Development of ternary iron vanadium oxide semiconductors for applications in photoelectrochemical water oxidation

Abstract

Herein, we report the synthesis of Fe–V-oxides via the drop casting of metal precursor solutions in different proportions onto an indium tin oxide (ITO) coated glass followed by annealing in air at 500 °C for 3 h. UV-vis spectroscopy of the Fe–V-oxides indicates absorption due to ‘direct’ and ‘indirect’ band gaps, although Fe-oxide shows a direct band gap nature. Scanning electron microscopy-energy dispersive X-ray (SEM-EDX) and X-ray diffraction (XRD) studies reveal different surface morphologies with variable crystalline phases for the Fe₂O₃, FeVO₄, FeV₂O₄ and Fe₂VO₄ semiconductors. The photoelectrochemical (PEC) water oxidation reaction over the different materials reveals that the FeV₂O₄ semiconductor exhibits the maximum photocurrent of 0.18 mA cm⁻² at an applied bias of +1.0 V (vs. Ag/AgCl) under the illumination of 100 mW cm⁻² compared to the other Fe₂O₃, FeVO₄ and Fe₂VO₄ semiconductors. Electrochemical impedance spectroscopic (Mott–Schottky) analysis confirms n-type semiconductivity for all the materials with highest donor density, in the order of 2.7 × 10²⁰ cm⁻³, for the FeV₂O₄ thin film, and PL spectra are useful for measuring the separation efficiency of the photo-generated charge carriers. Chronoamperometric studies under constant illumination of the best semiconductor (FeV₂O₄) indicate the significant stability of the material, and photoelectrochemical action spectra demonstrate 22% incident photon to current conversion efficiency (IPCE) and 60% absorbed photon to current conversion efficiency (APCE).

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1. Introduction

We prepared a stable and low cost Fe-based semiconductor (SC), which can absorb a large portion of solar photons while having a minimum band gap energy. Most stable semiconductors absorb almost exclusively in the UV region of the solar spectrum.^1,2 Iron(III) oxide (Fe₂O₃), which has a band gap of ∼2.2 eV, can absorb most of the visible light (300 to 560 nm), of solar radiation, which comprises 38% of the photons of sunlight in air mass (AM) 1.5 spectra. Even though the band gap of Fe₂O₃ is suitable to allow absorption of a significant amount of sunlight photons, its photoresponse is quite low, which is mainly due to its low electrical conductivity and high recombination rate of photogenerated electron–hole pairs.^3,4 To minimize these limitations, n-type iron(III) oxide SC has been modified by incorporating different proportions of vanadium^5,6 into its matrix with the aim to enhance its photoelectrochemical water oxidation^1,7,8 behavior. The goal of this work is to synthesize Fe–V-oxide films of different compositions using a drop cast method and determine their efficacy toward the photoelectrochemical water splitting process.^9,10 Since solar hydrogen is a sustainable and environment-friendly energy carrier, it is considered to replace fossil fuels in the near future. It can be generated by the splitting of water under solar light illumination.^11–13 The search for stable, efficient and affordable PEC electrode materials^14–17 (photoanodes and photocathodes) is an on-going quest. Hematite, α-Fe₂O₃, remains an intriguing choice as an oxygen-evolving photoanode material because of its environmentally benign chemical stability,^18,19 low price and visible absorptivity character.²⁰ Recent theoretical and computational works support that hematite remains a promising parent material for artificial photosynthesis²¹ by undergoing water oxidation and CO₂ reduction reactions. Its bulk and surface electronic structure has been under scrutiny for many decades and is considered for the improvement of efficiency.¹⁸

Herein, we report the synthesis of a visible light responsive V incorporated Fe-oxide compound SC. This composite demonstrates photocatalytic activity for oxygen evolution from water, under UV-vis illumination. Fe–V-oxide²² compound theoretical calculations predict a decrease of ∼2.8 eV in its band gap with the Fermi level in the middle of the valence band maximum and conduction band minimum.

This study further demonstrates the improvement in its visible-light photocatalytic behavior for both water splitting and solar-energy conversion.

2. Experimental section

2.1. Reagents

Iron(III) nitrate nonahydrate [Fe(NO₃)₃·9H₂O] and ammonium metavanadate (NH₄VO₃) were purchased from Sigma-Aldrich. NaH₂PO₄, Na₂HPO₄, Na₂SO₄, Na₂SO₃, and ethylene glycol (EG) were purchased from Merck (AR quality) and used as received. All metal precursor solutions were prepared in ethylene glycol (EG) at a 0.1 M concentration. Milli-Q grade water was used in all experiments to prepare solutions. 2.0 × 1.5 cm² ITO-coated glass slides (Xinyan technology Ltd, Hong Kong) were thoroughly cleaned via sonication in successive solutions of soap water followed by ethanol and finally rinsed with Milli-Q water.

