A highly crystalline Nb3O7F nanostructured photoelectrode: fabrication and photosensitisation

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Materials for energy and sustainability

Journal ofMaterials Chemistry Awww.rsc.org/MaterialsA Volume 1 | Number 22 | 14 June 2013 | Pages 6499–6694

PAPERHuijun Zhao et al.A highly crystalline Nb3O7F nanostructured photoelectrode: fabrication and photosensitisation

www.rsc.org/MaterialsARegistered Charity Number 207890

Showcasing research from Dr. Min Ji’s Laboratory,

School of Chemistry, Dalian University of Technology,

Dalian, China.

Title: Organic electron-rich N-heterocyclic compound as a

chemical bridge: building a Brönsted acidic ionic liquid confi ned

in MIL-101 nanocages

This work introduces a facile post-synthetic modifi cation strategy

to synthesize a novel functionalized MIL-101 material in which a

Brönsted acidic quaternary ammonium salt ionic liquid is confi ned

inside well-defi ned nanocages. It shows excellent catalytic

performance for acetalization.

As featured in:

See Q.-X. Luo et al.,

J. Mater. Chem. A, 2013, 1, 6530.

Journal ofMaterials Chemistry A

PAPER

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View Article OnlineView Journal | View Issue

aCentre for Clean Environment and Energy,

QLD 4222, Australia. E-mail: h.zhao@griffi

+61 7 55528261bCentre for Environmental and Energy Nanom

Chinese Academy of Sciences, Hefei 230031cQLD Micro- and Nanotechnology Centre, G

4111, AustraliadState Key Laboratory of Organic Geochemis

Chinese Academy of Sciences, Guangzhou 51eCentre for Microscopy & Microanalysis, The

4072, Australia

† Electronic supplementary informcharacterisation, supporting images, Umeasurement. See DOI: 10.1039/c3ta1104

Cite this: J. Mater. Chem. A, 2013, 1,6563

Received 13th March 2013Accepted 11th April 2013

DOI: 10.1039/c3ta11042g

www.rsc.org/MaterialsA

This journal is ª The Royal Society of

A highly crystalline Nb3O7F nanostructuredphotoelectrode: fabrication and photosensitisation†

Haimin Zhang,a Yibing Li,a Yun Wang,a Porun Liu,a Huagui Yang,a Xiangdong Yao,c

Taicheng An,d Barry J. Woode and Huijun Zhao*ab

A highly crystalline Nb3O7F nanostructured film composed of a bottom single crystal nanosheet layer with

ca. 1.5 mm thickness and a topmicrosphere layer with ca. 18.5 mm thickness has been successfully fabricated

on a FTO conducting substrate through a facile and one-pot hydrothermal method. The top Nb3O7F

microspheres with 2–5 mm diameter consist of transparent single crystal nanosheets with 10–40 nm

thickness. Without the need for further calcination, the as-synthesised Nb3O7F nanostructured films

possess excellent crystallinity and high mechanical stability, which can be directly used as photoanodes

for CdS quantum dot-sensitised solar cells (QDSSCs) and dye-sensitised solar cells (DSSCs). The

transparent single crystal nanosheets constituting top Nb3O7F microspheres possess exposed (100) and

(010) surfaces, which can play an important role in sensitiser loading. The photoelectrochemical

measurements indicate that the CdS quantum dot-sensitised Nb3O7F nanostructured photoanode with

seven chemical bath deposition (CBD) cycles (NbCdS-7) shows the best performance under visible light

irradiation (l > 400 nm) due to higher carrier concentration and longer electron lifetime in NbCdS-7.

QDSSCs made of NbCdS-7 photoanodes show an overall light conversion efficiency of 1.68%, which is

almost 1.4 and 1.9 times of the NbCdS-5 and NbCdS-10 photoanodes, respectively. DSSC measurement

indicates that an overall light conversion efficiency of 2.78% can be achieved for the Nb3O7F

nanostructured photoanode. This work demonstrates the possibility of direct growth of a highly

crystalline metal oxide-based nanostructured film on a FTO conducting substrate as a photoanode

material without the need for further calcination for solar energy conversion applications.

1 Introduction

Titanium dioxide (TiO2) has been the most widely investigatedsemiconductor material due to its excellent properties andextensive applications in photocatalysis, water splitting for thegeneration of hydrogen, sensing, biomedicine, lithium-ionbatteries, and solar energy conversion.1–11 Even so, considerableefforts have also beenmade to develop other alternativematerialssuch as ZnO,WO3, SnO2, andniobium-containing nanostructuresfor more extensive applications.12–17 Amongst them, there is

Griffith University, Gold Coast Campus,

th.edu.au; Fax: +61 7 55528067; Tel:

aterials, Institutes of Solid State Physics,

, China

riffith University, Nathan Campus, QLD

try, Guangzhou Institute of Geochemistry,

0640, China

University of Queensland, St Lucia, QLD

ation (ESI) available: DetailedV-vis spectra, EDS analysis and PL2g

Chemistry 2013

increasing interest in niobium-containing materials owing totheir structural diversity and attractive applications for photo-catalysis, gas sensing, lithium-ionbatteries, andsolar cells.15,16,18–21

