Ultrafine Rhodium-Chromium Mixed-Oxide Cocatalyst with Facet-Selective Loading for Excellent Photocatalytic Water Splitting

The development of water-splitting photocatalysts capable of generating green hydrogen (H2) from water and sunlight is crucial for achieving carbon neutrality. Further enhancement of the photocatalytic water-splitting activity is essential to realizing this objective. Photocatalysts with specific exposed crystal facets can facilitate efficient charge separation of electrons/holes, thereby achieving high activity for water splitting. However, there have been no reports of ultrafine (∼1 nm) cocatalysts being loaded onto specific crystal facets of photocatalysts, despite cocatalysts being the actual reaction sites for water splitting. This study establishes a novel method for achieving facet-selective loading of ultrafine H2-evolution cocatalysts onto the {100} facets, which are the H2-evolution facets, of a strontium titanate photocatalyst. The resulting photocatalyst exhibits the highest apparent quantum yield achieved to date for strontium titanate. This research holds the potential to further improve various types of advanced photocatalysts and is expected to accelerate the transition to carbon neutrality.

Spiral-Concave Prussian Blue Crystals with Rich Steps: Growth Mechanism and Coordination Regulation

Investigating the formation and transformation mechanisms of spiral-concave crystals holds significant potential for advancing innovative material design and comprehension. We examined the kinetics-controlled nucleation and growth mechanisms of Prussian Blue crystals with spiral concave structures, and constructed a detailed crystal growth phase diagram. The spiral-concave hexacyanoferrate (SC-HCF) crystals, characterized by high-density surface steps and a low stress-strain architecture, exhibit enhanced activity due to their facile interaction with reactants. Notably, the coordination environment of SC-HCF can be precisely modulated by the introduction of diverse metals. Utilizing X-ray absorption fine structure spectroscopy and in situ ultraviolet-visible spectroscopy, we elucidated the formation mechanism of SC-HCF to Co-HCF facilitated by oriented adsorption-ion exchange (OA-IE) process. Both experimental data, and density functional theory confirm that Co-HCF possesses an optimized energy band structure, capable of adjusting the local electronic environment and enhancing the performance of the oxygen evolution reaction. This work not only elucidates the formation mechanism and coordination regulation for rich steps HCF, but also offers a novel perspective for constructing nanocrystals with intricate spiral-concave structures.

Facet-controlled growth and soft-chemical exfoliation of two-dimensional titanium dioxide nanosheets

TiO2 remains one of the most popular materials used in catalysts, photovoltaics, coatings, and electronics due to its abundance, chemical stability, and excellent catalytic properties. The tailoring of the TiO2 structure into two-dimensional nanosheets prompted the successful isolation of graphene and MXenes. In this review, facet-controlled TiO2 and monolayer titanate are outlined, covering their synthesis route and formation mechanism. The reactive facet of TiO2 is usually controlled by a capping agent. In contrast, the monolayer titanate is achieved by ion-exchange and delamination of layered titanates. Each route leads to 2D structures with unique physical and chemical properties, which expands its utilisation into several niche applications. We elaborate the detailed outlook for the future use and research studies of facet-controlled TiO2 and monolayer titanates. Advantages and disadvantages of both structures are provided, along with suggested applications for each type of 2D TiO2 nanosheets.

Revisiting the photocharging effect on the BiVO4 (010) surface by identifying the charge reaction kinetics

The alteration of intermediates on the (010) facet of BiVO4 leads to variations in charge accumulation numbers before overcoming the rate-determining step, which accounts for the enhanced charge transfer for oxygen evolution. This discovery provides insights into the photocharging effect and the photoelectrochemical reaction mechanism.

Single Entity Electrocatalysis

Making a measurement over millions of nanoparticles or exposed crystal facets seldom reports on reactivity of a single nanoparticle or facet, which may depart drastically from ensemble measurements. Within the past 30 years, science has moved toward studying the reactivity of single atoms, molecules, and nanoparticles, one at a time. This shift has been fueled by the realization that everything changes at the nanoscale, especially important industrially relevant properties like those important to electrocatalysis. Studying single nanoscale entities, however, is not trivial and has required the development of new measurement tools. This review explores a tale of the clever use of old and new measurement tools to study electrocatalysis at the single entity level. We explore in detail the complex interrelationship between measurement method, electrocatalytic material, and reaction of interest (e.g., carbon dioxide reduction, oxygen reduction, hydrazine oxidation, etc.). We end with our perspective on the future of single entity electrocatalysis with a key focus on what types of measurements present the greatest opportunity for fundamental discovery.

Selective atomic sieving across metal/oxide interface for super-oxidation resistance

Surface passivation, a desirable natural consequence during initial oxidation of alloys, is the foundation for functioning of corrosion and oxidation resistant alloys ranging from industrial stainless steel to kitchen utensils. This initial oxidation has been long perceived to vary with crystal facet, however, the underlying mechanism remains elusive. Here, using in situ environmental transmission electron microscopy, we gain atomic details on crystal facet dependent initial oxidation behavior in a model Ni-5Cr alloy. We find the (001) surface shows higher initial oxidation resistance as compared to the (111) surface. We reveal the crystal facet dependent oxidation is related to an interfacial atomic sieving effect, wherein the oxide/metal interface selectively promotes diffusion of certain atomic species. Density functional theory calculations rationalize the oxygen diffusion across Ni(111)/NiO(111) interface, as contrasted with Ni(001)/NiO(111), is enhanced. We unveil that crystal facet with initial fast oxidation rate could conversely switch to a slow steady state oxidation.