2.2. Preparation of thin film electrodes through the drop casting technique

Appropriate solutions with Fe : V (v/v) ratios of 1 : 0, 1 : 1, 1 : 2 and 2 : 1 were prepared in EG in separate glass vials, and the mixtures were kept in an ultrasonication bath for ∼1 hour. 500 μL of each mixtures was dropped cast onto a cleaned ITO substrate to prepare semiconductor films. The thin films were annealed in air at 500 °C with a heating ramping rate of 1 °C min⁻¹ followed by soaking for 3 h to obtain uniform, well-adhered thin films. The thickness (t) of the films was measured gravimetrically using the following relation:

t = m/ρA (1)

where m and A stand for mass and area of the deposited film and ρ the density of the Fe₂O₃ and Fe–V-oxide films. The thickness was found to be 260 nm for Fe₂O₃, 396 nm for FeVO₄, 480 for FeV₂O₄ nm and 481 nm for Fe₂VO₄.

2.3. Optical characterization

Absorption spectra of the prepared thin films were obtained using a V-630 UV-vis spectrophotometer (Jasco, Japan) within the wavelength range of 350–700 nm, and the band gap energy was calculated from the absorption edge on the wavelength scale.

2.4. Photoluminescence study

Photoluminescence spectra of the Fe₂O₃ and Fe–V-oxide thin film semiconductors were obtained using a PerkinElmer LS 55 Fluorescence Spectrometer at an excitation wavelength of 360 nm.

2.5. Surface characterization via SEM, XRD and EDX measurements

The surface morphology and quality of the film on the ITO coated glass substrate were determined via scanning electron microscopic (SEM) analysis using a HITACHI S3400N Instrument. Elemental composition was further determined by energy dispersive X-ray (EDX) spectroscopic analyses of the samples²³ using a 7021-H HORIBA EMAX Instrument installed with the SEM. The crystalline behavior of the prepared semiconductor was evaluated using the powder X-ray diffraction (XRD) technique on a Rigaku MiniFlex II, operated with a Cu Kα target (λ = 1.54 Å) within the 2θ range from 20° to 80° at a fixed scan rate of 2° min⁻¹.

2.6. Photoelectrochemical measurement

Photocurrent density measurements were performed by linear sweep voltammetry (LSV) using a CHI-650 potentiostat (CH Instrument, Austin, TX) in a standard three-electrode configuration cell (0.27 cm² geometric area exposed, using one O-ring of the same inner area) with a scan rate of 10 mV s⁻¹. The as prepared Fe–V-oxide films served as the photoanode, Pt wire as the counter electrode and Ag/AgCl as the reference electrode and all the potentials reported herein are with respect to this reference electrode. Measurements were carried out either in 0.1 M Na₂SO₄ with 0.1 M Na₂SO₃ as a sacrificial electron donor within the potential range from −0.1 to 0.6 V or in the presence of 0.1 M Na₂SO₄ in 0.1 M phosphate buffer solution (PBS) for water oxidation within the range from 0.0 to 1.2 V. 100 mW cm⁻² illumination was maintained using a 300 W Xe arc lamp (Excelitas USA) as the light source in this study. Visible-light responsiveness of the semiconductors towards the photo-oxidation of water was determined by holding a UV cut-off filter (λ > 420 nm) in front of the Xe lamp. PEC measurements were conducted by irradiating the prepared electrodes through the electrolyte/electrode interface.

2.7. Chronoamperometric measurement

The stability of the thin-film semiconductor electrode undergoing a solar driven water oxidation reaction was evaluated via continuous photocurrent measurements for 30 min in 0.1 M Na₂SO₄–PBS when the electrode was held at a constant potential of 1.0 V under 100 mW cm⁻² illumination. A monochromator (Oriel) was used in combination with a power meter and a silicon detector (Newport) to measure the photocurrent spectrum. From the electrochemical action spectra, incident photon to current conversion efficiency (IPCE) and absorbed photon to current conversion efficiency (APCE) were calculated.

