Defects of Metal Halide Perovskites in Photocatalytic Energy Conversion: Friend or Foe?

Photocatalytic solar-to-fuel conversion over metal halide perovskites (MHPs) has recently attracted much attention, while the roles of defects in MHPs are still under debate. Specifically, the mainstream viewpoint is that the defects are detrimental to photocatalytic performance, while some recent studies show that certain types of defects contribute to photoactivity enhancement. However, a systematic summary of why it is contradictory and how the defects in MHPs affect photocatalytic performance is still lacking. In this review, the innovative roles of defects in MHP photocatalysts are highlighted. First, the origins of defects in MHPs are elaborated, followed by clarifying certain benefits of defects in photocatalysts including optical absorption, charge dynamics, and surface reaction. Afterward, the recent progress on defect-related MHP photocatalysis, i.e., CO2 reduction, H2 generation, pollutant degradation, and organic synthesis is systematically discussed and critically appraised, putting emphasis on their beneficial effects. With defects offering peculiar sets of merits and demerits, the personal opinion on the ongoing challenges is concluded and outlining potentially promising opportunities for engineering defects on MHP photocatalysts. This critical review is anticipated to offer a better understanding of the MHP defects and spur some inspiration for designing efficient MHP photocatalysts.

Vacancy-Rich CoSx@LDH@Co-NC Catalytic Membrane for Antibiotic Degradation with Mechanistic Insights.

Improving the wettability of carbon-based catalysts and overcoming the rate-limiting step of the Mn+1/Mn+ cycle are effective strategies for activating peroxymonosulfate (PMS). In this study, the coupling of Co-NC, layered double hydroxide (LDH), and CoSx heterostructure (CoSx@LDH@Co-NC) was constructed to completely degrade ofloxacin (OFX) within 10 min via PMS activation. The reaction rate of 1.07 min-1 is about 1-2 orders of magnitude higher than other catalysts. The interfacial effect of confined Co-NC and layered double hydroxide (LDH) not only enhanced the wettability of catalysts but also increased the vacancy concentration; it facilitated easier contact with the interface reactive oxygen species (ROS). Simultaneously, reduced sulfur species (CoSx) accelerated the Co3+/Co2+ cycle, acquiring long-term catalytic activity. The catalytic mechanism revealed that the synergistic effect of hydroxyl groups and reduced sulfur species promoted the formation of 1O2, with a longer lifespan and a longer migration distance, and resisted the influence of nontarget background substances. Moreover, considering the convenience of practical application, the CoSx@LDH@Co-NC-based catalytic membrane was prepared, which had zero discharge of OFX and no decay in continuous operation for 5.0 h. The activity of the catalytic membrane was also verified in actual wastewater. Consequently, this work not only provides a novel strategy for designing excellent catalysts but also is applicable to practical organic wastewater treatment.

Thin Zinc Oxide Layer Passivating Bismuth Vanadate for Selective Photoelectrochemical Water Oxidation to Hydrogen Peroxide.

Selective photoelectrochemical (PEC) water oxidation to hydrogen peroxide is an underexplored option as opposed to the mainstream oxygen reduction reaction. Albeit interesting, selective H2 O2 production via oxidative pathway is plagued by the noncontrollable two-electron transfer reaction and the overoxidation of the thus-formed H2 O2 to O2 . Here, ZnO passivator-coated BiVO4 photoanode is reported for selective PEC H2 O2 production. Both the H2 O2 selectivity and production rate increase in the range of 1.0-2.0 V versus RHE under simulated sunlight irradiation. The photoelectrochemical impedance spectra and open-circuit potentials suggest a flattened band bending and positively shifted quasi-Fermi level of BiVO4 upon ZnO coating, facilitating H2 O2 generation and suppressing the competitive reaction of O2 evolution. The ZnO overlayer also inhibits H2 O2 decomposition, accelerates charge extraction from BiVO4 , and serves as a hole reservoir under photoexcitation. This work offers insights into surface states and the role of the coating layer in manipulating two/four-electron transfer for selective H2 O2 synthesis from PEC water oxidation.

Atomically Dispersed Cu Catalysts on Sulfide-Derived Defective Ag Nanowires for Electrochemical CO2 Reduction.

