B-site ions with cations of lower or higher valence,respectively,producing compounds of AB (1àx )B x 0O 3?d .A common oxygen deficient perovskite structure is Brownmillerite (A 2B 2O 5),in which one-sixth of oxygen atoms are removed.A-site vacancies can be introduced by the substitution of A-site ions with cations of higher valence,giving compounds of stoichiometry of A 1àx A x 0BO 3.The substitution of A-site ions with lower-valence cations results in oxygen vacancy formation giving compounds of A (1àx )A x 0BO 3àd .B-site vacancies in perovskite oxides are not as common as they are not thermodynamically favoured because of the large charge and the small size of the B cations.A-site vacancies are more observed because the BO 3array in the perovskite structure forms a stable network,the large A cations at 12coordinated sites can be partially missing.Recently,intro-ducing appropriate defects onto the surface of metal oxide semiconductors has been intensively studied as a means of altering the electronic structures and optical absorption proper-ties of the parent materials.For example,hydrogenated TiO 2(black TiO 2)has been reported as an e?cient photocatalyst that can split water under UV or visible light irradiation.23From this point of view,perovskite materials provide a huge platform for
defect engineering to change the photocatalytic properties of perovskite photocatalysts.
3.Perovskite materials for photocatalysis
A wide range of perovskite photocatalysts have been developed for water splitting and organic pollutant degradation under UV or visible light irradiation during the past few decades.These representative examples and brief experimental results on them are summarized according to their structures,which can be classified into five groups.Specifically,ABO 3-type perovskites,AA 0BO 3,AB
B 0O 3and AB(ON)3-type perovskites,and AA 0BB 0O 3-type perovskites are listed in Table 1,Table 2and Table 3,respectively.3.1
ABO 3type
3.1.1Tantalates.NaTaO 3has long been recognized as an e?cient UV-light photocatalyst for overall water splitting.46–57It has a band gap of
4.0eV and can be synthesized by solid-state,46–48,53,56hydrothermal,49,52,54,55sol–gel 50,51and molten salt methods.57As a first example,Kato and Kudo reported highly efficient splitting of pure water into H 2and O 2over the NaTaO 3photocatalyst.The quantum yield of the NaTaO 3(0.05wt%NiO as a co-catalyst)photocatalyst prepared by the solid-state reaction method was 28%for water splitting at 270nm.46–48In order to increase the surface area of NaTaO 3bulk particles,many researchers attempted to use other synthetic routes to prepare nano-sized particles as an extension of the study on the NaTaO 3photocatalyst.Kondo et al.synthe-sized a colloidal array of NaTaO 3nanoparticles using three-dimensional mesoporous carbon as a template,which was replicated by the colloidal array of silica nanospheres.After burn-ing out the mesoporous carbon matrix,a colloidal array of NaTaO 3nanoparticles with a size of 20nm and a surface area of 34m 2g à1was https://www.sodocs.net/doc/499996297.html,pared to non-nanostructured bulk NaTaO 3particles,the nanostructured NaTaO 3exhibited more than 3times higher photocatalytic activity for overall water splitting.54Shortly afterwards,Shi and Li et al.developed a fast and facile method for the preparation of NaTaO 3nanocrystals via a microwave-assisted hydrothermal process.55An indirect transformation route from Ta 2O 5to Na 2Ta 2O 6and to NaTaO 3was proposed as the main reason for that pure NaTaO 3could be synthesized in a rather short time (less than 3h)under mild conditions.The water splitting efficiency of NaTaO 3nanocrystals prepared by this approach was more than two times higher than that prepared from conventional hydrothermal synthesis.Recently,the nano-sized NaTaO 3,pre-pared by an exo-template method with a crystallite size of about 25nm,showed an 18times higher hydrogen evolution rate than NaTaO 3synthesized by a solid-state reaction method (Fig.2).56The hydrogen production activity of NaTaO 3obtained from the exo-template synthesis could be further improved by about 30–40times by mixing with reduced graphene oxide and using Au as a co-catalyst.However,it did not show the improvement of activity for NaTaO 3prepared from solid-state reaction under identical conditions.
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Table 1
Summary of perovskite photocatalysts (ABO 3)for water splitting and degradation of pollutants
Perovskites Morphology Band gap (eV)Synthetic
method a Incident light Reaction conditions Co-catalyst (wt%)Activity (m mol g à1h à1)Ref.H 2
O 2
Pollutants b
NaTaO 3
Bulk 4.0SS UV Water
NiO (0.05)2180110046–48Bulk
3.96HT UV 5%methanol —36750
49Nanoparticle 3.9–4.1SG UV Water —2050100050—
4.0SG UV Water —2660133051Nanocube 4.0HT UV Water —
CH 3CHO
52Bulk 4.1SS UV Glucose NiO (0.2)1420053Colloidal —HT UV Water NiO (1.0)6600330054Nanocrystal —HT UV Water
NiO (0.4)110050055Nanocrystal —SS UV 50%methanol Au (0.2)13780—56Microsphere
4.08HT
UV Water NiO (0.3)261357La doped
Bulk 4.07SS UV Water NiO (0.05)5900290058Bulk 4.09SS UV Water
NiO (0.02)198********Bulk
4.1MS
UV 20%methanol Pt (1.0)1115—60Nanoparticle 4.01Microwave UV Water NiO (0.02)35701770
61Nanoparticle 3.9–4.0SG UV Water MB
62Nanoparticle 4.0SG UV Water
RuO 2(1.0)4108174363Nanoparticle 4.0SG UV 10%methanol 2860
—
64Nanocube 3.86HT UV Water MB
65Bulk
—
SS UV Water Au (3.0)14580694066Ca,Sr,Ba doped Bulk
4.0–4.1SS UV Water NiO
272001338067Ta 4+doped
Nanoclusters 1.70HT 4420nm Water 61
68Cr doped Nanocrystal 4.1–3.1HT UV Water MB 69Eu doped Bulk 44.0SS UV Water MB 70Bi doped Bulk
—SS 4420nm Water
MB
71Bulk 2.64SS 4390nm 20%methanol Pt (0.06)0.8672—
2.88HT 4400nm 5%methanol NiO (0.02)59.573Mn or Fe doped Bulk 2.82SS 4420nm
———74N doped Bulk 3.92SS UV Water
MB 75C doped Bulk 2.03HT
4400nm
NO x
76La/Cr co-doped Bulk
2.88Pyrolysis 4415nm 20%methanol Pt (0.5)146777Bulk
NG SS 4420nm 20%methanol Pt (0.5) 4.478La/N co-doped Bulk NG SS 4420nm 20%methanol
Ru (0.1)3579N/F co-doped Nanoparticle
3.8–
4.0HT UV Water RhB
80LiTaO 3
Bulk 4.7SS UV Water —43022047K TaO 3
Bulk 3.6SS UV Water —291347Zr doped Bulk
3.8SS UV Water NiO (1.5)122.357.481AgTaO 3
Bulk 3.4SS
UV Water
NiO (0.3)13863.382SrTiO 3
Single crystal —Commercial UV 20M NaOH ———83Single crystal —Commercial UV 45M NaOH ———84Bulk 3.2Commercial UV Water vapor NiO (1.7)0.10.0585Bulk 3.2Commercial UV Water vapor NiO ——86Bulk
3.2Commercial UV Water NiO
(1.5)
——
87Nanoparticle 3.2HT UV Water RhB 88Nanoparticle 3.2HT UV Water
RhB 89Nanocube 3.2HT UV 20%methanol Pt (1.0)202.690Microcube —HT UV Water
——RhB
91Nanocrystal
3.16SG UV 50%methanol Pt (0.5)276924400nm
188Bulk 3.2—3.3SS UV
Water
NiO (3.0)
2893
30nm 3.3–3.7HT 19.46.5nm
3.3–3.7SG 3.0Cr doped —
—HT 4420nm 5%methanol 27.9294Nanoparticle 2.3SG/HT 4420nm 20%methanol Pt (1.0)330
95Fe doped Nanoparticle —SG 4420nm Water
RhB
96Mn doped Bulk 2.7SS 4440nm
10%methanol 0.05M AgNO 3
Pt (0.5)
0.668.997
Ru doped Bulk 1.9SS 5.612.9Rh doped Bulk 1.7SS 56.760Ir doped Bulk 2.3SS 28.38 1.32Er doped —
NG SG
4420nm Na 2S/Na 2SO 3
Pt (1.0)46.2398
0.185M AgNO 3
44.23
Zn doped Bulk 3.15SG
UV 3%ethanol
732
99Ti 3+doped Bulk
NG Combustion Water
Pt (0.3)CO 2
100Surface Ti 3+Nanocrystal 3.2HT UV 25%methanol Pt (1.0)2200101
N doped Nanoparticle 2.9HT
4420nm Water MB,RhB,MO 102F doped
Nanoparticle 3.0Ball milling 4400nm Gas
NO 103
Cr/Sb co-doped
Bulk
2.4
SS
4420nm 8%methanol
Pt (0.3)
156
104
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Table 1
(continued )
Perovskites Morphology Band gap (eV)Synthetic
method a Incident light Reaction conditions Co-catalyst (wt%)Activity (m mol g à1h à1)Ref.
