Mo-doped BiVO₄ thin films – high photoelectrochemical water splitting performance achieved by a tailored structure and morphology

Abstract

The n-type semiconductor bismuth vanadate (BiVO₄) is one of the most promising ternary oxide materials for visible light-induced water oxidation, offering a theoretical solar-to-hydrogen efficiency of 9.1%. However, several factors strongly limit its actual efficiency and among these, poor charge transport has been identified as one major limitation to be overcome. Many efforts have been made to improve charge transport and charge separation in BiVO₄ including various doping strategies as well as nanostructuring the absorber to the dimension of its charge carrier diffusion length. In this article, we present a new wet chemical synthesis approach to fabricate pristine and Mo-doped BiVO₄ thin film photoanodes. Starting from a solution containing metalorganic Bi, V and Mo precursors, homogeneous and reproducible thin films can be fabricated by a simple procedure of dip-coating and subsequent calcination in air. Structural and morphological characterization reveals that the polycrystalline BiVO₄ thin films crystallize in the monoclinic scheelite structure in micrometer-sized randomly oriented, porous domains. The small band-gap scheelite structure is maintained upon Mo doping into the BiVO₄ lattice, yielding optimized light harvesting and improved charge carrier transport properties. As a result, the photoelectrochemical performance regarding water oxidation of the as-synthesized Mo-doped BiVO₄ thin film photoanodes is highly improved with respect to their pristine counterparts. Photocurrent densities of 1.9 mA cm⁻² and 4.6 mA cm⁻² at 1.23 V vs. RHE under visible light illumination (100 mW cm⁻²) are measured for unmodified and CoPi-modified Mo-doped BiVO₄ photoanodes respectively, both of which are amongst the highest values reported for modified BiVO₄ single-layer design photoanodes so far.

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Introduction

In consideration of the rising global energy demand and the environmental concerns raised by fossil fuel combustion, the production of clean and renewable fuels has become increasingly essential.¹ Because of its high energy density (142 MJ kg⁻¹)² and the possibility of its CO₂-emission free combustion in fuel cells,³ hydrogen is an interesting alternative to fossil fuels. In this context and in view of establishing a renewable hydrogen economy, emission free hydrogen production becomes crucial.

Photoelectrochemical (PEC) water splitting using sunlight as the energy source and suitable semiconductors as photocathode and photoanode materials in a tandem system is considered as the Holy Grail in sustainable hydrogen production.⁴ In the context of developing a high-performance photoanode for the water oxidation half reaction under visible light, a semiconductor with a suitable band gap (1.9 eV < E_g < 3.1 eV) and a suitable position of the valence band (i.e. more positive than the OER redox potential) decorated with a thin layer of water oxidation catalyst is required.^5–8

Since Kudo et al. discovered the ability of BiVO₄ to oxidize water under visible light,⁹ it has become one of the most investigated metal oxide photoanode materials for water oxidation because of its manifold beneficial properties, including suitable valence band position for water oxidation, small band gap of 2.4 eV for efficient visible light absorption, long hole diffusion length (i.e. photo-generated charge carriers are separated efficiently within this material), high stability in aqueous media under oxidizing conditions, low toxicity, earth-abundance of its building elements and reasonable price.¹⁰

Many synthesis strategies including precipitation reactions,^11–16 hydrothermal syntheses,^17–23 sol–gel-methods,^24–28 and sonochemical methods^29–33 have been reported for the preparation of BiVO₄ powders for photocatalytic water oxidation. However, for photoelectrocatalytic water oxidation, the construction of BiVO₄ photoanodes in the form of electrodes is required. Hence, development of facile and scalable deposition methods of high quality BiVO₄ thin films on transparent conductive substrates with tunable structure, morphology and thickness is essential. The thickness of the BiVO₄ photoanode should thereby match the charge carrier diffusion length of BiVO₄ (L_d ≈ 70–100 nm (ref. 34–36)) to ensure an optimized charge transport in the system.

Mullins et al. showed in 2011 that nanostructured BiVO₄ thin films can be synthesized on F:SnO₂-coated glass slides (FTO) by reactive ballistic deposition, involving co-evaporation of bismuth and vanadium in an oxygen atmosphere.³⁷ After deposition of a cobalt based water oxidation electrocatalyst, Mullins et al. achieved water oxidation photocurrents of ca. 1 mA cm⁻² at 1.23 V vs. RHE under simulated sunlight (AM 1.5 filter, 100 mW cm⁻²). By applying spray pyrolysis for the direct deposition of BiVO₄ thin films and an optimized deposition of CoPi (a cobalt based water oxidation catalyst developed by Kanan and Nocera³⁸), van de Krol et al. showed in 2013 that W-doping allows enhancing the charge transport efficiency in BiVO₄ thin film photoanodes, reaching water oxidation photocurrents of 2.3 mA cm⁻² at 1.23 V vs. RHE under simulated sunlight (AM 1.5 filter, 100 mW cm⁻²).³⁹ Introducing the concept of gradient doping in W-doped BiVO₄, van de Krol et al. further improved the charge separation in W-doped BiVO₄ photoanodes, yielding photocurrents of 4 mA cm⁻² at 1.23 V vs. RHE for CoPi functionalized photoanodes under simulated sunlight.⁴⁰ Nanostructuring of the BiVO₄ absorber provides another approach to improve the photoelectrochemical performance. For example, nanostructuring and deposition of a multilayer water oxidation catalyst allowed Choi and Kim to further improve the photocurrent density to 4.5 mA cm⁻² at 1.23 V under simulated sunlight.⁴¹

Despite the large progress made in the performance of BiVO₄ photoanodes over the last few years, the employed synthesis procedures are often technically advanced and their scalability and reproducibility is often an issue. A simple, wet chemical synthesis method for the reproducible synthesis of high-performance BiVO₄ thin film photoanodes for visible light induced water oxidation, i.e. with optimized light absorption, charge separation and water oxidation catalysis, is still missing.

In the present work, we present a new and facile synthesis method for high-performance single-layer Mo-doped BiVO₄ photoanodes for water oxidation under visible light. Our approach is based on non-aqueous sol–gel chemistry starting from metalorganic bismuth, vanadium and molybdenum precursors and allows the facile and direct deposition of Mo-doped BiVO₄ thin films on various substrates via simple dip-coating and subsequent calcination. Structural, morphological and photoelectrochemical characterizations reveal the as-synthesized BiVO₄ thin films to crystallize in the photoelectrochemically favorable monoclinic scheelite structure in large, micrometer sized, single crystalline, flake-like domains with optimized charge transport properties. The as-synthesized Mo-doped BiVO₄ photoanodes revealed remarkable water oxidation activity in a neutral electrolyte (potassium phosphate buffer at pH 7.3), yielding photocurrents as high as 1.9 mA cm⁻² and 4.6 mA cm⁻² at 1.23 V vs. RHE (100 mW cm⁻²; 400–700 nm) prior to and after CoPi functionalization respectively, both values being amongst the highest values reported so far for unmodified and water oxidation catalyst modified metal-doped single-layer BiVO₄-based photoanodes.

