Concentration-mediated Band Gap Reduction of Bi2MoO6

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Concentration-mediated Band Gap Reduction of Bi2MoO6 Photoanodes Prepared by Bi3+ Cation Insertions into Anodized MoO3 Thin Films: Structural, Optical and Photoelectrochemical Properties Shi Nee Lou, Rose Amal, Jason Scott, and Yun Hau Ng ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00675 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 14, 2018

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ACS Applied Energy Materials

Concentration-mediated Band Gap Reduction of Bi2MoO6 Photoanodes Prepared by Bi3+ Cation Insertions into Anodized MoO3 Thin Films: Structural, Optical and Photoelectrochemical Properties Shi Nee Loua*, Rose Amalb, Jason Scottb* and Yun Hau Ngb* a: School of Materials Science and Engineering, Nanyang Technological University, Singapore, 50 Nanyang Avenue, Singapore 639798 b: Particles and Catalysis Research Group, School of Chemical Engineering, The University of New South Wales, Sydney, Australia 2052. KEYWORDS: Bi2MoO6, Solar Energy Conversion, Photoelectrochemical Water Splitting, Photocatalysis, Thin Film, MoO3

ABSTRACT: A secondary cation insertion technique to fabricate ternary Bi2MoO6 thin films with reduced optical band gaps and shallow valence bands by the controllable insertion of Bi3+ cations into anodized MoO3 thin films has been established. Near complete conversion of the MoO3 thin film to a low temperature-phase γ(L)-Bi2MoO6 thin film was achieved when the MoO3 thin films were subject to hydrothermal treatment in a low Bi(NO3)3.5H2O

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solution concentration. In contrast, a bi-layered Bi2MoO6 / MoO3 thin film photoelectrode comprising pre-dominantly a high temperature-phase γ(H)-Bi2MoO6 oxide-electrolyte interface top region and a MoO3 oxide-collector interface bottom region was formed when a high Bi(NO3)3.5H2O solution concentration was utilized. UV-Vis spectroscopy shows both the γ(L)-Bi2MoO6 (Eg = 2.7 eV) and γ(H)-Bi2MoO6 (Eg = 3.05 eV) thin films exhibit smaller band gaps than MoO3 (Eg = 3.4 eV). For γ(L)-Bi2MoO6. The reduction in optical band gap was attributed to the formation of a higher-lying O 2p valence band maximum while for the γ(H)-Bi2MoO6 the film, hybridisation of the Bi 6s orbitals with the O 2p valence orbitals lowers the potential of the valence band maximum, leading to the reduced band gap. Overall, the Bi2MoO6 thin films with the highest γ(L)-Bi2MoO6 concentration exhibited the highest photocurrent density. The photocurrent enhancement can be attributed to two main reasons: firstly, the tri-layer Bi2MoO6 / MoO3 heterostructure obtained from the direct thin film assembly enables a smooth percolation of photoexcited charges from the surface generation sites to the charge collection sites at the Mo substrate, minimizing charge recombination losses; secondly, the MoO6 octahedra-coordinated γ(L)-Bi2MoO6 possesses a wide conduction band enabling fast separation and migration of delocalized charges. The secondary cation insertion technique has potential as a universal method to prepare complex oxides with narrow band gaps and shallow valence bands from insertion-type oxides for solar energy applications.

1. Introduction The greatest challenge faceing the large scale utilization of solar energy is the storage of the solar energy1. An ideal way to store solar energy for night time and transportation use is by directly splitting water molecules using sunlight into H2 fuel and O21. Photoelectrochemical water splitting is a potential cost-effective method to achieve solar water splitting1-3.


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However, the problem facing this approach is the requirement of an efficient water oxidation anode that is also cheap to manufacture and stable for long periods of application. In general, metal oxides tend to be more stable in aqueous electrolytes than non-oxide semiconductors because of their more positive O 2p valence orbitals (ca. 3.0 eV vs NHE) relative to the water oxidation potential (ca. 1.23 eV vs NHE). Their low-lying O 2p valence band potentials, which are well below the water oxidation (H2O/O2) potential, also make them well suited for O2 evolution from water. Additionally, most metal oxides are earth-abundant and readily available at low cost hence fulfilling the cost requirement. However, many metal oxides have wide band gaps and respond only to UV light. Considering, the photons in an AM 1.5G solar spectrum exist largely within the visible light range (400 nm < λ < 800 nm), it is imperative to develop metal oxide photoanodes which can absorb both UV and visible light from the solar spectrum and achieve more efficient photon energy harvesting and splitting of water molecules. Forming shallow valence bands in wide band gap metal oxides by hybridizing the O 2p valence orbitals with a foreign element with high-lying valence orbitals such as Bi(III), Ag(I), Sn(II), Pb(II) or Cu(I) is a promising strategy to narrow the band gap of metal oxides4-10. Orbitals of Bi 6s in Bi3+, Ag 4d in Ag+, Sn 5s in Sn2+, Pb 6s in Pb2+ and Cu 3d in Cu+ can form a valence band above the O 2p valence band in metal oxide photocatalysts4-10. The degree by which these metal cations contribute to the valence band shift generally depends on the crystal structure and the ratio of the incorporated metal4-10. Recently, ternary metal oxides containing Bi elements, such as BiVO411-13, Bi2WO614 and Bi2MoO615, have shown promise as semiconductor electrodes for solar energy conversion, due to their near-ideal band gaps and band edge positions. Among them, the Aurivillius-phase γ-Bi2MoO6 constitutes two distinct structures: (i) a low temperature phase, layered γ(L)-Bi2MoO6 and (ii) a fluorite-like, high temperature phase γ(H)-Bi2MoO6, which is formed at elevated temperatures16-17. The 3 ACS Paragon Plus Environment


