Solvothermal Synthesis of Molybdenum-Tungsten- Oxides and Their

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Solvothermal Synthesis of Molybdenum-Tungsten- Oxides and Their Application for Photoelectrochemical Water Splitting Dmitri Spetter, Muhammad Nawaz Tahir, Jan Hilgert, Ibrahim Khan, Ahsan Ul Haq Qurashi, Hao Lu, Tobias Weidner, and Wolfgang Tremel ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01370 • Publication Date (Web): 30 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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ACS Sustainable Chemistry & Engineering

Solvothermal Synthesis of Molybdenum-TungstenOxides and Their Application for Photoelectrochemical Water Splitting Dmitri Spetter,1 Muhammad Nawaz Tahir,*3 Jan Hilgert,1 Ibrahim Khan,2 Ahsanulhaq Qurashi,2,3 Hao Lu,4 Tobias Weidner,4 and Wolfgang Tremel*1 1

Institut für Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universität, Duesbergweg 10-14, 55128 Mainz, Germany 2

Center of Excellence in Nanotechnology, Dhahran, 31262, Kingdom of Saudi Arabia

3

Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Kingdom of Saudi Arabia

4

Max-Planck-Institut for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany

*

Correspondence should be addressed to: [email protected], [email protected]

Keywords: Molybdenum oxide, tungsten oxide, molybdenum-tungsten oxide, solvothermal synthesis, nanocatalysis; photoelectrochemical hydrogen evolution

Abstract Molybdenum and tungsten oxides are of interest as semiconductors for the production of clean and sustainable energy. Here we show that synergistic effects arising from a combination of non-crystallinity and plasmonic resonance in mixed molybdenum/tungsten oxides can lead to improved efficiency for the photoelectrochemical (PEC) splitting of water. The quasibinary Mo/W oxides were synthesized solvothermally on a gram scale. Size, structure, morphology and electronic properties of the as-prepared microspheres were characterized by scanning and transmission electron microscopy (SEM, TEM), X-ray diffraction (XRD), Raman, optical absorption (UV-vis) and X-ray photoelectron spectroscopy (XPS). Molybdenum oxide benefits from W-substitution and the concomitant metal reduction. The increased number of charge carriers lead to higher photocurrents for Mo0.5W0.5O2.1 (5.25 mA cm-2), the most reduced phase compared to Mo0.89W0.11O2.7 (1.75 mA cm-2). Long-term photocurrent stability tests (2000 s) under photo-illumination confirmed the chemical stability of Mo/W oxides under sunlight. The improved PEC performance is attributed to synergistic effect of increased charge carrier concentration due to metal reduction, suppressing the formation of crystalline metallic oxides through disorder and tuning the absorption in the visible and NIR range by the formation of W5+ and Mo5+ sites. 1 ACS Paragon Plus Environment

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Introduction The development of new sustainable energy technologies based on renewable or inexhaustible resources (solar, wind, or thermal energy) could be a carbon-free alternative energy source for the future. The production of hydrogen fuel from water using photo/electrocatalysis in a hydrogen evolution reaction (HER) is one of the most encouraging solutions because of its eco-friendliness and the abundance of water.1-3 Still, the synthesis of new photocatalysts which are chemically and electrically stable and can generate a high flux of charge carriers (i.e. electron (e-) and hole (h+) pairs) with suitable band-edge potentials for water splitting remains a challenge. Many catalysts with a variety of compositions ranging from transition metals or alloys, metal oxides, or metal chalcogenides have been employed as catalysts, but not a single semiconductor has yet been identified that meet all necessary criteria.4-7 Therefore, approaches based on band gap engineering of semiconductors, the integration of different semiconductors in a single particle or the integration of electrocatalysts with semiconductor surfaces have been pursued to achieve maximum efficiency.8,9 MoO3 and WO3 are semiconducting transition metal oxides (TMOs), which are cheap, naturally abundant, environmentally compatible and chemically stable. Therefore, they have been widely used in heterogeneous catalysis,10 electrocatalysis,11 and lithium-ion battery research.12-13 These TMOs qualify as candidates for photoelectrocatalysis14,15 because of their band gaps (~ 2.6-2.8 eV), good Hall mobilities and long carrier diffusion lengths. Molybdenum and tungsten oxides have been used not only as photoanodes in solar cells,16 in optoelectronic and electrochromic devices17-19 but also for water splitting20,21 and in photocatalysis.22 A drawback of pure MoO3 and WO3 in photoelectrocatalysis is their low reduction potential of electrons due to lower conduction band (in comparison to frequently used TiO2) which results in the low light energy conversion efficiency.23 Doping (i.e. solid solutions) is one way to tailor the electronic structure of these TMOs.24-26 As the radii of tungsten and molybdenum ions are nearly identical,27 the substitution of W by Mo (and vice versa) is possible over a wide compositional range without affecting the crystal structure.28,29 Thus, quasi-binary oxides of molybdenum/tungsten (MoxW1-xOy) provide an opportunity to enhance the photo-conversion efficiency.30 The recent exploration of the plasmonic properties of MoO3 nanosheets and WO3 nanorods31, 32 could be beneficial for an efficient conversion of solar to chemical energy.33,34 Several studies reported the preparation of Mo/W-mixed metal ox3 ACS Paragon Plus Environment


