Focus on Photochemical or Photothermal Catalysts for Solar Fuel Conversion

The random disposal and immature recycling of post-consumer polyethylene terephthalate (PET) packages lead to a severe threaten to the ecological system owing to slow natural degradation kinetics of PET plastic, and meanwhile cause a waste of carbon resources stored in PET plastics. Many methods have been developed to recycle PET plastics, such as mechanical recycling, which induces a reduced quality relative to the virgin PET. In recent years, the photocatalytic conversion of PET plastic wastes into chemicals has received considerable attention due to their unique advantages, including mild conditions, less energy consumption, and simple operation. In this review, we have summarized the latest achievements in photoreforming of PET plastics into value-added chemicals. Primarily, we described the mechanism for bond cleavage during PET photoreforming, the emerging pretreatment methodologies for PET plastics, and the advantages of photocatalytic PET plastics conversion. Then, we introduced electro-/bio-assisted photocatalysis technologies for PET disposal and commented their strengths and limitations. Finally, we put forward the challenges and potential advances in the domain of photocatalytic PET plastics conversion.

Photocatalytic CO₂ reduction is considered to be an appealing way of alleviating environmental pollution and energy shortages simultaneously under mild condition. However, the activity is greatly limited by the poor separation of the photogenerated carriers. Ion doping is a feasible strategy to facilitate the charge transfer. In this work, Ni-doped Bi₄O₅I₂ photocatalyst is successfully fabricated using a one-pot hydrothermal method. A few doping levels appear in the energy band of Bi₄O₅I₂ after Ni doping, which are used as springboards for electrons transition, thus promoting photoexcited electrons and holes separation. As a consequence, a remarkably enhanced yield of CO and CH₄ (6.2 and 1.9 μmol g⁻¹ h⁻¹) is obtained over the optimized Bi₄O₅I₂-Ni15, which is approximately 2.1 and 3.8 times superior to pure Bi₄O₅I₂, respectively. This work may serve as a model for the subsequent research of Bi-based photocatalysts to implement high-performance CO₂ photoreduction.

This work investigates the implication of graphene and Cu₂ZnSnS₄ (CZTS) quantum dots (QDs) incorporation in the hematite thin film for its use in a photoelectrochemical cell. The thin film has been prepared by decorating the CZTS QDs over graphene-hematite composite by simple chemical approach. In Comparison to graphene modification and CZTS QDs modification separately over hematite thin film, the combination of both has produced more photocurrent. The photocurrent density obtained for CZTS QDs and graphene modified hematite thin film is 1.82 mA cm⁻² at 1.23 V/RHE, which is 1.75 higher than pristine hematite. The presence of CZTS QDs over hematite-graphene composite enhances the absorption properties of composite along with creating the p–n junction heterostructure which aids the transportation of the charge carriers. The thin films have been characterized using x-ray diffractometer, Raman spectroscopy, field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy, and diffuse reflectance UV–vis spectroscopy for phase, morphology and optical properties analysis. The enhancement in photoresponse has been justified by Mott-Schottky and transient open circuit potential analysis.

An efficient broadband responsive two-dimensional (2D) heterometallic Zn-/Co-porphyrin conjugated polymer (ZnCoP-F CP) with its Co-porphyrin bridging unit bearing two perfluorophenyls is coupled with 2.0 wt% Pt-loaded graphite carbon nitride (PCN) to fabricate a novel 2D/2D nanocomposite (ZnCoP-F/PCN). The resultant ZnCoP-F/PCN composite with an optimal mass ratio exhibits broadband (UV–vis–NIR) responsive H₂ evolution reaction (HER) activity up to 432 μmol h⁻¹, 5.2 and 2.8 times higher than that of the ZnCoP-F CP (83 μmol h⁻¹) and PCN (151 μmol h⁻¹) alone, respectively. Furthermore, the ZnCoP-F/PCN displays excellent apparent quantum yields (AQY) of 18.2%, 18.3%, 17.6%, 16.5%, 13.9%, 8.7%, 5.1%, 4.3%, 1.9%, 0.95% and 0.62% at 350, 380, 420, 450, 500, 550, 600, 700, 785, 850 and 950 nm, which are also higher than that of ZnCoP-F CP illuminated at the respective monochromatic light. The enhanced broadband responsive HER performance of ZnCoP-F/PCN can be attributed to the easily assembled ZnCoP-F CP and PCN nanosheets through strong π–π stacking interaction, which can facilitate the fast charge transfer from ZnCoP-F CP to PCN for HER. This work opens a new pathway to fabricate porphyrin polymer-based nanocomposite for more efficiently converting solar radiation and water into H₂.

