Novel CdIn2S4 nano-octahedra/TiO2 hollow hybrid heterostructure: In-situ synthesis,synergistic effect and enhanced dual-functional photocatalytic activities

The development of highly efficient and multifunctional composite photocatalysts for both energy conversion and environmental governance has obtained great concerns. Here, a novel CdIn₂S₄/TiO₂ (CIS/THS) hollow composite photocatalyst was firstly designed and synthesized via a facile in-situ growth process, where the CdIn₂S₄ nano-octahedra densely attached on the surface of TiO₂ hollow spheres to form the unique hybrid heterostructure. The as-synthesized CIS/THS heterojunctions exhibit much superior photocatalytic activities for hydrogen evolution and Methyl Orange (MO) decomposition in comparison to pure CdIn₂S₄ and TiO₂ hollow spheres. The experimental results display that the CIS/THS-3 sample with the 30 wt% of TiO₂ presents the optimal photocatalytic H₂ production efficiency and its generation rate is 3.38 and 2.56 times as high as those of pure TiO₂ and CdIn₂S₄. Besides, the as-synthesized CIS/THS-3 hybrid also possesses the best MO photodegradation performance and its rate constant is 11.43 and 8.34 times higher than those of pure TiO₂ and CdIn₂S₄. The enhanced photocatalytic activities can be assigned to the synergistic effect, optimized light-harvesting capacity and the formation of hybrid heterostructure for boosting interfacial charge transfer and separation. Furthermore, based on the trapping experiments and ESR analysis, the possible type-Ⅱ interface charge transport mechanism was also proposed. Our study may provide the direct guidance for constructing other hollow TiO₂-based composite photocatalysts with superior photocatalytic water splitting and degradation performances.     

Non-Metal-Doped Porous Carbon Nitride Nanostructures for Photocatalytic Green Hydrogen Production

Photocatalytic green hydrogen (H₂) production through water electrolysis is deemed as green, efficient, and renewable fuel or energy carrier due to its great energy density and zero greenhouse emissions. However, developing efficient and low-cost noble-metal-free photocatalysts remains one of the daunting challenges in low-cost H₂ production. Porous graphitic carbon nitride (gCN) nanostructures have drawn broad multidisciplinary attention as metal-free photocatalysts in the arena of H₂ production and other environmental remediation. This is due to their impressive catalytic/photocatalytic properties (i.e., high surface area, narrow bandgap, and visible light absorption), unique physicochemical durability, tunable electronic properties, and feasibility to synthesize in high yield from inexpensive and earth-abundant resources. The physicochemical and photocatalytic properties of porous gCNs can be easily optimized via the integration of earth-abundant heteroatoms. Although there are various reviews on porous gCN-based photocatalysts for various applications, to the best of our knowledge, there are no reviews on heteroatom-doped porous gCN nanostructures for the photocatalytic H₂ evolution reaction (HER). It is essential to provide timely updates in this research area to highlight the research related to fabrication of novel gCNs for large-scale applications and address the current barriers in this field. This review emphasizes a panorama of recent advances in the rational design of heteroatom (i.e., P, O, S, N, and B)-doped porous gCN nanostructures including mono, binary, and ternary dopants for photocatalytic HERs and their optimized parameters. This is in addition to H₂ energy storage, non-metal configuration, HER fundamental, mechanism, and calculations. This review is expected to inspire a new research entryway to the fabrication of porous gCN-based photocatalysts with ameliorated activity and durability for practical H₂ production.     

Nanosized Au Particles as an Efficient Cocatalyst for Photocatalytic Overall Water Splitting

The effect of the photodeposition of gold particles onto several photocatalysts on the photocatalytic activities was studied. The photocatalytic activities of K₄Nb₆O₁₇, Sr₂Nb₂O₇, KTaO₃ NaTaO₃, and NaTaO₃ doped with La for water splitting were improved when gold particles were deposited. The latter were nanoparticles, consistent with their surface plasmon absorption. The nanosized gold particle functioned as an efficient cocatalyst for photocatalytic water splitting by assisting H₂ evolution.     