ides,35-41 as well as studies of the formation mechanism42,43 and properties,44-51 but mostly with a focus on the Mo-doping of WO3 nanoparticles. Zhou et al.52 reported on the tunability of the band-gaps in mixed oxides of MoxW1-xO3·0.33H2O obtained hydrothermally. Li et al.53 highlighted the enhanced photocatalytic properties of Modoped tungsten oxides for HER by water splitting. Most recently, Jin et. al54 reported on the activity of amorphous oxygen-deficient MoO3-x spheres in photoanodes for water oxidation. Although they obtained well-dispersed microspheres composed of MoO3x

with cetyltrimethylammonium bromide (CTAB) as surface surfactant but could

achieve current density 3.8 mA/cm2. Here we report the solvothermal synthesis of a solid solution series of spherical Mo1-xWxO3-y microparticles with homogeneous size distribution and morphology without employing any extra organic surfactant. The Mo1-xWxO3-y microparticles revealed strong disorder as demonstrated by X-ray-powder diffraction (XRD). UV-visible spectroscopy (UV-vis) revealed the change in composition to be accompanied by a continuous color change in the visible and particularly the NIR regime. The combined effects lead to enhanced photocurrents and photoelectrocatalytic properties in the water splitting reaction with current density 5.25 mA/cm2. The compositions of the as-synthesized Mo1-xWxO3-y microparticles was determined using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron (XPS) and Raman spectroscopy.

Experimental Materials. Molybdenum (V) chloride (99.99%, MoCl5), tungsten (VI) chloride (99.99%, WCl6) and ethanol (99.8%) were purchased from Sigma Aldrich and used without further purification. Synthesis of Mo1-xWxO3-y molybdenum-tungsten oxide microspheres. 100 mg (0.366 mmol) of molybdenum (V) chloride and Y mmol of tungsten (VI) chloride (where Y = 0.538, 0.355, 0.177, 0.089, 0.044) was dissolved in ethanol to form a clear solution. The as-prepared solutions were transferred to a 50 mL Teflon-lined vessel. After sealing the vessel in a stainless steel autoclave the solvothermal reaction was carried out by heating the autoclave at 180 °C for 12 h in an electric oven. After the reaction, black precipitates were collected using centrifugation, which were rinsed with ethanol two times and dried in vacuo for 12 h. Synthesis of molybdenum oxide microspheres. Pure MoO3-x microspheres were synthesized as reference compound to compare the photoelectrocatalytic properties. 4 ACS Paragon Plus Environment

