Surface Passivation of GaN Nanowires for Enhanced Photoelectrochemical Water-Splitting

High-Speed and High-Responsivity Blue Light Photodetector with an InGaN NR/PEDOT:PSS Heterojunction Decorated with Ag NWs

InGaN nanorods possessing larger and wavelength selective absorption by regulating In component based visible light photodetectors (PDs) as one of the key components in the field of visible light communication have received widespread attention. Currently, the weak photoelectric conversion efficiency and slow photoresponse speed of InGaN nanorod (NR) based PDs due to high surface states of InGaN NRs impede the actualization of high-responsivity and high-speed blue light PDs. Here, we have demonstrated high-performance InGaN NR/PEDOT:PSS@Ag nanowire (NW) heterojunction blue light photodetectors utilizing surface passivation and a localized surface plasmon resonance effect. The dark current is significantly reduced by passivating the InGaN NR surface states using PEDOT:PSS. The photoelectric conversion efficiency is significantly increased by increasing light absorption due to the electromagnetic field oscillation of Ag NWs. The responsivity, external quantum efficiency, detectivity, and fall/off time of the InGaN NR/PEDOT:PSS@Ag NW PDs are up to 2.9 A/W, 856%, 6.64 × 1010 Jones, and 439/725 μs, respectively, under 1 V bias and 420 nm illumination. The proposed device design presents a novel approach toward the development of low-cost, high-responsivity, high-speed blue light photodetectors for applications involving visible light communication.

Alternating 3rd- to 2nd-Order Charge Reaction Kinetics on Bismuth Vanadate Photoanodes with Ultrathin Bismuth Metal-Organic-Frameworks

The most challenging obstacle for photocatalysts to efficiently harvest solar energy is the sluggish surface redox reaction (e. g., oxygen evolution reaction, OER) kinetics, which is believed to originate from interface catalysis rather than the semiconductor photophysics. In this work, we developed a light-modulated transient photocurrent (LMTPC) method for investigating surface charge accumulation and reaction on the W-doped bismuth vanadate (W : BiVO4) photoanodes during photoelectrochemical water oxidation. Under illuminating conditions, the steady photocurrent corresponds to the charge transfer rate/kinetics, while the integration of photocurrent (I~t) spikes during the dark period is regarded as the charge density under illumination. Quantitative analysis of the surface hole densities and photocurrents at 0.6 V vs. reversible hydrogen electrode results in an interesting rate-law kinetics switch: a 3rd-order charge reaction behavior appeared on W : BiVO4, but a 2nd-order charge reaction occurred on W : BiVO4 surface modified with ultrathin Bi metal-organic-framework (Bi-MOF). Consequently, the photocurrent for water oxidation on W : BiVO4/Bi-MOF displayed a 50 % increment. The reaction kinetics alternation with new interface reconstruction is proposed for new mechanism understanding and/or high-performance photocatalytic applications.

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.

Advances and challenges in the modification of photoelectrode materials for photoelectrocatalytic water splitting

With the increasing consumption of fossil fuels, the development of clean and renewable alternative fuels has become a top priority. Hydrogen (H2) is an ideal primary clean energy source for its extremely high gravimetric energy density, carbon-free combustion, and abundant natural resources. Photoelectrocatalytic (PEC) water splitting is among the most promising approaches for converting sunlight and water into H2. However, the cost-effectiveness and the overall solar to hydrogen conversion efficiency of PEC water splitting are still big challenges. In the past few decades, several studies have been devoted to this technology, and it is essential to develop economical photoelectrocatalysts with high conversion efficiency. Therefore, there is an urgent need for a comprehensive and updated review of recent advances in the design, manufacture, and modification of efficient PEC water splitting systems. This review first starts with the basic mechanism of photoelectrochemical water splitting. Then the problems in PEC water splitting are discussed, and the methods of photoelectrode modulation such as nanostructure fabrication, doping engineering, surface modification, and heterojunction construction are introduced. Finally, the critical challenges and future trends/perspectives in the PEC water splitting are discussed.