Single-atom catalysts (SACs) have shown potential for achieving an efficient electrochemical CO2 reduction reaction (CO2RR) despite challenges in their synthesis. Here, Ag2S/Ag nanowires provide initial anchoring sites for Cu SACs (Cu/Ag2S/Ag), then Cu/Ag(S) was synthesized by an electrochemical treatment resulting in complete sulfur removal, i.e., Cu SACs on a defective Ag surface. The CO2RR Faradaic efficiency (FECO2RR) of Cu/Ag(S) reaches 93.0% at a CO2RR partial current density (jCO2RR) of 2.9 mA/cm2 under -1.0 V vs RHE, which outperforms sulfur-removed Ag2S/Ag without Cu SACs (Ag(S), 78.5% FECO2RR with 1.8 mA/cm2jCO2RR). At -1.4 V vs RHE, both FECO2RR and jCO2RR over Cu/Ag(S) reached 78.6% and 6.1 mA/cm2, which tripled those over Ag(S), respectively. As revealed by in situ and ex situ characterizations together with theoretical calculations, the interacted Cu SACs and their neighboring defective Ag surface increase microstrain and downshift the d-band center of Cu/Ag(S), thus lowering the energy barrier by ∼0.5 eV for *CO formation, which accounts for the improved CO2RR activity and selectivity toward related products such as CO and C2+ products.

Low-bias photoelectrochemical water splitting via mediating trap states and small polaron hopping.

Metal oxides are promising for photoelectrochemical (PEC) water splitting due to their robustness and low cost. However, poor charge carrier transport impedes their activity, particularly at low-bias voltage. Here we demonstrate the unusual effectiveness of phosphorus doping into bismuth vanadate (BiVO4) photoanode for efficient low-bias PEC water splitting. The resulting BiVO4 photoanode shows a separation efficiency of 80% and 99% at potentials as low as 0.6 and 1.0 VRHE, respectively. Theoretical simulation and experimental analysis collectively verify that the record performance originates from the unique phosphorus-doped BiVO4 configuration with concurrently mediated carrier density, trap states, and small polaron hopping. With NiFeOx cocatalyst, the BiVO4 photoanode achieves an applied bias photon-to-current efficiency of 2.21% at 0.6 VRHE. The mechanistic understanding of the enhancement of BiVO4 properties provides key insights in trap state passivation and polaron hopping for most photoactive metal oxides.

Modulating the Active Sites of Oxygen-Deficient TiO2 by Copper Loading for Enhanced Electrocatalytic Nitrogen Reduction to Ammonia.

The electrocatalytic nitrogen reduction reaction (NRR) provides a sustainable route for NH3 synthesis. However, the process is plagued by the strong NN triple bond and high reaction barrier. Modification of catalyst surface to increase N2 adsorption and activation is crucial. Herein, copper nanoparticles are loaded on the oxygen-deficient TiO2 , which exhibits an enhanced NRR performance with NH3 yield of 13.6 µg mgcat -1 h-1 at -0.5 V versus reversible hydrogen electrode (RHE) and Faradaic efficiency of 17.9% at -0.4 V versus RHE compared to the pristine TiO2 . The enhanced performance is ascribed to the higher electrochemically active surface area, promoted electron transfer, and increased electron density originated from the strong metal-support interaction (SMSI) between Cu nanoparticles and oxygen-deficient TiO2 . The SMSI effect also results in lopsided local charge distribution, which polarizes the adsorbed N2 molecules for better activation. This work provides a facile strategy toward the electrocatalyst design for efficient NRR under ambient conditions.

Green synthesis of graphite-based photo-Fenton nanocatalyst from waste tar via a self-reduction and solvent-free strategy.

