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A facile synthesis of anatase TiO2 nanosheets-based hierarchical spheres

A facile synthesis of anatase TiO 2nanosheets-based hierarchical spheres with over 90%{001}facets for dye-sensitized solar cells w

Weiguang Yang,ab Jianming Li,a Yali Wang,b Feng Zhu,a Weimin Shi,b Farong Wan c and Dongsheng Xu*a

Received 17th August 2010,Accepted 19th November 2010DOI:10.1039/c0cc03312j

Anatase TiO 2nanosheets-based hierarchical spheres with over 90%{001}facets synthesized via a diethylene glycol–solvothermal route were used as photoanodes of dye-sensitized solar cells,which generated an energy conversion e?ciency of 7.51%.Dye-sensitized solar cells (DSSCs),which emerged as a new generation photovoltaic device,have been studied extensively because of their high e?ciency,low cost and facile fabrication process.1The heart of the DSSC system is a wide bandgap semiconductor nanoparticle ?lm that provides a large surface area for the adsorption of light-harvesting molecules.Because attached to the surface of the nanocrystalline ?lm is a monolayer of the charge transfer dye,an increase of the surface roughness factor of the nanocrystalline ?lm is very necessary to meet the requirement of e?cient harvesting of sunlight.Classically,thick mesoporous wide-band gap oxide semiconductor ?lms with an enormous internal surface area,typically a thousand times larger than that of bulk ?lms,were used as the photoelectrode of DSSCs.2However,the high surface area of a nanocrystalline ?lm should bring about many opportunities for the recombination of photoinjected electrons and the oxidized dye and/or the electron acceptors in the electrolyte.In addition,since the sensitizers (typically,N3or N719)have low absorption e?ciency in the red region,large particles are incorporated into the ?lm to enhance the photoresponse to red light by means of light scattering e?ects.However,the reduction of surface area counteracts the enhancement e?ect of light scattering on the optical absorption.Moreover,improving the absorption of red and near-infrared light through thickening the nanoparticle ?lm to increase its optical density is limited by the electron di?usion length through the nanoparticle network.3

To balance the light scattering and surface area e?ects,hierarchically mesoporous microspheres,which are comprised of polydisperse aggregates consisting of nanosized crystallites,have been studied.4–6The mesoporous microspheres are submicrometre-sized and,thus,can function as e?cient light scatters,while the nanocrystallites provide the ?lms with the

necessary mesoporous structure and large internal surface area.However,a decrease of the electron di?usion length arose by large amounts of electron traps while keeping high surface area is still a big problem for those mesoporous microspheres assembled by nanosized crystallites.Herein,we report a facile and nontoxic solvothermal route of preparation of anatase TiO 2nano-sheets-based hierarchical spheres (ATNHSs)as the photoanodes in DSSC for the improvement of energy conversion e?ciency.Although several studies by research groups worldwide have developed new routes to prepare more anatase TiO 2sheets with exposed {001}facets,7the synthesis of 3D hierarchitectures self-assembled with sheetlike anatase TiO 2as building blocks is up to date extremely sparse.8Compared to the mesoporous spheres consisting of nanosized crystallites,we proposed that the nanosheet-like architectures can e?ectively minimize the grain interface e?ect,and the anatase (001)surfaces with high surface energy have strong ability to dissociatively absorb (COOH)group.9As a result,an overall energy conversion e?ciency up to 7.51%has been achieved,indicating a 43%increase compared to the standard Degussa P25photoanode (5.26%).

ATNHSs with dominantly exposed {001}facets have been synthesized by a diethylene glycol (DEG)–solvothermal method,in which TiF 4was used as a Ti precursor.In a typical 5mL of acetic acid was added in a 0.01–0.03M TiF 4–DEG solution under magnetic stirring for 3h at room temperature,and then transferred to a Te?on-lined stainless steel autoclave of 50mL capacity and heated for 8h at 1801C with a heating rate of 11C min à1.Fig.1shows the scanning electron microscopy (SEM)images of the products prepared at di?erent TiF 4concentrations,indicating a hierarchical structure with numerous randomly arranged TiO 2The as-prepared ATNHSs

Fig.1SEM images of the hierarchically structured TiO 2nanosheets prepared at di?erent TiF 4concentrations:(a and b)10mM;(c and d)30mM.Scale bars:(a,c)500nm;(b,d)100nm.