Unusual facet and co-catalyst effects in TiO2-based photocatalytic coupling of methane

Photocatalytic coupling of methane to ethane and ethylene (C2 compounds) offers a promising approach to utilizing the abundant methane resource. However, the state-of-the-art photocatalysts usually suffer from very limited C2 formation rates. Here, we report our discovery that the anatase TiO2 nanocrystals mainly exposing {101} facets, which are generally considered less active in photocatalysis, demonstrate surprisingly better performances than those exposing the high-energy {001} facet. The palladium co-catalyst plays a pivotal role and the Pd2+ site on co-catalyst accounts for the selective C2 formation. We unveil that the anatase {101} facet favors the formation of hydroxyl radicals in aqueous phase near the surface, where they activate methane molecules into methyl radicals, and the Pd2+ site participates in facilitating the adsorption and coupling of methyl radicals. This work provides a strategy to design efficient nanocatalysts for selective photocatalytic methane coupling by reaction-space separation to optimize heterogeneous-homogeneous reactions at solid-liquid interfaces.

Vertically Oriented Perovskites with Minimized Intrinsic Boundaries for Efficient Photovoltaics

Intrinsic boundaries formed by grain stacks of randomly oriented perovskite crystallites seriously restrict charge transport in the resultant photovoltaic devices, whereas direct passivation of these defects remains unexplored, and it is desirable to modulate perovskite growth with uniform orientation. Herein, we report a simple but effective method to regulate perovskite crystallization by employing a volatile and polymerizable monomer of hydroxyethyl methacrylate (HEMA), which can simultaneously interact with both FA+ and Pb2+ via hydrogen and coordination bonding, respectively, to seed perovskite crystallization with accelerated nucleation and retarded crystal growth. Upon thermal annealing, the gradual volatilization and partial self-condensation of the HEMA drive the perovskite growth perpendicularly to the substrate, leading to largely suppressed defect states, improved crystallinity, and a reduced Young's modulus of the perovskite film. As a result, champion efficiencies exceeding 24 and 22% with improved operational and mechanical stability of the optimized perovskite solar cells based on rigid and flexible substrates were finally achieved.

Halogen-Site Regulation in Cs3Bi2X9 Quantum Dots for Efficient and Selective Oxidation of Benzyl Alcohol Driven by Solar Light

Sluggish charge kinetics and low selectivity limit the solar-driven selective organic transformations under mild conditions. Herein, an efficient strategy of halogen-site regulation, based on the precise control of charge transfer and molecule activation by rational design of Cs3Bi2X9 quantum dots photocatalysts, is proposed to achieve both high selectivity and yield of benzyl-alcohol oxidation. In situ PL spectroscopy study reveals that the Bi─Br bonds formed in the form of Br-associated coordination can enhance the separation and transfer of photoexcited carriers during the practical reaction. As the active center, the exclusive Bi─Br covalence can benefit the benzyl-alcohol activation for producing carbon-centered radicals. As a result, the Cs3Bi2Br9 with this atomic coordination achieves a conversion ratio of 97.9% for benzyl alcohol and selectivity of 99.6% for aldehydes, which are 56.9- and 1.54-fold higher than that of Cs3Bi2Cl9. Combined with quasi-in situ EPR, in situ ATR-FTIR spectra, and DFT calculation, the conversion of C6H5-CH2OH to C6H5-CH2* at Br-related coordination is revealed to be a determining step, which can be accelerated via halogen-site regulation for enhancing selectivity and photocatalytic efficiency. The mechanistic insights of this research elucidate how halogen-site regulation in favor of charge transfer and molecule activation toward efficient and selective oxidation of benzyl alcohol.

Current trends on dry photocatalytic oxidation technology for BTX removal: Viable light sources and highly efficient photocatalysts

One of the main gaseous pollutants released by chemical production industries are benzene, toluene and xylene (BTX). These dangerous gases require immediate technology to combat them, as they put the health of living organisms at risk. The development of heterogeneous photocatalytic oxidation technology offers several viewpoints, particularly in gaseous-phase decontamination without an additional supply of oxidants in air at atmospheric pressure. However, difficulties such as low quantum efficiency, ability to absorb visible light, affinity towards CO2 and H2O synthesis, and low stability continue to limit its practical use. This review presents recent advances in dry-phase heterogeneous photodegradation as an advanced technology for the practical removal of BTX molecules. This review also examines the impact of low-cost light sources, the roles of the active sites of photocatalysts, and the feasible concentration range of BTX molecules. Numerous studies have demonstrated a significant improvement in the efficiency of the photodegradation of volatile organic compounds by enhancing the photocatalytic reactor system and other factors, such as humidity, temperature, and flow rate. The mechanism for BTX photodegradation based on density functional theory (DFT), electron paramagnetic resonance (EPR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) investigations is also discussed. Finally, the present research complications and anticipated future developments in the field of heterogeneous photocatalytic oxidation technology are discussed.

Estimating Nonradiative Excited-State Lifetimes in Photoactive Semiconducting Nanostructures

The time evolution of the exciton generated by light adsorption in a photocatalyst is an important feature that can be approached from full nonadiabatic molecular dynamics simulations. Here, a crucial parameter is the nonradiative recombination rate between the hole and the electron that form the exciton. In the present work, we explore the performance of a Fermi's golden rule-based approach on predicting the recombination rate in a set of photoactive titania nanostructures, relying solely on the coupling of the ground and first excited state. In this scheme the analysis of the first excited state is carried out by invoking Kasha's rule thus avoiding computationally expensive nonadiabatic molecular dynamics simulations and resulting in an affordable estimate of the recombination rate. Our results show that, compared to previous ones from nonadiabatic molecular dynamics simulations, semiquantitative recombination rates can be predicted for the smaller titania nanostructures, and qualitative values are obtained from the larger ones. The present scheme is expected to be useful in the field of computational heterogeneous photocatalysis whenever a complex and computationally expensive full nonadiabatic molecular dynamics cannot be carried out.

