Construction of a ZnO Heterogeneous Structure Using Co3O4 as a Co-Catalyst to Enhance Photoelectrochemical Performance

Recently, heterostructured photocatalysts have gained significant attention in the field of photocatalysis due to their superior properties compared to single photocatalysts. One of the key advantages of heterostructured photocatalysts is their ability to enhance charge separation and broaden the absorption spectrum, thereby improving photocatalytic efficiency. Zinc oxide is a widely used n-type semiconductor with a proper photoelectrochemical activity. In this study, zinc oxide nanorod arrays were synthesized, and then the surfaces of ZnO nanorods were modified with the p-type semiconductor Co3O4 to create a p-n junction heterostructure. A significant increase in the photocurrent for the ZnO/Co3O4 composite, of 4.3 times, was found compared to pure ZnO. The dependence of the photocurrent on the morphology of the ZnO/Co3O4 composite allows for optimization of the morphology of the ZnO nanorod array to achieve improved photoelectrochemical performance. The results showed that the ZnO/Co3O4 heterostructure exhibited a photocurrent density of 3.46 mA/cm2, while bare ZnO demonstrated a photocurrent density of 0.8 mA/cm2 at 1.23 V. The results of this study provide a better understanding of the mechanism of charge separation and transfer in the heterostructural ZnO/Co3O4 photocatalytic system. Furthermore, the results will be useful for the design and optimization of photocatalytic systems for water splitting and other applications.

Photophysical Characterization of Ru Nanoclusters on Nanostructured TiO2 by Time-Resolved Photoluminescence Spectroscopy

Despite the promising performance of Ru nanoparticles or nanoclusters on nanostructured TiO₂ in photocatalytic and photothermal reactions, a mechanistic understanding of the photophysics is limited. The aim of this study is to uncover the nature of light-induced processes in Ru/TiO₂ and the role of UV versus visible excitation by time-resolved photoluminescence (PL) spectroscopy. The PL at a 267 nm excitation is predominantly due to TiO₂, with a minor contribution of the Ru nanoclusters. Relative to TiO₂, the PL of Ru/TiO₂ following a 267 nm excitation is significantly blue-shifted, and the bathochromic shift with time is smaller. We show by global analysis of the spectrotemporal PL behavior that for both TiO₂ and Ru/TiO₂ the bathochromic shift with time is likely caused by the diffusion of electrons from the TiO₂ bulk toward the surface. During this directional motion, electrons may recombine (non)radiatively with relatively immobile hole polarons, causing the PL spectrum to red-shift with time following excitation. The blue-shifted PL spectra and smaller bathochromic shift with time for Ru/TiO₂ relative to TiO₂ indicate surface PL quenching, likely due to charge transfer from the TiO₂ surface into the Ru nanoclusters. When deposited on SiO₂ and excited at 532 nm, Ru shows a strong emission. The PL of Ru when deposited on TiO₂ is completely quenched, demonstrating interfacial charge separation following photoexcitation of the Ru nanoclusters with a close to unity quantum yield. The nature of the charge-transfer phenomena is discussed, and the obtained insights indicate that Ru nanoclusters should be deposited on semiconducting supports to enable highly effective photo(thermal)catalysis.

Hydrothermal Synthesis of Co-Exposed-Faceted WO3 Nanocrystals with Enhanced Photocatalytic Performance

In this paper, rod-shaped, cuboid-shaped, and irregular WO₃ nanocrystals with different co-exposed crystal facets were prepared for the first time by a simple hydrothermal treatment of tungstic acid colloidal suspension with desired pH values. The crystal structure, morphology, specific surface area, pore size distribution, chemical composition, electronic states of the elements, optical properties, and charge migration behavior of as-obtained WO₃ products were characterized by powder X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), fully automatic specific surface area and porosity analyzer, UV-vis absorption spectra, photoluminescence (PL) spectra, and electrochemical impedance spectroscopy (EIS). The photocatalytic performances of the synthesized pHx-WO₃ nanocrystals (x = 0.0, 1.5, 3.0, 5.0, and 7.0) were evaluated and compared with the commercial WO₃ (CM-WO₃) nanocrystals. The pH7.0-WO₃ nanocrystals with co-exposed {202} and {020} facets exhibited highest photocatalytic activity for the degradation of methylene blue solution, which can be attributed to the synergistic effects of the largest specific surface area, the weakest luminescence peak intensity and the smallest arc radius diameter.