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100 mg (0.366 mmol) of molybdenum pentachloride (MoCl5) were dissolved in 100 mL of ethanol and subsequently transferred to two 100 mL Teflon-lined vessels. The vessels were sealed in stainless steel autoclaves, which were kept in an electric oven for 12 h at 180 °C. Afterwards, the mixture was allowed to cool naturally to ambient temperature. The black precipitate was collected by centrifugation, rinsed with ethanol two times and dried in vacuo for 12 h. Analytical data for the MoO3-x reference sample (S6) are given in the Supporting Information. Synthesis of W18O49 nanowires. Tungsten oxide (W18O49) nanowires were synthesized as reference compound as reported in ref. 55. The starting solution was prepared by dissolving 500 mg of tungsten hexachloride (WCl6) in 25 mL of ethanol by sonication. Next, 5 mL of the precursor solution were injected into a 50 mL Teflon-lined vessel containing another 30 mL of ethanol. After sealing the vessel in a stainless steel autoclave the reaction was started by putting the autoclave in an electric oven for 12 h at 180 °C. After the reaction, dark blue particles were collected using centrifugation, rinsed with ethanol two times and dried in vacuo for 12 h. Analytical data for the W18O49 reference sample (S7) are given in the Supporting Information Photoanode Fabrication. Photoanodes and working electrode were fabricated by dip coating. FTO electrodes (1x2 cm2) were used as conducting substrates. For the preparation of each photoelectrode 20 mg of catalyst were dissolved in 2 mL of ethanol under vigorous sonication to obtain a homogenous mixture. 0.5 volume % of Nafion® was added to the mixture to achieve a stable deposition of the catalysts on the substrate. The photoelectrodes were immersed in the electrolyte and kept there for 5 min to build the charge equilibrium before starting the photoelectrochemical measurements.

Figure 1. a) Scheme of the solvothermal synthesis of molybdenum-tungsten oxide microspheres along with a SEM image (upper image) and possibility to make gram amount of the product (lower image). b) Digital photographs of dispersions of the microspheres in water.



Chemical, structural and morphological characterization.

The chemical composition of the catalysts was determined by inductively coupled plasma atomic emission spectroscopy (ICP–AES, Spectro Ciros Visions). The samples were digested with a mixture of HNO3, HCl, and H2O. At designated time intervals, 0.2 mL of the suspension were collected and centrifuged for 5 minutes at 5000 rpm to separate the solution from larger particles for analysis. The supernatant was acidified to stop the reaction with nitric acid (8 M) and diluted with deionized water. Calibration was performed by taking into account matrix-matching with suitable molybdenum and tungsten standards. Electron microscopy. SEM images were taken on a FEI Nova NanoSEM 600, equipped with an Everhart-Thornley detector (ETD) and a low-voltage high-contrast detector (vCD), operated under the high vacuum mode with an acceleration voltage of 5-10 kV. The powdered samples were drop casted on an aluminium stub using an adhesive conductive carbon tape. A built-in EDAX-Genesis detector was used to confirm the chemical composition of the non-sputtered samples. Samples for TEM studies were prepared by diluting a nanoparticle dispersion with ethanol. Subsequently, 3-5 drops of the solution (depending on concentration) were placed onto a carbon coated copper grid. The sample grid was dried in air overnight. The images were recorded on a Tecnai G2 Spirit transmission electron microscope with an acceleration voltage of 120 kV. X-ray diffraction (XRD). Powder X-ray diffraction (PXRD) patterns were recorded on a Bruker AXS D8 discover diffractometer equipped with a HiStar detector using graphite monochromated Cu Kα radiation. The samples were attached to a glass substrate without an adhesive. Individual frames were typically recorded at 2θ = 24, 34, 44, 54, 64, 74, and 84° (detector distance 150 mm, detector range D(2q) = 358) in 0.028 steps covering a 2θ range from 5 to 85°. The X-ray diffraction patterns were integrated from individual frames with the Bruker AXS GADDS software package and merged with Bruker AXS EVA. Crystalline phases were identified using the PDF-2 database56 and the Bruker AXS EVA program suite. Raman spectra were recorded on a Horiba Yvon Lab RAM HR 800 spectrometer equipped with a microscope (Olympus BX41) and a CCD detector. The entrance slit was set to 100 µm, the laser focal spot was 2x2 µm. A Nd:YAG laser (532.12 nm) with a laser power of 2 mW was used for excitation. The laser light was focused onto the fine powder sample using a 50x long working distance objective. All Raman spectra were recorded in backscattering geome6 ACS Paragon Plus Environment

