Z-scheme Photocatalyst Systems Research Articles

Critical assessment of a TiO2-Ag-ZnO nanocomposite photocatalyst on improved photocatalytic activity under mixed UV-visible light

In recent years, semiconductor photocatalysts such as zinc oxide (ZnO) and titanium dioxide (TiO2) have been studied to utilize solar light for energy production and environmental remediation. However, their wide band gap energy and fast charge recombination limit their practical applications. So far, various approaches, such as noble metal incorporation, metal oxide coupling, and the design of Z-scheme photocatalyst systems, among others, have been explored to enhance the lifetime of photogenerated charge carriers in ZnO and TiO2. In this study, a novel TiO2-Ag-ZnO nanocomposite photocatalyst has been successfully synthesized by a combination of wet chemistry, photochemistry, and plasma-based processes. Simple spin coating of ZnO thin films was coupled with subsequent photodeposition of metallic Ag nanoparticles (NPs). Afterward, well-uniform TiO2 NPs (roughly 18 nm) were deposited on this unique Ag-ZnO surface via a gas cluster aggregation source (GAS) process. The results showed that the activity of the ZnO photocatalyst under mixed UV-visible light was significantly increased by the TiO2 and Ag contributions. A single mechanism to be responsible for the photocatalytic activity can be excluded. Instead, a critical assessment of the possible mechanisms corresponding to the different interfaces present at the TiO2-Ag-ZnO surface including a Z–scheme is provided.

Recent advances in metal oxide photocatalysts for photocatalytic degradation of organic pollutants: A review on photocatalysts modification strategies

Wastewater from industries that predominantly consist of organic pollutants significantly contributes to water pollution and harms the environment, demanding urgent solutions. Among the available wastewater treatment technologies, photocatalysis has garnered considerable interest due to its high efficiency, cleanliness, and sustainability. However, metal oxide photocatalysts, despite extensive study, possess limitations such as agglomeration, rapid electron-hole recombination, and photo corrosion. These limitations hinder the practical design and synthesis of photocatalysts. To address these challenges, researchers have explored various photocatalyst modification approaches, including doping with noble or non-noble metals, crystal facet engineering, physical deposition, dye sensitization, and the implementation of the Z-scheme photocatalyst system. These modifications aim to enhance the catalytic properties of photocatalysts and improve the degradation of organic pollutants. This review article highlights recent advances in the modification strategies of metal oxide photocatalysts for the photocatalytic degradation of organic pollutants. The future prospect and conclusions were also discussed. This review is expected to provide an in-depth understanding of metal oxide photocatalyst development, thus accelerating the evolution of photocatalytic degradation of pollutants.

In-situ synthesis of direct Z-scheme 2D/2D ZnIn2S4@CeO2 heterostructure toward enhanced photodegradation and Cr(VI) reduction

The rationally designed semiconductor photocatalytic materials have attracted significant attention because of their huge advantages in the use of solar light. Herein, a series of direct Z-scheme heterojunction nanocomposites were synthesized via in situ growth of zinc indium sulfide nanosheets on ceria nanosheets (ZnIn2S4@CeO2) and employed for effective photodegradation and Cr(VI) reduction. Advanced characterizations such as XPS, EPR spectra, and DFT calculations verified that the formation of direct Z-scheme charges transfer mechanism promoted rapid charge separation and retained strong redox capacity. As a result, the optimized ZnIn2S4@CeO2 sample showed significantly promoted photocatalytic removal rates for tetracycline (TC) and Cr(VI) at a 3.7 and 4.5 times enhancement by comparison with the pristine ZnIn2S4, respectively. Additionally, after four consecutive cyclic tests, the as-synthesized nanocomposite exhibited remarkable photocatalytic activity and stability. The present work provides an integrated engineering strategy for Z-scheme photocatalyst systems with high photocatalytic efficiency and extensive applications.