H 2O 2Pollutants b
50mM AgNO 3 1.8
Cr/N co-doped Nanoparticle 2.4SG/HT 4420nm 18.5%methanol Pt (0.5)213.4105Cr/Ta co-doped Bulk
—SS 4440nm 6.5%methanol Pt (1.0)70106Cr/La co-doped Nanoparticle 2.12HT 4400nm Methanol/NaOH Pt (0.5)
1089
107Ni/La co-doped Nanoparticle —SG 4400nm Water MG
108S/C co-doped Nanoparticle 2.0SS 4420nm Water 2-Propanol 109N/La co-doped Nanoparticle —SG 4410nm Gas
2-Propanol
110Ni/Ta co-doped Bulk 2.8SS 4420nm
10%methanol Pt (0.1)811150mM AgNO 3
1.7La/Rh co-doped Sheet —SS 4420nm Water Ru,RuO x —
—
112BaTiO 3Bulk 3.33SG UV Water Pesticide 113Bulk
3.33SG UV Water Aromatics 114Nanotube —HT UV Water
Ag
MO
115Rh doped Bulk
—SG 4420nm 10%methanol Pt (0.25)308
116Fe doped Nanoparticle 2.81SG 4420nm Photocurrent
117CaTiO 3
Bulk 3.5HT UV 0.2M NaOH Pt (0.1)5220118Bulk 3.5SS UV Water/methane Pt (0.04)——
119Cu doped ——SG 4400nm Water
NiO x 22.7120Rh doped
——SS 4420nm 10%methanol Pt (0.1)16.6121Ag/La co-doped ——SG 4400nm 5%methanol 10.1122PbTiO 3Bulk
2.75MS/SS 4420nm 20%methanol Pt (1.0)27.4123RuO 218
3.1
Bulk 2.95HT UV 10%methanol Pt (1.0)70124KNbO 3
Bulk
3.12HT UV Water
Ni (0.1)11.7126Nanowire 3.2HT UV 12%methanol Pt (0.5)5170
127Nanowire 3.8HT 4420nm Water
Au RhB
128Microcube 3.0–3.2HT 4250nm 25%methanol Pt (1.5)1242129Microcrystal 3.24HT 4300nm 25%methanol Pt (1.0)
1242
130Nanowire/cube NG HT UV Water RhB
131Microcube NG HT 4420nm Water
Au (3.0–6.0)H 2O 2/MB
132N doped Nanocube 2.76HT
4390nm
20%methanol RuO 2(0.5) 6.7
1330.05M AgNO 358
NaNbO 3Single crystal 3.8–3.9—
UV Water RhB
134Nanoparticle 3.7Impregnation UV Air 2-Propanol 135Thin film 3.5SS UV Water MB
136N doped Bulk —SS Air 2-Propanol 137N doped Bulk
—SS Visible Air 2-Propanol 138Ru doped Nanocube/wire 2.3HT 4425nm Water Phenol 139AgNbO 3
Bulk 2.86SS UV
Water MB
140Bulk 2.9SG 4420nm Water
Chlorophenol
141Bulk 2.8HT 4420nm 5mM AgNO 375142Bulk 2.8MS 4420nm 20%methanol Pt (1%) 5.9
143La doped Bulk 2.8SS 4400nm Gas
2-Propanol
144Sr 1àx NbO 3
Bulk
1.9SS
4420nm Oxalic acid 44.8
145
5mM AgNO 324
Bulk
1.8–1.9SS 4420nm Oxalic acid 46.14
146BiFeO 3
Nanoparticle 2.18SG UV-Vis Water MO
147Hollow sphere 2.1Spray 4420nm Water RhB,4-CP 148Nanoparticle 2.12HT 4420nm Water RhB 149Microsphere 1.8–2.3HT 4400nm Water CR 150Film
2.5—
4400nm Water
CR
151Epitaxial film 2.74Sputtering Photocurrent 152Film
2.1,2.0Laser 4420nm Water
Photocurrent
153Nanowire 2.35HT 4380nm 4mM FeCl 3Au (1.0)400
154
Microsphere 2.1ST 4420nm Water RhB 155Bulk
2.5SS 470nm Water Ag reduction 156Ba doped Nanofiber —ES Visible Water CR 157Ca doped Nanofiber —ES Visible Water CR 158Ba or Mn doped Nanofiber —
ES Visible Water CR 159Ca or Mn doped Bulk 2.1–2.4HT UV-Vis Water RhB 160Gd doped Nanoparticle
2.03SG
4420nm Water RhB 161LaFeO 3
Nanoparticle —Combustion 4340nm Water Methylphenol 162Nanoparticle 2.4SG 4400nm Water RhB 163Nanosheet 2.1HT 4400nm Water
RhB,MB 164
Nanoparticle
2.1SG
4420nm
10%methanol 8600
165Combustion 0.05M AgNO 34266
Nanoparticle 2.07SG Ethanol
Pt
3315
166Microsphere
2.1
HT
4400nm Water
RhB
167
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In addition to the nanostructure engineering of NaTaO 3,much e?ort has been made to achieve a higher activity of NaTaO 3by doping with lanthanide 58–66or alkaline-earth cations.67Kudo et al.found that the photocatalytic activity of NiO-loaded NaTaO 3:La was 9times higher than that of the non-doped sample.58,59The maximum apparent quantum yield of the NiO/NaTaO 3:La photocatalyst reached 56%at 270nm (high-est record).It was explained that La doping reduced the particle size of NaTaO 3from 2–3m m to 0.1–0.7m m and created an ordered surface nanostructure with many characteristic steps,which accounts for the enhancement of photocatalytic activity.The bulk recombination of photogenerated electrons and holes was less in smaller particles with high crystallinity.In addition,the back recombination of H 2and O 2was inhibited because of the effective separation of the reaction sites for H 2and O 2evolution.Afterwards,La-doped NaTaO 3powders with high surface area and good crystallinity were prepared using a microwave irradiation method 61and a H 2O 2assisted sol–gel route.65The highest H 2production rate of 2.86mmol h à1g à1was obtained for a 2.0mol%La-doped sample from a 10%methanol solution.65In addition to La-doping,Kudo et al.also systematically investigated the effect of alkaline earth metal ion dopants (Ca,Sr and Ba)on water splitting with NaTaO 3powder.67Interestingly,doping of Ca,Sr and Ba can also create surface nanostep structures on doped NaTaO 3when the amount of dopants was larger than 0.5mol%.The formation of a nanostep structure could dramatically enhance water splitting efficiency.However,both positive and negative effects existed for Sr-doping as proved by photoluminescence measurements.A small amount of Sr dopant enhanced charge separation,whereas a large amount of Sr dopant created a significant amount of defects as recombination centers.These studies demonstrated that doping of La and alkaline earth metals is a useful approach for suppressing electron–hole pair recombi-nation by reducing the particle size of NaTaO 3as well as forming a surface nanostep structure,however the concen-tration of dopants needs to be considered for maximizing the photocatalytic performance.
Recently,a major research topic on NaTaO 3is trying to extend the absorption spectrum of NaTaO 3into visible light by doping of metals such as Ta 4+,68Cr,69Eu,70Bi,71–73Mn and Fe,74and non-metals such as N,75C,76or co-doping of La/Cr,77,78La/N 79and N/F.80The doping mechanism on band gap narrowing of NaTaO 3is generally accepted that d orbitals of metal elements and s or p orbitals of non-metal elements contribute some intermittent energy levels within the band gap of NaTaO 3,which are mainly determined by Ta 5d and O 2p orbitals.74For example,Chen et al.prepared 5%Bi-doped NaTaO 3powders by solid-state reaction by varying the ratio of Na/Ta in starting materials.71,72They have found that the Na/Ta molar ratio strongly influenced the doping sites of Bi in the lattice of NaTaO 3and optical and photocatalytic properties.
Table 1
(continued )
Perovskites Morphology Band gap (eV)Synthetic
method a Incident light Reaction conditions Co-catalyst (wt%)
Activity (m mol g à1h à1)Ref.
H 2
O 2
Pollutants b
Nanoparticle —SG Water
Chlorophenol 168Nanocube 2.01HT
4400nm Water RhB 169Nanorod 2.05Nanosphere 2.1Ca doped Nanoparticle ——4400nm Water MB 170LnFeO 3(Pr,Y)Bulk
2.5SG
4400nm Water RB 171SrFeO 3àx Nanoparticle —Ultrasonic 4410nm Water
Phenol 172SrFeO 3Nanoparticle —SS Water
MB
173GaFeO 3Nanoparticle 2.7SG 4395nm Water 10.0 5.0
174BaZrO 3Nanoparticle —SG UV Water MB 177Nanosphere 4.8HT UV Water MO
178Bulk 4.8SS UV Water 500
179Mg doped Bulk —SS
UV Water
MB
180Ta doped Bulk —Precipitation Water
900
450181SrSnO 3
Bulk 4.1SS UV Water RuO 2(1.25)227.2113.5182Nanorod 4.1HT
UV 15%methanol
Pt (0.5)8200183
15mM AgNO 3
2500
Nanoparticle 4.04Precipitation UV Water NiO x 254
184Bulk —Microwave UV Water MO
185CaSnO 3Bulk
4.4SS UV Water RuO 292
47186SrSrO 3 4.1151.475.6BaSnO 3 3.1 2.82 1.22BaCeO 3Bulk
SG
UV
Water RuO 2(1.0)
59
26
187LaCoO 3
Nanoparticle —Microwave 4410nm Water MO
188Hollow sphere 2.07Adsorption UV Water MB,MO 189Nanofiber —ES
UV Water RhB
190C doped
Bulk 2.16Chelation UV-Vis Water CO 2reduction 191C/Fe co-doped 2.63SG 4400nm Water CO 2reduction 192LaNiO 3
Nanoparticle 2.6SG Visible Water
MO
193Nanoparticle
2.42
SG
4420nm
12.5%HCHO
33
194
a
HT:hydrothermal;SS:solid-state;MS:molten salt;SG:sol–gel;ES:electronspun.b
RhB:rhodamine B;MO:methyl orange;MB:methylene blue;
4-cp:4-chlorophenol;MG:malachite green;CR:congo red.
Chem Soc Rev Review Article
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The visible light absorption of Bi-doped NaTaO 3prepared under mild Na-rich conditions was extended up to 550nm,because Bi dopants were located both at Na and Ta sites.As a result,this sample was the most active one for the degradation of methylene blue under visible light irradiation compared to others prepared under Na-deficient or strongly Na-rich
Table 2
Summary of perovskite photocatalysts (AA 0BO 3,ABB 0O 3and AB(ON)3)for water splitting and degradation of pollutants
Perovskites Morphology Band gap (eV)Synthetic
method a Incident light Reaction conditions
Co-catalyst
(wt%)Activity (m mol g à1h à1)
Ref.H 2
O 2Pollutants b AA 0BO 3
Bi 0.5Na 0.5TiO 3
Nanoparticle 3.08HT o 365nm Water
MO
195Microsphere 2.8–2.9HT UV
20%methanol Pt 325.4
196Nanotube 3.0HT 4420nm Gas NO
197Na 1àx K x TaO 3Bulk
3.75SG UV
Water 11000
5500
198La 0.7Sr 0.3MnO 3Nanoparticle 1.6SG Solar light Water MO
199La 0.5Ca 0.5NiO 3Nanoparticle —SG o 365nm Water RB5200La 0.5Ca 0.5CoO 3Nanoparticle —
SG UV
Water CR
201Sr 1àx Ba x SnO 3Nanoparticle 4.0–2.8SS UV 254nm Water Azo-dye
202ABB 0O 3
K 0.95Ta 0.92Zr 0.08O 3
Bulk
3.8SS UV Water NiO (1.0)122.357.42033.5SS UV Water Dyes/Pt 575280.4204
3.5SS UV Water Porphyrin/Pt
513
257
205and 206CaCo 1/3Nb 2/3O 3Bulk 2.80SS 4420nm Water
NiO x (1.0)
1.72207SrCo 1/3Nb 2/3O 3
2.46 1.72BaCo 1/3Nb 2/3O 3 2.46 2.74CaIn 1/2Nb 1/2O 3Bulk 4.17SS UV 20%methanol Pt (0.2)608208SrIn 1/2Nb 1/2O 3
3.96114BaIn 1/2Nb 1/2O 3 3.51102
BaM 1/3N 2/3O 3
(M =Ni,Zn;N =Nb,Ta)Bulk
3.3–
4.5
SS UV 20%methanol Pt (0.5)136.4–1416.4
209BaZn 1/3Nb 2/3O 3Bulk 3.90SS UV
Water NiO x /RuO 2291.2
145.6
210BaCo 1/2Nb 1/2O 3
Nanoparticle 2.26SG 4400nm Water MB
211Ba(In 1/3Pb 1/3M 1/3)O 3(M =Nb,Ta)
Bulk 1.5SS 4420nm MB,4-CP 212A(In 1/3Nb 1/3B 1/3)O 3
(A =Sr,Ba;B =Sn,Pb)Bulk
1.5–3.5
SS 4420nm Water
MB,4-CP
213SrTi x M 1àx O 3
(M =Ru,Rh,Ir,Pt,Pd)Nanoparticle 3.1–2.0HT UV visible 10%methanol Pt (1%)67021478
SrTi (1àx )Fe x O (3àd )Bulk
—SS Visible Water MB 215SrTi 0.1Fe 0.9O 3àd Nanoparticle —SG Solar light Water MO 216SrFe 0.5Co 0.5O 3àd Nanoparticle —SG Water
CR
217SrFe 1/2Nb 1/2O 3Bulk
2.06SS 4420nm 15%methanol Pt (0.2)
45218LaNi 0.7Cu 0.3O 3Nanoparticle 2.8SG 4400nm 12.5%HCHO 582219LaNi 1àx Cu x O 3Nanoparticle 2.5–2.8SG 4400nm 12.5%HCHO 1180
220LaFe 1/2Ti 1/2O 3Nanoparticle —
SG UV Water Phenol
221BaZr 1àx Sn x O 3 4.8–3.3
UV Water
690185
222CaTi 1àx Zr x O 3
Nanoparticle 3.60SG UV
20%ethanol Pt (1.0)1400223Bi(Mg 3/8Fe 2/8Ti 3/8)O 3Bulk 1.86MS 4420nm Water
MO 224NaBi x Ta 1àx O 3Bulk 2.8–3.4SP 4415nm 20%methanol NiO (0.2)1335225NaTi 1àx Cu x O 3Bulk —SS 4400nm 10%methanol NiO (0.3)69.3226AgTa 1àx Nb x O 3Bulk 2.8–3.4HT 405/420nm Water
NiO
—
—227AB(ON)3LaTiO 2N
Bulk 2.1SS flux 4420nm 50mM AgNO 3CoO x (2)3680228
Bulk 2.1SS 4420nm 10%methanol Pt (3.0)——229and 230
10mM AgNO 3
——Bulk 2.1SS flux 4420nm 10mM AgNO 3CoO x (2)2600231and 232Bulk
2.1SS
4420nm
20%methanol Pt (3.0)600
233
20mM AgNO 31500
LaTi(ON)3
Bulk 2.3–2.6SG 4420nm Gas
Acetone 234(CaLa)TiO 2.25N 0.75Bulk 2.0SS 4420nm 10mM AgNO 3IrO 2(2.0)500
230
CaTaO 2N Bulk 2.5Nitridation 4420nm 20%methanol Pt (0.3)
250241–243SrTaO 2N 2.1420BaTaO 2N 2.0500
W-BaTaO 2N Bulk —SS 4420nm 10mM AgNO 3IrO 2(1.5)220
244CaNbO 2N
Bulk
2.0
SS
4420nm 10%methanol Pt (1.0)10
246
0.01M AgNO 3
312
a
HT:hydrothermal;SS:solid-state;MS:molten salt;SG:sol–gel.b
MO:methyl orange;MB:methylene blue;4-cp:4-chlorophenol;RB5:reactive
blue 5;CR:congo red;NO:nitrogen monoxide.