To date, Nb2O5 with a wide bandgap of about 3.4 eV has been themost widely studied niobium-containing material because of itsunique properties and extensive applications.15,19,20,22 Varioussynthetic methods have been employed to fabricate Nb2O5 nano-structures with different morphologies, such as nanoparticles,nanotubes, nanobelts and nanoforests, which have exhibitedpromising applications in different elds.15,19,20,22–24However, onlya few studies on other forms of niobium-containing materialshave been reported to date in the literature.16,21,25–28 Very recently,our group developed a facile hydrothermal method to directlygrow a Nb3O7(OH) single crystal nanorod lm on a FTOsubstrate.16 The fabricated Nb3O7(OH) single crystal nanorodspossess high crystallinity and large surface area, which can bedirectly used as photoanodes for dye-sensitised solar cells (DSSCs)without the need for further calcination to achieve an impressiveoverall light conversion efficiency of 6.77%.16 This conceptmay beextended to fabricate a highly crystalline metal oxide-basednanostructured lm on a exible substrate for exible solar cellsbecause the fabricatedlmdoesnotneedfurther calcination, thusavoiding the exible substrate damage.

J. Mater. Chem. A, 2013, 1, 6563–6571 | 6563

http://dx.doi.org/10.1039/c3ta11042g

http://pubs.rsc.org/en/journals/journal/TA

http://pubs.rsc.org/en/journals/journal/TA?issueid=TA001022

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Several early reports have indicated that Nb3O7F is a class ofimportant niobium-containing material with high crystallinity,which can be readily transformed into Nb2O5 with differentcrystal phases through thermal conversion.25–31 In their studies,the Nb3O7F structures are usually obtained under the condi-tions of high temperature and high pressure.25–31 In 1964,Andersson reported the synthesis of Nb3O7F by the reaction ofNb2O5 and NbO2F under the conditions of high temperatureand high pressure.25 His work conrmed that the Nb3O7Fstructure consists of blocks of the ReO3-structure type which arefused together by means of octahedra sharing edges, with theunit-cell dimensions of a ¼ 20.67 A, b ¼ 3.833 A, and c ¼ 3.927A.25 Permer reported the synthesis of Nb3O7F structures bythermal decomposition of NbO2F under high temperature andhigh pressure.26,31 In their studies, lithium ion insertion into theNb3O7F structure and its thermal decomposition products wereinvestigated in detail by using various characterisation tech-niques.26,31 Their studies demonstrated that the one set ofcrystallographic shear planes in Nb3O7F can obviously slowdown the Li-insertion reaction and stabilise the structure.31 Thestudy on thermal decomposition of the Li-inserted sample ofLi3xNb3O7F (0 # x # 1.4) resulted in various thermal decom-position products, such as LiF, NbO2, LiNb3O8, and LiNbO3.26

Recently, Zhu et al. reported the template free synthesis of 3DNb3O7F hierarchical nanostructures.21 The resulting product asa photocatalyst showed good photocatalytic activity towardphotocatalytic degradation of different dyes under UV irradia-tion.21 To the best of our knowledge, the synthesised Nb3O7Fstructures in all reports to date are exclusively in powder form,which have not been investigated as photoanode materials forphotosensitisation.

Herein, we report for the rst time a facile and one-pothydrothermal method to directly grow highly crystalline Nb3O7Fnanostructured lms on FTO conducting substrates. Theobtained Nb3O7F nanostructured lm consists of a bottomsingle crystal nanosheet layer and a top microsphere layer. Thetop microspheres are composed of transparent Nb3O7F singlecrystal nanosheets. The formation process of the as-synthesisedNb3O7F nanostructured lm has been investigated and dis-cussed in this work. The as-synthesised Nb3O7F nanostructuredlms possess excellent crystallinity and high mechanicalstability, which can be directly used as photoanodes for solarcells (CdS quantum dot-sensitised and dye-sensitised solarcells) without the need for further calcination, displaying greatpotential as promising photoanode materials for solar energyconversion applications.

2 Experimental sectionFabrication of Nb3O7F nanostructured lms

In a typical synthesis, 0.5403 g of niobium(V) chloride (NbCl5,Aldrich) was dissolved in 40 mL of 1.0% (v/v) hydrouoric acid(HF, 48%, Sigma-Aldrich) solution. Aer ultrasonic treatmentfor 1 min, the resultant solution was transferred into a Teon-lined stainless steel autoclave with a volume of 100 mL.Subsequently, a piece of pre-treated FTO conducting glass (15 U

per square, Nippon Sheet Glass, Japan) with the conductive side

6564 | J. Mater. Chem. A, 2013, 1, 6563–6571

facing up was immersed in the above solution. The hydro-thermal reaction was carried out at 200 �C for 3 h. Aer reaction,the autoclave was cooled to room temperature and then the FTOsubstrate was taken out, rinsed adequately with deionised waterand allowed to dry in a nitrogen stream for further character-isation and measurement. The Nb2O5 nanostructured lmswere obtained by thermal treatment of highly crystallineNb3O7F nanostructured lms at 550 �C for 2 h.