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crystal structure of γ(L)-Bi2MoO6 consists of layers of corner-sharing MoO6 octahedra sandwiched between alternating Bi2O2 layers whereas γ(H)-Bi2MoO6 has a fluorite-related derivative-type framework with each bismuth atom having eight oxygen neighbours and the molybdenum atom surrounded tetrahedrally by four oxygen neighbours, as illustrated in Scheme 1. The γ(L)-Bi2MoO6, with corner-shared MoO6 octahedra, possesses a relatively narrower optical band gap of 2.7 eV whereas γ(H)-Bi2MoO6 with MoO4 tetrahedron coordination exhibits a wider optical band gap of 3.0 eV15,18-20. Kudo and co-workers found γ(L)-Bi2MoO6 to be an active photocatalyst for O2 evolution from water containing a Fe3+/Fe2+ redox couple or AgNO3 under visible light irradiation18-20. However, γ(H)-Bi2MoO6 was an inactive photocatalyst for O2 evolution under both visible and UV light19. Using density functional theorem (DFT), Kudo and co-workers found the visible light response of γ(L)-Bi2MoO6 to be attributable to a charge transition between the O 2p and Mo 4d orbitals within the corner-sharing MoO6 octahedra. In addition, the mobility of delocalized charges within the octahedron-coordinated structure were significantly faster than for the tetrahedrally-coordinated counterpart due to the relatively wider conduction band of the MoO6 octahedron structure, further contributing to the visible light photocatalytic O2 evolution performance19. The discovery of molybdenum-based oxide photocatalysts for O2 evolution under visible light irradiation prompted the present interest to develop and investigate γ-Bi2MoO6 as a thin film-type photoanode for PEC water splitting devices. Conventional methods to prepare Bi-ternary metal oxide electrodes are via powder intermediate techniques, such as by pressing pre-synthesized Bi2MoO6 powders on conducting glass substrates or dipping the conducting glass in amorphous heteronuclear complexes21-24. Other thin film preparation techniques from preformed powders include dropcasting, spin-coating, spray coating and doctor-blading. These methods for assembling metal oxide thin film electrodes from powder precursors are effective but less ideal owing to the 4 ACS Paragon Plus Environment

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generally weaker interaction between the pre-synthesized particles and the conducting substrate. The particles may detach from the conductive surface during applications resulting in deteriorating activities and inaccurate measurements. Consequently, a direct complex oxide thin film assembly technique with improved particle adhesion between the film and the conducting substrate is desirable for preparing photoelectrodes for PEC water splitting14-15, 25. Here, a controllable secondary cation insertion technique to engineer the band gap structures of wide band gap insertion-type metal oxide thin films so as to extend their light absorption into the visible light region is demonstrated. New oxides with reduced optical band gaps and shallow valence bands can be generated. Orthorhomic-phase MoO3 (α-MoO3) is a wide band gap layered structure metal oxide (Eg = 3.4 eV) with ion intercalation properties26-31. Intercalating alkali cations, such as Li+, Na+ and Mg2+, in a MoO3 electrode has already been used to develop high energy density batteries26-28 and solar-chargeable intercalation batteries29-31. We have previously reported a two-step anodization-hydrothermal processing method to prepare Bi2MoO6 and Bi2WO6 thin films14, 15. The as-anodized MoO3 and WO3 thin films were subjected to hydrothermal processing in a bismuth(III) nitrate pentahydrate (Bi(NO3)3.5H2O) solution, as shown in Scheme 1 for Bi2MoO6 thin film synthesis. The pressurized conditions of the hydrothermal process impelled diffusion of the large Bi3+ cations into the Van der Waal’s gaps of pre-synthesized MoO3 thin films or the tunnelled-like channels of WO3 thin films. A final calcination step crystalized and stabilized the resulting films to form crystalline Bi2MoO6 or Bi2WO6 thin films14,15. The direct Bi-ternary oxide thin film synthesis approach by Bi3+ cation insertion into a host oxide framework without needing preformed particles resulted in both a more stable photoelectrode and hybridisation of the Bi 6s and O 2p orbitals, shifting the valence band maximum potential of the parent oxide towards a more negative potential, which narrowed the optical band gap of the native oxide. However, the hydrothermal condition utilized in the previous study was demonstrated to 5 ACS Paragon Plus Environment