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conditions.Although doping of metal or non-metal elements into NaTaO 3(likewise doping of TiO 2)can be realized by various synthetic methods,it is worth mentioning that Cao and Hu et al.developed self-doped (Ta 4+)NaTaO 3nanoclusters by a facile one-pot low-temperature solvothermal method.68Compared to foreign element doping,the self-doping method benefits from a more homogeneous feature.Furthermore,low-temperature synthesis avoids the increase of the particle size and aggregation during prolonged high temperature synthesis.Doping of Ta 4+greatly reduced the band gap of NaTaO 3from 3.94eV to 1.70eV as indicated by the reddish colour of the doped NaTaO 3sample.In addition,Zhou and Li et al.theoretically calculated the band structures of Mn and Fe-doped NaTaO 3and they found that Mn-doped NaTaO 3,with a band gap of 2.82eV,was only active for the photo-oxidation of water.Fe-doped NaTaO 3(2.03eV)was capable of overall water splitting.74
Following the early studies on NaTaO 3,other tantalate photocatalysts including ATaO 3(A =Li and K)and AgTaO 3were also reported.Kudo et https://www.sodocs.net/doc/499996297.html,pared the alkali tantalate ATaO 3(A =Li,Na and K)photocatalysts for water splitting under UV-light irradiation.47The ilmenite LiTaO 3photocatalyst showed higher activity than NaTaO 3and KTaO 3,when no co-catalyst was employed.However,in the presence of a NiO co-catalyst,NaTaO 3exhibited the highest activity and the activity of the NiO/NaTaO 3was enhanced by 1order of magni-tude compared to the pure NaTaO 3.Furthermore,the activity of KTaO 3could be enhanced by Zr-doping as an acceptor.The Zr dopant plays the roles of reducing the charge density in the sample and increasing the lifetime of photoexcited charges.81Shortly afterwards,they also found that AgTaO 3(3.4eV)can split water with or without co-catalysts under UV-light irradiation.82
Tantalate-based perovskite oxides are considered as a group of promising UV-light photocatalysts with wide band gaps.The band structures are mainly determined by Ta 5d and O 2p orbitals,whereas the A site cations (A =Li,Na,K and Ag)have little e?ect.Through nanostructure engineering and doping modification,the photocatalytic performance of the parent tantalates could be dramatically enhanced,as demonstrated by the fact that the apparent quantum yield of La-doped NaTaO 3(with NiO co-catalyst)reached 56%at 270nm.58,59In addition,doping of tantalates with appropriate metals or non-metals makes them active under visible light.68–80
3.1.2Titanates.SrTiO 3,one of the earliest studied perovskite photocatalysts,has a band gap of 3.2eV and suitable band levels for water splitting.A SrTiO 3single crystal was found to generate H 2from highly alkaline conditions ([NaOH]45M)under UV-light illumination.83,84The hydroxide ions at or near the photocatalyst surface were suggested as facile hole acceptors to increase the lifetime of electrons for proton reduction.Reduced Ti 3+surface species were found on the illuminated crystal surface,which might be involved in the production of H 2under UV-light
Table 3
Summary of perovskite photocatalysts (AA 0BB 0O 3)for water splitting and degradation of pollutants
Perovskites Morphology
Band gap (eV)Synthetic method a Incident light Reaction conditions Co-catalyst
(wt%)Activity (m mol g à1h à1)
Ref.H 2
O 2Pollutants b (Ag 0.75Sr 0.25)(Nb 0.75Ti 0.25)O 3Bulk
2.8SS 4400nm Gas phase CH 3CHO
248(AgNbO 3)1àx (SrTiO 3)x
Bulk 2.7–3.2SS 4410nm 5mM AgNO 3324249(BaZrO 3)x –(BaTaO 2N)1àx
Bulk —
SS 4420nm
NaI solution
Pt (0.3)11030250Pt/WO 3
(BaZrO 3)0.05–(BaTaO 2N)0.95
Nanoparticle 1.8SS 4420nm 1mM NaI Pt
44093251(BaZrO 3)0.05–(BaTaO 2N)0.95Nanoparticle 1.8SS 4420nm 10%methanol Pt (0.3)
14125210mM AgNO 3IrO 2(1.5)
77253LaMg x Ta 1àx O 1+3x N 2à3x Bulk 1.9–2.14420nm Water RhCrO y 5
2.5254CaZrO 3–CaTaO 2N Nanoparticle 2.6–4.0SS 4420nm 10%HCOOH Pt (1.0)12.4–52.4255(SrTiO 3)1àx (LaTiO 2N)x Bulk 2.0–
3.2SS 4420nm 18%methanol
66.7
256
10mM AgNO 3
53.0
La 0.8Ba 0.2Fe 0.9Mn 0.1O 3àx Nanoparticle —SG Solar light Water
MO
257Na 1àx La x Fe 1àx Ta x O 3Bulk
2.24SS 4390nm 20%methanol Pt (0.05)0.81258Na 0.5La 0.5TiO 3–LaCrO 3
Nanocube 2.25HT 4420nm 18%methanol Pt (1.0)8.2
259Cu-(Sr 1ày Na y )(Ti 1àx Mo x )O 3Nanoparticle —HT 4400nm Gas
Propanol
260Na 1àx La x Ta 1àx Cr x O 3Bulk
—SS 4420nm 20%methanol Pt (0.2)9.0
261BiFeO 3–(Na 0.5Bi 0.5)TiO 3Macropore 2.1SG 4400nm
RhB
262Sr 1àx Bi x Ti 1àx Cr x O 3
Bulk
—
SS/HT
4420nm 10%methanol Pt (1)
37
263
a
HT:hydrothermal;SS:solid-state;MS:molten salt;SG:sol–gel.b
MO:methyl orange;RhB:rhodamine B;CH 3CHO:
acetaldehyde.
Fig.2Comparison of hydrogen generation rates from NaTaO 3prepared by solid-state reaction (SSR)and exo-template method (EM)with loading gold or/and reduced graphene oxide.Reprinted with permission from ref.56.Copyright r 2014,American Chemical Society.
Chem Soc Rev Review Article
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illumination.Almost simultaneously,Domen et al.also reported the photocatalytic splitting of water vapor on NiO (1.7wt%)loaded SrTiO 3.85Later on,they found that liquid water splitting was 3times faster than water vapor splitting with the NiO/SrTiO 3photocatalyst.86The NiO loading was crucial for the significant enhancement (100times)of the water splitting performance.Through further study of the structure of the NiO co-catalyst,87it was revealed that a core/shell structure was formed on the surface of SrTiO 3during the pre-treatment process (Fig.3).The NiO on the surface of SrTiO 3has undergone a reduction and re-oxidation process forming a Ni/NiO core/shell structure.The Ni metal in contact with SrTiO 3was important for the photocatalytic activity.On the other hand,the NiO in the outer layer was partly changed to Ni(OH)2during the reaction with water under irradia-tion.This work reminds us that the co-catalyst engineering (e.g.structure,morphology,distribution and composition tuning)is also vitally important to enhance the photocatalyst performance.These early studies have demonstrated that SrTiO 3is an excellent material for water splitting under UV-light irradiation.Recently,various nano-structured SrTiO 3particles with high surface area were prepared by hydrothermal 88–91or sol–gel 92methods to improve the performance.For example,single-crystal-like porous SrTiO 3nanocube assemblies were prepared via a facile hydrothermal reaction at 1501C using layered protonated titanate hierarchical spheres of sub-micrometer size as a precursor template.90The hierarchical 3D assemblies were formed by oriented stacking of SrTiO 3nanocubes of 60–80nm,as a result of topographic transformation in crystallography between the layered titanate and perovskite structure as well as Ostwald-ripening assisted oriented attachment.Similarly,single-crystal-like mesoporous SrTiO 3sub-micrometer spheres with a wormlike structure were synthesized by a hydrothermal method using polyvinyl alcohol as an additive.89Despite the well-documented roles in nanoscale engineering of solid photo-catalysts,Osterloh et al.have observed that the activity for overall water splitting of SrTiO 3decreases from 28m mol H 2per g per h (bulk SrTiO 3)to 19.4m mol H 2per g per h (30nm SrTiO 3),and 3.0m mol H 2per g per h/(6.5nm SrTiO 3).93The reasons for this decrease were ascribed to an increase of the water oxidation overpotential for the smaller particles and reduced light absorption due to a quantum size effect.They suggested that
the catalyst particles based on SrTiO 3should be larger than 30nm for overall water splitting under UV-light irradiation.The contro-versial results on the nanoscale engineering of solid photocatalysts should be considered in future studies.
Recent studies on SrTiO 3are concentrated on doping SrTiO 3for achieving visible light activity.Doping of single metal elements such as Cr,94,95Fe,96Mn,97Ir,97Ru,97Rh,97Er,98Zn 99and Ti(III ),100,101and non-metal elements such as N 102and F 103as well as co-doping of Cr/Sb,104Cr/N,105Cr/Ta,106Cr/La,107La/Ni,108S and C,109N/La,110Ni/Ta 111and La/Rh 112have been intensively investigated for showing photocatalytic activities under visible light irradiation.Among the di?erent noble metal ions,Rh ion was the best one than the others like Ru and Ir.The donor level located at ca.1.0eV above the valence band was formed by Rh 3+doping,which behaved as a visible light absorption centre and a surface reaction centre for oxidation of methanol (hole scavenger).97Later on,Cr-doped SrTiO 3was also evaluated for photocatalytic hydrogen production under visible light irradiation.The Sr 0.95Cr 0.05TiO 3sample prepared by the hydrothermal method extended its visible light absorp-tion up to 540nm and exhibited 3times higher activity than that synthesized by solid-state reaction due to the increased surface area.95Note that the Cr 3+substitution of Ti 4+sites in SrTiO 3would create oxygen defects and/or generate Cr 6+ions to keep the charge balance,which may increase the recombina-tion between photogenerated electrons and holes.The positive doping e?ect for Cr 3+was suggested only for the substitution of Sr 2+sites in SrTiO 3.Recently,La and Rh co-doped SrTiO 3and Mo-doped BiVO 4powders embedded into a gold layer in a Z-scheme system have been developed for water splitting with a solar-to-hydrogen energy conversion e?ciency of over 1%and an apparent quantum yield of over 30%at 419nm was achieved.Another important feature is that the photocatalyst sheet is scalable by screen-printing an ink containing the mixed photocatalysts.112In addition to the foreign element doping,self-doped SrTiO 3àx was prepared through a one-step combus-tion method followed by annealing at high temperatures under an Ar atmosphere.100The oxygen vacancy played dual roles:enhancing the visible light absorption and chemical adsorption of CO 2onto catalysts,which improved the artificial photo-synthesis to produce hydrocarbon fuels from CO 2and H 2O as a result of the synergetic e?ect.However,as observed from Sun’s group,oxygen vacancy on the surface of SrTiO 3induced by chemical reduction can only improve the UV-light photo-catalytic activity due to the enhancement of charge separation,but it had little e?ect on the visible light activity.101In the case of doping of non-metal elements,mesoporous N-doped SrTiO 3was prepared using glycine as both a nitrogen source and a mesopore creator.102The doped sample has a higher specific surface area (52.3m 2g à1)with a lower band gap of 2.9eV,and exhibits excellent activity in photodegradation of dyes.