A novel Z-scheme NH2-MIL-125(Ti)/Ti3C2 QDs/ZnIn2S4 photocatalyst with fast interfacial electron transfer properties for visible light-driven antibiotic degradation and hydrogen evolution

Herein, a novel all-solid-state Z-scheme NH2-MIL-125(Ti)/Ti3C2 MXene quantum dots/ZnIn2S4 (Ti-MOF/QDs/ZIS) photocatalyst was prepared, then applied for photocatalytic antibiotic (tetracycline and sulfamethazine) degradation and hydrogen evolution under visible light. The Z-scheme Ti-MOF/QDs/ZIS photocatalyst afforded very efficient degradation of tetracycline (TC, 96% in 50 min) and sulfamethazine (SMZ, 98% in 40 min), as well as a high rate of hydrogen generation (2931.9 μmol g-1 h−1) under visible light irradiation. Trapping studies showed superoxide radicals (•O2−) to be the main active species for antibiotic degradation, with the intermediates formed during antibiotic photo-oxidation identified by liquid chromatography-mass spectrometry. Photocatalyst recycling studies showed the Z-scheme composite photocatalyst to be stable during both antibiotic photodegradation and hydrogen generation experiments. The Ti3C2 MXene quantum dots facilitated electron transfer between the semiconducting components of Z-scheme composite photocatalyst. This work identifies a promising new Z-scheme photocatalyst system for water purification and solar hydrogen production.

Synthesis of hierarchical tandem double Z-scheme heterojunctions for robust photocatalytic H2 generation

The design of advanced structures for semiconductor photocatalysts is one of the most tremendous approaches to stimulate their conversion performance from solar energy to chemical energy. Nonetheless, it is challenging to harvest the full solar spectrum in semiconductor photocatalysts for excessive activity. Herein, a hierarchical CuCo2O4/graphdiyne/CuO nanocages (CCOGC NCs) tandem heterojunction was once effectively synthesized by means of liquid–liquid interface method for the first time and a facile calcination method. The graphdiyne nanosheets (GDY NSs) not only function as a cost-effective cocatalyst but also act as a bridge to connect two light-harvesting semiconductors into a tandem heterojunction where the CuCo2O4 (CCO) NCs and CuO Quantum dots (QDs) absorb visible light on both sides efficiently, coupling with the GDY NSs into a double Z-scheme photocatalyst system. Benefiting from the novel hierarchical tandem nanocages structure, the optimized CCOGC NCs photocatalyst manifests high activity under visible light irradiation with the hydrogen (H2) generation rate of 9617 µmol h−1 g−1, which is much enhanced compared with those of the CCO NCs and CCO/GDY NCs (CCOG NCs), as well CCO/CuO (CCOC NCs).

