Dual-Functional Surfactant-Templated Strategy for Synthesis of an In Situ N2 -Intercalated Mesoporous WO3 Photoanode for Efficient Visible-Light-Driven Water Oxidation

Cs-Doped WO3 with Enhanced Conduction Band for Efficient Photocatalytic Oxygen Evolution Reaction Driven by Long-Wavelength Visible Light

Cesium doped WO3 (Cs-WO3) photocatalyst with high and stable oxidation activity was successfully synthesized by a one-step hydrothermal method using Cs2CO3 as the doped metal ion source and tungstic acid (H2WO4) as the tungsten source. A series of analytical characterization tools and oxygen precipitation activity tests were used to compare the effects of different additions of Cs2CO3 on the crystal structure and microscopic morphologies. The UV-visible diffuse reflectance spectra (DRS) of Cs-doped material exhibited a significant red shift in the absorption edge with new shoulders appearing at 440-520 nm. The formation of an oxygen vacancy was confirmed in Cs-WO3 by the EPR signal, which can effectively regulate the electronic structure of the catalyst surface and contribute to improving the activity of the oxygen evolution reaction (OER). The photocatalytic OER results showed that the Cs-WO3-0.1 exhibited the optimal oxygen precipitation activity, reaching 58.28 µmol at 6 h, which was greater than six times higher than that of WO3-0 (9.76 μmol). It can be attributed to the synergistic effect of the increase in the conduction band position of Cs-WO3-0.1 (0.11 V) and oxygen vacancies compared to WO3-0, which accelerate the electron conduction rate and slow down the rapid compounding of photogenerated electrons-holes, improving the water-catalytic oxygen precipitation activity of WO3.

Temperature-Controlled Transformation of WO3 Nanowires into Active Facets-Exposed Hexagonal Prisms toward Efficient Visible-Light-Driven Water Oxidation

A unique transformation of WO₃ nanowires (NW-WO₃) into hexagonal prisms (HP-WO₃) was demonstrated by tuning the temperature of the (N₂H₄)WO₃ precursor suspension prepared from tungstic acid and hydrazine as a structure-directing agent. The precursor preparation at 20 °C followed by calcination at 550 °C produced NW-WO₃ nanocrystals (ca. <100 nm width, 3-5 μm length) with anisotropic growth of monoclinic WO₃ crystals to (002) and (200) planes and a polycrystalline character with randomly oriented crystallites in the lateral face of nanowires. The precursor preparation at 45 °C followed by calcination at 550 °C produced HP-WO₃ nanocrystals (ca. 500-1000 nm diameter) with preferentially exposed (002) and (020) facets on the top-flat and side-rectangle surfaces, respectively, of hexagonal prismatic WO₃ nanocrystals with a single-crystalline character. The HP-WO₃ electrode exhibited the superior photoelectrochemical (PEC) performance for visible-light-driven water oxidation to that for the NW-WO₃ electrode; the incident photon-to-current conversion efficiency (IPCE) of 47% at 420 nm and 1.23 V vs RHE for HP-WO₃ was 3.1-fold higher than 15% for the NW-WO₃ electrode. PEC impedance data revealed that the bulk electron transport through the NW-WO₃ layer with the unidirectional nanowire structure is more efficient than that through the HP-WO₃ layer with the hexagonal prismatic structure. However, the water oxidation reaction at the surface for the HP-WO₃ electrode is more efficient than the NW-WO₃ electrode, contributing significantly to the superior PEC water oxidation performance observed for the HP-WO₃ electrode. The efficient water oxidation reaction at the surface for the HP-WO₃ electrode was explained by the high surface fraction of the active (002) facet with fewer grain boundaries and defects on the surface of HP-WO₃ to suppress the electron-hole recombination at the surface.

Controllable Synthesis of N2-Intercalated WO3 Nanorod Photoanode Harvesting a Wide Range of Visible Light for Photoelectrochemical Water Oxidation

A highly efficient visible-light-driven photoanode, N2-intercalated tungsten trioxide (WO3) nanorod, has been controllably synthesized by using the dual role of hydrazine (N2H4), which functioned simultaneously as a structure directing agent and as a nitrogen source for N2 intercalation. The SEM results indicated that the controllable formation of WO3 nanorod by changing the amount of N2H4. The β values of lattice parameters of the monoclinic phase and the lattice volume changed significantly with the nW: nN2H4 ratio. This is consistent with the addition of N2H4 dependence of the N content, clarifying the intercalation of N2 in the WO3 lattice. The UV-visible diffuse reflectance spectra (DRS) of N2-intercalated exhibited a significant redshift in the absorption edge with new shoulders appearing at 470–600 nm, which became more intense as the nW:nN2H4 ratio increased from 1:1.2 and then decreased up to 1:5 through the maximum at 1:2.5. This addition of N2H4 dependence is consistent with the case of the N contents. This suggests that N2 intercalating into the WO3 lattice is responsible for the considerable red shift in the absorption edge, with a new shoulder appearing at 470−600 nm owing to formation of an intra-bandgap above the VB edges and a dopant energy level below the CB of WO3. The N2 intercalated WO3 photoanode generated a photoanodic current under visible light irradiation below 530 nm due to the photoelectrochemical (PEC) water oxidation, compared with pure WO3 doing so below 470 nm. The high incident photon-to-current conversion efficiency (IPCE) of the WO3-2.5 photoanode is due to efficient electron transport through the WO3 nanorod film.