Beijing National Laboratory for Molecular Sciences,

State Kay Laboratory for Structural Chemistry of Unstable and Stable Species,College of Chemistry and Molecular Engineering,Peking University,Beijing 100871,China.E-mail:dsxu@https://www.sodocs.net/doc/8a18292640.html,;Fax:+8610-62760360;Tel:+8610-62753580b

School of Materials Science and Engineering,Shanghai University,Shanghai 200444,China c

Beijing Key Lab of Advanced Energy Material and Technology,Department of Materials Physics and Chemistry,University of Science and Technology Beijing,Beijing100085,China

w Electronic supplementary information (ESI)available:Experimental details and ?gures.See DOI:10.1039/c0cc03312j

COMMUNICATION https://www.sodocs.net/doc/8a18292640.html,/chemcomm |ChemComm

D o w n l o a d e d b y N o r t h e a s t N o r m a l U n i v e r s i t y o n 06 D e c e m b e r 2011P u b l i s h e d o n 06 D e c e m b e r 2010 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 0C C 03312J

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prepared at a TiF 4concentration of 10mM are quite uniform with an average diameter of 250nm (Fig.1a).The higher magni?cation micrograph shows that the TiO 2nanosheets in the hierarchical structures are 7nm in thickness and 30–180nm in diameter (Fig.1b).With increasing the TiF 4concentration to 30mM (Fig.1c and d),the average diameter of the obtained microspheres decreases from 250nm to 200nm and the amounts of constituent nanosheets tend to increase,while the thickness of the constituent nanosheets remains about 7nm.

Fig.2a displays the X-ray di?raction (XRD)pattern of the as-prepared ATNHSs with an average diameter of 200nm,giving a pure anatase TiO 2phase.The relative intensities of the (200)and (004)re?ections represent the a and c axes,respectively.The (200)re?ection of the as-synthesized product is signi?cantly more intense and sharper than both the (004)and (101)peaks,indicating a domain crystalline growth along the a axis.Fig.2b shows the nitrogen adsorption–desorption isotherm of the ATNHSs.The isotherm displays the typical type IV curve with a H3hysteresis according to the IUPAC classi?cation,which is usually ascribed to slit-like mesopores formed by sheet-like particles.10The Barrett–Joyner–Halenda (BJH)pore size distribution (Fig.4b)obtained from the isotherm indicates that the BET speci?c surface area is B 63.5m 2g à1and the ATNHS sample contains two types of the pores.The ?rst type of pores may be ascribed to a smaller portion of mesopores (about 2.5nm)attributed to the parallel assembly of the nanosheets.The second type comprises a majority of the slit-like mesopores (about 17.9nm)which resulted from randomly assembled nanosheets.Besides,the voids formed by interaggregated ATNHSs may also contribute to the second type of pores.

The microstructures of the ATNHSs were further characterized by transmission electron microscopy (TEM).Fig.3a reveals that the entire structure of the architecture is built from several dozens of self-organized TiO 2nanosheets.A representative higher-resolution TEM image of a single

nanosheet is given in Fig.3b.Two sets of the lattices with an equal fringe spacing of 0.19nm and an interfacial angle of 901are in well agreement with the (200)and (020)planes of anatase TiO 2.7a The corresponding fast Fourier transform (FFT)pattern (the inset in Fig.3b)can be indexed to di?raction spots of the [001]zone.Furthermore,the HRTEM image from the vertical nanosheet (Fig.3c)shows that the lattice fringes spacing of 0.235nm parallel to the top and bottom facets are assigned to (001)planes of anatase TiO 2.On the basis of the above structural information,the percentage of exposed {001}facets of the ATNHSs can be estimated to be over 90%.

The formation mechanism of such a nanosheet-based hierarchical structure could be understood by the capping e?ect of ?uorine ions on the exposed {001}facets.Generally,the synergistic functions of ?uorine ions can markedly reduce the surface energy of the {001}facets to a level lower than that of {101}facets.7a The emerging high-energy (001)facets are most e?ectively stabilized by ?uorine ions formed in the hydrolysis reaction of TiF 4,which signi?cantly retards their growth along the [001]direction.In addition,we found that in a control experiment,with the replacement of DEG by triethylene glycol,no nanosheet-like structure was observed (Fig.S1,ESI w ).The unsaturated Ti 4+cations on the (001)and (101)surfaces are inclined to be coordinatively bound to DEG or deprotonated DEG molecules.11Therefore,it is hypothesized that DEG could also acts as a protective capping agent and the growth rate along the (001)direction relative to the (101)direction is further retarded.