(Invited) Photocatalytic Water Splitting Using SWCNT/TiO2 Nanohybrids

Water splitting photocatalysts based on semiconducting single-walled carbon nanotubes (s-SWCNTs) have been attracted much attention from the viewpoint of solar fuel production.[1-7] Since s-SWCNTs have chirality dependent bandgap covering a majority of the solar spectrum (400 – 2000 nm), s-SWCNTs are expected for an efficient solar-light absorber.[8] However, upon photoirradiation, the large exciton binding energy in s-SWCNTs prevents mobile carrier generation. Then, in order to make a particulate photocatalyst consisting of s-SWCNTs, an electron-extracting material is necessary. In this context, we have developed CNT-photocatalysts that contain a s-SWCNT/C60 coaxial heterojunction fabricated by a physical modification of s-SWCNTs using fullerodendron.[4] Here, the C60 acts as the electron-extracting material to generate a charge-separated state, s-SWCNT•+/C60 •–, followed by electron transfer to a co-catalyst that produces hydrogen from water. Z-scheme photocatalyst system employing BiVO4 as an oxygen evolution photocatalyst (OEP) and s-SWCNT/C60 coaxial hybrids as a hydrogen evolution photocatalyst (HEP) produced H2 and O2 from water, of which solar-to-hydrogen conversion efficiency (STH) was 0.089%.[7] We conducted an H2 evolution reaction using s-SWCNT/C60/Ru(III) upon monochromatic irradiation at 570, 650, and 680 nm, which are E22 absorption of (6,5), (7,5), and (8,3)tubes, respectively, and estimated the external quantum yields to be 0.22% for (6,5), 0.16% for (7,5), and 0.70% for (8,3). These EQY values were considerably lower than that using 1000-nm-light (E11 absorption of (6,5) and (8,3)tubes) irradiation, 12.8%.[7] The result might be attributed to branching pathways from the higher exciton (E22 excitation) to other than the lower exciton (E11 excitation). Blackburn and co-workers reported such a branching ratio is crucial for the PL quantum yield of s-SWCNTs.[9] Cui and co-workers describe E22 excitation makes interfacial electrotransfer inhibited in the (6,5)tube/C60 heterojunction, although ultrafast electron transfer process can be seen in E11 excitation by a DFT-based nonadiabatic dynamics simulation.[10] In this context, direct electron extraction from E22 excitation, which is faster than other branching pathways, is expected to improve the photocatalytic activity of CNT-photocatalysts. Recently, Blackburn reported higher-energy second excitonic s-SWCNT transitions produce more photocurrent by the use of submonolayer coverages of s-SWCNTs on single crystal TiO2 electrode, demonstrating carrier injection rates are competitive with fast hot-exciton relaxation processes.[11] These backgrounds prompt us to investigate a CNT-photocatalyst containing the s-SWCNT/TiO2 heterojunction. This paper describes fabrication of s-SWCNT/TiO2/Pt nanohybrids and their photocatalytic activity for H2 evolution from water. Interestingly, their EQYs of H2 evolution reaction using E22 excitation, 12% for (6,5)tube/TiO2/Pt and 43% for (8,3)tube/TiO2/Pt is much higher than those of (6,5)tube/C60/Ru(III) and (8,3)tube/C60/Ru(III), 0.22% and 0.70%.

Synthesis of polyaniline decorated with ZnO and CoMoO4 nanoparticles for enhanced photocatalytic degradation of imidacloprid pesticide under visible light

Structural design in a semiconductor photocatalyst is significant to improve the photocatalytic efficiency to solve current energy or environmental problems. In this study, we developed a Z-scheme photocatalyst system consisting of PANI/ZnO-CoMoO4 for degradation photocatalytic processes. The structural composition and morphologies of the PANI/ZnO-CoMoO4 nanocomposite were analyzed by FTIR, FESEM-EDS, and XRD techniques. Subsequently, the pesticide-degradation experiments were performed for the degradation of imidacloprid (IM) using as-synthesized PANI/ZnO-CoMoO4 nanocomposite under visible-light irradiation to investigate its photocatalytic activity. The influence of PANI/ZnO-CoMoO4 amount, pH, and IM concentration on the performance of the degradation reaction has been considered by the experimental design. Under optimal conditions (4.5 ppm of IM, pH = 4, and 163.5 mg photocatalyst), about 97% of the IM was degraded within 180 min of irradiation. The prepared photocatalyst can be readily recovered and reutilized with the centrifugation process.

Construction of Z-scheme Photocatalyst Containing ZnIn2S4, Co3O4-Photodeposited BiVO4 (110) Facets and rGO Electron Mediator for Overall Water Splitting into H2 and O2

