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Haimin Zhang , Yun Wang , Dongjiang Yang , Yibing Li , Hongwei Liu , Porun Liu , Barry J. Wood , and Huijun Zhao *

Directly Hydrothermal Growth of Single Crystal Nb 3 O 7 (OH) Nanorod Film for High Performance Dye-Sensitized Solar Cells

As a promising alternative to conventional silicon-based solar cells, dye-sensitized solar cells (DSSCs) have attracted exten-sive interest in recent years due to their potential low-cost and high-effi ciency. [ 1–6 ] The photoanode is an essential com-ponent of DSSCs that plays a key role to determine the dye loading and photoelectron transfer, hence the light conversion effi ciency. [ 1–5 , 7 , 8 ] Although nanostructured TiO 2 has been the most widely used photoanode material, much effort has also been paid to develop alternative photoanode materials such as Nb 2 O 5 , ZnO, SnO 2 , SrTiO 3 , and other composites. [ 9–14 ] A main objective of these research activities is to develop alternative photoanode materials that possess one or combined advan-tages of large surface area and/or rich surface functionality to improve the dye loading capacity; suitable structure and good crystallinity to facilitate the photoelectron transfer and enhance the current collection effi ciency; and more negative conduction band (CB) edge positions versus TiO 2 to achieve high open-circuit voltage ( V oc ). [ 5 , 7 , 15 , 16 ] Development of new photoanode materials which can be used for assembly of the fl exible DSSCs is also an important aspect of such research activities. [ 17 , 18 ]

As a wide bandgap semiconductor, Nb 2 O 5 has been widely investigated for photocatalysis, lithium-ion battery, and solar cells. [ 19–24 ] A higher V oc is expected for a DSSC assembled with Nb 2 O 5 photoanode because of its more negative CB edge posi-tion relative to TiO 2 . [ 21 , 22 ] Le Viet et al . recently investigated the CB edge potentials of different Nb 2 O 5 crystal structures. [ 22 ] Their investigation suggests that the CB edge potentials of H-Nb 2 O 5 (pseudohexagonal), O-Nb 2 O 5 (orthorhombic) and M-Nb 2 O 5 (monoclinic) are of 0.82, 0.78 and 0.87 eV more nega-tive than that of TiO 2 . For DSSC application, 3.05% conversion effi ciency can be obtained for H-Nb 2 O 5 photoanode, which is signifi cantly higher than that of DSSCs made from O-Nb 2 O 5 and M-Nb 2 O 5 photoanodes due to the high surface area of

© 2012 WILEY-VCH Verlag G

Dr. H. M. Zhang , Dr. Y. Wang , Dr. D. J. Yang , Y. B. Li , Dr. H. W. Liu , Dr. P. R. Liu , Prof. H. J. Zhao Centre for Clean Environment and Energy and Griffi th School of Environment Griffi th University Gold Coast Campus, QLD 4222, Australia E-mail: h.zhao@griffi th.edu.au Dr. B. J. Wood Centre for Microscopy & Microanalysis The University of Queensland Brisbane, QLD 4072, Australia

DOI: 10.1002/adma.201104650

Adv. Mater. 2012, DOI: 10.1002/adma.201104650

H-Nb 2 O 5 nanowires. [ 22 ] To date, Nb 2 O 5 with various nanostruc-tures including nanoparticles, nanobelts, nanowires and nano-forests have been synthesized by hydrothermal method, electro-spinning technique and pulsed laser deposition (PLD). [ 21 , 22 , 25–27 ] However, as the photoanode materials for DSSCs using these nanostructures, the obtained light-to-electricity conversion effi -ciencies are still unsatisfactory (≤5.0%). [ 21 , 22 , 25–27 ] The low effi -ciency has mainly been attributed to the insuffi cient surface area for dye loading. [ 21 , 22 ] Therefore, development of niobium oxide nanostructure with high surface area is highly desired. In 1978, Kodama et al . reported the synthesis of Nb 3 O 7 (OH) nano-rods by hydrothermally treating niobic acid or triniobium chlo-ride heptaoxide with sulfuric acid at 250–350 ° C and 15 MPa. [ 28 ] However, no detailed structure information was given in their study.