Hollow anatase TiO2 tetrakaidecahedral crystals with an active {001}/{110} redox interface toward high-performance photocatalytic activity

The existence of the oxidation/reduction interface can promote the performance of a photocatalyst, due to its effect on the separation of photogenerated carriers and the surface reactivity. However, it is difficult to construct two sets of oxidation/reduction interfaces in a single crystal and compare their separation efficiency for photogenerated carriers. Introducing a high proportion of active facets into the co-exposed facets is even more challenging. Herein, a hollow anatase TiO2 tetrakaidecahedron (HTT) with two sets of oxidation/reduction interfaces ({001}/{101} and {001}/{110}) is synthesized by directional chemical etching. Theoretical and experimental results indicate that the {001}/{110} interface is a dominant oxidation/reduction interface, showing a better promotion on the separation of photogenerated carriers than the {001}/{101} interface. In the HTT, the ratio of dominant {001}/(110) is increased and the proportion of the active {110} facet is about 40% (generally about 15%). Therefore, the HTT shows excellent catalytic activity for photocatalytic reductive (hydrogen production) and oxidative (selective oxidation of sulfides) reactions. The HTT also demonstrates favorable photocatalytic activity for the cross-dehydrogenative coupling reaction, where both photogenerated electrons and photogenerated holes are involved, further verifying its high separation efficiency of photogenerated carriers and surface reactivity. This work provides an important guideline for developing advanced structures with a predetermined interface toward desired applications.

Surface-Controlled CdS/Ti3 C2 MXene Schottky Junction for Highly Selective and Active Photocatalytic Dehydrogenation-Reductive Amination

Photocatalytic valorization and selective transformation of biomass-derived platform compounds offer great opportunities for efficient utilization of renewable resources under mild conditions. Here, the novel three-dimensional hierarchical flower-like CdS/Ti3 C2 Schottky junction (MCdS) composed of surface-controlled CdS and pretreated Ti3 C2 MXene is created for photocatalytic dehydrogenation-reductive amination of biomass-derived amino acid production under ambient temperature with unprecedented activity and selectivity. Schottky junction efficiently promotes photoexcited charge migration and separation and inhibits photogenerated electron-hole recombination, which results in a super-high activity. Meanwhile, CdS with the reduced surface energy supplies sufficient hydrogen sources for imine reduction and induces the preferential orientation of alanine, thus contributing superior selectivity. Moreover, a wide range of hydroxyl acids are successfully converted into corresponding amino acids and even one-pot conversion of glucose to alanine is easily achieved over MCdS. This work illustrates the mechanism of crystal orientation control and heterojunction construction in controlling catalytic behavior of photocatalytic nanoreactor, providing a paradigm for construction of MXene-based heterostructure.

Tetragonal-zircon BiVO4: a better polymorph for the formation of coherent type-II heterostructures for water splitting applications

The monoclinic-scheelite (m-s) polymorph of BiVO4 has the highest photocatalytic activity, whereas tetragonal-zircon (t-z) has the lowest photocatalytic activity, which may be due to a higher band gap. However, t-z has the highest crystal symmetry, which makes it a more suitable candidate to form coherent type-II interfaces for the efficient separation of electron-hole pairs. Furthermore, the method of preparation (e.g. low temperature and moderate pH) of t-z is more facile compared to the m-s polymorph. Hence, in this report, we construct coherent isomaterial and heteromaterial type-II heterostructures by facet engineering of low index surfaces of t-z polymorph with different semiconductor materials (e.g. ZnO, TiO2, CdSe, and ZnS) by screening the band gap, band edge positions, and lattice misfit strain. On the basis of the calculated band-edge positions, the polymorphs of BiVO4 can form 212 combinations of the type-II interface, which reduces to 17 coherent interfaces with lattice misfit strain between 1.55% to 28.5% when translational symmetry, atomic positions, lattice mismatch, and bond complementarity have been imposed. Furthermore, the current study suggests that t-z polymorphs can form more coherent interfaces (4 out of 168), which may be due to its highest symmetry structure in comparison to previously formed 67 isomaterial and heteromaterial type-II heterostructure combinations of BiVO4 (1 out of 67), which suggests that t-z can be a suitable candidate for the formation of type-II coherent interfaces for PEC/PC applications.

Role of N Doping in the Reduction of Titania Nanostructures

The effect of N-doping of titania (TiO2) nanoparticles (NPs) on their reduction through neutral O vacancy (Ovac) formation is investigated using all electron density functional theory-based calculations, including hybrid density functionals, and taking the bipyramidal anatase (TiO2)84 NP as a realistic model. The location of the N dopant is systematically analyzed, including O substitution in the (TiO2)84 structure and N occupying interstitial regions. Our computational study concludes that interstitial N doping is more favorable than N substituting O atoms and confirms that the presence of N reduces the energy gap. In the N-doped NP, Ovac formation is more favored than in undoped NP but less than in the N-doped bulk, which has important consequences.

Photocatalytic Conversion of Methane: Current State of the Art, Challenges, and Future Perspectives

With 28-34 times the greenhouse effect of CO2 over a 100-year period, methane is regarded as the second largest contributor to global warming. Reducing methane emissions is a necessary measure to limit global warming to below 1.5 °C. Photocatalytic conversion of methane is a promising approach to alleviate the atmospheric methane concentrations due to its low energy consumption and environmentally friendly characteristics. Meanwhile, this conversion process can produce valuable chemicals and liquid fuels such as CH3OH, CH3CH2OH, C2H6, and C2H4, cutting down the dependence of chemical production on crude oil. However, the development of photocatalysts with a high methane conversion efficiency and product selectivity remains challenging. In this review, we overview recent advances in semiconductor-based photocatalysts for methane conversion and present catalyst design strategies, including morphology control, heteroatom doping, facet engineering, and cocatalysts modification. To gain a comprehensive understanding of photocatalytic methane conversion, the conversion pathways and mechanisms in these systems are analyzed in detail. Moreover, the role of electron scavengers in methane conversion performance is briefly discussed. Subsequently, we summarize the anthropogenic methane emission scenarios on earth and discuss the application potential of photocatalytic methane conversion. Finally, challenges and future directions for photocatalytic methane conversion are presented.