The self-assemble 0.5Pt-ZnIn2S4/rGO/Co3O4-BiVO4 (110) Z-scheme system photocatalysts were synthesized successfully, in which 0.5Pt-ZnIn2S4, rGO and Co3O4-BiVO4 (110) were as H2-photocatalyst, electron mediator and O2-photocatalyst, respectively. Herein, the preferred exposed (110) crystal facets of BiVO4 have great contribution to optimizing its photocatalytic oxidation performance. The Z-scheme system samples were used for photocatalytic water splitting to H2 and O2 in stoichiometric ratio without any sacrificial agents under visible light irradiation. SEM results clearly revealed the morphology and structure of Pt-ZnIn2S4, rGO and Co3O4-BiVO4 (110), suggesting that the three samples closely contacted with each other. Thus the electrons in the Z-scheme photocatalyst system could flow continuously in the photocatalytic water splitting process, which would induce the highly separation rate of photoinduced charge carriers and then accelerated the photocatalytic performance. Furthermore, rGO and Co3O4 were shown to be the critical factors for the improvment of photoinduced electron–hole pairs, thereby influencing the photocatalytic performance. In 0.5Pt-ZnIn2S4/2rGO/5Co3O4-BiVO4 (110) photocatalyst, the amount of the H2 and O2 could reach 294.3 μmol/g and 143.4 μmol/g, respectively (in 12 h). The mechanism for the photocatalytic overall water splitting was also discussed in detail. We confirmed that Z-scheme 0.5Pt-ZnIn2S4/rGO/Co3O4-BiVO4 (110) exhibit photocatalytic overall water splitting into H2 and O2 (294.3 and 143.4 μmol/g in 12 h). Here, 0.5Pt-ZnIn2S4, rGO and Co3O4-BiVO4 (110) act as H2-photocatalyst, electron-mediator and O2-photocatalyst, respectively. And the contact interface is favourable and then helpful for the electrons flow continuously during the photocatalytic reaction process. The electron mediator rGO and cocatalyst Co3O4 are responsible for the photoactivity.

A novel Z-scheme Ag6Si2O7/AgI nanocomposite photocatalyst: Study on the degradation of various refractory compounds and reduction of vanadium (V)

The Z-scheme photocatalyst system, with independent reduction and oxidation reaction processes and efficient separation of carriers, has been widely concerned in the field of wastewater treatment. Herein, a Z-scheme Ag6Si2O7/AgI heterojunction nanocomposite was successfully constructed through co-precipitation method and subsequently investigated as a novel photocatalyst for the first time. The macroscopic physical structure, microscopic chemical element composition and optical properties of samples as-prepared samples were adequately detected by various characterization techniques. The photocatalytic properties of Ag6Si2O7/AgI heterostructure with different composite ratios were investigated by degrading methyl orange (MO) under visible light irradiation (λ > 420 nm), and the degradation efficiency of optimal sample (1/3 Ag6Si2O7/AgI) can achieve 99.7% within 8 min, which was about 15.3 and 124.6 folds higher than that of individual Ag6Si2O7 (6.5%) and AgI (0.8%), respectively. Meanwhile, Ag6Si2O7/AgI showed an excellent capability of phenol degradation and V reduction, which might be ascribed to the strong visible light absorption, rapid separation efficiency of photo-induced charge carriers and direct Z-scheme charge transfer mechanism verified by the results of UV–vis diffuse reflection spectra, transient photocurrent response spectra and XPS measurements, respectively. Moreover, Ag6Si2O7/AgI nanocomposite presented a good stability and reusability after four runs. Finally, a possible Z-scheme mechanism is proposed to illustrate the enhanced photocatalytic performance of the Ag6Si2O7/AgI nanocomposite.

Z-scheme photocatalyst systems employing Rh- and Ir-doped metal oxide materials for water splitting under visible light irradiation.