The increasing energy and environmental problems have made clean energy-driven catalysis a hot research topic. Methane is an earth-abundant raw material but difficult to be converted by thermochemical processes. It is of great significance to seek novel strategies to convert methane into high-value chemicals. Herein, we synthesize a series of transition metal catalysts based on layered double hydroxide precursors which were used for photothermal methane nonoxidative coupling reactions. The strong photothermal and chemisorption effects of the derived transition metal nanostructures allow the efficient activation of methane molecules. Among them, alumina-supported metallic Ni and NiCo-alloy catalysts show excellent methane nonoxidative coupling activities, achieved hydrogen production rates of 4816.53 μmol g⁻¹ h⁻¹ and 5130.9 μmol g⁻¹ h⁻¹, accompanied by liquid fuels production rates of 59.2 mg g⁻¹ h⁻¹ and 63 mg g⁻¹ h⁻¹, respectively. The findings, therefore, provide a new strategy for methane nonoxidative coupling driven by light energy at mild conditions.

Photocatalytic conversion of carbon dioxide into fuels and valuable chemicals is a promising method for carbon neutralization and solving environmental problems. Through a simple thermal-oxidative exfoliation method, the O element was doped while exfoliated bulk g-C₃N₄ into ultrathin structure g-C₃N₄. Benefitting from the ultrathin structure of g-C₃N₄, the larger surface area and shorter electrons migration distance effectively improve the CO₂ reduction efficiency. In addition, density functional thory computation proves that O element doping introduces new impurity energy levels, which making electrons easier to be excited. The prepared photocatalyst reduction of CO₂ to CO (116 μmol g⁻¹ h⁻¹) and CH₄ (47 μmol g⁻¹ h⁻¹).

Photochemical conversion of CO₂ into solar fuels is one of the promising strategies to reducing the CO₂ emission and developing a sustainable carbon economy. For the more efficient utilization of solar spectrum, several approaches were adopted to pursue the visible-light-driven SrTiO₃. Herein, oxygen vacancy was introduced over the commercial SrTiO₃ (SrTiO_3−x) via the NaBH₄ thermal treatment, to extend the light absorption and promote the CO₂ adsorption over SrTiO₃. Due to the mid-gap states resulted from the oxygen deficiency, combined with the intrinsic energy level of SrTiO₃, the SrTiO_3−x catalyst exhibited excellent CO productivity (4.1 μmolˑg⁻¹ˑh⁻¹) and stability from the CO₂ photodissociation under the visible-light irradiation (λ > 400 nm). Then, surface alkalization over SrTiO_3−x (OH-SrTiO_3−x) was carried out to further enhance the CO₂ adsorption/activation over the surface base sites and provide the OH ions as hole acceptor, the surface alkali OH connected with Sr site of SrTiO₃ could also weaken the Sr–O bonding thus facilitate the regeneration of surface oxygen vacancy under the light illumination, thus resulting in 1.5 times higher CO productivity additionally. This study demonstrates that the synergetic modulation of alkali OH and oxygen vacancy over SrTiO₃ could largely promote the CO₂ photodissociation activity.

Photocatalytic production of H₂O₂ from water and atmospheric oxygen has been recognized as a green and sustainable chemical process, due to the abundance of raw materials and sustainable solar energy. Herein, flower-like hierarchical ZnO microspheres were prepared by hydrothermal method followed by calcination at different temperatures, and their photocatalytic performance in H₂O₂ production was examined under simulated sunlight irradiation. The calcination temperature plays a vital role in the structure, morphology, and surface area of the final ZnO products as well as their optical and electrochemical properties, which are determining factors in their photocatalytic activity. The ZnO calcined at 300 °C (Zn-300) exhibits the highest activity and optimal stability, showing productivity of 2793 μmol l⁻¹ within 60 min of irradiation, which was 6.5 times higher than that of uncalcined ZnO precursor. The remarkable photocatalytic activity is attributed to enhanced light utilization, large surface area, abundant exposed active sites, improved separation efficiency, and prolonged carrier lifespan. Moreover, the results from cycling experiments indicate the as-prepared ZnO samples exhibit good stability and long-time performance. This work provides useful information for the preparation of hierarchical ZnO photocatalysts.