Experimental

Reagents and materials

Chloroform (CHCl₃, 99.9%, anhydrous), vanadium(V) oxytriethoxide (VO(OEt)₃, 95%) and molybdenum(VI) dioxydiacetylacetonate (MoO₂(acac)₂) were purchased from Sigma Aldrich. Bismuth(III) 2-ethyl-hexanoate (C₂₄H₄₅BiO₆, 92% in 2-ethylhexanoic acid, “Bi(OHex)₃”) was purchased from Alfa Aesar (maximum amount of metal impurity: 1%). Potassium dihydrogen phosphate (KH₂PO₄, ≥99%) and dipotassium hydrogen phosphate (K₂HPO₄, 99%) were purchased from Carl-Roth. All chemicals were used without further purification. Fluorine-doped tin oxide coated glass slides (FTO, 30 × 30 cm, 8–12 Ω per square) were purchased from Sigma-Aldrich and were cut into 3 × 1 cm² pieces and cleaned by ultrasonication in ethanol, isopropanol and acetone before use.

Synthesis of BiVO₄ thin films

A solution of 123 μl (0.7 mmol) of VO(OEt)₃ in 1.5 ml CHCl₃ was prepared. After stirring for 10 min the red solution was added to 445 mg (0.7 mmol) Bi(2-ethylhexanoate)₃ and stirred for 4 h. The obtained solution was used for film deposition by dip-coating silicon wafers or FTO-coated glass slides under controlled conditions (<30% relative humidity, 30 °C, 300 mm min⁻¹ withdrawal speed). After solvent evaporation all deposited films were aged at 100 °C for 12 h followed by calcination at 450 °C for 2 h, heating ramp 0.5 °C min⁻¹.

Synthesis of Mo:BiVO₄ thin films

A solution of 123 μl (140 mg, 0.7 mmol) of VO(OEt)₃ in 1.5 ml CHCl₃ was prepared. An amount of 11.3 mg (0.035 mmol), 22.6 mg (0.07 mmol), 33.9 mg (0.104 mmol) or 45.2 mg (0.138 mmol) of MoO₂(acac)₂, which corresponds to an atomic Mo composition with respect to the Bi content of 5, 10, 15 and 20 at% in the resulting Mo:BiVO₄ precursor solution, respectively, was added. After stirring for 10 min the homogeneous red solution was added to 445 mg (0.700 mmol) of Bi(2-ethylhexanoate)₃ and stirred for 4 h. The obtained red solution was used for dip coating silicon wafers or FTO-coated glass slides under controlled conditions (relative humidity <30%, 30 °C, 300 mm min⁻¹ withdrawal speed). After solvent evaporation, all deposited films were aged at 100 °C for 12 h followed by calcination at 450 °C for 2 h (heating ramp: 0.5 °C min⁻¹). In Table 1 the exact composition of the precursor solutions is summarized. Note: the use of chloroform as a solvent comes along with several concerns about health and safety. The use of a more sustainable solvent is desired and under investigation at the moment.

Table 1 Composition of the precursor solution used for the synthesis of X% Mo:BiVO₄ thin film photoanodes

	Bi(OHex)3	VO(OEt)3	MoO2(acac)2
0% Mo:BiVO4	445 mg/0.70 mmol	140 mg/123 μl/0.70 mmol	—
5% Mo:BiVO4	445 mg/0.70 mmol	140 mg/123 μl/0.70 mmol	11.3 mg/0.036 mmol
10% Mo:BiVO4	445 mg/0.70 mmol	140 mg/123 μl/0.70 mmol	22.6 mg/0.072 mmol
15% Mo:BiVO4	445 mg/0.70 mmol	140 mg/123 μl/0.70 mmol	33.9 mg/0.104 mmol
20% Mo:BiVO4	445 mg/0.70 mmol	140 mg/123 μl/0.70 mmol	45.2 mg/0.138 mmol

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Synthesis of BiVO₄ and Mo:BiVO₄ powders

For the powder synthesis, the solutions used for the deposition of BiVO₄ and Mo:BiVO₄ thin films were poured into a ceramic crucible. Once the solvent was completely evaporated, the left-over was aged (similar to the thin films) for 12 h at 100 °C and subsequently calcined for 2 h at 450 °C (heating ramp 0.5 °C min⁻¹) to obtain BiVO₄ and Mo:BiVO₄ reference powders.

Characterization

A PANalytical X'Pert Pro powder diffractometer equipped with Cu-K_α radiation was used for powder X-ray diffraction (P-XRD) analysis of Mo:BiVO₄ powders. The structural refinements were performed with the program FullProf Suite Version 2009 by applying a pseudo-Voigt function.⁴² A Seifert XRD 3003 TT with Cu-K_α radiation equipped with a multilayer mirror was used for XRD analysis of the thin films in grazing incidence (GI-XRD). The elemental composition of the thin films and of the reference powders (obtained by calcination of dried precursor solutions (12 h 100 °C, 2 h 450 °C)) was determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) using a Varian ICP-OES 715 ES equipped with a radial plasma torch. For ICP-OES analysis either 10 mg of powder or 10 X% Mo:BiVO₄ thin films were dissolved in 1 M HCl by ultrasonication. The specific surface area of the X% Mo:BiVO₄ thin films was determined by Kr physisorption (5 point BET measurement) using a Quantachrome Instrument ASI-C-11 operated at 77 K. Thin films have been coated onto double side polished (dsp) Si wafers and were cut into pieces of 0.8 cm × 3.5 cm to fit the sample holder. Usually 10 to 12 of the thin films deposited onto dsp-Si wafers were used. Prior to measurements the samples were degassed under vacuum overnight at 100 °C. The light absorption of the BiVO₄ thin films was investigated by UV/Vis spectroscopy using a Perkin Lambda 20 spectrometer operated in transmission mode. The morphology and structure of the BiVO₄ thin films was characterized by scanning electron microscopy (SEM) using a FEG-SEM SU8030 from Hitachi equipped with a 30 mm² silicon drift detector (SDD) from EDAX for energy dispersive X-ray spectroscopy (EDX). In addition, transmission electron microscopy (TEM) and selective area electron diffraction (SAED) were performed using a FEI Tecnai G² 20 S-TWIN transmission electron microscope. Scanning transmission electron microscopy (STEM) was performed using a FEI Titan Themis microscope operated at 300 kV with aberration-corrected probe-forming lenses. All TEM samples were prepared as follows: (a) chunks of BiVO₄ thin films were scraped off the substrate and transferred to a TEM grid or (b) a TEM lamella was cut out of a thin film using a FEI Helios Nanolab 600 focussed ion beam device (FIB). For electron backscatter diffraction (EBSD) analysis, BiVO₄ thin films were analyzed using a Zeiss DSM 982 GEMINI equipped with an EDAX Hikari XP detector.