Various types of Z-scheme systems for water splitting under visible light irradiation were successfully developed by employing Rh- and Ir-doped metal oxide powdered materials with relatively narrow energy gaps (EG): BaTa2O6:Ir,La (EG: 1.9-2.0 eV), NaTaO3:Ir,La (EG: 2.1-2.3 eV), SrTiO3:Ir (EG: 1.6-1.8 eV), NaNbO3:Rh,Ba (EG: 2.5 eV) and TiO2:Rh,Sb (EG: 2.1 eV), with conventional SrTiO3:Rh (an H2-evolving photocatalyst) or BiVO4 (an O2-evolving photocatalyst), and suitable electron mediators. The Z-scheme systems were classified into three groups depending on the combination of H2- and O2-evolving photocatalysts and electron mediator. The Z-scheme systems combining BaTa2O6:Ir,La with BiVO4, and NaTaO3:Ir,La with BiVO4 were active when a [Co(bpy)3]3+/2+ redox couple was used rather than an Fe3+/2+ one. The combination of SrTiO3:Ir with SrTiO3:Rh gave an activity when the [Co(bpy)3]3+/2+ and Fe3+/2+ redox couple ionic mediators were used. The Z-scheme systems combining NaNbO3:Rh,Ba and TiO2:Rh,Sb with SrTiO3:Rh showed activities by using the [Co(bpy)3]3+/2+ and Fe3+/2+ redox couples and also via interparticle electron transfer by just contact with/without reduced graphene oxide (RGO). These suitable combinations can be explained based on the impurity levels of doped Rh3+ and Ir3+ toward the redox potentials of the ionic mediators for the Z-scheme systems employing ionic mediators, and p-/n-type and onset potentials of the photocurrent in the photoelectrochemical properties of those photocatalyst materials for the Z-scheme systems working via interparticle electron transfer.

NiS2 as trapezoid conductive channel modified ternary Z-scheme photocatalyst system, NiGa2O4/NiS2/WO3, for highly photocatalytic simultaneous conversions of NO2− and SO32−

In this work, a novel ternary Z-scheme photocatalyst, NiGa2O4/NiS2/WO3, is prepared by sol-hydrotherma and calcination methods. The photocatalytic performance of prepared photocatalyst is evaluated through the simultaneous conversions of NO2− and SO32− under simulated solar-light irradiation. At the same time, the effects of solution acidity, simulated solar-light irradiation time, e− and h+ scavengers, used times and photocatalyst species on photocatalytic conversions of NO2− and SO32− are also studied. Due to the presence of NiS2 as conducting pathway, which can quickly transfer photo-generated electron (e−) and efficiently promote separation of electrons (e−) and holse (h+), the prepared ternary Z-scheme photocatalyst, NiGa2O4/NiS2/WO3, exhibits excellent performance in the simultaneous conversions of NO2− and SO32− under simulated solar-light irradiation. The conversion ratios of NO2− and SO32− can reach 87.03% and 97.47% under optimal experimental conductions, respectively.

Building a direct Z-scheme heterojunction photocatalyst by ZnIn2S4 nanosheets and TiO2 hollowspheres for highly-efficient artificial photosynthesis

Facing the requirement of modern industrial development, selective reduction of 4-nitroaniline (4-NA) into p-phenylenediamine (PPD) via a facile, economic and eco-friendly photocatalysis technique has attracted more and more attention in recent years. In this study, ZnIn2S4 (ZIS) nanosheets grew in-situ on the surface of TiO2 hollowspheres through a hydrothermal method and formed a direct Z-scheme heterojunction composite, which exhibited exceedingly better photocatalytic 4-NA reduction performance than bare ZIS and TiO2. Such enhancement was mainly attributed to the dissimilar dimensionalities, intimate contact, and matchable band edge positions between ZIS and TiO2 semiconductors. Moreover, the substantial mechanism for the photocatalytic reaction in the ZIS/TiO2 system was reasonably proposed. Inspired by recent applications of the Z-scheme heterostructured materials in solar energy conversion and artificial photosynthesis, this work would arouse an increasing interest in designing more Z-scheme photocatalyst systems with high efficiency and extensive applications.