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try at a resolution of 0.5 cm-1 in the range between 150 and 1200 cm-1. At least five spectra were measured for each sample to improve the statistics. X-ray photoelectron spectroscopy (XPS) samples were prepared by drop-casting a suspension of the particles on a gold-sputtered surface multiple times, until a homogeneous thin film was obtained. The XPS measurements were conducted on a Kratos Axis UltraDLD spectrometer (Kratos, Manchester, England) using an Al Kα excitation source with a photon energy of 1487 eV. The data was acquired in hybrid mode using a 0° take-off angle, defined as the angle between the surface normal and the axis of the analyser lens. The spectra were collected with setting the pass energy of the analyser at 20 eV. A neutralizer was always used during spectra collection to compensate charge build-up on the samples. The binding energy (BE) scale was calibrated according to Au 4f7/2 emission peak at 84 eV.57 The W 4f XPS spectra of the as-synthesized products in Figure 3 were fitted with one doublet and one broader singlet peak at higher binding energy. The doublet was restricted with the same full width at half-maximum, a suitable spin orbit splitting of 2.18 eV, and branching ratio of 4/3 for 4f7/2/4f5/2.58 The broader peak is a combinational feature from emissions from Mo 4p and W 5p3/2. The atomic composition of W% and Mo%, in Table 1 was calculated by quantifying the intensities of W 4f (derived from fitting procedure), Mo 3d, and O 1s core-level emissions with the CasaXPS software. A linear background was used for all peak quantifications, and the peak areas were normalized with the sensitivity factors supplied by the manufacturer. The optical absorption (UV-Vis) spectra were recorded with a Cary Varian 5G UVVis-NIR spectrometer using a 1 mL micro-cuvette with a size of 12.5 x 12.5 x 45 mm from Brand, filled with a 1 mg mL-1 solution of the appropriate product.

Photoelectrochemical (PEC) water splitting. The reactions were carried out in a clear Pyrex container 3-electrode cell with a 0.5 M Na2SO4 electrolyte (pH 7.0 ± 0.1) connected to argon gas cylinder. A Pt wire was used as auxiliary electrode, FTO coated with different MoxW1-xOy composites as photoanode (or working electrode), and a saturated calomel electrode (SCE) as reference electrode. All photoelectrochemical parameters were controlled with a Metrohm Autolab potentiostat (PGSTAT302N) and a solar light source (Oriel sol 3A class AAA solar simulator-Newport). The solar simulator was provided with several specifications: 100 mW cm−2 power (1 SUN), IEC/JIS/ASTM-certified containing a 450 Watt Xenon lamp, Air Mass 1.5G Filter, UV cut off filter (λ > 420 nm) and 2×2-inch aperture for output beam. The power of the solar light was calibrated with standard silicon diodes and fixed at 1 SUN energy (100 mW cm-2). Before each sample run oxygen was removed from the cell by flush7 ACS Paragon Plus Environment


ing with an argon stream for 10 min. The PEC plots were transformed to values vs. RHE using the Nernst equation (pH dependence).

Results and Discussion Structural and morphological characterization of the molybdenum-tungsten oxides. The solvothermal synthesis in ethanol yielded uniform Mo1-xWxO3-y microspheres. The as-prepared samples had the compositions Mo0.5W0.5O2.1 (S1), Mo0.62W0.38O2.3 (S2), Mo0.75W0.25O2.4 (S3), Mo0.85W0.15O2.6 (S4), and Mo0.89W0.11O2.7 (S5); the increasing number indicates increasing amounts of Mo. The synthesis procedure along with SEM image is illustrated in Figure 1a. The digital micrographs at the bottom trace (Figure 1b) highlight the dispersibility as well as the continuous change of the colour (i.e. the absorption spectrum) properties as a function of composition. Spherical MoO3-x microparticles were prepared as reference material starting from MoCl5 (Figure S1, further referred to as sample S6). W18O49 nanowires were obtained under a similar set of conditions using WCl6 as starting material (Figure S1, sample S7). The as-prepared particles were spherical throughout the Mo1-xWxO3-y solid solution series (samples S1 to S5). The average size of the microspheres was approximately ~1.3 µm. A size histogram of the products is shown in Figure S3. The elemental Mo and W compositions were confirmed by ICP-AES.