Herein, we report a facile, one-pot hydrothermal method to directly grow the three-dimensional (3D) high crystallinity Nb 3 O 7 (OH) single crystal nanorod fi lm on FTO substrate. The obtained 3D nanorod fi lm possesses a high surface area of 104 m 2 /g with an average pore size of 19.5 nm. The fi lm composes of Nb 3 O 7 (OH) single crystal nanorods with an average diameter of 22 nm and an average length of 230 nm. In addition to the large surface area, the as-synthesized Nb 3 O 7 (OH) nanorod fi lm displays an excellent crystallinity, which can be directly used as the photoanode for DSSCs without need for further calcination to achieve an impressive overall effi ciency of 6.77%, the highest among all reported DSSCs assembled with niobium oxide-based photoanodes. [ 21 , 22 , 25–27 ] Such a distinctive feature of the as-synthesized Nb 3 O 7 (OH) nanorod fi lm makes it a promising candidate for fabrication of fl exible photoanodes. [ 17 , 18 ] To the best of our knowledge, this is for the fi rst time a directly grown 3D Nb 3 O 7 (OH) single crystal nanorod fi lm photoanode is used for DSSCs without the calcination. Additionally, the crystal structure of Nb 3 O 7 (OH) has been precisely identifi ed based on the experimental data and theoretical calculations.

The XRD pattern of the as-synthesized sample ( Figure 1 A) can be indexed to an orthorhombic Nb 3 O 7 (OH) structure with lattice parameters of a = 20.74 Å, b = 3.823 Å and c = 3.936 Å (JCPDS, Card No. 31-0928). [ 28 ] The SEM image of the as-synthesized Nb 3 O 7 (OH) shown in Figure 1 B displays a uniform nanorod fi lm. High magnifi cation SEM image indicates that the obtained nanorods with an average diameter of 22 nm and an average length of 230 nm (inset in Figure 1 B). The cross-sectional SEM image reveals that the as-synthesized nanorod fi lm has an average thickness of 11.5 μ m, with uniform porous structure across the entire fi lm (Figure 1 C and inset). TEM image of an individual Nb 3 O 7 (OH) nanorod is given in Figure 1 D.

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Figure 1 . (A) XRD pattern of the as-synthesized sample obtained at 210 ° C for 24 h. (B) SEM image of the as-synthesized Nb 3 O 7 (OH) nanorod fi lm (low magnifi cation) and inset of high magnifi cation SEM image. (C) Cross-sectional SEM image of the obtained Nb 3 O 7 (OH) nanorod fi lm (low magnifi cation) and inset of high magnifi cation SEM image. (D) TEM image of an indi-vidual Nb 3 O 7 (OH) nanorod with insets of SAED pattern (top) and HRTEM image. (E) Atomic structure models of Nb 3 O 7 (OH). Atom colour code: green–Nb, red–O, black–H.

The SAED pattern (top inset in Figure 1 D) and HRTEM image (bottom inset in Figure 1 D) reveal a good single crystalline nature of the as-synthesised Nb 3 O 7 (OH) nanorods. The SAED data confi rm a preferred growth along [010] direction, while HRTEM image confi rms the fringe spacings of 0.376 nm and 1.04 nm, which are consistent with the d values of (110) and (200) planes of the orthorhombic Nb 3 O 7 (OH), respectively. [ 28 ] Known precise atomic structure of crystals is critical for under-standing their properties. For the single crystal Nb 3 O 7 (OH), the

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insightful knowledge on the atomic structure, especially the H atom in the atomic structure, is specially interested. The density functional theory (DFT) is therefore performed to gain such insight information. The locations of Nb and O atoms can be readily derived from XRD data for building the atomic structure model. However, the precise H atom location in the Nb 3 O 7 (OH) atomic structure cannot be confi rmed by the XRD data. Four struc-tural confi gurations based on the symmetry of the systems involving possible H atom locations are therefore considered for DFT model (Figure S1, Supporting Information). The calculation results suggest that the intra-molecular hydrogen bonds play a critical role capable of reducing the system energy by 0.2–0.3 eV. After the lattice constants and atomic coordinates are fully optimized, the most stable structure (Figure 1 E) is found to be confi gured by the H atom bonding to one O atom at the (002) plane and interacting with another nearest O atom at the same plane via a hydrogen bond to form O − H·O (Figure S1A, Supporting Information). [ 29 ]

