Tuning the surface Fermi level on p-type gallium nitride nanowires for efficient overall water splitting

Controlling crystallization in covalent organic frameworks to facilitate photocatalytic hydrogen production

The catalytic performance, depending on the surface nature, is ubiquitous in photocatalysis. However, surface engineering for organic photocatalysts through structural modulation has long been neglected. Here, we propose a zone crystallization strategy for covalent organic frameworks (COFs) that enhances surface ordering through regulator-induced amorphous-to-crystalline transformation. Dynamic simulations show that attaching monofunctional regulators to the surface of spherical amorphous precursor improves surface dynamic reversibility, increasing crystallinity from the inside out. The resulting COF microspheres display surface-enhanced crystallinity and uniform spherical morphology. The visible photocatalytic hydrogen evolution rate reaches 126 mmol g-1 h-1 for the simplest β-ketoenamine-linked COF and 350 mmol gCOF-1 h-1 for SiO2@COF with minimal Pt cocatalysts. Mechanism studies indicate that surface crystalline domains build the surface electrical fields to accumulate photogenerated electrons and diminish electron transfer barriers between the COF and Pt interface. This work bridges the gap between microscopic molecules and macroscopic properties, allowing tailored design of crystalline organic photocatalysts.

Photocatalytic Conversion of Methane to Ethane and Propane Using Cobalt-Cluster-Activated GaN Nanowires

The photocatalytic nonoxidative coupling of methane (PNOCM) offers a promising route to synthesize valuable C2+ hydrocarbons while minimizing side reactions. Oxide-based photocatalysts have been predominant in this field, but suffering from limited conversion rates, selectivity, and durability due to poor C-C coupling as well as overoxidation of CH4 by lattice oxygen. Here, we introduce an advancement in PNOCM for methane conversion into ethane and propane using GaN, one of the most produced semiconductors, together with trace amounts of metallic cobalt clusters (0.1 wt %). The photocatalytic system exhibits outstanding stability, maintaining performance over 110 hours, achieving conversion rates of approximately 192.3 mmol g-1 h-1 for ethane and ~17.9 mmol g-1 h-1 for propane, with virtually no coke byproducts detected, representing the highest activity and stability ever reported to our knowledge. This high activity is attributed to the critical methane activation and C-C coupling on Co cluster, which can be greatly accelerated via the ultrafast photogenerated charge transfer from p-GaN to Co cluster. Additionally, the GaN support further synergistically enhances methane activation by in situ generating N-H and O-H species under reaction, as well as provides a vital anti-overoxidation effect to CH4 for high selectivity and stability.

An Active and Robust Catalytic Architecture of NiCo/GaN Nanowires for Light-Driven Hydrogen Production from Methanol

On-site hydrogen production from liquid organic hydrogen carriers e.g., methanol provides an emerging strategy for the safe storage and transportation of hydrogen. Herein, a catalytic architecture consisting of nickel-cobalt nanoclusters dispersed on gallium nitride nanowires supported by silicon for light-driven hydrogen production from methanol is reported. By correlative microscopic, spectroscopic characterizations, and density functional theory calculations, it is revealed that NiCo nanoclusters work in synergy with GaN nanowires to enable the achievement of a significantly reduced activation energy of methanol dehydrogenation by switching the potential-limiting step from *CHO → *CO to *CH3O → *CH2O. In combination with the marked photothermal effect, a high hydrogen rate of 5.62 mol·gcat-1·h-1 with a prominent turnover frequency of 43,460 h-1 is achieved at 5 Wcm-2 without additional energy input. Remarkably, the synergy between Co and Ni, in combination with the unique surface of GaN, renders the architecture with outstanding resistance to sintering and coking. The architecture thereby exhibits a high turnover number of >16,310,000 over 600 h. Outdoor testing validates the viability of the architecture for active and robust hydrogen evolution under natural concentrated sunlight. Overall, this work presents a promising architecture for on-site hydrogen production from CH3OH by virtually unlimited solar energy.

Facile Semiconductor p-n Homojunction Nanowires with Strategic p-Type Doping Engineering Combined with Surface Reconstruction for Biosensing Applications

Photosensors with versatile functionalities have emerged as a cornerstone for breakthroughs in the future optoelectronic systems across a wide range of applications. In particular, emerging photoelectrochemical (PEC)-type devices have recently attracted extensive interest in liquid-based biosensing applications due to their natural electrolyte-assisted operating characteristics. Herein, a PEC-type photosensor was carefully designed and constructed by employing gallium nitride (GaN) p-n homojunction semiconductor nanowires on silicon, with the p-GaN segment strategically doped and then decorated with cobalt-nickel oxide (CoNiOx). Essentially, the p-n homojunction configuration with facile p-doping engineering improves carrier separation efficiency and facilitates carrier transfer to the nanowire surface, while CoNiOx decoration further boosts PEC reaction activity and carrier dynamics at the nanowire/electrolyte interface. Consequently, the constructed photosensor achieves a high responsivity of 247.8 mA W-1 while simultaneously exhibiting excellent operating stability. Strikingly, based on the remarkable stability and high responsivity of the device, a glucose sensing system was established with a demonstration of glucose level determination in real human serum. This work offers a feasible and universal approach in the pursuit of high-performance bio-related sensing applications via a rational design of PEC devices in the form of nanostructured architecture with strategic doping engineering.

Pseudocapacitance-Induced Synaptic Plasticity of Tribo-Phototronic Effect Between Ionic Liquid and 2D MoS2

Contact-induced electrification, commonly referred to as triboelectrification, is the subject of extensive investigation at liquid-solid interfaces due to its wide range of applications in electrochemistry, energy harvesting, and sensors. This study examines the triboelectric between an ionic liquid and 2D MoS2 under light illumination. Notably, when a liquid droplet slides across the MoS2 surface, an increase in the generated current and voltage is observed in the forward direction, while a decrease is observed in the reverse direction. This suggests a memory-like tribo-phototronic effect between ionic liquid and 2D MoS2 . The underlying mechanism behind this tribo-phototronic synaptic plasticity is proposed to be ion adsorption/desorption processes resulting from pseudocapacitive deionization/ionization at the liquid-MoS2  interface. This explanation is supported by the equivalent electrical circuit modeling, contact angle measurements, and electronic band diagrams. Furthermore, the influence of various factors such as velocity, step size, light wavelength and intensity, ion concentration, and bias voltage is thoroughly investigated. The artificial synaptic plasticity arising from this phenomenon exhibits significant synaptic features, including potentiation/inhibition, paired-pulse facilitation/depression, and short-term memory (STM) to long-term memory (LTM) transition. This research opens up promising avenues for the development of synaptic memory systems and intelligent sensing applications based on liquid-solid interfaces.