DSSCs consisting of TiO 2?lms with the ATNHSs were tested by measuring the current–voltage behaviour under illumination with a power density of 100mW cm à2.For comparison,the TiO 2photoanodes made of commercial Degussa P25TiO 2nanoparticles were also investigated.Fig.4a shows the typical current density versus voltage curves of the ATNHSs and P25photoanodes.It can be seen that the TiO 2photoanodes made from the ATNHSs show a J sc of 17.9mA cm à2and a V oc of 0.65V with the energy conversion e?ciency (Z )of 7.51%,whereas the photoanodes made from P25nanoparticles give a J sc of 11.6mA cm à2and a

Fig.2(a)XRD pattern and (b)nitrogen adsorption–desorption isotherm and Barret–Joyner–Halenda (BJH)pore size distribution plot (inset)of the as-prepared sample prepared at the TiF 4concen-tration of 30mM.

Fig.3(a)Low-magni?cation TEM and (b and c)high-resolution TEM images of the ATNHSs prepared at a TiF 4concentration of 30mM.The insets show the corresponding FFT patterns.

V oc of 0.63V with a Z of 5.26%,indicating 54%and 43%increases in J sc and Z compared to the P25photoanodes of the similar thickness,respectively.Meanwhile,a decrease of the dark current demonstrated that the nanosheet-like architecture can e?ectively minimize the grain interface e?ect and reduce the electron loss.To analyze in more detail the enhanced cell performance,incident monochromatic photo-to-electron conversion e?ciency (IPCE)measurements were conducted.Fig.4b shows the IPCE as a function of wavelength measured at short circuit.The IPCE is determined by the light absorption e?ciency of the dye,the quantum yield of electron injection,and the e?ciency of collecting the injected electrons at the conducting glass substrate,12which is strongly a?ected by the morphology and surface area of the photoelectrode.13Compared with the P25TiO 2?lm,the ?lm of the ATNHSs had a higher IPCE from 400nm to 750nm wavelength ranges.At the maximum value of the IPCE spectra at 520nm,the IPCE of the ?lm of the ATNHSs was approximately 67%higher than that of the P25TiO 2?lm.

The great improvement in the J sc and Z of the ATNHSs can be attributed to the following outstanding features:(i)the anatase (001)surfaces with high surface energy have strong ability to dissociatively absorb (COOH)group.9During this process,the carbonyl O atom binds to 5-fold Ti (Ti 5C )on the (001)surfaces,while the hydroxy H is transferred to O 2C .14This can increase the electronic coupling between the sensitizer and the TiO 2semiconductor and favor electron injection from the excited state of the sensitizer into the TiO 2conduction band;15(ii)the ATNHSs have high surface area,which is essential to increase dye loading,and meanwhile,the submicrometre-sized TiO 2spheres can promote the light harvesting of the N719dye within the photoanodes due to the light scattering e?ect (Fig.S2,ESI w );(iii)the unique hierarchical structure built from nanosheets with large particle

sizes can e?ectively minimize the grain interface e?ect and thus reduce the electron loss.

In summary,we have developed a facile solvothermal method for the synthesis of ATNHSs with dominantly exposed {001}facets.The obtained hierarchical TiO 2spheres consist of ultrathin anatase nanosheets (B 7nm in thickness and 30–180nm in lateral size),with diameters of 200–250nm and a speci?c surface area of B 63m 2g à1.Inspired by the unique structure,the photoanodes of DSSC made from these hierarchical spheres exhibit great improvement in J sc and Z .We expect that such ATNHSs will have a good potential for the applications in energy conversion and catalysis,because their dominantly exposed {001}surfaces with high surface energy and high surface area provide multiple bene?ts.

This work was supported by NSFC (Grant No.50821061)and MSTC (MSBRDP,Grant Nos.2006CB806102,2007CB936201).

Notes and references

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Fig.4(a)Comparison of the current–voltage characteristics of DSSCs based on the ATNHSs and P25TiO 2.(b)IPCE spectra as a function of wavelength in the dye-sensitized ATNHSs and P25?lms with the similar thickness (about 14m m).

A facile synthesis of anatase TiO2 nanosheets-based hierarchical spheres

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