CdS quantum dot sensitisation

CdS particles were deposited on the highly crystalline Nb3O7Fnanostructured lms by the sequential chemical bath deposi-tion (CBD) method.32,33 Typically, the Nb3O7F lm was succes-sively immersed in four different beakers for about 30 s in eachbeaker. One beaker contained 0.05 M Cd(NO3)2 solution,another contained 0.05 M Na2S, and the other two containeddistilled water to rinse the samples from the excess of eachprecursor solution. Such an immersion cycle was repeated 1, 3,5, 7 and 10 times, and the obtained samples were denoted asNbCdS-1, NbCdS-3, NbCdS-5, NbCdS-7, and NbCdS-10, respec-tively. The resultant samples were tested in photo-electrochemical experiments and solar cells. For comparison,the Nb2O5 nanostructured lms were also sensitised by CdSquantum dots with 7 CBD cycles as photoanodes for photo-electrochemical measurement.

Characterisation

SEM (JSM-7001F), TEM (Philips F20), and XRD (Shimadzu XRD-6000 diffractometer) were employed for characterisation thefabricated samples. The chemical compositions of the sampleswere analysed by X-ray photoelectron spectroscopy (XPS, KratosAxis ULTRA, incorporating a 165 mm hemispherical electronenergy analyzer). UV-vis diffuse reectance spectra of thesamples were recorded using a Varian Cary 5E UV-VIS-NIRspectrophotometer (Varian, US). The photoluminescent (PL)spectra of CdS quantum dot-sensitised samples were recordedon a F-7000 Fluorescence Spectrophotometer (Hitachi).Nitrogen adsorption–desorption isotherms of the samples wereobtained on a Quantachrome Autosorb-1 surface area and poresize analyser.

Measurements

The photoelectrochemical experiments were carried out at 23 �Cin a photoelectrochemical cell with a quartz window for illu-mination.34 It consisted of a nanostructured photoanode, asaturated Ag/AgCl reference electrode, and a platinum meshcounter electrode. A voltammograph (CV-27, BAS) was used forthe application of potential bias. Potential and current signalswere recorded using a Macintosh (AD Instruments). The illu-minated area of the photoanode was 0.785 cm2. In photo-electrochemical experiments, 0.5 M Na2S solution was used asthe supporting electrolyte. Illumination was carried out using a150 W Xe lamp (TrustTech, China). The wavelengths of theincident light were greater than 400 nm through a UV-400 lter.The intensity of the incident light was 100 mW cm�2. For solarcell measurements, a series of cells were fabricated with a

This journal is ª The Royal Society of Chemistry 2013


Paper Journal of Materials Chemistry A

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traditional sandwich type conguration by using a CdSquantum dot-sensitised nanostructured lm (or a dye sensi-tised nanostructured lm) and a platinum counter electrode. Amask with a window area of 0.15 cm2 was applied on the pho-toanode lm side to dene the active area of the cells. A 500 WXe lamp (Trusttech Co., Beijing) with an AM 1.5G lter (Scien-cetech, Canada) was used as the light source. The light intensitywas measured using a radiant power meter (Newport, 70260)coupled with a broadband probe (Newport, 70268). The photo-voltaic measurements of solar cells were performed using ascanning potentiostat (Model 362, Princeton Applied Research,US). The electrolyte for CdS quantum dot-sensitised solar cellmeasurement contained 0.5 M Na2S, 2 M S, and 0.2 M KCl. Fordye-sensitised solar cell measurement, the electrolyte was aDYESOL high efficiency electrolyte (EL-HPE). The IPCE as afunction of wavelength was measured with a QE/IPCEmeasurement kit (NewSpec). Impedance measurements wereperformed with a computer-controlled potentiostat (PAR2273,US). The frequency range is 0.1 Hz to 1 M Hz. The applied biasvoltage and the magnitude of the modulation signal were set atan open-circuit voltage of solar cells and 10 mV, respectively.

Fig. 1 (A) XRD pattern of the as-synthesised sample obtained in 40 mL of 1.0%(v/v) HF solution at 200 �C for 3 h. (B) Surface SEM image of the as-synthesisedNb3O7F film. (C) High magnification SEM image of an individual Nb3O7F micro-sphere. (D and E) Cross-sectional SEM images of the obtained Nb3O7F nano-structured film. (F) TEM image of an individual Nb3O7F nanosheet with insets ofthe SAED pattern (top) and HRTEM image (bottom). (G) Atomic structure modelsof Nb3O7F. Atom colour code: green – Nb, red – O, green octahedron: [NbO6]. (H)High magnification SEM image of Nb3O7F nanosheets with exposed (100) and(010) planes.