2.8. Electrochemical impedance spectroscopic analysis of the semiconductor–electrolyte interface

Electrochemical impedance spectroscopic (EIS) measurements were conducted using a similar experiment setup as stated above, at a constant applied potential of 1.0 V within the frequency range from 20 mHz to 100 kHz with the oscillation amplitude of 10 mV. Experiments were conducted in dark and illuminated (UV-vis and only visible) conditions. A Mott–Schottky experiment was conducted in 0.1 M Na₂SO₄ in 0.1 M phosphate buffer solution (PBS) using different frequencies of 200, 500 and 1000 Hz at an applied potential ranging from −0.4 to 0.8 V, maintaining an ac amplitude of 10 mV at each of the potentials.

3. Results and discussion

3.1. Physicochemical characterization

Thin film Fe–V-oxide semiconductors were synthesized via drop-casting using Fe³⁺ and V⁵⁺ as precursors in the EG solution. The colors of the individual precursor solution and the corresponding semiconductor thin films developed on ITO coated glass substrates are presented in Fig. S1(a and b) (ESI†).

It has been observed that with the gradual addition of V to the pure Fe₂O₃ matrix, the color of the solution as well as that of the films changes from deep reddish to light yellow. Fig. 1a represents the UV-visible absorption spectra of the different thin film semiconductors, and Fig. 1b (inset) represents the Tauc plot for the calculation of band gap energy as well as to determine the nature of the band-to-band transition pattern. The Tauc plot indicates absorption characteristics due to the distinct ‘direct’ band gap at ∼1.9 eV and ‘indirect’ band gap at 2.6–2.7 eV for the Fe–V-oxides, which impart yellow color to the film, whereas the presence of a pure ‘direct’ band gap ∼2.0 eV for the Fe-oxide causes the film to be reddish in color. The thickness of the different materials is found to vary as 260 nm for Fe₂O₃, 396 nm for FeVO₄, 480 for FeV₂O₄ nm and 481 nm for Fe₂VO₄.

Fig. 1 (a) Absorption spectra for the determination of the band gap energy of the Fe–V-oxide compound thin film semiconductor prepared with 500 μL solution on the glass substrate. (b) Inset: Tauc plot of same film semiconductors showing ‘direct’ and ‘indirect’ gaps.

Photoluminescence (PL) emission spectra characterize the recombination of free carriers, and thus highlight the separation efficiency of the photo-generated charge carriers in a semiconductor.^24–26 The higher the PL intensity, the greater the probability of charge carrier recombination.²⁷ A comparison of the PL spectra (excited at 360 nm) of Fe₂O₃ and the Fe–V-oxide compounds at room temperature is presented in Fig. 2. The PL spectrum indicates a strong peak at ∼480 nm, which corresponds to the high-level transition in Fe–V-oxide. Among the different samples, FeV₂O₄, which has the lowest intensity in that particular region, demonstrates that the recombination process can be effectively controlled inside the matrix. Therefore, FeV₂O₄ is very effective for separating the photo-generated charge carriers, which in turns triggers the charge transfer process at a faster rate than the other semiconductor thin films.

Fig. 2 PL spectra of the Fe–V-oxide semiconductors excited at 360 nm.

SEM analysis reveals the surface morphology of the thin-film semiconductors. Fig. 3a–d represent the SEM images for the Fe–V-oxide compounds grown over ITO coated glass substrates. For the pure iron oxide material, as presented in Fig. 3a, the surfaces are found to be covered with agglomerated particles. With the addition of 33% V to the matrix, the surface morphology (Fig. 3b) changes significantly with the appearance of ‘cubic’ particles of approximate size of 250–300 nm. Upon the addition of 50% V, the morphology changes to a ‘3D triclinic’ structure of average length 800–1000 nm along with presence of some cubic structures (Fig. 3c). With the further addition of to V (70% V), the surface was found to be covered with small particles along with ‘cylindrical’ shaped particles with an average diameter in the order of 300–400 nm (Fig. 3d).

Fig. 3 (a–d) SEM images of (a) Fe₂O₃; (b) Fe₂VO₄; (c) FeVO₄ and (d) FeV₂O₄ films on ITO coated glass substrates annealed at 500 °C for 3 h.

The EDX analysis of the different semiconductor thin films measures the individual elemental compositions (atomic percentage, %) of the constituents. Fig. 4a–d represent the elemental mapping of the Fe–V-oxide compounds, whereas Fig. 4e (bar plot) indicates the relative proportions of the individual elements in the semiconductor matrix. The composition analysis corresponds well to the prepared semiconductors developed via the drop-casting techniques.

