Crystal Facet-Dependent CO2 Photoreduction over Porous ZnO Nanocatalysts

Asymmetric Atomic Dual-Sites for Photocatalytic CO2 Reduction

Atomically dispersed active sites in a photocatalyst offer unique advantages such as locally tuned electronic structures, quantum size effects, and maximum utilization of atomic species. Among these, asymmetric atomic dual-sites are of particular interest because their asymmetric charge distribution generates a local built-in electric potential to enhance charge separation and transfer. Moreover, the dual sites provide flexibility for tuning complex multielectron and multireaction pathways, such as CO2 reduction reactions. The coordination of dual sites opens new possibilities for engineering the structure-activity-selectivity relationship. This comprehensive overview discusses efficient and sustainable photocatalysis processes in photocatalytic CO2 reduction, focusing on strategic active-site design and future challenges. It serves as a timely reference for the design and development of photocatalytic conversion processes, specifically exploring the utilization of asymmetric atomic dual-sites for complex photocatalytic conversion pathways, here exemplified by the conversion of CO2 into valuable chemicals.

Ruthenium Single Atomic Sites Surrounding the Support Pit with Exceptional Photocatalytic Activity

Single-metal atomic sites and vacancies can accelerate the transfer of photogenerated electrons and enhance photocatalytic performance in photocatalysis. In this study, a series of nickel hydroxide nanoboards (Ni(OH)x NBs) with different loadings of single-atomic Ru sites (w-SA-Ru/Ni(OH)x) were synthesized via a photoreduction strategy. In such catalysts, single-atomic Ru sites are anchored to the vacancies surrounding the pits. Notably, the SA-Ru/Ni(OH)x with 0.60 wt % Ru loading (0.60-SA-Ru/Ni(OH)x) exhibits the highest catalytic performance (27.6 mmol g-1 h-1) during the photocatalytic reduction of CO2 (CO2RR). Either superfluous (0.64 wt %, 18.9 mmol g-1 h-1; 3.35 wt %, 9.4 mmol-1 h-1) or scarce (0.06 wt %, 15.8 mmol g-1 h-1; 0.29 wt %, 21.95 mmol g-1 h-1; 0.58 wt %, 23.4 mmol g-1 h-1) of Ru sites have negative effect on its catalytic properties. Density functional theory (DFT) calculations combined with experimental results revealed that CO2 can be adsorbed in the pits; single-atomic Ru sites can help with the conversion of as-adsorbed CO2 and lower the energy of *COOH formation accelerating the reaction; the excessive single-atomic Ru sites occupy vacancies that retard the completion of CO2RR.

Structure-driven tuning of catalytic properties of core-shell nanostructures

The annual increase in demand for renewable energy is driving the development of catalysis-based technologies that generate, store and convert clean energy by splitting and forming chemical bonds. Thanks to efforts over the last two decades, great progress has been made in the use of core-shell nanostructures to improve the performance of metallic catalysts. The successful preparation and application of a large number of bimetallic core-shell nanocrystals demonstrates the wide range of possibilities they offer and suggests further advances in this field. Here, we have reviewed recent advances in the synthesis and study of core-shell nanostructures that are promising for catalysis. Particular attention has been paid to the structural tuning of the catalytic properties of core-shell nanostructures and to theoretical methods capable of describing their catalytic properties in order to efficiently search for new catalysts with desired properties. We have also identified the most promising areas of research in this field, in terms of experimental and theoretical studies, and in terms of promising materials to be studied.

