Synthesis and characterization of Nb2O5 mesostructures with tunable morphology and their application in dye-sensitized solar cells

Accepted Manuscript Synthesis and characterization of Nb2O5 mesostructures with tunable morphology and their application in dye-sensitized solar cells Riccardo Panetta, Alessandro Latini, Ida Pettiti, Carmen Cavallo PII:

S0254-0584(17)30734-4

DOI:

10.1016/j.matchemphys.2017.09.030

Reference:

MAC 19997

To appear in:

Materials Chemistry and Physics

Received Date: 1 March 2017 Revised Date:

11 September 2017

Accepted Date: 16 September 2017

Please cite this article as: R. Panetta, A. Latini, I. Pettiti, C. Cavallo, Synthesis and characterization of Nb2O5 mesostructures with tunable morphology and their application in dye-sensitized solar cells, Materials Chemistry and Physics (2017), doi: 10.1016/j.matchemphys.2017.09.030. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Synthesis and Characterization of Nb2O5 Mesostructures with Tunable Morphology and Their Application in Dye-Sensitized

Riccardo Panetta1, Alessandro Latini1*, Ida Pettiti1, Carmen Cavallo1 1

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Solar Cells

Dipartimento di Chimica, Università degli Studi di Roma "La Sapienza", Piazzale Aldo Moro, 5

00185 Roma, Italy. *

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Corresponding Author. Email: [email protected]

ABSTRACT: Mesoporous submicrometric particles of orthorhombic Nb2O5 were prepared by a very simple route consisting in the hydrolysis of niobium ethoxide Nb(OEt)5 in an alcoholic medium containing 1-hexadecylamine as structure-directing agent followed by a hydrothermal treatment. The effects related to the variation of the length of the aliphatic chain of the alcohol solvent (C2 to C4) and the ramification of the alcohol (primary, secondary and tertiary), as well as

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the reactants to solvent ratio were analyzed in terms of morphology, crystal structure, specific surface area and porosity. The obtained solids, once thermally treated to remove the organics, were thoroughly characterized by powder XRD, UV-Vis spectroscopy (band gap measurement), photovoltage measurements (conduction band edge determination), BET-BJH surface analysis and

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FESEM. By simply modifying the solvent and reactants/solvent ratio, different morphologies spanning from nearly monodisperse beads to peanut-shaped particles, sintered spheres aggregates, a

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mix of different morphologies can be achieved. The obtained materials were tested as photoanodes in dye-sensitized solar cells. After the optimization of the thickness of the photoanode and of its chemical treatment in order to improve the inter-particle connections, DSSCs were prepared by using N719 dye and a non-volatile iodide-based electrolyte. The cells were tested by J-V curves under AM 1.5 G illumination and dark, IPCE measurements and electrochemical impedance spectroscopy. A remarkable efficiency value of 3.4% under 1 sun illumination was achieved by employing peanut-shaped particles obtained by using 2-propanol as solvent with the highest used reactant/solvent ratio. Keywords: niobium pentoxide; mesoporous materials; dye-sensitized solar cells.

ACCEPTED MANUSCRIPT 1. Introduction Niobium (V) oxide, Nb2O5, is an important industrial material, used for the extraction of niobium metal [1], for the control of refractive index in glass manufacturing [2], for the synthesis of the important ferroelectric material LiNbO3 [3,4], and in the electronic industry (relaxors [5] and ceramic capacitors [1]). The hydrated forms of Nb2O5 are also heterogeneous catalysts, being strong

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solid acids, and thus they have been used in many reactions such as alkenes polymerization (e.g., propylene) and isomerization, hydrolysis and esterification reactions [6,7]. Nb2O5 is a wide band gap semiconductor with a gap value exceeding 3 eV; the exact value depends on the polymorph and on the nature of the transition (direct or indirect, allowed or forbidden [8,9]). The semiconducting

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properties of Nb2O5, together with its band gap value and the energetic position of the band edges [10], make it an interesting photoanode material for hydrogen photoelectrosynthesis [11] and for dye-sensitized solar cells (DSSCs). DSSCs represent an interesting low-cost class of photovoltaic

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devices. DSSCs are made up of four main components: (i) a conductive glass substrate acting as a support for the nanocrystalline semiconductor, (ii) a dye-sensitized wide band gap semiconductor film, (iii) a redox electrolyte, and (iv) a counter electrode. When a photon possessing sufficient energy is adsorbed, an HOMO (Highest Occupied Molecular Orbital)-LUMO (Lowest Unoccupied Molecular Orbital) transition occurs in the dye. The excited electron is injected into the

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semiconductor’s conduction band where it diffuses through the nanostructured semiconductor film. Then, it is routed through the external circuit to the counter electrode, where it reduces the oxidized form of the redox couple present in the electrolyte. The dye is regenerated by the reduced form of the redox couple of the electrolyte. Such a mechanism allows for light conversion into electrical

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energy without any permanent chemical modification in the device. For DSSCs applications, Nb2O5 has not received yet by scientists and technologists as much attention as other semiconductors such as TiO2 and ZnO [10,12,13].

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The reason behind the relatively low interest of the scientific community for the use of Nb2O5 in DSSCs may be related to the difficulty to prepare it in a form that is thermally stable up to at least 500 °C and which possesses at the same time a quite high specific surface area, probably due to the fact that all the polymorphs of Nb2O5 show very large crystalline unit cells [10]. The important role played by porous materials in DSSCs stems from the necessity of chemisorbing a high number of dye molecules per unit device area. Furthermore, large specific surface area materials need to be coupled with adequate morphologies in order to maximize the probability of light absorption. Different synthetic approaches are present in literature in order to prepare high specific surface area Nb2O5 [7]. As examples, we can cite the synthesis using long chain primary aliphatic amines as

ACCEPTED MANUSCRIPT templating agents to prepare an amorphous, TMS type oxide [14], the electrochemical anodization processes [15,16], the combination of template reaction and antisolvent precipitation [17], K4Nb6O16 exfoliation [18], and the block copolymers templated syntheses [19]. Our laboratory has been active in the research on new mesoporous semiconducting oxides for photoanodes of DSSCs for quite a few years [20-22]. As stated before, Nb2O5 is an interesting candidate for such an

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application, obviously if a simple, not expensive route can be used for preparing it in mesoporous forms (for high dye loadings) and with suitable optical properties. In the present paper we show the results regarding the preparation Nb2O5 samples by the simple hydrolysis of Nb (V) ethoxide in alcohol solvents containing 1-hexadecylamine as templating agent, followed by a mild

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hydrothermal treatment of the obtained precipitate and a calcination in air at 500 °C for 2 hours of the final solid in order to remove the organic matter. The choice of this synthetic approach arises from two considerations. The first one is that niobium compounds in the (V) oxidation state show