In contrast to forming donor/acceptor levels within the band gaps through doping of metal or non-metal ions,a series of upconversion luminescent materials (e.g.Er 3+doped SrTiO 3photo-catalysts)have been demonstrated to be active for visible light driven water splitting with sacrificial agents.98The
upconversion
Fig.3Schematic illustration of the core/shell structure of NiO on the SrTiO 3catalyst.Adapted with permission from ref.87.Copyright r 1986,American Chemical Society.
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luminescent agents (e.g.rare earth elements)such as Eu 3+,Nd 3+and Er 3+can emit high-energy photons by absorbing two or more low-energy photons.By this approach,visible light photons can be firstly up-converted into UV-light photons so as to excite the large band-gap semiconductors if the upconversion material is properly trapped in the large band gap semiconductor host as shown in Fig.4.
Other titanates including BaTiO 3,113–115Rh or Fe-doped BaTiO 3,116,117CaTiO 3118,119and Cu,120Rh,121Ag and La-doped CaTiO 3122and PbTiO 3123,124were also reported as UV or visible light photocatalysts.It was interestingly found that the Pt loaded CaTiO3photocatalyst exhibited an enhanced production rate of hydrogen from a mixture of water vapor and methane due to the simultaneous photocatalytic steam reforming of methane and water decomposition.119The activated methane species or reaction intermediates would accelerate the water splitting or suppress the reverse reaction.This result implies that renewable sources such as methane from biogas can be utilized for producing H 2by photocatalytic steam reforming.BaTiO 3,an n-type semiconductor with a band gap of 3.0eV,was studied by doping Rh species in order to generate a new absorption band in the visible light region.The Rh-doped BaTiO 3powder prepared by the polymerized complex method was examined for water reduction with a sacrificial agent under visible light irradiation.116Interestingly,the Rh-doped BaTiO 3or SrTiO 3electrode 125generated a stable cathodic photocurrent,in con-trast to the anodic photocurrent generated from the un-doped BaTiO 3electrode under UV light irradiation.Thus,Rh-doped BaTiO 3is regarded as a p-type semiconductor,which is rarely observed for doped metal oxides.Similar to the reported Rh-doped SrTiO 3,the substitution of Ti 4+sites with Rh 3+ions without forming oxygen vacancies yields positive holes due to the charge compensation and thus changes the semiconducting properties from n-type to p-type.However,it should be noted that doping Rh into n-type oxide semiconductors does not always produce p-type character.
Titanate photocatalysts,represented by SrTiO 3as a prototype perovskite,are attractive in the research field of photo(electro)-catalysis.In a similar manner to that employed in tantalate photocatalysts,nanostructure engineering and doping modifi-cation are two of the most commonly adopted strategies to improve the photocatalytic performance of titanates.However,the smaller particle size of the solid photocatalyst cannot
guarantee higher water splitting e?ciency due to the quantum size e?ect.On the other hand,SrTiO 3is a good candidate for investigating the surface defects related to visible light activity and adsorption properties due to the flexible conversion between Ti 3+and Ti 4+.
3.1.3Niobates.Alkaline niobates such as KNbO 3and NaNbO 3have been widely investigated as UV-light photocatalysts.The bulk KNbO 3with a band gap of 3.12eV was synthesized by hydro-thermal reaction under supercritical water conditions in an earlier report.126By heating at 4001C,a single phase of K 4Nb 6O 17was gradually transformed into a single phase of KNbO 3in 24h.However,the mixed phases of K 4Nb 6O 17and KNbO 3were found to be more photoactive for water splitting than pure K 4Nb 6O 17and KNbO 3under UV-light irradiation.Nano-structured KNbO 3with di?erent morphologies of nano-wires,nanorods,nanocubes,nanocrystals and microcubes can be prepared by hydrothermal synthesis,127–133which basically involves the hydrothermal treatment of a mixed aqueous solution of KOH and Nb 2O 5at about 180–2001C in an autoclave for several hours.The variation of hydrothermal parameters such as the temperature,reaction time,pH of solution and concentration of precursors changes the morphologies.For example,cubic,orthorhombic and tetragonal KNbO 3microcubes were prepared by varying the ratio of KOH and Nb 2O 5in a precursor solution.129,130The cubic KNbO 3,with a relatively larger BET surface area and band gap compared to the tetragonal and orthorhombic samples,exhibited the highest H 2production rate.At the same time,Yi et al.proposed a dissolu-tion–recrystallization mechanism to explain the formation of the corresponding nanostructures.131In addition,gold nano-particle deposited KNbO 3microcubes were prepared for utilizing visible light induced surface plasmon resonance effects on gold nanoparticles.132N-doped KNbO 3nanocubes with a band gap of 2.76eV were prepared and tested for water splitting with sacrificial agents under visible light.133
Compared to KNbO 3,NaNbO 3showed a relatively lower photo-catalytic activity probably because of its relatively larger band gap.134,135However,an interesting phenomenon was found that the KNbO 3film exhibited photoinduced hydrophilicity under UV-light irradiation,even though the photocatalytic oxidation of the dye by the KNbO 3film was negligible.136Normally,photoinduced hydrophilicity was observed for the TiO 2film because of the photocatalytic decomposition of organic compounds re-generating a hydrophilic surface.These results confirm that photoinduced hydrophilicity was not caused solely by the photocatalytic oxidation.In addition,N-doped NaNbO 3137,138and Ru-doped NaNbO 3nanocubes and nanowires 139were also reported for the photocatalytic decom-position of phenol and 2-propanol under visible light irradiation.In contrast to the alkaline niobates,AgNbO 3has a smaller band gap of about 2.8eV and can be prepared by solid-state,sol–gel,solvothermal and molten-salt flux techniques.140–143For example,a polyhedron-shaped AgNbO 3photocatalyst with surface nanosteps was prepared by a solvothermal method.142The well-defined edges and corners on the polyhedron-shaped AgNbO 3were found to be able to enhance its
photocatalytic
Fig.4Schematic illustration of the main processes of the photocatalytic reaction on the Er 3+doped SrTiO 3sample.Reprinted with permission from ref.98.Copyright r 2012,Wiley-VCH Verlag GmbH &Co.KGaA,Weinheim.
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activity,in a similar manner to that observed on La-doped NaTaO 3.Likewise,the formation of a 20–50nm terraced surface microstructure was also observed on the AgNbO 3nanoparticles synthesized by molten-salt flux,whereas there was no well-defined morphology and microstructure for the sample pre-pared from solid-state synthesis.143In order to further enhance the photocatalytic activity of AgNbO 3,La-doped AgNbO 3was prepared by a solid-state reaction method.144The Ag 0.88La 0.12NbO 3sample showed a 12-fold higher rate for photocatalytic decomposi-tion of gaseous 2-propanol under visible light irradiation.The enhanced performance was assumed to be due to several possible reasons such as the enlarged surface area,enhanced mobility of photo-generated electrons,deposition of metallic silver and A-site defects in the perovskite structure.
Recently,nonstoichiometric Sr 1àx NbO 3(x =0.1–0.2)has been reported as a novel metallic visible light photocatalyst by Irvine et al.145,146A series of strongly coloured red materials with a band gap energy of 1.9eV were obtained by controlling the nonstoichiometry in the perovskite (Fig.5).The band structures of metallic oxide conductors are assumed to be different from semiconductor oxides,in which the valence and conduction bands are clearly distinguished.For metallic Sr 1àx NbO 3,it was described that the band below the conduction band as the highest fully occupied band (B à1)and that above it as the lowest unoccupied band (B 1),respectively.Therefore,photon excitation by visible light might be involved either from B à1to the conduction band or from the conduction band to B 1.The high conductivity of the sample might allow the fast separation and mobility of charge carriers.As a result,Sr 0.9NbO 3showed the highest efficiency for H 2or O 2evolution from a sacrificial aqueous solution.
Niobate perovskite photocatalysts are less studied than those tantalate and titanate-based photocatalysts,probably because of their relatively lower activities and stabilities.Most of the studies on alkaline niobate photocatalysts are focused on the preparation of nano-structured niobates with di?erent morphologies.127–133,140–143In an exceptional case,nonstoi-chiometric Sr 1àx NbO 3(x =0.1–0.2)has been demonstrated as
a novel metallic visible light photocatalyst,145,146which opens up a new research direction in searching for visible light photocatalysts.
3.1.4Ferrites.Magnetic BiFeO 3,known as the one of the multiferric materials in magnetoelectric applications,was also studied as a visible light photocatalyst for water splitting and degradation of organic pollutants due to its small band gap (ca. 2.2eV).Both BiFeO 3nanoparticle powders and films were prepared for evaluating their photo(electro)catalytic performance.147–156In an early report,BiFeO 3with a band gap of about 2.18eV synthesized by a citric acid assisted sol–gel method has shown its visible light photocatalytic activity by the decomposition of methyl orange dye.147The following studies on BiFeO 3are mainly focused on the preparation of novel structured BiFeO 3with different morphologies.For example,Lin and Nan et al.synthesized BiFeO 3uniform microspheres and microcubes by a controlled hydrothermal method as shown in Fig.6.150The band gaps of BiFeO 3materials were estimated to be about 1.82eV for BiFeO 3microspheres and 2.12–2.27eV for microcubes.The clear shift for the absorption edge among these samples was influenced by the particle size,morphology and crystal-field strength.The microcube sample exhibited the highest efficiency for the photocatalytic degradation of congo red dye under visible light irradiation because of the relatively larger band https://www.sodocs.net/doc/499996297.html,ter on,a facile aerosol-spraying approach was developed to prepare mesoporous BiFeO 3hollow spheres with enhanced activity for the removal of RhB dye and 4-chlorophenol,due to enhanced light absorbance resulting from multiple light reflections in a hollow chamber and a higher surface area.148
In addition,a remarkably enhanced water oxidation activity on Au nanoparticle loaded BiFeO 3nanowires under visible light irradiation was reported.154The Au–BiFeO 3hybrid structure was induced by the electrostatic interaction of positively charged BiFeO 3nanowires and negatively charged Au nanoparticles at pH 6.0according to their different isoelectric points.An enhanced absorbance between 500and 600nm was observed for Au/BiFeO 3samples due to the typical Au surface plasmon band in the visible light region.The amount of O 2produced from Au/BiFeO 3nano-wires with 1wt%Au loading was 30times higher than that from parent BiFeO 3nanowires during the first 4h reaction.More interestingly,a synergistic effect was found between the Au nanoparticles and the BiFeO 3nanowire support,as the photo-catalytic activity of self-assembled Au/BiFeO 3nanowires was much higher than the composite of Au and BiFeO 3
nanowires.
Fig.5Ultraviolet-visible absorbance spectra of Sr 1àx NbO 3.Kubelka–Munk transformation of the absorption curves is shown in the top inset.The colour of sample pellets is shown in the bottom inset.Reprinted with permission from ref.145.Copyright r 2012,Nature Publishing
group.
Fig.6SEM images of BiFeO 3:(a)microspheres and (b)microcubes.The magnified images are shown in the top insets.Reprinted with permission from ref.150.Copyright r 2010,American Chemical Society.