Lattice Distortion in a Confined Structured ZnS/ZnO Heterojunction for Efficient Photocatalytic CO2 Reduction

It is a promising strategy to effectively promote "carbon neutrality" by reducing CO₂ to small energy molecules through photocatalysis technology. However, due to low light utilization and recombination of photogenerated carriers, photocatalysts usually have low activity and low selectivity for products. Herein, a hollow spherical ZnS/ZnO heterojunction with a spatial confinement effect photocatalyst was synthesized toward CO₂ photoreduction through preciously controlling the nano-/microstructure. The local lattice distortions were introduced into the surface of the hollow ZnS/ZnO microsphere, which activated lattice oxygen and provided additional active reaction sites. Furthermore, the heterojunction constructed between ZnS and ZnO interfaces facilitated the separation of photoinduced charge carriers. Combined with the natural advantage of enhanced light capture and absorption for a hollow confined structure, as a result, the systemic design in the electronic and confined structures for the photocatalyst has brought an excellent CO₂ reduction performance with a CO yield rate as high as 35.85 μmol g^-1h^-1 and durability under a 300 W Xe lamp irradiation without any sacrificial agent and cocatalyst.

Effect of the Synthetic Parameters over ZnO in the CO2 Photoreduction

Zinc oxide (ZnO) is an attractive semiconductor material for photocatalytic applications, owing to its opto-electronic properties. Its performances are, however, strongly affected by the surface and opto-electronic properties (i.e., surface composition, facets and defects), in turn related to the synthesis conditions. The knowledge on how these properties can be tuned and how they are reflected on the photocatalytic performances (activity and stability) is thus essential to achieve an active and stable material. In this work, we studied how the annealing temperature (400 °C vs. 600 °C) and the addition of a promoter (titanium dioxide, TiO₂) can affect the physico-chemical properties of ZnO materials, in particular surface and opto-electronic ones, prepared through a wet-chemistry method. Then, we explored the application of ZnO as a photocatalyst in CO₂ photoreduction, an appealing light-to-fuel conversion process, with the aim to understand how the above-mentioned properties can affect the photocatalytic activity and selectivity. We eventually assessed the capability of ZnO to act as both photocatalyst and CO₂ adsorber, thus allowing the exploitation of diluted CO₂ sources as a carbon source.

Tuning the Microstructures of ZnO To Enhance Photocatalytic NO Removal Performances

Effective removal of kinetically inert dilute nitrogen oxide (NO, ppb) without NO₂ emission is still a challenging topic in environmental pollution control. One effective approach to reducing the harm of NO is the construction of photocatalysts with diversified microstructures and atomic arrangements that could promote adsorption, activation, and complete removal of NO without yielding secondary pollution. Herein, microstructure regulations of ZnO photocatalysts were attempted by altering the reaction temperature and alkalinity in a unique ionic liquid-based solid-state synthesis and further investigated for the removal of dilute NO upon light irradiation. Microstructure observations indicated that as-tuned photocatalysts displayed unique nucleation, diverse morphologies (spherical nanoparticles, short and long nanorods), defect-related optical characteristics, and enhanced carrier separations. Such defect-related surface-interface aspects, especially V_o″-related defects of ZnO devoted them to the 4.16-fold enhanced NO removal and 2.76 magnitude order decreased NO₂ yields, respectively. Improved NO removal and toxic product inhabitation in as-tuned ZnO was disclosed by mechanistic exploitations. It was revealed that regulated microstructures, defect-related charge carrier separation, and strengthened surface interactions were beneficial to active species production and molecular oxygen activation in ZnO, subsequently contributing to the improved NO removal and simultaneous avoidance of NO₂ formation. This investigation shed light on the facile regulation of microstructures and the roles of surface chemistry in the oxidation of low concentration NO in the ppb level upon light illumination.