New aspects of heterogeneous photocatalysts for water decomposition

Several new photocatalysts for overall water splitting are described. Under UV light irradiation (270 nm), La-doped NaTaO₃ modified with NiO decomposed water into H₂ and O₂ with extremely high quantum efficiency. Under an optimized condition, the apparent quantum efficiency, which was estimated with numbers of irradiated photons and evolved H₂ molecules, reached 56%. New stable photocatalytic materials containing elements with d¹⁰ electronic configuration such as In³⁺ Sn⁴⁺ and Sb⁵⁺ were developed for overall water splitting. Some mesoporous oxides were proved to be effective photocatalysts. (Oxy)nitrides of some early transition metals, i.e., Ta, Nb and Ti, were found to be stable materials having potentials for H₂ and O₂ evolutions under visible light irradiation (⪯600 nm). The electronic structures of these photocatalysts are also discussed based on DFT calculation.     

Visible light induced photocatalytic overall water splitting over micro-SiC driven by the Z-scheme system

We present the micro-sized SiC powder applying to overall water splitting under visible light irradiation without adding any sacrificial compounds. The stoichiometric amounts of H₂ and O₂ in 2:1 were obtained when using WO₃ nanoparticle and Pt-loaded micro-SiC grains as the O₂ and H₂ evolution photocatalysts, respectively. The relatively low efficiency step for splitting water reaction is the H⁺ reduction in the solution. Under optimal condition, the apparent quantum efficiency reaches up to 0.021% at 420 nm. Our results provide a method to enhance the photocatalytic activity of micro-sized SiC powder for overall water splitting using solar energy.     

Solar light-driven photocatalytic hydrogen evolution over ZnIn<Subscript>2</Subscript>S<Subscript>4</Subscript> loaded with transition-metal sulfides

A series of Pt-loaded MS/ZnIn₂S₄ (MS = transition-metal sulfide: Ag₂S, SnS, CoS, CuS, NiS, and MnS) photocatalysts was investigated to show various photocatalytic activities depending on different transition-metal sulfides. Thereinto, CoS, NiS, or MnS-loading lowered down the photocatalytic activity of ZnIn₂S₄, while Ag₂S, SnS, or CuS loading enhanced the photocatalytic activity. After loading 1.0 wt.% CuS together with 1.0 wt.% Pt on ZnIn₂S₄, the activity for H₂ evolution was increased by up to 1.6 times, compared to the ZnIn₂S₄ only loaded with 1.0 wt.% Pt. Here, transition-metal sulfides such as CuS, together with Pt, acted as the dual co-catalysts for the improved photocatalytic performance. This study indicated that the application of transition-metal sulfides as effective co-catalysts opened up a new way to design and prepare high-efficiency and low-cost photocatalysts for solar-hydrogen conversion.     

Hydrogen Production Using Highly Active Titanium Oxide-based Photocatalysts

Applying the principle of water decomposition over photoelectrochemical cells to heterogeneous photocatalytic systems using powdered semiconductors is now a field of great interest, encouraging new fundamental investigations into chemical reactions which take place at the electrode surfaces in the electrochemical cells. In the present review, we have focused on systems, which can convert solar energy into chemical energy by using TiO₂ photocatalysts. The photocatalytic decomposition of water under UV light irradiation has been achieved with systems using various nanoparticle photocatalysts such as TiO₂. However, recently, visible light-responsive TiO₂ thin films photocatalysts have been successfully prepared by a radio frequency magnetron sputtering (RF-MS) deposition method. These thin film photocatalysts were found to have enough potential for the separate evolution of H₂ and O₂ from water under solar light. The TiO₂ thin films were prepared on metal substrates by RF-MS deposition and mounted on H-type containers filled with water. This unique system enabled the separate evolution of H₂ and O₂ from water under sunlight irradiation, opening new opportunities for the practical on-site production of pure and clean H₂ from water using abundant and clean sunlight in a safe, environmentally harmonious way.     

Photocatalytic Degradation of Organic Pollutants Under Visible Light Irradiation

Several TiO₂-based photocatalytic systems that have considerable visible light response have been developed, such as the photodegradation of organic pollutants on sensitized TiO₂ by visible light, construction of visible-light-active novel TiO₂ photocatalysts by matrix or surface modification. In this paper, we review briefly our recent progress in the TiO₂ photocatalytic degradation of organic pollutants by visible light, some related work by other groups is also involved.     