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The photoluminescence study suggested the occurrence of efficient charge transfer from BiFeO 3to Au,which explained the enhancement of the photocatalytic activity.In addition,Ba,Ca,Mn and Gd doped BiFeO 3nanofibers and nanoparticles have shown obvious room temperature ferromagnetic behaviours and photocatalytic activity for the decomposition of dyes.157–161
Various nanostructured LaFeO 3with di?erent morphologies such as nanoparticles,nanosheets,nanotubes,nanorods,and nanospheres have also been prepared as visible light photo-catalysts for water splitting and degradation of organic dyes.162–170Other ferrites like PrFeO 3,171SrFeO 3172,173and GaFeO 3174were reported as novel visible light photocatalysts.Particularly,GaFeO 3,with a band-gap of 2.7eV,exhibited an attractive overall water splitting activity without any co-catalyst loading from pure water.The hydrogen and oxygen yields were about 10.0and 5.0m mol g à1h à1under visible light irradiation.
Considering the ferroelectric and magnetoelectric properties of ferrite materials,it is more attractive to develop multifunc-tional photocatalysts as demonstrated by the study of BiFeO 3and LnFeO 3,even though their photocatalytic activities are not as good as their counterparts such as titanate and tantalate perovskites.For example,ferroelectric BiFeO 3has recently been used in photovoltaic devices for coupling of light absorption with other functional properties.175,176
3.1.5Others.Other perovskites such as BaZrO 3,177–179Mg,Ta-doped BaZrO 3,180,181MSnO 3(M =Ca,Sr,Ba)182–186and BaCeO 3187were barely studied as UV-light photocatalysts with relatively large band gaps of over
4.0eV.Among them,SrSnO 3nanorods prepared by the hydrothermal method exhibited much better photocatalytic water splitting performance.183The hydrogen and oxygen yields with Pt-loaded SrSnO 3in the sacrificial systems were 8200and 2500m mol g à1h à1under UV-light irradiation,respectively.In addition,LaCoO 3,188–190C,Fe-doped LaCoO 3191,192and LaNiO 3193,194are active for the photocatalytic degradation of dyes and water splitting under visible light irradiation.3.2
AA 0BO 3type
The substitution of an A ion in ABO 3by an A 0ion with a di?erent valence state will alter the valence state of the B metal ion or induce some defects and oxygen vacancies into the structure,which would significantly influence optical and photo-catalytic activities.Thus,this strategy o?ers us more options to design new perovskite photocatalysts by rational combination of dual metal ions with consideration of the charge balance.For illustration,sodium bismuth titanate (Bi 0.5Na 0.5TiO 3)has been widely used for piezoelectric,ferroelectric and pyroelectric devices.It was also studied as a UV-light photocatalyst with a band gap of ca.3.0eV.195–197Hierarchical micro/nanostructured Bi 0.5Na 0.5TiO 3was synthesized by in situ self-assembly of Bi 0.5Na 0.5TiO 3nanocrystals under controlled hydrothermal con-ditions,during which the growth mechanism was studied in detail.195It was proposed that the hierarchical nanostructure was built through a process of nucleating and growth and aggregation of nanoparticles and subsequent in situ dissolution–recrystallization of the microsphere nanoparticles with prolonged
heating time and increased temperature or basic conditions.The 3D hierarchical Bi 0.5Na 0.5TiO 3exhibited much higher photo-catalytic activity for the removal of methyl orange dye due to the increased surface area and adsorption of dye molecules.The activities of Na 0.5Bi 0.5TiO 3were also evaluated by photocatalytic production of H 2from water and removal of nitric oxide in the gas phase.196,197Another example is that K-doping in the Na 1àx K x TaO 3photocatalyst transformed the distorted perovskite NaTaO 3to a pseudo-cubic phase,which significantly promoted the photocatalytic water splitting activity.198In terms of the crystalline structure,approximately 1801(a value ideal for the delocalization of the excited energy in tantalates)Ta–O–Ta bond linkage caused by K-doping facilitates the separation of photogenerated charges as to enhance the photocatalytic https://www.sodocs.net/doc/499996297.html, 0.7Sr 0.3MnO 3,as a visible light photocatalyst,was investigated for solar photocatalytic degradation of methyl orange.199In addition,La 0.5Ca 0.5NiO 3,200La 0.5Ca 0.5CoO 3àx 201and Sr 1àx Ba x SnO 3(x =0–1)202nanoparticles were prepared for showing enhanced photocatalytic degradation of dyes.3.3
ABB 0O 3type
In a similar manner to A-site substitution,B-site substitution by a di?erent cation is another option for tuning the physico-chemical or photocatalytic properties of perovskites.KTaO 3is well known as a good UV light photocatalyst for water splitting.Ishihara et al.systematically investigated the e?ect of doping a series of cations (Zn 2+,Y 3+,Al 3+,Ga 3+,In 3+,Ce 4+,Ti 4+,Zr 4+,Hf 4+,Si 4+,Ge 4+,Nb 5+,Sb 5+and W 6+)to substitute Ta in KTaO 3.203It was found that 8%doping of Zr 4+exhibited the highest rate for water splitting under UV-light irradiation.The increased activity was proposed to the enhancement of the lifetime of photoexcited charge due to the decreased charge https://www.sodocs.net/doc/499996297.html,ter on,they further combined KTa(Zr)O 3with various organic dyes for water splitting in a Z-scheme system.204The cyanoco-balamin sensitized K 0.95Ta 0.92Zr 0.08O 3exhibited the highest photocatalytic water splitting e?ciency with a formation rate of 575.0and 280.4m mol g à1h à1for H 2and O 2,respectively.The enhanced charge transfer mechanism on the porphyrinoid modified K 0.95Ta 0.92Zr 0.08O 3was further studied by photolumi-nescence spectroscopy in detail as shown in Fig.7.205Unlike dye sensitization,photo-excited electrons transferred from K 0.95Ta 0.92Zr 0.08O 3to dyes and Z-type excitation was successfully achieved.The photogenerated charges were spatially separated between KTa(Zr)O 3and Cr-TPP dye as in a photosynthetic system.This work shows great promise for organic–inorganic hybrid Z-scheme systems for water splitting,compared to all-solid-state Z-scheme systems.
Another series of M(N x Nb 1àx )O 3,(M =Ca,Sr and Ba;N =Co,In and Zn)solid solution samples were synthesized by solid state reaction and their performance was evaluated for water splitting under UV-light irradiation.206–211Among these different compositions,BaZn 1/3Nb 2/3O 3seemed to be the most active photocatalyst for pure water splitting and generation of H 2from a methanol containing aqueous solution under UV-light irradiation.Raman spectra indicated that different binding modes of M–O–Nb may be the dominant factors in the migration
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of photogenerated charge carriers and affecting the photocatalytic activity.Furthermore,highly electronegative non-transition metal Pb or Sn ions were incorporated into the perovskite lattice of (BaIn 1/3M 1/3M 1/30)O 3(M =Sn,Pb;M 0=Nb,Ta).212,213The Pb-containing quaternary metal oxides Ba(In 1/3Pb 1/3M 1/3)O 3possess a much narrower band gap of ca. 1.50eV when compared to those of the ternary oxides Ba(In 1/2M 1/2)O 3(2.97–3.30eV)and the Sn-containing Ba(In 1/3Sn 1/3M 1/3)O 3derivatives (2.85–3.00eV).These results provided a new method of developing efficient visible light photocatalysts by doping electronegative non-transition metal cations.212In addition,A-site strontium based perovskites such as SrTi (1àx )Fe x O 3àd ,SrTi 0.1Fe 0.9O 3àd ,SrCo 1/2Fe 1/2-O 3àd ,and SrNb 1/2Fe 1/2O 3compounds were synthesized through solid-state reaction and sol–gel methods,and were investigated for the degradation of dyes under visible light irradiation.214–218Another group of A-site lanthanum based perovskites such as LaNi 1àx Cu x O 3and LaFe 1/2Ti 1/2O 3were demonstrated as effi-cient visible light photocatalysts for the generation of H 2from HCHO aqueous solution and degradation of p -chlorophenol under visible light irradiation.219–221The other ABB 0O 3type photocatalysts including Ba(ZrSn)O 3,222Ca(TiZr)O 3,223Bi(MgFeTi)O 3,224Na(BiTa)O 3,225Na(TiCu)O 3,226and Ag(TaNb)O 3227have also been reported.
Compared to AA 0BO 3type perovskites,the ABB 0O 3type structure provides more flexibility in composing the perovskite photocatalyst,because normally the B-site cations in ABO 3mainly determine the level of the conduction band,in addition to build the framework of perovskite structure with oxygen atoms.The band structures of photocatalysts can be finely tuned by rationally combining dual or ternary metal cations at the B-site,or varying the ratio of multiple cations,which has been well demonstrated by the examples given above.Further investigations on ABB 0O 3type photocatalysts are expected to explore their new exciting photocatalytic performance.3.4
AB(ON)3type
In addition to A-or B-site doping,nitridation of perovskite oxides to form oxynitride-type perovskite AB(ON)3is another
e?ective approach to reduce the band gap of ABO 3and enhance the photo(electro)catalytic performance under visible light.Since the N 2p orbitals can introduce new intermittent energy levels above the valance band edge constructed by O 2p orbitals,most of the AB(ON)3materials have strong visible light absorption up to 600–650nm.The development of oxynitride photocatalysts successfully makes visible light driven water splitting possible at an irradiation wavelength of over 600nm.For example,LaTiO 2N developed by Domen and co-workers has been studied as a visible light photocatalyst for water splitting.228–234It has a band gap of 2.1eV and exhibits both photocatalytic H 2and O 2evolution from a sacrificial aqueous system under visible light irradiation up to ca.600nm.229,230By loading LaTiO 2N with a CoO x co-catalyst,the O 2evolution efficiency can be greatly enhanced.228,231However,it shows a relatively lower activity for H 2evolution even with a Pt co-catalyst.The infrared spectroscopic analysis suggests that photoexcited electrons in LaTiO 2N cannot efficiently transfer to the Pt co-catalyst.232It was thought that LaTiO 2N prepared by the nitridation process may contain a lot of defects,especially in the surface region,which would prevent the charge transfer at the interfaces between the photocatalyst and co-catalyst.Recently,eliminating such surface defect layers on LaTiO 2N with appropriate acid etching gave rise to significant improve-ments in the photocatalytic activity for both H 2and O 2evolution reactions.233Furthermore,the LaTiO 2N powder can be fabricated into a photoanode for photoelectrochemical water splitting.235–240The LaTiO 2N photoanode decorated with IrO 2co-catalysts exhibited a markedly improved anodic photocurrent based on water oxidation.
Tantalum or niobium oxynitride series (ABO 2N,A:Ca,Sr and Ba;B:Ta and Nb)are other successful examples that can generate H 2or O 2from aqueous sacrificial solution under visible light irradiation up to ca.600nm.In the case of ATaO 2N,the band gaps decrease with increasing radius of alkaline-earth metals with 2.5,2.1,2.0eV for A =Ca,Sr and Ba,respectively.241Only H 2was produced from an aqueous methanol solution with these photocatalysts,while no O 2evolved even from the silver nitrate solution.Thus,ATaO 2N is usually used as a H 2produc-tion photocatalyst in a Z-scheme water splitting system.242,243Recently,doping of pentavalent W-species into BaTaO 2N signi-ficantly improved the activity for O 2evolution from aqueous silver nitrate solution in the presence of an IrO 2co-catalyst.244The optimum ratio of W/Ta =0.005was found for the O 2evolution.Since the W-doping did not largely alter the band gap structure of BaTaO 2N,a plausible explanation for the enhanced oxidation of water was proposed to the pronounced upward band-bending,because holes in the valence band are able to migrate easily to the surface according to the upward band-bending.The BaTaO 2N fabricated photoanode can achieve water splitting under visible light irradiation up to a 660nm wavelength.The IPCE value was estimated to be about ca.10%at 1.2V vs.RHE under 600nm,which is the highest record among the photoanodes excited beyond 600nm for water oxidation.245Niobium-based oxynitrides generally have much smaller band-gaps than the corresponding
tantalum
Fig.7Schematic mechanism for the photocatalytic water splitting into H 2and O 2on dye (Cr-TPP)modified KTa(Zr)O 3at pH 11.Reprinted with permission from ref.205.Copyright r 2009,Wiley-VCH Verlag GmbH &Co.KGaA,Weinheim.