Research Progress of Tungsten Oxide-Based Catalysts in Photocatalytic Reactions

Photocatalysis technology is a potential solution to solve the problem of environmental pollution and energy shortage, but its wide application is limited by the low efficiency of solar energy conversion. As a non-toxic and inexpensive n-type semiconductor, WO3 can absorb approximately 12% of sunlight which is considered one of the most attractive photocatalytic candidates. However, the narrow light absorption range and the high recombination rate of photogenerated electrons and holes restrict the further development of WO3-based catalysts. Herein, the studies on preparation and modification methods such as doping element, regulating defects and constructing heterojunctions to enlarge the range of excitation light to the visible region and slow down the recombination of carriers on WO3-based catalysts so as to improve their photocatalytic performance are reviewed. The mechanism and application of WO3-based catalysts in the dissociation of water, the degradation of organic pollutants, as well as the hydrogen reduction of N2 and CO2 are emphatically investigated and discussed. It is clear that WO3-based catalysts will play a positive role in the field of future photocatalysis. This paper could also provide guidance for the rational design of other metallic oxide (MOx) catalysts for the increasing conversion efficiency of solar energy.

High-ratio {100} plane-exposed ZnO nanosheets with dual-active centers for simultaneous photocatalytic Cr(VI) reduction and Cr(III) adsorption from water

The development of an efficient catalyst for the simultaneous removal of Cr(VI) and Cr(III) from water is required to eliminate the risk of Cr(III) reconversion in the photocatalytic Cr(VI) reduction process. ZnO with large regions of high-energy {001} and {101} surfaces is often used to degrade various pollutants due to its high activity. However, the more readily available low-energy facets have relatively limited its applications. Here, we report a new strategy that employs a high proportion of {100} plane-exposed ZnO nanosheets for simultaneous photocatalytic Cr(VI) reduction and Cr(III) adsorption. The mechanism of Zn-O co-exposed on the {100} plane as the dual-active centers to jointly promote Cr(VI) reduction and Cr(III) adsorption was clarified at the atomic level. ZnO nanosheets with a high exposure ratio of the {100} plane achieve a total Cr removal rate of over 90% within 120 min under simulated sunlight irradiation, neutral conditions, and a negligible difference in the band structure.

Exploring the Effects of Crystal Facet Orientation at the Heterojunction Interface on Charge Separation for Photoanodes

As one of the most effective strategies to promote the spatial separation of charges, constructing heterojunction has received extensive attention in recent years. However, it remains unclear whether the crystal facet orientation (CFO) at the heterojunction interface is contributory to charge separation. Herein, three types of TiO₂/CdS heterojunction films with different CFOs at the heterojunction interface were produced by adjusting the CdS CFO through in situ conversion. Among them, the TiO₂/CdS film with a mixed CdS CFO showed the maximum photocurrent density and charge separation efficiency. In contrast, the TiO₂/CdS film with a uniform CdS (100) (CdS-100) performed worst. According to the results of experimentation and DFT calculation, these three types of TiO₂/CdS films varied significantly in electron transport time. This is attributable to the different Fermi levels of CdS CFO and the formation of different built-in electric fields upon coupling with TiO₂. The rise in the Fermi level of CdS can increase the driving force required for charge migration at the heterojunction interface. Additionally, a stronger built-in electric field is conducive to charge separation. To sum up, these results highlight the significant impact of CFO at the heterojunction interface on charge separation.

The Photocatalytic Conversion of Carbon Dioxide to Fuels Using Titanium Dioxide Nanosheets/Graphene Oxide Heterostructure as Photocatalyst

Carbon dioxide (CO₂) photoreduction to high-value products is a technique for dealing with CO₂ emissions. The method involves the molecular transformation of CO₂ to hydrocarbon and alcohol-type chemicals, such as methane and methanol, relying on a photocatalyst, such as titanium dioxide (TiO₂). In this research, TiO₂ nanosheets (TNS) were synthesized using a hydrothermal technique in the presence of a hydrofluoric acid (HF) soft template. The nanosheets were further composited with graphene oxide and doped with copper oxide in the hydrothermal process to create the copper-TiO₂ nanosheets/graphene oxide (CTNSG). The CTNSG exhibited outstanding photoactivity in converting CO₂ gas to methane and acetone. The production rate for methane and acetone was 12.09 and 0.75 µmol h^-1 g_cat^-1 at 100% relative humidity, providing a total carbon consumption of 71.70 µmol g_cat^-1. The photoactivity of CTNSG was attributed to the heterostructure interior of the two two-dimensional nanostructures, the copper-TiO₂ nanosheets and graphene oxide. The nanosheets-graphene oxide interfaces served as the n-p heterojunctions in holding active radicals for subsequent reactions. The heterostructure also directed the charge transfer, which promoted electron-hole separation in the photocatalyst.