Anodic Reconstructed p+-GaAs/a-InAsN for Stable and Efficient Photoelectrochemical Hydrogen Evolution

The extremely poor solution stability and massive carrier recombination have seriously prevented III-V semiconductor nanomaterials from efficient and stable hydrogen production. In this work, an anodic reconstruction strategy based on group III-V active semiconductors is proposed for the first time, resulting in 19-times photo-gain. What matters most is that the device after anodic reconstruction shows very superior stability under the protracted photoelectrochemical (PEC) test over 8100 s, while the final photocurrent density does not decrease but rather increases by 63.15%. Using the experiment and DFT theoretical calculation, the anodic reconstruction mechanism is elucidated: through the oxidation of indium clusters and the migration of arsenic atoms, the reconstruction formed p+-GaAs/a-InAsN. The hole concentration of the former is increased by 10 times (5.64 × 1018 cm-1 increases up to 5.95 × 1019 cm-1) and the band gap of the latter one is reduced to a semi-metallic state, greatly strengthening the driving force of PEC water splitting. This work turns waste into treasure, transferring the solution instability into better efficiency.

Towards the quantification of the chemical mechanism of light-driven water splitting on GaN photoelectrodes

We present results from a study addressing the unbiased water-splitting process and its side reactions on GaN-based photoelectrodes decorated with NiOx, FeOx, and CoOx nanoparticles. Observations involving physicochemical analyses of liquid and vapour phases after the experiments were performed in 1 M NaOH under ambient conditions. A water-splitting process with GaN-based photoelectrodes results in the generation of hydrogen gas and hydrogen peroxide. Quantification of the water-splitting chemical mechanism gave numerical values indicating an increase in the device performance and restriction of the GaN electrocorrosion with surface modifications of GaN structures. The hydrogen generation efficiencies are ηH2(bare GaN) = 1.23%, ηH2(NiOx/GaN) = 4.31%, ηH2(FeOx/GaN) = 2.69%, and ηH2(CoOx/GaN) = 2.31%. The photoelectrode etching reaction moieties Qetch/Q are 11.5%. 0.21%, 0.26% and 0.20% for bare GaN, NiOx/GaN, FeOx/GaN, and CoOx/GaN, respectively.

Two-dimensional lateral anatase-rutile TiO2 phase junctions with oxygen vacancies for robust photoelectrochemical water splitting

Exploiting the photoelectrode materials with broad solar light response, high-efficient separation of photogenerated charges and abundant active sites is extremely vital yet enormously challenging. Herein, an innovative two-dimensional (2D) lateral anatase-rutile TiO₂ phase junctions with controllable oxygen vacancies perpendicularly aligned on Ti mesh is presented. Our experimental observations and theoretical calculations corroborate explicitly that the 2D lateral phase junctions together with three-dimensional arrays not only exhibit the high-efficient photogenerated charges separation guaranteed by the build-in electric field at the side-to-side interface, but also furnish enriching active sites. Moreover, the interfacial oxygen vacancies generate new defect energy levels and serve as electron donors, hence extending visible light response and further accelerating the separation and transfer of photogenerated charges. Profiting from these merits, the optimized photoelectrode yield a pronounced photocurrent density of 1.2 mA/cm² at 1.23 V vs. RHE with Faradic efficiency of 100%, which is approximately 2.4 times larger than that of pristine 2D TiO₂ nanosheets. Furthermore, the incident photon to current conversion efficiency (IPCE) of the optimized photoelectrode is also boosted within both ultraviolet and visible light regions. This research is envisioned deliver the new insight in developing the novel 2D lateral phase junctions for PEC applications.

Oxynitrides enabled photoelectrochemical water splitting with over 3,000 hrs stable operation in practical two-electrode configuration

Solar photoelectrochemical reactions have been considered one of the most promising paths for sustainable energy production. To date, however, there has been no demonstration of semiconductor photoelectrodes with long-term stable operation in a two-electrode configuration, which is required for any practical application. Herein, we demonstrate the stable operation of a photocathode comprising Si and GaN, the two most produced semiconductors in the world, for 3,000 hrs without any performance degradation in two-electrode configurations. Measurements in both three- and two-electrode configurations suggest that surfaces of the GaN nanowires on Si photocathode transform in situ into Ga-O-N that drastically enhances hydrogen evolution and remains stable for 3,000 hrs. First principles calculations further revealed that the in-situ Ga-O-N species exhibit atomic-scale surface metallization. This study overcomes the conventional dilemma between efficiency and stability imposed by extrinsic cocatalysts, offering a path for practical application of photoelectrochemical devices and systems for clean energy.

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.

