Realizing efficient exciton dissociation in an all-organic heterojunction photocatalyst for highly improved photocatalytic H2 evolution

Although graphitic carbon nitride is a promising photocatalyst in the field of energy conversion and environmental purification, the intrinsic properties like excitonic effects and sluggish charge transfer restrict further photocatalytic applications. To circumvent these limitations, the novel all-organic heterojunction photocatalysts were constructed by anchoring organic carbon dots (O-dots) on porous graphitic carbon nitride nanosheets (O-dots/CNS). Results demonstrated that excitons can be e?ectively dissociated into electrons and holes at the interface of O-dots/CNS heterojunction, followed by holes injected to O-dots and electrons accumulated in CNS to realize efficient charge separation. Consequently, the O-dots/CNS with the optimized hydrogen (H₂) evolution performance could be reached 1564.5 μmol h^?1g^?1 under the visible light irradiation. This work not only presents new ideas for rational design photocatalytic reaction system from exciton and charge carrier, but also broaden the applications of this new kind of organic dots in the field of energy conversion.     

Wet chemistry synthesis of CuFe2O4/CdSe heterojunction for enhanced efficient photocatalytic H2 evolution under visible irradiation

A heterostructure of CuFe₂O₄/CdSe was synthesized as H₂ evolution photocatalyst under visible light. The optical absorption onset of the CuFe₂O₄/CdSe heterostructures was red-shifted to 2.30–2.48 eV, compared to that of the bare CuFe₂O₄ (2.55 eV), leading to better utilization of visible light. Furthermore, the CuFe₂O₄/CdSe samples exhibited a higher specific surface area than the bare CuFe₂O₄, due to the introduction of CdSe nanospheres. Compared to the bare CuFe₂O₄, the CuFe₂O₄/CdSe heterostructure promoted H₂ production from water splitting. The enhanced photocatalytic performance of the CuFe₂O₄/CdSe catalyst was attributed to the more efficient charge separation and lower charge transfer resistance, confirmed by fluorescence decay measurements and Nyquist plots, respectively. The band alignment between CuFe₂O₄ and CdSe resulted in an interfacial p-n junction, which directed the electron transfer from CdSe to CuFe₂O₄ and the hole transfer from CuFe₂O₄ to CdSe, achieving improved spatial separation of charge carriers.     

Engineering highly efficient photocatalysts for hydrogen production by simply regulating the solubility of insoluble compound cocatalysts

A series of heterostructured composites composed of insoluble copper (II) compounds (CuX) loaded on P25, in which the CuX possess different solubility products (K_sp), have been fabricated to compare the charge separation efficiencies and photocatalytic hydrogen production activities. The results indicate that the K_sp of CuX in the as-prepared photocatalysts strongly correlates with the charge separation efficiencies and photocatalytic activities for hydrogen production. The as-optimized Cu₂(OH)₂CO₃/P25 photocatalyst shows an excellent photocatalytic activity for hydrogen production with an apparent quantum efficiency up to 31.9%, far exceeding that of bare P25 by 485 times. An innovative strategy for constructing highly efficient insoluble compound-semiconductor heterostructured photocatalysts is proposed, where regulating the reduction potential (φ) of the insoluble compounds can simultaneously control both the separation efficiency of photogenerated charge carriers and the reduction ability of the transferred electrons. This design strategy shows an obvious advantage that changing K_sp through selecting right Xⁿ⁻ can easily modulate the φ of the insoluble compounds to significantly enhance the photocatalytic activity of the heterostructured photocatalyst. The results reported here not only inspire us to engineer highly efficient photocatalysts by the utilization of insoluble compounds as cocatalysts, but also offer an innovative possibility for the enhanced separation of photogenerated charge carriers in solar cells.     

Electronic structure and photocatalytic properties of copper-doped CaTiO3

Perovskite oxides, CaTi_1−xCu_xO₃, with the density of copper ions, x, ranging from 0.01 to 0.04, were prepared by sol-gel method coupled with ultrasonic technique for the first time. The determination by X-ray diffraction pattern of crystal structure and UV–visible light adsorption studied by ultraviolet-visible absorption spectroscopy (UV–vis) were reported. Electronic structures were investigated by density functional theory (DFT). It has been found that CaTiO₃-doped with 2 mol% Cu²⁺ exhibits the highest activity to the photocatalytic decomposition of water. Photocatalytic activity of doped CaTiO₃ powder for hydrogen evolution under UV light is increased dramatically about 8 times than that of pure CaTiO₃ powder when the NiO_x is used for cocatalyst. The results of DFT calculation illuminate that absorption of visible light is mainly due to the transition from the donor levels formed by Cu²⁺ to the conduction band of copper-doped CaTiO₃.     