Chitosan Fibers Loaded with Limonite as a Catalyst for the Decolorization of Methylene Blue via a Persulfate-Based Advanced Oxidation Process

Wastewater generated from industries seriously impacts the environment. Conventional biological and physiochemical treatment methods for wastewater containing organic molecules have some limitations. Therefore, identifying other alternative methods or processes that are more suitable to degrade organic molecules and lower chemical oxygen demand (COD) in wastewater is necessary. Heterogeneous Fenton processes and persulfate (PS) oxidation are advanced oxidation processes (AOPs) that degrade organic pollutants via reactive radical species. Therefore, in this study, limonite powder was incorporated into porous regenerated chitosan fibers and further used as a heterogeneous catalyst to decompose methylene blue (MB) via sulfate radical-based AOPs. Limonite was used as a heterogeneous catalyst in this process to generate the persulfate radicals (SO₄^-·) that initiate the decolorization process. Limonite-chitosan fibers were produced to effectively recover the limonite powder so that the catalyst can be reused repeatedly. The formation of limonite-chitosan fibers viewed under a field emission scanning electron microscope (FESEM) showed that the limonite powder was well distributed in both the surface and cross-section area. The effectiveness of limonite-chitosan fibers as a catalyst under PS activation achieved an MB decolorization of 78% after 14 min. The stability and reusability of chitosan-limonite fibers were evaluated and measured in cycles 1 to 10 under optimal conditions. After 10 cycles of repeated use, the limonite-chitosan fiber maintained its performance up to 86%, revealing that limonite-containing chitosan fibers are a promising reusable catalyst material.

Insight into the Crystal Facet Effect of {101} and {100} Facets of CeVO4 in the Photochemical Property and Photocatalysis

To investigate the photochemical property of specific crystal facets, two well-defined CeVO₄ dodecahedrons with exposed {101} and {100} facets are prepared, which have distinguishing appearances and unequal {101}/{100} area ratios (A_{101}/A_{100}), i.e., compressed dodecahedra (CeVO₄ CD, A_{101}/A_{100} ≈ 1) and elongated dodecahedra (CeVO₄ ED, A_{101}/A_{100} ≈ 0.3). During the visible-light-irradiated process, the {101} and {100} facets are certified to selectively deposit photogenerated holes (h⁺) and electrons (e^-), thus exhibiting the photooxidability and photoreducibility, respectively. Meanwhile, a surface heterojunction could form at the adjacent facet interface and facilitate the spatial separation of carriers. Benefiting from the large exposure extent of the {101} facet and the rational A_{101}/A_{100} (∼1), the CeVO₄ CD shows a superior photocatalytic performance for the degradation of tetracycline to the CeVO₄ ED. Finally, simulation calculations reveal that the energy deviations of the valence band (VB) and conduction band (CB) between CeVO₄{101} and CeVO₄{100} impel the photogenerated h⁺ and e^- to transfer in opposite directions, resulting in the facet-dependent photoactivity of the CeVO₄ dodecahedron.