Manipulating Surface Band Bending of III-Nitride Nanowires with Ambipolar Charge-Transfer Characteristics: A Pathway Toward Advanced Photoswitching Logic Gates and Encrypted Optical Communication

The operational principle of semiconductor devices critically relies on the band structures that ultimately govern their charge-transfer characteristics. Indeed, the precise orchestration of band structure within semiconductor devices, notably at the semiconductor surface and corresponding interface, continues to pose a perennial conundrum. Herein, for the first time, this work reports a novel postepitaxy method: thickness-tunable carbon layer decoration to continuously manipulate the surface band bending of III-nitride semiconductors. Specifically, the surface band bending of p-type aluminum-gallium-nitride (p-AlGaN) nanowires grown on n-Si can be precisely controlled by depositing different carbon layers as guided by theoretical calculations, which eventually regulate the ambipolar charge-transfer behavior between the p-AlGaN/electrolyte and p-AlGaN/n-Si interface in an electrolyte environment. Enabled by the accurate modulation of the thickness of carbon layers, a spectrally distinctive bipolar photoresponse with a controllable polarity-switching-point over a wide spectrum range can be achieved, further demonstrating reprogrammable photoswitching logic gates "XOR", "NAND", "OR", and "NOT" in a single device. Finally, this work constructs a secured image transmission system where the optical signals are encrypted through the "XOR" logic operations. The proposed continuous surface band tuning strategy provides an effective avenue for the development of multifunctional integrated-photonics systems implemented with nanophotonics.

Ag2Se Nanorod Arrays with Ultrahigh Room Temperature Thermoelectric Performance and Superior Mechanical Properties

Ag₂Se is an intriguing material for room-temperature energy harvesting. Herein, we report the fabrication of Ag₂Se nanorod arrays by glancing angle deposition technique (GLAD) followed by simple selenization in a two-zone furnace. Ag₂Se planar films of different thickness were also prepared. The unique tilted Ag₂Se nanorod arrays show excellent zT = 1.14 ± 0.09 and a power factor of 3229.21 ± 149.01 μW/m-K², respectively, at 300 K. The superior thermoelectric performance of Ag₂Se nanorod arrays compared to planar Ag₂Se films could be ascribed to the unique nanocolumnar architecture that not only facilitates efficient electron transport but also significantly scatters phonons at the interfaces. Furthermore, the nanoindentation measurements were performed to explore mechanical properties of the as-prepared films. The Ag₂Se nanorod arrays showed hardness values of 116.51 ± 4.25 MPa and elastic modulus of 10,966.01 ± 529.61 MPa, which are lowered by 51.8 and 45.6%, compared to Ag₂Se films, respectively. The synergetic dependence between the tilt structure and thermoelectric properties accompanied with the simultaneous improvement in mechanical properties opens a new avenue for the practical applications of Ag₂Se in next-generation flexible thermoelectric devices.

Light-Induced Bipolar Photoresponse with Amplified Photocurrents in an Electrolyte-Assisted Bipolar p-n Junction

The p-n junction with bipolar characteristics sets the fundamental unit to build electronics while its unique rectification behavior constrains the degree of carrier tunability for expanded functionalities. Herein, a bipolar-junction photoelectrode employed with a gallium nitride (GaN) p-n homojunction nanowire array that operates in electrolyte is reported, demonstrating bipolar photoresponse controlled by different wavelengths of light. Significantly, with rational decoration of a ruthenium oxides (RuO_x ) layer on nanowires guided by theoretical modeling, the resulting RuO_x /p-n GaN photoelectrode exhibits unambiguously boosted bipolar photoresponse by an enhancement of 775% and 3000% for positive and negative photocurrents, respectively, compared to the pristine nanowires. The loading of the RuO_x layer on nanowire surface optimizes surface band bending, which facilitates charge transfer across the GaN/electrolyte interface, meanwhile promoting the efficiency of redox reaction for both hydrogen evolution reaction and oxygen evolution reaction which corresponds to the negative and positive photocurrents, respectively. Finally, a dual-channel optical communication system incorporated with such photoelectrode is constructed with using only one photoelectrode to decode dual-band signals with encrypted property. The proposed bipolar device architecture presents a viable route to manipulate the carrier dynamics for the development of a plethora of multifunctional optoelectronic devices for future sensing, communication, and imaging systems.

Nanostructured Gallium Nitride Membrane at Wafer Scale for Photo(Electro)catalytic Polluted Water Remediation

Photo(electro)catalysis methods have drawn significant attention for efficient, energy-saving, and environmental-friendly organic contaminant degradation in wastewater. However, conventional oxide-based powder photocatalysts are limited to UV-light absorption and are unfavorable in the subsequent postseparation process. In this paper, a large-area crystalline-semiconductor nitride membrane with a distinct nanoporous surface is fabricated, which can be scaled up to a full wafer and easily retrieved after photodegradation. The unique nanoporous surface enhances broadband light absorption, provides abundant reactive sites, and promotes the dye-molecule reaction with adsorbed hydroxyl radicals on the surface. The superior electric contact between the nickel bottom layer and nitride membrane facilitates swift charge carrier transportation. In laboratory tests, the nanostructure membrane can degrade 93% of the dye in 6 h under illumination with a small applied bias (0.5 V vs Ag/AgCl). Furthermore, a 2 inch diameter wafer-scale membrane is deployed in a rooftop test under natural sunlight. The membrane operates stably for seven cycles (over 50 h) with an outstanding dye degradation efficiency (>92%) and satisfied average total organic carbon removal rate (≈50%) in each cycle. This demonstration thus opens the pathway toward the production of nanostructured semiconductor layers for large-scale and practical wastewater treatment using natural sunlight.