[Ru(tmphen)3]2[Fe(CN)6] and [Ru(phen)3][Fe(CN)5(NO)] complexes and formation of a heterostructured RuO2-Fe2O3 nanocomposite as an efficient alkaline HER and OER electrocatalyst

Water electrolysis is one of the most capable processes for supplying clean fuel. Herein, two novel ionic Ru(II)-Fe(II) complexes, [Ru(tmphen)₃]₂[Fe(CN)₆] and [Ru(phen)₃][Fe(CN)₅(NO)], where tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline and phen = 1,10-phenanthroline, were synthesized and characterized by UV-Vis spectroscopy, elemental analysis, FT-IR, and single-crystal X-ray structural analysis. By thermally decomposing the [Ru(tmphen)₃]₂[Fe(CN)₆] complex at 600 °C for 4 h, a heterostructured RuO₂-Fe₂O₃ nanocomposite was fabricated through a facile one-pot treatment and then characterized by FT-IR, XRD, FT-Raman, UV-Vis (DRS), ICP-OES, FE-SEM, TEM, TGA/DTG, BET, and XPS analyses, which revealed the formation of highly crystalline RuO₂-Fe₂O₃ nanoparticles with an average size of 8-12 nm. The prepared nanocomposite was an efficient heterostructured electrocatalyst for performing water-splitting redox reaction processes, including hydrogen and oxygen evolution reactions (HER and OER) in alkaline solutions. In this regard, RuO₂ and Fe₂O₃ samples were also prepared through thermal decomposition of [Ru(tmphen)₃](NO₃)₂ and K₄[Fe(CN)₆] precursors, respectively, as control experiments to compare their HER and OER electrocatalytic activity with that of the RuO₂-Fe₂O₃ nanocomposite. Specifically, the RuO₂-Fe₂O₃ nanocomposite exhibited significant electrocatalytic performance, generating 10 mA cm^-2 current density at -148 and 292 mV overpotentials, and the Tafel slope results from fitting the LSV curves to the Tafel equation were -43 and 56.08 mV dec^-1 for the HER and OER, respectively. Therefore, the heterostructured RuO₂-Fe₂O₃ nanocomposite can be viewed as a bi-functional electrocatalyst for HER and OER because it exploits the synergistic effects of heterostructures and active sites at its interface.

Regulation of Ferroelectric Polarization to Achieve Efficient Charge Separation and Transfer in Particulate RuO2 /BiFeO3 for High Photocatalytic Water Oxidation Activity

Exploiting spontaneous polarization of ferroelectric materials to achieve high charge separation efficiency is an intriguing but challenging research topic in solar energy conversion. This work shows that loading high work function RuO₂ cocatalyst on BiFeO₃ (BFO) nanoparticles enhances the intrinsic ferroelectric polarization by efficient screening of charges to RuO₂ via RuO₂ /BFO heterojunction. This leads to enhancement of the surface photovoltage of RuO₂ /BFO single nanoparticles nearly 3 times, the driving force for charge separation and transfer in photocatalytic reactions. Consequently, efficient photocatalytic water oxidation is achieved with quantum efficiency as high as 5.36 % at 560 nm, the highest activity reported so far for ferroelectric materials. This work demonstrates that, unlike low photocurrent density in film-based ferroelectric devices, high photocatalytic activity could be achieved by regulating the ferroelectric spontaneous polarization using appropriate cocatalyst to enhance driving force for efficient separation and transfer of photogenerated charges in particulate ferroelectric semiconductor materials.

Synergistic Effect of Fe Doping and Plasmonic Au Nanoparticles on W18O49 Nanorods for Enhancing Photoelectrochemical Nitrogen Reduction

Photoelectrochemical (PEC) nitrogen fixation has opened up new possibilities for the production of ammonia from water and air under mild conditions, but this process is confronted by the inherent challenges associated with theoretical and experimental works, limiting the efficiency of the nitrogen reduction reaction. Herein, we report for the first time a novel and efficient photoelectrocatalytic system, which has been prepared by assembling plasmonic Au nanoparticles with Fe-doped W₁₈O₄₉ nanorods (denoted as WOF-Au). (i) The introduction of exotic Fe atoms into nonstoichiometric W₁₈O₄₉ can eliminate bulk defects of the W₁₈O₄₉ host, which resulted in narrowing bandgap energy and facilitating electron-hole separation and transportation. (ii) Meanwhile, Au nanoparticles combined with a semiconductor induce the localized surface plasmon resonance and generate energetic (hot) electrons, increasing electron density on W₁₈O₄₉ nanorods. Consequently, this plasmonic WOF-Au system shows an NH₃ production yield of 9.82 μg h^-1 cm^-2 at -0.65 V versus Ag/AgCl, which is ∼2.5-folds higher than that of the WOF (without Au loading), as well as very high stability, and no NH₃ formation was found for the bare W₁₈O₄₉ (WO). This high activity can be associated with the synergistic effects between the Fe dopant and plasmonic Au NPs on the host semiconductor W₁₈O₄₉. This work can bring some insights into the target-directed design of efficient plasmonic hybrid systems for N₂ fixation and artificial photocatalysis.