Preparation of Unique TiO2 Nano-particle Photocatalysts by a Multi-gelation Method for Control of the Physicochemical Parameters and Reactivity

A novel multi-gelation method to prepare TiO₂ nano-particle photocatalysts showed good performance in controlling the important parameters determining the photocatalytic reactivity, i.e., the particle size, surface area, crystallinity, pore-volume, pore-diameter as well as the anatase and rutile phase composition of the catalysts. In particular, this method at higher pH swing times could prevent the phase transition from anatase to rutile, leading to higher photocatalytic activity. By adopting variations in the pH swing, the TiO₂ nano-particle photocatalysts showed significantly higher photocatalytic reactivity for the complete oxidation of 2-propanol diluted with water into CO₂ and H₂O. It can be considered a viable alternative method for the preparation of high performance TiO₂ nano-particle photocatalysts for widespread commercial applications.     

S-scheme bimetallic sulfide ZnCo2S4/g-C3N4 heterojunction for photocatalytic H2 evolution

Transition bimetallic sulfides have attracted widespread attention because of their superior electrochemical characteristics compared to their parent materials. Herein, ternary ZnCo₂S₄ was deposited on g-C₃N₄ (CN) to enhance the photocatalytic water splitting reactivity of CN. The hydrogen (H₂) evolution rate of 25 wt%-ZnCo₂S₄/CN reached 6619 μmol h⁻¹ g⁻¹, which was 55.2 times higher than that of CN alone. Under the same conditions, ZnS/CN and Co₃S₄/CN were also synthesized, and their H₂ evolution rates were both inferior to that of ZnCo₂S₄/CN. Investigations showed that the presence of both zinc and cobalt ions in ZnCo₂S₄ lowered the H₂ evolution overpotential and charge recombination rate, leading to excellent H₂ release activity. In addition, the composite maintained its activity even after reacting for 20 h, and the charge transfer mechanism between ZnCo₂S₄ and CN was subject to the S-scheme charge transfer route according to trapping experiments for active species. This work revealed a promising and efficient bimetallic sulfide heterojunction to enhance H₂ evolution during water splitting and thus achieved improved conversion efficiency for solar energy applications.     

Synthesis of Cu2O/La2CuO4 nanocomposite as an effective heterostructure photocatalyst for H2 production

A perovskite-like compound La₂CuO₄-based material, a well-known high-temperature superconductor, was studied as a photocatalyst. H₂ was produced over La₂CuO₄ from aqueous solutions containing methanol as a sacrificial reagent under high-pressure Hg lamp irradiation. The photocatalytic activity of La₂CuO₄ is superior to that of the commercial anatase TiO₂, and can be greatly improved by loading with Cu₂O. Photoelectrochemical measurements revealed that the conduction band minimum of the La₂CuO₄ photocatalysts is thermodynamically feasible for the reduction reaction of water and Cu₂O efficiently increase the electron–hole separation on La₂CuO₄ under photon irradiation to account for its higher photocatalytic activity.     

Photocatalytic oxidation of ethylene to CO2 and H2O on ultrafine powdered TiO2 photocatalysts: Effect of the presence of O2 and H2O and the addition of Pt

The complete photocatalytic oxidation of C₂H₄ with O₂ into CO₂ and H₂O has been achieved on ultrafine powdered TiO₂ photocatalysts and the addition of H₂O was found to enhance the reaction. The photocatalytic reaction has been studied by IR, ESR, and analysis of the reaction products. UV irradiation of the photocatalysts at 275 K led to the photocatalytic oxidation of C₂H₄ with O₂ into CO₂, CO, and H₂O. The large surface area of the photocatalyst is one of the most important factors in achieving a high efficiency in the photocatalytic oxidation of C₂H₄. The photoformed OH species as well as O ₂ ⁻ and O ₃ ⁻ anion radicals play a significant role as a key active species in the complete photocatalytic oxidation of C₂H₄ with O₂ into CO₂ and H₂O. Interestingly, small amount of Pt addition to the TiO₂ photocatalyst increased the amount of selective formation of CO₂ which was the oxidation product of C₂H₄ and O₂.     

The Synergistic Effects of Two Co-catalysts on Zn2GeO4 on Photocatalytic Water Splitting

Zinc orthogermanate was prepared via a hydrothermal method and a remarkable synergistic effect on the photocatalytic activity for overall water splitting was found for Zn₂GeO₄ co-loaded with noble metals (Pt, Rh, Pd, Au) and metal oxides (RuO₂, IrO₂). The photocatalytic activity of Pt-RuO₂/Zn₂GeO₄ for overall water splitting is 2.2 times of Pt/Zn₂GeO₄ and 3.3 times of RuO₂/Zn₂GeO₄. Photocatalytic half reactions evaluation of water splitting for H₂ and O₂ productions shows that Pt plays the major roles in H₂ production and RuO₂ promotes the O₂ production. The roles and valence states of co-catalysts and the mechanism of photocatalytic reaction are discussed.     