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Figure 2. Representative SEM and low magnification TEM images of the as-synthesized molybdenum-tungsten oxide microspheres: (a, b) Mo0.5W0.5O2.1, (c) Mo0.62W0.38O2.3, (d) Mo0.75W0.25O2.4, (e) Mo0.85W0.15O2.6, and (f) Mo0.89W0.11O2.7.

Figure 2 shows respective SEM and TEM images of the Mo1-xWxO3-y products. The HR-TEM image as inset to Figure 2b shows a representative disordered, defect-rich edge of the microparticles. Due to the non-crystalline structure of the product, no lattice fringes (resulting from a superposition of a transmitted wave and diffracted wave from one lattice plane of a crystal) could be identified. All samples were characterized by powder X-ray diffraction to check for their phase identity and crystallinity. The presence of weak and very broad reflections in X-ray diffractograms (Figure 3a) revealed the samples to be mostly non-crystalline, which precludes their structural identification or a determination of the composition and oxygen deficiency in Mo1-xWxO3-y. Only a single reflection could be identified at 2θ ≈ 41° for samples S1, S2 and S3, which might be attributed to traces of remnant Mo4O11 (JCPDS No.13-0142). Additionally, in sample S1 a broad feature centered around 2θ ≈ 35° may be compatible with the (114) reflection of W18O49. Broad reflections centered around 2θ ≈ 13° and 26° might be assigned to the (020) and (040) reflections of orthorhombic MoO3 (JCPDS No. 35-0609). The XRD pattern of sample S5 shows four weak, but sharp reflections at 2θ ≈ 29°, 41°, 47°, and 57° which indicate the presence of remnants of Mo4O11. This suggests that a large excess of Mo precursor leads to preferential formation of the non-crystalline and non-stoichiometric MoO3-x, domains while the evolution of WO3-x phases is impeded. Although the atomic radii of Mo (134 pm) and W (137 pm) are nearly identical,27 molybdenum and tungsten form a number of very distinct non-stoichiometric oxides (Magnéli phases).59-61 In contrast, the molybdenum ions can be substituted by tungsten over a wide compositional range in these non-crystalline phases, which prevents a phase separation of crystalline (and also metallic) compounds with Magneli-type structure. A clear statement on the incorporation of W into the MoO3-x structure or Mo into the WO3-x structure cannot be made solely on the basis of XRD data. Raman spectroscopy is a valuable tool that provides information for non-crystalline phases and allows for a discrimination between different localized structural building blocks and different compositions, respectively. Figure 3b shows Raman spectra of the as-prepared sam-



ples. In the sequel, M (= Mo or W) is used to illustrate that in the binary oxides every incorporated W atom can occupy Mo sites and vice versa.

Figure 3. (a) Powder XRD patterns and (b) Raman spectra of the as-synthesized molybdenum-tungsten oxides Mo0.5W0.5O2.1 (S1), Mo0.62W0.38O2.3 (S2), Mo0.75W0.25O2.4 (S3), Mo0.85W0.15O2.6 (S4), and Mo0.89W0.11O2.7 (S5).

The typical spectra of molybdenum-tungsten mixed oxides (S1-S5) exhibit four main peaks. The two strongest bands, located at 809 and 710 cm-1 in S1, can be assigned to the stretching mode of the terminal v(M=O) bond and stretching vibrations of bridging oxygen atoms v(O-M-O), respectively. Two additional bands with intermediate intensity in the low wavenumber region (at 260 and 329 cm-1) can be attributed to δ(O-M-O) deformation modes. As the amount of W decreases from 0.538 mmol to 0.044 mmol in samples S1 to S5, the positions of the characteristic bands continuously shift to higher wavenumbers. Interestingly, the terminal W=O stretching mode of W18O49 is located at 804 cm-1, while both the stretching vibrations of Mo=O (symmetrical, asymmetrical) in MoO3-x are observed at 819 and 992 cm-1 (see Table S1, Supporting Information). Thus, it can be concluded that the Raman bands represent a mean value of all possible M=O contributions, and they are gradually blue-shifted upon W substitution. In addition, the profile of the M=O band becomes gradually sharper, and the significant


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vas(Mo=O) stretch gains intensity as the molybdenum content in MoxW1-xOy increases. This is in good agreement with the results obtained from XRD.