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convert MoO3 into γ(H)-Bi2MoO6, where the extent of optical band gap narrowing was limited due to the intrinsic larger band gap of γ(H)-Bi2MoO6. A more controllable reduction in optical band gap can occur if a richer concentration of γ(L)-Bi2MoO6 can be obtained within a Bi2MoO6 thin film. In the present study, phases rich in γ(L)-Bi2MoO6 were prepared from an as-anodized MoO3 thin films by varying the concentration of a bismuth(III) nitrate pentahydrate (Bi(NO3)3.5H2O) solution during hydrothermal treatment (Scheme 1). The Bi2MoO6 / MoO3 (BMO) thin films were fabricated using three different Bi(NO3)3.5H2O hydrothermal solutions (1, 5 and 10 mM Bi(NO3)3.5H2O). Bi3+ cation diffusion into the MoO3 thin films was found to be significantly enhanced when a lower concentration of Bi(NO3)3.5H2O solution was employed during the hydrothermal treatment. Diffusion of Bi3+ cations in the asanodized MoO3 thin film was most favourable using low concentration of bismuth nitrate solution which enables a smoother passage of Bi3+ cations from the MoO3 film surface to the entire depth of the film. Near complete conversion of the as-anodized MoO3 thin film into a Bi2MoO6 thin film was achieved for the 1 mM Bi(NO3)3.5H2O hydrothermal solution. The optical band gaps of the resulting Bi2MoO6 photoanodes were red-shifted from 3.4 eV to 2.7 eV as the concentration of the Bi2MoO6 consitutent increases within the film. In addition, the relative proportion of the γ(L)-Bi2MoO6 with respect to γ(H)-Bi2MoO6 was found to vary with Bi(NO3)3.5H2O precursor solution concentration which influenced the Bi2MoO6 band gap. Structural evolution of the BMO thin films arising from Bi3+ cation insertion was examined by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy elemental dispersive spectroscopy (TEM-EDS). Photoelectrochemical experiments were also performed on the Bi2MoO6 thin film photoanodes to evaluate the band positions and wavelength dependence of the photocurrent.


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2. Results and Discussion 2.1. Crystal Structures and Optical Properties of the Bi2MoO6 Thin Films Fig.1a shows the XRD patterns of the as-prepared MoO3 and Bi2MoO6 thin films. α-MoO3 thin film (ICDD-JCPDS N0. 05-0508) was derived on calcination of the as-anodized MoO3 thin film (no hydrothermal treatment) at 450oC for 2 h, as shown in Fig. 1 a(i). XRD patterns of the Bi2MoO6 thin films obtained from hydrothermal treatment in 10 mM and 5 mM Bi(NO3)3.5H2O solutions (BMO-10 and BMO-5, respectively) show the presence of both Bi2MoO6 and MoO3 diffraction peaks indicating partial conversion of MoO3 to Bi2MoO6, Fig. 1a (ii-iii). Hydrothermal treatment of the as-anodized MoO3 thin film in 10 mM and 5 mM Bi(NO3)3.5H2O solutions yield both γ(H)-Bi2MoO6 (ICDD-JCPDS no. 082-2067) and γ(L)-Bi2MoO6 (ICDD-JCPDS no. 021-0102) constituents. The γ(H)-Bi2MoO6 is observed to be the dominant phase in both films as indicated by the strong intensities of the γ(H)Bi2MoO6 (-341) and (341) diffraction peaks while the (131) peak of γ(L)-Bi2MoO6 appeared as a weak shoulder peak in both samples. The diffraction peaks associated with the (020), (110) and (040) planes of α-MoO3 phase within the BMO-10 and BMO-5 thin films are observed to left-shift to a lower angle compared to the α-MoO3 thin film (with no hydrothermal treatment). Left-shifting of the MoO3 diffraction peaks indicates the layered structure of MoO3 undergoes expansion upon Bi3+ cation intercalation. When the as-anodized MoO3 thin film was hydrothermally treated in a 1 mM Bi(NO3)3.5H2O solution (BMO-1, Fig. 1a, iv), the MoO3 thin film was fully converted to Bi2MoO6 as indicated by the diminished MoO3 peaks and an increased in intensity of the γ(L)-Bi2MoO6 (131) peak. The almost equivalent intensities of the γ(H)-Bi2MoO6 (341) and γ(L)-Bi2MoO6 (131) diffraction peaks indicate the BMO-1 thin film contained nearly equal concentrations of γ(H)- and γ(L)Bi2MoO6. The γ(H)-Bi2MoO6 (341) and γ(L)-Bi2MoO6 (131) diffraction peaks of the BMO-1



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thin film are also better resolved compared to the Bi2MoO6 thin films synthesized at higher Bi(NO3)3.5H2O solution concentrations implying there is both an improved conversion of MoO3 to Bi2MoO6 using a low concentration of Bi(NO3)3.5H2O solution and an improved ordering of the γ-Bi2MoO6 crystal structure within the film. Fig. 1 b and c show the UV-Vis spectra of the as-prepared MoO3 and Bi2MoO6 thin films. The onset absorption of MoO3 is positioned at 365 nm, corresponding to an optical band gap of 3.4 eV. The absorption spectra of BMO thin films are red-shifted from 3.4 eV to 2.7 eV as the proportion of γ-Bi2MoO6 phase increases, as seen in Fig. 1b and c. BMO-10 and BMO-5 thin films, which contained predominantly γ(H)-Bi2MoO6, show strong absorptions at 400 nm and 410 nm, respectively, corresponding to optical band gaps of 3.1 eV and 3.05 eV, respectively, see Fig. 1c, ii and iii. Although the XRD patterns of the BMO-10 and BMO-5 (Fig. 1a, ii-iii) indicate the presence of small amount of γ(L)-Bi2MoO6 within the films, visible light absorption from γ(L)-Bi2MoO6, which is expected at 460 nm and a corresponding bandgap of 2.7 eV, is not observed. Instead secondary onset absorption edges at shorter wavelengths of 420 nm and 430 nm for BMO-10 and BMO-5, respectively, are apparent, see Fig. 1c, ii and iii. The visible light absorption at these wavelengths could arise from an intermediate form of γ(L)-phase Bi2MoO6, which is usually observed when a γ(L)Bi2MoO6 transforms into a γ(H)-Bi2MoO6 by a reconstructive sequence upon heating at elevated temperature and pressure16-17. The BMO-1 thin film shows two strong absorption edges at 460 nm and 415 nm due to the presence of both γ(L)- and γ(H)-Bi2MoO6 in almost equal amounts, (Fig. 1c, iv), in good agreement with the XRD pattern (Fig. 1a, iv). The band gaps of γ(L)- and γ(H)-Bi2MoO6 are estimated to be 2.7 eV and 3.0 eV, respectively (Fig. 1c, iv). On the whole, the optical band gap values measured from the Bi2MoO6 thin films are in good agreement with literature reported values for both γ(L)- and γ(H)-Bi2MoO6 15, 18-20.