Sr2NiWO6 Double Perovskite Oxide as a Novel Visible-Light-Responsive Water Oxidation Photocatalyst

Screening of stable visible-light-responsive water oxidation semiconductor photocatalysts is highly desired for the development of photocatalytic water splitting systems. Herein, a visible-light-absorbing Sr₂NiWO₆ double perovskite oxide photocatalyst was successfully prepared via a conventional solid-state reaction method. The intrinsic Sr₂NiWO₆ shows photocatalytic oxygen evaluation reaction (OER) activity of 60 μmol h^-1 g^-1, even without loading any cocatalysts. The DFT calculation indicates that the Ni species on the surface is the active site for the OER. The photocatalytic OER activity was further improved by loading Pt and RuO₂ dual redox cocatalysts on the surface of Sr₂NiWO₆ to achieve a photocatalytic OER activity of 420 μmol h^-1 g^-1, which corresponds to a remarkable apparent quantum efficiency (AQE) of 8.6% (λ ≈ 420 nm). The result indicates that Sr₂NiWO₆ is one of the best double perovskite oxide-based photocatalysts for the photocatalytic OER, and the activity is even comparable to the benchmark BiVO₄-based photocatalyst. The improvement of the photocatalytic OER activity is due to the provision of more active redox sites as well as the synergetic effect of the dual redox cocatalysts in facilitating charge separation and transfer. This work demonstrates that double perovskite oxides may serve as a novel class of efficient and stable oxide-based semiconductor photocatalysts for water splitting.

Designing of WO3@Co3O4 Heterostructures to Enhance Photoelectrochemical Performances

Heterostructured photocatalysts are superior to single photocatalysts because they offer better charge separation and broaden light harnessing abilities. Although WO₃ is considered an oxygen-evolving photocatalyst with decent stability and proper band gap, its lower photocatalytic efficiency is ascribed to high charge recombination. In this research, a WO₃@Co₃O₄ heterostructure reduced the recombination of photocatalytic charges and extended light absorption abilities, resulting in improved photocatalytic activity. The presence of Co₃O₄ nanoparticles improved light absorption and charge transfer of tungsten oxide films for photoelectrochemical reactions. For photoelectrochemical water oxidation, WO₃@Co₃O₄ nanostructures generated a photocurrent 20 times higher than that of pure WO₃. Both electrodeposition and sol gel techniques were utilized to synthesize the WO₃@Co₃O₄ photoelectrode. Scanning electron microscopy and X-ray diffraction were used to characterize the formation of the above photocatalyst. A photocurrent study was done to investigate the charge separation mechanism to explain the enhanced photocatalytic activity.

Experimental and DFT Studies of Au Deposition Over WO3/g-C3N4 Z-Scheme Heterojunction

A typical Z-scheme system is composed of two photocatalysts which generate two sets of charge carriers and split water into H₂ and O₂ at different locations. Scientists are struggling to enhance the efficiencies of these systems by maximizing their light absorption, engineering more stable redox couples, and discovering new O₂ and H₂ evolutions co-catalysts. In this work, Au decorated WO₃/g-C₃N₄ Z-scheme nanocomposites are fabricated via wet-chemical and photo-deposition methods. The nanocomposites are utilized in photocatalysis for H₂ production and 2,4-dichlorophenol (2,4-DCP) degradation. It is investigated that the optimized 4Au/6% WO₃/CN nanocomposite is highly efficient for production of 69.9 and 307.3 µmol h^-1 g^-1 H₂ gas, respectively, under visible-light (λ > 420 nm) and UV-visible illumination. Further, the fabricated 4Au/6% WO₃/CN nanocomposite is significant (i.e., 100% degradation in 2 h) for 2,4-DCP degradation under visible light and highly stable in photocatalysis. A significant 4.17% quantum efficiency is recorded for H₂ production at wavelength 420 nm. This enhanced performance is attributed to the improved charge separation and the surface plasmon resonance effect of Au nanoparticles. Solid-state density functional theory simulations are performed to countercheck and validate our experimental data. Positive surface formation energy, high charge transfer, and strong non-bonding interaction via electrostatic forces confirm the stability of 4Au/6% WO₃/CN interface.

Crystal Facet Engineering of Photoelectrodes for Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is a promising approach for solar-driven hydrogen production with zero emissions, and it has been intensively studied over the past decades. However, the solar-to-hydrogen (STH) efficiencies of the current PEC systems are still far from the 10% target needed for practical application. The development of efficient photoelectrodes in PEC systems holds the key to achieving high STH efficiencies. In recent years, crystal facet engineering has emerged as an important strategy in designing efficient photoelectrodes for PEC water splitting, which has yet to be comprehensively reviewed and is the main focus of this article. After the Introduction, the second section of this review concisely introduces the mechanisms of crystal facet engineering. The subsequent section provides a snapshot of the unique facet-dependent properties of some semiconductor crystals including surface electronic structures, redox reaction sites, surface built-in electric fields, molecular adsorption, photoreaction activity, photocorrosion resistance, and electrical conductivity. Then, the methods for fabricating photoelectrodes with faceted semiconductor crystals are reviewed, with a focus on the preparation processes. In addition, the notable advantages of the crystal facet engineering of photoelectrodes in terms of light harvesting, charge separation and transfer, and surface reactions are critically discussed. This is followed by a systematic overview of the modification strategies of faceted photoelectrodes to further enhance the PEC performance. The last section summarizes the major challenges and some invigorating perspectives for future research on crystal facet engineered photoelectrodes, which are believed to play a vital role in promoting the development of this important research field.