Advances in CO2 utilization employing anisotropic nanomaterials as catalysts: a review

Anisotropic nanomaterials are materials with structures and properties that vary depending on the direction in which they are measured. Unlike isotropic materials, which exhibit uniform physical properties in all directions, anisotropic materials have different mechanical, electrical, thermal, and optical properties in different directions. Examples of anisotropic nanomaterials include nanocubes, nanowires, nanorods, nanoprisms, nanostars, and so on. These materials have unique properties that make them useful in a variety of applications, such as electronics, energy storage, catalysis, and biomedical engineering. One of the key advantages of anisotropic nanomaterials is their high aspect ratio, which refers to the ratio of their length to their width, which can enhance their mechanical and electrical properties, making them suitable for use in nanocomposites and other nanoscale applications. However, the anisotropic nature of these materials also presents challenges in their synthesis and processing. For example, it can be difficult to align the nanostructures in a specific direction to impart modulation of a specific property. Despite these challenges, research into anisotropic nanomaterials continues to grow, and scientists are working to develop new synthesis methods and processing techniques to unlock their full potential. Utilization of carbon dioxide (CO₂) as a renewable and sustainable source of carbon has been a topic of increasing interest due to its impact on reducing the level of greenhouse gas emissions. Anisotropic nanomaterials have been used to improve the efficiency of CO₂ conversion into useful chemicals and fuels using a variety of processes such as photocatalysis, electrocatalysis, and thermocatalysis. More study is required to improve the usage of anisotropic nanomaterials for CO₂ consumption and to scale up these technologies for industrial use. The unique properties of anisotropic nanomaterials, such as their high surface area, tunable morphology, and high activity, make them promising catalysts for CO₂ utilization. This review article discusses briefly about various approaches towards the synthesis of anisotropic nanomaterials and their applications in CO₂ utilization. The article also highlights the challenges and opportunities in this field and the future direction of research.

Theoretical screening of single atom doping on β-Ga2O3 (100) for photoelectrochemical water splitting with high activity and low limiting potential

Photoelectrochemical (PEC) water splitting combined with renewable energy is an appealing approach for solar energy conversion and storage. Monoclinic gallium oxide (β-Ga₂O₃) has been identified as a promising photoelectrode for PEC because of its good electrical conductivity and chemical and thermal stability. However, the wide bandgap (around 4.8 eV) and the recombination of photogenerated electrons and holes inside β-Ga₂O₃ limit its performance. Doping β-Ga₂O₃ is a practical strategy to enhance photocatalytic activity, but studies on doped β-Ga₂O₃ based photoelectrodes are lacking. In this study, we evaluate the doping effect of ten different dopants for β-Ga₂O₃ photoelectrode at the atomic level using density functional theory calculations. In addition, the oxygen evolution performance is evaluated on doped structures as it is considered the bottleneck reaction in water slitting on the anode of the PEC cell. Our results suggest that rhodium doping is optimal as it demonstrated the lowest overpotential for oxygen evolution reaction. We performed further electronic structure analysis, indicating the narrower bandgap and enhanced photogenerated electron-hole transfer comparing with β-Ga₂O₃ are the main reasons for the improved performance after Rh doping. This study demonstrates that doping is an attractive strategy for the development of efficient Ga₂O₃-based photoanodes and it will be of great importance in helping the design of other semiconductor-based photoelectrodes for practical application.

Vanadium Oxide: Phase Diagrams, Structures, Synthesis, and Applications

Vanadium oxides with multioxidation states and various crystalline structures offer unique electrical, optical, optoelectronic and magnetic properties, which could be manipulated for various applications. For the past 30 years, significant efforts have been made to study the fundamental science and explore the potential for vanadium oxide materials in ion batteries, water splitting, smart windows, supercapacitors, sensors, and so on. This review focuses on the most recent progress in synthesis methods and applications of some thermodynamically stable and metastable vanadium oxides, including but not limited to V₂O₃, V₃O₅, VO₂, V₃O₇, V₂O₅, V₂O₂, V₆O₁₃, and V₄O₉. We begin with a tutorial on the phase diagram of the V-O system. The second part is a detailed review covering the crystal structure, the synthesis protocols, and the applications of each vanadium oxide, especially in batteries, catalysts, smart windows, and supercapacitors. We conclude with a brief perspective on how material and device improvements can address current deficiencies. This comprehensive review could accelerate the development of novel vanadium oxide structures in related applications.

pH- and Facet-Dependent Surface Chemistry of TiO2 in Aqueous Environment from First Principles

TiO₂ is a relevant catalytic material, and its chemistry in aqueous environment is a challenging aspect to address. Also, the morphology of TiO₂ particles at the nanoscale is often complex, spanning from faceted to spherical. In this work, we study the pH- and facet-dependent surface chemistry of TiO₂/water interfaces by performing ab initio molecular dynamics simulations with the grand canonical formulation of species in solution. We first determined the acid-base equilibrium constants at the interface, which allows us to estimate the pH at the point of zero charge, an important experimental observable. Then, based on simulated equilibrium constants, we predict the amount of H⁺, OH^-, and adsorbed H₂O species present on the surfaces as a function of the pH, a relevant aspect for water splitting semi-reactions. We approximated the complex morphology of TiO₂ particles by considering the rutile (110) and (011), and anatase (101), (001), and (100) surfaces.