Manipulation of Photoelectrochemical Water Splitting by Controlling Direction of Carrier Movement Using InGaN/GaN Hetero-Structure Nanowires

We report the improvement in photoelectrochemical water splitting (PEC-WS) by controlling migration kinetics of photo-generated carriers using InGaN/GaN hetero-structure nanowires (HSNWs) as a photocathode (PC) material. The InGaN/GaN HSNWs were formed by first growing GaN nanowires (NWs) on an Si substrate and then forming InGaN NWs thereon. The InGaN/GaN HSNWs can cause the accumulation of photo-generated carriers in InGaN due to the potential barrier formed at the hetero-interface between InGaN and GaN, to increase directional migration towards electrolyte rather than the Si substrate, and consequently to contribute more to the PEC-WS reaction with electrolyte. The PEC-WS using the InGaN/GaN-HSNW PC shows the current density of 12.6 mA/cm² at -1 V versus reversible hydrogen electrode (RHE) and applied-bias photon-to-current conversion efficiency of 3.3% at -0.9 V versus RHE. The high-performance PEC-WS using the InGaN/GaN HSNWs can be explained by the increase in the reaction probability of carriers at the interface between InGaN NWs and electrolyte, which was analyzed by electrical resistance and capacitance values defined therein.

Solar-to-hydrogen efficiency of more than 9% in photocatalytic water splitting

Production of hydrogen fuel from sunlight and water, two of the most abundant natural resources on Earth, offers one of the most promising pathways for carbon neutrality^1-3. Some solar hydrogen production approaches, for example, photoelectrochemical water splitting, often require corrosive electrolyte, limiting their performance stability and environmental sustainability^1,3. Alternatively, clean hydrogen can be produced directly from sunlight and water by photocatalytic water splitting^2,4,5. The solar-to-hydrogen (STH) efficiency of photocatalytic water splitting, however, has remained very low. Here we have developed a strategy to achieve a high STH efficiency of 9.2 per cent using pure water, concentrated solar light and an indium gallium nitride photocatalyst. The success of this strategy originates from the synergistic effects of promoting forward hydrogen-oxygen evolution and inhibiting the reverse hydrogen-oxygen recombination by operating at an optimal reaction temperature (about 70 degrees Celsius), which can be directly achieved by harvesting the previously wasted infrared light in sunlight. Moreover, this temperature-dependent strategy also leads to an STH efficiency of about 7 per cent from widely available tap water and sea water and an STH efficiency of 6.2 per cent in a large-scale photocatalytic water-splitting system with a natural solar light capacity of 257 watts. Our study offers a practical approach to produce hydrogen fuel efficiently from natural solar light and water, overcoming the efficiency bottleneck of solar hydrogen production.

Bipolar charge collecting structure enables overall water splitting on ferroelectric photocatalysts

Ferroelectrics are considered excellent photocatalytic candidates for solar fuel production because of the unidirectional charge separation and above-gap photovoltage. Nevertheless, the performance of ferroelectric photocatalysts is often moderate. A few studies showed that these types of photocatalysts could achieve overall water splitting. This paper proposes an approach to fabricating interfacial charge-collecting nanostructures on positive and negative domains of ferroelectric, enabling water splitting in ferroelectric photocatalysts. The present study observes efficient accumulations of photogenerated electrons and holes within their thermalization length (~50 nm) around Au nanoparticles located in the positive and negative domains of a BaTiO3 single crystal. Photocatalytic overall water splitting is observed on a ferroelectric BaTiO3 single crystal after assembling oxidation and reduction cocatalysts on the positively and negatively charged Au nanoparticles, respectively. The fabrication of bipolar charge-collecting structures on ferroelectrics to achieve overall water splitting offers a way to utilize the energetic photogenerated charges in solar energy conversion.

Optimizing the semiconductor–metal-single-atom interaction for photocatalytic reactivity

Metal single-atom (MSA) catalysts with 100% metal atom utilization and unique electronic properties are attractive cocatalysts for efficient photocatalysis when coupled with semiconductors. Owing to the absence of a metal-metal bond, MSA sites are exclusively coordinated with the semiconductor photocatalyst, featuring a chemical-bond-driven tunable interaction between the semiconductor and the metal single atom. This semiconductor-MSA interaction is a platform that can facilitate the separation/transfer of photogenerated charge carriers and promote the subsequent catalytic reactions. In this Review, we first introduce the fundamental physicochemistry related to the semiconductor-MSA interaction. We highlight the ligand effect on the electronic structures, catalytic properties and functional mechanisms of the MSA cocatalyst through the semiconductor-MSA interaction. Then, we categorize the state-of-the-art experimental and theoretical strategies for the construction of the efficient semiconductor-MSA interaction at the atomic scale for a wide range of photocatalytic reactions. The examples described include photocatalytic water splitting, CO₂ reduction and organic synthesis. We end by outlining strategies on how to further advance the semiconductor-MSA interaction for complex photocatalytic reactions involving multiple elementary steps. We provide atomic and electronic-scale insights into the working mechanisms of the semiconductor-MSA interaction and guidance for the design of high-performance semiconductor-MSA interface photocatalytic systems.

Metal-Support Interaction-Promoted Photothermal Catalytic Methane Reforming into Liquid Fuels

Clean and renewable photocatalytic technology for methane reforming into high-value liquid fuels, such as methanol, is a promising strategy for commercial industrial applications. However, poor charge separation, sluggish methane activation, and excessive oxidation collectively inhibit the production of methanol from photocatalytic methane reforming. Herein, we have developed enhanced metal-support interactions between a GaN nanowire photocatalyst and a Cu nanoparticle (CuNP) cocatalyst via p-doping in GaN. CuNP-loaded p-type GaN (Cu/p-GaN) with enhanced metal-support interaction has 3.5-fold higher activity (12.8 mmol g^-1 h^-1, higher than previous reports) for methanol production in photothermal catalytic methane reforming with oxygen as an oxidant and sunlight as the sole energy source than CuNP-loaded intrinsic GaN (Cu/i-GaN) or n-type GaN (Cu/n-GaN). In-situ IR measurements indicate that enhanced metal-support interaction significantly promotes activation of methane and formation of methanol. Combining with X-ray photoelectron spectroscopy (XPS), density functional theory (DFT) simulations demonstrate that this enhanced metal-support interaction in Cu/p-GaN greatly improves electron transfer from p-GaN photocatalysts to the 3d states of CuNP cocatalysts through the interface between them. Catalytic pathway simulations further reveal that the enhanced metal-support interaction in Cu/p-GaN also desirably decreases the reaction energy of rate-determining methanol desorption, which decreases the excessive oxidation of the produced methanol and accelerates the regeneration of surface catalytic sites. These studies and findings offer critical insights into the design and development of metal nanoparticle-loaded photocatalysts for photocatalysis-based methane reforming into methanol.