The effect of thermal treatment on the morphology and crystal structure of the Nb 3 O 7 (OH) nanorods was investigated. The XRD pattern of the calcined sample can be indexed to Nb 2 O 5 , suggesting a struc-tural transformation from orthorhombic Nb 3 O 7 (OH) to monoclinic Nb 2 O 5 (JCPDS, Card No. 71-0005) has occurred (Figure S2A, Supporting Information). [ 28 ] Despite the crystal structural changes during the calcina-tion process, the obtained SEM image shows no obvious dimensional and morphological changes after calcination, except the forma-tion of some nanorod bundles (Figure S2B, Supporting Information). The TEM image of an individual Nb 2 O 5 nanorod is given in Figure S2C (Supporting Information). The SAED data indicate that the obtained Nb 2 O 5 nanorods are single crystals with high crystallinity (top inset in Figure S2C, Sup-porting Information). The fringe spacings of 0.374 nm and 1.02 nm are consistent with the d values of (110) and (101) planes of the monoclinic Nb 2 O 5 , respectively (bottom inset in Figure S2C, Supporting Information). In

order to transform Nb 3 O 7 (OH) to Nb 2 O 5 during calcination, the OH groups in the (002) plane are removed via a dehydration process, leading to a serious deformation of the Nb 3 O 7 (OH) crystal structure along z axis. However, the deformation along x and y directions are minor, as shown in Figure 2 . Indeed, the obtained lattice constants changes when the Nb 3 O 7 (OH) is con-verted to Nb 2 O 5 do support the theoretical prediction (Table S1, Supporting Information). The structure transformation caused by the calcination is also confi rmed by the FT-IR spectra. The

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Figure 2 . Schematic illumination of atomic structure transformation from orthorhombic Nb 3 O 7 (OH) to monoclinic Nb 2 O 5 during calcination. (A) orthorhombic Nb 3 O 7 (OH); (B) monoclinic Nb 2 O 5 . The locations of OH groups are highlighted by blue cycles in Figure 2 A. Atom colour code: green–Nb, red–O, black–H; green octahedron: [NbO 6 ].

Figure 3 . Photocurrent as a function of photovoltage for DSSCs assem-bled with single crystal Nb 3 O 7 (OH) and Nb 2 O 5 nanorod and Nb 2 O 5 nanoparticle fi lm photoanodes.

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as-synthesized Nb 3 O 7 (OH) exhibits a strong absorption peak at ∼ 3400 cm − 1 due to the stretching vibration of Nb − OH, [ 28 , 30 , 31 ] a peak at 1680 cm − 1 attributed to OH in-plane bending vibra-tion, [ 28 , 30 ] and two peaks at 873 cm − 1 and 530 cm − 1 assigned to Nb − O stretching and Nb − O − Nb angular vibrations, respectively (curve a in Figure S3, Supporting Information). [ 31 ] The disap-pearance of absorption peaks at 3400 cm − 1 and 1680 cm − 1 for the calcined sample (curve b in Figure S3, Supporting Infor-mation) confi rms the removal of OH groups. [ 28 ] Importantly, the peak resulting from the Nb − O stretching vibration is blue-shifted from 873 cm − 1 to 912 cm − 1 after calcination, suggesting the existence of theoretically predicted O − H·O hydrogen bond in Nb 3 O 7 (OH). [ 29 ] The crystal structure change was also con-fi rmed by XPS data. The XPS spectra of Nb 3d can be assigned to the binding energy of Nb 3d 5/2 and Nb 3d 3/2 electrons, respec-tively (Figure S4A, Supporting Information). [ 20 , 30 ] For O 1s, a main peak at 530.2 eV due to the oxygen anions (O 2 − ) bound to the niobium in the lattice is recorded for both Nb 3 O 7 (OH) and Nb 2 O 5 , but an additional peak at 532.8 eV is obtained from the as-synthesized Nb 3 O 7 (OH) sample, confi rming the presence of OH groups (Figure S4B, Supporting Information). [ 20 , 32 , 33 ]

When NbCl 5 is used as the precursor, a precise controlled hydrolysis rate is the key for direct growth of single crystal Nb 3 O 7 (OH) nanorod fi lm on FTO substrate. In this work, HCl is used to control an apt acidic reaction environment. With high concentration HCl (e.g., 10.2 M), a porous structure resulted from the substrate etching was observed (Figure S5A, Sup-porting Information). No formation of niobium oxide on the

© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, WeinAdv. Mater. 2012, DOI: 10.1002/adma.201104650