Experimental and theoretical evaluation of crystal facet exposure on the charge transfer and SERS activity of ZnO films

Semiconductors exhibit great potential as a surface enhanced Raman scattering (SERS) substrate due to their low cost, good stability and biocompatibility. However, the extensive application of semiconductors has been restricted by their intrinsically low SERS sensitivity. It is urgently required to design uniform metal oxide substrates with enhanced charge transfer and SERS activity. Herein, three facet-defined ({101̄0}, {0001} and {101̄1}) ZnO films were synthesized via an electrodeposition procedure with the assistance of KCl or ethylenediamine. According to the results, the ZnO films with {0001} and {101̄1} exposed facets exhibit appreciable SERS enhancement factors (EFs) of 1.6 × 10⁴ and 2.8 × 10⁴ for 4-nitrobenzenethiol (4-NBT), as well as a relatively low limit of detection (LOD) down to 1 × 10^-6 M and 5 × 10^-7 M, respectively. Simultaneously, the electrodeposited ZnO films deliver good repeatability and SERS stability, with relative standard deviation (RSD) less than 6% and 85.2% of their original activity retained after 40 days. Theoretical calculations verified that the {0001} and {101̄1} facets can transfer more electrons from ZnO to the molecules on account of their low facet-related electronic work functions, thus generating the noticeable improvement of SERS activity. The current study provides theoretical and technical support for the crystal facet engineering and property improvement of semiconductors.

Recent review of BixMOy (M=V, Mo, W) for photocatalytic CO2 reduction into solar fuels

The utilization of solar energy for CO₂ conversion not only enables a green and low-carbon recycling of CO₂ with renewable energy, but also solves ecological problems. Bi_xMO_y (M = V, Mo, W) materials have typical layered structures and unique electronic properties that provide suitable band gaps and potential to meet the basic conditions for CO₂ reduction. However, pristine Bi_xMO_y faces with problems such as small specific surface area, insufficient active sites, low charge carriers' separation and utilization efficiency. This review comprehensively described the basic principles and reaction pathways of photocatalytic CO₂ reduction, and further presented the research progress of Bi_xMO_y catalysts in CO₂ conversion reactions. In this perspective, we further focus on the design concepts and modification strategies to improve the photocatalytic CO₂ reduction activity of Bi_xMO_y, such as morphology control, constructing surface vacancies and heterojunction fabrication. Finally, based on representative researches, the present review will be expected to provide updated information and insights for developing advanced Bi_xMO_y materials to further improve CO₂ reduction activity and selectivity.

Co-embedding oxygen vacancy and copper particles into titanium-based oxides (TiO2, BaTiO3, and SrTiO3) nanoassembly for enhanced CO2 photoreduction through surface/interface synergy

Photocatalytic CO₂ reduction into valuable fuel and chemical production has been regarded as a prospective strategy for tackling with the issues of the increasing of greenhouse gases and shortage of sustainable energy. A composite photocatalysis system employing a semiconductor enriched with oxygen vacancy and coupled with metallic cocatalyst can facilitate charge separation and transfer electrons. In this work, mesoporous TiO₂ and titanium-based perovskite oxides (BaTiO₃ and SrTiO₃) nanoparticle assembly incorporated with abundant oxygen vacancy and copper particles have been successfully synthesized for CO₂ photoreduction. As an example, the TiO₂ decorated with different amounts of Cu particles has an impact on photocatalytic CO₂ reduction into CH₄ and CO. Particularly, the optimal TiO₂/Cu-0.1 exhibits nearly 13.5 times higher CH₄ yield (22.27 μmol g^-1 h^-1) than that of pristine TiO₂ (1.65 μmol g^-1 h^-1). The as-obtained BaTiO₃/Cu-0.1 and SrTiO₃/Cu-0.1 also show enhanced CH₄ yields towards photocatalytic CO₂ reduction compared with pristine ones. Based on the temperature programmed desorption (TPD) and photo/electrochemical measurements, the co-embedding of Cu particles and abundant oxygen vacancy into the titanium-based oxides could promote CO₂ adsorption capacity as well as separation and transfer of photoinduced electron-hole pairs, and finally result in efficient CO₂ photoreduction upon the TiO₂/Cu, SrTiO₃/Cu, and BaTiO₃/Cu composites.