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2.2. Morphological Structure of the Bi2MoO6 Thin Films Fig. 2 shows the morphological structures of the BMO thin films. With no hydrothermal treatment, direct calcination of the as-anodized MoO3 thin film resulted in a planar α-MoO3 thin film with a plate-shaped morphology and well-defined (0k0) and (hk0) crystal facets 15, 30-31

. The SEM images of the BMO thin films after hydrothermal and calcination treatments

show two distinct morphological features: (i) a top layer with dense coverage of micron-sized island-like structures, as shown in Fig. 2a; and (ii) an under layer consisting of cuboid-shaped particles, as shown in Fig. 2b. The BMO thin films synthesized using different concentrations of Bi(NO3)3.5H2O solutions show similar cube or cuboid-like structures on the under layer, however, the morphologies of the island structures differ significantly. Fig. 2c-e show SEM images of the cross-section of an island structure of BMO-10, BMO-5 and BMO-1, respectively, taken at a 45o tilt. The top surface of the BMO-10 island structures consists of a conformal layer of fused-quasi-spherical nanoparticles formed above micron-sized platelets. The BMO-5 island structures comprise a significantly higher density of the fused-quasispherical nanoparticles compared to the BMO-10 thin films. Within some regions of the island structures, the fused-quasi-spherical nanoparticles form the island completely while in other regions two distinctive layers of fused-quasi-spherical nanoparticles (top) and micronsized platelets (bottom) are apparent, Fig. 2d. The BMO-1 island structures are entirely different to the BMO-5 and BMO-10 cases. The BMO-1 island structures comprise relatively larger fused-aggregates, Fig. 2e. The micron-sized platelets that were observed for BMO-5 and BMO-10 are not found in the BMO-1 thin film. The contrast in morphologies between the BMO-1 thin film and the BMO-5 and BMO-10 thin films agrees well with the XRD patterns (Fig. 1a), which indicated a complete conversion of MoO3 to Bi2MoO6 for BMO-1 whereas BMO-5 and BMO-10 comprised composite films of MoO3 and Bi2MoO6.



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To elucidate the elemental compositions of the BMO thin films, TEM-EDS line-scan analyses were conducted across the cross-sections of the BMO thin films. The cross-sectional TEM sample was prepared by milling a thin cross-section of the Bi2MoO6 thin film perpendicular to the substrate using Focus Ion Beam/Scanning Electron Microscopy (FIB/SEM). Before the milling process, the thin film surface was covered with a sacrificial protective layer of Pt to protect the top surface of the film from ion beam damage during milling. The TEM images and TEM-EDS line-scan of BMO-10 and BMO-1 thin film crosssections can be seen in Fig. 3. The yellow arrows show the locations on the cross-sectional TEM samples where the TEM-EDS line-scans were conducted. The TEM-EDS line scan profiles indicate the thickness of the BMO-10 and BMO-1 thin films are 2.0 µm and 2.15 µm, respectively. For the BMO-10 thin film, elemental Mo, O and Bi are simultaneously detected from the surface of the film to ~0.35 µm deep, as indicated by region 1 on the BMO-10 line scan profile. This implies the thin conformal layer of fused-quasi-spherical-nanoparticles at the top layer of the island structures observed in the cross-section of the SEM image in Fig. 2c is Bi2MoO6. Beyond the conformal layer, elemental Bi was no longer detected while O and Mo signals remain (region 2 on the line scan profile of BMO-10). This indicates the micron-size platelets observed in the bulk of the island structures and the cuboid-shaped particles of the barrier oxide layer are MoO3. For the BMO-1 thin film, elemental Mo, O and Bi are simultaneously detected from the surface of the film to a depth of ~1.65 µm (region 1 on the line scan profile), indicating the fused-aggregate particles which make-up the island structures of the BMO-1 thin film (SEM image Fig. 2e) are Bi2MoO6. The decreasing concentration of Bi from the film surface to the bulk of the oxide thin film indicates formation of the Bi2MoO6 thin film is facilitated by Bi diffusion into the as-anodized MoO3 thin film during the hydrothermal processing. Beneath the island structures, only Mo and O signals were detected, implying a thin layer of pure10 ACS Paragon Plus Environment