Thermochemical conversion of biomass yields large quantities of tar as a by-product, which is a potential precursor for the synthesis of renewable carbon-based functional materials. In this study, high-performance photo-Fenton catalyst of graphite­carbon-supported iron nanoparticles was synthesized using a self-reduction and solvent-free approach. The results showed that the tar-derived catalyst had unique properties including a defect-rich graphitic structure, high surface area, and an abundant porous structure resulting from the inherent properties of biomass tar. The iron nanoparticles were highly dispersed within the prepared catalysts and were stably anchored on the carbonaceous surface by the FeC bond. The prepared nanocatalyst showed the highest decomposition constant (91.87 × 10-3 min-1) for 20 mM H2O2, and 40 mg/L RhB can be completely degraded within 2 h under catalyst dosage of 1 g/L and H2O2 addition of 20 mM. The degradation mechanism under the photo-Fenton catalyst/H2O2/light system included the heterogeneous Fenton reaction of iron nanoparticles and photo-Fenton reaction of iron oxide, and the efficient RhB degradation was mainly ascribed to the heterogeneous Fenton reaction. In addition, recycling and leaching tests demonstrated that the photo-Fenton catalyst had excellent reusability and stability, where only 7.3% catalytic reactivity was reduced after five cycles. This work provided a green, sustainable, and facile approach for synthesizing photo-Fenton catalysts by value-added utilization of tar wastes.

Noble-Metal-Free Multicomponent Nanointegration for Sustainable Energy Conversion.

Global energy and environmental crises are among the most pressing challenges facing humankind. To overcome these challenges, recent years have seen an upsurge of interest in the development and production of renewable chemical fuels as alternatives to the nonrenewable and high-polluting fossil fuels. Photocatalysis, photoelectrocatalysis, and electrocatalysis provide promising avenues for sustainable energy conversion. Single- and dual-component catalytic systems based on nanomaterials have been intensively studied for decades, but their intrinsic weaknesses hamper their practical applications. Multicomponent nanomaterial-based systems, consisting of three or more components with at least one component in the nanoscale, have recently emerged. The multiple components are integrated together to create synergistic effects and hence overcome the limitation for outperformance. Such higher-efficiency systems based on nanomaterials will potentially bring an additional benefit in balance-of-system costs if they exclude the use of noble metals, considering the expense and sustainability. It is therefore timely to review the research in this field, providing guidance in the development of noble-metal-free multicomponent nanointegration for sustainable energy conversion. In this work, we first recall the fundamentals of catalysis by nanomaterials, multicomponent nanointegration, and reactor configuration for water splitting, CO2 reduction, and N2 reduction. We then systematically review and discuss recent advances in multicomponent-based photocatalytic, photoelectrochemical, and electrochemical systems based on nanomaterials. On the basis of these systems, we further laterally evaluate different multicomponent integration strategies and highlight their impacts on catalytic activity, performance stability, and product selectivity. Finally, we provide conclusions and future prospects for multicomponent nanointegration. This work offers comprehensive insights into the development of cost-competitive multicomponent nanomaterial-based systems for sustainable energy-conversion technologies and assists researchers working toward addressing the global challenges in energy and the environment.

Manipulating the Fate of Charge Carriers with Tungsten Concentration: Enhancing Photoelectrochemical Water Oxidation of Bi2 WO6.

Bismuth tungstate (Bi2 WO6 ) thin film photoanode has exhibited an excellent photoelectrochemical (PEC) performance when the tungsten (W) concentration is increased during the fabrication. Plate-like Bi2 WO6 thin film with distinct particle sizes and surface area of different exposed facets are successfully prepared via hydrothermal reaction. The smaller particle size in conjunction with higher exposure extent of electron-dominated {010} crystal facet leads to a shorter electron transport pathway to the bulk surface, assuring a lower charge transfer resistance and thus minimal energy loss. In addition, it is proposed based on the results from conductive atomic force microscopy that higher W concentration plays a crucial role in facilitating the charge transport of the thin film. The "self-doped" of W in Bi2 WO6 will lead to the higher carrier density and improved conductivity. Thus, the variation in the W concentration during a synthesis can be served as a promising strategy for future W based photoanode design to achieve high photoactivity in water splitting application.

A CuNi Alloy-Carbon Layer Core-Shell Catalyst for Highly Efficient Conversion of Aqueous Formaldehyde to Hydrogen at Room Temperature.