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many similarities with the corresponding titanium (IV) compounds, as obvious being Ti and Nb in diagonal relationship in the periodic table. For instance, both the most common alkoxides of these elements, i.e. the isopropoxide for Ti (IV) and the ethoxide for Nb (V) tend to form complexes with aliphatic primary amines. The second consideration is that these complexes self-assemble to give, after hydrolysis, a 3-D mesoporous oxide network. The morphology of this latter depends on the

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experimental conditions [14,23]. Thus, we decided to try to use the same approach we adopted in the preparation of TiO2 mesoporous beads [20,22] to prepare Nb2O5 mesoporous beads using Nb(V) ethoxide instead of Ti(IV) isopropoxide. The aim of this approach is twofold, i.e. to have a simple and quantitative synthesis of a morphologically adequate material for DSSCs that combines the

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properties of the transparent and the scattering semiconductor layers normally used in these devices [24]. The optimization of the morphology of the material for use in DSSCs was performed by the variation of two experimental parameters, i.e. the structure of the solvent and the reactants/solvent

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ratio. The effect of the variation of the structure of the alcohol solvent (aliphatic chain length up to C4, primary, secondary and tertiary alcohol) was studied, as well as the effect of the reactants/solvent ratio in order to vary the hydrolysis reaction rate. In all cases mesoporous Nb2O5 with orthorhombic crystal structure was obtained. However, the morphology as well as the pore size distribution turned out to be highly solvent-dependent, while the effect of the solvent/reactants ratio was found out to be more pronounced on the porosity of the samples rather than on their morphology.

2. Experimental

ACCEPTED MANUSCRIPT 2.1 Materials. Niobium (V) ethoxide (Nb(OC2H5)5 99.9+%) was purchased from Strem Chemicals. Lithium iodide (ultra dry, 99.999%), iodine (99.9985%), potassium chloride (99%) have been were purchased from Alfa Aesar. Titanium (IV) chloride (99.9%), 4-tert-butylpyridine (TBP) (96%), acetonitrile, di-tetrabutylammonium cis-bis (isothiocyanato) bis (2,2′ -bipyridyl-4,4′ -dicarboxylato) ruthenium(II) (N719 dye, 95%), absolute ethanol, 1-propanol, 2-propanol, n-butanol, tert-butyl

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alcohol, hydrogen peroxide solution (34.5–36.5%), acetic acid (99–100%), ammonium hydroxide solution (30–33%), hydrochloric acid (≥ 37%), nitric acid (70%), anhydrous terpineol, 5–15 mPa·s ethyl cellulose (48.0–49.5% w/w ethoxyl basis), 30–70 mPa·s ethyl cellulose (48.0–49.5% w/w ethoxyl basis), 1-hexadecylamine (HDA) (technical, 90%), guanidinium thiocyanate (GuSCN, ≥

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99%), 1,1’-dimethyl-4,4’-bipyridinium dichloride hydrate (methyl viologen, 98%) and benzonitrile (99.9%) have been were purchased from Sigma Aldrich. Hydrogen hexachloroplatinate (IV) hydrate (40% Pt by weight) has been was purchased from Chempur. 1-ethyl-3 methylimidazolium

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iodide (EMII, > 98%) was purchased from Iolitec. Potassium nitrate was purchased from Merck. All the chemicals were used as received, with the exception of benzonitrile, which was dried over activated 3 Å molecular sieves, and 4-tert-butylpyridine, which was vacuum distilled. Ultrapure water was used during the experiments (resistivity: 18 MΩ·cm).

2.2 Mesostructures syntheses. The syntheses were performed with two reactants (reactants are

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Nb(OC2H5)5 and 0.1 M aqueous KCl) to solvent ratios (the solvent is a solution of HDA in the appropriate alcohol solvent), keeping fixed the concentration of HDA and the Nb(OC2H5)5/0.1 M aqueous KCl ratio. In a typical synthesis 1.99 g of HDA were added to 200 mL of alcohol in a 250 mL beaker under magnetic stirring at room temperature until complete dissolution. Then, 0.8 mL

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(highest ratio) or 0.4 mL (lowest ratio) of 0.1 M KCl were added to the solution. 3.8 mL (highest ratio) or 1.9 mL (lowest ratio) of Nb(OC2H5)5 were finally added under vigorous magnetic stirring, and after some seconds a white precipitate forms. The suspension was kept under stirring for 1

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minute, then the magnetic stirrer was removed and the beaker covered with parafilm. The suspension was left undisturbed overnight. The day after the suspension was filtered on a büchner funnel, left under suction for 30 min and the obtained solid was placed in a 300 mL Teflon lined stainless steel autoclave with 61 mL of ultrapure water, 120 mL of ethanol and 5 mL of 33% aqueous ammonia. The autoclave was sealed and kept at 160 °C for 16 hours. After cooling to room temperature, the solid was recovered by filtration on a büchner funnel, washed 3 times with 100 mL of ethanol, left under suction for 30 min and finally calcined at 500 °C in air for 2 hours (heating slope: 5 °C/min) to remove the organic matter. 2.3 Materials characterization

ACCEPTED MANUSCRIPT 2.3.1 XRD. Powder diffraction analysis of the prepared materials after calcination was performed using a Panalytical X’Pert Pro MPD diffractometer (Cu Kα radiation, λ = 0.154184 nm) equipped with a X’Celerator ultrafast RTMS detector. The angular range was 10–90° (in 2θ). The Rietveld analysis of the diffraction patterns was performed using the MAUD software package [25], obtaining the values of the unit cell axes and the mean crystallite size.

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2.3.2 BET-BJH textural analysis. Surface area, Brunauer–Emmett–Teller (BET) multipoint method [26] and textural analysis were carried out using a Micromeritics 3-Flex 3500 analyzer. The pore size distribution was determined from the adsorption curve by the Barret–Joyner–Halenda (BJH) method [27]. The total pore volume was determined by the rule of Gurvitsch [28]. The materials

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were tested after calcination.

2.3.3 UV-Vis spectroscopy. UV-Vis spectroscopy in absorption mode was employed for the determination of the dye loading in the DSSC photoanodes, while diffuse reflectance mode was

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used for the determination of the band gap of Nb2O5 samples. A Shimadzu (Japan) UV2600 UV-Vis spectrophotometer was used for the above measurements. For the determination of the dye loadings, the dye was desorbed from sensitized photoanodes by treatment with aqueous 0.02 M NaOH. A ISR-2600 Plus integrating sphere was connected to the spectrophotometer for measurements in diffuse reflectance condition. BaSO4 powder was used as reflectance reference. Due to the

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ambiguity present in literature on the nature of the transition [8,10], the gap value was calculated by determining the absorption edges using the Kubelka-Munk function. Transition nature was determined by comparing the Kubelka-Munk function absorption edges and Tauc's plots [29] with different exponents.