Zinc-doped g-C3N4/BiVO4 as a Z-scheme photocatalyst system for water splitting under visible light

A two-step photocatalytic water splitting system, termed a “Z-scheme system”, was achieved using Zn-doped g-C3N4 for H2 evolution and BiVO4 for O2 evolution with Fe2+/Fe3+ as a shuttle redox mediator. H2 and O2 were evaluated simultaneously when the doping amount of zinc was 10%. Moreover, Zn-doped (10%) g-C3N4 synthesized by an impregnation method showed superior active ability to form the Z-scheme with BiVO4 than by in-situ synthesis. X-ray diffraction, UV-Vis spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy were used to characterize the samples. It was determined that more Zn–N bonds could be formed on the surface of g-C3N4 by impregnation, which could facilitate charge transfer.

Construction of highly efficient and stable ternary AgBr/Ag/PbBiO2Br Z-scheme photocatalyst under visible light irradiation: Performance and mechanism insight

In this work, the novel ternary AgBr/Ag/PbBiO2Br Z-scheme photocatalysts were synthesized via a CTAB-assisted calcination process. The AgBr/Ag/PbBiO2Br composites were employed for the degradation of rhodamine B (RhB) and antibiotic bisphenol A (BPA) under visible light irradiation. Results showed that the obtained AgBr/Ag-3/PbBiO2Br displayed optimal photocatalytic performance, which could remove almost all RhB within 25 min and effectively decompose 82.3% of BPA in 120 min. Three-dimensional excitation-emission matrix fluorescence spectra (3D EEMs) were utilized for the purposes of fully grasping the behaviors of RhB molecules during the reaction process. Meanwhile, the effects of initial RhB concentration and co-existent electrolytes were investigated from the viewpoint of practical application. In addition, there was no obvious loss in degradation efficiency even after four cycles. The enhanced photocatalytic performances of AgBr/Ag/PbBiO2Br could be credited to the accelerated interfacial charge transfer process and the improved separation of the photogenerated electron-hole pairs. The existence of a small amount of metallic Ag played a significant role in preventing AgBr from being further photocorroded, resulting in the formation of a stable Z-scheme photocatalyst system. This study demonstrated that using metallic Ag as an electron mediator to construct Z-scheme photocatalytic system provided a feasible strategy in promoting the stability of Ag-based semiconductors.

(Keynote) Photocatalytic and Photoelectrochemical Water Splitting and CO2 Reduction As Artificial Photosynthesis