A copper (Cu) material is catalytically active for formaldehyde (HCHO) dehydrogenation to produce H2, but the unsatisfactory efficiency and easy corrosion hinder its practical application. Alloying with other metals and coating a carbon layer outside are recognized as effective strategies to improve the catalytic activity and the long-term durability of nonprecious metal catalysts. Here, highly dispersed CuNi alloy-carbon layer core-shell nanoparticles (CuNi@C) have been developed as a robust catalyst for efficient H2 generation from HCHO aqueous solution at room temperature. Under the optimized reaction conditions, the CuNi@C catalyst exhibits a H2 evolution rate of 110.98 mmol·h-1·g-1, which is 1.5 and 4.9 times higher than those of Cu@C and Ni@C, respectively, which ranks top among the reported nonprecious metal catalysts for catalytic HCHO reforming at room temperature to date. Furthermore, CuNi@C also displays excellent stability toward the catalytic HCHO reforming into H2 in tap water owing to the well-constructed carbon sheath protecting CuNi nanocrystals from oxidation in an alkaline medium. Combined with density functional theory calculations, the superior catalytic efficiency of CuNi@C for H2 generation results from the synergistic contribution between the massive active species from HCHO decomposition on the Cu sites and the remarkable H2 evolution activity on Ni sites. The improved performance of CuNi@C highlights the enormous potential of advancing noble-metal-free nanoalloys as cost-effective and recyclable catalysts for energy recovery from industrial HCHO wastewater.

Coupled porosity and heterojunction engineering: MOF-derived porous Co3O4 embedded on TiO2 nanotube arrays for water remediation.

Strive to develop the interaction and efficient co-catalysts is one of the vital projects in realizing hybrid photocatalytic systems for water remediation. In this work, p-type porous Co3O4 was embedded onto n-type vertical TiO2 nanotube via an in-situ thermal etching method. ZIF-67 was employed as the structural template for Co3O4, which then augmented the light harvesting ability of the resultant photocatalyst. Such improvement was prompted by the light reflecting and directing attributes of porous Co3O4. Therefore, a remarkable MB removal rate was attained under sunlight irradiation, with superoxide radical being identified as the major reactive species. Photoelectric properties evaluation also verified that the p-n heterojunction developed herein exhibits outstanding charges separation ability with low impedance, particularly under light irradiation. This work highlights the idea on coupling both porous and p-n heterojunction engineering in augmenting photoactivity of catalyst, while offering insights in such structure-mediating approach.

Metal-Organic Framework Decorated Cuprous Oxide Nanowires for Long-lived Charges Applied in Selective Photocatalytic CO2 Reduction to CH4.

Improving the stability of cuprous oxide (Cu2 O) is imperative to its practical applications in artificial photosynthesis. In this work, Cu2 O nanowires are encapsulated by metal-organic frameworks (MOFs) of Cu3 (BTC)2 (BTC=1,3,5-benzene tricarboxylate) using a surfactant-free method. Such MOFs not only suppress the water vapor-induced corrosion of Cu2 O but also facilitate charge separation and CO2 uptake, thus resulting in a nanocomposite representing 1.9 times improved activity and stability for selective photocatalytic CO2 reduction into CH4 under mild reaction conditions. Furthermore, direct transfer of photogenerated electrons from the conduction band of Cu2 O to the LUMO level of non-excited Cu3 (BTC)2 has been evidenced by time-resolved photoluminescence. This work proposes an effective strategy for CO2 conversion by a synergy of charge separation and CO2 adsorption, leading to the enhanced photocatalytic reaction when MOFs are integrated with metal oxide photocatalyst.

Visible-light-driven photoelectrocatalytic activation of chloride by nanoporous MoS2@BiVO4 photoanode for enhanced degradation of bisphenol A.

The massive emission of bisphenol A (BPA) has imposed adverse effects on both ecosystems and human health. Herein, nanoporous MoS2@BiVO4 photoanodes were fabricated on fluorine-doped tin oxide (FTO) substrates for photoelectrocatalytic degradation of BPA. The photocurrent density of the optimized photoanode (MoS2-3@BiVO4) was 5.4 times as that of BiVO4 photoanode at 1.5 V vs. Ag/AgCl under visible light illumination, which was ascribed to the reduced recombination of photogenerated charge carriers of the well-designed hybrid structure. 10 ppm of BPA could be completely degraded in 75 min by MoS2-3@BiVO4 photoanode, with a bias of 1.5 V vs. Ag/AgCl and 100 mM of NaCl as the supporting electrolyte. The electron paramagnetic resonance (EPR) and free radicals scavenging experiments confirmed that chlorine oxide radical (•ClO) played a dominant role in the degradation of BPA. 14 intermediates were detected and identified during photoelectrocatalytic degradation of BPA by MoS2-3@BiVO4 photoanode and 3 pathways were proposed based on the above intermediates. The hybrid film exhibited high stability and reusability, and promising application potential in photoelectrocatalytic degradation of organic pollutants in aqueous solution.