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2.3.4 Conduction band edge measurements. The absolute positions of the conduction band edge levels (Ecb) on the energy scale was performed on three Nb2O5 samples by using the powder suspension photovoltage method as reported in literature [30]. The samples were chosen according

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to the DSSC performances, i.e. the best, the worst and an intermediate performing sample. A sample of highly crystalline anatase (Tioxide A-HR, free sample kindly provided by Huntsman) was also tested for comparison purposes. A Crison GLP 21 pH meter calibrated against three buffers (pH 4.01, 7.01 and 9.21) was used to control the pH during the measurements. An Ag/AgCl/3M KCl electrode (Amel Instruments) was used as reference. Data acquisition was performed with the electrochemical interface described in § 2.4.2. Illumination of the suspension was provided by the solar simulator also described in § 2.4.2. The measurements were performed at 25.0 °C. The Ecb values are calculated at pH=7.

ACCEPTED MANUSCRIPT 2.3.5 FESEM. Field emission scanning electron microscopy analysis of the morphology of the samples was conducted using a Zeiss Auriga FESEM. The SEM is equipped with a Schottky field emission Gemini column. Operating range 100 V-30 kV. Resolution: 1.0 nm at 15 kV. 2.4 Dye sensitized solar cells 2.4.1 Cells preparation and assembly. Dye sensitized solar cells photoanodes were prepared

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according to the same procedure used for the TiO2-based cells [20,22], i.e. by screen printing ethylcellulose-based pastes containing the prepared Nb2O5 samples through a 34T mesh polyester screen. The screen printing process was repeated up to the desired photoanode thickness. The thickness of the photoanodes was determined by using an A.P.E. Research (Italy) MAP3D-25 stylus

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profilometer. The quantity of Nb2O5 used in the preparation of each paste was adjusted in order to have the same volume of solid used in the preparation of TiO2-based pastes, taking into account the different densities of the two materials.

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The thickness of Nb2O5 photoanodes was optimized on the basis of its different electronic properties compared to TiO2, especially the electron diffusion length, that needs to be much higher than the photoanode thickness.

The photoanodes were screen printed on TiCl4-treated FTO glass slides (thickness 3 mm, sheet resistance 10 Ω/□, XOP Fisica, Spain). After the sintering process of the photoanodes, they were

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impregnated with a 0.2 M solution of Nb(OC2H5)5 in ethanol (previously dried on 3 Å molecular sieves) and then calcining them at 500 °C for 30 minutes in air. This treatment improves the particles interconnectivity and the dye adsorption properties of the photoanode in the same way that 40 mM aqueous TiCl4 does for TiO2 photoanodes [31].

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A different sensitization process compared to the ones commonly used for TiO2 photoanodes [32,33] was employed, because in these last conditions Nb2O5 photoanodes do not sensitize completely.

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The sensitization process was performed by autoclaving at 90 °C for 12 hours the photoanodes in a 300 mL teflon lined stainless steel autoclave with 100 mL of a 0.5 mM solution of N719 in a 1:1 vol:vol mixture of acetonitrile and tert-butyl alcohol. Once the autoclave cooled down to room temperature, the photoanodes were removed from it, washed thoroughly with ethanol and acetonitrile and left until complete drying. After drying, DSSCs were assembled using a platinized FTO glass slide (thickness 3 mm, sheet resistance 15 Ω/□, XOP Fisica, Spain) prepared according to literature procedure [32] as counter electrode. The cells were sealed with 25 µm Surlyn gasket (Dyenamo, Sweden) at 110 °C. After sealing and cooling to room temperature, the space between the electrodes was filled with a non-volatile electrolyte containing 0.6 M EMII, 0.5 M TBP, 0.1 M GuSCN, 0.1 M LiI and 0.03 M I2 in benzonitrile [34] through a 1 mm diameter hole that had been

ACCEPTED MANUSCRIPT previously drilled through the counter electrode. The hole for electrolyte insertion was then sealed by melting over it a 60 µm Surlyn gasket (Solaronix, Switzerland) covered with a thin microscope glass slide using a hot solder tip. Copper wires were soldered on each electrode by using MBR Electronics USS-9210 ultrasonic soldering system in conjunction with the Cerasolzer CS246-150 soldering alloy. A Kynar PVDF 502-CUH-HC film (free sample by Arkema Inc.) was used as anti-

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reflection and UV blocking layer (< 400 nm) on the photoanode side. A minimum of 3 cells were prepared for each Nb2O5 sample.

2.4.2 DSSCs test. The assembled DSSCs were tested through a wide range of techniques. J-V curves were recorded using a Solartron Analytical 1286 electrochemical interface (EI) coupled with

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a Solartron Analytical 1260 frequency response analyzer (FRA) under simulated AM 1.5 G solar radiation produced by an Asahi Spectra (Japan) HAL-320 class A solar simulator, and in dark. A calibrated Asahi Spectra Sun Checker was used to verify the intensity of the simulated solar

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radiation (within ± 1% of 1 sun). Electrochemical impedance spectra (EIS) under 1 sun simulated solar radiation at (VOC-0.35) V bias (around the maximum power point) were acquired by using the FRA. These spectra were used to calculate electron transport resistances (Rt), recombination resistances (Rr), electron diffusion lengths (Le) and charge collection efficiencies. The data acquisition for the EI and FRA was performed using the Full Combo ZPLOT/CorrWare software by

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Scribner Associates Inc., USA. The fit of the impedance spectra was carried out with the ZView software using the transmission line model [35]. The incident photon to current conversion efficiency (IPCE) curves were recorded in DC mode without white light bias using a custom-made

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apparatus controlled by a custom made, LabVIEW-based software [20].

3. Results and discussion

3.1.1 XRD. All the samples were made of orthorhombic [36] Nb2O5 but with a different degree of

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crystallinity, as can be inferred by the different signal/background ratios. The sample with the lowest signal/background ratio (lowest crystallinity, panel A) and the sample with the highest signal/background ratio (highest crystallinity, panel B) are displayed in Figure 1. The XRD patterns of the remaining samples are reported in Figures S1-S8 of the supporting information. The results of the Rietveld analysis of the diffraction patterns are reported in Table 1. The c cell parameter is in excellent agreement with the reference values, while a small (~0.9%) but clear increase of the a axis and a decrease of the b axis (~0.4%) were observed in all samples. This distortion of the ab plane with respect to the reference structure can be explained considering that the oxygen planes lie on it and Nb2O5 is naturally oxygen-defective. Vacant ionic sites in crystals produce distortions of the lattice due to locally unbalanced repulsions of the counterions of the first coordination sphere [37].