FTO substrate was observed, due to the com-pletely suppressed NbCl 5 hydrolysis. In the absence of HCl (water was used as reaction solution), NbCl 5 precursor rapidly hydrolyzed to form Nb(OH) 5 , which is subsequently dehydrolyzed to form Nb 2 O 5 precipitates (Figure S5B, Supporting Information). The formation of Nb 3 O 7 (OH) could occur only when the rate of hydrolysis of NbCl 5 is pre-cisely controlled by a suitable concentration of HCl (Figure 1 B). [ 28 ]

The as-synthesized Nb 3 O 7 (OH) single crystal nanorod fi lms on FTO substrate without calcination were dyed and directly used as the photoanodes for DSSCs. For comparison, Nb 2 O 5 single crystal nanorod fi lms and Nb 2 O 5 nanoparticle fi lms were also used as photoanodes for DSSCs meas-urement. The Nb 2 O 5 single crystal nanorod fi lms with an average thickness of 11.2 μ m (Figure S6A, Supporting Information) were obtained by thermal treatment of Nb 3 O 7 (OH) single crystal nanorod fi lms at 450 ° C for 2 h. The Nb 2 O 5 nanoparticle fi lms with an average thickness of 12.1 μ m (Figure S6B, Supporting Information) were fabricated on FTO substrates by combining sol-gel method and screen-printing technique. [ 21 ] All photoanodes were sensitized with dye N719 (3 × 10 − 4 mol/L) for 24 h before DSSCs measurement. [ 34 ]

The performance of DSSCs assembled with the dye sensitized photoanodes was evaluated under the standard AM 1.5 simu-lated sunlight (100 mW/cm 2 ). Figure 3 shows typical photocur-rent density-photovoltage curves ( J– V curves) of the resulting

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Figure 4 . (A) Diffusive refl ectance spectra and (B) Incident photon to current conversion effi ciency (IPCE) curves of the single crystal Nb 3 O 7 (OH) and Nb 2 O 5 nanorod and Nb 2 O 5 nanoparticle fi lms with similar thickness.

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DSSCs. The Nb 3 O 7 (OH) photoanode possesses a highest short-circuit current densities ( J sc ) of 15.00 mA/cm 2 that is almost 1.3 times of both Nb 2 O 5 nanorod and nanoparticle photoanodes. A 740 mV of open-circuit voltages ( V oc ) was obtained from the Nb 3 O 7 (OH) photoanode, which is 9 mV less than that of the Nb 2 O 5 nanorod photoanode, but 78 mV greater than that of the Nb 2 O 5 nanoparticle photoanode. The Nb 2 O 5 nanoparticle photo anode exhibits a lower V oc than that of the Nb 2 O 5 nanorod photoanode could be attributed to a number of reasons such as the higher electron transport resistance of the nanoparti-cles and larger dark current resulting from the reduction of triiodide at the nanoparticle photoanode. [ 35 ] The similar V oc values obtained from the single crystal Nb 3 O 7 (OH) and Nb 2 O 5 nanorod photoanodes could be due to the similarity in their conduction band edge positions, which deserves a further inves-tigation. The measured fi ll factor of 61.0% from the Nb 3 O 7 (OH) photoanode is the same when compares to the Nb 2 O 5 nano-particle photoanode but 8% lower than that of Nb 2 O 5 nanorod photoanode. As a result, the Nb 3 O 7 (OH) photoanode exhibits a highest conversion effi ciency ( η = 6.77%) that is 12% and 45% higher than that of the Nb 2 O 5 nanorod and nanoparticle photo anodes, respectively. The key characteristics of these photoanodes are summarized in Table 1 . It is obvious that the highest J sc is the main attribute for the dramatically improved DSSCs performance using the Nb 3 O 7 (OH) photoanode, which could be ascribed to the attributes such as dye loading capacity, light utilization effi ciency and photoelectron transport.