Facet Engineering of Advanced Electrocatalysts Toward Hydrogen/Oxygen Evolution Reactions

The crystal facets featured with facet-dependent physical and chemical properties can exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) attributed to their anisotropy. The highly active exposed crystal facets enable increased mass activity of active sites, lower reaction energy barriers, and enhanced catalytic reaction rates for HER and OER. The formation mechanism and control strategy of the crystal facet, significant contributions as well as challenges and perspectives of facet-engineered catalysts for HER and OER are provided.

The electrocatalytic water splitting technology can generate high-purity hydrogen without emitting carbon dioxide, which is in favor of relieving environmental pollution and energy crisis and achieving carbon neutrality. Electrocatalysts can effectively reduce the reaction energy barrier and increase the reaction efficiency. Facet engineering is considered as a promising strategy in controlling the ratio of desired crystal planes on the surface. Owing to the anisotropy, crystal planes with different orientations usually feature facet-dependent physical and chemical properties, leading to differences in the adsorption energies of oxygen or hydrogen intermediates, and thus exhibit varied electrocatalytic activity toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, a brief introduction of the basic concepts, fundamental understanding of the reaction mechanisms as well as key evaluating parameters for both HER and OER are provided. The formation mechanisms of the crystal facets are comprehensively overviewed aiming to give scientific theory guides to realize dominant crystal planes. Subsequently, three strategies of selective capping agent, selective etching agent, and coordination modulation to tune crystal planes are comprehensively summarized. Then, we present an overview of significant contributions of facet-engineered catalysts toward HER, OER, and overall water splitting. In particular, we highlight that density functional theory calculations play an indispensable role in unveiling the structure–activity correlation between the crystal plane and catalytic activity. Finally, the remaining challenges in facet-engineered catalysts for HER and OER are provided and future prospects for designing advanced facet-engineered electrocatalysts are discussed.

Effects of oxygen pressure on the morphology and surface energetics of β-PbO2: insight from DFT calculations

For over a century, lead dioxide (PbO₂) has been investigated in lead-acid batteries and extensively utilized in a variety of applications. Identifying the surface properties and equilibrium morphology of β-PbO₂ (rutile phase) particles is mandatory for industrial utilization and surface engineering. Using density-functional calculations within the generalized gradient approximation revised for solids (PBEsol), we investigate a variety of surface properties of β-PbO₂. The surface energies of low-Miller-index stoichiometric surfaces are firstly determined, and the (110) surface is found to be the most thermodynamically stable. The relative energetics of these surfaces are represented by a Wulff construction which shows an acicular shape, mostly dominated by the (110) and (100) surfaces. Besides, we investigate the surface chemistry of β-PbO₂ under reduction and oxidation conditions as a function of oxygen pressure, finding that most surfaces except for (100) and (110) are likely to be oxidized. Under oxygen pressure at 1 atm and oxygen-rich limit, the (101) surface is the most thermodynamically stable, dominating the Wulff construction with pyramidal shapes. Our results indicate that the growth conditions that cause non-stoichiometry of the surface could modify the equilibrium Wulff shape of β-PbO₂. Our predicted Wulff shapes and dominant facets agree with the experimental results in which the pyramidal shape of the β-PbO₂ grains has often been observed with the (101) preferred orientation.

Lead-Free Metal Halide Perovskite Nanocrystals: From Fundamentals to Applications

Lead (Pb) halide perovskite nanocrystals, with the general formula APbX₃ , where A=CH₃ NH³⁺ , CH(NH₂ )²⁺ , or Cs⁺ and X=Cl^- , Br^- , or I^- , have emerged as a class of materials with promising properties due to their remarkable optical properties and solar cell performance. However, important issues still need to be addressed to enable practical applications of these materials, such as instability, mass production, and Pb toxicity. Recent studies have carried out the replacement of Pb by various less-toxic cations as Sn, Ge, Sb, and Bi. This variety of chemical compositions provide Pb-free perovskite and metal halide nanostructures with a wide spectral range, in addition to being considered less toxic, therefore having greater practical applicability. Highlighting the necessity to address and solve the toxicity problems related to Pb-containing perovskite, this review considers the prospects of the Pb-free perovskite, involving synthesis methods, and properties of them, including advantages, disadvantages, and applications.

Fabrication and Photocatalytic Activity of Single Crystalline TiO2 Hierarchically Structured Microspheres

Single crystalline anatase TiO2 microspheres with co-exposed {001}/{101} facets were prepared by a facile one-pot hydrothermal method using NaF as a morphology controlling agent. The influences of the NaF amount on the morphology and also on the photocatalytic activity were investigated systematically. The obtained microspheres possessed better morphology when the concentration of NaF was chosen at 0.1 mol/L, and the experimental results indicated that the crystal structure and morphology played important roles on the photocatalytic activity, based on the experimental results it was found that the photocatalytic degradation efficiency of TiO2 microspheres on Tetracycline hydrochloride could reach 76.4% in 2 h. Finally, a growth mechanism was proposed by investigating the growth process, i.e., a synergistic effect of F ions modified Ostwald ripening and oriented attachment.

Increased photocorrosion resistance of ZnO foams via transition metal doping

ZnO is a widely studied photocatalyst, but practical use is hindered by its low resistance to photocorrosion in water, which leads to metal leaching and loss of performance over time. In this work, highly porous and mechanically stable ZnO foams, called MolFoams, were doped by adding 1% or 2% Co, Ni or Cu salts to the starting Zn salt, followed by air insufflation during a sol-gel rection and sintering. The resulting doped foams showed a major increase in stability, with a 60-85% reduction in Zn2+ leaching after irradiation, albeit with a reduction in photocatalytic activity. A systematic analysis using XRD, Raman, XPS and XANES allowed for the identification of dopant species in the foams revealing the presence of Co3O4, NiO and Cu2O within the ZnO lattice with doping leading to a reduced band gap and significant increases in the resistance to photocorrosion of ZnO while identifying the cause of the reduction in photocatalytic activity to be shifting of the band edge positions. These results provide a pathway to significantly reduce the photocorrosion of ZnO in water, with further work required to maintain the photocatalytic activity of undoped ZnO.