ACCEPTED MANUSCRIPT The materials are all nanocrystalline, with crystallite sizes in the range 24-41 nm. For all the alcohols used as solvents, the crystallite size is always slightly higher at the lower reactants/solvent ratio. This fact can be easily justified by the slower kinetics brought about by these conditions that produce a smaller number of critical nuclei available for subsequent growth. 3.1.2 BET-BJH textural analysis. All the samples display a quite high specific surface area, in the

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36-61 m2 g-1 range and pore size distributions with diameter values in the 2-15 nm mesopores range. The results are summarized in Table 2. The pore size distribution values indicate that the access of N719 molecules inside the mesopores is allowed, being the diameter of N719 molecule about 1.8 nm [38]. An interesting linear relationship between the total pores volume and the number

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of carbon atoms in the primary alcohols was found for both reactants/solvent ratios (Figures 2 and 3). A similar correlation between the maximum values (ranging from 4 to 10 nm) of the pores size distribution and the ramification of the aliphatic chain of the alcohol, i.e. between ethanol (primary),

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2-propanol (secondary) and tert-butyl alcohol (tertiary), was found. However, in this case only for the highest reactants/solvent ratio (Figure 4). In all cases a decreasing trend was observed. These effects can be understood in terms of a decreasing affinity of the solvent (decreasing polarity with increasing aliphatic chain length) for the oxide surface. The effect is more pronounced for the highest reactants/solvent ratio, probably because of the faster kinetics that allows a higher quantity

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of solvent to be trapped inside the forming solid. This explanation is supported by 2 facts: 1. the decreasing trend of the maximum values of the pore size distribution vs. the ramification of the aliphatic chain of the alcohol (and thus the decrease of its polarity) cannot be observed for the lowest reactants/solvent ratio (Figure 5), where the slower kinetics allow for a stabilization of the

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pore structure regardless of the solvent;

2. the much less pronounced slope of the trend shown in Figure 3 compared to the one of Figure 2, i.e. the total pore volume is less dependent on the aliphatic chain length of the alcohol solvent for

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the samples with the lowest reactants/solvent ratio. 3.1.3 UV-Vis Spectroscopy. The dye loadings for the different samples are displayed in Table 3. With only one exception, dye loadings increase linearly with total pore volume (Figure S9 of the supporting information). By estimating the weight of the films (using the total pore volumes obtained by BET-BJH surface analyses, the density of Nb2O5, the geometrical areas and the thickness values of the films) and calculating their approximate surface area from the BET specific surface area values, the number of N719 molecules necessary to have a monolayer can be roughly calculated (by using a value of 1.65 nm2 as the area per N719 molecule [39]). The agreement between the experimental values of the number of molecules derived from the dye loading measurements and the calculated ones, both reported in Table 3, is excellent, thus confirming the

ACCEPTED MANUSCRIPT effectiveness of the autoclavation procedure to achieve a full sensitization of the films. Such a dye loading level is not achievable with the common procedures used for the sensitization of TiO2 films. A proof of this is given by the quantity of N719 desorbed from a film made with the best DSSCperforming Nb2O5 (sample E) sensitized with the standard procedure [32]. The amount of adsorbed dye is 0.14±0.01 mg cm-2, which is almost two times lower than the one obtained by the

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autoclavation procedure (0.26±0.04 mg cm-2). Considering now the band gap values, their calculation by the Tauc’s plot was not straightforward because we had to assess the nature of the transition without which the exponent of the αhν term would be unknown. Therefore, the reflectance spectra were transformed into absorbance by using

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the Kubelka-Munk function and the band gap value was determined the absorption onset energy value. Then by making use of the Tauc’s plot and varying the exponents, we determined the nature of the transition by comparing the gap values obtained with different exponents with the one

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obtained by the absorption onsets. The results are shown in Table 4. The direct forbidden transition gave the best agreement for all samples with the exception of sample C, where the best agreement was found with an allowed indirect transition. This last sample is the one that displays the lowest crystallinity by XRD (highest background to noise ratio), thus, its higher structural disorder may be responsible for the different electronic structure. The gap values are always lower for the samples

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with the lowest reactants/solvent ratio, and a plausible reason is their higher crystallite dimensions. 3.1.4 Conduction band edge measurements. The photovoltage vs. pH curves are displayed in figure 6: 1 is the TiO2 reference, 2 (sample E) the best, 3 (sample D) the intermediate and 4 (sample H) the worst DSSC-performing Nb2O5. The TiO2 Ecb value (-0.49 V vs NHE) is in excellent agreement

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with literature [40]. The Ecb values for the Nb2O5 samples are -0.56 V, -0.45 V and -0.53 V (all versus NHE) for samples reported in panel B, C and D, respectively. The diagram reported in Figure 7 shows the relative positions of the band edges (the valence band position is calculated by

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adding the band gap energy obtained by UV-Vis spectroscopy), together with the positions of the fundamental and excited states of the N719 dye [41]. The photovoltage increase for B during the experiment was about 0.54 V, slightly lower than that obtained with the reference A (0.66 V), and the increase obtained with C and D was sensibly lower (0.34 V and 0.31 V, respectively). This last effect is obviously related to the higher defect concentration in these last two samples that favors e-h+ recombination with respect to electron transfer to methyl viologen [42]. This higher concentration of defects is responsible for their lower performances in DSSCs compared to B. The higher performance of C compared to D in DSSCs is probably linked to the fact that its conduction band edge is in a more favorable position than that of D with respect to the excited state of N719 dye in the energy scale, thus facilitating electron injection.

ACCEPTED MANUSCRIPT 3.1.5 FESEM. In Figure 8, the highest and the lowest reactants/solvent ratio samples are grouped on the left and on the right, respectively, with insets showing higher magnifications. While ethanol and 1-propanol tend to give regular, nearly monodisperse spheroidal particles with both ratios without evident morphological differences, 1-butanol, 2-propanol and tert-butyl alcohol tend to give aggregates of irregular shape (1-butanol, tert-butyl alcohol) or peanut-shaped (2-propanol) particles.

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The smallest particles are observed with 2-propanol and tert-butyl alcohol, but in the case of 2propanol with the highest and in that of tert-butyl alcohol with the lowest reactants/solvent ratio, the particles are highly merged in large, compact aggregates. In all cases, the particles are larger for the highest reactants/solvent ratio. This effect may be due to the surface free energy, that is higher in

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case of smaller crystallites (which obviously possess a higher surface to volume ratio) and favors crystallites aggregation.