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phase MoO3 (~0.5 µm) exists between the Bi2MoO6 upper-layer (~1.65 µm) and the Mo substrate. Overall, the TEM-EDS line scan profiles show the diffusion of Bi3+ cations into the as-anodized MoO3 thin film is more favorable when the hydrothermal processing is conducted in lower Bi(NO3)3.5H2O solution concentrations. Taken together, the XRD and TEM-EDS line scan results show Bi3+ cation diffusion into the as-anodized MoO3 thin film during hydrothermal processing is governed by the concentration of Bi3+ cations at the solution-oxide interface. When a high concentration of Bi(NO3)3.5H2O solution is present during hydrothermal processing, the Bi3+ concentration gradient at the solution-oxide interface is high leading to a rapid diffusion of Bi3+ cations from the hydrothermal solution into the MoO3 thin film. The fast diffusion rate of the Bi3+ cations into the layered MoO3 structures consequently block the open channels of the MoO3 thin film and impinge further diffusion of Bi3+ cations into the film. As a result, the formation of Bi2MoO6 is confined to the surface of the film. In contrast, when a low concentration of Bi(NO3)3.5H2O solution is employed during the hydrothermal processing, the Bi3+ concentration gradient at the solution-oxide interface is comparatively lower. This reduces the diffusion rate of Bi3+ cations into the MoO3 thin film enabling a smoother passage of Bi3+ cations from the metal oxide surface into the entire depth of the film. A near complete conversion of the MoO3 into Bi2MoO6 was achieved for a 1 mM Bi(NO3)3.5H2O solution. The subtle diffusion of Bi3+ cations at a low Bi(NO3)3.5H2O solution concentration facilitates both a higher conversion of MoO3 to Bi2MoO6 and allows the layered structure of the parent MoO3 thin film to be better preserved. This is demonstrated by the XRD patterns in Fig 1a, which show a higher proportion of layered γ(L)-Bi2MoO6 is obtained as the concentration of the Bi(NO3)3.5H2O solution is lowered during hydrothermal treatment. 2.3. Photoelectrochemical Properties



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Fig. 4 shows the dark and illuminated current-voltage profiles of the MoO3 and BMO thin films. The four electrodes show a similar onset potential for photocurrent generation at ~0.93 V vs Ag/AgCl (pH 7). The onset potential, defined as the minimum potential required for a continuous anodic photocurrent to be measured, can be used to indicate the flat-band potential of a n-type semiconductor32. In brief, at potentials negative to the flat-band potential, an accumulation layer exists, and the electrode acts as a cathode, so reductive currents are measured both in the dark and under illumination, Fig. 4. At potentials that are positive relative to the flat-band potential a depletion layer exists so there can be no oxidative current in the dark. However, illuminating the electrode with a photon energy greater than the optical band gap width of the semiconductor can induce a photocurrent at a potential that is negative relative to the redox potential of the electrolyte, as some of energy required for oxidation is now provided by the light. Therefore, n-type semiconductors are dark cathodes and photoanodes and, by the same reasoning, p-type semiconductors are dark anodes and photocathodes. As the flat-band potential of an n-type semiconductor is located at a very close proximity to its conduction band, the flat-band potential can be used to estimate the conduction band potential of a n-type semiconductor. Thus, the four electrodes show a similar conduction band potential of approx. -0.93 V vs Ag/AgCl (pH 7) or -0.31 eV vs NHE (pH = 0), Fig. 4. Therefore, the reduction in optical band gaps of the Bi2MoO6 thin films as seen in the diffused reflection spectroscopy, Fig. 1b-c, can be attributed to a negative shift of the valence band potentials of the BMO thin films. The conduction and valence band structures of the MoO3 and BMO-10, BMO-5 and BMO-1 thin films can be derived by combining the optical band gaps of the corresponding thin films and their conduction band potentials, as illustrated in Fig. 5a. To elucidate the contribution of the orbitals to the conduction and valence bands of the BMO thin films and the mechanism for the reduction in optical band gap, a DFT model for γ(L)12 ACS Paragon Plus Environment

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Bi2MoO6 and γ(H)-Bi2MoO6, reported by Kudo and co-workers, was utilized19. Figure 5b illustrates the band structures of MoO3, γ(H)-Bi2MoO6 and γ(L)-Bi2MoO6. Pure-phase MoO3 is a wide band gap layered semiconductor structure consisting of Mo 4d and O 2p conduction and valence orbitals. The γ(H)-Bi2MoO6 possesses a fluorite-related structure formed by MoO4 tetrahedra. DFT indicates the valence band of γ(H)-Bi2MoO6 consists mainly of O 2p orbitals. However, the top of the valence band (HOMO) is also partly formed by a hybridisation of the Bi 6s orbitals with the O 2p orbitals, as seen in BiVO4 and Bi2WO6. The conduction band of γ(H)-Bi2MoO6 is predominantly derived from Mo 4d orbitals with a secondary contribution from Bi 6p. Hence, the Bi 6s orbitals of the γ(H)-Bi2MoO6 contribute to the negative shift in the valence band resulting in a narrowing of its optical band gap. For γ(L)-Bi2MoO6, which comprises a layered structure formed by alternating layers of cornersharing MoO6 octahedra and Bi2O2 layers, DFT indicates the valence band top (HOMO) of γ(L)-Bi2MoO6 is comprised of only the O 2p orbitals while the Bi 6s orbital band is located below the HOMO. This is unlikely the case for γ(H)-Bi2MoO6, where the HOMO is formed by hybridisation of the Bi 6s and O 2p orbitals. In contrast, the conduction band of the γ(L)Bi2MoO6 consists of hybridised Mo 4d orbitals and Bi 6p orbitals, similar to γ(H)-Bi2MoO6. Therefore, the visible light absorption property of γ(L)-Bi2MoO6 can be attributed to a charge transition from the valence band comprising the O 2p orbitals to the conduction band derived from Mo 4d orbitals and Bi 6p orbitals. In addition, the density contour maps for various bismuth molybdates show the Mo 4d orbitals of MoO6 octahedra typically invoke a wider conduction band than structures containing MoO4 tetrahedra. Consequently, bismuth molybdates containing higher degrees of MoO6 octahedra typically exhibit smaller optical band gaps than their tetrahedrally-coordinated counterparts. Hence, γ(L)-Bi2MoO6 with corner-sharing MoO6 octahedra shows a smaller optical band gap than γ(H)-Bi2MoO6 with MoO4 tetrahedra coordination. On the whole, the DFT indicates the optical band gap red-



