Oriented CuWO₄ Films for Improved Photoelectrochemical Water Splitting

Abstract

Hydrogen generation through photoelectrochemical (PEC) technology is one of the most appropriate ways for delivering sustainable fuel. Simultaneously, anisotropic properties will be exhibited by the materials with low crystal symmetry, allowing the tuning of the PEC properties by controlling the crystallographic orientation and exposed facets. Therefore, we synthesized copper tungstate films (CuWO 4 ) with highly exposed (100) crystal facets by regulating anions in the precursor solution. According to experimental characterization and density functional theory calculations, the CuWO 4 film with a high exposure ratio of the (100) crystal facet has promoted charge transport with trapfree mode and reduced recombination of electrons and holes. Meanwhile, the oxygen evolution reaction is promoted on the (100) facet because of the relatively low energy barrier. Compared to the CuWO 4 with other mixed exposure facets, CuWO 4 with a highly exposed (100) facet presents a twofold current density (0.38 mA/cm 2 ) and one-fifteenth electron transit time (0.698 ms) and also has great stability (more than 6 h). These results provide an easy way to enhance the PEC performance by modulating the exposure facets of the film electrode.

Enhanced Photoactivity by Doping of Cu–W Oxide-Based Photoanodes by Trivalent Al3+, Ga3+, and In3+ Ions─A Combinatorial Study

“…69,70 Thus, the enhanced photoactivity of Cu 45 In 1 Ga 3 W 51 O x and Cu 46 In 2 Ni 2 W 50 O x can also be promoted by the preferential formation of (100) facets that also show lower charge transfer resistance in undoped CuWO 4 . 68 The observed unchanged morphology of Cu 46 In 2 Ni 2 W 50 O (Figure 9M,N) suggests a high stability in alkaline solution, which was further confirmed by photocurrent transients for 30 min (Figure 9O). The measured photocurrent remains stable over the duration of the experiment and does not show a decay.…”

“…The different intensities for different crystal planes correlate with the dominating facet at the crystal surfaces of the materials, which also correspond to different crystal morphologies in the SEM images in Figure . The crystallization process and thus the preferential formation of particular facets can be influenced by the addition of foreign atoms (such as cations or anions) to the precursor solution. , Thus, the enhanced photoactivity of Cu 45 In 1 Ga 3 W 51 O x and Cu 46 In 2 Ni 2 W 50 O x can also be promoted by the preferential formation of (100) facets that also show lower charge transfer resistance in undoped CuWO 4 …”

Nature of Charge Carrier Recombination in CuWO4 Photoanodes for Photoelectrochemical Water Splitting

“…Recently, facet control along the 100 crystal facet showed improved conductivity and enhanced current density (0.38 mA cm −2 ) in bulk 1.5 μm thick CuWO 4 films. 43 Nanostructuring provides efficient charge carrier transport. 59 Indeed, CuWO 4 electrodes with nanoflake morphology 22,29,[44][45][46][47]52 facilitate hole extraction from the oxide flakes because their 30−60 nm width is within the hole diffusion length in CuWO 4 .…”

“…The thickness of the best-performing photoanode (580 nm) is similar to that of transparent electrodes prepared by Tian et al with a spin-coating synthesis (500 nm), suggesting that the optimal balance between absorption and charge separation in undoped, bulk, and polycrystalline CuWO 4 is within 5–600 nm. Recently, facet control along the 100 crystal facet showed improved conductivity and enhanced current density (0.38 mA cm –2 ) in bulk 1.5 μm thick CuWO 4 films …”

Interfacial engineering of CuWO4/WO3 thin films precisely fabricated by ultrasonic spray pyrolysis for improved solar water splitting

“…Among the potential semiconductors for paring with CuWO 4 , WO 3 is one of the optimal ones due to the following merits: (1) the close chemical composition ensures good compatibility with CuWO 4 ; 29 (2) WO 3 shows an intrinsic long hole diffusion length and high mobility; 30,31 (3) the bandgap of WO 3 matches well with that of CuWO 4 because of the similar conduction band contributed by W 5d and the slightly different valence band position; 32 (4) the well-matched lattice parameter with CuWO 4 is favorable for establishing stable interfaces with a strong built-in electric field, enabling improved photogenerated charge separation and transfer (see discussion below). 33 Although there were several reports for the construction of the CuWO 4 /WO 3 heterojunction, [34][35][36][37] the underlying molecular mechanism for improved electrocatalytic PEC performance is still unclear and needs to be further clarified. While several studies have reported the construction of CuWO 4 /WO 3 heterojunctions, the molecular mechanism underlying the improved electrocatalytic PEC performance remains unclear and requires further elucidation.…”