Dr Cui Ying Toe

Harnessing solar energy for the production of storable and transportable chemicals via photoelectrochemical (PEC) reactions offers a promising solution to overcome the intermittence of solar irradiation. Kesterites have been known as cost-efficient, environmentally friendly, and efficient semiconductor photoelectrode materials for PEC solar fuel production. While significant progress has been made in water splitting, there is increasing attention paid to extending applications to CO2 reduction, ammonia synthesis, and more. However, when efficient kesterite-based photoelectrodes are designed for water splitting and beyond, it is crucial to comprehensively consider both photoelectrode activity and reaction selectivity. This review elaborates on strategies for rationally designing kesterite-based photoelectrodes by optimizing photoactivity in terms of photogenerated charge migration and regulating the surface catalytic sites through nanoscale engineering. More importantly, it discusses optical management and system integration to advance PEC device design for future scalable applications. The perspectives and challenges are also proposed for future solar fuel applications.

Photoelectrochemical (PEC) processes will play a crucial role in future clean energy systems, however severe charge recombination and sluggish charge transfer kinetics have hindered their practical adoption. Exploiting ferroelectric polarization-controlled charge dynamics promises an additional lever that can potentially enable the performance limits of traditional static photoelectrodes to be surpassed. Here one of the most notable ferroelectric polarization-induced photocurrent enhancements is reported, using a heterostructure of the multiferroic bismuth ferrite (BFO) and the photoactive bismuth vanadate (BVO) in a neutral pH electrolyte. In contrast to previous works, enhancements for both poling directions are reported, of 136% for down-poled BFO/BVO and 70% for up-poled BFO/BVO at 1.23¿VRHE in comparison to the unpoled sample, delivering a Faradaic efficiency of >95% for prolonged oxygen evolution reaction. Extensive PEC and surface analyses complemented by density functional theory (DFT) calculations reveal the improvements are attributed to the modulation of gradients in the BFO band energies, as well as changes in band-bending and offsets at the interfaces. Given the scalability of the employed sol¿gel synthesis method and the use of environmentally benign materials and PEC conditions, these findings pave the way for multifunctional materials as new-generation agile and dynamic catalysts and photoelectrode systems.

Copper sulfides are appealing semiconductors for optoelectronics applications requiring visible-light activity, such as solar water splitting, due to their relatively low band gaps and earth-abundance. This study investigates the effect of deposition conditions, including substrate temperature and background gas pressure, on the characteristics and photoelectrochemical performance of copper sulfide (CuxS) films grown by pulsed laser deposition (PLD) on Si (100) substrates. The results reveal that varying the deposition parameters influences the crystalline phases, morphology, and optoelectronic properties of the films. For deposition at room temperature, the films exhibited covellite CuS phase, while elevated deposition temperatures (250 °C and 500 °C) led to the formation of Cu1.8S and Cu2S phases, resulting in narrowing of the band gap, with the increased temperature also enhancing the film crystallinity and surface roughness. As a result of the changes in film morphology and crystal structure, photocurrent density magnitudes increased with higher deposition temperatures, reaching 0.747 mA/cm2 at -0.8 V for films deposited at 500 °C under vacuum, at least an order of magnitude higher than reported in comparable previous studies of the photoelectrochemical performance of CuxS. Background gas pressure only slightly affected the photocurrent density, with changes in photoactivity also correlating with changes in film roughness and band gap. The results demonstrate the good visible-light activity and behavior of CuxS as a stand-alone photoelectrode material, with tunability based on the fabrication conditions, suggesting their potential use in photoelectrochemical and other solar energy conversion applications.

Restoring ammonia from waste nitrate stands as a promising strategy for reducing reliance on the energy-intensive Haber-Bosch process and tackling environmental pollutants. Advancing the catalytic aspects of photoelectrochemical (PEC) ammonia synthesis via waste nitrate reduction is of great importance to enhance its viability for sustainable chemical production. However, this process still suffers from low ammonia faradaic efficiency (FE) with high operational potential due to its involvement in multi-electron reactions. Herein, we integrated a cobalt-doped TiOx (Co-TiOx) cocatalyst and Ag nanowires (NWs) electron extraction layer onto TiOx/CdS/Cu2ZnSnS4 (CZTS) photocathode, achieving nearly 100 % ammonia FE and an onset potential of ~0.49 V vs. RHE. Evidenced by the in-situ synchrotron-radiated FTIR (SR-FTIR) and theoretical calculations, the increased ratio of surface oxygen vacancy sites (Vo) induced by Co-TiOx is crucial for the key reaction intermediates adsorption (i.e. *NO3 and *NO2) for subsequent ammonia production. Moreover, the transparent Ag NWs facilitates the electron extraction from TiOx/CdS/CZTS to the surface catalytic sites. Powered by CZTS solar cells, a standalone solar-to-ammonia system has been demonstrated with outstanding activity and catalytic performance.