Noble metal-free cuprous oxide/reduced graphene oxide for enhanced photocatalytic hydrogen evolution from water reduction

Cu₂O loaded reduced graphene oxide (Cu₂O/RGO) was prepared via a one-step in-situ reduction method. Composition and structure of the Cu₂O/RGO were characterized by X-ray diffraction, high resolution transmission electron microscope and X-ray photoelectron spectroscopy. With eosin Y (EY) and rose bengal (RB) as co-sensitizers, the activity of hydrogen evolution over the Cu₂O/RGO dramatically increased and achieved a maximum when the loading amount of Cu on the RGO was about 3 wt.%. It exceeded that of RGO and Cu₂O by a factor of 7.3 and 4.2 at the same conditions, respectively. It could be even comparable to that of the Pt/RGO under the same reaction conditions. This work showed a possibility of utilizing Cu₂O as an alternative for noble metals (such as Pt) due to its low cost and high performance in photocatalytic hydrogen production.     

ZnO–Bi2O3/graphene oxide photocatalyst with high photocatalytic performance under visible light

Abstract

Photocatalytic nanomaterials are attracting more and more attention because of their potential for solving environmental problems. ZnO, as one of the most promising photocatalysts, can only be excited by ultraviolet (UV) or near UV radiation. The objective of the study is to describe an efficient visible light driven ZnO based photocatalyst. In this regard, we communicate the preliminary research on the synthesis, characterisation and photocatalytic properties of ZnO–Bi₂O₃/graphene oxide (GO) composite materials. It was found that the photodegradation of methylene blue in the presence of ZnO–Bi₂O₃/GO reached 99·62% after irradiation with visible light for 2 h. The presence of GO enhances the stability of ZnO–Bi₂O₃ and reduces the recombination of charge carriers. ZnO–Bi₂O₃/GO also shows high photocatalytic activity for the degradation of acid blue, acid yellow, reactive red, acid red, reactive yellow and reactive blue under visible light irradiation. The novel aspect is the combination of GO and Bi₂O₃ doped ZnO. The use of GO enhances the efficiency of photocatalysis, and Bi₂O₃ doping ZnO excites the absorption of visible light. The impact of the research concerns the study of ZnO–Bi₂O₃/GO, which can be used as a promising photocatalyst for the treatment of textile wastewater.     

Oxide content optimized ZnS–ZnO heterostructures via facile thermal treatment process for enhanced photocatalytic hydrogen production

The ZnS–ZnO heterostructured photocatalysts are synthesized by thermal treatments of the ZnS materials at various thermal processing temperatures (150 °C – 550 °C) with controlling O₂ partial pressures (7.8 kPa – 33.8 kPa). The ZnS–ZnO composite structure shows much higher photocatalytic hydrogen production than those from the ZnS and ZnO pure substances. This phenomenon is mainly caused by effective charge separation between the photoexcited electrons and holes. The thermal oxidation of ZnS materials proceeds at temperatures higher than 500 °C. In addition to the thermal processing temperature, O₂ partial pressure is also chosen for an experimental variable in order to control the atomic composition minutely. The ZnS–ZnO photocatalyst composite fabricated at 500 °C under 16.9 kPa of O₂ partial pressure shows the highest hydrogen production rate of 494.8 μmol g⁻¹ h⁻¹ under 1 sun irradiation condition, and it is 37 times higher than that (13.5 μmol g⁻¹ h⁻¹) from the ZnS pure substance. At this optimized production rate, the Zn/S/O atomic compositions are measured as 45.9/46.9/7.2 (XPS) and 53.3/42.1/4.6 (ICP-AES), respectively.     