Water splitting and CO2 fixation of uphill reactions can be regarded as artificial photosynthesis, because light energy is converted to chemical energy. In the present paper, I mention the trends of photocatalytic water splitting to generate solar hydrogen and then introduce our results of various metal oxide and sulfide photocatalysts, and photoelectrochemical cells for water splitting and CO2 reduction of uphill reactions, aiming at artificial photosynthesis.1) Rh and Sb-codoped SrTiO3 photocatalyst loaded with IrO2 is active for water splitting into H2 and O2 under visible light and simulated sunlight irradiations as a single particle type photocatalyst. This photocatalyst responds to 500 nm.2) SrTiO3:Rh of a H2-evolving photocatalyst and BiVO4 of an O2-evolving photocatalyst construct various type of Z-schematic photocatalyst systems with Fe3+/Fe2+, [Co(bpy)3]3+/2+, [Co(phen)3]3+/2+, and a conductive reduced graphene oxide (RGO) as an electron mediator and even without an electron mediator. It is noteworthy that a sheet photocatalyst consisting of SrTiO3:Rh,La and BiVO4 powders with a Au contacting layer shows a quite high activity.3) On the other hand, SnNb2O6 with 2.4 eV of the band gap that has previously been reported as a powdered photocatalyst been has arisen as a candidate of an alternative of a BiVO4 photoanode.4) Metal sulfide photocatalysts that are normally unstable for water splitting into H2 and O2 in the absence of an electron donor can be employed for Z-schematic photocatalyst systems for water splitting. Z-schematic photocatalyst systems combining metal sulfide photocatalysts as a H2-evolving photocatalyst with TiO2 (RGO/TiO2)5) and BiVO4+Co complex (an electron mediator) as an O2-evolving photocatalyst show activity for water splitting into H2 and O2.5) These photocatalyst materials can also be employed for photoelectrochemical system for solar water splitting.6,7) Ag/BaLa4Ti4O15 and Ag/NaTaO3:M (M=Ca, Sr, Ba, and La) photocatalysts with wide bandgaps show activities for CO2 reduction to form CO and HCOOH in an aqueous medium without any sacrificial reagents.8,9) O2 evolved with a stoichiometric amount indicating that water reacted as an electron donor indicating that an uphill reaction of CO2 reduction accompanied with water oxidation was achieved. CuGaS2-RGO/BiVO4 of a Z-scheme photocatalyst system is active for water splitting and CO2 reduction to CO under visible light irradiation without any sacrificial reagents. This is the first time to demonstrate CO2 reduction using water as an electron donor in a Z-schematic powdered photocatalyst system with visible light response.10) References 1) A. Kudo, Y. Miseki, Chem. Soc. Rev., 2009, 38, 253. 2) R. Asai, H. Nemoto, Q. Jia, K. Saito, A. Iwase, A. Kudo, Chem. Commun. 2014, 50, 2543. 3) Q. Wang, T. Hisatomi, Q. Jia, H. Tokudome, M. Zhong, C. Wang, Z. Pan, T. Takata, M. Nakabayashi, N. Shibata, Y. Li, I. Sharp, A. Kudo, T. Yamada, and K. Domen, Nature Mater, 2016, 15, 611. 4) R. Niishiro, Y. Takano, Q. Jia, M. Yamaguchi, A. Iwase, Y. Kuang, T. Minegishi, T. Yamada, K. Domen, and A. Kudo, Chem. Commun., 2017, 53, 629. 5) K. Iwashina, A. Iwase, Y. Hau Ng, R. Amal, A. Kudo, J. Am. Chem. Soc., 2015, 137, 604. 6) T. Kato, Y. Hakari, S. Ikeda, Q. Jia, A. Iwase, A. Kudo, J. Phys. Chem. Lett., 2015, 6, 1042. 7) Q. Jia, K. Iwashina, A. Kudo, Proc. Natl. Acad. Sci. USA, 2012, 109, 11564. 8) K. Iizuka, T. Wato, Y. Miseki, K. Saito, and A. Kudo, J. Am. Chem. Soc., 2011, 133, 20863. 9) H. Nakanishi, K. Iizuka, T. Takayama, A. Iwase, A. Kudo, ChemSusChem, 2017, 10, 112. 10) A. Iwase, S. Yoshino, T. Takayama, Y. H. Ng, R. Amal, A. Kudo, J. Am. Chem. Soc., 2016, 138, 10260.

Facile fabrication of direct Z-scheme MoS2/Bi2WO6 heterojunction photocatalyst with superior photocatalytic performance under visible light irradiation

Abstract Direct Z-scheme photocatalyst systems with efficient charge separation as well as outstanding oxidation and reduction ability have been under immense investigation. In our work, direct Z-scheme MoS 2 /Bi 2 WO 6 heterojunctions were prepared by a facile method. The heterostructure was formed by loading MoS 2 nanoparticles on the surface of Bi 2 WO 6 nanosheets. It was found that the as-prepared MoS 2 /Bi 2 WO 6 composites showed superior photocatalytic activity than that of individual MoS 2 and Bi 2 WO 6 . The optimal composite with 4 at% MoS 2 content exhibited the highest photocatalytic activity. Based on the active species trapping experiments, photoluminescence spectroscopy analysis, and photocurrent responses, the possible enhanced photocatalytic mechanism could be ascribed to a direct Z-scheme heterojunction system which can not only increase the separation efficiency of photogenerated electron-hole pairs but also possess a splendid oxidation and reduction ability for high photocatalytic performances. This work provides an effective approach to develop other Bi 2 WO 6 -based direct Z-scheme photocatalytic systems for environmental purification and energy conversion.