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shifting exhibited by the BMO thin films (from BMO-10 to BMO-5 and BMO-1) is attributable to the formation of higher-lying valence band maxima in association with the increasing ratio of the γ(L)-Bi2MoO6 with respect to the γ(H)-Bi2MoO6 within the films. For γ(L)-Bi2MoO6, the negative shift in the valence band and narrowing of the optical band gap is attributed to higher-lying O 2p orbitals while in the case of γ(H)-Bi2MoO6, hybridisation of the Bi 6s and O 2p orbitals lowers the potential of the valence band maximum, which reduces the optical band gap. Fig. 6a provides the chrono-amperometry measurements of BMO-1, 5 and 10 thin films under visible light illumination (λ ≥ 420 nm) at an applied bias of 0.1 V vs Ag/AgCl and 50s light-on/100s light-off illumination cycles. The BMO-10 thin film, which contains the highest proportion of γ(H)-Bi2MoO6 relative to γ(L)-Bi2MoO6, exhibits the lowest photocurrent density (~55 µA/cm2) due to the larger band gap of γ(H)-Bi2MoO6. As the relative proportion of the smaller band gap γ(L)-Bi2MoO6 constituent in the film increases (BMO-10 < BMO-5 < BMO-1), the light harvesting property of the film improves and the photocurrent density of the film increases. The BMO-1 thin film exhibited the highest photocurrent density of ~150 µA/cm2, which is almost three times higher than the BMO-10 thin film. The BMO-5 thin film imparts an intermediate photocurrent density of ~100 µA/cm2, which is around twice that of the BMO-10 thin film. Fig. 6b shows the chrono-amperometry measurements of the BMO-1 thin film and MoO3 thin film under full spectrum illumination from a 300 W Xe lamp at a higher applied bias of 0.4 V vs Ag/AgCl whilst similar 50s light-on/100s light-off illumination cycle. With a wide band gap of 3.4 eV, MoO3 can only be photoexcited by the UV component of the Xe lamp spectrum. The BMO-1 thin film exhibited a photocurrent density of ~580 µA/cm2, which is about ~3.5 times higher than that of MoO3 thin film at ~160 µA/cm2. The photocurrent


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densities of the Bi2MoO6 cells are also reproducible on repeated light-chopping indicating the stability of the Bi2MoO6 and MoO3 particles for light harvesting, Fig. 6a-b. The high photocurrent performance of the BMO-1 thin film can be attributed to both the smaller optical band gap of γ(L)-Bi2MoO6, which enables enhanced light-harvesting, and, more importantly, an enhanced charge transport across the tri-layer Bi2MoO6 / MoO3 heterostructure interfaces that are formed directly on the Mo foil electron collecting substrate, which minimizes charge recombination losses. The ease of charge mobility within the octahedron-coordinated γ(L)-Bi2MoO6, due to the wider conduction band of a MoO6 octahedron, also plays an important role in the enhancing photocurrent generation by the BMO-1 thin film. Fig, 6c shows the wavelength-dependent photocurrent action spectrum together with the UV-vis absorption spectrum of the BMO-1 thin film. The photocurrent spectrum of the BMO-1 thin film closely matches the film’s absorption spectrum, confirming the photocurrent originates from band gap excitation of the Bi2MoO6 and not from a parasitic reaction. Compared to a γ(L)-Bi2MoO6 thin film prepared by pressing a hydrothermallysynthesized powder onto conducting ITO-glass24, the BMO-1 thin film delivers superior photocurrent activity. The photocurrent action spectra of the pressed-powder γ(L)-Bi2MoO6 electrode under chopped irradiation in a 0.5 M Na2SO4 electrolyte, at 0.5 V vs. Ag/AgCl and under a wavelength range of 320 nm to 500 nm displays photocurrent densities in the range of 2 – 15 uA cm-2. In our study, the photocurrent action spectra were collected at a lower voltage (0.1 V vs Ag/AgCl), a more dilute (0.1 M Na2SO4) electrolyte whilst under chopped irradiation over a similar wavelength range (340 nm to 540 nm). Despite the less favorable experimental conditions, the photocurrent densities of the BMO-1 thin film are an order of magnitude higher than those for the pressed γ(L)-Bi2MoO6 electrode indicating the effectiveness of our direct thin film synthesis approach in assembling a robust ternary Bi2MoO6 anode for photoelectrochemical water splitting. However, the dark current densities



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of the BMO thin film cells are observed to rise with the light chopping, as shown in the chrono-amperometry profiles in Fig. 6a and b. In contrast, the dark current density of the MoO3 cell (without hydrothermal treatment) is stable, Fig. 6b. The rising dark current densities of the BMO thin film cells with repeated on-off illuminations are likely caused by oxidation of the Mo foils underneath the oxide structures. As the dark current density of the MoO3 thin film cell is stable, it implies the relatively thin barrier oxide layers of the BMO thin films are not sufficiently thick to protect the Mo foils from contact with the electrolyte, causing the Mo foil oxidation and contributing to the dark current. Further studies on suppressing the dark current of the BMO thin films are currently underway. Nevertheless, the secondary cation insertion technique presented in this work is effective for engineering the band gap structure of wide band gap insertion oxides, turning them into complex oxides with reduced optical band gaps and improved light-to-current conversion performance. Not limited to anodized metal foil, this secondary cation insertion technique can be a universal method to prepare complex oxides with narrow band gaps and shallow valence bands for solar energy applications. For example, Kudo and co-workers have recently developed a visible-lightresponsible CuLi1/3Ti2/3O2 (band gap 2.1 ev) photoabsorber for sacrificial H2 evolution by the intercalation of Cu+ into monoclinic Li2TiO35. Beside Bi(III) and Cu(I), Ag(I), Sn(II) and Pb(III), which exhibit high-lying valence orbitals, are also potential elements that can be investigated to form new insertion oxides with reduced band gaps and shallow valence bands4-10. 3.