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analogues.The band-gaps of ANbO 2N are 2.0,1.8,1.7and 1.6eV for A =Ca,Sr,Ba and La,respectively.246However,only CaNbO 2N is photoactive for H 2and O 2evolution under sacri-ficial conditions.SrNbO 2N powder showed a weak photo-catalytic O 2evolution activity even in the presence of silver nitrate,but generated an anodic photocurrent due to the water oxidation upon irradiation with visible light photons up to 700nm,even without an externally applied potential.247Under visible light irradiation with an applied potential of 1.0–1.55V vs.RHE,stoichiometric H 2and O 2evolution was achieved on the SrNbO 2N electrode decorated with a colloidal IrO 2co-catalyst.
The nitridation approach can e?ciently narrow the band gap of ABO 3oxide and increase the capability of visible light absorption,and thus has been widely employed for developing new materials in both photochemical and photoelectro-chemical systems.The development of nitridized materials is indeed a great success in extending visible light response of the photocatalysts and photoelectrodes.However,it is not faultless.Nitridized materials su?er from stability challenge during the water oxidation process,as the nitrogen component would be easily oxidized to nitrogen gas instead of water oxidation.233Therefore,how to improve the stability of nitridized materials is important and requires further investigation.3.5
ABO 3-A 0B 0O 3(A 0B 0O 2N)type
Because of the high capacity of accommodating a wide range of cations and valences at both A-and B-sites,ABO 3type perovskite oxides are promising candidates for producing solid-solution photocatalysts.In this case,both the A and B cations can be replaced by equivalent cations resulting in a perovskite with the formula of (ABO 3)x (A 0B 0O 3)1àx .Since most of the reported visible light photocatalysts are not capable of generating H 2and O 2simultaneously due to the unsuitable band edge positions,it is thus expected that the band structures of solid solution materials can be tuned by combining a H 2evolution photocatalyst and an O 2evolution photocatalyst,in order to achieve the purpose of overall splitting of water on a single-phase material under visible light irradiation.
For instance,SrTiO 3(3.2eV)and AgNbO 3(2.7eV)have been reported as H 2and O 2evolution photocatalysts,respectively,due to their di?erent band gap energies and band edge potentials.Ye et al.developed a series of solid-solution samples (AgNbO 3)1àx (SrTiO 3)x (0o x o 1),which have shown modu-lated band structures and enhanced visible light photocatalytic activity.248,249Rietveld refinement revealed that the perovskite-type solid solutions (AgNbO 3)1àx (SrTiO 3)x were crystallized in an orthorhombic phase (0o x o 0.9)or a cubic phase (0.9o x o 1).(Ag 0.75Sr 0.25)(Nb 0.75Ti 0.25)O 3exhibited the best visible light activities for O 2evolution and decomposition of gaseous 2-propanol.
Another example is BaZrO 3–BaTaO 2N solid solution photo-catalysts developed by Maeda and Domen et al.250–253BaTaO 2N with a band gap of 1.8–1.9eV has been shown as a H 2evolution photocatalyst over 600nm irradiation.However,the apparent quantum yield is very low (o 0.1%at 420–440nm)in a two-step
water splitting process,when combined with Pt-loaded WO 3as an O 2evolution photocatalyst.By composing a solid solution with BaZrO 3(4.8eV),the BaZrO 3–BaTaO 2N solid solution (Zr/Ta =0.05)exhibited a much higher activity for H 2evolution in aqueous NaI solution under visible light (4420nm)than BaTaO 2N.250,251When Pt–WO 3was employed as an O 2evolu-tion catalyst in NaI solution,stoichiometric water splitting into H 2and O 2was achieved under visible light.The apparent quantum yield was calculated to be about 0.6%at 420–440nm,which was at least six times higher than that obtained with the optimized Pt/BaTaO 2N.Furthermore,the BaZrO 3–BaTaO 2N solid solution has been demonstrated to be active for both photocatalytic water reduction and oxidation under visible light irradiation.252,253The overall water splitting on the BaZrO 3–BaTaO 2N based material was also tested in a photoelectro-chemical cell system.This is the first single photoanode material with a band gap smaller than 2.0eV for overall water splitting.It should be mentioned that IrO 2-loading is indis-pensable to achieve stable water oxidation over BaTaO 2N-based photocatalysts owing to self-oxidation.When colloidal IrO 2was deposited on the solid solution anode,the anodic photocurrent was significantly improved.In addition,the onset potential was shifted to ca.à0.3V vs.RHE,indicating that colloidal IrO 2loaded onto BaTaO 2N promoted water oxidation,which was consistent with the results of photocatalytic reactions.Recently,Pan and Domen et al.tried to solve the self-oxidation problem by double-coating a mixture of silica and titania layer (SiO x H/TiO x H)on a complex perovskite-type oxy-nitride,LaMg x Ta 1àx O 1+3x N 2à3x (x Z 1/3),namely the solid solution of oxynitride LaTaON 2and the complex oxide LaMg 2/3-Ta 1/3O 3.254The amorphous coating layer successfully prevented N 2evolution due to the accumulated hole oxidation of nitrogen species.By employing RhCrO y as a H 2evolution co-catalyst as shown in Fig.8,TiO x H/SiO x H-deposited LaMg 1/3Ta 2/3O 2N exhibited stable overall water splitting performance at wave-lengths of up to 600nm.However,the quantum efficiency of overall water splitting is still low (ca.0.03%at 440?30
nm).
Fig.8Reaction mechanism for water splitting on a surface coated LaMg x Ta 1àx O 1+3x N 2à3x photocatalyst.Reprinted with permission from ref.254.Copyright r 2015,Wiley-VCH Verlag GmbH &Co.KGaA,Weinheim.
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Other solid solution samples including CaZrO 3–CaTaO 2N,255SrTiO 3–LaTiO 2N,256La 0.8Ba 0.2Fe 0.9Mn 0.1O 3àx ,257Na 1àx La x -Fe 1àx Ta x O 3,258Na 0.5La 0.5TiO 3–LaCrO 3,259Cu-(Sr 1ày Na y )-(Ti 1àx Mo x )O 3,260Na 1àx La x Ta 1àx Cr x O 3,261
BiFeO 3–(Na 0.5Bi 0.5)TiO 3,262and Sr 1àx Bi x Ti 1àx Cr x O 3263have been reported as visible light photocatalysts for water splitting in a sacrificial system and degradation of organics.
In brief,with the hope of increasing the visible light harvesting,charge separation and transfer,and surface photocatalytic e?ciency,research on the ABO 3type and modified ABO 3type perovskite photocatalysts has been heavily focused on nano-structuring,morphology control,band-gap engineering by doping of metal and non-metal dopants and forming a solid solution.Among these ABO 3-related photocatalysts,La and Rh co-doped SrTiO 3has been demonstrated to be the so far best candidate for hydrogen evolution under visible light irradiation as demonstrated by Domen et al.112On the other hand,ABO 2N and its solid solution compounds have been recorded with ‘‘benchmark’’performance by extending the threshold of exci-tation wavelength near to 700nm for water splitting.247How-ever,the solar energy conversion e?ciency and the stability of these nitridized perovskites need to be further improved.Although great progress on the ABO 3perovskite photocatalysts has been achieved during recent decades,considerable research is needed to develop new perovskite-based photocatalysts or to design more e?cient composites based on the reported materials.
https://www.sodocs.net/doc/499996297.html,yered perovskite materials for photocatalysis
In addition to the ‘‘ideal’’ABO 3and the modified ABO 3materials discussed above,a number of layered perovskite photocatalysts are also reported.According to their structural characteristics,the reported layered perovskite photocatalysts are classified and discussed along with the summary as shown in Table 4.4.1
Ruddlesden–Popper (RP)phase
In the general formula of the RP phase,A n à1A 20B n X 3n +1,A and A 0represent alkali,alkaline earth,or rare earth metals while B refers to transition metals.The A cations are located in the perovskite layer and have a 12-fold cuboctahedral coordination to the anions.The A 0cations have a coordination number of nine and are located at the perovskite boundary with an intermediate block layer.The B cations are located inside the anionic octahedra,pyramids and squares.A series of RP-type layered tantalates,A 02ATa 2O 7(A 0=H,Li,K and Rb;A =La 2/3,Ca and Sr),as well as their hydrated products were presented as UV-light photocatalysts for water splitting.264–268The band gaps of these RP-type compounds are about 3.9–4.1eV.The first example of RP-type layered tantalates with a hydrated inter-layer,A 2SrTa 2O 7án H 2O (A =H,Li,K and Rb),was prepared by conventional solid state reaction and cation exchange methods.264Overall water splitting was achieved on H 2SrTa 2O 7án H 2O and K 2SrTa 2O 7án H 2O.The hydrated catalysts showed higher H 2and
O 2evolution rates than anhydrous Li 2SrTa 2O 7and KTaO 3.From the study of photoluminescence,they concluded that the recombination of photogenerated electrons and holes in hydrated tantalates was less.Thus,the photogenerated charges can be more effectively reacted with water to generate H 2and O 2.In addition,they further substituted Sr in A 2SrTa 2O 7with La,and the hydrated A 2La 2/3TaO 7exhibited higher activity than anhydrous perovskites (KTaO 3and La 1/3TaO 3).265The incorporation of Ni(II )species into the interlayer space,as the effective sites for H 2evolution,was found to enhance the photocatalytic water splitting activity.Other tantalates such as K 2Sr 1.5Ta 3O 10,266N -alkyl chain grafted H 2CaTa 2O 7,267Li 2CaTa 2O 7,268and H 1.81Sr 0.81Bi 0.19Ta 2O 7269were studied as UV-light photocatalysts for degradation of dyes.