Fig. 4 (a–e) EDX elemental mapping of (a) Fe₂O₃, (b) Fe₂VO₄, (c) FeVO₄, and (d) FeV₂O₄. (e) Atomic percentage (%) of Fe (red), V (yellow) and O (green).

The crystalline nature of the as-prepared samples was examined via the XRD technique. Fig. 5 represents the XRD pattern for pure Fe-oxide, which is identified as α-Fe₂O₃ (hematite) compounds with a ‘rhombohedral’ geometry (JCPDS file 79-1741). The addition of different levels of vanadium to the Fe-oxide leads to a change in the crystallinity of the semiconductors to a significant extent. The material with 33% V added has been identified as Fe₂VO₄ using JCPDS file 75-1519, which has a ‘cubic’ structure, whereas the 50% V added film is identified as FeVO₄ with JCPDS file 71-1592, which shows a preferential ‘triclinic’ nature. The compound containing 70% of V shows a ‘cubic’ structure, as indicated by JCPDS file 75-0317 and has been identified as FeV₂O₄. The different peaks in the XRD pattern for all the materials have been indexed with the corresponding (hkl) values compared with the standard JCPDS data file.

Fig. 5 XRD patterns of Fe–V-oxide compound semiconductor films annealed at 500 °C for 3 h on an ITO conducting glass.

The crystallize size, D, (in Å), of the strongest peaks for each of the XRD patterns was analyzed using the Scherrer formula:

D = (0.9λ)/(β cos θ) (2)

where ‘λ’ is the wavelength of X-ray (1.54 Å), ‘β’ FWHM (full width at half maxima, in degrees), and ‘θ’ is the diffraction angle (in degrees). The value of d, which is the interplanar spacing between the atoms, is calculated using the Bragg's law.

nλ = 2d sin θ (3)

The average sizes of the crystallites corresponding to the strongest peaks for each of the Fe₂O₃ (104), FeVO₄ (311), FeV₂O₄ (311) and Fe₂VO₄ (112) were calculated as 21 nm, 23 nm, 23 nm and 26 nm, respectively.

3.2. Photocurrent measurements

The three-electrode cell setup with an area of 0.27 cm² exposed SC surface as the working electrode (WE) was used to study the photoelectrochemical (PEC) behavior of the compounds under periodic chopped UV-visible illumination of 100 mW cm⁻². Fig. 6a represents the linear sweep voltammograms for photo oxidation of the sacrificial reagent (SO₃²⁻) over the different SC surfaces at a scan rate of 10 mV s⁻¹, within the potential range from −0.1 to 0.6 V. Fig. 6b inset represents the bar plot, i.e. variation of photocurrent as measured from LSV (Fig. 6a) at a potential of 0.5 V, for the compounds which indicate that FeV₂O₄ exhibits the highest photocurrent compared to others.

Fig. 6 (a) Linear sweep voltammograms of Fe–V-oxide semiconductor electrodes in 0.1 M SO₃²⁻–SO₄²⁻ solution in the presence of UV-vis light (intensity: 100 mW cm⁻²). (b, inset) Variation of photocurrent (I_Ph) at 0.5 V for the different materials.

The water oxidation behavior of the SC materials was evaluated through LSV plots. The plot in Fig. 7a was obtained in 0.1 M SO₄²⁻ and 0.1 M phosphate buffer solution (pH 7) within the potential range from 0.0 to 1.2 V and the variation of photocurrent for the different materials at 1.2 V is presented in the bar plot (Fig. 7b, inset). Similar to that observed for sacrificial oxidation, in this case, the highest photocurrent is also observed for the FeV₂O₄ compound SC, which attains a value of 0.18 mA cm⁻² at 1.2 V. The visible-light responsiveness of the semiconductors towards water oxidation was determined by holding a UV cut-off filter (λ > 420 nm) in front of the Xe lamp. Fig. S2 (ESI†) represents the comparative behavior of the FeV₂O₄ film undergoing the water oxidation process, wherein the material retains ∼30% photo-performance when exposed to periodic UV-vis and visible illumination.

Fig. 7 (a) Linear sweep voltammograms of Fe–V-oxide semiconductor electrodes in 0.1 M SO₄²⁻ phosphate buffer solution (for water oxidation) in the presence of UV-vis light (intensity: 100 mW cm⁻²). (b, inset) Variation of photocurrent (I_Ph) at 1.2 V for the different materials.