Conclusions

The findings presented here detail a secondary cation insertion technique to prepare ternary Bi2MoO6 thin films directly from layered MoO3 thin films by the controllable insertion of Bi3+ cations into pre-anodized MoO3 thin films. Bi3+ cation diffusion into the Van der Waal’s


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gaps of MoO3 thin films was significantly enhanced when the anodized MoO3 thin films were hydrothermally processed in low Bi(NO3)3.5H2O solution concentrations. The optical band gaps of the resulting Bi2MoO6 / MoO3 thin films were red-shifted as the concentration of the γ(L)-phase Bi2MoO6 constituent increases within the films. The reduction in optical band gap width of the BMO thin films with increased γ(L)- and γ(H)-Bi2MoO6 concentrations was attributed to the formation of higher-lying O 2p and hybridized Bi 6s and O 2p valence band maxima, respectively. The Bi2MoO6 / MoO3 thin films with a higher γ(L)-Bi2MoO6 concentration exhibited higher photocurrent densities owing to the enhanced mobility of the delocalized charges within the MoO6 octahedron structure, improved charge transport across the continuous tri-layered heterostructure interfaces (Bi2MoO6, MoO3 and Mo substrate which minimize charge recombination losses), a reduced optical band gap and an improved light harvesting capacity. The secondary cation insertion technique can be a versatile method for assembling new complex oxides with a reduced band gap and shallow valence band for solar energy harvesting applications. Experimental Information Thin film synthesis and characterization An amorphous MoO3 (A-MoO3) thin film was synthesised by anodising a Mo foil (SigmaAldrich, ≥99.99, 0.1 mm thick) in 0.5 wt% NaF (Sigma-Aldrich, 99.99%) aqueous electrolyte at a voltage of 2.5 V for 20 min. Prior to anodisation, the Mo foils were polished using sandpaper to remove any oxide resulting from ambient oxidation. The polished Mo foils were degreased by sonicating twice in acetone, followed by rinsing with acetone, ethanol and distilled water and then dried in an oven at 110oC. The anodization reactor comprised a twoelectrode cell with a platinum foil as the counter electrode and the pretreated Mo foil as the working electrode. The Mo foil was positioned at the base of the anodization reactor, 17 ACS Paragon Plus Environment


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contacted with a copper spring and pressed against a rubber O-ring to expose an area of 4.15 cm2 to the electrolyte. To fabricate the Bi2MoO6 thin films, the as-anodized MoO3 thin films were immersed in 1, 5 and 10 mM of bismuth nitrate solution (Bi(NO3)3.5H2O, Sigma-Aldrich, 99%) prepared by mixing solid Bi(NO3)3.5H2O in 70 mL of deionised water. The mixtures were initially sonicated for 10 minutes followed by vigorous magnetic stirring for 30 minutes. The pH of the resulting bismuth nitrate solutions was adjusted to 9 using aqueous ammonia solution (28 wt%) so as to preserve the Mo foil, which will otherwise dissolve in the strongly acidic Bi(NO3)3.5H2O solution (pH < 1). The as-anodized MoO3 thin film and bismuth nitrate precipitate solution was added to a Teflon-lined hydrothermal reactor with the hydrothermal reaction performed for 24 hours at 180oC. Thermal treatment of the hydrothermally treated MoO3 thin films was conducted at a temperature of 450oC in air for 2 h at a ramping rate of 2.5oC per min to obtain the crystalline Bi2MoO6 thin films. The crystalline MoO3 (α-MoO3) thin film was obtained by calcining the as-anodized MoO3 thin film at a temperature of 450oC in air for 2 h at a ramping rate of 2.5oC per min. Crystalline structures of the produced thin films were characterized by glancing angle X-ray diffraction using a Cu Kα radiation (λ = 1.54 Å) source with a potential of 40 kV and a current of 30 mA. Scanning electron micrographs (SEM) of the thin films were taken using a NanoSEM 450 operating at an accelerating voltage of 5 kV. Transmission electron microscopy (TEM) analysis was performed on a Phillips CM200 facility. Thin film sample preparation for TEM-EDS analysis was conducted using a Carl Zeiss focused ion-beam scanning electron microscope (FIB-SEM). Diffuse reflection ultraviolet and visible (DRUVvis) spectra of the samples were recorded using a Shimadzu UV-3600 Spectrophotometer.


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The reflectance of the sample was measured and the corresponding absorbance (F(R)) was calculated using Kubelka-Munk theory. Photoelectrochemical measurements Chrono-amperometry and cyclic-voltammetry (CV) analyses were performed in a 0.1 M sodium sulphate (Na2SO4, Fluka, ≥ 99%) electrolyte at room temperature using an Autolab potentiostat (Model PGSTAT302N) in a three-electrode PEC cell with Pt as the counter electrode, Ag/AgCl as the reference electrode and α-MoO3 or Bi2MoO6 thin films as the working electrode. The electrolyte solution was purged with nitrogen gas for 10 min prior to measurement and the purging was continued for the duration of photocurrent measurements to remove any dissolved oxygen in the cell. Illumination was provided by a 300 W Xe lamp (Perkin Elmer, CERMAX, LC-300BUV). To remove any heating of the electrolyte solution during UV-illumination, a water jacket was placed between the Xe lamp and PEC cell. The illuminated area was 1 cm-2. Wavelength dependent photocurrent measurements of the samples were taken in a 0.1 M Na2SO4 electrolyte solution at a voltage of 0.1 V vs Ag/AgCl at different wavelengths (λ ≥ 320, 345, 365, 400, 420, 435, 450, and 500 nm) by applying different cut-off filters to the 300 W Xe lamp. Illumination was provided for 50 s in each cycle.