A 2La 2Ti 3O 10(A =K,Rb and Cs)and doped A 2La 2Ti 3O 10comprise another family of RP-type layered titanates.Domen et al.firstly reported the ion-exchangeable layered perovskites with a general formula of A 2àx La 2Ti 3àx Nb x O 10(A =K,Rb,Cs;x =0,0.5,1)for water splitting.270As shown in Fig.9,Rb 2La 2Ti 3O 10showed the highest activity (869and 430m mol g à1h à1for H 2and O 2,respectively)with 4%Ni loading under UV-light irradiation among all of these compositions.A partial substitution of Ti 4+by Nb 5+reduced the number of alkaline metal cations located to keep the charge balance at the interlayer space.As a result,the hydration of the interlayer in Nb 5+substituted layered perovskites was inhibited,which reduced the photocatalytic activity conse-quently.Furthermore,a K 2La 2Ti 3O 10sample was also prepared by polymerized complex and hydrothermal methods.271,272The poly-merized complex synthesis helps to reduce the calcination time and enhance the purity of the sample.The optimized Ni/KaLa 2Ti 3O 10prepared by this method was more photoactive than conventional Ni/K 2La 2Ti 3O 10for water splitting,even though they have similar surface areas and band https://www.sodocs.net/doc/499996297.html,ter on,a series of Sn,Cr,Zn,V,Fe,Ni,W and N-doped K 2La 2Ti 3O 10samples were prepared to reduce the band gap of K 2La 2Ti 3O 10for performing photocatalysis tests under UV and visible light irradiation.273–278However,only Sn-doping effectively reduced the band gap of K 2La 2Ti 3O 10from ca.3.6eV to 2.7eV.The band gap of N-doped K 2La 2Ti 3O 10was estimated to be about 3.4eV.Other RP-type titanates such as Sr 3Ti 2O 7,279Sr 4Ti 3O 10,280Sr 2SnO 4,281Cr-doped Sr 2TiO 4,282Rh and Ln-doped Ca 3Ti 2O 7283and Na 2Ca 2Nb 4O 13284have also been investigated.4.2
Aurivillius phase (AL)
The Aurivillius phase is a form of perovskite represented by the general formula of (Bi 2O 2)2+(A n à1B n O 3n +1)2à,where A repre-sents the 12-fold coordinated cation with low valence in the perovskite sub-lattice,B denotes the octahedral site occupied by ions with high valence,and n is the number of perovskite layers between the [Bi 2O 2]2+layers.Bi 2MO 6(M =W and Mo),composed of perovskite-like [MO 4]2àlayers sandwiched between bismuth oxide [Bi 2O 2]2+layers,are the simplest members of the Aurivillius family and the most studied samples in this family.They were firstly reported by Kudo and co-workers for photocatalytic water splitting under visible light irradiation.285–288Bi 2WO 6(2.8eV)exhibits higher O 2evolution efficiency than
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Table 4
Summary of layered perovskite photocatalysts for water splitting and degradation of pollutants
Perovskites
Band gap (eV)Synthetic method a Incident light Reaction conditions Co-catalyst
(wt%)Activities (m mol g à1h à1)
Ref.H 2O 2Pollutants b Ruddlesden–Popper H 2SrTa 2O 7án H 2O 3.9SS/EX UV Water 770
358264H 2La 2/3Ta 2O 7 4.0SS/EX UV Water 158
77265K 2Sr 1.5Ta 3O 10 4.1SS UV Water RuO 2(0.5)39
11.8
266H 2CaTa 2O 7 3.9SS/EX UV Water RhB 267Li 2CaTa 2O 7
4.36SS UV Water RhB 268H 1.81Sr 0.81Bi 0.19Ta 2O 7
3.64SS/EX UV Water 24601110269A 2La 2Ti 3O 10(A =K,Rb,Cs)—SS UV Water
869430270K 2La 2Ti 3O 10
SG UV 0.1M KOH Ni (3.0)
21861131
271—HT UV 10%methanol 2.6
272Sn 2+and N 3àdoped 2.67SS/EX 4400nm
Water
RhB
273Cr doped —SS UV 0.1M KOH Ni (3.0)
32701650274Zn doped —SG UV 56mM I à126.6275V doped —SG UV 56mM I à
96276N doped
3.44SS UV 0.1M methanol 7.2277K 2La 2Ti 3àx M x O 10+a (M =Fe,Ni,W)—SS UV Na 2S Na 2SO 322278Sr 3Ti 2O 7 3.2SS/SG UV Water NiO (3.0)14472279Sr 4Ti 3O 10 3.2SS/SG UV Water NiO (3.0)170280Sr 2SnO 4
SS UV Water
RuO 2(1)42281Cr doped Sr 2TiO 4
SS 4400nm 0.05M Na 2SO 3Pt (1.0)170282Rh and Ln doped Ca 3Ti 2O 7—SS 4420nm 10%methanol Pt (0.1) 3.3283Na 2Ca 2Nb 4O 13 3.3MS 4300
nm
20%methanol
Pt (1.0)1355284Aurivillius Bi 2WO 6 2.8SS 4420nm 5.4%methanol
Pt (1.0)
1.634285
Bi 2MoO 6 3.00.05M AgNO 3
0.01
2.1Bi 2WO 6 2.69SS 4420nm 5mM AgNO 3 4.0CHCl 3286CH 3CHO
Bi 2MoO 6 2.7–2.8HT 4420nm 0.05M AgNO 375287Bi 2MoO 6 2.7SS 4420nm 0.05M AgNO 3
110288Bi 2Mo 2O 9 3.1 3.6Bi 2Mo 3O 12 2.8815.2PbBi 2Nb 2O 9
2.88SS 4420nm 30%methanol Pt (1.0)
7.65202930.05M AgNO 3
PbBi 2Nb 2O 9 2.88SS 4420nm 15%methanol 371429294PbBi 4Ti 4O 15
3.04400nm 0.05M AgNO 310.61716W doped PbBi 2Nb 2O 9 2.74SS 4420nm 15%methanol 0.05M AgNO 3
Pt (0.1)
15.3
631
295SrBi 2Nb 2O 9
3.5SG 254nm Water Aniline 2963.4
SS UV
RhB 297ABi 2Nb 2O 9(A =Sr,Ba) 3.34–3.54SG 254nm Water MO
298ABi 2Nb 2O 9(A =Ca,Sr,Ba) 3.46–3.30SS UV 12.5%methanol
366053029910mM AgNO 3
ALa 4Ti 4O 15(A =Ca,Sr,Ba) 3.8–3.9SS UV Water/CO 2Ag 363
168
CO 2300Bi 5Ti 3FeO 15
2.08HT 4420nm Water
RhB
301CH 3CHO 2.38SS 4420nm Isopropyl alcohol IPA 302Bi 5àx La x Ti 3FeO 15
2.0–2.7SS Solar light Water
RhB
303K 0.5La 0.5Bi 2M 2O 9(M =Ta,Nb) 3.4SS Water
2.9517304Bi 4Ti 3O 12
3.1SS UV 5%methanol Pt (1.0)
0.6 3.0285BaBi 4Ti 4O 15 3.30.05M AgNO 38.2 3.7Bi 3TiNbO 9
3.13331
Cr doped Bi 4Ti 3O 12
—
SG 4400nm 5%methanol 58.1
305Bi 2ASrTi 2TaO 12(A =Bi,La) 3.48,3.32SS UV Water RhB
306Dion–Jacobsen RbNdTa 2O 7
3.8SS UV Water 23
4.8126.4307SS RbLnTa 2O 7(Ln =La,Pr,Nd,and Sm)
3.8,3.9
SS UV Water NiO (0.5)586
293.5
308SS MLnTa 2O 7(M =Cs,Rb,Na,and H;
Ln =La,Pr,Nd,and Sm)
3.6–
4.2SS UV Water NiO x (0.5)277.5131.5309MCa 2Ta 3O 10(M =Cs,Na,H,C 6H 13-NH 3)4.0–4.3
SS/IE UV Water NiO x (0.5)1540790310MCa 2Ta 3O 10(M =Li,Na,K,Rb,Cs) 4.2–4.3SS UV
Water
NiO x (0.5)35401665311SS N-doped CsCa 2Ta 3O 10
2.0SS 4400nm 0.01M AgNO 321.6
312RbPb 2Nb 3O 10,HPb 2Nb 3O 10
—
SS/EX 4420nm 16%methanol
Pt (0.1)
15
313
SS
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Bi 2MoO 6(3.0eV)from aqueous AgNO 3solution under visible light irradiation.Due to the suitable band gap energy,relatively high photocatalytic activity and good stability,Bi 2MO 6com-pounds have been intensively studied as the AL-type visible light photocatalysts.More than one hundred papers related to the Bi 2WO 6and Bi 2MoO 6photocatalysts have been reported so far.Most of the studies in the literature are focused on the preparation of various nanostructured Bi 2WO 6and Bi 2MoO 6including nanosheets,nanofibers,hierarchical architectures,ordered arrays,nanoplates,inverse opals,hollow spheres and films etc.,by different preparation methods such as hydrothermal,solvothermal,molten salt,electrospinning,microwave and thermal evaporation deposition.The hydrothermal synthetic route has been mostly employed to control the morphologies and shapes of the particles.The photocatalytic activities of these nanostructured materials are mainly evaluated by the degradation of organic pollutants and selective organic transformations.In addition to the studies on the bare Bi 2WO 6and Bi 2MoO 6,doping of Zn,Mo,F,Er,N,Zr,Gd and W into Bi 2WO 6and Bi 2MoO 6was investi-gated for improving the photocatalytic behaviour under visible light.A summary of these Bi 2MO 6photocatalysts is not given here,since more detailed discussions can be found in several specific reviews.289–292
ABi 2Nb 2O 9(A =Ca,Sr,Ba and Pb)is another member of the AL-type layered perovskite with n =2.293–299PbBi 2Nb 2O 9with a band gap of 2.88eV was firstly reported as an undoped,single-phase oxide photocatalyst working under visible light.293Under visible light irradiation,Pt/PbBi 2Nb 2O 9gave a QE of 0.95%for H 2evolution from aqueous methanol solution,and a QE of 29%for O 2evolution from AgNO 3solution.However,the other
Table 4
(continued )
Perovskites Band gap (eV)Synthetic method a Incident light Reaction conditions Co-catalyst (wt%)Activities (m mol g à1h à1)
Ref.
H 2O 2
Pollutants b HCa 2Nb 3O 10
—SS/EX UV 10%methanol Pt (0.1)8400314SS 3.3SG/EX 450nm EDTA/Ru-dye Pt (0.3)17603213.3SG/EX 4420nm EDTA/Ru-dye Pt (0.3)4400322HSr 2Nb 3O 10 3.3SS/EX UV 1M 2-propanol Pt (0.3)900320KCa 2Nb 3O 10
SS UV 10%methanol Pt (1.0)5500315ACa 2Nb 3O 10(A =Li,Na,K)—SS UV Water
RuO x (0.25)389168316Pt/KCa 2Nb 3O 10
SS UV 10mM NaI Pt (1.0)17070317ASr 2Ta x Nb 3àx O 10(A =K,H) 3.3–4.3SS/EX UV 10%methanol Pt 9300318HCa 2Ta x Nb 3àx O 10/(ZnS,PbS) 3.5–3.7SS/EX UV Na 2S/Na 2SO 311200319AgLaNb 2O 7
2.98Flux UV 20%methanol Pt (1.0)2102323Ag/RbLaNb 2O 7,RbA 2Nb 3O 10 2.4–
3.7SS/EX UV 20%methanol Pt (0.1)13616324HLaNb 2O 7
3.9–
4.2SG/EX UV 10%methanol Pt
4800325H 1àx LaNb 2àx Mo x O 7 3.1,2.3SS/EX UV 10%methanol 3570326{111}layered Ba 5Nb 4O 15 3.9SG UV
Water NiO (0.7)47322278
327Ba 5Ta 4O 15
3.75HT 254nm Water
RhB 3284.5SS UV 10%methanol Rh (0.025)1600329M 5Nb 4O 15(M =Sr,Ba) 3.9–4.0SS UV
Water
NiO (0.5)80423944330N-doped Ba 5Ta 4O 15
1.78SS 4400nm 20%methanol Pt (0.1)49.5331N doped Sr 5Ta 4O 15or Ba 5Ta 4O 15
2.2
SS
4420nm
20%methanol Pt (0.3)91.7
332
10mM AgNO 3CoO x 132
{110}layered Sr 2Nb 2O 7 4.1SS UV Water
Ni (0.1)
402333
La 4CaTi 5O 17 3.8499
Sr 2Nb 2O 7
4.0HT UV Water RuO 2(0.5)475220
334Sr 2(Ta 1àx Nb x )2O 7
4.5–3.9SS UV Water NiO (0.15)1000500335Sr 2Nb x Ta 2àx O 7(x =0–2) 3.9–4.5SG UV Water NiO (0.15)35171733336Sr 2Ta 2O 7
4.6SS UV Water NiO (0.15)1000480337N-doped Sr 2Ta 2O 7 2.3SS AM 1.520%methanol Pt (0.5)439.5338La 2Ti 2O 7
3.2SS UV Water Ni (0.1)4413333.87SG UV Water Ni (1.0)960478
339—SS UV Water Ni (1.0)1373403.8MS UV Methanol Pt (1.0)1403413.40HT UV Water 72.43422.92HT UV 20%ethanol 7503433.1SS UV 0.5mM TMAH Ni (1.0)53286107344Ln 2Ti 2O 7(Ln =La,Pr,Nd) 3.0–3.8SS UV Water NiO x 400
200
345Ba,Sr,Ca doped SS UV Water NiO x
2010346
Cr,Fe doped 2.2,2.6SS 4420nm 33.3%methanol Pt (1.0)15347and 348N doped
2.51HT 4420nm Water
MO
349Rh doped La 2Ti 2O 7
—SS,MS,SG 4420nm 10%methanol Pt (0.5)8.3
350
a
HT:hydrothermal;SS:solid-state;MS:molten salt;SG:sol–gel;EX:ion-exchange.b
RhB:rhodamine B;MO:methyl orange;IPA:isopropyl
alcohol.