Electrochemical investigations

Electrochemical measurements were carried out in a home-built 3-electrode-setup consisting of the BiVO₄ thin films as the working electrode (photoanode), a Pt-wire counter electrode, and a reversible hydrogen reference electrode (RHE) from Gaskatel. The electrode area exposed to the electrolyte was circle-shaped with a diameter of 0.79 cm resulting in a geometric area of 0.5 cm². The potential of the working electrode with respect to the reference electrode was controlled by a potentiostat SP150 from Biologic. The photoanode was illuminated by a 150 W Xe-lamp from Lumatec Superlite, equipped with a UV cut-off filter and illuminating the spectral range between 400 and 700 nm. The lamp was mounted at a reproducible and constant distance of 5 cm from the BiVO₄ photoanode surface. The light intensity at the electrode surface was adjusted to 100 mW cm⁻² using a Si-diode light meter from Extech. j–V-measurements were conducted in 0.1 M potassium phosphate buffer (KPi) at pH 7.3 using a scan rate of 20 mV s⁻¹.

For determination of the faradaic efficiency, the amount of dissolved oxygen in the electrolyte (0.1 M phosphate buffer at pH 7.3) was monitored using a Clark electrode (Strathkelvin Instruments oxygen meter model 782). A potential of 0.8 V vs. RHE was applied under illumination of white light (400–700 nm, 100 mW cm⁻², backside illumination) over a time period of 50 minutes. Prior to measurements the electrolyte was purged with N₂ for 12 h. The theoretical O₂ amount was calculated according to the number of transferred electrons (assuming 100% faradaic efficiency). The faradaic efficiency was calculated by comparing the experimentally observed O₂ amount with that obtained by the theoretical results.

For chronoamperometric measurements of the photocurrent transients a potential of 1.23 V vs. RHE was applied and blue light (440 nm) at an intensity of 2 mW cm⁻² was used.

Staircase potentio-electrochemical impedance spectroscopy (PEIS, Mott–Schottky) was carried out in 0.5 M KPi buffer at pH 7.3 scanning from anodic to cathodic and using a sinusoidal modulation of 10 mV at frequencies of 500 Hz and 1 kHz. Estimation of the amount of free charge carriers in the thin film and evaluation of the flat band potential (V_FB) was performed by Mott–Schottky analysis (vide infra).

Co-catalyst deposition

A photo-assisted electrodeposition method as already reported by Durrant et al. was used to deposit a cobalt phosphate water oxidation catalyst (CoPi) onto the BiVO₄ electrode surface.⁴³ The BiVO₄ working electrode was immersed in 0.1 M potassium phosphate buffer at pH 7.3 containing 0.5 mM cobalt nitrate (Co(NO₃)₂). A constant potential of 1.2 V vs. RHE was applied under an illumination of 100 mW cm⁻² of white light (400–700 nm) from the electrolyte/BiVO₄/FTO side. To ensure a comparable amount of deposited CoPi co-catalyst for all BiVO₄ samples, the deposition was stopped after a charge of 130 mC was passed in all cases.

Results and discussion

Synthesis, structural and morphological characterization of Mo:BiVO₄ thin films

Synthesis. A novel, non-aqueous, wet chemical synthesis method based on metalorganic precursors of bismuth, vanadium and molybdenum dissolved in chloroform was applied to prepare thin films of pristine and molybdenum-doped BiVO₄ with tunable amounts of Mo. The synthesis steps are depicted schematically in Fig. 1 and were followed by TEM and SAED for pristine BiVO₄ thin films, as seen in Fig. 2.

Fig. 1 Synthesis scheme: (a) precursor solution, (b) as-deposited amorphous thin film and (c) calcined, crystalline BiVO₄ thin film.

Fig. 2 Synthesis steps followed by TEM for a pristine BiVO₄ thin film: (a) as-deposited amorphous thin film as revealed by SAED and (b) crystalline BiVO₄ thin film with the FFT pattern of the HR-TEM image.

As demonstrated by the group of Vest in the early nineties, metal hexanoates are suitable compounds for solution-based synthesis of metal oxide thin films because they are quantitatively transformed into metal oxides when pyrolized.⁴⁴ For a bimetallic compound like BiVO₄, the exclusive use of separate bismuth and vanadium hexanoates is however not ideal because these metal hexanoates do not tend to react with each other, eventually leading to certain compositional inhomogeneity. According to Lange however, once mixed with metal alkoxides, metal hexanoates react to form so-called “double-alkoxide” precursors with defined cationic ratios between the two metals.⁴⁵ As such, by using hexanoate and alkoxide precursors of different metals, metalorganic single-source precursors can be formed with a 1 : 1 stoichiometry as demonstrated for example for PbTiO₃.⁴⁵

Herein, we adapted this synthesis concept to the synthesis of BiVO₄, i.e. by mixing bismuth 2-ethyl-hexanoate and vanadium oxytriethoxide to form in situ a bismuth–vanadium double-alkoxide precursor in the starting precursor solution. Regarding the incorporation of molybdenum, molybdenum dioxydiacetylacetonate was chosen to take advantage of the condensation inhibiting properties of the acetylacetonate ligand.⁴⁶ All compounds were mixed to form a clear, deep red precursor solution after 4 hours of stirring (Fig. 1a). After stirring, the aged solution was used to deposit thin films on various substrates (silicon wafers, FTO) by dip coating. The dip-coating process led to the formation of soapy, amorphous thin films on the substrates (as can be seen in the TEM images and SAED diffraction pattern in Fig. 1b and 2a).

Calcination at 450 °C leads to grain/crystallite growth driven by the change in free energy due to elimination of grain boundaries by either coalescence or dissolution of smaller grains as well as decomposition of the organic residues,^45,47 resulting in the formation of BiVO₄ thin films comprised of large, porous, single-crystalline 2D-domains (vide infra). Homogeneous crystalline thin films with good substrate coverage and homogeneous film thicknesses (200 to 250 nm) were obtained for both silicon and FTO coated glass as substrates, as demonstrated by SEM top-view, SEM cross-section and HRTEM investigations (see Fig. 6 for FTO substrates and Fig. S1† for silicon substrates). In the case of Mo-doped thin films, similar results were obtained.

As demonstrated by ICP-OES and EDX, the amount of Mo incorporated within the BiVO₄ thin films can be adjusted between 0 and 10.5% (at% with respect to Bi) by varying the Mo concentration in the precursor solution. However, one has to note (see Table 2) that the Mo-content incorporated in the thin films is always lower (by ca. 50%) than the actual Mo-concentration in the precursor solution. This discrepancy, especially for higher Mo-concentrations, can be explained by the limited solubility of the Mo-precursor in the precursor solution, as revealed by the increasing turbidity and hence higher absorbance of the precursor solution (see ESI, Fig. S2† for UV/Vis spectra of Mo-containing synthesis solutions). In the following discussion, the nominal Mo-content of the precursor solution is used to label the five samples, as X% Mo:BiVO₄ (X = 0, 5, 10, 15 or 20).