Titanium nitride as a promising sodium-ion battery anode: interface-confined preparation and electrochemical investigation

The search for new electrode materials for sodium-ion batteries (SIBs), especially for enhancing the specific capacity and cycling stability of anodes, is of great significance for the development of new energy conversion and storage materials. Here, a new type of titanium nitride composite anode (TiN@C) coated with 2D carbon nanosheets was prepared for the first time using a rationally designed topochemical conversion approach of interface-confinement. Subsequently, the electrochemical performance and Na⁺ storage mechanism of TiN@C as an anode for SIBs was investigated. The quantum-dot-sized TiN anodes exhibited shorter ionic transport pathways, while the 2D ultrathin carbon nanosheets reinforced the structural stability of the composite and provided a high electron transformation rate. As a result, the TiN/C composite anode can deliver a high reversible capacity of 170 mA h g^-1 and 149 mA h g^-1 after 5000 cycles at a current density of 0.5 A g^-1 and 1 A g^-1, indicating excellent electrochemical properties. This work provides new opportunities to explore the convenient and controllable preparation of metal nitride anodes for other energy conversion and storage applications.

Catalytic radiosensitization: Insights from materials physicochemistry

Radiotherapy is indispensable in clinical cancer treatment, but because both tumor and normal tissues have similar sensitivity to X-rays, their clinical curative effect is intrinsically limited. Advanced nanomaterials and nanotechnologies have been developed for radiotherapy sensitization, typically employing high atomic number (high-Z) materials to enhance the energy deposition of X-rays in tumor tissues, but the efficiency is largely limited by the toxicity of heavy metals. A new and promising approach for radiosensitization is catalytic radiosensitization, which takes advantage of the catalytic activity of nanomaterials triggered by radiation. The efficiency of catalytic radiosensitization can be greatly enhanced by electron modulation and energy conversion of nanocatalysts upon X-ray irradiation, further enhancing the clinical curative effect. In this review, we highlight the challenges and opportunities in cancer radiosensitization, discuss novel approaches to catalytic radiosensitization, and finally describe the development of catalytic radiosensitization based on an in-depth understanding of radio-nano interactions and catalysis-biological interactions.

Molecular Design of Two-Dimensional Covalent Heptazine Frameworks for Photocatalytic Overall Water Splitting under Visible Light

Photocatalytic water splitting sustainably offers clean hydrogen energy, but it is challenging to produce low-cost photocatalysts that split water stoichiometrically into H₂ and O₂ without sacrificial agents under visible light. Here, we designed 17 two-dimensional (2D) covalent heptazine frameworks (CHFs) by topologically assembling heptazine and benzene-containing molecular units that provide active sites for hydrogen and oxygen evolution reactions, respectively. Among them, 12 CHFs have band gap values of <3.0 eV with band margins straddling the chemical reaction potential of H₂/H⁺ and O₂/H₂O. In particular, a 2D H@DBTD CHF based on heptazine and 4,7-diphenyl-2,1,3-benzothiadiazole is a potential photocatalyst with a band gap of 2.47 eV for overall water splitting, which was confirmed with the calculated Gibbs free energy, non-adiabatic molecular dynamics, and preliminary experiment. This study presents an experimentally feasible molecular design of 2D CHFs as metal-free photocatalysts for overall water splitting under visible light.

Crystallographic Effects of GaN Nanostructures in Photoelectrochemical Reaction

Tuning the surface structure of the photoelectrode provides one of the most effective ways to address the critical challenges in artificial photosynthesis, such as efficiency, stability, and product selectivity, for which gallium nitride (GaN) nanowires have shown great promise. In the GaN wurtzite crystal structure, polar, semipolar, and nonpolar planes coexist and exhibit very different structural, electronic, and chemical properties. Here, through a comprehensive study of the photoelectrochemical performance of GaN photocathodes in the form of films and nanowires with controlled surface polarities we show that significant photoelectrochemical activity can be observed when the nonpolar surfaces are exposed in the electrolyte, whereas little or no activity is measured from the GaN polar c-plane surfaces. The atomic origin of this fundamental difference is further revealed through density functional theory calculations. This study provides guideline on crystal facet engineering of metal-nitride photo(electro)catalysts for a broad range of artificial photosynthesis chemical reactions.

Recent progress of heterostructures based on two dimensional materials and wide bandgap semiconductors

Recent progress in the synthesis and assembly of two-dimensional (2D) materials has laid the foundation for various applications of atomically thin layer films. These 2D materials possess rich and diverse properties such as layer-dependent band gaps, interesting spin degrees of freedom, and variable crystal structures. They exhibit broad application prospects in micro-nano devices. In the meantime, the wide bandgap semiconductors (WBS) with an elevated breakdown voltage, high mobility, and high thermal conductivity have shown important applications in high-frequency microwave devices, high-temperature and high-power electronic devices. Beyond the study on single 2D materials or WBS materials, the multi-functional 2D/WBS heterostructures can promote the carrier transport at the interface, potentially providing novel physical phenomena and applications, and improving the performance of electronic and optoelectronic devices. In this review, we overview the advantages of the heterostructures of 2D materials and WBS materials, and introduce the construction methods of 2D/WBS heterostructures. Then, we present the diversity and recent progress in the applications of 2D/WBS heterostructures, including photodetectors, photocatalysis, sensors, and energy related devices. Finally, we put forward the current challenges of 2D/WBS heterostructures and propose the promising research directions in the future.