Figure 4. High-resolution XPS spectra of the W 4f core level region of the mixed molybdenum-tungsten oxides Mo0.5W0.5O2.1 (S1), Mo0.62W0.38O2.3 (S2), Mo0.75W0.25O2.4 (S3), Mo0.85W0.15O2.6 (S4), and Mo0.89W0.11O2.7 (S5).

The effect of W substitution on the oxidation states of MoO3-x in the MoxW1-xO3-y molybdenum-tungsten oxide samples was monitored by XPS. The presence of Mo, W, O, and an impurity of C was detected in the XPS survey spectra (Figure S4). C may be attributed to remnants of ethanol adsorbed on the particle surface. Figure 4 shows the evolution of the W 4f core level region as a function of the Mo content. A spin-orbit doublet with peaks at binding energies of 37.6 ± 0.10 (W 4f5/2) and 35.5 ± 0.10 eV (W 4f7/2) characterizes the W 4f core level spectra of the mixed oxides, which can be deconvoluted into contributions from the W6+ and W5+ oxidation states. As expected, the intensity of the doublet decreases with increasing Mo content, accompanied by a narrowing of the W 4f peaks (from sample S5 to S1). This clearly demonstrates that a substantial amount of the W6+ species is consecutively reduced to W5+ in the mixed oxides. The broad peak around 41 ± 0.10 eV is attributed to W 5p3/2, which always can be observed in association with the W 4f peaks.62 To determine the relative bulk composition of the particles, EDS coupled with SEM was used. The measurements were conducted over more than 10 positions to obtain reliable information. It appears that molybdenum and tungsten atoms are distributed homogenously in the products. Moreover, both EDS and XPS confirm the atomic ratio of the Mo host 11 ACS Paragon Plus Environment


and incorporated W to resemble the proposed composition based on the synthetic setup (Table 1). Table 1. Analytically determined elemental compositions of the MoxW1-xO3-y microparticles.

The amount of oxygen in the different samples was determined and indicates that the solvothermal synthesis of the microparticles yields oxygen-deficient products. Based on the XPS and EDS data a defined composition of the as-synthesized amorphous Mo/W oxides was calculated. As a result, S1 exhibits the structure with the largest oxygen deficiency, whereas S5 shows considerable higher O saturation. Since the introduction of oxygen vacancies in the structures of MoO3 and WO3 generates charge carriers (and leads to electrical conductivity in crystalline compounds), it can be concluded that the substoichiometric compounds S1-S5 behave as “heavily doped” semiconductors.63


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Figure 5. (a) Photocurrent-voltage responses measurements vs. RHE for the MoxW1-xO3-y molybdenum-tungsten oxide microsphere electrode, under a potential of 1.23 V. (b) UV-vis absorbance spectra of MoxW1-xO3-y molybdenum-tungsten oxide microparticles dispersed in water. The inset shows the strong absorption band of S1 at 690 nm, characteristic for a localized surface plasmon resonance.

Photoelectrocatalytical measurements of molybdenum-tungsten oxide photoanodes. The PEC water splitting function of MoxW1-xO3-y photoanodes was evaluated

photoelectrochemically

using

voltammetry

and

chronoamperometry.