Table 2 Mo-content in the precursor solution and corresponding X% Mo:BiVO₄ thin films and powder samples. The Mo-content was determined by ICP-OES. The Mo-contents are indicated in atom percent with respect to the determined amount of Bi

Mo% in the synthesis solution	Mo% in the BiVO4 thin films	Mo% in the BiVO4 powders
0	0	0
5	3	4
10	6	9
15	8.5	15
20	10.5	18

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Structural characterization. In order to obtain information about the crystal structure of the Mo-doped BiVO₄ thin films, grazing incidence X-ray diffraction (GI-XRD) was performed. Fig. 3a gives an overview of the obtained diffraction patterns. The GI-XRD data clearly reveal the BiVO₄ thin films to be polycrystalline with no preferred orientation. However, due to reflection broadening, unambiguous determination of the crystal structure proved to be extremely difficult. Indeed, as reported in the literature,^48,49 BiVO₄ exhibits three different crystal structures: the tetragonal zircon-type structure, the monoclinic scheelite-type structure and the tetragonal scheelite-type. Heat treatment at 400–500 °C irreversibly transforms the tetragonal zircon-type structure into the monoclinic scheelite structure⁴⁹ (see ESI, S3† for an overview of BiVO₄ phase transitions). It should be mentioned that the crystal structures of the monoclinic and tetragonal scheelite modifications are very similar. The space group of the monoclinic low temperature phase I2/b, a non-standard setting of C2/c, is a maximal non-isomorphic subgroup (translationengleich, index 2) of I4₁/a, the space group of the tetragonal scheelite-type polymorph. The monoclinic crystal structure of BiVO₄ shows only slight deviations from the tetragonal one, which is expressed for example by the only minimal change in the gamma angle between the monoclinic and the tetragonal phase (monoclinic gamma-angle ∼90.3°; tetragonal gamma-angle ∼90°). As such, the diffraction patterns of the monoclinic and the tetragonal scheelite phase are difficult to distinguish, especially when approaching the transition, by temperature or doping effects. For this reason, high-quality powder diffraction measurements were performed on reference powder samples where structural refinement using the Rietveld method is applicable for reliable structural determination. To reveal eventual composition effects on the crystal structure as well as gather information about the ideal (crystal) and real (defect) structure of the pristine and Mo-doped BiVO₄ thin film samples, reference powder samples were synthesized by the same synthesis route as applied for the thin films: the solvent was left to evaporate from the precursor solutions and the leftovers were aged and calcined under the same conditions.

Fig. 3 Diffraction pattern of (a) X% Mo:BiVO₄ thin films and (c) X% Mo:BiVO₄ reference powders. (b) Magnified diffraction pattern of X% Mo:BiVO₄ thin films and (d) magnified powder diffraction pattern of X% Mo:BiVO₄ reference powders. (e) Correlation between the Mo-content in the precursor solution and the ratio between the cell parameters a and b as well as the gamma-angle, respectively. The 2θ range focuses on the (200) and (020) reflections of the monoclinic phase merging into the (200) reflection in the tetragonal phase. Note: reflections marked with a star are measurement artifacts from the sample holder.

Fig. 3a and c give an overview of the diffraction patterns of the X% Mo:BiVO₄ thin films and powders. Details about the powder diffraction measurements as well as the outcome of the Rietveld refinements can be found in the ESI (S4, Fig. S5, and Table S1†). From these data (see Fig. 3e), one can see that a continuous phase transition from the monoclinic scheelite to the tetragonal scheelite phase takes place with increasing molybdenum concentration. As can be seen in Fig. 3d, the monoclinic scheelite structure forms for the BiVO₄, 5% Mo:BiVO₄, 10% Mo:BiVO₄ and 15% Mo:BiVO₄ samples. Only the 20% Mo:BiVO₄ sample crystallizes in the tetragonal scheelite structure, as revealed e.g. by the merging of the (200) and (020) reflections of the monoclinic scheelite structure into a single reflection corresponding to the (200) plane of the tetragonal scheelite structure. This molybdenum induced phase transition agrees with previous literature reports and is related to Mo⁶⁺ cations with a larger ionic radius (0.41 Å, coordination number 4) substituting V⁵⁺ cations (0.35 Å, coordination number 4).⁵⁰ For a better understanding of the Mo-doped materials, a closer look was given at the possible defects arising from substituting V⁵⁺ by Mo⁶⁺. In the ESI (S4, and Table S4†), all reasonable defect models are summarized. The observed under-occupation of the bismuth position (see site occupation factor, s.o.f. in S4, Table S2† determined by Rietveld refinement) clearly points to the presence of bismuth vacancies as the main defects in the Mo-doped BiVO₄ structure, which accordingly should be written as Bi_1−x/3Mo_xV_1−xO₄. This is in good agreement with single crystal diffraction results on such phases, as reported for example by Cesari et al.⁵¹

Starting from the structural information gained from the powder samples, one would expect the thin films to exhibit the same crystal structure as the corresponding BiVO₄ powders given a similar Mo composition. Considering that the Mo contents within the Mo-doped BiVO₄ thin films are below 11 at% (see ICP-OES results of the powders and thin film samples summarized in Table 2), all synthesized X% Mo:BiVO₄ thin films are expected to crystallize in the monoclinic scheelite structure.

UV/Vis spectroscopy. UV/Vis spectroscopy was performed to determine the optical absorption properties of the X% Mo:BiVO₄ thin films deposited on FTO coated glass substrates. The UV-Vis results and the photographs of the measured samples are shown in Fig. 4. All samples show typical light absorption in the UV and visible wavelength range. For the pristine BiVO₄ sample, an absorption onset of λ ≈ 515 nm can be estimated by extrapolation of the linear region of the absorbance curve. For all X% Mo:BiVO₄ samples, except for the 20% one, an absorption onset of 530 nm can be estimated. These results are in good agreement with the reported band gaps of scheelite-type BiVO₄ ranging from 2.34 eV to 2.40 eV.⁴⁹ The absorbance of the thin films increases with increasing Mo content, with the highest absorbance observed for the 15% Mo:BiVO₄ sample. This correlates well with the increasing turbidity of the deposited thin films, probably a result of a change in the thin film morphology with increasing Mo amounts (vide infra, see SEM investigations in top view and cross-section of X% Mo:BiVO₄ thin films with X ranging from 0 to 20% presented in Fig. 6).

Fig. 4 UV/Vis spectra and photographs of BiVO₄ and Mo:BiVO₄ thin films deposited on FTO coated glass substrates. The absorbance of a pristine FTO substrate was subtracted.