Amorphous nickel tungstate films prepared by SILAR method for electrocatalytic oxygen evolution reaction

Development of electrocatalyst using facile way from non-noble metal compounds with high efficiency for effective water electrolysis is highly demanding for production of hydrogen energy. Nickel based electrocatalysts were currently developed for electrochemical water oxidation in alkaline pH. Herein, amorphous nickel tungstate (NiWO₄) was synthesized using the facile successive ionic layer adsorption and reaction method. The films were characterized by X-ray diffraction, Raman spectroscopy, Fourier transfer infrared spectroscopy, scanning electron microscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy techniques. The electrochemical analysis showed 315 mV of overpotential at 100 mA cm^-2 with lowest Tafel slope of 32 mV dec^-1 for oxygen evolution reaction (OER) making films of NiWO₄ compatible towards electrocatalysis of water in alkaline media. The chronopotentiometry measurements at 100 mA cm^-2 over 24 h showed 97% retention of OER activity. The electrochemical active surface area (ECSA) of NW120 film was 25.5 cm^-2.

CuS-Decorated GaN Nanowires on Silicon Photocathodes for Converting CO2 Mixture Gas to HCOOH

Hybrid materials consisting of semiconductors and cocatalysts have been widely used for photoelectrochemical (PEC) conversion of CO₂ gas to value-added chemicals such as formic acid (HCOOH). To date, however, the rational design of catalytic architecture enabling the reduction of real CO₂ gas to chemical has remained a grand challenge. Here, we report a unique photocathode consisting of CuS-decorated GaN nanowires (NWs) integrated on planar silicon (Si) for the conversion of H₂S-containing CO₂ mixture gas to HCOOH. It was discovered that H₂S impurity in the modeled industrial CO₂ gas could lead to the spontaneous transformation of Cu to CuS NPs, which resulted in significantly increased faradaic efficiency of HCOOH generation. The CuS/GaN/Si photocathode exhibited superior faradaic efficiency of HCOOH = 70.2% and partial current density = 7.07 mA/cm² at -1.0 V_RHE under AM1.5G 1 sun illumination. To our knowledge, this is the first demonstration that impurity mixed in the CO₂ gas can enhance, rather than degrade, the performance of the PEC CO₂ reduction reaction.

Ambient-Stable Black Phosphorus-Based 2D/2D S-Scheme Heterojunction for Efficient Photocatalytic CO2 Reduction to Syngas

Black phosphorus (BP), an emerging remarkable photocatalytic semiconductor, is arousing strong interests in this field of solar-driven CO₂ reduction, but its stability and activity are still facing huge challenges. Here, an ambient-stable and effective 2D/2D heterostructure of BP/bismuth tungstate (Bi₂WO₆) with oxygen vacancy is innovatively designed for syngas production via photocatalytic CO₂ reduction. This work, not only resolves the stability problem of BP nanosheets by anchoring ultrasmall platinum (Pt) nanoparticles (∼2 nm) but also greatly improves the charge transfer efficiency by constructing S-scheme 2D/2D heterostructure with coupled oxygen defects. As a result, the generation rates of carbon monoxide (CO) and hydrogen (H₂) remarkably reach 20.5 and 16.8 μmol g^-1 h^-1, respectively, which are much higher than that of reported BP-based materials, and the accomplished CO/H₂ ratios (1:1-2:1) are exactly the most desirable syngas for industrial applications. Thus, this work constructs an efficient and ambient-stable BP-based photocatalyst for syngas production by CO₂ reduction at mild conditions.

Piezocatalytic Effect Induced Hydrogen Production from Water over Non-noble Metal Ni Deposited Ultralong GaN Nanowires

Piezoelectric material-based catalysis that relies on an external stress-induced piezopotential has been demonstrated to be an effective strategy toward various chemical reactions. In this work, non-noble metal Ni-decorated ultralong monocrystal GaN nanowires (NWs) were prepared through a chemical vapor deposition (CVD) technique, followed by a photodeposition method. The piezocatalytic activity of the GaN NWs was enhanced by ∼9 times after depositing the Ni cocatalyst, generating hydrogen gas of ∼88.3 μmol·g^-1·h^-1 under ultrasonic vibration (110 W and 40 kHz), which is comparable to that of Pt-loaded GaN NWs. Moreover, Ni/GaN NWs with smaller diameters (∼100 nm) demonstrated superior piezocatalytic efficiency, which can be attributed to the large piezoelectric potential evidenced by both finite-element analysis and piezoresponse force microscopy measurements. These results demonstrate the promising application potential of non-noble metal loaded GaN nanostructures in hydrogen generation driven by weak mechanical energy from the surrounding environment.

Nanoscale redox mapping at the MoS2-liquid interface

Layered MoS₂ is considered as one of the most promising two-dimensional photocatalytic materials for hydrogen evolution and water splitting; however, the electronic structure at the MoS₂-liquid interface is so far insufficiently resolved. Measuring and understanding the band offset at the surfaces of MoS₂ are crucial for understanding catalytic reactions and to achieve further improvements in performance. Herein, the heterogeneous charge transfer behavior of MoS₂ flakes of various layer numbers and sizes is addressed with high spatial resolution in organic solutions using the ferrocene/ferrocenium (Fc/Fc⁺) redox pair as a probe in near-field scanning electrochemical microscopy, i.e. in close nm probe-sample proximity. Redox mapping reveals an area and layer dependent reactivity for MoS₂ with a detailed insight into the local processes as band offset and confinement of the faradaic current obtained. In combination with additional characterization methods, we deduce a band alignment occurring at the liquid-solid interface.

Here, high-resolution atomic force microscopy and scanning electrochemical microscopy are used to investigate the electron transfer behaviour of layered MoS₂ flakes in organic solutions, offering insights on the electronic band alignment at the solid-liquid interface.