Superior photoelectrocatalytic performance of ternary structural BiVO4/GQD/g-C3N4 heterojunction.

Herein, we performed an encyclopedic analysis on the photoelectrocatalytic hydrogen production of BiVO4/g-C3N4 decorated with reduced graphene oxide (RGO) or graphene quantum dots (GQDs). The differences between RGO and GQDs as an electron mediator was revealed for the first time in the perspective of theoretical DFT analysis and experimental validation. It was found that the incorporation of GQDs as an electron mediator promotes better photoelectrocatalytic hydrogen performance in comparison to the RGO. The addition of GQD can significantly improve the activity by 25.2 and 75.7% in comparison to the BiVO4/RGO/g-C3N4 and binary composite samples, respectively. Correspondingly, the BiVO4/GQD/g-C3N4 attained the highest photocurrent density of 19.2 mA/cm2 with an ABPE of 0.57% without the presence of any sacrificial reagents. This enhancement is stemming from the low photocharge carrier transfer resistance which was further verified via DFT study. The DFT analysis revealed that the BiVO4/GQD/g-C3N4 sample shared their electronic cloud density through orbital hybridization while the BiVO4/RGO/g-C3N4 sample show less mutual sharing. Additionally, the charge redistribution of the GQDs-composite at the heterostructure interface articulates a more stable and stronger heterojunction than the RGO-composite. Notably, this study provides new insights on the effect of different carbonaceous materials (RGO and GQDs) which are often used as an electron mediator to enhance photocatalytic activity.

Visible-light photocatalysis and charge carrier dynamics of elemental crystalline red phosphorus.

Elemental red phosphorus (red P) is a new class of photocatalysts with a desirable bandgap of ∼1.7 eV and has a strong visible-light response. Here, we show that the efficiency of red P is limited by severe electron trapping at deep traps that are intrinsic to the different crystal facets of the red P. To overcome this, we synthesized the red P/RGO (reduced graphene oxide) composite in a one-step ampoule chemical vapor deposition synthesis that formed a conducive interface between the red P photocatalyst and the RGO acceptor for efficient interfacial charge transport. As substantiated through photoelectrochemical characterization and ultrafast (femtoseconds) transient absorption spectroscopy, the interfacing with RGO provided a rapid pathway for the photocharges in red P to be interfacially separated, thereby circumventing the slower the charge trapping process. As a result, up to a sevenfold increase in the photocatalytic hydrogen production rate (apparent quantum yield = 3.1% at 650 nm) was obtained for the red P/RGO relative to the pristine red P.

Solid Nanoporosity Governs Catalytic CO2 and N2 Reduction.

Global demand for green and clean energy is increasing day by day owing to ongoing developments by the human race that are changing the face of the earth at a rate faster than ever. Exploring alternative sources of energy to replace fossil fuel consumption has become even more vital to control the growing concentration of CO2, and reduction of CO2 into CO or other useful hydrocarbons (e.g., C1 and C≥2 products), as well as reduction of N2 into ammonia, can greatly help in this regard. Various materials have been developed for the reduction of CO2 and N2. The introduction of pores in these materials by porosity engineering has been demonstrated to be highly effective in increasing the efficiency of the involved redox reactions, over 40% increment for CO2 reduction to date, by providing an increased number of exposed facets, kinks, edges, and catalytically active sites of catalysts. By shaping the surface porous structure, the selectivity of the redox reaction can also be enhanced. In order to better understand this area benefiting rational design for future solutions, this review systematically summarizes and constructively discusses the porosity engineering in catalytic materials, including various synthesis methods, characterization of porous materials, and the effects of porosity on performance of CO2 reduction and N2 reduction.