Three dimensional Z-scheme (BiO)2CO3/MoS2 with enhanced visible light photocatalytic NO removal

The Z-scheme photocatalyst systems, with the redox ability of photogenerated electrons and holes unchanged and the efficient separate of carriers, have been under extensive investigation. Herein, we developed a facile method to prepare three dimensional Z-scheme (BiO)2CO3/MoS2 photocatalysts. Due to the enhanced visible light absorption, fast electrons and holes separate and transfer, the resulting three dimensional (BiO)2CO3/MoS2 photocatalysts exhibited high visible photocatalytic activity in the removal of NO in air, which exceeded those of the pristine (BiO)2CO3 and MoS2. Electron spin resonance trapping results indicated that h+ and O2− were the main reactive species for NO removal. Although (BiO)2CO3 is a n-type semiconductor while MoS2 is a p-type semiconductor, a unique Z-scheme electron/hole transport mechanism instead of the p-n junction mechanism was proposed for the interfacial interaction between MoS2 and (BiO)2CO3 under visible light irradiation. The present work reveals a unique 3D Z-scheme photocatalyst system with p- and n-type semiconductors, and opens up new opportunities in designing hybrid photocatalysts with enhanced photocatalytic properties.

Co(bpy)3]3+/2+ and [Co(phen)3]3+/2+ Electron Mediators for Overall Water Splitting under Sunlight Irradiation Using Z-Scheme Photocatalyst System

[Co(bpy)3](3+/2+) and [Co(phen)3](3+/2+) redox couples were revealed to play as electron mediators for Z-scheme photocatalyst systems composed of Ru/SrTiO3:Rh and BiVO4 powders for overall water splitting under visible light irradiation. These electron mediators were effective for only the combination of SrTiO3:Rh with BiVO4. They did not work when nondoped SrTiO3 and TiO2 of H2-evolving photocatalysts and WO3 of O2-evolving photocatalysts were employed. These results indicated that the affinity between photocatalysts and the Co-complex electron mediators was important. The photocatalytic activity depended on pH. Neutral pH conditions gave the highest activity for overall water splitting. Overall water splitting by the present system steadily proceeded for a long time. The Z-scheme photocatalyst system was also confirmed to split water under sunlight irradiation at the rates depending on weather. Moreover, overall water splitting by the Z-scheme photocatalyst system with the Co-complex electron mediator using a reaction cell in which the Ru/SrTiO3:Rh suspension was divided from BiVO4 suspension by a membrane filter resulted in H2 evolution separated from that of O2.

Z-scheme photocatalyst systems for water splitting under visible light irradiation

Abstract

Development of Visible-Light-Driven TiO<sub>2</sub> and SrTiO<sub>3</sub> Photocatalysts Doped with Metal Cations for H<sub>2</sub> or O<sub>2</sub> Evolution

This review paper represents photocatalytic properties of metal cation-doped TiO2 (rutile) and SrTiO3 photocatalysts for O2 evolution from an aqueous silver nitrate solution and H2 evolution from an aqueous methanol solution under visible light irradiation. Photocatalytic activities for the O2 evolution of Cr/Sb and Rh/Sb-codoped TiO2 are strongly dependent on the codoping ratio and the amount of doped chromium and rhodium. The codopant controls the oxidation number of doped chromium and rhodium. Rh-doped SrTiO3 in which the doped Rh species possesses a reversible redox property is active for the H2 evolution reaction under visible light irradiation. Overall water splitting under visible light irradiation proceeds with Z-scheme photocatalyst systems consisting of the Rh-doped SrTiO3 as a H2 evolution photocatalyst combined with BiVO4 as an O2 evolution photocatalyst and an Fe3+/Fe2+ electron mediator.