Construction of ZnIn2S4/ZnO heterostructures with enhanced photocatalytic decomposition and hydrogen evolution under blue LED irradiation

The novel ZnIn₂S₄/ZnO heterostructures can prepare by combining a facile hydrothermal and wet chemical methods. Various microscale and nanoscale characterization techniques analyzed the surface topography, structural and optical properties of the ZnIn₂S₄/ZnO heterostructures. The addition amount of ZnIn₂S₄ nanosheets plays an essential role in controlling the optical properties of ZnIn₂S₄/ZnO heterostructures. ZnIn₂S₄/ZnO heterostructures can improve charge carriers separation and specific surface area for enhancing the photocatalytic decomposition of the 4-aminobenzoic acid solution and hydrogen evolution under blue LED irradiation. Furthermore, ZnIn₂S₄/ZnO heterostructures also revealed high photocatalytic stability and reusability for long-term reaction processes. Therefore, ZnIn₂S₄/ZnO heterostructures can be used as efficient visible-light-induced photocatalysts for the practical applications in water splitting and wastewater treatment.     

Fabrication of In(OH)3–In2S3–Cu2O nanofiber for highly efficient photocatalytic hydrogen evolution under blue light LED excitation

This study synthesized the In(OH)₃–In₂S₃ nanosheet via a simple hydrothermal method. The different amounts of In(OH)₃–In₂S₃ nanosheet used to synthesize In(OH)₃–In₂S₃–Cu₂O composite by a wet chemical method. FESEM, FETEM, XRD, XPS, BET, UV–vis DRS, and PL spectroscopy characterized the In(OH)₃–In₂S₃–Cu₂O composite. Compared with In(OH)₃–In₂S₃ nanosheet, In(OH)₃–In₂S₃–Cu₂O composite can exhibit the synergistic effects of higher specific surface area, higher light-harvesting capacity, and accelerating the separation and migration of photogenerated charge carriers. In addition, In(OH)₃–In₂S₃–Cu₂O nanofiber was fabricated via a facile electrospinning process at the different amounts of In(OH)₃–In₂S₃ nanosheet. The manufacturing process offers many advantages, such as simplicity, low temperature, and without templates. The effect of operational parameters (such as the reaction conditions of photocatalysts, pH values, sacrificial reagents, and light sources) on the photocatalytic hydrogen production of In(OH)₃–In₂S₃–Cu₂O nanofiber was investigated. The results indicate that the appropriate reaction conditions of In(OH)₃–In₂S₃–Cu₂O nanofiber can reveal higher efficient photocatalytic water splitting than commercial TiO₂ or ZnO nanofiber under blue light LED excitation. Furthermore, In(OH)₃–In₂S₃–Cu₂O nanofiber can provide a simple fabrication process, high photocatalytic activity, and high reusability shall be beneficial for the practical application of photocatalytic hydrogen production.     

Effects of anions on the photocatalytic H2 production performance of hydrothermally synthesized Ni-doped Cd0.1Zn0.9S photocatalysts

A series of Ni²⁺ doped Cd_0.1Zn_0.9S photocatalysts were prepared with different salt by hydrothermal method. The prepared photocatalysts were characterized by XRD, UV–Vis, BET and SEM. The effects of SO₄²⁻, CH₃COO⁻, Cl⁻ and NO₃⁻ anions on the photocatalytic hydrogen production performance of these photocatalysts were examined. Experimental results showed that the photocatalysts prepared with acetates and chlorides have the highest hydrogen production activity, and their initial hydrogen production rate reaches to 76.52 μmol/h and 80.75 μmol/h under visible-light irradiation with the apparent quantum yield of 12.30% and 14.36% at 420 nm without any noble metal loading, respectively. The absence of noble metal is propitious to reduce the cost of photocatalyst preparation.     

Photocatalytic activities of Sr2Ta2O7 nanosheets synthesized by a hydrothermal method

Sr₂Ta₂O₇ nanosheets have been synthesized by a hydrothermal method without using any template. The thickness, widths, and lengths of Sr₂Ta₂O₇ nanosheets are about 10–50 nm, 50–150 nm, and 500 nm, respectively. The optimum conditions for the formation of the nanosheets are maintaining the reactants at 260 °C for 7 days. On basis of the experimental data, a possible formation mechanism of the nanosheets under the hydrothermal conditions was proposed. The photocatalytic activity for water splitting was investigated under ultraviolet irradiation. It has been found that Sr₂Ta₂O₇ nanosheets, compared to the bulk Sr₂Ta₂O₇ sample, showed a higher photocatalytic activity even in the absence of a cocatalyst. The higher activity of the hydrothermally synthesized sample is attributed to its larger surface areas and nanoscale structure.