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3.2 Dye sensitized solar cells

3.2.1 J-V curves under 1 sun AM 1.5G illumination

Considering the electron diffusion lengths that, as shown later in the EIS results, are always shorter for Nb2O5 in comparison to the values typically found for TiO2 [22], and the optimal thickness for photoanodes made of this material (12-14 µm [32]), this last quantity should be smaller or at most

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equal for Nb2O5. Figure 9 shows the J-V curves of cells with photoanodes made of Nb2O5 (sample A) of different thicknesses. As clearly evident, the thickest film (11 µm) is the most performant, consequently, all the photoanodes used in the experiment were prepared with a thickness around this value. In addition to the photoanode thickness optimization, we also tried to improve the

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performances of the cells by a chemical treatment similar to the aqueous TiCl4 treatments performed on TiO2 photoanodes, that are well known for sensibly improving their performances by enhancing interparticle connectivity and surface roughness [43]. The Nb2O5 DSSC photoanodes

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treatment with 0.2 M Nb(OEt)5 in dry ethanol is reported to have beneficial effect on their performances for the same reasons [31]. Thus, we tested this last treatment on a photoanode made of the same Nb2O5 sample used for thickness optimization. The comparison between untreated and treated photoanode J-V curves is given in Figure 10. The beneficial effect of the treatment is clear, and consequently all the cells presented in the following discussion were prepared using ethanolic Nb(OEt)5 treated photoanodes. The J-V curves for all cells are displayed in Figure 11, while the parameters extrapolated from them are grouped in Table 5. The highest efficiency was obtained by the cell obtained using Nb2O5 synthesized in 2-propanol with the highest reactants/solvent ratio (sample E). The efficiency value (3.4%) is remarkable, considering that, among other oxide semiconductors, only TiO2 and ZnO allow to achieve sensibly higher efficiencies (12.3% and 7.5%,

ACCEPTED MANUSCRIPT respectively). For instance, SnO2 and Zn2SnO4 have reached similar efficiency values (3.2% and 3.8%, respectively), while the maximum efficiency obtained with WO3 is reported to be lower than 2 % (1.46%) [10]. In all cases (with the exception of tert-butyl alcohol) the cells were more efficient for the samples obtained from the highest reactants/solvent ratio. For these, there is an approximately linear trend

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between efficiency and total pore volume, while no trend was observed in the case of the lowest reactants/solvent ratio samples (Figures S10-S11 of the supporting information, respectively). With some exceptions, there is also a linear relationship between the efficiency and the dye loading values (Figure S12 of the supporting information). The samples out of this last trend are those

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possessing particular optical characteristics, as can be envisaged by their IPCE spectra (see below). For example, sample L shows one of the highest efficiency values despite of having one of the lowest dye loadings. On the other hand, sample G that shows the highest dye loading, is not the best

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performing one. These results are indicative of the importance of particles shape and morphology. In fact, samples E and L show the biggest most compact and irregular aggregates that favor light scattering, to an extent that, in sample L, overcomes the relatively low dye loading capability. On the contrary, in sample G, the small size and the regular and small dimensions of the aggregates hinders the light scattering to an extent that the high dye loading capability is not able to

3.2.2 Dark current

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compensate for. These findings are supported by IPCE spectra as can be read below.

J-V curves in dark under forward bias (Figure S13 of the supporting information) were acquired for all cells to obtain J0 (a measurement of recombination processes) and the ideality factor m, that with the equation:

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accounts for the deviation of the behaviour of the cell from that of an ideal diode fitting the data

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ln ≈ ln +

 (1)

 

where q represents the elementary charge, kB the Boltzmann constant and T the absolute temperature. The data are reported in Table 5 together with those obtained under simulated solar radiation,. The J0 values are small for all the devices, being in the nA cm-2 range for all of them. This implies a low level of charge recombination. The m values for all cells are around 2, a significant deviation from the ideal diode; but it is not surprising for cells using a low conductivity electrolyte as in the present case [22]. 3.2.3 Electrochemical impedance spectroscopy (EIS) The results obtained by fitting the EIS plots are reported in Table 6. While the Rr values do not differ sensibly from those commonly found in TiO2-based devices [44], the Rt and, consequently, the Le and charge collection efficiency values are sensibly different, being Rt about 1 order of

ACCEPTED MANUSCRIPT magnitude higher for the niobium pentoxide photoanodes. These high electron transport resistances are very probably one of the most important factors responsible for the lower efficiencies of Nb2O5 devices compared to TiO2 ones. In fact, when Nb2O5 systems have very ordered structures (i.e. with less defects, and/or with lower concentrations of traps and grain boundaries that contrast electron transport) like single crystal nanorods [45] or anodically grown tubular structures, the Rt and

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conversion efficiency become comparable to those of anatase [16]. The higher Rt is confirmed also by the value of series resistances (Rs) derived from the J-V curves under illumination and reported in Table 5, which are sensibly higher than those of similar TiO2based DSSCs [22]. Conversion efficiency values display an approximately linear trend versus Rs, as

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evident in Figure 12. This series resistances contain all the contributions stemming from from Rt, contacts, electrolyte and charge transfer at the counterelectrode. Being all the contributions to Rs practically identical to those found for TiO2 with the exception of Rt, this justifies the differences in

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Rs values and confirms, with an independent measurement, the results obtained by fitting the EIS spectra.

The lowest efficiency cell (with photoanode made of Nb2O5 sample H) shows the highest Rt value; this fact can be related to the high defectivity of the sample, as proved by Ecb measurements. Hence, these defects act as electron traps that increase Rt thus reducing the efficiency of the DSSC.

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3.2.4 Incident photon to current conversion efficiency (IPCE)

The quantum efficiency curves of DSSCs made with the same samples tested by photovoltage measurements are reported in Figure 13, panel A. All the spectra show the features of the N719 light absorption, with intensities reflecting the DSSCs efficiency trend. Some samples, as seen in §

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3.2.1, are out of the linear efficiency vs. dye loading trend, and this reflects into the IPCE spectra. As an example, a comparison between the spectrum of the best performing sample (E) and sample L is shown in Figure 13, panel B. They display similar intensities, especially for λ>500 nm (and

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also their efficiency values are similar), thus indicating superior light scattering properties of the sample with lower dye loading in that wavelength range.