3 Results and discussionStructural characteristics

Fig. 1A shows the XRD pattern of the as-synthesised sample,which can be indexed to an orthorhombic Nb3O7F structurewith lattice parameters of a¼ 20.67 A, b¼ 3.833 A and c¼ 3.927A (JCPDS no. 74-2363).21,25,28 Fig. 1B shows the surface SEMimage of the as-synthesised sample, displaying a uniformmicrosphere lm with microsphere sizes of 2–5 mm diameter. Ahigh magnication SEM image indicates that the obtainedmicrosphere is composed of nanosheets with 10–40 nm thick-ness (Fig. 1C). Interestingly, the cross-sectional SEM imagereveals that the formed Nb3O7F nanostructured lm consists ofa bottom nanosheet layer with 1.5 mm thickness and a topmicrosphere layer with 18.5 mm thickness, as shown in Fig. 1Dand E. The thickness of the whole lm is about 20 mm. For solarenergy conversion applications, the bottom Nb3O7F nanosheetlayer may play an important role in prohibiting electron leakageat the interfaces between FTO and the microsphere layer, thusimproving conversion efficiency.35,36 The diffuse reectancespectra of the Nb3O7F nanostructured lm (Fig. SI-1, ESI†) showa higher reectance in the wavelength range between 400 and800 nm due to its large microsphere size, which can effectivelyimprove the sunlight utilisation efficiency of the photoanode,and thus the solar cell performance.37 From this point of view,the fabricated Nb3O7F nanostructured lm may have potentialas a promising photoanode candidate for solar energy conver-sion applications. A TEM image of an individual Nb3O7Fnanosheet from an ultrasound treated sample is shown inFig. 1F. The SAED pattern (top inset in Fig. 1F) and the HRTEMimage (bottom inset in Fig. 1F) reveal a good single crystallinenature of the Nb3O7F nanosheets. Moreover, the nanosheets aretransparent. The SAED data conrm a preferred growth alongthe [010] direction, while the HRTEM image conrms the fringespacings of 0.393 nm and 0.383 nm, which are consistent with


the d values of the (001) and (010) planes of the orthorhombicNb3O7F, respectively.25 Our experiments also demonstrate thatthe nanosheets originating from the bottom layer and topmicrospheres have the same crystal structure. Fig. 1G sche-matically shows the atomic conguration of the (100) and (010)planes of the Nb3O7F nanosheet. In a previous study reported byanother group, the oxygen and uorine atoms are assumed tosubstitute each other in a random way in the Nb3O7F crystalstructure.25 The structure of Nb3O7F consists of blocks of theReO3-structure type which are fused together by means ofoctahedra having edges in common (Fig. 1G).25 Based on the

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above results, the Nb3O7F nanosheets originating from the topmicrospheres have (100) and (010) exposed planes, as shown inFig. 1H. These exposed crystal planes could be advantageous forfurther sensitiser loading (e.g., CdS quantum dots).

In this work, the effect of different hydrothermal conditionson the resultant Nb3O7F morphology has been investigated. Asshown in Fig. SI-2 (ESI†), when the concentration of HF solutionwas set at 0.5% (v/v), only dispersed nanosheets were observedon the FTO substrate (Fig. SI-2A, ESI†). Moreover, the formednanosheet lm possesses weak mechanical stability and is easyto peel off from the FTO substrate. When the concentration ofHF solution was increased to 3.0% (v/v), particle-shaped struc-tures with an average diameter of 100 nm were observed on theFTO substrate (Fig. SI-2B, ESI†), which could be due to theetching role of high concentration HF.38 Similarly, the formedlm has weak mechanical stability. Our experiments indicatethat an optimum concentration of HF solution is 1.0% (v/v) fordirect growth of a highly crystalline Nb3O7F nanostructured lmon a FTO substrate. Keeping other reaction conditions constant,the experiments on the inuence of NbCl5 concentration indi-cate that the Nb3O7F nanostructured lm can be formed on aFTO substrate in the concentration range of 0.03–0.06 M. Eithertoo low or too high concentration of NbCl5 is not favorable forthe formation of a stable Nb3O7F nanostructured lm (Fig. SI-3Aand B, ESI†). Further investigation demonstrates that a highlycrystalline Nb3O7F nanostructured lm can be rmly formed ona FTO substrate at hydrothermal reaction temperature higherthan 180 �C (Fig. SI-4B, ESI†). While at a low hydrothermalreaction temperature (e.g., 150 �C), only Nb3O7F microspheresassembled with nanorods can be observed (Fig. SI-4A, ESI†).Compared to the Nb3O7F lms formed at higher reactiontemperature ($180 �C), the microsphere lm obtained at 150 �Cis not very rm and is easy to peel off from the FTO substrate. Toinvestigate the formation process of the Nb3O7F nanostructuredlm obtained at 200 �C, the effect of the hydrothermal reactiontime was investigated in this work. When the hydrothermalreaction time was set at 0.5 h, the Nb3O7F nanoparticle lm(Fig. 2A) started to form on the FTO substrate due to thehydrolysis of the NbCl5 precursor in HF solution to formNb3O7F based on the following equations:21

Fig. 2 Schematic illustration of the growth process of the Nb3O7F nanostructured

6566 | J. Mater. Chem. A, 2013, 1, 6563–6571

NbCl5 þ 5H2O��! ��

Hþ

200 �CNbðOHÞ5 þ 5HCl (1)

2NbðOHÞ5 ��!200 �C

Nb2O5Yþ 5H2O (2)

3Nb2O5 þ 2HF��!200 �C

2Nb3O7FYþH2O (3)