The stabilities of the semiconductor electrodes were verified via chronoamperometry by holding a fixed potential of 1.0 V vs. Ag/AgCl for the photoelectrochemical oxidation of water under the constant illumination of 100 mW cm⁻². Fig. 8 represents the chronoamperometric (current–time) diagram of all the semiconductors, wherein FeV₂O₄ indicates fairly stable water oxidation behavior for at least 30 minutes. The gradual decrease in photocurrent for much longer exposure may be ascribed to the steady recombination of the photo-generated electrons and holes within the bulk of the SC matrix.

Fig. 8 Current–time response (chronoamperometry) curves of the Fe–V-oxide semiconductors at an applied potential of 1.0 V in 0.1 M Na₂SO₄ with a 0.1 M phosphate buffer solution (pH 7) under UV-vis light irradiation.

Electrochemical action spectra were obtained under periodic chopped monochromatic illumination. Fig. 9 represents the photoelectrochemical action spectra for all the semiconductors in 0.1 M Na₂SO₄ in the presence of PBS, which were measured at 1.0 V with a gradual change in monochromatic light wavelength from 250 to 620 nm. The corresponding photocurrent at each wavelength was calculated from the action spectra, which is presented in Fig. S3 (ESI†). The onset of photocurrent for the different semiconductors (Fe₂O₃, Fe₂VO₄, FeVO₄ and FeV₂O₄) was found to be 560, 580, 600 and 610 nm, which correspond to the ‘true’ or photoelectrochemical band gap of the materials of 2.21, 2.13, 2.06 and 2.03 eV, respectively.

Fig. 9 Photoelectrochemical action spectra of the Fe–V-oxide semiconductors, which were calculated from the photocurrent spectra of the material.

IPCE measurement showed the clear wavelength dependence of the photocurrent generation. The incident photon to current conversion efficiency (IPCE) measurement was performed with monochromatic irradiation employing a 300 W Xe arc lamp attached with an optical power meter (Oriel). The IPCE (%) was calculated from the following equation:

IPCE (%) = (1240/λ) × (I_Ph/P_in) × 100 (4)

where λ is the wavelength of the incident light in nm, I_Ph is the photocurrent in mA cm⁻² and P_in is the power of the incident beam in 100 mW cm⁻² of monochromatic light. The applied potential was monitored against an Ag/AgCl reference, and the absorbed photon to current conversion efficiency (APCE) was further calculated by considering the absorbance (A_λ) of the material at that particular wavelength using the following equation:

APCE (%) = (IPCE/1 − 10^−A_λ) × 100 (5)

Fig. 10a and b represent the variation of % IPCE and % APCE, respectively, for the different materials, and it has been observed that the FeV₂O₄ semiconductor exhibits the highest photoconversion efficiency of IPCE as 22%, and corresponding APCE as 60%, compared to the other Fe–V-oxides.

Fig. 10 Variation of (a) % IPCE and (b) % APCE of Fe–V-oxide semiconductors in 0.1 M Na₂SO₄–PBS (pH 7) at applied potential 1.0 V vs. Ag/AgCl reference electrode.

3.3. Capacitance measurement

The Mott–Schottky (M–S) plot, which is the linear variation of 1/C² (space charge capacitance in F cm⁻²) with respect to the applied potential, can be expressed using the relation shown in eqn (6).where ε_s is the dielectric constant of the semiconductor, ε₀ is the permittivity of free space, e is the electronic charge in C, N_D is the donor density (per cm³), E is the applied potential in V, E_fb is the flat band potential in V, k_B is the Boltzmann constant and T is the temperature in absolute scale. The flat band potential of the semiconductor–electrolyte interface can be obtained from the intercept on the potential axis, whereas the slope of the straight line is inversely related to the carrier concentration, N_D The positive slope of the M–S plots, as shown in Fig. 11, confirms the n-type semiconductivity of all the materials and the estimated flat band potentials for Fe₂O₃, Fe₂VO₄, FeVO₄ and FeV₂O₄ are obtained from the intercept of the tangent line of the M–S plots on the potential axis as −0.15, −0.10, 0.08 and 0.07 V, respectively. The positive shifting of the flat band potentials with the gradual addition of % V to the matrix may favor the overall charge transfer kinetics across the semiconductor–electrolyte interface. The donor densities of the Fe–V-oxide compound semiconductors of different compositions were calculated from the slope of the respective M–S plots using the respective dielectric constants (ε_s) of the materials to be Fe₂O₃: 14.2;²⁸ FeVO₄: 14.78 (ref. 29) and FeV₂O₄: 7.0,³⁰ and the value for Fe₂VO₄ is considered to be the same as that for FeVO₄ due to the unavailability of data. The order of the slopes of the M–S plots is found to be quite similar to the values reported for similar Fe-oxide based semiconductor materials.¹⁸ A relatively higher carrier concentration of FeV₂O₄ matrix (2.7 × 10²⁰ cm⁻³) may lead to a reduction in the ohmic resistivity of the film, which thereby improves the photoelectrochemical water oxidation performance, as observed in the LSV plots. M–S plots measured at 200, 500 and 1000 Hz using FeV₂O₄ are presented in Fig. S4 (ESI†), which indicate a frequency independent behavior from the analysis.