Photoreforming enables simultaneous H2 production and organic synthesis in a one-pot system. In this study, a single-step synthesis approach was employed to fabricate atomically dispersed Ni in Zn3In2S6 (NixZIS) for benzyl alcohol photoreforming. While neat ZIS exhibits high selectivity toward hydrobenzoin via C-H activation and C-C coupling, its H2 evolution rate remains below feasible levels. Incorporating Ni single atoms significantly enhances ZIS activity by improving the carrier dynamics, resulting in an optimal H2 evolution rate of 9.13 mmol g-1 h-1 on Ni4ZIS, over five times higher than neat ZIS. The presence of Ni single atoms also alters selectivity, suppressing C-C coupling products and promoting benzaldehyde generation. The Ni single atoms induce facile O-H activation following the C-H activation of benzyl alcohol on ZIS, inhibiting the desorption of carbon radicals and causing consecutive oxidation to benzaldehyde. This study elucidates the role of Ni single atoms in driving activity and selectivity during organic photoreforming.

Photocatalytic solar hydrogen generation, encompassing both overall water splitting and organic reforming, presents a promising avenue for green hydrogen production. This technology holds the potential for reduced capital costs in comparison to competing methods like photovoltaic-electrocatalysis and photoelectrocatalysis, owing to its simplicity and fewer auxiliary components. However, the current solar-to-hydrogen efficiency of photocatalytic solar hydrogen production has predominantly remained low at ¿1¿2% or lower, mainly due to curtailed access to the entire solar spectrum, thus impeding practical application of photocatalytic solar hydrogen production. This review offers an integrated, multidisciplinary perspective on photocatalytic solar hydrogen production. Specifically, the review presents the existing approaches in photocatalyst and system designs aimed at significantly boosting the solar-to-hydrogen efficiency, while also considering factors of cost and scalability of each approach. In-depth discussions extending beyond the efficacy of material and system design strategies are particularly vital to identify potential hurdles in translating photocatalysis research to large-scale applications. Ultimately, this review aims to provide understanding and perspective of feasible pathways for commercializing photocatalytic solar hydrogen production technology, considering both engineering and economic standpoints.

Photoelectrochemical oxidation (PECO) is a promising advanced technology for treating micropollutants in stormwater. However, it is important to understand its operation prior to practical validation. In this study, we introduced a flow PECO system designed to evaluate its potential for full-scale applications in herbicides degradation, providing valuable insights for future large-scale implementations. The PECO flow reactor demonstrated the ability to treat a larger volume of stormwater (675 mL, approximately 10 times more than previous batch experiments) with effective removal rates of 92 % for diuron and 22 % for atrazine over 6 h of operation at 2 V. To address the large volume issue in stormwater treatment, a multiple module parallel application design is being considered to increase the treatment capacity of the PECO flow reactor. During the flow reactor operations, flow rate was found to have a notable impact on removal performance, particularly for diuron. At a flow rate of 610 mL min-1, approximately 90 % removal of diuron was achieved, while at 29 mL min-1, the removal efficiency decreased to 60 %. While light intensity had minimal effect on diuron degradation (all settings achieved over 90 % removal), it enhanced atrazine degradation from 9 % to 31 % with an increase in intensity from 63 mW cm-2 to 144 mW cm-2. Remarkably, the PECO flow system exhibited excellent removal performance (>90 % removal) for diuron even at extremely high initial pollutant concentrations (240 µg L-1), demonstrating its capacity to handle varying contaminant loads in stormwater. Energy consumption analysis revealed that flow rate as the primary factor influenced the specific energy consumption rate. Higher flow rate (e.g., 610 mL min-1) were preferable in flow reactor due to its well-balanced performance between removal and energy consumption. These findings confirm that the PECO flow system offers an efficient and promising approach for stormwater treatment applications.

Seawater splitting by piezocatalysis and piezo-photocatalysis recently has been developed as a strategy for efficient energy conversion from mechanical energy and/or renewable solar energy. However, the piezocatalytic hydrogen evolution reaction (HER) performance currently remain worse than those from electrolysis or photocatalysis, suffering from serious charge recombination and catalyst leaching in seawater. Herein, the present work reveals a novel nanostructure with self-generated Na0.5Bi0.5TiO3/Na0.5Bi4.5Ti4O15 (NBT/NBT4) heterojunctions, which can be engineered systematically by regulating the NaOH precursor concentration (2.5 M, 7.5 M, 12.5 M) during hydrothermal synthesis. This engineering facilitates the substitution of Bi3+ by Na+ in the NBT solid solution, which also enables the regulation of oxygen vacancy and modification of the band structure. Experimental results and associated theoretical simulation reveal the formation of heterojunction between NBT and NBT4, thereby promoting both charge transfer and charge separation. Consequently, the heterojunction (NBT-12.5 M) exhibited an efficient HER rate of 140 µmol/g/h from DI water through piezo-photocatalysis. More significantly, the heterojunction also causes notable seawater splitting capability, with the HER rates of 68 µmol/g/h from simulated seawater and 58 µmol/g/h from natural seawater. The unique nanostructure effectively suppresses the leaching of Na+ ions in simulated seawater, indicating the potential for durable and practical seawater splitting.