4. Conclusion

Mesoporous Nb2O5 oxides were synthesized by a simple hydrolysis route in alcohol solvents with a templating agent (hexadecylamine). Morphology (nearly monodisperse beads, beads aggregates, peanut-shaped particles), crystallinity and porosity were tuned by varying the alcohol solvents (carbon chain length and/or ramification) and the reactants/solvents ratios. The crystal structure turned out not being affected by the different synthetic procedures (always orthorhombic). Besides the structural properties, also the electronic ones are deeply influenced by the synthetic routes, as

ACCEPTED MANUSCRIPT evidenced by UV-Vis spectroscopy and photovoltage measurements. The differences in physicochemical properties among the various materials reflects in their performances as photoanodes in DSSCs: their optical properties, porosity and defectivity deeply affect the efficiency of the devices, with η values ranging from 1.76 to 3.4%.

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Associated content Supporting information is available.

Acknowledgements

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The Authors are grateful to Università degli Studi di Roma “La Sapienza” for financial support. The Authors wish to thank Dr. Simone Quaranta of UOIT (Canada) for fruitful scientific discussion and Dr. Francesco Mura of the Nanotechnology and Nanoscience Laboratory of the Università degli

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Studi di Roma “La Sapienza” for FESEM analyses.

Additional Information

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The Authors declare no competing financial interests.

ACCEPTED MANUSCRIPT References [1] S. Albrecht, C. Cymorek, J. Eckert, Niobium and Niobium Compounds. Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, 2005. [2] T. Ichimura, Niobium Oxide in Optical Glass Manufacture, Niobium-Proceedings of the International Symposium, New York 1984, pp. 603-614.

Company, Amsterdam 1978, pp.481-601. [4] T. Wearden, New Electronics, 20th Mar. 1984, 29-31.

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[3] A. Räuber, Current Topics in Materials Science, vol. I, E. Kaldis, North-Holland Publishing

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[9] C. Nico, T. Monteiro, M. P. F. Graça, Niobium oxides and niobates physical properties: Review and prospects. Progress in Materials Science 80 (2016) 1-37. [10] L. Alibabaei, H. Luo, R. L. House, P. G. Hoertz, R. Lopez, T. J. Meyer, Applications of metal oxide materials in dye sensitized photoelectrosynthesis cells for making solar fuels: let

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the molecules do the work. J. Mater. Chem. A 1 (2013) 4133-4145. [11] H. Luo, W. Song, P. G. Hoertz, K. Hanson, R. Ghosh, S. Rangan, M. K. Brennaman, J. J. Concepcion, R. A. Binstead, R. A. Bartynski, R. Lopez, T. J. Meyer, A Sensitized Nb2O5

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Photoanode for Hydrogen Production in a Dye-Sensitized Photoelectrosynthesis Cell. Chem. Mater. 25 (2013) 122-131. [12] R. Jose, V. Thavasi, S. Ramakrishna, Metal Oxides for Dye-Sensitized Solar Cells. J. Am. Ceram. Soc. 92 (2009) 289-301. [13] C. Cavallo, F. Di Pascasio, A. Latini, M. Bonomo, D. Dini, Nanostructured Semiconductor Materials for Dye-Sensitized Solar Cells. J. Nanomater. 2017, Article ID 5323164, 31 pp. [14] D. A. Antonelli, J. Y. Ying, Synthesis of a Stable Hexagonally Packed Mesoporous Niobium Oxide Molecular Sieve Through a Novel Ligand-Assisted Templating Mechanism. Angew. Chem. Int. Ed. Engl. 35 (1996) 426-430.

ACCEPTED MANUSCRIPT [15] R. A. Rani, A. Sabirin Zoolfakar, J. Subbiah, J. Zhen Ou, K. Kalantar-zadeh, Highly ordered anodized Nb2O5 nanochannels for dye-sensitized solar cells. Electrochem. Commun. 40 (2014) 20-23. [16] J. Zhen Ou, R. A. Rani, M. Ham, M. R. Field, Y. Zhang, H. Zheng, P. Reece, S. Zhuiykov, S. Sriram, M. Bhaskaran, R. B. Kaner, K. Kalantar-zadeh, Elevated Temperature Anodized

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Catalysts. ChemCatChem 4 (2012) 1675- 1682.

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[19] P. Yang, T. Deng, D. Zhao, P. Feng, D. Pine, B. F. Chmelka, G. M. Whitesides, G. D. Stucky, Hierarchically Ordered Oxides. Science 282 (1998) 2244-2246. [20] A. Latini, C. Cavallo, F. K. Aldibaja, D. Gozzi, D. Carta, A. Corrias, L. Lazzarini, G. Salviati, Efficiency Improvement of DSSC Photoanode by Scandium Doping of

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Mesoporous Titania Beads. J. Phys. Chem. C 117 (2013), 25276-25289. [21] A. Latini, R. Panetta, C. Cavallo, D. Gozzi, S. Quaranta, A Comparison of the Performances of Different Mesoporous Titanias in Dye-Sensitized Solar Cells. J. Nanomater., 2015, Article ID 450405, 8 pp.

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[22] C. Cavallo, A. Salleo, D. Gozzi, F. Di Pascasio, S. Quaranta, R. Panetta, A. Latini, Solid Solutions of Rare Earth Cations in Mesoporous Anatase Beads and Their Performances in Dye-Sensitized Solar Cells. Sci. Rep. 5 (2015) 16785, 15 pp.

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[23] D. Chen, L. Cao, F. Huang, P. Imperia, Y. B. Cheng R. A. Caruso, Synthesis of Monodisperse Mesoporous Titania Beads with Controllable Diameter, High Surface Areas, and Variable Pore Diameters (14-23 nm). J. Am. Chem. Soc. 132 (2010) 4438-4444. [24] F. Sauvage, D. Chen, P. Comte, F. Huang, L. P. Heiniger, Y. B. Cheng, R. A. Caruso, M. Graetzel, Dye-Sensitized Solar Cells Employing a Single Film of Mesoporous TiO2 Beads Achieve Power Conversion Efficiencies Over 10%. ACS Nano 4 (2010) 4420-4425. [25] L. Lutterotti, D. Chateigner, S. Ferrari, J. Ricote, Texture, Residual Stress and Structural Analysis of Thin Films using a Combined X-Ray Analysis. Thin Solid Films 450 (2004) 3441.

ACCEPTED MANUSCRIPT [26] S. J. Gregg, K. S. W. Sing, Adsorption, Surface Area and Porosity. Second Edition. Academic Press, Inc. London. 1982. [27] E. P. Barrett, L. G. Joyner, P. P. Halenda, The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms J. Am. Chem. Soc. 73 (1951) 373-380.

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[28] Gurvitsch, L. J. Phys. Chem. Soc. Russ. 47 (1915) 805-827. [29] J. Tauc, R. Grigorovici, A. Vancu, Optical Properties and Electronic Structure of Amorphous Germanium. Phys. Status Solidi 15 (1966) 627-637.