Surface and Interface Engineering for the Catalysts of Electrocatalytic CO2 Reduction

The massive use of fossil fuels releases a great amount of CO₂ , which substantially contributes to the global warming. For the global goal of putting CO₂ emission under control, effective utilization of CO₂ is particularly meaningful. Electrocatalytic CO₂ reduction reaction (eCO₂ RR) has great potential in CO₂ utilization, because it can convert CO₂ into valuable carbon-containing chemicals and feedstock using renewable electricity. The catalyst design for eCO₂ RR is a key challenge to achieving efficient conversion of CO₂ to fuels and useful chemicals. For a typical heterogeneous catalyst, surface and interface engineering is an effective approach to enhance reaction activity. Herein, the development and research progress in CO₂ catalysts with focus on surface and interface engineering are reviewed. First, the fundaments of eCO₂ RR is briefly discussed from the reaction mechanism to performance evaluation methods, introducing the role of the surface and interface engineering of electrocatalyst in eCO₂ RR. Then, several routes to optimize the surface and interface of CO₂ electrocatalysts, including morphology, dopants, atomic vacancies, grain boundaries, surface modification, etc., are reviewed and representative examples are given. At the end of this review, we share our personal views in future research of eCO₂ RR.

Recent progress of crystal orientation engineering in halide perovskite photovoltaics

Manipulating the crystallographic orientation of semiconductor crystals plays a vital role in fine-tuning their facet-dependent properties, such as surface properties, charge transfer properties, trap state density, and lattice strain. The success in crystal orientation engineering enables the preferential growth orientation of perovskite thin films with favorable crystal planes by precise nucleation manipulation and growth condition optimization, rendering the films with the unique optoelectronic properties to further improve the efficiency of perovskite solar cells (PSCs). However, the origin and impact of preferential crystallographic orientation of perovskite thin films on the corresponding photovoltaic performance of PSCs are still far from being well understood. Herein, we explore the crystal orientation-dependent optoelectronic properties of halide perovskites and their influence on the photovoltaic performance of PSCs. We summarize the basic strategies for crystal facet engineering in the fabrication of preferentially oriented perovskite thin films, with a focus on the oriented growth mechanism during thin film formation. Based on the above knowledge and the recent research progress in terms of crystal orientation engineering in PSCs, a brief outlook on the remaining challenges and perspectives are provided.

Self-Doping Based Facet Junctions and Oxygen Vacancies in Ferroelectric Bi3TixNb2-xO9 Nanosheets for Boosting Photocatalytic Degradation and Antibacterial Activity

Constructing facet junction in semiconductor photocatalysts has been demonstrated as an effective method to promote charge-carrier separation and suppress carrier recombination. Herein, we proposed a novel but facile self-doping strategy to regulate the crystal facet exposure ratio in ferroelectric Bi3TixNb2-xO9 single-crystalline nanosheets, thereby optimizing its facet junction effect. Through tuning the atomic ratio of Ti and Nb, the exposure ratio of {001} and {110} crystal planes in Bi3TixNb2-xO9 nanosheets can be delicately modulated, and more {110} facets were exposed with the increase of the Ti/Nb atomic ratio as evidenced by the X-ray diffraction and scanning electron microscopy results. A facet junction between {110} and {001} crystal planes was verified based on the density functional theory calculation and photodeposition experiment results. Photogenerated electrons tend to accumulate in {110}, while holes gathered in {001} crystal planes. Owing to the optimal facet junction effect, the sample of Ti1.05 shows the most efficient charge-carrier separation and transportation compared to Ti0.95 and Ti1.00 as supported by the photoluminescence, surface photovoltage, photoelectrochemistry, and electron paramagnetic resonance (EPR) results. In addition, the oxygen vacancy arising from the inequivalent substitution of Nb5+ by Ti4+ as proved by X-ray photoelectron spectroscopy and EPR results and the enhanced ferroelectricity supported by P-E loops can also assist charge-carrier separation and migration. Benefiting from these properties, Ti1.05 outperformed Ti0.95 and Ti1.00 in the photodegradation of organic dye and antibiotic molecules. Meanwhile, the excellent antibacterial activity of Ti1.05 under visible light was also demonstrated by the Escherichia coli sterilization experiment. This work not only presents a novel pathway to adjust the facet junction but also provides new deep insights into the crystal facet engineering in ferroelectrics as photocatalysts.

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.

Nanoetching TiO2 nanorod photoanodes to induce high-energy facet exposure for enhanced photoelectrochemical performance

Crystal facet engineering is considered as an effective way to improve photoelectrochemical (PEC) performance. Here, we have developed a nanoetching technology (TiO₂ → TiO₂/Bi₄Ti₃O₁₂ → TiO₂/BiVO₄ → etching-TiO₂) to treat rutile TiO₂ nanorod films. Interestingly, the technology can induce the exposure of a large number of high energy (101) faces, and the etching-TiO₂ film (E-TiO₂) showed a significantly enhanced PEC performance. A dynamic study indicates that charge separation and transfer have been obviously improved by such a nanoetching technology. In particular, the charge transfer efficiency (η_trans) of E-TiO₂ reaches 93.4% at 1.23 V vs. RHE without any loaded cocatalyst. The mechanism of PEC performance enhanced by the strategy is experimentally and theoretically unraveled. The improvement of PEC performance is mainly attributed to the shorter distance between H and the neighboring O-b for the HO* intermediates of the rutile (101) facet, which can reduce the energy barrier for the OER. Besides, the driving force for spatial charge separation between the (110) and (101) facets can promote charge separation. This work offers a new and versatile nanotechnology to induce the exposure of the high energy crystal facets and improve the PEC performance.