Trace Amounts of Co3O4 Nano-Particles Modified TiO2 Nanorod Arrays for Boosted Photoelectrocatalytic Removal of Organic Pollutants in Water

Trace amounts of Co₃O₄ modified TiO₂ nanorod arrays were successfully fabricated through the photochemical deposition method without adding any nocuous reagents. The Co₃O₄/TiO₂ nanorod arrays fabricated in acid solution had the highest photo-electrochemical activity. We elaborated on the mechanism of Co₃O₄-TiO₂ fabricated in different pH value solutions. The Co₃O₄-TiO₂ had a more remarkable photo-electrochemical performance than the pure TiO₂ nanorod arrays owing to the heterojunction between Co₃O₄ and TiO₂. The degradation of methylene blue and hydroquinone was selected as the model reactions to evaluate the photo-electrochemical performance of Co₃O₄-TiO₂ nanorod arrays. The Co₃O₄/TiO₂ nanorod arrays had great potential in waste water treatment.

Nanocarbon-Enhanced 2D Photoelectrodes: A New Paradigm in Photoelectrochemical Water Splitting

Solar-driven photoelectrochemical (PEC) water splitting systems are highly promising for converting solar energy into clean and sustainable chemical energy. In such PEC systems, an integrated photoelectrode incorporates a light harvester for absorbing solar energy, an interlayer for transporting photogenerated charge carriers, and a co-catalyst for triggering redox reactions. Thus, understanding the correlations between the intrinsic structural properties and functions of the photoelectrodes is crucial. Here we critically examine various 2D layered photoanodes/photocathodes, including graphitic carbon nitrides, transition metal dichalcogenides, layered double hydroxides, layered bismuth oxyhalide nanosheets, and MXenes, combined with advanced nanocarbons (carbon dots, carbon nanotubes, graphene, and graphdiyne) as co-catalysts to assemble integrated photoelectrodes for oxygen evolution/hydrogen evolution reactions. The fundamental principles of PEC water splitting and physicochemical properties of photoelectrodes and the associated catalytic reactions are analyzed. Elaborate strategies for the assembly of 2D photoelectrodes with nanocarbons to enhance the PEC performances are introduced. The mechanisms of interplay of 2D photoelectrodes and nanocarbon co-catalysts are further discussed. The challenges and opportunities in the field are identified to guide future research for maximizing the conversion efficiency of PEC water splitting.

GaN nanowires grown by halide chemical vapour deposition as photoanodes for photo-electrochemical water oxidation reactions

Manifold morphologies of GaN nanowires (NWs) were fabricated using halide chemical vapour deposition (HCVD) on an n-Si (111) substrate and demonstrated to be a promising photoelectrode for photo-electrochemical (PEC) water splitting applications. We report a substantial enhancement in the photocurrent for vertically-grown GaN NWs on a buffer layer as compared to other counterparts such as GaN whiskers, tapered nanostructures and thin films. GaN NWs grown on Si have advantages due to the absorption of photons in a wide spectral range from ultraviolet to infrared and thus are directly involved in PEC reactions. A GaN NW photoanode was demonstrated with a saturation photocurrent density of 0.55 mA cm-2 under 1 sun of illumination, which is much greater than its counterparts. The role of the buffer layer and the carrier density on the PEC performance of vertically-grown GaN NW photoanodes is further elucidated. Photo-electrochemical impedance spectroscopy and Mott-Schottky characterizations were employed to further explain the PEC performance of GaN NW embedded photoanodes. Here, photoanodes based on diverse GaN nanostructures were examined for a better PEC evaluation in order to support the conclusion. The results may pave the way for the fabrication of efficient photoelectrodes and GaN as a protective layer against corrosion for improved photo-stability in an NaOH electrolyte for enhancing the efficiency of water splitting.

GaN nanowires as a reusable photoredox catalyst for radical coupling of carbonyl under blacklight irradiation

Employing photo-energy to drive the desired chemical transformation has been a long pursued subject. The development of homogeneous photoredox catalysts in radical coupling reactions has been truly phenomenal, however, with apparent disadvantages such as the difficulty in separating the catalyst and the frequent requirement of scarce noble metals. We therefore envisioned the use of a hyper-stable III-V photosensitizing semiconductor with a tunable Fermi level and energy band as a readily isolable and recyclable heterogeneous photoredox catalyst for radical coupling reactions. Using the carbonyl coupling reaction as a proof-of-concept, herein, we report a photo-pinacol coupling reaction catalyzed by GaN nanowires under ambient light at room temperature with methanol as a solvent and sacrificial reagent. By simply tuning the dopant, the GaN nanowire shows significantly enhanced electronic properties. The catalyst showed excellent stability, reusability and functional tolerance. All reactions could be accomplished with a single piece of nanowire on Si-wafer.

Recent advances in engineering active sites for photocatalytic CO2 reduction

The photocatalytic conversion of green-house gas CO₂ into high value-added carbonaceous fuels and chemicals through harvesting solar energy is a great promising strategy for simultaneously tackling global environmental issues and the energy crisis. Considering the vital role of active sites in determining the activity and selectivity in photocatalytic CO₂ reduction reactions, great efforts have been directed toward engineering active sites for fabricating efficient photocatalysts. This review highlights recent advances in the strategies for engineering active sites on surfaces and in open frameworks toward photocatalytic CO₂ reduction, referring to surface vacancies, doped heteroatoms, functional groups, loaded metal nanoparticles, crystal facets, heterogeneous/homogeneous single-site catalysts and metal nodes/organic linkers in metal organic frameworks.

Directional Change of Interfacial Electric Field by Carbon Insertion in Heterojunction System TiO2/WO3

Z-scheme transfer is an ideal photocatalytic system with stronger redox ability, but its design and construction still lack understanding. Herein, the work function difference and the band bending are found to be the determining factors for the construction of the Z-scheme transfer mechanism of photoexcited charges in TiO₂/WO₃. The control of work function and band bending achieved by carbon insertion results from the hybridization of orbitals and redistribution of electron density, as demonstrated by ultraviolet photoelectron spectroscopy and photocatalytic analysis. The heterojunction system, TiO₂/WO₃, with controlled work function and band bending, shows 2 times faster ^•OH radical formation rate (0.011 μmol min^-1) compared to the undisturbed system. First-principles calculation reveals that the changes in work function and band bending result in an interfacial electric field, which shifts the charge transfer mechanism from type II to Z-scheme. This work proves that the design of work function and band bending allows reconstructing charge transfer mechanism by forming the interfacial electric field in heterojunction systems.