[30] D. Mitoraj, H. Kisch, Analysis of Electronic and Photocatalytic Properties of

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Semiconductor Powders through Wavelength-Dependent Quasi-Fermi Level and Reactivity Measurements. J. Phys. Chem. C 113 (2009) 20890-20895.

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Electrode Sensitized by a Ruthenium Dye. Chem. Mater. 10 (1998) 3825-3832. [32] S. Ito, T. N. Murakami, P. Comte, P. Liska, C. Grätzel, M. K. Nazeeruddin, M. Grätzel, Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%. Thin Solid Films 516 (2008), 4613-4619. [33] M. K. Nazeeruddin, R. Splivallo, P. Liska, P. Comte, M. Grätzel, A swift dye uptake

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procedure for dye sensitized solar cells. Chem. Commun. (2003) 1456-1457. [34] A. Latini, F. K. Aldibaja, C. Cavallo, D. Gozzi, Benzonitrile based electrolytes for best operation of dye sensitized solar cells. J. Power Sources 269 (2014) 308-316. [35] F. Fabregat-Santiago, G. Garcia-Belmonte, I. Mora-Serò, J. Bisquert, Characterization of

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nanostructured hybrid and organic solar cells by impedance spectroscopy. Phys. Chem. Chem. Phys. 13 (2011) 9083-9118. [36] International Center for Diffraction Data, database JCPDS, card 71-0336, 2001.

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[37] A. R. West, Solid State Chemistry and its Applications. Second Edition John Wiley & Sons, 2014.

[38] C. Lin, F. Tsai, M. Lee, C. Lee, T. Tien, L. Wang S. Tsai, Enhanced performance of dyesensitized solar cells by an Al2O3 charge recombination barrier formed by low-temperature atomic layer deposition. J. Mater. Chem. 19 (2009) 2999-3003. [39] A. Hagfeldt, M. Grätzel, Molecular Photovoltaics. Acc. Chem. Res. 33 (2000) 269-277. [40] J. Park, G. Viscardi, C. Barolo, N. Barbero Near-infrared Sensitization in Dye-sensitized Solar Cells. Chimia 67 (2013) 129-135. [41] M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Müller, P. Liska, N. Vlachopoulos, M. Grätzel, Conversion of light to electricity by cis-X2bis(2,2'-bipyridyl-4,4'-

ACCEPTED MANUSCRIPT dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = Cl-, Br-, I-, CN-, and SCN-) on nanocrystalline titanium dioxide electrodes. J. Am. Chem. Soc. 115 (1993) 6382-6390. [42] S. Ikeda, N. Sugiyama, S. Murakami, H. Kominami, Y. Kera, H. Noguchi, K. Uosaki, T. Torimoto, B. Ohtani, Quantitative analysis of defective sites in titanium(IV) oxide photocatalyst powders. Phys. Chem. Chem. Phys. 5 (2003) 778-783.

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[43] S. Ito, P. Liska, P. Comte, R. Charvet, P. Péchy, U. Bach, L. Schmidt-Mende, Zakeeruddin, S. M. Kay, A. Nazeeruddin M. K. Grätzel, M. Control of dark current in photoelectrochemical (TiO2/I--I3-) and dye-sensitized solar cells. Chem. Commun. (2005) 4351-4353.

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[44] F. Fabregat-Santiago, J. Bisquert, G. Garcia-Belmonte, G. Boschloo, A. Hagfeldt, Influence of electrolyte in transport and recombination in dye-sensitized solar cells studied by impedance spectroscopy. Sol. Energ. Mat. Sol. C. 87 (2005) 117-131.

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[45] H. Zhang, Y. Wang, D. Yang, Y. Li, H. Liu, P. Liu, B. J. Wood, H. Zhao, Directly Hydrothermal Growth of Single Crystal Nb3O7(OH) Nanorod Film for High Performance

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Dye-Sensitized Solar Cells. Adv. Mater. 24 (2012) 1598-1603.

ACCEPTED MANUSCRIPT Figure legends

Figure 1. Powder XRD patterns of the sample with the highest crystallinity (sample I, panel A) and with the lowest crystallinity (sample C, panel B).

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Figure 2. Plot of the total pore volume versus the number of carbon atoms in the primary alcohol solvent for samples synthesized with the highest reactants/solvent ratio.

Figure 3. Plot of the total pore volume versus the number of carbon atoms in the primary alcohol

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solvent for samples synthesized with the lowest reactants/solvent ratio.

Figure 4. Plot of the total pore size distribution maximum versus the ramification of the alcohol

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solvent for samples synthesized with the highest reactants/solvent ratio.

Figure 5. Plot of the total pore size distribution maximum versus the ramification of the alcohol solvent for samples synthesized with the lowest reactants/solvent ratio.

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Figure 6. Variation of photovoltage with pH for selected samples of Nb2O5 and for a reference sample of anatase TiO2. The conduction band edges have been calculated at pH=7 and T=25 °C according to the relationship Ecb (V vs NHE)=-0.445+0.059(pH0-7) given in ref. 30, where pH0 is the abscissa of the inflection point of the curve. Panel 1 is the TiO2 reference, panel 2 sample E,

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panel 3 sample D and panel 4 sample H.

Figure 7. Diagram showing the positions of the valence and conduction bands of selected samples

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of Nb2O5 versus NHE. The energy levels of N719 dye are also shown together with anatase TiO2 as reference. The letters in parentheses h, m and l stand for highest, medium and lowest performance in DSSCs, respectively.

Figure 8. SEM micrographs of the Nb2O5 samples. Each sample is indicated according to a letter as reported in Table 1. For each sample a low magnification (big panel) and a high magnification (small panel) micrograph is provided in order to better appreciate the morphology.

Figure 9. J-V curves of DSSCs assembled with photoanodes made of sample A with different thicknesses.

ACCEPTED MANUSCRIPT Figure 10. J-V curves of DSSCs assembled with photoanodes made of sample A (thickness: 11±1 µm) with and without treatment with 0.2 M ethanolic Nb(OEt)5. Figure 11. J-V curves under AM 1.5 simulated solar radiation of DSSCs assembled with all Nb2O5

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samples.

Figure 12. Plot of efficiency vs Rs values.

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Figure 13. Comparison of the IPCE spectra of samples E, D and H (panel A) and E and L (panel

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B).