Accurate band-gap prediction is essential for designing and discovering new materials with desired properties. However, current methods for calculating band gaps based on local and semilocal functionals lead to significant underestimation, hindering the effectiveness of in silico and high-throughput screening of materials. We present a machine learning model with domain adaptation to rapidly yield accurate band-gap prediction of semiconductors (oxides, chalcogenides, nitrides, phosphides, etc.). The approach circumvents the prerequisite for a large amount of physically measured band-gap data, which is notoriously scarce. It instead sources knowledge from a large dataset with underestimated band gaps and subsequently transfers knowledge to train a crystal graph convolution neural network (CGCNN) using a small dataset of accurate, physically measured band gaps. The prediction model shows a low mean absolute error (MAE) of 0.23 eV, outperforming those using Perdew-Burke-Ernzerhof (PBE) functionals (MAE = 0.87 eV). Visualization of the learned crystal graph using the t-distributed stochastic neighbor embedding (t-SNE) algorithm revealed that the crystal structure and composition have a strong influence on the material band gaps.

Photoreforming ethanol to simultaneously produce hydrogen and value-added organic products was realized over defected TiO2. Chemically induced defects in TiO2 promoted light absorption and charge separation, enhancing overall photoactivity. The induced defects also regulated product selectivity, leading to greater hydrogen purity and liquid to gaseous carbon ratio. The optimal catalyst generated 0.08 mmol/hr of hydrogen with a purity greater than 99 % and 0.08 mmol/hr of liquid acetaldehyde over a 6 hr timeframe. This was three times greater than the untreated TiO2. Active species trapping revealed that the preferred ethanol oxidation pathway was direct hole transfer, indicating the selectivity relies on surface chemisorption. Surface defects decreased the acetaldehyde adsorption energy, instigating its prompt desorption and suppressing overoxidation into CO2, thus improving the selectivity towards hydrogen and liquid hydrocarbon products. The work offers an alternative approach towards sustainable energy by coupling photocatalysis with waste organic utilization.

Solar water splitting by means of photoelectrochemical (PEC) cells offers the promise to produce cost-effective renewable and clean fuel from abundant sunlight and water. Lately, the realization of promise of concurrent hydrogen (H2) production along with alternate oxidation reaction (which is less energetically demanding than the water oxidation reaction) has also become a subject of intense global research interests. At present, developing inexpensive, non-toxic, and earth-abundant semiconductor-based photoelectrodes (i.e. photocathode and photoanode) with a high stability is of great importance in achieving economically viable H2 production and value-added chemicals. This review summarizes recent advances in these photoelectrodes along with contemporary understanding of key factors responsible for high solar-to-hydrogen efficiency, device stability, and highlights a promising new research trend of alternate oxidation reactions at photoanodes. First, we outline recent developments of novel photoelectrode materials using high-throughput computational screening integrated with ab-initio calculations. We proceed to discuss the merits and major challenges of these novel and existing photoelectrodes and links the strategies used to overcome these challenges to achieve economically viable solar H2 generation. Several important studies on the emerging new trend of alternate oxidations reactions at photoanodes toward value-added chemicals are then detailed with particular emphasis is placed on dependency of photoanode design on type of organic feedstocks and desired products from the oxidation reaction. We also emphasize the development of tandem devices for overall water splitting using these photoelectrodes with high onset potentials. Finally, we provide not only promising future directions for each material system, but also a critical assessment and outlook on how these earth-abundant photoelectrodes could lead to a potential large-scale implementation of water splitting devices.

Oxide-derived Cu catalysts from Cu2O microcrystals are capable of electrochemically converting CO2 into various value-added chemicals. However, their structural transformation and associated preferred products remain unclear, requiring further investigation. Herein, Cu2O microcrystals with controllable low- and high-index facets exposure are fabricated to differentiate the effects of initial exposed facets on their structural reconstruction and product selectivity in electrochemical CO2 reduction reaction. Combined in situ characterizations and theoretical investigation reveal the direct correlations of Cu2O reconstruction and product selectivity to its initial facet exposure. The Cu2O low-index facet, being more stable with a high energy barrier on material reduction, tends to partially maintain its original crystalline structure and larger Cu2O particle size throughout the transformation. The derived flatter surface and limited Cu2O/Cu interfaces result in a favorable selectivity toward 2-electron transfer products. The chemically active Cu2O high-index facet (311) is energetically favorable to be reduced owing to the feasible protonation process, thus experiencing a drastic reconstruction with rich newly formed Cu nanoparticles and evolved fine Cu2O grains; Such a reconstruction creates uncoordinated Cu species and abundant boundaries, benefiting charge transfer and increasing the local pH by confining OH-, thus leading to a high selectivity toward C2+ products.