Electron microscopy. To investigate the morphology of the Mo-doped BiVO₄ thin films, scanning electron microscopy (SEM) was performed. Morphological features identified from top-view and cross-section images are summarized in Fig. 5. All films consist of differently sized porous areas (orange areas in Fig. 5a, referred to as domains) separated from each other by thin grooves (orange areas in Fig. 5b). As revealed by SEM cross-section images, the thin groove separating the porous domains goes throughout the entire film thickness down to the substrate. Within a domain, wormlike pores of several tens of nanometers enclose materials referred to as inner domain material streaks (areas delimited by arrows in Fig. 5a).

Fig. 5 Illustration of discussed thin film features, highlighted in (a) an SEM top view image and (b) a cross-section image of a 5% Mo:BiVO₄ thin film on an Si substrate.

Top view and cross-sectional images with corresponding EDX spectra of pristine BiVO₄ and Mo-doped BiVO₄ thin films (deposited onto FTO substrates) are shown in Fig. 6. All films have similar thicknesses ranging from 215 to 270 nm, are porous and homogeneously cover the rough FTO surface. EDX of all X% Mo:BiVO₄ samples proves the thin films to consist of Bi, V, O and Mo. Interestingly, the molybdenum content strongly influences the morphology of the thin films, as can be seen from Fig. 6. Both the average domain size and the size of the inner domain material streaks depend on the Mo content. As summarized in Table 3, undoped BiVO₄ has an average domain size of 7.2 ± 3.3 μm² and an average inner domain material streak size of 68 ± 7 nm. With increasing Mo content, the average domain size decreases (down to 1 μm² for 20% Mo–BiVO₄) while the inner domain material streak size increases (up to 139 ± 8 nm for 20% Mo–BiVO₄, see Table 3). Interestingly, the inner domain material streak sizes of the 5%, 10% and 15% Mo:BiVO₄ samples, which are 80 ± 7 nm, 84 ± 9 nm and 105 ± 9 nm, respectively, match the BiVO₄ diffusion length of 70–100 nm.^34–36 The specific geometrical surface area, as determined by krypton physisorption, decreases with increasing Mo content from 8.2 cm_film² cm_substrate⁻² for the pristine BiVO₄ thin films down to 2.9 cm_film² cm_substrate⁻² for the 20% Mo:BiVO₄ sample, which correlates with the coarsening of the inner domain material streaks. Similar morphology was observed for films deposited onto Si (see S1, and Fig. S1†), suggesting that there is no influence of the substrate on the microstructure and morphology of the BiVO₄ films.

Fig. 6 SEM images (top view and cross-section [insets]) and EDX spectra of the differently doped Mo:BiVO₄ thin films deposited on FTO substrates. The thin grooves separating the domains from each other and FTO/BiVO₄ interfaces are highlighted. The Mo-L peak overlaps with the Bi-M peak at 2.3 keV.

Table 3 Nominal molybdenum content, molybdenum content determined by ICP-OES, average domain size, average inner domain streak size and average film thickness (determined by SEM) as well as specific geometrical surface area (determined by Kr-physisorption) of BiVO₄ and X% Mo:BiVO₄ thin films deposited on FTO substrates

Nominal Mo content (%, atomic ratio)	Molybdenum content determined by ICP-OES (at% with respect to Bi)	Average geometrical domain size by SEM (μm^2)	Average inner domain streak size (nm)	Average film thickness (nm)	Specific geom. surface area by Kr-adsorption (cmfilm^2 cmsubstrate^−2)
0	0	7.2 ± 3.3	68 ± 7	269 ± 5	8.2
5	3	6.4 ± 2.4	80 ± 7	245 ± 12	8.0
10	6	2.8 ± 1.7	84 ± 9	238 ± 10	7.0
15	8.5	1.7 ± 1.1	105 ± 9	223 ± 12	5.5
20	10.5	<1	139 ± 8	214 ± 14	2.9

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The thin film crystallinity and orientation was further investigated by selected area electron diffraction (SAED) and electron backscatter diffraction (EBSD). TEM/SAED of a 5% Mo:BiVO₄ sample are exemplarily shown in Fig. 7a. SAED analysis performed at three different spots in one thin film domain reveals identical single-crystalline diffraction pattern close to the [1̄10] zone axis. This result shows that the whole domain exhibits one preferred orientation. EBSD analysis shown in Fig. 7b further confirms that each domain has a preferred orientation. Furthermore, the misorientation between domains is random, so that the thin film is overall polycrystalline, in agreement with our GI-XRD result, which shows no preferred orientation/texture in the film.

Fig. 7 (a) TEM/SAED and (b) EBSD analysis of a 5% Mo:BiVO₄ thin film displaying the single-crystalline character of each Mo-doped BiVO₄ domain. Note: the TEM sample could not exactly be aligned to the zone axis due to fast beam damage.

The 10% Mo-doped BiVO₄ thin film sample, which features the best PEC performance for water oxidation (vide infra), was further examined by high resolution scanning transmission electron microscopy (HR-STEM). The cross-sectional TEM lamella was prepared by focused ion beam (FIB) cutting. The interface between the FTO back-contact and the Mo:BiVO₄ is shown in Fig. 8a and FFT analysis was performed at several areas across the interface. The FFT pattern of the Mo:BiVO₄ crystal (red square) exhibits only one set of reflections corresponding to a d-spacing of 2.9 Å and the (004) plane. In the adjacent area (green square), the (110) plane with a corresponding d-spacing of 3.4 Å of the FTO is shown. At the interface region with a projected width of approximately 5 nm, spatial frequencies of both crystals were found within the FFT. No amorphous interlayer between the FTO substrate and the Mo-doped BiVO₄ thin film was found. The crystalline character of the interface can facilitate an efficient interfacial electron transfer by reducing the ohmic resistivity. The distribution of Mo within the 10% Mo:BiVO₄ thin film was investigated by STEM-EDX. As shown in Fig. 8b, the STEM-EDX elemental mapping proves the homogeneous distribution of Mo all over the thin film, indicating the homogeneous incorporation of Mo within the BiVO₄ lattice and excluding any phase segregation, in agreement with the XRD results. Furthermore, the determined amount of Mo (≈5 at%) is in agreement with the results obtained by ICP-OES (6 at%) for the 10% Mo:BiVO₄ sample. From the STEM-EDX mapping one can further see that the Mo-surface concentration (i.e. the Mo-concentration in the first 10 nm of the photoanode surface) is quite similar to the bulk concentration in the film underneath (see the inset, no shadowing). According to recent reports,⁵² the superficial dopant content in BiVO₄ can be significantly different from the dopant content in the bulk, which in some cases can significantly influence the photocatalytic performance regarding water oxidation. According to our STEM/EDX investigations, no increased Mo concentration at the electrode surface could be detected, so that this effect can most likely be excluded in the present study.

Fig. 8 (a) HR-STEM image of the interface between 10% Mo-doped BiVO₄ and the FTO substrate with the corresponding FFT patterns; inset: overview of the TEM lamella, the circle marks the area shown in (a). A Pt protection layer was deposited during the FIB lamella preparation. (b) STEM image of 10% Mo:BiVO₄ lamella and corresponding elemental EDX maps. The EDX quantification was performed for the cation sites Bi, V, and Mo.