Synergistic Effect of Sr-O Divacancy and Exposing Facets in SrTiO3 Micro/Nano Particle: Accelerating Exciton Formation and Splitting, Highly Efficient Co2+ Photooxidation

As a typical perovskite-type crystal, polyhedral strontium titanate (SrTiO₃ ) has shown anisotropic charge transport behavior in recent studies, however, the carrier transportation and transition of which has not been explained very clearly. This work present the existence of Sr and O divacancies in the novel rhombicuboctahedron SrTiO₃ micro/nano particles (Sr_{1- x} TiO_{3- x} /TiO_{2- x} ) with exposing (100), (110) and (111) facets and the diameter of 300-700 nm synthesized via hydrothermal synthesis, and also summarizes the dissociation mechanism of self-trapped excitons (STEs) caused by the divacancy and facet effect. In addition, most importantly, the metastable STEs with ultra-low binding energy (E_b  < 3 meV) under illumination are discovered. Combining the model of S-scheme heterojunction, a conversion mechanism of photoinduced carriers is proposed. The photocatalytic reaction of Co²⁺ is used as the probe reaction, and the unique Sr_{1- x} TiO_{3- x} /TiO_{2- x} possesses a high photooxidation efficiency of Co²⁺ , by which 70.3% of Co²⁺ is oxidized to Co³⁺ (CoOOH) in 5 min. This finding may provide a guideline for an optimal design of the photocatalytic materials for the recovery and extraction of metal ions based on SrTiO₃ .

strong synergistic effect of the (110) and (100) facets of the SrTiO3 perovskite micro/nanocrystal: decreasing the binding energy of exciton and superb photooxidation capability for Co2

Crystal facet regulation is an effective method for preparing SrTiO₃ or other perovskite semiconductor materials with high photochemical catalysis performance. In general, the edge-truncated cube of SrTiO₃ micro-nano particles has been widely reported because of the multiple crystal facets exposed at the same time. However, the effect of the (110) facet and the interaction between the (100) and (110) facets on the properties of photo-induced carriers is still not very clear. In this article, we have designed and prepared two edge-truncated cube SrTiO₃-a small and large area proportion of the (110) facet, respectively. In addition to the morphological and structural characterization, high-resolution XPS and femtosecond multiphoton transient absorption (fs-TA) spectroscopy were used to detect the atomic vacancy and were applied to confirm the state of carrier transition. The results showed that the larger (110) facet led to two influences-more Sr vacancies and more self-trapping excitons (STEs) with an ultra-low binding energy (E_b = 2.13 meV), about 1.17 meV lower than that of the sample with the smaller (110) facet. In particular, the larger (110) facet also caused a much higher photooxidation performance for Co²⁺ to Co³⁺. This study not only enriches the arsenal of SrTiO₃ materials but also sheds new insights into the understanding of the synergistic effect essence of the (100) and (110) facets, which could promote the development of new perovskite photocatalytic materials, particularly in the recovery of heavy metals.

Carbon-doped anatase titania nanoparticles: similarities and differences with respect to bulk and extended surface models

C-Doping of titania nanoparticles is analyzed by using all-electron density functional theory-based calculations considering the (TiO₂)₈₄ nanoparticle as a realistic representative of nanoparticles in the scalable regime. Several sites are evaluated including substituting oxygen (C_O) and titanium (C_Ti) sites as well as interstitial (C_i) situations. The formation energy of such a doped structure is studied as a function of the oxygen chemical potential (or oxygen partial pressure). Our calculations predict that low partial oxygen pressure favors the formation of C-doped (TiO₂)₈₄ NPs at oxygen and interstitial sites. For the former, the most stable situation is for O sites at the inner part of the nanoparticle. Interestingly, the substitution of O by C at facet sites requires formation energies as those reported in previous studies where the bulk anatase and surfaces models were considered. However, C-doping - at other low coordinated sites not presented in extended models - is even more favorable which shows the need to employ more realistic models for nanostructures involved in photocatalytic processes.

Determination of Crystallographic Orientation and Exposed Facets of Titanium Oxide Nanocrystals

Titanium dioxide (TiO₂ ) nanocrystals have attracted great attention in heterogeneous photocatalysis and photoelectricity fields for decades. However, contradicting conclusions on the crystallographic orientation and exposed facets of TiO₂ nanocrystals frequently appear in the literature. Herein, using anatase TiO₂ nanocrystals with highly exposed {001} facets as a model, the misleading conclusions that exist on anatase nanocrystals are clarified. Although TiO₂ -001 nanocrystals are recognized to be dominated by {001} facets, in fact, anatase nanocrystals with both dominant {001} and {111} facets always co-exist due to the similarities in the lattice fringes and intersection angles between the two types of facets (0.38 nm and 90° in the [001] direction, 0.35 nm and 82° in the [111] direction). A paradigm for determining the crystallographic orientation and exposed facets based on transmission electron microscopy (TEM) analysis, which provides a universal methodology to nanomaterials for determining the orientation and exposed facets, is also given.