Fabrication of an Efficient N, S Co-Doped WO3 Operated in Wide-Range of Visible-Light for Photoelectrochemical Water Oxidation

In this work, a highly efficient wide-visible-light-driven photoanode, namely, nitrogen and sulfur co-doped tungsten trioxide (S-N-WO₃), was synthesized using tungstic acid (H₂WO₄) as W source and ammonium sulfide ((NH₄)₂S), which functioned simultaneously as a sulfur source and as a nitrogen source for the co-doping of nitrogen and sulfur. The EDS and XPS results indicated that the controllable formation of either N-doped WO₃ (N-WO₃) or S-N-WO₃ by changing the n_W:n_(NH4)2S ratio below or above 1:5. Both N and S contents increased when increasing the n_W:n_(NH4)2S ratio from 1:0 to 1:15 and thereafter decreased up to 1:25. The UV-visible diffuse reflectance spectra (DRS) of S-N-WO₃ exhibited a significant redshift of the absorption edge with new shoulders appearing at 470–650 nm, which became more intense as the n_W:n_(NH4)2S ratio increased from 1:5 and then decreased up to 1:25, with the maximum at 1:15. The values of n_W:n_(NH4)2S ratio dependence is consistent with the cases of the S and N contents. This suggests that S and N co-doped into the WO₃ lattice are responsible for the considerable redshift in the absorption edge, with a new shoulder appearing at 470–650 nm owing to the intrabandgap formation above the valence band (VB) edge and a dopant energy level below the conduction band (CB) of WO₃. Therefore, benefiting from the S and N co-doping, the S-N-WO₃ photoanode generated a photoanodic current under visible light irradiation below 580 nm due to the photoelectrochemical (PEC) water oxidation, compared with pure WO₃ doing so below 470 nm.

Visible Light-Driven Water Oxidation on an In Situ N2 -Intercalated WO3 Nanorod Photoanode Synthesized by a Dual-Functional Structure-Directing Agent

With a view to developing a photoanode for visible light-driven water oxidation in solar water splitting cells, pure-monoclinic WO₃ nanorod crystals with N₂ intercalated into the lattice were synthesized by using hydrazine with a dual functional role-as an N atom source for the in situ N₂ intercalation and as a structure-directing agent for the nanorod architecture-to gain higher incident photon-to-current conversion efficiency at 420 nm than with most previously reported WO₃ electrodes.

Simple eco-friendly synthesis of the surfactant free SnS nanocrystal toward the photoelectrochemical cell application

A simple, low cost, non-toxic and eco-friendly pathway for synthesizing efficient sunlight-driven tin sulfide photocatalyst was studied. SnS nanocrystals were prepared by using mechanical method. The bulk SnS was obtained by evaporation of SnS nanocrystal solution. The synthesized samples were characterized by using XRD, SEM, TEM, UV-vis, and Raman analyses. Well crystallized SnS nanocrystals were verified and the electrochemical characterization was also performed under visible light irradiation. The SnS nanocrystals have shown remarkable photocurrent density of 7.6 mA cm^-2 under 100 mW cm^-2 which is about 10 times larger than that of the bulk SnS under notably stable operation conditions. Furthermore, the SnS nanocrystals presented higher stability than the bulk form. The IPCE(Incident photon to current conversion efficiency) of 9.3% at 420 nm was obtained for SnS nanocrystal photoanode which is strikingly higher than that of bulk SnS, 0.78%. This work suggests that the enhancement of reacting area by using SnS nanocrystal absorbers could give rise to the improvement of photoelectrochemical cell efficiency.

Insight into Charge Separation in WO3/BiVO4 Heterojunction for Solar Water Splitting

Recently, the WO₃/BiVO₄ heterojunction has shown promising photoelectrochemical (PEC) water splitting activity based on its charge transfer and light absorption capability, and notable enhancement of the photocurrent has been achieved via morphological modification of WO₃. We developed a graft copolymer-assisted protocol for the synthesis of WO₃ mesoporous thin films on a transparent conducting electrode, wherein the particle size, particle shape, and thickness of the WO₃ layer were controlled by tuning the interactions in the polymer/sol-gel hybrid. The PEC performance of the WO₃ mesoporous photoanodes with various morphologies and the individual heterojunctions with BiVO₄ (WO₃/BiVO₄) were characterized by measuring the photocurrents in the absence/presence of hole scavengers using light absorption spectroscopy and intensity-modulated photocurrent spectroscopy. The morphology of the WO₃ photoanode directly influenced the charge separation efficiency within the WO₃ layer and concomitant charge collection efficiency in the WO₃/BiVO₄ heterojunction, showing the smaller sized nanosphere WO₃ layer showed higher values than did the plate-like or rod-like one. Notably, we observed that photocurrent density of WO₃/BiVO₄ was not dependent on the thickness of WO₃ film or its charge collection time, implying slow charge flow from BiVO₄ to WO₃ can be a crucial issue in determining the photocurrent, rather than the charge separation within the nanosphere WO₃ layer.