Electrochemical investigations

To investigate the photoelectrochemical performance of the differently doped BiVO₄ thin films with respect to water oxidation, an electrochemical cell equipped with a reversible hydrogen electrode (RHE) as the reference electrode and a coiled Pt wire as the counter electrode was used. BiVO₄ thin films were used as working electrodes and were, if not indicated otherwise, backside-illuminated (i.e. via the FTO back-contact) by a 150 W white light source (Xe lamp, cut-off filter, 400–700 nm) adjusted to an intensity of 100 mW cm⁻². Details about the setup for electrochemical measurements are given in the ESI (see S6†).

Fig. 9a shows j–V curves of undoped and Mo-doped BiVO₄ thin films supported on FTO. For the undoped BiVO₄ samples, low photocurrents were observed, accounting for 0.2 mA cm⁻² at 1.23 V vs. RHE. With increasing Mo content, the photocurrents dramatically increase by a factor of 10, with maximum photocurrent densities of 1.7 mA cm⁻² and 1.9 mA cm⁻² at 1.23 V vs. RHE being reached for the 5% Mo:BiVO₄ and 10% Mo:BiVO₄ samples, respectively. Monitoring the amount of dissolved evolved oxygen in the electrolyte using a Clark electrode revealed a faradaic efficiency of 93% in the case of the 10% Mo:BiVO₄ photoanode, a result which confirms that the obtained photocurrents can almost completely be assigned to the water oxidation reaction (see ESI, Fig. S9†). This result is in line with the faradaic efficiency determined for other BiVO₄ photoanode systems (see Table S4†).

Fig. 9 j–V curves for X% Mo:BiVO₄ thin film photoanodes: (a) without a water oxidation catalyst, (b) with a thin deposited CoPi layer. Forward scan of the second cycle is shown.

To rationalize the photocurrent increase by Mo-doping, frontside/backside illumination tests were performed. In the case of frontside illumination (i.e. illumination through the absorber film), photo-generated electrons generated at the top surface of the absorber film need to travel longer distances towards the FTO back-contact than in the case of backside illumination through the FTO. In the case of a poor electron conducting material, recombination is hence more likely to occur under frontside than under backside illumination, leading to a large discrepancy between frontside and backside currents. As such, frontside/backside illumination tests are commonly used to assess limitations due to insufficient electron/hole transport properties. As summarized by the ratios of photocurrents obtained in frontside and backside illumination in Fig. 10 (see also ESI, Fig. S7† for j–V curves in frontside and backside illumination), the photocurrents obtained for pristine BiVO₄ in frontside-illumination are about 5 times lower than the photocurrent yielded in backside-illumination. This clearly indicates that bulk recombination due to poor electron transport through the BiVO₄ thin film is a limiting factor in the pristine system. In contrast, the photocurrents under frontside and backside illumination are highly comparable for the Mo-doped samples. This clearly points to an improved electronic conductivity and decreased bulk recombination in the Mo-doped materials, results which will be further corroborated by Mott–Schottky and photocurrent transient analysis (vide infra).

Fig. 10 j(Frontside)/j(backside) for X% Mo:BiVO₄ photoanodes (a) without a water oxidation catalyst, (b) with CoPi deposited.

Further improvement of the PEC performance of the as-synthesized BiVO₄ and Mo-doped BiVO₄ photoanodes could be achieved by CoPi deposition at the semiconductor–electrolyte interface. j–V curves of BiVO₄ and Mo-doped BiVO₄ photoanodes with a deposited layer of CoPi are shown in Fig. 9b. As can be seen, CoPi deposition enhances the photocurrents in all cases, yielding a maximum value of 4.6 mA cm⁻² at 1.23 V vs. RHE for the 10% Mo:BiVO₄ samples. Quantification of the amount of evolved oxygen revealed a faradaic efficiency of 88% in the case of the CoPi-modified 10% Mo:BiVO₄ photoanode, a result which confirms that the obtained photocurrents can be almost completely assigned to the water oxidation reaction (see ESI, Fig. S9†).

Table 4 gives a brief overview on well-performing BiVO₄-based photoanodes with respect to water oxidation under illumination of 100 mW cm⁻² at neutral pH. To make a fair comparison, we compare our single-layered BiVO₄ photoanode with comparable single-layered BiVO₄-based systems. Despite the comparatively simple synthesis method applied in our work, our 10% Mo:BiVO₄/CoPi photoanode easily keeps up with most of the listed photoanodes consisting of a single BiVO₄ thin film layer modified with a water oxidation co-catalyst (see Table 4). For a more detailed summary of well-performing BiVO₄ photoanode systems the reader is referred to a recent review by Tolod et al.⁵³ Future work will concentrate on the application of our simple synthesis route to more complex heterojunction photoanode architectures like, for instance, those given in Table 4.

Table 4 Overview of water oxidation photocurrents at 1.23 V vs. RHE of highly active BiVO₄ photoanodes without and with water oxidation co-catalysts under an illumination of 100 mW cm⁻²

Anode/catalyst	j at 1.23 V vs. RHE without a co-catalyst (mA cm^−2)	j at 1.23 V vs. RHE with a co-catalyst (mA cm^−2)	Buffer/pH	Author/reference	Publication year
Single-layer BiVO 4 /co-catalyst photoanodes
Mo:BiVO4/CoPi	0.2	1.0	0.5 M Na2SO4, pH = 7	Pilli et al.^54	2011
Mo:BiVO4/FeOOH	—	2.1	0.1 M KH2PO4, pH = 6.8	Chen et al.^55	2015
Mo:BiVO4/RhO2	n.a.	2.9	Natural seawater	Luo et al.^56	2011
Mo:BiVO4/FeOOH	1.1	3.0	0.1 M KH2PO4, pH = 7	Park et al.^57	2014
BiVO4/FeOOH	0.2	2.3	0.1 M KH2PO4, pH = 7	Seabold et al.^58	2012
BiVO4/CoOx/NiO	1.1	3.5	0.1 M KH2PO4, pH = 7	Zhong et al.^59	2015
W:BiVO4/CoPi	1.1	4.0	0.1 M KH2PO4, pH = 7.3	Abdi et al.^40	2013
BiVO4/FeOOH/NiOOH	1.9	4.5	0.5 M KH2PO4, pH = 7	Kim et al.^41	2014
Mo:BiVO 4 /CoPi	1.9	4.6	0.1 M KH 2 PO 4 , pH = 7.3	This work	2017
H2-treated Mo:BiVO4/CoPi	2.5	4.9	0.1 M KH2PO4, pH = 7	Kim et al.^60	2015
H2-treated Mo:BiVO4/CoPi before H2-treatment	1.5	3.0	0.1 M KH2PO4, pH = 7	Kim et al.^60	2015
N:BiVO4/FeOOH	3.0	5.0	0.5 KH2PO4, pH = 7	Kim et al.^61	2015
More complex BiVO 4 -based heterojunction photoanodes
WO3 helices/BiVO4/FeOOH/NiOOH	3.8	5.4	0.5 KH2PO4, pH = 7	Shi et al.^62	2014
SiOx/Pt/SnO2 nanocones/Mo:BiVO4/Fe(Ni)OOH	4.1	5.8	0.5 KH2PO4, pH = 7	Qiu et al.^63	2016
WO3 nanorods/BiVO4/FeOOH/NiOOH	—	6.7	KH2PO4, pH = 7	Pihosh et al.^64	2015