With increasing hydrothermal reaction time, the Nb3O7Fconcentration in reaction solution was further enhanced, whichresulted in the formation of sphere-shaped structures on theFTO substrate (Fig. 2B). The formed Nb3O7F sphere-shapedstructure is composed of nanoparticles (Fig. 2B). When thehydrothermal reaction time was set at 1.5 h, it was found thatthe size of the sphere-shaped structures was further increased(Fig. 2C). Importantly, the surface of the sphere-shaped struc-ture started to form interlaced connected streak structures atthis reaction stage (Fig. 2C). Based on the foregoing TEManalysis, the exposed crystal planes of the interlaced connectedstreak structures should be (010) planes, which are favored forfurther growth of Nb3O7F nanosheets with a preferred growthalong the [010] direction. Further increasing the reaction timeto 2 h, nanosheet structures on the Nb3O7F microsphere surfacewere obviously observed, as shown in Fig. 2D, indicating thegrowth of Nb3O7F nanosheets. Accompanying the growth of thetop Nb3O7F microspheres, the bottom nanosheet layer alsoformed. A highly crystalline Nb3O7F hierarchically structuredlm was rmly grown on the FTO substrate at a hydrothermalreaction time of 3 h (Fig. 1B–E).

CdS quantum dot sensitisation

Quantum dot-sensitised solar cells (QDSSCs) have attractedmuch attention because of their efficient charge separation,transport, and potential performance.39,40 To date, most studieson QDSSCs mainly focus on the use of TiO2 as a photoanodematerial.39–41 Without doubt, exploration of more alternativematerials is highly desired to develop high performanceQDSSCs. Recently, Kang et al. investigated a Nb2O5 nanowirephotoanode sensitised by a composition-tuned CdSxSe1�x shellfor photoelectrochemical cells.42 In comparison with the

film on the FTO substrate.




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TiO2–CdSxSe1�x nanowire photoanode, the Nb2O5–CdSxSe1�xnanowire photoanode exhibits excellent photoconversionefficiency.42

In this work, a directly grown highly crystalline Nb3O7Fnanostructured lm on a FTO substrate was sensitised with CdSquantum dots by the chemical bath deposition (CBD)method.32,33 During sensitisation, the CBD process was repeatedup to N times (N ¼ 1, 3, 5, 7, 10), and the corresponding elec-trodes are denoted as “NbCdS-N”. The CdS quantum dot-sen-sitised Nb3O7F nanostructured lms as photoanodes weresubsequently investigated in QDSSCs.

Fig. 3 shows the surface SEM images of CdS quantum dot-sensitised Nb3O7F lms with different CBD cycles. It can beseen from Fig. 3 that some CdS crystallites begin to deposit onthe nanosheets during initial cycles (e.g., NbCdS-1, Fig. 3B).With further increasing CBD cycles, CdS crystallites form intoaggregates of quantum dots, and more and more aggregateswere observed on the surface of Nb3O7F nanosheets (Fig. 3C–E).Aer 10 CBD cycles, the whole Nb3O7F microsphere was almostcompletely covered by aggregates (NbCdS-10, Fig. 3F). Somestudies have indicated that more CBD cycles can result in the

Fig. 3 (A) SEM image of the Nb3O7F nanostructured film before CdS sensitisa-tion. (B–F) SEM images of the Nb3O7F nanostructured films with different CBDcycles. (G) TEM image of the Nb3O7F nanosheet after CdS sensitisation with 7 CBDcycles. (H) HRTEM image of the Nb3O7F nanosheet after CdS sensitisation with 7CBD cycles. The insets are their corresponding photographs.


increase of particle sizes of CdS aggregates, which can bedisadvantageous to improve the resultant solar cell perfor-mance.43–45 From Fig. 3, it was also observed that CdS quantumdots are preferentially deposited on the top exposed (010)crystal planes of nanosheets within initial CBD cycles. Withincreasing CBD cycles, CdS particles appear on the (100) planesof Nb3O7F nanosheets. Fig. 3G shows a TEM image of theNb3O7F nanosheet aer depositing CdS quantum dots with 7CBD cycles. Obviously, the exposed surface including (010) and(100) surfaces of the nanosheet was uniformly covered by CdSparticles, which can be benecial to improve the resulting solarcell efficiency. HRTEM analysis (Fig. 3H) indicates that theobserved 0.335 nm and 0.393 nm fringes correspond to the(111) plane of the cubic phase of CdS (JCPDS no. 65-2887) andthe (001) plane of the orthorhombic Nb3O7F, respectively. Theinsets in Fig. 3A–F show the corresponding photographs of CdSquantum dot-sensitised Nb3O7F samples with different CBDcycles. It can be seen that there was an obvious change in thecolour of the sample surface from white for the Nb3O7F samplewithout CdS sensitisation to light yellow, deep yellow, andorange yellow with increasing CBD cycles, suggesting anincrease in the deposition amount of CdS quantum dots.43

UV-vis diffuse reectance spectra of CdS quantum dot-sen-sitised samples show that the absorption edge of the samplesappears to be red shied with successive CBD cycles, which ismainly due to the increase of the CdS particle size withincreasing CBD cycles (Fig. SI-5, ESI†).43 In this work, we alsoperformed energy dispersive spectroscopy (EDS) analysis for thesamples before and aer CdS sensitisation (Fig. SI-6, ESI†).Obviously, compared to the Nb3O7F sample (Fig. SI-6A, ESI†),Cd and S elements appear in the EDS spectra aer CdS depo-sition (take NbCdS-7 as an example) (Fig. SI-6B, ESI†). Moreover,the molar ratio of Cd to S in the NbCdS-7 sample is approxi-mately 1 : 1, conrming stoichiometric formation of CdS.