Fig. 11 Capacitance–voltage profile for Fe–V-oxide compounds recorded at 1000 Hz with an ac amplitude of 10 mV at each potential.

The energy band diagram for the different Fe–V-oxide compounds is presented in Fig. 12, which shows the conduction band (CB) position near their flat band positions and the corresponding valence band (VB), considering their individual band gap energies. The redox potential of the photo-generated holes (+1.8–1.9 V vs. Ag/AgCl) is high enough to drive the water oxidation reaction (H₂O → O₂, E⁰ +0.59 V vs. Ag/AgCl at pH 7), and the material exhibits excellent photoelectrochemical oxidation behavior.

Fig. 12 Schematic band diagrams of Fe–V-oxides semiconductor–electrolyte interface.

3.4. Impedance measurements

Frequency dispersive impedance data were collected for all the materials and a typical Nyquist plot showing the variation of real and imaginary part of the impedance, under UV-vis illumination, is presented in Fig. 13a. The Nyquist plots were analyzed based on the equivalent circuit model (Fig. 13b), which includes a solution resistance (R_s), charge-transfer resistance (R_ct) and double-layer capacitance (C_ct) associated with the charge transfer process. In the present case, the charge-transfer resistance (R_ct) values of the different materials are found to vary for Fe₂O₃, FeVO₄, Fe₂VO₄ and FeV₂O₄ as 102, 45, 25 and 23 kohm, respectively. In fact, a material with a low R_ct and high C_ct value is generally considered suitable for use in PEC cells as a photo-electrode due to its efficiency in the transfer of charge carriers across the semiconductor–electrolyte interface and better semiconductivity of the film matrix.

Fig. 13 (a) Nyquist plot of Fe–V-oxide semiconductor films in 0.1 M Na₂SO₄ with PBS aqueous solution under a UV-vis light source. (b, inset) Equivalent circuit diagram to analyze the Nyquist plot.

In the present case, the minimum value of R_ct and relatively higher C_ct for the FeV₂O₄ compound semiconductor support the superior photoelectrochemical oxidation behavior of this material. Fig. S5 (ESI†) represents the impedance data of FeV₂O₄ in dark, visible and UV-visible conditions, which indicates the facile charge transfer reaction in the presence of UV-vis light.

4. Conclusion

Iron vanadium oxide semiconductor thin films of different compositions have been prepared on an ITO coated glass substrate at 500 °C. EDX analysis further confirms the presence of iron, vanadium and oxygen in the films, almost in the same order as that prepared through the drop-casting technique. The semiconductor electrodes are found to remain almost stable, at least for half an hour in aqueous electrolytes at pH 7 under light illumination of 100 mW cm⁻². The Fe–V-oxides show a direct band gap at ∼1.9 eV and indirect gap at ∼2.6–2.7 eV, which result in the significant visible-light responsiveness of the materials for water oxidation reaction. The FeV₂O₄ semiconductor, which has a distinct surface morphology, exhibits the highest photocurrent due to its high carrier concentration, low charge transfer resistance and higher double layer capacitance. The photoconversion efficiency of 22% IPCE and 60% APCE has been recorded with the FeV₂O₄ semiconductor material toward the O₂ evolution reaction from water.

Acknowledgements

Financial support through the sponsored project grant of the Govt. of India from SERB-DST (Sanction Order No. SB/S1/PC-042/2013, dt. 28-05-2014), the BRNS-DAE (Sanction Order No. 2013/37C/61/BRNS, dt. 03-12-2013) and the DST, Govt. of West Bengal, (Sanction order no. 902(Sanc.)/ST/P/S & T/4G – 1/2013, dt. 08/01/2015) to the Department of Chemistry, IIEST, Shibpur is gratefully acknowledged. Instrumental support from the MHRD, UGC-SAP to the Department of Chemistry, IIEST, Shibpur is also gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra22586h

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