A higher dye loading on photoanodes normally leads to a higher photocurrent density. [ 15 , 34 ] The Nb 3 O 7 (OH) photoanode shows a highest dye loading of 1.62 × 10 − 7 mol/cm 2 , which is 16.6% and 44.6% higher than that of Nb 2 O 5 nanorod and Nb 2 O 5 nanoparticle photoanodes, respectively (Table 1 ). This superior dye loading capacity of the Nb 3 O 7 (OH) nanorod fi lm could be attributed to its large surface area as demonstrated in Figure S7 (Supporting Information). A specifi c surface area of 104 m 2 /g with a narrow pore size distribution centered at 19.5 nm was obtained from the Nb 3 O 7 (OH) nanorod fi lm, which is 1.46 and 3.97 times higher than that of Nb 2 O 5 nanorod (71.0 m 2 /g) and nanoparticle (26.2 m 2 /g) fi lms, respectively (Table 1 ). The decreased specifi c surface area for the Nb 2 O 5 nanorod fi lm resulted from the calcination of the Nb 3 O 7 (OH) nanorod fi lm could be due to the formation of nanorod bundles during the calcination process (Figure S2B, Supporting Information). These confi rm the high amount of dye loading benefi ted from

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Table 1 . Photovoltaic properties of the DSSCs assembled by using photo anodes made of Nb 3 O 7 (OH) and Nb 2 O 5 nanorods and Nb 2 O 5 nanoparticles with similar thickness.

Photoanodes Thickness [ μ m]

Surface Area [m 2 /g]

J sc [mA/cm 2 ]

V oc [mV]

FF [%]

η [%]

Adsorbed Dye [ × 10 − 7 mol/cm 2 ]

Nb 3 O 7 (OH)

Nanorod

11.5 104 ± 0.6 15.00 740 61.0 6.77 1.62

Nb 2 O 5

Nanorod

11.2 71.0 ± 0.3 12.20 749 66.0 6.03 1.39

Nb 2 O 5

Nanoparticle

12.1 26.2 ± 0.1 11.57 662 61.1 4.68 1.12

the high specifi c surface area is a major contribute for the improved performance of DSSCs assembled with Nb 3 O 7 (OH) nanorod photoanode. However, the presence of the surface functional group − OH in Nb 3 O 7 (OH) could also be a attribute for the high dye loading capacity.

A high J sc could also be resulted from enhanced light utilization effi ciency. [ 8 , 9 , 15 , 34 , 36 ] Figure 4 A shows the diffuse refl ectance spectra of the three photoanode fi lms with similar thickness. Among the three fi lms, the Nb 2 O 5 nanorod fi lm exhibits the highest light refl ectance ability over the entire visible and near-infrared regions, indicating a superior light scattering property of the Nb 2 O 5 nanorod fi lm. [ 8 , 9 , 15 , 34 , 36 ] This could be due to the formation of nanorod bundles during the calcination process (Figure S2B, Sup-porting Information). Although the Nb 2 O 5 nanorod photoanode exhibits higher light utilization effi ciency, the signifi cantly lower surface area makes its dye loading capacity much lower than that of the Nb 3 O 7 (OH) nanorod photoanode, leading to a lower conver-sion effi ciency, as confi rmed by the measured incident photon to current conversion effi ciency (IPCE) spectra (Figure 4 B).

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The electron transport inside photoanode is also a key factor affecting the J sc that can be investigated by an electrochemical impedance spectroscopy (EIS) technique. [ 22 , 37 ] The Nyquist plots of DSSCs assembled by the three photoanodes display a small and a large semicircle within low and high frequency regions, attributing to the electron transfer at the oxide/dye/electrolyte interfaces and the redox reaction of I − /I 3 − at the Pt/electrolyte interface, respectively (Figure S8, Supporting Information). [ 10 , 37 ] The measured semicircular radius within the low frequency region suggests a charge transfer resistance order of Nb 2 O 5 nanoparticle fi lm > Nb 3 O 7 (OH) nanorod fi lm > Nb 2 O 5 nanorod fi lm. The electrons lifetime ( τ n ) was calculated from the Bode plots (inset in Figure S8, Supporting Information) using τ n = 1/( 2 π f max ), where f max refers to the peak frequency. [ 22 , 38 ] The cal-culated τ n values for Nb 3 O 7 (OH) nanorod, Nb 2 O 5 nanorod and nanoparticle photoanodes are 17.7, 22.7 and 7.98 ms, respec-tively. The τ n value of the Nb 2 O 5 nanorod fi lm is slightly higher than that of the Nb 3 O 7 (OH) nanorod fi lm, attributing to the high crystallinity of single crystal nanorods. These results also suggest that the high dye loading capacity of the Nb 3 O 7 (OH) nanorod photoanode benefi ted from the large surface area is the decisive factor for the obtained high DSSCs effi ciency.