Prior to evaluation of solar cell performance using NbCdSseries photoanodes, the photoelectrochemical activity of thefabricated CdS-sensitised Nb3O7F nanostructured lms asphotoanodes was rst measured in a 0.5 M Na2S supportingelectrolyte under visible light irradiation (l > 400 nm, lightintensity ¼ 100 mW cm�2).43 Fig. 4A shows the voltammogramsof the photoanodes made of Nb3O7F and NbCdS-7 lms in a0.5 M Na2S supporting electrolyte. Apparently, no photocurrentresponse was observed for the Nb3O7F photoanode undervisible light irradiation. For the NbCdS-7 photoanode withoutvisible light irradiation, only a negligible dark current wasobserved. Under visible light illumination, the NbCdS-7 pho-toanode shows good visible light response. Fig. 4B shows thetransient photocurrent responses of the NbCdS series photo-anodes obtained at �0.60 V (vs. Ag/AgCl) applied potential.Obviously, the NbCdS-7 photoanode was found to show the bestphotoelectrocatalytic activity in all investigated photoanodes.The increase in photocurrent response for NbCdS series pho-toanodes from NbCdS-1 to NbCdS-7 is mainly ascribed to theincrease of the CdS deposition amount with CBD cycles. Furtherincreasing CBD cycles can result in further growth and forma-tion of additional CdS particles to form a thicker CdS particlelm (e.g., NbCdS-10). Although a thicker CdS particle lm may

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Fig. 4 (A) Voltammograms obtained fromNb3O7F and NbCdS-7 photoanodes ina 0.5 M Na2S supporting electrolyte solution. (B) Transient photocurrentresponses of different NbCdS series photoanodes obtained at an appliedpotential of �0.60 V in a 0.5 M Na2S supporting electrolyte solution. Visible lightintensity of 100 mW cm�2.

Fig. 5 Mott–Schottky plots of CdS sensitised Nb3O7F nanostructured photo-anodes with different CBD cycles.


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be benecial to visible light utilisation, the recombination ofphotoelectrons and holes occurs more easily, and thusdecreases photocurrent response.43 This has been furtherconrmed by the photoluminescence (PL) spectrum. Manystudies have indicated that PL analysis can be used to investi-gate the separation efficiency of photogenerated electrons andholes of photocatalysts.46,47 As shown in Fig. SI-7 (ESI†), NbCdS-10 shows a stronger PL intensity than that of NbCdS-7, indi-cating an improved recombination of photoelectrons and holesof NbCdS-10, and thus decreasing photocurrent response. Theabove results demonstrate that the photoanode with anappropriate CdS deposition amount can cause the photoelec-trons produced by CdS sites to be injected more effectively intothe Nb3O7F, thus improving visible light activity of the photo-anode. This can be highly advantageous for photo-electrocatalytic and photovoltaic applications. For comparison,we also performed photoelectrochemical experiments using thephotoanode made from Nb2O5/CdS-7 (7 CBD cycles). The Nb2O5

lm was rst obtained by thermal treatment of the Nb3O7Fnanostructured lm at 550 �C for 2 h. Aer calcination, the XRDpattern of the calcined sample can be indexed to Nb2O5, sug-gesting a structural transformation from orthorhombic Nb3O7Fto monoclinic Nb2O5 (JCPDS no. 43-1042) (Fig. SI-8A, ESI†).

6568 | J. Mater. Chem. A, 2013, 1, 6563–6571

Compared to the XRD result obtained from the as-synthesisedNb3O7F sample, the calcined sample seems to have poor crys-tallinity owing to the rearrangement of the crystal structureduring calcination.48 The SEM image of the calcined sampledemonstrates that no obvious change in morphology hasoccurred aer annealing, indicating a high thermal stability ofthe nanosheet assembled microsphere structure (Fig. SI-8B,ESI†). XPS data (survey spectrum) demonstrate that aerannealing at 550 �C, the uorine element completely disap-pears, further indicating the structural transformation fromorthorhombic Nb3O7F to monoclinic Nb2O5 (Fig. SI-9, ESI†).Fig. SI-10 (ESI†) shows the transient photocurrent responses ofNbCdS-7 and Nb2O5/CdS-7 photoanodes at an applied potentialof �0.6 V (vs. Ag/AgCl). Obviously, the photocurrent response ofthe NbCdS-7 photoanode is higher than that of the Nb2O5/CdS-7photoanode. This can be attributed to the decrease in crystal-linity and surface area of Nb2O5 nanostructures aercalcination.