Biorenewable hydrogen production through biomass gasification: A review and future prospects.

Hydrogen is recognized as one of the cleanest energy carriers, which can be produced from renewable biomass as a promising feedstock to achieve sustainable bioeconomy. Thermochemical technologies (e.g., gasification and pyrolysis) are the main routes for hydrogen production from biomass. Although biomass gasification, including steam gasification and supercritical water gasification, shows a high potential in field-scale applications, the selectivity and efficiency of hydrogen production need improvement to secure cost-effective industrial applications with high atom economy. This article reviews the two main-stream biomass-to-hydrogen technologies and discusses the significance of operating conditions and considerations in the catalytic system design. Challenges and prospects of hydrogen production via biomass gasification are explored to advise on the critical information gaps that require future investigations.

Light-Induced Formation of MoOxSy Clusters on CdS Nanorods as Cocatalyst for Enhanced Hydrogen Evolution.

Metal and metal-oxide particles are commonly photodeposited on photocatalysts by reduction and oxidation reactions, respectively, consuming charges that are generated under illumination. This study reveals that amorphous MoOxSy clusters can be easily photodeposited at the tips of CdS nanorods (NRs) by in situ photodeposition for the first time. The as-prepared MoOxSy-decorated CdS samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and inductively coupled plasma (ICP) to determine the composition and the possible formation pathways of the amorphous MoOxSy clusters. The MoOxSy-tipped CdS samples exhibited better hydrogen evolution performance than pure CdS under visible-light illumination. The enhanced activity is attributed to the formation of intimate interfacial contact between CdS and the amorphous MoOxSy clusters, which facilitates the charge separation and transfer. Through time-resolved photoluminescence (TRPL) measurements, it was clearly observed that all MoOxSy-decorated CdS samples with different loadings of MoOxSy showed a faster PL decay when compared to pure CdS, resulting from the effective trapping of photogenerated electrons by the MoOxSy clusters. Kelvin probe force microscopy (KPFM) was further used to study the surface potentials of pure CdS NRs and MoOxSy-decorated CdS NRs. A higher surface potential on MoOxSy-decorated CdS NRs was observed in the dark, indicating that the loading of MoOxSy resulted in a lower surface work function compared to pure CdS NRs. This contributed to the effective electron trapping and separation, which was also reflected by the increased photoelectrochemical response. Thus, this study demonstrates the design and facile synthesis of MoOxSy-tipped CdS NRs photocatalysts for efficient solar hydrogen production.

Photocatalytic and Photoelectrochemical Systems: Similarities and Differences.

Photocatalytic and photoelectrochemical processes are two key systems in harvesting sunlight for energy and environmental applications. As both systems are employing photoactive semiconductors as the major active component, strategies have been formulated to improve the properties of the semiconductors for better performances. However, requirements to yield excellent performances are different in these two distinctive systems. Although there are universal strategies applicable to improve the performance of photoactive semiconductors, similarities and differences exist when the semiconductors are to be used differently. Here, considerations on selected typical factors governing the performances in photocatalytic and photoelectrochemical systems, even though the same type of semiconductor is used, are provided. Understanding of the underlying mechanisms in relation to their photoactivities is of fundamental importance for rational design of high-performing photoactive materials, which may serve as a general guideline for the fabrication of good photocatalysts or photoelectrodes toward sustainable solar fuel generation.

An Oxygen Paradox: Catalytic Use of Oxygen in Radical Photopolymerization.

A peculiar radical polymerization reaction is presented in which oxygen serves as a cocatalyst, alongside triethylamine, to provide activation with light in the far-red (690 nm, 3 mW cm-2 ) of the PET-RAFT process in the presence of zinc(II) (2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin) as photocatalyst. Apart from the ability to exert temporal control by switching the light on or off, this system possesses the exciting capability of inducing temporal control by removal or reintroduction of oxygen. Furthermore, this multicomponent catalytic system was typified by controlled polymerizations of various acrylate and acrylamide monomers, which all resulted in well-defined polymers with low dispersity (<1.2). The process displayed excellent living characteristics that were demonstrated through chain extensions and a range of degrees of polymerization (200-1600).