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To gather more information about the origin of the high photoelectrochemical performance, Mott–Schottky-type electrochemical impedance spectroscopy was applied. This method relies on measuring the capacitance of the space charge region C_SC at the semiconductor/electrolyte junction. According to the Mott–Schottky-equation (eqn (1)) the flat band potential V_FB and the carrier concentration N can be estimated by the linear region of the plot of 1/C_SC² as a function of the applied potential V, as seen in Fig. 11.

Fig. 11 (a) Mott–Schottky plot of Mo-doped BiVO₄ thin films, (b) Mott–Schottky plots with undoped BiVO₄ sample included, (c) Mott–Schottky-plot of a 10% Mo:BiVO₄ sample at two different frequencies.

Mott–Schottky equation.

In eqn (1), ε_r represents the relative permittivity of the semiconductor, ε₀ the permittivity in vacuum, A the surface area of the semiconductor electrode, e the charge of an electron, k_b the Boltzmann constant, and T the temperature. Considering the BiVO₄/FTO junction structure, it was uncertain whether accurate results would be provided by the Mott–Schottky analysis. The obtained positive slopes of the linear region of the Mott–Schottky plot, typical of n-type semiconductors, and the V_FB values obtained (summarized in Table 5) are (i) frequency independent as exemplarily shown for the 10% Mo:BiVO₄ sample (Fig. 11c), and (ii) in line with previously reported V_FB values for BiVO₄ electrodes.^36,41 As can be seen in Fig. 11a and b, the slope of the Mott–Schottky plot for undoped BiVO₄ is much steeper than for the Mo-doped BiVO₄ samples. This result reveals a drastic increase of free charge carriers in the Mo-doped BiVO₄ electrodes.

Table 5 V _FB values determined by Mott–Schottky-analysis measured at 500 Hz and 1 kHz

Sample	V FB vs. RHE (V) measured at 500 Hz	V FB vs. RHE (V) measured at 1 kHz
BiVO4	0.11	0.07
5% Mo:BiVO4	0.17	0.16
10% Mo:BiVO4	0.15	0.15
15% Mo:BiVO4	0.13	0.13
20% Mo:BiVO4	0.11	0.12

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To further clarify the origin of the different photoelectrochemical performances of the X% Mo:BiVO₄ photoanodes, chronoamperometric investigations of the photocurrent transients of the pristine BiVO₄ and the 10% Mo:BiVO₄ samples were performed. The results are shown in Fig. 12a and b. The transients were acquired at an applied potential of 1.23 V vs. RHE using blue light (440 nm) with an intensity of 2 mW cm⁻². To analyze the transients a phenomenological approach commonly used in the literature was used.^65–68 As can be seen in Fig. 12a and b, the BiVO₄ photoanodes respond to light on/light off illumination with a characteristic “spike and overshoot” photocurrent transient. This transient response is very typical of photoanode systems with pronounced surface electron–hole recombination. When the illumination is turned on, holes generated in the space charge region are rapidly displaced towards the semiconductor–electrolyte interface. Due to the sluggish kinetics of the water oxidation reaction, holes initially accumulate at the semiconductor–electrolyte interface. In the steady state, the rate of hole arrival is then balanced by the rates of hole transfer towards the electrolyte and the rate of electron–hole surface recombination. As such, the resulting photocurrent transient is the sum of the hole and electron contributions. The instantaneous photocurrent (j_ini) measured when the illumination is switched on corresponds to a charging or displacement current due to the initial movement of photo-generated holes towards the surface. By contrast, the steady-state photocurrent (j_ss) corresponds to the flux of holes that are transferred successfully to the electrolyte without undergoing recombination with electrons at the surface. As a consequence, the ratio between j_ss and j_ini is a measure of the hole transfer efficiency η_transfer (see eqn (2)). Using a simple approach and assuming that both hole transfer and surface recombination follow a pseudo-first order law in terms of surface hole concentration, the hole transfer efficiency can be expressed by the corresponding first order rate constants of hole transfer, k_trans, and hole recombination, k_rec.

Fig. 12 Photocurrent transients, exemplarily shown for an undoped and a 10% Mo:BiVO₄ sample. (a) Undoped BiVO₄, (b) 10% Mo:BiVO₄, (c) undoped BiVO₄ with CoPi deposited, (d) 10% Mo:BiVO₄ with CoPi deposited. Blue light (440 nm, 2 mW cm⁻²) switched on and off at an applied potential of 1.23 V vs. RHE was used to record the transients. Transients for other X% Mo:BiVO₄ samples are given in the ESI.†

Hole transfer efficiency expressed as a function of j_ss and j_ini as well as a function of hole transfer and hole surface recombination rate constants.

The exponential decay of the initial photocurrent j_ini towards the steady state photocurrent j_ss is characterized by the time constants (k_rec + k_trans)⁻¹. The decay can be analyzed and k_trans and k_rec can be separated using eqn (2). After switching the light off, a cathodic (photo-)current overshoot is observed. This results from electrons flowing back to the electrode surface to recombine with holes (rapid) and with accumulated holes at surface states (slow), a phenomenon which is called back-electron–hole recombination.^69–71 Detailed results of the analysis carried out in this work for all samples are given in the ESI (S8, and Table S5†) and the essential outcome is described in the following.

As can be seen in Fig. 12a, the 0% Mo:BiVO₄ sample shows a typical “spike and overshoot” behavior during light-on/off experiments, accounting for an initial photocurrent j_ini of 0.055 mA cm⁻² which is rapidly decreasing towards the steady state photocurrent j_ss accounting for 0.002 mA cm⁻². This response is typical for a system with pronounced surface electron–hole recombination. Consequently, the hole transfer efficiency of the pristine BiVO₄ sample was determined to be very low accounting for 0.03 (see ESI, Table S5†).

In the case of the Mo-doped BiVO₄ samples the situation is different (Fig. 12b). Only a moderate photocurrent spike is observed for the Mo-doped samples while any overshoot is suppressed. This result is in good agreement with previous reports about charge carrier dynamics in undoped and doped BiVO₄ photoanodes.^71,72 The absence of any overshoot can be explained by two effects related to n-type doping in BiVO₄: (a) decrease of the space charge width due to increased donor concentration reducing back electron–hole recombination with surface accumulated holes and (b) removal of surface trapping states upon doping, therefore reducing the number of surface accumulated holes which are prone to back electron–hole recombination when the light is switched off. Consequently, Mo-doping also substantially increases the hole transfer efficiencies η_transfer from 0.45 for the 5% Mo:BiVO₄ sample up to 0.53 for the 20% Mo:BiVO₄ sample (see Table S5†).