Preparation of ZnIn2S4/K2La2Ti3O10 composites and their photocatalytic H2 evolution from aqueous Na2S/Na2SO3 under visible light irradiation

ZnIn₂S₄/K₂La₂Ti₃O₁₀ composite photocatalysts were synthesized via a hydrothermal route. The photocatalysts were characterized by the X-ray diffraction, scanning electron microscopy, ultraviolet–visible (UV–vis) diffuse reflection spectra and photoluminescence measurements. The UV–vis results indicated that ZnIn₂S₄/K₂La₂Ti₃O₁₀ has a strong absorption in the visible light region. The compositions of ZnIn₂S₄/K₂La₂Ti₃O₁₀ composite photocatalysts were optimized according to the photocatalytic activity for hydrogen production from aqueous Na₂S/Na₂SO₃. The composite photocatalyst loading 25 wt.% ZnIn₂S₄ exhibited the highest photocatalytic activity, the amount of H₂ production was 6.29 mmol/g after 3 h irradiation under visible light irradiation.     

Doped 1D Nanostructures of Transition-metal Oxides: First-principles Evaluation of Photocatalytic Suitability

The splitting of water molecules under the influence of solar light on semiconducting electrodes is a clean and renewable source for the production of hydrogen fuel. Its efficiency depends on the relative position of the band-gap edges or the induced defect levels with a proper band alignment relative to the redox H⁺/H₂ and O₂/H₂O potentials. For example, TiO₂ and ZnO bulk, as well as thick slabs (whose band gaps are ∼3.2–3.4 eV), can be active only for photocatalytic applications under UV irradiation (possessing ∼1 % solar energy conversion efficiency). Nevertheless, by adjusting the band gap through formation of nanostructures and further doping, the efficiency can be increased up to ∼15 % (for 2.0–2.2 eV band gap). We analyse results of DFT (density functional theory) calculations on TiO₂ nanotubes and ZnO nanowires, both pristine and doped (e.g., by Ag_Zn, C_O, Fe_Ti, N_O and S_O substitutes). To reproduce the energies of one-electron states better, we have incorporated the Hartree-Fock (HF) exchange into the hybrid DFT+HF Hamiltonian. Both the atomic and electronic structure of nanomaterials, simulated by us, are analysed to evaluate their photocatalytic suitability, including positions of the redox potential levels inside the modified band gap, the width of which corresponds to visible-light energies. Analysis of the densities of states (DOS) for considered nanostructures clearly shows that photocatalytic properties can be significantly altered by dopants. The chosen hybrid methods of first-principles calculations significantly simplify selection of suitable nanomaterials possessing the required photocatalytic properties under solar light irradiation.     

Review on photocatalytic conversion of carbon dioxide to value-added compounds and renewable fuels by graphitic carbon nitride-based photocatalysts

Photocatalytic reduction of CO₂ is known as one of the most promising methods to produce valuable fuels and value-added compounds. To overcome selectivity and efficiency downsides, various photocatalysts have been designed and developed. This review discusses the state-of-the-art in photo-conversion of CO₂ over graphitic carbon nitride (g-C₃N₄)-based composites. The modification strategies to improve photocatalytic activity of g-C₃N₄ were classified into different categories and discussed as structural modifications, elemental doping, copolymerization, fabricating heterojunctions between g-C₃N₄ and other semiconductors, Z-scheme heterojunctions, noble metal/g-C₃N₄ photocatalysts, and design of ternary nanocomposites based on g-C₃N₄. Finally, perspectives and future research works in this field were also outlined.     

Rational design of MoS2/g-C3N4/ZnIn2S4 hierarchical heterostructures with efficient charge transfer for significantly enhanced photocatalytic H2 production