Simultaneous detection of trace Ag(I) and Cu(II) ions using homoepitaxially grown GaN micropillar electrode

Driven by the motivation to quantitively control and monitor trace metal ions in water, the development of environmental-friendly electrodes with superior detection sensitivity is extremely important. In this work, we report the design of a stable, ultrasensitive and biocompatible electrode for the detection of trace Ag⁺ and Cu²⁺ ions by growing n-type GaN micropillars on conductive p-type GaN substrate. The electrochemical measurement based on cyclic voltammetry indicates that the GaN micropillars exhibit quasi-reversible and mass-controlled reaction in redox probe solution. In the application of trace Ag⁺ and Cu²⁺ determination, the GaN micropillars show superior sensitivity and excellent conductivity by presenting a detection limit of 3.3 ppb for Ag⁺ and 3.3 ppb for Cu²⁺. Comparative studies on the electrochemical response of GaN micropillars and GaN film in the simultaneous Ag⁺ and Cu²⁺ detection reveal that GaN micropillars show three orders of magnitude higher stripping peak current than GaN film. It is assumed that the microarray morphology with large active area and the hydrophilia nature of GaN micropillars are responsible for the excellent sensitivity. This work will open up some opportunities for GaN nanostructure electrodes in the application of trace metal ions detection.

Catalytic Growth of Gallium Nitride Nanowires on Wet Chemically Etched Substrates by Chemical Vapor Deposition

Growth of gallium nitride nanowires on etched sapphire and GaN substrates using binary catalytic alloy were investigated by manipulating the growth time and precursor-to-substrate distance. The variations in behavior at different growth conditions were observed using X-ray diffractometer, Raman spectroscopy, X-ray photoelectron spectroscopy, cathodoluminescence spectroscopy, optical microscopy, atomic force microscopy, and scanning electron microscopy. It was noticed that, in respect of both the substrates, when growth time and/or precursor-to-substrate distance is increased, thickness of the nanowires around the etch pits remains unaltered, but there is variation in the density of nanowires. In addition, formation of gallium nitride microwires within the etch pits was also observed on etched sapphire substrates. Similarly, the thickness and density of the microwires were found to increase with increase in growth time and decrease with increase in precursor-to-substrate distance. The dimensionality scaling of gallium nitride was found to have a positive effect in improving the luminescence property and band gap of the grown nanowires. This method of nanowire growth can be helpful in increasing the probability of multiple reflections in the materials which makes them a suitable candidate for optoelectronic devices.

Remarkable Visible-Light Photocatalytic Activity Enhancement over Au/p-type TiO2 Promoted by Efficient Interfacial Charge Transfer

Metal-induced photocatalysis has emerged as a promising approach for exploiting visible-light-responsive composite materials for solar energy conversion, which is generally hindered by low photocatalytic efficiency. Herein, for the first time, an Au/p-TiO₂ (p-type TiO₂) strategy with the hole transfer mechanism is developed, remarkably promoting visible-light photocatalytic performance. An efficient acetone evolution rate (138 μmol·g^-1·h^-1) in the photocatalytic isopropyl alcohol (IPA) degradation under λ_ex = 500 nm light (light intensity, 5.5 mW/cm²) was achieved over Au/p-TiO₂, which is approximately 5 times as high as that over Au/n-TiO₂ under the same conditions. Photoluminescence and electrochemical impedance spectroscopy measurements indicate enhanced charge carrier separation and transfer for Au/p-TiO₂. In an elaborate study, apparent quantum efficiency and transmission electron microscopy characterization on selective PbO₂ deposition over p-TiO₂ revealed that visible-light-excited holes other than electrons generated in the Au interband transition transferred to p-TiO₂, which is opposite to the general route in Au/n-TiO₂ (n-type TiO₂). Energetic holes generated in the d band of Au led to a fluent transfer across the Schottky barrier, which is further confirmed by the IPA photodegradation mechanism study with different scavengers over Au/p-TiO₂. This discovery opens up new opportunities in designing and developing efficient metal semiconductor composite materials with visible-light response.

Photo-induced selective etching of GaN nanowires in water

Nanowire (NW) based devices for solar driven artificial photosynthesis have gained increasing interest in recent years due to the intrinsically high surface to volume ratio and the excellent achievable crystal qualities. However, catalytically active surfaces often suffer from insufficient stability under operational conditions. To gain a fundamental understanding of the underlying processes, the photochemical etching behavior of hexagonal and round GaN NWs in deionized water under illumination are investigated. We find that the crystallographic c-plane remains stable, whereas the m-planes are photochemically etched with rates up to 11 nm min-1, depending on the applied UV light intensity. By investigating nanowalls, we achieve control of the exposed crystallographic facets and find an enhanced stability of the a-plane compared to the m-plane. Photo-excited holes, which drift to the side facets due to the upward surface band bending in nominally n-type (not intentionally doped) GaN, are identified as the driving force of the process, which allows the development of concepts for the stabilization of the nanostructures. A geometrically enhanced absorption of periodic NW arrays is correlated with a dependence of the etch rate on the NW pitch and diameter. Further, we find selective photochemical etching of the NW base in the presence of sub-band gap illumination, which is attributed to defect-related absorption in this region. These results provide improved understanding of the roles of inhomogeneous defect distribution, light excitation profiles, and different surface facets on the photochemical stability of nanostructures and provide viable strategies for improving stabilities under light-driven reaction conditions.

Enhanced luminescence from InGaN/GaN nano-disk in a wire array caused by surface potential modulation during wet treatment

Here we have demonstrated the profound impact of surface potential on the luminescence of an array of InGaN/GaN nano-disk in a wire heterostructure. The change in surface potential is brought about by a combination of dry and successive wet-processing treatments. The photoluminescence (PL) properties are determined as a function of size and height of this array of nano-disks. The observed characteristics are coherently explained by considering a change in quantum confinement induced by the change in surface potential, quantum-confined Stark effect, exciton binding energy and strain relaxation for varying surface potential. The change in hole bound state energy due to parabolic potential well near the side-wall is found to be the dominating factor. The PL peak position, full width at half-maximum, strain relaxation and integrated PL intensity are studied as a function of incident power and temperature. The devices demonstrate higher integrated PL intensity and slope efficiency.