Deposition of a layer of CoPi onto the undoped BiVO₄ anode increases the steady state photocurrent by a factor of 4, as a result of increased water oxidation kinetics (Fig. 12c). However, the CoPi deposition does not affect the spike and overshoot behavior positively, indicating that the undoped CoPi-modified BiVO₄ sample still suffers from a high degree of electron–hole recombination, which most probably occurs because of its limited electron transport.³⁵

For all Mo:BiVO₄ samples the observed photocurrents are largely increased after CoPi deposition indicating a drastic reduction of surface recombination due to CoPi-enhanced heterogeneous electron transfer rates at the semiconductor–electrolyte interface. In line, the recombination rates k_rec become very small compared to the hole transfer rates k_trans for the 5% Mo:BiVO₄ sample and the 10% Mo:BiVO₄ sample (factor of 6 and 7.5, respectively). A maximum hole transfer efficiency η_transfer of 0.88% was yielded for the 10% Mo:BiVO₄ sample, which is in good agreement with the obtained j–V-curves and photocurrent densities, respectively (see Fig. 9 and ESI, S8†).

As such, in line with other literature reports,³⁵ the main limiting factor for the PEC performance of the undoped BiVO₄ photoanodes was figured out to be the poor electron transport properties through the BiVO₄ bulk. According to our results, this problem can be overcome by Mo-doping, which increases the charge carrier density in the material. Depositing a layer of CoPi enhances water oxidation kinetics and preeminently reduces possible surface recombination.

Discussion

In summary, the new bottom-up synthesis approach for the direct deposition of X% Mo:BiVO₄ thin films allows adjusting the anode's charge transport properties and morphology for optimal photoelectrochemical performance. As demonstrated by frontside/backside illumination assays and Mott–Schottky analysis, Mo-doping increases the amount of free charge carriers in the as-synthesized Mo-doped BiVO₄ thin film photoanodes, hence decreasing the probability of hole recombination with the majority charge carriers in the bulk. As such, a better PEC performance for water oxidation is observed for all Mo-doped samples. The thin film features of the Mo-doped BiVO₄ photoanodes are further dependent on the Mo content. While specific geometrical surface area, film thickness and average geometrical domain size decrease with increasing Mo content, the average inner domain material streak size increases with increasing Mo content. An optimal compromise between inner domain streak size, which should be adjusted to the hole diffusion length in BiVO₄, film thickness and specific surface area, which should be rather big for high PEC performance, can be found for the 10% Mo:BiVO₄ sample, which also exhibits the highest PEC performance for water oxidation (see Fig. 13). A closer analysis of photocurrent transients reveals that Mo-doping generally increases the hole transfer efficiency η_transfer, which is expressed by a significant decrease of electron–hole recombination at the semiconductor surface and by largely reduced k_rec values when compared to the undoped case. As a result, Mo-doping leads to an increased hole transfer efficiency at the semiconductor–electrolyte interface (Fig. 13b). Further improvement of the PEC performance for all BiVO₄ samples could be achieved by CoPi deposition, as reflected by an increased hole transfer efficiency. As revealed by the photocurrent transient analysis, this mainly results from (a) slightly increased water oxidation kinetics (slightly enhanced k_trans values) and (b) dramatically reduced surface recombination (very small k_rec values) probably resulting from removal of surface states during the CoPi deposition (Fig. 13c). A summary of these structure–function correlations can be found in Fig. 13.

Fig. 13 Morphological features (a) and photoelectrochemical properties without (b) and with (c) CoPi of the X% Mo:BiVO₄ thin film photoanodes illustrated as a function of Mo content.

The applied bias photon to current efficiency (ABPE) curves for the X% Mo:BiVO₄ samples calculated from the j–V curves are shown in Fig. 14. ABPE corresponds to the solar-to-hydrogen efficiency (STH) if an external bias is needed to enable photoelectrolysis, i.e. the electrical power output is corrected by the electrical power input provided by the external bias according to eqn (3):

Fig. 14 ABPE curves for X% Mo:BiVO₄ photoanodes calculated from j–V curves.

Applied bias to photon to current efficiency.

As can be seen in Fig. 14, the 10% Mo:BiVO₄ yields the best ABPE without and with CoPi deposited accounting for 0.22% and 1.09%, respectively. Aiming at higher efficiencies, the 10% Mo:BiVO₄ thin film photoanodes in principle can easily be coupled to a suitable photocathode or be biased externally by a coupled solar cell. However, in consideration of a reasonable target STH value for light-driven water splitting of 16%, as for instance proposed by Laurence Peter,⁷³ there is still much room for improvement.

Conclusion

A new one-pot synthesis method based on metalorganic Bi, V and Mo precursors was applied to synthesize FTO-supported Mo-doped BiVO₄ thin film photoanodes for ligth-induced water oxidation by facile dip coating and subsequent calcination. The synthesis does not involve advanced coating technology, and is easy to reproduce and in principle easy to scale up. With up to 15% Mo doping, the thin films crystallize in the monoclinic scheelite structure, the favorable crystal structure for BiVO₄ based PEC water oxidation. The thin films are porous and consist of micrometer-sized domains wetting the FTO substrate with homogeneous thickness. With increasing Mo content, the domain size and the film thickness decrease, and the inner domain material streaks increase. Electrochemical investigations of photoelectrocatalytic properties of pristine and CoPi-decorated BiVO₄ and X% Mo:BiVO₄ thin films (X = 5, 10, 15, 20% Mo) showed high activity concerning water oxidation for all samples in their pristine or Co-Pi decorated state. In line with their crystal structure, morphology, domain size, streak size, film thickness and free charge carrier concentration, the highest PEC performance was achieved for the 10% Mo:BiVO₄ sample. To the best of our knowledge, the measured photocurrents of 1.9 and 4.6 mA cm⁻² at 1.23 V vs. RHE (100 mW cm⁻² visible light) for the unmodified and CoPi-modified 10% Mo:BiVO₄, respectively, are among the highest values reported to date for comparably simple single-layered thin film BiVO₄ photoanodes for water oxidation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Dr Dirk Berger (ZELMI TU Berlin) and Christoph Fahrenson (ZELMI TU Berlin) are gratefully acknowledged for TEM lamella preparation and EBSD measurements. Financial support by the DFG funded SPP1613 (FI 1885/1-2, LE 781/13-2 and SCHE 634/12-2) as well as by the Cluster of Excellence “UNICAT” of the DFG (Deutsche Forschungsgemeinschaft) is gratefully acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7se00301c

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