In summary, the Nb 3 O 7 (OH) single crystal nanorod photo-anode with high surface area of 104 m 2 /g have been success-fully synthesized onto FTO substrates via a facile hydrothermal method. An overall light conversion effi ciency of 6.77% can be obtained from a DSSC assembled with the Nb 3 O 7 (OH) single crystal nanorod fi lm photoanode without the need for further calcination. The high DSSCs performance is attributed to the high dye loading capacity of the photoanode resulted from the high specifi c surface area. This work demonstrates a feasibility of assembly high performance DSSCs using alternative metal oxide photoanode materials such as Nb 3 O 7 (OH) single crystal nanorod fi lm.

Experimental Section Synthesis of single crystal Nb 3 O 7 (OH) nanorod fi lm. In a typical synthesis, 0.5403 g of niobium (V) chloride (NbCl 5 , Aldrich) was dissolved in a 40 mL of 5.1 M hydrochloric acid (HCl, 32%, Sigma-Aldrich) solution. After stirring for 2 min, the resulting reaction solution was transferred into a Tefl on-lined stainless steel autoclave with an volume of 100 mL. Subsequently, a piece of pre-treated FTO conducting glass (15 Ω /square, Nippon Sheet Glass, Japan) with conductive facing up was immersed into the above solution. The hydrothermal reaction was performed at 210 ° C for 24 h. After hydrothermal reaction, the autoclave was cooled to room temperature and then FTO substrate was taken out, rinsed adequately with deionized water and allowed to dry in a nitrogen stream. The dried Nb 3 O 7 (OH) nanorod samples were further characterized and tested in DSSCs. The single crystal Nb 2 O 5 nanorod fi lms were obtained by thermal treatment of single crystal Nb 3 O 7 (OH) nanorod fi lms at 450 ° C for 2 h. The Nb 2 O 5 nanoparticle fi lm was fabricated by combination of sol-gel method and scree-printing technique. [ 21 ] Nb 2 O 5 nanoparticles were fi rst obtained by sol-gel method and calcination at 450 ° C for 2 h. Subsequently, the fabricated Nb 2 O 5 nanoparticles were scree-printed on FTO substrates. After annealing at 450 ° C for 1 h, the Nb 2 O 5 nanoparticle fi lm was used as photoanode for DSSC measurement.

Characterization : SEM (JSM-6300F), TEM (Philips F20), and XRD (Shimadzu XRD-6000 diffractometer) were employed for characterizing the prepared samples. FT-IR analysis of the samples was performed

© 2012 WILEY-VCH Verlag GmAdv. Mater. 2012, DOI: 10.1002/adma.201104650

using a Nicolet Nexus 870 FT-IR spectrometer with a smart Endurance single-bounce diamond ATR cell. Chemical compositions of the samples were analyzed by X-ray photoelectron spectroscopy (XPS, Kratos Axis ULTRA incorporating a 165 mm hemispherical electron energy analyzer). Nitrogen adsorption-desorption isotherms of the samples were obtained on a Quantachrome Autosorb-1 surface area and pore size analyser. The pore size distributions of the samples were derived from the adsorption branches of the isotherms based on the Barett–Joyner–Halenda (BJH) model. Diffuse refl ectance spectra of the fi lms were recorded on a Varian Cary 5E UV-vis-NIR spectrophotometer. Dye loading measurements were conducted by desorbing the dye molecules from the dye-anchored photoanode fi lms in NaOH ethanolic solution (10 − 4 M). [ 39 ] The loading amount was calculated from the absorbance of the completely desorbed dye solutions by the spectrophotometer (Varian, Cary 4500).

Theoretical Calculations : All computations are performed using the Vienna ab initio simulation package (VASP) based on the all-electron projected augmented wave (PAW) method. [ 40 , 41 ] A plane-wave basis set is employed to expand the smooth part of wave functions with a kinetic energy cut-off of 520 eV. For the electron-electron exchange and correlation interactions, the functional of PBE, [ 42 ] a form of the general gradient approximation (GGA), is used throughout. The Brillouin-zone integrations were performed using Monkhorst-Pack grids of special points, with gamma-point centered (2 × 8 × 8) k -points meshes used for the bulk cells. When the geometry is optimized, all atoms are allowed to relax. A variable cell technique is employed to simultaneously optimize the lattice constant and the atomic structure to encounter the complexity and low-symmetry of the Nb 3 O 7 (OH) crystals. And the lattice constants and geometric structure are optimized until the residual forces were below 0.001 eV/Å.