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ACCEPTED MANUSCRIPT b/nm

c/nm

D/nm

A

0.6226±1x10-4

2.9056±3x10-4

0.39312±4x10-5

33±3

B

Ethanol

lowest

0.6229±1x10-4

2.9050±1x10-4

0.3929±1x10-4

41±5

C

1-Propanol

highest

0.6230±2x10-4

2.903±1x10-3

0.39324±1x10-5

24±3

D

1-Propanol

lowest

0.6229±1x10-4

2.9072±4x10-4

0.39333±4x10-5

31±3

E

2-Propanol

highest

0.6229±1x10-4

2.9071±4x10-4

0.39311±4x10-5

27±2

F

2-Propanol

lowest

0.6229±1x10-4

2.9061±4x10-4

0.3930±1x10-4

29±1

G

1-Butanol

highest

0.6232±1x10-4

2.9042±4x10-4

0.3932±1x10-4

34±4

H

1-Butanol

lowest

0.6229±1x10-4

2.9054±4x10-4

0.3932±1x10-4

36±4

I

Tert-butyl alcohol

highest

0.6229±1x10-4

2.9049±3x10-4

0.39307±4x10-5

35±3

L

Tert-butyl alcohol

lowest

0.6229±1x10-4

2.9037±3x10-4

0.39312±4x10-5

40±4

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a/nm

Ethanol

Reactants / solvent ratio highest

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Solvent

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Sample

Table 1. Results of the Rietveld refinement analysis performed on the XRD patterns of the Nb2O5

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samples.

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A B C D E F G H I L

Total pore volume/ cm3 g-1

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Table 2. Results of the BET-BJH textural analysis.

0.135 0.105 0.101 0.0908 0.122 0.118 0.0606 0.0718 0.0894 0.0864

Pore size distribution maximum/ nm 10.2 7 4.1 8.7 9.2 6.7 5.1 6.5 8.0 7.5

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Specific surface area/ m2 g-1 47.8±0.3 37.2±0.2 36.4±0.2 41.7±0.2 51.7±0.3 61.4±0.3 44.3±0.2 41.2±0.2 42.8±0.5 38.2±0.5

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Sample

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A B C D E F G H I L

0.29±0.04 0.24±0.03 0.21±0.03 0.19±0.03 0.26±0.04 0.26±0.03 0.29±0.04 0.16±0.02 0.17±0.03 0.16±0.02

Molecules per monolayer 9.6x1015 1.1x1016 1.3x1016 1.0x1016 1.0x1016 1.1x1016 9.8x1015 1.1x1016 1.4x1016 1.0x1016

Molecules adsorbed 1.0x1016 1.4x1016 1.5x1016 9.2x1015 9.7x1015 1.4x1016 1.1x1016 1.6x1016 1.6x1016 8.8x1015

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Dye loading/ mg cm-2

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Table 3. Dye loading values of the photoanodes prepared with the Nb2O5 sample (second column), the calculated numbers of molecules necessary to form a monolayer (third column) and the

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estimated numbers of molecules adsorbed on each photoanode from the dye loadings (fourth

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ACCEPTED MANUSCRIPT Eg allowed direct/eV 3.635±0.008 3.324±0.005 3.595±0.007 3.436±0.006 3.729±0.008 3.651±0.007 3.620±0.007 3.490±0.005 3.564±0.006 3.336±0.003

Eg forbidden direct/eV 3.252±0.003 3.104±0.002 3.369±0.003 3.150±0.001 3.234±0.004 3.298±0.005 3.360±0.005 3.172±0.001 3.193±0.001 3.077±0.001

Eg allowed indirect/eV 3.123±0.002 3.047±0.001 3.284±0.003 3.063±0.002 3.103±0.002 3.169±0.002 3.267±0.003 3.098±0.001 3.120±0.001 3.001±0.001

Eg forbidden indirect/eV 2.916±0.002 2.947±0.001 3.102±0.002 2.964±0.001 2.867±0.003 2.980±0.001 3.084±0.003 2.969±0.002 2.997±0.002 2.893±0.002

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A B C D E F G H I L

Eg absorption onset/eV 3.267±0.004 3.109±0.004 3.213±0.004 3.173±0.004 3.309±0.004 3.295±0.004 3.324±0.004 3.186±0.004 3.207±0.004 3.098±0.004

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Table 4. Band gap values of the Nb2O5 samples obtained from the absorption onset of the UV-Vis

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A B C D E F G H I L

0.6696±0.0001 0.6929±0.0001 0.7026±0.0001 0.6832±0.0001 0.6832±0.0001 0.7201±0.0001 0.6579±0.0001 0.6803±0.0001 0.6540±0.0001 0.6890±0.0001

JSC/ mA cm-2 7.0±0.1 4.08±0.04 5.58±0.04 4.9±0.01 6.6±0.01 4.5±0.1 4.80±0.04 3.79±0.03 4.49±0.04 6.0±0.1

η/ %

FF

Rs/ Ω

3.26±0.04 2.14±0.03 2.81±0.03 2.47±0.04 3.4±0.1 2.27±0.04 2.31±0.03 1.76±0.03 2.20±0.03 3.03±0.04

0.69±0.01 0.76±0.01 0.72±0.01 0.74±0.01 0.76±0.01 0.69±0.01 0.73±0.01 0.68±0.01 0.49±0.01 0.73±0.01

140±10 196±10 130±10 170±5 110±5 200±10 160±5 200±10 150±10 130±10

J0/ nA cm-2 1.9±0.2 8±2 300±30 4±1 2.1±0.3 2.8±0.3 1.4±0.1 12±1 19±4 21±2

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VOC/ V

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Table 5. DSSC cells parameters obtained from J-V curves under illumination and in dark.

m 1.88±0.02 2.22±0.04 2.00±0.02 2.00±0.03 1.86±0.02 2.12±0.01 1.92±0.02 2.56±0.03 2.2±0.1 2.30±0.01

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Le/ µm

A B C D E F G H I L

390±40 220±20 300±20 290±30 330±30 260±30 170±10 460±50 150±20 260±20

1620±150 2180±180 1030±90 1110±100 1590±160 1950±200 2330±190 1560±160 4490±450 1400±130

22±1 40±10 22±1 21±1 22±2 27±3 50±10 18±1 60±40 26±2

Charge collection efficiency 0.8±0.1 0.9±0.1 0.8±0.1 0.8±0.1 0.8±0.1 0.9±0.1 0.9±0.1 0.8±0.1 1.0±0.1 0.8±0.1

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Rt/ Ω

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Table 6. Electrical parameters of the DSSCs obtained by fitting the EIS spectra with the

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Mesoporous niobium (V) oxide structures with tunable morphology were synthesized. The synthesis is based on a simple, controlled hydrolysis of niobium (V) ethoxide. The morphology can be easily tuned by varying the alcohol solvent. DSSCs with efficiencies up to 3.4% were assembled with the synthesized materials. DSSC performances are correlated with the structure and morphology of the materials.

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