Recent Advances in Photocatalytic Oxidation of Methane to Methanol

Methane is one of the promising alternatives to non-renewable petroleum resources since it can be transformed into added-value hydrocarbon feedstocks through suitable reactions. The conversion of methane to methanol with a higher chemical value has recently attracted much attention. The selective oxidation of methane to methanol is often considered a “holy grail” reaction in catalysis. However, methanol production through the thermal catalytic process is thermodynamically and economically unfavorable due to its high energy consumption, low catalyst stability, and complex reactor maintenance. Photocatalytic technology offers great potential to carry out unfavorable reactions under mild conditions. Many in-depth studies have been carried out on the photocatalytic conversion of methane to methanol. This review will comprehensively provide recent progress in the photocatalytic oxidation of methane to methanol based on materials and engineering perspectives. Several aspects are considered, such as the type of semiconductor-based photocatalyst (tungsten, titania, zinc, etc.), structure modification of photocatalyst (doping, heterojunction, surface modification, crystal facet re-arrangement, and electron scavenger), factors affecting the reaction process (physiochemical characteristic of photocatalyst, operational condition, and reactor configuration), and briefly proposed reaction mechanism. Analysis of existing challenges and recommendations for the future development of photocatalytic technology for methane to methanol conversion is also highlighted.

Beyond Janus Geometry: Characterization of Flow Fields around Nonspherical Photocatalytic Microswimmers

Catalytic microswimmers that move by a phoretic mechanism in response to a self-induced chemical gradient are often obtained by the design of spherical janus microparticles, which suffer from multi-step fabrication and low yields. Approaches that circumvent laborious multi-step fabrication include the exploitation of the possibility of nonuniform catalytic activity along the surface of irregular particle shapes, local excitation or intrinsic asymmetry. Unfortunately, the effects on the generation of motion remain poorly understood. In this work, single crystalline BiVO₄ microswimmers are presented that rely on a strict inherent asymmetry of charge-carrier distribution under illumination. The origin of the asymmetrical flow pattern is elucidated because of the high spatial resolution of measured flow fields around pinned BiVO₄ colloids. As a result the flow from oxidative to reductive particle sides is confirmed. Distribution of oxidation and reduction reactions suggests a dominant self-electrophoretic motion mechanism with a source quadrupole as the origin of the induced flows. It is shown that the symmetry of the flow fields is broken by self-shadowing of the particles and synthetic surface defects that impact the photocatalytic activity of the microswimmers. The results demonstrate the complexity of symmetry breaking in nonspherical microswimmers and emphasize the role of self-shadowing for photocatalytic microswimmers. The findings are leading the way toward understanding of propulsion mechanisms of phoretic colloids of various shapes.

Engineering Noble Metal-like Bi onto Hierarchical SrWO4 for the Enhancement of Photocatalytic Activity

Solar-driven hydrogen production from water has attracted increasing attention due to sustainable H2 fuel generation with zero-emissions. However, the design of a photocatalyst without noble metals to enable efficient water splitting is still critical for practical applications. In this study, the hierarchical microspheres of SrWO4 assembled with well-defined metallic Bi nanoparticles were synthesized through mild hydrothermal aging. The resultant photocatalyst with optimum proportion exhibited the competitive performance of a high hydrogen-generation rate at 4.5 mmol·h−1·g−1, outperforming SrWO4 and Bi by factors of 2.0 and 2.4, respectively. Both the experimental characterization and active analysis revealed that the synergistic effect of noble metal-like behavior of Bi and their electron-sink capacity mainly contribute to boosting the overall photocatalytic efficiency. This work may provide advanced insights into the application of economical bismuth elements as co-catalysts, instead of noble metals, to improve photocatalytic efficiency in solar-fuel conversion and environmental remediation.

Crystal Facet Dependent Energy Band Structures of Polyhedral Cu2O Nanocrystals and Their Application in Solar Fuel Production

We demonstrated a facile hydrothermal method to synthesize the (100)-, (110)- and (111)-oriented Cu₂O nanocrystals (NCs) by controlling the concentration of the incorporated anions (CO₃^2- and SO₃^2-). The crystal facet dependent activity of the orientation controlled Cu₂O NCs in the rhodamine B (RhB) photodegradation and photocatalytic hydrogen (H₂) evolution was found to follow the trend: (111) > (110) > (100). The mechanism was investigated by characterizing the optical property, energy band structure, interfacial charge carrier dynamics and reducing ability. The results indicated that the (111)-oriented Cu₂O NCs exhibit the higher conduction band (CB) potential as compared with the (110)-oriented and (100)-oriented Cu₂O NCs, which resulted in the largest driving force of interfacial electron transfer for (111)-oriented Cu₂O NCs to carry out solar fuel generation. The current study offers an easy strategy for crystal facet engineering of semiconductors and provides important physical insights into their electronic properties for the desired solar energy conversions.

Synthesis of 2D anatase TiO2 with highly reactive facets by fluorine-free topochemical conversion of 1T-TiS2 nanosheets

Two-dimensional (2D) anatase titanium dioxide (TiO₂) is expected to exhibit different properties as compared to anatase nanocrystallites, due to its highly reactive exposed facets. However, access to 2D anatase TiO₂ is limited by the non-layered nature of the bulk crystal, which does not allow use of top-down chemical exfoliation. Large efforts have been dedicated to the growth of 2D anatase TiO₂ with high reactive facets by bottom-up approaches, which relies on the use of harmful chemical reagents. Here, we demonstrate a novel fluorine-free strategy based on topochemical conversion of 2D 1T-TiS₂ for the production of single crystalline 2D anatase TiO₂, exposing the {001} facet on the top and bottom and {100} at the sides of the nanosheet. The exposure of these faces, with no additional defects or doping, gives rise to a significant activity enhancement in the hydrogen evolution reaction, as compared to commercially available Degussa P25 TiO₂ nanoparticles. Because of the strong potential of TiO₂ in many energy-based applications, our topochemical approach offers a low cost, green and mass scalable route for production of highly crystalline anatase TiO₂ with well controlled and highly reactive exposed facets.