The rational design of hierarchical heterojunction photocatalysts with efficient spatial charge separation remains an intense challenge in hydrogen generation from photocatalytic water splitting. Herein, a noble-metal-free MoS₂/g-C₃N₄/ZnIn₂S₄ ternary heterostructure with a hierarchical flower-like architecture was developed by in situ growth of 3D flower-like ZnIn₂S₄ nanospheres on 2D MoS₂ and 2D g-C₃N₄ nanosheets. Benefiting from the favorable 2D-2D-3D hierarchical heterojunction structure, the resultant MoS₂/g-C₃N₄/ZnIn₂S₄ nanocomposite loaded with 3 wt% g-C₃N₄ and 1.5 wt% MoS₂ displayed the optimal hydrogen evolution activity (6291 μmol g^?1 h^?1), which was a 6.96-fold and 2.54-fold enhancement compared to bare ZnIn₂S₄ and binary g-C₃N₄/ZnIn₂S₄, respectively. Structural characterizations reveal that the significantly boosted photoactivity is closely associated with the multichannel charge transfer among ZnIn₂S₄, MoS₂, and g-C₃N₄ components with suitable band-edge alignments in the composites, where the photogenerated electrons migrate from g-C₃N₄ to ZnIn₂S₄ and MoS₂ through the intimate heterojunction interfaces, thus enabling efficient electron-hole separation and high photoactivity for hydrogen evolution. In addition, the introduction of MoS₂ nanosheets highly benefits the improved light-harvesting capacity and the reduced H₂-evolution overpotential, further promoting the photocatalytic H₂-evolution performance. Moreover, the MoS₂/g-C₃N₄/ZnIn₂S₄ ternary heterostructure possesses prominent stability during the photoreaction process owing to the migration of photoinduced holes from ZnIn₂S₄ to g-C₃N₄, which is deemed to be central to practical applications in solar hydrogen production.     

Photoreduction of Carbondioxide on Surface Functionalized Nanoporous Catalysts

Nanoporous photocatalysts have been designed to exhibit unique photocatalytic activities through framework substitution of titanium species or surface immobilization of rhenium complex onto mesoporous silica. This article summarizes recent work on the synthesis, characterization and photocatalytic activities of the designed porous photocatalysts performed by the present authors. Various spectroscopic investigations revealed that the photo-excited states of these catalysts play a vital role in the photocatalytic reactions and their photocatalytic reactivities are strongly dependent on structures of active sites, which are confined and immobilized in the restricted framework structure of the mesoporous silica. Highly dispersed titanium oxide species incorporated in the framework of mesoporous silica exhibited high and unique photocatalytic reactivity for the reduction of CO₂ with H₂O to produce CH₄ and CH₃OH under UV irradiation, its reactivity being much higher than bulk TiO₂. The cationic rhenium(I) complex was encapsulated into a mesoporous AlMCM-41 material by ion-exchange method, yielding a visible light photocatalyst to be active for photocatalytic reduction of CO₂.     

Electrostatic self-assembly of 2D-2D CoP/ZnIn2S4 nanosheets for efficient photocatalytic hydrogen evolution

Constructing efficient and cost-effective photocatalysts are highly desirable for photocatalytic hydrogen evolution. Herein, we prepare a unique 2D-2D architecture photocatalyst composed of CoP and ZnIn₂S₄ (ZIS) nanosheets through electrostatic self-assembly method. The constructed 2D-2D CoP/ZIS exhibit a remarkably enhanced photocatalytic performance with hydrogen production rate of 8.775 mmol g⁻¹ h⁻¹, and this value is much higher than ZIS and most of other ZIS-based nanohybrids. Additionally, the nanohybrids possess excellent stability with 96.3% of initial activity remaining after 24 hours of testing. These satisfactory results are attributed to the large/intimate contact interface and the photo/electro-chemical properties of ZIS and CoP, which improves light absorption, facilitates photoelectron transport and suppresses charge recombination. This work not only demonstrates ZIS nanosheet can serve as a versatile and effective platform supporting non-noble metal nanosheets to boost their photocatalytic performance, but also offers a general and simple electrostatic self-assembly method to design 2D-2D-based heterostructures for hydrogen conversion from water splitting.     

Developments in photocatalytic antibacterial activity of nano TiO<Subscript>2</Subscript>: A review

TiO₂, which is one of the most explored materials, has emerged as an excellent photocatalyst material for environmental and energy fields, including air and water purification, self-cleaning surfaces, antibacterial and water splitting. This review summarizes recent research developments of TiO₂-based photocatalyst used for photocatalytic antibacterial applications. Several strategies to enhance the efficiency of TiO₂ photocatalyst are discussed, including doping with metal ions, noble metals, non-metals, and coupling with other materials. The mechanism of photocatalytic antibacterial activity in the presence of nano-sized TiO₂ is also discussed. The modified TiO₂ photocatalyst significantly inhibits the growth of bacterial cells in response to visible light illumination. TiO₂ photocatalysis appears to be promising as a route of advanced oxidation process for environmental remediation.