Crystal-facet and microstructure engineering in ZnO for photocatalytic NO oxidation

Photocatalysis is believed to be an important way of reducing NO pollutant in air and the facet engineering of semiconducting oxides could enhance the efficiency of the photocatalysis. ZnO nanoparticles with different exposed crystalline facets were successfully synthesized using a hydrothermal method and their photocatalytic degradation towards NO was investigated. The crystals from ZnCl₂ precursor were hexagonal mesoporous ones with exposed (0002) facet, while those from zinc acetate were in the form of flakes or wheat ears with enhanced exposure of (101(-)1) facet. Calcination in air imparted an enhanced the textural coefficient of the orientated facets as well as the oxygen defects. The nanocrystals with enhanced (0002) facet and lower flat-band energy did better in photoelectrochemical water-oxidation than those with exposed (101(-)1) facet that showed superior photocatalytic activity (approaching 76.7 ± 0.6% under 365 nm photons) for NO oxidation. According to theoretical calculations, (101(-)1) facet with O termination showed much higher affinity to NO molecules than other configurations, and the oxygen vacancy in ZnO played an minor role in the photocatalytic oxidation of NO. A high quantum efficiency approaching 97.5 ± 1.4% under 275 nm photons was obtained for the ZnO crystals from zinc acetate with mixed (0002) and (101(-)1) facets. This research explores the special characteristics of ZnO with different exposed facets and is important for the future design of highly efficient photocatalyst for hazardous material removal.

Chemical conversion based on the crystal facet effect of transition metal oxides and construction methods for sharp-faced nanocrystals

In-depth research has found that the nanocrystal facet of transition metal oxides (TMOs) greatly affects their heterogeneous catalytic performance, as well as the property of photocatalysis, gas sensing, electrochemical reaction, etc. that are all involved in chemical conversion processes. Therefore, the facet-dependent properties of TMO nanocrystals have been fully and carefully studied by combining systematic experiments and theoretical calculations, and mechanisms of chemical reactions are accurately explained at the molecular level, which will be closer to the essence of reactions. Evidently, as an accurate investigation on crystal facets, well-defined TMO nanocrystals are the basis and premise for obtaining relevant credible results, and shape-controlled synthesis of TMO nanocrystals thereby has received great attention and development. The success in understanding of facet-dependent properties and shape-controlled synthesis of TMO nanocrystals is highly valuable for the control of reaction and the design of high-efficiency TMO nanocrystal catalysts as well as other functional materials in practical applications.

Construction of Low-Cost Z-Scheme Heterostructure Cu2 O/PCN for Highly Selective CO2 Photoreduction to Methanol with Water Oxidation

Solar-driven CO₂ reaction with water oxidation into alcohols represents a promising approach to achieve real artificial photosynthesis. However, rapid recombination of photogenerated carriers seriously restricts the development of artificial photosynthesis. Herein, a facile method is explored to construct low-cost Z-Scheme heterostructure Cu₂ O/polymeric carbon nitride (PCN) by in situ growth of Cu₂ O hollow nanocrystal on PCN. The protective PCN layer and Z-schematic charge flow can make robust Cu₂ O/PCN photocatalysts, and the spatial separation of electrons and holes with high redox potentials of E_CB (-1.15 eV) and E_VB (1.65 eV) versus NHE can efficiently drive CO₂ photoreduction to methanol in pure water, which is further confirmed by DFT calculation. The Z-scheme heterostructure Cu₂ O/PCN exhibits a high methanol yield of 276 µmol g^-1 in 8 h with ca. 100% selectivity, much superior to that of isolated Cu₂ O and PCN, and all the reported Cu₂ O-based heterostructures. This work provides a unique strategy to efficiently and selectively promote the conversion of CO₂ and H₂ O into high-value chemicals by constructing a low-cost Z-scheme heterostructure.

Metal halide perovskites as an emergent catalyst for CO2 photoreduction: a minireview

The present minireview summarizes recent advances in the application of metal halide perovskite for CO₂ photoreduction.