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niobates like ABi 2Nb 2O 9(A =Ca,Sr,Ba)have the larger band
gaps of about 3.3–3.5eV.296–299Under UV-light irradiation,Ag/ALa 4Ti 4O 15(A =Ca,Sr and Ba)photocatalysts with ca.3.8eV of band gaps can reduce CO 2to CO and HCOOH by bubbling CO 2gas into an aqueous suspension of the photocatalyst powder without any sacrificial reagents.300Among these perovskites,Ag/BaLa 4Ti 4O 15is the most active photocatalyst.Under specific loading of an Ag co-catalyst on the edge of BaLa 4Ti 4O 15,the main reduction product from Ag/BaLa 4Ti 4O 15suspension was CO rather than H 2.As shown in Fig.10,it was proposed that the edge and the basal plane of BaLa 4Ti 4O 15were the reduction and water oxidation sites,respectively.Loading fine Ag particles (10–20nm)onto BaLa 4Ti 4O 15by impregnation and H 2reduction or a liquid phase reduction method,CO 2reduction to CO and HCOOH predominated over water reduction to form H 2.The stoichiometric ratio of reduction and oxidation products (H 2+CO :O 2=2:1)suggested that water was consumed as an electron donor for the CO 2production.
In addition to unique magnetoelectric properties,a nano-structured four-layered Bi 5FeTi 3O 15perovskite with a band gap of ca. 2.1eV also exhibits visible light photocatalytic activities.301,302The hydrothermal synthesis can produce nanoplate-based,flower-like hierarchical morphology,and the detailed growth process,from nanonets to nanoplate-built microflowers was revealed.The photocatalytic activity of the as-prepared Bi 5FeTi 3O 15was evaluated by the photodegradation of acetaldehyde and rhodamine B under visible light irradiation.301By degradation of rhodamine B under solar-light irradiation,the photocatalytic performance of La substituted Bi 5àx La x Ti 3FeO 15(x =1,2)with band gaps of 2.0–2.7eV was also evaluated.303Other AL-type compounds such as K 0.5La 0.5Bi 2M 2O 9(M =Ta,Nb),304Bi 4Ti 3O 12,285BaBi 4Ti 4O 15,285Bi 3TiNbO 9,285Cr-doped Bi 4Ti 3O 12,305and Bi 2ASrTi 2TaO 12(A =Bi,La)306have been studied as UV light photocatalysts for water splitting.Among AL-type perovskites,only Bi 2MO 6(M =W or Mo),PbBi 2Nb 2O 9and Bi 5Ti 3FeO 15are active under visible light.Considering the band edge positions,these perovskites are more useful for water oxidation and pollutant degradation owing to the deep valence band positions,whereas their activities for H 2production are much lower even loading with a Pt co-catalyst.PbBi 2Nb 2O 9is a unique AL-type perovskite that shows a QE of 29%for O 2evolution in 0.05M AgNO 3solution,293yet the presence of toxic lead in the compounds is still a concern from the environmental point of view.4.3
Dion–Jacobsen (DJ)phase
A series of layered lanthanide tantalates and their ion-exchanged compounds with the general formula of A(Ln n à1Ta n O 3n +1)(A =K,Rb,Cs,Ag and H;Ln =La,Pr,Nd,Sm;n =2and 3,respectively)were studied as DJ-type photocatalysts.307–309The band gaps of these tantalates are in the range of 3.8–4.3eV.Upon UV-light irradiation,RbNdTa 2O 7was firstly demonstrated by Machida et al.for efficient evolution of stoichiometric H 2/O 2even without loading metal catalysts.307Later on,they further investigated the H 2and O 2evolution from MLnTa 2O 7tantalates (M =H,Na,Rb and Cs;Ln =La,Pr,Nd and Sm).In the case of M =Rb,the activity follows in the order of Ln:Nd 4Sm 4La 4Pr.308The effect of Ln was explained from the aspect of the energy level of the Ln 4f bands and the degree of Ln–O–Ta hybridization in the band structure dominated by Ta 5d and O 2p orbitals.Furthermore,they evaluated the effect of ion exchange of interlayer cations (M =H,Na and Rb)on the photocatalytic activities of MLnTa 2O 7.309The photocatalytic activity of hydrated HLaTa 2O 7was very low regardless of the unchanged band gap energy.The hydrated interlayer would lead to a considerable modification of the valence band structure and the formation of structural defects as evident from the XRD study,which was suggested as the possible reason for the reduced activity.Unlike the hydrated forms of layered perovskites such as K 4Nb 6O 17and K 2La 2Ti 3O 10,which are highly active for overall water splitting,the hydration of MLaTa 2O 7may not allow successive photooxidation reactions inside the interlayer.To further elucidate the effect of interlayer hydration,another group of DJ-type tantalates (MCa 2Ta 3O 10,M =Cs,Na,H and C 6H 13NH 3)with a triple-layer structure
was
Fig.10Mechanism of photocatalytic CO 2reduction over BaLa 4Ti 4O 15with an Ag co-catalyst loaded by several methods.Reprinted with permission from ref.300.Copyright r 2011,American Chemical Society.
Fig.9Proposed reaction mechanism of water splitting on the layered A 2La 2Ti 3O 10catalyst.Reprinted with permission from ref.270.Copyright r 1997,American Chemical Society.
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synthesized.310,311In the presence of 0.5wt%NiO x ,the hydrated Na-exchanged phase exhibited an order of magnitude higher activity compared to the anhydrous Cs phase and the hydrous H phase for overall water splitting.While the organic C 6H 13NH 3-intercalated phase was not stable in the photooxidation reactions.The hydrated interlayer was found as the active sites for water splitting by H 2O/D 2O isotopic experiment.Thus,the effect of hydration,associated with the mobility of water molecules inside the interlayer space,is different among the DJ-type layered perovskites.On the other hand,N-doped CsCa 2Ta 3O 10with a band gap of 2.0eV was reported as the first example on the utilization of N-doped ion-exchangeable layered perovskite photo-catalysts for water oxidation under visible light.312
In the case of layered niobates with the DJ phase,APb 2Nb 3O 10(A =K,Rb,Cs and H)was initially studied for H 2evolution from an aqueous alcohol solution.313The band gaps of most of the layered niobates are wide,however,RbPb 2Nb 3O 10sheets have an absorption band in the visible light region up to ca.500nm with a broad band tail absorption,which may be due to the existence of defects in niobate sheets.RbPb 2Nb 3O 10was not e?cient for H 2evolution under visible light,even loading with a Pt co-catalyst.However,when Rb +ions were exchanged by H +ions,the hydrated HPb 2Nb 3O 10exhibits remarkable enhancement of H 2evolution activity.This enhancement was suggested due to the fast migra-tion of reactants (H 2O and CH 3OH)into the interlayer space of HPb 2Nb 3O 10.Another series of layered AB 2Nb 3O 10(A =H,Li,Na and K;B =Ca and Sr)based materials have relatively larger band gaps.314–322The colloidal KCa 2Nb 3O 10nanosheet suspension was immediately flocculated when added to an aqueous alkali hydroxide solution.The delaminated nanosheets were restacked together with accommodating alkali metal ions and water mole-cules between the nanosheets.Such an exfoliation/restacking process generated porous aggregates with a high surface area and enhanced the photocatalytic activity for hydrogen evolution from an aqueous methanol solution under UV-light irradiation.315Furthermore,by incorporation of RuO x into the exfoliated/restacked material as active sites for water oxidation,overall water splitting with a stoichiometric ratio was achieved upon UV-light illumination.316Mallouk’s group found that the photo-catalytic activity of restacked triple-layered nanosheets (HSr 2Nb 3O 10)was an order of magnitude higher than that of the double-layered HLaNb 2O 7nanosheets,even though there was little difference in the physicochemical characteristics.320In addition,they constructed a visible light H 2production system from HCa 2Nb 3O 10nanosheets utilizing Ru–bipyridine dye as a sensitizer.321,322Recently,Pt nanoclusters with a diameter smaller than 1nm were deposited on the interlayer nanospace of KCa 2Nb 3O 10.The Pt incorporated material exhibited an eight-fold greater photocatalytic activity for water splitting than the previous RuO 2loaded sample.317
The interlayer cation e?ect was also studied by silver-exchange of A-site cations in RbLaNb 2O 7and RbA 2Nb 3O 10(A:Ca and Sr).323,324The substitution of silver cations into the interlayer spacing of these layered compounds is able to reduce the band gap by about 0.5–1.0eV.The Ag-exchanged products exhibited significantly improved activity by an order
of magnitude than those prior to Ag-exchange for H 2production under UV-light irradiation.The Ag-exchanged RbCa 2Nb 3O 10with a loaded 1.0wt%Pt co-catalyst exhibited the highest photo-catalytic H 2production rate (ca.13616m mol g à1h à1)from 20%methanol solution under UV-light irradiation,but it was not active under visible light irradiation.Ag +at the particle surfaces was reduced to Ag particles during prolonged UV-light irradia-tion,which was one of the reasons for enhanced activities.In addition,hydrated HLaNb 2O 7and H 1àx LaNb 2àx Mo x O 7were also studied for H 2production from methanol solution under UV-light irradiation.325,3264.4
{111}layered perovskites
Layered perovskite A 5B 4O 15(A =Ba and Sr,B =Nb and Ta)with a plane in parallel with {111}was investigated as a series of UV-light photocatalysts.A Ba 5Nb 4O 15photocatalyst was firstly prepared by conventional solid-state reaction and polymer-complex methods for water splitting under UV-light irradiation.327By loading a suitable amount of NiO co-catalysts,Ba 5Nb 4O 15exhibited remarkably enhanced H 2and O 2evolution rates of 4.8and 2.4mmol h à1g à1,respectively.At the same time,Zhu et al.synthesized monomolecular-layered Ba 5Ta 4O 15nano-sheets with a hexagonal structure by a hydrothermal method.328The thickness of the nanosheets is ca.1.1nm,which corre-sponds to a monolayer of Ba 5Ta 4O 15.The mono-layered structure of Ba 5Ta 4O 15facilitates the migration of electron–hole pairs to the sample surface,as demonstrated by the degradation of rhodamine B and gaseous formaldehyde.H 2evolution on the Rh-loaded Ba 5Ta 4O 15from an aqueous methanol solution was also reported with a rate of 1600m mol g à1h à1under UV-light irradiation.329After that,the photophysical and photocatalytic properties of Sr 5Nb 4O 15and Ba 5Nb 4O 15were compared with La 4Ti 3O 12and ALa 4Ti 4O 15(A =Ca,Sr and Ba)layered perovskites.330Their band gaps are ca.3.7–4.1eV.The NiO x loaded BaLa 4Ti 4O 15and NiO x /Ba 5Nb 4O 15are the most active photocatalysts for water splitting among the tested titanates and niobates.
Recently,N-doped layered Ba 5Ta 4O 15and Sr 5Ta 4O 15were studied for water splitting under visible light irradiation.331,332N-doping markedly reduced the band gaps from ca.4.0eV to 1.78eV for the doped Ba 5Ta 4O 15and 2.2eV for the doped Sr 5Ta 4O 15,respectively.The strong absorption of visible light for the N-doped Ba 5Ta 4O 15was also demonstrated by the red-shift of the absorption band in the UV-Vis absorption spectrum as shown in Fig.11.331For the doped Sr 5Ta 4O 15,both conduction and valence band edges of Sr 5Ta 4O 15àx N x estimated from Mott–Schottky measurements possess sufficient poten-tials for the respective water reduction and oxidation.This is the first example of nitrogen-doped tantalum-based layered oxide that is able to achieve both the two half reactions of water splitting under visible light illumination.4.5
{110}layered perovskites
{110}layered perovskite materials have a general composition of A n B n O 3n +2(A =Sr,La;B =Ta,Nb,Ti;n =4,5).For example,Sr 2M 2O 7(B =Ta and Nb)with the perovskite slabs parallel to
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