Improved solar hydrogen production by engineered doping of InGaN/GaN axial heterojunctions

InGaN-based nanowires (NWs) have been investigated as efficient photoelectrochemical (PEC) water splitting devices. In this work, the InGaN/GaN NWs were grown by molecular beam epitaxy (MBE) having InGaN segments on top of GaN seeds. Three axial heterojunction structures were constructed with different doping types and levels, namely n-InGaN/n-GaN NWs, undoped (u)-InGaN/p-GaN NWs, and p-InGaN/p-GaN NWs. With the carrier concentrations estimated by Mott-Schottky measurements, a PC1D simulation further confirmed the band structures of the three heterojunctions. The u-InGaN/p-GaN and p-InGaN/p-GaN NWs exhibited optimized stability in pH 0 electrolytes for over 10 h with a photocurrent density of about -4.0 and -9.4 mA/cm², respectively. However, the hydrogen and oxygen evolution rates of the Pt-treated u-InGaN/p-GaN NWs exhibited a less favorable stoichiometric ratio. On the other hand, the Pt-decorated p-InGaN/p-GaN NWs showed the best PEC performance, generating approximately 1000 µmol/cm² hydrogen and 550 µmol/cm² oxygen in 10 h. The band-engineered p-InGaN/p-GaN axial NWs-heterojunction demonstrated a great potential for highly efficient and durable photocathodes.

On the interpretation of cathodoluminescence intensity maps of wide band gap nanowires

It is commonly assumed in the spatially resolved cathodoluminescence (CL) studies of wide band gap (WBG) nanowires (NWs) that the CL intensity maps of deep-level (DL) and near band edge (NBE) emission reflect the spatial distribution of defects in these structures. On this basis, crucial conclusions about the technological growth conditions of NWs are drawn. However, here we showed using three-dimensional finite element analysis that in the case of WBG NWs, which exhibit surface band bending, the CL intensity maps of DL and NBE emission do not reflect the distribution of defects but, instead, the electric field strength in NWs. In particular, we found that independently of the defect concentration distribution, the DL emission intensity is always the highest in the areas where the electric field is the strongest and the lowest where the electric field is absent, while the NBE emission intensity exhibits the opposite trends. We explained this finding by the strong influence of the electric field on the spatial distribution of radiative recombination rates. Overall, our results indicate that (i) the frequently observed spatially inhomogeneous CL intensity distribution in WBG NWs can result from the presence of electric fields but not, as widely accepted, non-uniform defect distribution and (ii) spatially resolved CL spectroscopy measurements on the WBG NWs in most cases can not provide any quantitative information about the DL defect distribution.

Reactive Inorganic Vapor Deposition of Perovskite Oxynitride Films for Solar Energy Conversion

The synthesis of perovskite oxynitrides, which are promising photoanode candidates for solar energy conversion, is normally accomplished by high-temperature ammonolysis of oxides and carbonate precursors, thus making the deposition of their planar films onto conductive substrates challenging. Here, we proposed a facile strategy to prepare a series of perovskite oxynitride films. Taking SrTaO₂N as a prototype, we prepared SrTaO₂N films on Ta foils under NH₃ flow by utilizing the vaporized SrCl₂/SrCO₃ eutectic salt. The SrTaO₂N films exhibit solar water-splitting photocurrents of 3.0 mA cm^-2 at 1.23 V vs. RHE (reversible hydrogen electrode), which increases by 270% compared to the highest photocurrent (1.1 mA cm^-2 at 1.23 V vs. RHE) of SrTaO₂N reported in the literature. This strategy may also be applied to directly prepare a series of perovskite oxynitride films on conductive substrates such as ATaO₂N (A = Ca, Ba) and ANbO₂N (A = Sr, Ba).

Photodeposition of a conformal metal oxide nanocoating

A variety of conformal metal oxide nanocoatings including Cr₂O₃, Al₂O₃, ZnO, and In₂O₃ can be accessed via simple photodeposition.

High Efficiency Si Photocathode Protected by Multifunctional GaN Nanostructures

Photoelectrochemical water splitting is a clean and environmentally friendly method for solar hydrogen generation. Its practical application, however, has been limited by the poor stability of semiconductor photoelectrodes. In this work, we demonstrate the use of GaN nanostructures as a multifunctional protection layer for an otherwise unstable, low-performance photocathode. The direct integration of GaN nanostructures on n⁺-p Si wafer not only protects Si surface from corrosion but also significantly reduces the charge carrier transfer resistance at the semiconductor/liquid junction, leading to long-term stability (>100 h) at a large current density (>35 mA/cm²) under 1 sun illumination. The measured applied bias photon-to-current efficiency of 10.5% is among the highest values ever reported for a Si photocathode. Given that both Si and GaN are already widely produced in industry, our studies offer a viable path for achieving high-efficiency and highly stable semiconductor photoelectrodes for solar water splitting with proven manufacturability and scalability.

Stannous oxide promoted charge separation in rationally designed heterojunction photocatalysts with a controllable mechanism

Due to the sluggish mobility of holes, the low charge-separation rate remains an intrinsic issue that limits further increase of the photocatalytic conversion efficiency. Herein, we proposed an in situ hydrothermal method to expedite the charge transfer with enhanced photocatalytic H2 evolution rate and photodegradation activities via introducing SnO microplates into TiO2. As compared to bare TiO2, the SnO/TiO2 heterojunction achieves remarkable 470% and 150% higher efficiency for the photocatalytic H2 evolution rate and photodegradation of rhodamine B, respectively. In particular, it is demonstrated that the charge transfer mechanism of SnO/TiO2 can be switched from the Z-scheme to type II by Pt loading, leading to a significant enhancement of photocatalytic performances. Furthermore, the photocatalytic H2 evolution activities of ZnO and C3N4 can also be improved by introducing SnO via simple mechanical mixing. This work provides not only a new versatile stimulant for enhancing photocatalytic activities but also in-depth understanding of the charge transfer mechanism of heterointerfaces of semiconductors.