Measurements : A series of DSSCs were fabricated with traditional sandwich type confi guration by using a dye-anchored nanostructured fi lm and a platinum counter electrode deposited on FTO conducting glass. A mask with a window area of 0.15 cm 2 was applied on the niobium oxide photoanode fi lm side to defi ne the active area of the cells. A 500 W Xe lamp (Trusttech Co., Beijing) with an AM 1.5G fi lter (Sciencetech, Canada) was used as the light source. The Light intensity was measured by a radiant power meter (Newport, 70260) coupled with a broadband probe (Newport, 70268). The photovoltaic measurements of DSSCs were recorded by a scanning potentiostat (Model 362, Princeton Applied Research, US). The IPCE as a function of wavelength was measured with QE/IPCE measurement kit (NewSpec). Impedance measurements were performed with a computer-controlled potentiostat (CHI660D, CH Instruments). The frequency range is 0.1 Hz to 1 M Hz and the magnitude of the modulation signal is 10 mV.

Acknowledgements This work was fi nancially supported by Australian Research Council (ARC) Projects.

Received: December 5, 2011 Revised: January 29, 2012

Published online:

[ 1 ] B. O’Regan , M. Graetzel , Nature 1991 , 353 , 737 . [ 2 ] M. Gratzel , Nature 2001 , 414 , 338 . [ 3 ] M. Graetzel , Inorg. Chem. 2005 , 44 , 6841 . [ 4 ] P. Wang , S. M. Zakeeruddin , J. E. Moser , M. K. Nazeeruddin ,

T. Sekiguchi , M. Graetzel , Nature Mater. 2003 , 2 , 402 . [ 5 ] A. Hagfeldt , G. Boschloo , L. Sun , L. Kloo , H. Pettersson , Chem. Rev.

2010 , 110 , 6595 . [ 6 ] Q. Zhang , G. Cao , Nano Today 2011 , 6 , 91 . [ 7 ] S. H. Kang , S.-H. Choi , M.-S. Kang , J.-Y. Kim , H.-S. Kim , T. Hyeon ,

Y.-E. Sung , Adv. Mater. 2008 , 20 , 54 . [ 8 ] Y. Qiu , W. Chen , S. Yang , Angew. Chem., Int. Ed. 2010 , 49 , 3675 .

5wileyonlinelibrary.combH & Co. KGaA, Weinheim

6

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[ 9 ] Q. Zhang , C. S. Dandeneau , X. Zhou , G. Cao , Adv. Mater. 2009 , 21 ,

4087 . [ 10 ] J. Qian , P. Liu , Y. Xiao , Y. Jiang , Y. Cao , X. Ai , H. Yang , Adv. Mater.

2009 , 21 , 3663 . [ 11 ] J.-H. Lee , N.-G. Park , Y.-J. Shin , Sol. Energy Mater. Sol. Cells 2010 , 95 ,

179 . [ 12 ] F. Lenzmann , J. Krueger , S. Burnside , K. Brooks , M. Graetzel , D. Gal ,

S. Ruehle , D. Cahen , J. Phys. Chem. B 2001 , 105 , 6347 . [ 13 ] R. Jose , V. Thavasi , S. Ramakrishna , J. Am. Ceram. Soc. 2009 , 92 , 289 . [ 14 ] S. G. Chen , S. Chappel , Y. Diamant , A. Zaban , Chem. Mater. 2001 ,

13 , 4629 . [ 15 ] D. Chen , F. Huang , Y.-B. Cheng , R. A. Caruso , Adv. Mater. 2009 , 21 ,

2206 . [ 16 ] X. Feng , K. Shankar , M. Paulose , C. A. Grimes , Angew. Chem., Int.

Ed. 2009 , 48 , 8095 . [ 17 ] X. Fan , Z. Chu , F. Wang , C. Zhang , L. Chen , Y. Tang , D. Zou , Adv.

Mater. 2008 , 20 , 592 . [ 18 ] F. Huang , D. Chen , L. Cao , R. A. Caruso , Y.-B. Cheng , Energy Environ.