Fig. 5 shows Mott–Schottky plots of NbCdS series photo-anodes under visible light irradiation (100mW cm�2). The slopeof the tangent line in a Mott–Schottky plot is proportional to1/ND (ND is the number of donors).49 For NbCdS series photo-anodes, the slope order is: NbCdS-7 < NbCdS-5 < NbCdS-10 <NbCdS-3 < NbCdS-1. This indicates that the order of the carrierconcentration in NbCdS series photoanode lms is NbCdS-7 >NbCdS-5 > NbCdS-10 > NbCdS-3 > NbCdS-1. Higher carrierconcentration means that more photoelectrons can be effec-tively transferred to an external circuit in a photoelectrocatalyticreaction, leading to a higher photocurrent response. The aboveresults are consistent with the results of photoelectrocatalyticexperiments of NbCdS series photoanodes. Based on Mott–Schottky plots, the at band potential of NbCdS-N photoanodesis approximately �0.87 V for NbCdS-7, �0.84 V for NbCdS-5,�0.79 V for NbCdS-10, �0.78 V for NbCdS-3, and �0.76 V forNbCdS-1, respectively (Fig. 5). Obviously, compared to otherNbCdS photoanodes, a negative shi occurs in the at bandpotential of NbCdS-7, which can result in an increase in theopen-circuit voltage of solar cells.43

In this work, we performed QDSSC evaluation using NbCdS-5, NbCdS-7 and NbCdS-10 photoanodes as examples under the



Table 1 Photovoltaic properties of the QDSSCs assembled with the NbCdS-5,NbCdS-7, and NbCdS-10 photoanodes

Samples Jsc (mA cm�2) Voc (mV) FF (%) h (%)

NbCdS-5 4.48 491 53.5 1.18NbCdS-7 5.91 511 55.6 1.68NbCdS-10 3.77 483 47.2 0.86


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standard AM 1.5 simulated sunlight (100 mW cm�2). Fig. 6Ashows typical photocurrent density–photovoltage curves ( J–Vcurves) of the resulting QDSSCs. Apparently, the NbCdS-7photoanode possesses the highest short-circuit current density( Jsc) of 5.91 mA cm�2 that is almost 1.3 and 1.6 times of theNbCdS-5 and NbCdS-10 photoanodes, respectively. The highphotocurrent density of the NbCdS-7 photoanode can be due tothe higher carrier concentration generated in the NbCdS-7photoanode lm in comparison with those in NbCdS-5 andNbCdS-10 photoanode lms. An open-circuit voltage (Voc) of 511mV was obtained from the NbCdS-7 photoanode, which is 20mV and 28 mV greater than those obtained from the NbCdS-5and NbCdS-10 photoanodes. Compared to the NbCdS-5 andNbCdS-10 photoanodes, the higher Voc of the NbCdS-7 photo-anode is mainly ascribed to an apt CdS deposition amount ofthe Nb3O7F nanostructured lm causing the negative shi ofthe at band potential, and thus resulting in Voc improvement.43

As a result, the NbCdS-7 photoanode exhibits an overall lightconversion efficiency of 1.68% that is almost 1.4 and 1.9 timesof the NbCdS-5 and NbCdS-10 photoanodes, respectively. Thekey characteristics of these photoanodes are summarised inTable 1. Fig. 6B shows the IPCE spectra of NbCdS-5, NbCdS-7and NbCdS-10 photoanodes. As shown in Fig. 6B, the NbCdS-7photoanode displays the highest IPCE amongst all photo-anodes, further conrming the signicant role of an apt CdSdeposition amount in the Nb3O7F nanostructured lm.

Fig. 6 (A) J–V characteristics of the NbCdS-5, NbCdS-7, and NbCdS-10 photo-anodes. (B) Incident photon to current conversion efficiency (IPCE) curves of theNbCdS-5, NbCdS-7, and NbCdS-10 photoanodes.


The electron transport inside the photoanode has animportant inuence on the resultant solar cell performance,which can be investigated by an electrochemical impedancespectroscopy (EIS) technique.16,33,50 Fig. 7A shows the Nyquistplots of the QDSSCs assembled by the NbCdS-5, NbCdS-7 andNbCdS-10 photoanodes. As shown, a small and a large semi-circle within high-frequency andmiddle-frequency regions wereobserved for all photoanodes, which is due to the electrontransfer at the Pt–electrolyte interface (redox reaction of S2�/Sx

2�) and the Nb3O7F–CdS–electrolyte interfaces, respectively.Obviously, Nyquist plots of all photoanodes indicate that thecharge transfer resistance order is NbCdS-7 < NbCdS-5 < NbCdS-10. The electron lifetime (sn) in the CdS sensitised Nb3O7Fnanostructured lms can be estimated using sn ¼ 1/2pfmax,where fmax is the maximum frequency of the middle-frequencypeak.16 As shown in Fig. 7B, the fmax values are 8.3 Hz for NbCdS-7, 22.9 Hz for NbCdS-5, and 35.1 Hz for NbCdS-10, respectively.

Fig. 7 Electrochemical impedance spectra (EIS) of QDSSCs assembled with theNbCdS-5, NbCdS-7, and NbCdS-10 photoanodes. (A) Nyquist plots of three films.(B) Bode-phase plots of three films.

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Correspondingly, the electron lifetime (sn) is calculated to be19.2, 7.0 and 4.5 ms for the cells assembled by the NbCdS-7,NbCdS-5 and NbCdS-10 photoanodes, respectively. The aboveresults further demonstrate that an apt CdS deposition amountof the Nb3O7F nanostructured lm can result in higher carrierconcentration and longer electron lifetime, thus improvingsolar energy conversion efficiency.