Exploring advanced technologies to efficiently produce green hydrogen energy is imperative to alleviate the energy crisis and environmental pollution. Conventional overall water electrolysis (OWE) has been regarded as a promising approach for effective H2 production, however, it is largely restricted by the sluggish kinetics of the anodic oxygen evolution reaction (OER). Coupling kinetically favorable anodic reactions, such as biomass-derived compound oxidation and pollutant degradation, with the hydrogen evolution reaction (HER) in hybrid water electrolysis (HWE), can not only solve the biomass recycling and pollutant emission problems but also save the energy cost for clean H2 generation. Hence, various advanced earth-abundant electrocatalysts have been developed to catalyze those promising anodic reactions, yet some problems such as tedious preparation and unsatisfactory performance still exist. Given the gap between research and practical applications, this review summarizes the recent progress in electrocatalysts for diverse alternative anodic oxidation reactions over the last five years together with their application in HWE systems. An in-depth understanding of different reaction mechanisms and assessments toward electrocatalysts is discussed to further enhance anodic efficiency. The advantages, differences, and critical issues of different HWE systems are thoroughly discussed as well, providing a new avenue for low-voltage H2 production from renewable resources and waste products.

Graphene edges exhibit a highly localized density of states that result in increased reactivity compared to its basal plane. However, exploiting this increased reactivity to anchor and tune the electronic states of single atom catalysts (SACs) remains elusive. To investigate this, a method to anchor Pt SACs with ultra-low mass loadings at the edges of edge-rich vertically aligned graphene (as low as 0.71¿µg¿Pt¿cm¿2) is developed. Angle-dependent X-ray absorption spectroscopy and density-functional theory calculations reveal that edge-anchored Pt SACs has a robust coupling with the p-electrons of graphene. This interaction results in a higher occupancy of the Pt 5d orbital, shifting the d-band center toward the Fermi level, improving the adsorption of *H for the hydrogen evolution reaction (HER). Pt primarily coordinated to the graphene edge shows improved alkaline HER performance compared to Pt coordinated in mixed environments (turnover frequencies of 22.6 and 10.9¿s¿1 at an overpotential of 150¿mV, respectively). This work demonstrates an effective route to engineering the coordination environment of Pt SACs by using the graphene edge for enhanced energy conversion reactions.

Reconstructing a catalyst with tunable properties is essential for achieving selective electrochemical CO2reduction reaction (CO2RR). Here, a reduction-oxidation-reduction (ROR) electrochemical treatment is devised to advisedly reconstruct copper nanoparticles on vertical graphene. Undercoordinated sites and oxygen vacancies constructed on the Cu active sites during the ROR treatment enhance the CO2RR activity. Moreover, by varying the oxidation potential while maintaining the reduction potential during the ROR treatment, CO2RR selectivity can be tuned between *COOH- and *OCHO-derived products. Specifically, rich grain boundaries are formed on the ROR catalyst with a high oxidation potential (+1.2 VRHE), favoring the *COOH/*OCCO adsorption and leading C-C coupling to *COOH-derived products, while the catalyst undergoing ROR at a low oxidation potential (+0.8 VRHE) lacks grain boundaries, resulting in highly selective formate (*OCHO-derived) production. Our findings are evidenced by combined in situ and ex situ characterizations and theoretical calculations.

The intrinsic carrier dynamics of cuprous oxide (Cu2O) are known to have a crucial influence on photocatalytic performances. The photoactivity of rhombic dodecahedral Cu2O with dominant {110} facets (RD-Cu2O) is demonstrated to surpass that of cubic Cu2O with {100} surfaces (CB-Cu2O). Time resolved microwave conductivity (TRMC) measurements reveal the higher carrier mobility of RD-Cu2O when compared to CB-Cu2O. Additionally, modulated surface photovoltage (SPV) measurements further supported the better charge separation efficiency of RD-Cu2O. Although CB-Cu2O exhibited more pronounce SPV signals, the homogeneous distribution of electrical fields drives the majority charge inward and led to detrimental charge recombination. In contrast, the weak SPV signals for RD-Cu2O were attributed to a modulated distribution of charges towards the facets and facet boundaries, demonstrating a better charge separation. This study shows that carrier dynamics and defect density should also be regarded as facet-dependent properties that can have deciding influence on the photocatalytic activity. This journal is

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.