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Scheme 1: The top row shows the material flowchart to fabricate a Bi2MoO6 thin film directly on a metal substrate by anodizing Mo foil and a subsequent hydrothermal treatment of the as-anodized MoO3 thin film in an aqueous Bi-containing nitrate solution at 180oC for 24 hours. Bottom row shows a cartoon illustration of the Bi2MoO6 thin film synthesis process whereby Mo foil anodization forms layered MoO3. Bismuth insertion into the layered MoO3 structure occurs during the hydrothermal reaction. A final calcination process crystallizes the oxide to form a γ(L) / γ(H) Bi2MoO6 composite thin film. Formation of layered γ(L)Bi2MoO6 is favoured when a low concentration of bismuth(III) nitrate solution is applied during the hydrothermal reaction. A fluorite-like γ(H)-Bi2MoO6 phase with a channelled structure is pre-dominantly formed when a high concentration of bismuth(III) nitrate solution is utilized in the hydrothermal processing. (Grey octahedron represents MoO6 octahedron, pink and red spheres represent Bi atom and O atom respectively)


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Figure 1: (a) XRD patterns of (i) the MoO3 thin film obtained after calcining the as-anodized MoO3 thin film at 450oC for 2 h and (ii-iv) the Bi2MoO6 thin films obtained after hydrothermally-treating the as-anodized MoO3 thin films in different solution concentrations of Bi(NO3)3.5H2O with subsequent calcination at 450oC for 2h, (ii) 10 mM (BMO-10), (iii) 5 mM (BMO-5) and (iv) 1 mM (BMO-1); UV-Vis absorption spectra of MoO3 and Bi2MoO6 thin films (BMO-10, BMO-5 and BMO-1) (b) overlaid and (c) as individual spectra (i) MoO3, (ii) BMO-10, (iii) BMO-5 and (iv) BMO-1 highlighting the various absorption edges.



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Figure 2: (a) Representative plain-view SEM image of the Bi2MoO6 thin films obtained after hydrothermal processing of the as-anodized MoO3 thin films in various Bi(NO3)3.5H2O solution concentrations with subsequent calcination at 450oC for 2 h; (b) cube-like particles beneath the island-like structures (BMO-1); SEM images showing a cross-section of an island structure of the Bi2MoO6 thin films for (c) BMO-10, (d) BMO-5 and (e) BMO-1.


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Figure 3: TEM images of the cross-section of the BMO thin films prepared by FIB/SEM microscopy and corresponding TEM EDS line-scan profile of the BMO thin films, (Top) BMO-1 and (Bottom) BMO-10. A sacrificial layer of Pt was deposited on the film surface during FIB/SEM microscopy to protect the film surface during milling.



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Figure 4: Current-voltage curves under dark and light conditions for (a) MoO3, (b) BMO-1, (c) BMO-5 and (d) BMO-10 thin films (scan rate of 20 mV s-1 in 0.1 M Na2SO4 aq. electrolyte vs. Ag/AgCl, illumination source: 300 W Xe lamp).


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Figure 5: (a) Conduction and valence band structures of MoO3, BMO-10, BMO-5 and BMO-1 thin films and (b) band structure of MoO3, γ(L)-Bi2MoO6 and γ(H)-Bi2MoO6 derived from DFT ref. 19.



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Figure 6: (a) Chrono-amperometry of BMO-1, BMO-5 and BMO-10 thin films (in 0.1 M Na2SO4 aq. electrolyte, illumination source: 300 W Xe lamp, 50 s illumination cycles and applied voltage vs. Ag/AgCl at 0.1 V); (b) Chrono-amperometry of BMO-1 and MoO3 (in 0.1 M Na2SO4 aq. electrolyte, illumination source: 300 W Xe lamp, 50 s illumination cycles and applied voltage vs. Ag/AgCl at 0.4 V); and (c) UV-Vis absorption spectra together with the photocurrent action spectra (0.1 M Na2SO4 aq. electrolyte, applied potential of 0.1 V vs Ag/AgCl, 300 W Xe lamp with cut-off filters, λ ≥ 345, 365, 370, 400, 420, 435, 450, 520 and 530 nm). 26 ACS Paragon Plus Environment

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ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author E-mail: [email protected]; [email protected]; [email protected] Author Contributions Y.H Ng and S.N. Lou conceived the idea. S.N. Lou developed the experiment plan. S.N. Lou conducted the experiments and wrote the manuscript. All authors have given approval to the final version of the manuscript. Funding Sources This work was financially supported by the Australian Research Council under the Laureate Fellowship Scheme – FL140100081. ACKNOWLEDGMENT The authors appreciate the facility and technical support provided by the UNSW Mark Wainwright Analytical Centre. In particular, Mr. Yin Yao from the UNSW’s Electron Microscopy Unit for his generous help in FIB/SEM microscopy.

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Concentration-mediated Band Gap Reduction of Bi2MoO6

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