Sci. 2011 , 4 , 2803 . [ 19 ] A. G. S. Prado , L. B. Bolzon , C. P. Pedroso , A. O. Moura , L. L. Costa ,

Appl. Catal. B 2008 , 82 , 219 . [ 20 ] A. Le Viet , M. V. Reddy , R. Jose , B. V. R. Chowdari , S. Ramakrishna ,

J. Phys. Chem. C 2010 , 114 , 664 . [ 21 ] P. Guo , M. A. Aegerter , Thin Solid Films 1999 , 351 , 290 . [ 22 ] A. Le Viet , R. Jose , M. V. Reddy , B. V. R. Chowdari , S. Ramakrishna ,

J. Phys. Chem. C 2010 , 114 , 21795 . [ 23 ] B. Gao , J. Fu , K. Huo , W. Zhang , Y. Xie , P. K. Chu , J. Am. Ceram. Soc.

2011 , 94 , 2330 . [ 24 ] J. Kim , J. Kim , J. Nanosci. Nanotechnol. 2011 , 11 , 7335 . [ 25 ] M. A. Aegerter , Sol. Energy Mater. Sol. Cells 2001 , 68 , 401 . [ 26 ] M. Wei , Z.-m. Qi , M. Ichihara , H. Zhou , Acta Mater. 2008 , 56 ,

2488 .

wileyonlinelibrary.com © 2012 WILEY-VCH Verlag G

[ 27 ] R. Ghosh , M. K. Brennaman , T. Uher , M.-R. Ok , E. T. Samulski , L. E. McNeil , T. J. Meyer , R. Lopez , Acs Appl. Mater. Interf. 2011 , 3 , 3929 .

[ 28 ] F. Izumi , H. Kodama , Z. Anorg. Allg. Chem. 1978 , 441 , 196 . [ 29 ] A. P. Atkinson , E. Baguet , N. Galland , J.-Y. Le Questel , A. Planchat ,

J. Graton , Chem. Eur. J. 2011 , 17 , 11637 . [ 30 ] S. Ueno , S. Fujihara , Electrochim. Acta 2011 , 56 , 2906 . [ 31 ] A. Esteves , L. C. A. Oliveira , T. C. Ramalho , M. Goncalves ,

A. S. Anastacio , H. W. P. Carvalho , Catal. Commun. 2008 , 10 , 330 .

[ 32 ] L. C. A. Oliveira , T. C. Ramalho , M. Goncalves , F. Cereda , K. T. Carvalho , M. S. Nazzarro , K. Sapag , Chem. Phys. Lett. 2007 , 446 , 133 .

[ 33 ] R. Wojcieszak , A. Jasik , S. Monteverdi , M. Ziolek , M. M. Bettahar , J. Mol. Catal. A 2006 , 256 , 225 .

[ 34 ] H. Zhang , Y. Han , X. Liu , P. Liu , H. Yu , S. Zhang , X. Yao , H. Zhao , Chem. Commun. 2010 , 46 , 8395 .

[ 35 ] M. K. Nazeeruddin , A. Kay , I. Rodicio , R. Humphry-Baker , E. Mueller , P. Liska , N. Vlachopoulos , M. Graetzel , J. Am. Chem. Soc. 1993 , 115 , 6382 .

[ 36 ] F. Huang , D. Chen , X. L. Zhang , R. A. Caruso , Y.-B. Cheng , Adv. Funct. Mater. 2010 , 20 , 1301 .

[ 37 ] S. Lee , J. H. Noh , H. S. Han , D. K. Yim , D. H. Kim , J.-K. Lee , J. Y. Kim , H. S. Jung , K. S. Hong , J. Phys. Chem. C 2009 , 113 , 6878 .

[ 38 ] R. Kern , R. Sastrawan , J. Ferber , R. Stangl , J. Luther , Electrochim. Acta 2002 , 47 , 4213 .

[ 39 ] X. Liu , Y. Luo , H. Li , Y. Fan , Z. Yu , Y. Lin , L. Chen , Q. Meng , Chem. Commun. 2007 , 2847 .

[ 40 ] G. Kresse , J. Furthmller , Comput. Mat. Sci. 1996 , 6 , 15 . [ 41 ] G. Kresse , D. Joubert , Phys. Rev. B 1999 , 59 , 1758 . [ 42 ] J. P. Perdew , W. Burke , M. Ernzerhof , Phys. Rev. Lett. 1996 , 77 ,

3865 .

mbH & Co. KGaA, Weinheim Adv. Mater. 2012, DOI: 10.1002/adma.201104650