Fig. 8 (A) UV-vis absorption spectra of the as-synthesised Nb3O7F nano-structured film. (B) XPS valence band spectra of the as-synthesised Nb3O7Fnanostructured film. (C) Determined valence band and conduction band edges ofthe as-synthesised Nb3O7F nanostructured film photoanode.

Dye sensitisation

In our previous report, a single crystal Nb3O7(OH) nanorod lmwas directly grown on a FTO substrate by a facile hydrothermalmethod.16 Without the need for further calcination, the singlecrystal Nb3O7(OH) nanorod lm can be directly used as a pho-toanode, showing an impressive solar energy conversion effi-ciency (h ¼ 6.77%) owing to the high crystallinity and largesurface area of nanorods.16 In this work, a highly crystallineNb3O7F nanostructured lm was also investigated in DSSCs.Without the need for further calcination, the Nb3O7F nano-structured lm as a photoanode material was rst sensitisedwith N719 dye (3 � 10�4 mol L�1) for 24 h before DSSCmeasurement. For comparison, the Nb2O5 nanostructured lmobtained by thermal treatment of the Nb3O7F nanostructuredlm at 550 �C was also sensitised with N719 dye to investigateits DSSC performance. Aer calcination, the formed Nb2O5

nanostructured lm possesses a thickness of ca. 18.6 mm. Thedecrease in lm thickness compared to the Nb3O7F lm (ca.20 mm) can be ascribed to the lm structure collapse duringhigh temperature calcination. This concurrently results in thedecrease in the surface area of the calcined sample (BET ¼ 26.1m2 g�1) compared to that of the Nb3O7F sample (BET ¼ 35.7 m2

g�1). This may not be benecial to high dye loading for theNb3O7F lm. Fig. SI-11 (ESI†) shows the current–voltage char-acteristics of DSSCs assembled with the Nb3O7F and Nb2O5

photoanodes. As shown, the DSSCs assembled with the Nb3O7Fphotoanodes display a short-circuit current density ( Jsc) of 6.02mA cm�2, an open-circuit voltage (Voc) of 697 mV, a ll factor(FF) of 66.2% and an overall conversion efficiency (h) of 2.78%,whereas DSSCs assembled with Nb2O5 photoanodes possess aJsc of 5.54 mA cm�2, an Voc of 686 mV, a FF of 55.3% and anoverall conversion efficiency (h) of 2.10%. Compared withNb2O5 photoanodes, a 32.4% improvement in the overallconversion efficiency was achieved by using Nb3O7F photo-anodes. Such a signicant increase in the overall conversionefficiency could be due to the high crystallinity and relativelylarge surface area of Nb3O7F nanostructures in comparison withNb2O5 nanostructures. The above results demonstrate that thehighly crystalline Nb3O7F nanostructured lm can become apromising candidate as a photoanode material to furtherimprove the DSSC performance.

It is well known that the matching of the conduction bandenergies between the photoanode material and the sensitiserplays a key role in the construction of high performancephotovoltaics.43 For this, we performed an X-ray photoelectronvalence-band (VB) spectra experiment with the as-synthesisedNb3O7F nanostructured lm. Based on the UV-vis diffusereectance spectra of the as-synthesised Nb3O7F

6570 | J. Mater. Chem. A, 2013, 1, 6563–6571

nanostructured lm (Fig. 8A), a band gap of 3.12 eV can beobtained for the Nb3O7F nanostructured lm. The X-rayphotoelectron valence band (VB) spectra reveal a VB maximumof ca. 1.94 eV for Nb3O7F, which is very close to the VB value ofTiO2 (Fig. 8B).51,52 Due to similar values of the band gap and thevalence band with TiO2, the as-synthesised Nb3O7F should besuitable as a promising candidate for CdS quantum dot (dye)-sensitised solar cells from the point of view of the matching ofthe conduction band energies (Fig. 8C).

4 Conclusions

In summary, the highly crystalline Nb3O7F nanostructuredlms have been successfully fabricated on FTO substratesthrough a facile and one-pot hydrothermal method. Withoutthe need for further calcination, the highly crystalline Nb3O7Fnanostructured lms can be directly sensitised with CdSquantum dots by the chemical bath deposition (CBD) method.The resultant CdS quantum dot-sensitised Nb3O7F lms withoptimised CBD cycles (7 CBD cycles in our case) show excellentphotoelectrocatalytic activity under visible light irradiation. Anoverall light conversion efficiency of 1.68% can be achievedfrom a quantum dot-sensitised solar cell (QDSSC) assembledwith the NbCdS-7 photoanode. An appropriate CdS depositionamount on Nb3O7F nanostructures can produce high carrierconcentration and long electron lifetime, thus resulting inexcellent photoelectrocatalytic activity and relatively highQDSSC efficiency. Without the need for further calcination, thehighly crystalline Nb3O7F nanostructured lms have also beeninvestigated in dye-sensitised solar cells (DSSCs), exhibiting anoverall light conversion efficiency of 2.78%. This study indicatesthe feasibility of developing alternative photoanode materialsfor solar energy conversion applications.




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