Cur-

rent−voltage (I-V) curves were achieved by linear sweeping voltammetry (LSV). The range of sweep voltage ranges from 0.00 V to 1.50 V (vs RHE) in the dark and in light (Figure 5a). The dotted lines correspond to the dark current of the photoanodes, which is negligible even at higher voltage as no significant dark current was observed. On the other hand, a significant photocurrent was generated under light irradiation. This shows that MoxW1-xO3-y photoanodes are active under light and therefore efficiently split water in the given voltage range. For all photoanodes the onset potential was observed beyond 0.6 V. Subsequently, the current shows a significant elevation with voltage. The best photocurrent character was observed for the Mo0.5W0.5O2.1 (S1) photoanode, the most oxygen-deficient mixed metal oxide. The optimum composition maximizes the charge carrier density that compensates for recombination, which is likely to occur radiatively by an electron-hole pair at a defect in the non-crystalline material. Alternative pathways could be non-radiative by self-trapping of a free exciton or by tunneling 13 ACS Paragon Plus Environment


between defects. Moreover, Mo0.5W0.5O2.1 microspheres showed the best optical performance by strongly absorbing in the UV and NIR (Figure 5b). Incident photon to current efficiency (%IPCE) was determined to analyze the effect of light absorption on photocurrent for different wavelengths of light. The %IPCE plot is provided in Figure S10. The %IPCE shows maximum values between 350 and 450 nm, which can be correlated with the higher light absorption as suggested by the UV-vis spectra. The significant %IPCE value for S1 (Figure S9) can be related to the efficient interfacial reaction of electron and holes. The water splitting capacity (i.e. the charge recombination) is minimized. The optical behavior, i.e. the relative position of the valence and conductance bands, can be tuned by substitution in the MoxW1-xO3-y solid solution series.64,65 Additionally, this sample showed the highest photocurrent (Figure 6). The photocurrents for S1, S2, S3, S4, and S5 were 5.25, 3.6, 3.1, 0.9, and 1.75 mAcm-2 (at the thermodynamic water oxidation voltage of 1.23 V vs RHE). The periodic ON-OFF current-time (I-t) graph in Figure 6a was derived from chronoamperometric measurements of the photoanodes at the water oxidation potential (i.e., 1.23 V vs RHE). The light ON/OFF periodicity was maintained at ~20 s. The graphs clearly indicate that in the absence of light the photocurrent drops off to the base line as no current is generated photocatalytically or electrocatalytically, even at high potential. The photocurrent strongly increased under illumination for all photoanodes, which indicates photocurrent generation for the MoxW1-xO3-y molybdenum-tungsten oxides under solar light. The photocurrent was maintained during periodic light chopping as observed in the LSV. Many cycles were measured to analyze the performance of the photoelectrodes. Even after multiple cycles the photocurrent values were stable. To further confirm the stability of the photoanodes, a long-term stability test was performed for 2000 s (Figure 6b). As expected, the higher current density was maintained by the Mo0.5W0.5O2.1 (S1) photoanode. A negligible photocurrent drop was observed, which shows the efficiency and resilience of Mo0.5W0.5O2.1 against chemical processes at the interface in the electrolyte. The long-term (three hours) stability data is provided as Table S2. The sample with composition Mo0.5W0.5O2.1 photoanode showed the best stability even for extended periods of time. Moreover, the mechanical and chemical stability of the photoanode before (as-prepared) and after measuring by using SEM. Representative scanning electron micrographs showing both mechanical and chemical stability are shown in Figure S10.


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The PEC water splitting results clearly indicate that the Mo0.5W0.5O2.1 photoanode shows the highest photocurrent, the best long-term stability and overall performance. This can be attributed to the number of charge carriers, as Mo0.5W0.5O2.1 shows the highest degree of reduction, which compensates for charge recombination and leads to improved charge separation in the strongly disordered material.66 The additional charge carriers lead to an enhanced absorption in the visible and particularly the NIR range through the plasmon excitation.31,32 Compared to other reports (Table 2), the asprepared mixed oxides show siginificantly increased generation of current densities.

Figure 6. (a) Photocurrent-time responses of the mixed oxides under chopped 1 SUN illumination in 0.5 M Na2SO4 electrolyte solution. (b) Chronoamperometry analysis of the as-prepared photoanodes. Table 2. Comparison of photoanodes of molybdenum/tungsten oxides and their PEC performances, prepared via different techniques. Compound

Preparation method

Current density (mA/cm2)

Reference

Mo0.5W0.5O2.1

Solvothermal

5.25 (1.23 V vs. RHE)

this work

Mo-doped W18O49 (urchins)

Hydrothermal

4.50 (0.3 V vs. RHE)

30

MoO3-x (microspheres)

Solvothermal (CTAB-assisted)

3.80 (1.23 V vs. RHE)

54

W/WO3 (nanoporous)

Anodization

3.50 (1.7 V vs. RHE)

68

WO3 (platelets)

sol-gel (microwave)

2.7 (1 V vs. RHE)

67

WO3 (columns)

RF Magnetron sputtering

2.7 (1.9 V vs. RHE)

69

W/WO3 (nanorods)

Hydrothermal

2.7 (1.8 V vs. RHE)

70

* RHE (reversible hydrogen reference elctrode)



Conclusions We demonstrated the solvothermal synthesis of spherical MoxW1-xO3-y microparticles with enhanced performance as photoanode materials in the PEC water splitting reaction. The asprepared materials show superior chemical long-term stability and higher photocurrent values than other semiconducting compounds.71 This can be attributed to the higher charge carrier density compared to binary stoichiometric WO3 and MoO3 and resulting from the partial reduction and the concomitant presence of W5+ (and Mo5+) as demonstrated by XPS spectroscopy. Alternative pathways could be non-radiative by self-trapping of a free exciton or by tunneling between defects. Raman spectra showed that W substitutes homogeneously for Mo. The Mo/W substitution prevents the formation of metallic and crystalline Magnéli-type phases. The non-crystalline behavior was demonstrated by powder XRD. The additional charge carriers arising from W5+ and Mo5+ sites lead to enhanced absorption in the visible and the NIR range through plasmon excitation. Thus, the improved PEC performance can be attributed to the synergistic effects of (i) increasing the charge carrier concentration by metal reduction, (ii) suppressing the formation of metallic oxides and associated metal-semiconductor transitions through non-crystallinity and (iii) tuning the absorption in the visible and NIR range due to the presence of W5+ and Mo5+ sites.

Associated Content Supporting Information Electronic supplementary information (ESI) available: Electronic Supporting Information (ESI) Available: Representative SEM image of MoO3-x microspheres and TEM image W18O49 nanowires (Figure S1), size-distribution histograms of all synthesized microspheres (Figure S2), powder XRD patterns and Raman spectra of amorphous MoO3-x microspheres and monoclinic W18O49 (Figure S3), XPS survey spectra of all as-synthesized Mo/W-oxides (Figure S4), UV-Vis absorption spectra of MoO3-x microspheres and TEM image W18O49 nanowires (Figure S5), photocurrent-voltage responses of the MoO3-x microspheres (a) and W18O49 nanowires (b) under chopped 1 SUN illumination in 0.5 M Na2SO4 electrolyte solution (Figure S6), photocurrent-time responses measurements vs. SCE for the MoO3-x microspheres (a) and W18O49 nanowires (b) electrode under a potential of 1.23 V (Figure S7), Chronoamperometry analysis of the as-prepared photoanodes of MoO3-x microspheres (a) and W18O49 nanowires (b) (Figure S8), significant %IPCE values (Figure S9), representative scanning electron mi-


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crographs showing both mechanical and chemical stability of the photoanode (Figure S10). See DOI:10.1039/b00000x/

Author Information Corresponding Authors * [email protected], [email protected] All authors have given approval to the final version of the manuscript.

Acknowledgements This work was partially supported by the priority program SPP1959 “Manipulation of Matter Controlled by Electric and Magnetic Fields: Towards Novel Synthesis and Processing Routes of Inorganic Materials”.

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TOC-Graphic

Synopsis Mixed-metal Mo/W oxides show high efficiency for the photoelectrochemical (PEC) splitting of water (photocurrent for Mo0.5W0.5O2.1: 5.25 mA cm-2) due to the synergistic effect of charge carrier concentration, noncrystallinity and vis/NIR absorption.


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Solvothermal Synthesis of Molybdenum-Tungsten- Oxides and Their

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