Environmental sensitivity of GaN nanofins grown by selective area molecular beam epitaxy

Nanostructures exhibit a large surface-to-volume ratio, which makes them sensitive to their ambient conditions. In particular, GaN nanowires and nanofins react to their environment as adsorbates influence their (opto-) electronic properties. Charge transfer between the semiconductor surface and adsorbed species changes the surface band bending of the nanostructures, and the adsorbates can alter the rate of non-radiative recombination in GaN. Despite the importance of these interactions with the ambient environment, the detailed adsorption mechanisms are still not fully understood. In this article, we present a systematic study concerning the environmental sensitivity of the electrical conductivity of GaN nanofins. We identify oxygen- and water-based adsorbates to be responsible for a quenching of the electrical current through GaN nanofins due to an increased surface band bending. Complementary contact potential difference measurements in controlled atmospheres on bulkm- andc-plane GaN reveal additional complexity with regard to water adsorption, for which surface dipoles might play an important role besides an increased surface depletion width. The sensitive reaction of the electrical parameters to the environment and surface condition underlines the necessity of a reproducible pre-treatment and/or surface passivation. The presented results help to further understand the complex adsorption mechanisms at GaN surfaces. Due to the sensitivity of the nanofin conductivity on the environment, such structures could perform well as sensing devices.

Nanowire photochemical diodes for artificial photosynthesis

Artificial photosynthesis can provide a solution to our current energy needs by converting small molecules such as water or carbon dioxide into useful fuels. This can be accomplished using photochemical diodes, which interface two complementary light absorbers with suitable electrocatalysts. Nanowire semiconductors provide unique advantages in terms of light absorption and catalytic activity, yet great control is required to integrate them for overall fuel production. In this review, we journey across the progress in nanowire photoelectrochemistry (PEC) over the past two decades, revealing design principles to build these nanowire photochemical diodes. To this end, we discuss the latest progress in terms of nanowire photoelectrodes, focusing on the interplay between performance, photovoltage, electronic band structure, and catalysis. Emphasis is placed on the overall system integration and semiconductor-catalyst interface, which applies to inorganic, organic, or biologic catalysts. Last, we highlight further directions that may improve the scope of nanowire PEC systems.

van der Waals integration of mixed-dimensional CeO2@Bi heterostructure for high-performance self-powered photodetector with fast response speed

Heterostructures have been extensively investigated for optoelectronic devices owing to their fantastic physicochemical properties. Herein, a mixed-dimensional van der Waals heterostructure (vdWH) CeO₂@Bi, 1D ceria (CeO₂) loaded with 0D bismuth quantum dots (Bi QDs), is synthesized through a facile hydrothermal bottom-up method. It is found that the fabricated CeO₂@Bi-based photoelectrochemical (PEC)-type photodetector (PD) shows self-powered photodetection capability with a fast photoresponse speed of 0.02 s. Besides, a photocurrent of 2.00 μA cm^-2 and a photoresponsivity of 888.89 μA W^-1 under 365 nm illumination are obtained. Furthermore, good long-term cycle stability is also observed after 1 month in a harsh environment, indicating the great potential for practical applications. These results are further supported by density functional theory (DFT) calculations. We believe that the presented work is expected to provide a new pathway for the future utilization of vdWHs for high-performance optoelectronics.

Tailoring of the photocatalytic activity of CeO2 nanoparticles by the presence of plasmonic Ag nanoparticles

The present study investigates basic features of a photoelectrochemical system based on CeO₂ nanoparticles fixed on gold electrodes. Since photocurrent generation is limited to the absorption range of the CeO₂ in the UV range, the combination with metal nanoparticles has been studied. It can be shown that the combination of silver nanoparticles with the CeO₂ can shift the excitation range into the visible light wavelength range. Here a close contact between both components has been found to be essential and thus, hybrid CeO₂@Ag nanoparticles have been prepared and analyzed. We have collected arguments that electron transfer occurs between both compositional elements of the hybrid nanoparticles.The photocurrent generation can be rationalized on the basis of an energy diagram underlying the necessity of surface plasmon excitation in the metal nanoparticles, which is also supported by wavelength-dependent photocurrent measurements. However, electrochemical reactions seem to occur at the CeO₂ surface and consequently, the catalytic properties of this material can be exploited as exemplified with the photoelectrochemical reduction of hydrogen peroxide. It can be further demonstrated that the layer-by layer technique can be exploited to create a multilayer system on top of a gold electrode which allows the adjustment of the sensitivity of the photoelectrochemical system. Thus, with a 5-layer electrode with hybrid CeO₂@Ag nanoparticles submicromolar hydrogen peroxide concentrations can be detected.

Observation of polarity-switchable photoconductivity in III-nitride/MoSx core-shell nanowires

III-V semiconductor nanowires are indispensable building blocks for nanoscale electronic and optoelectronic devices. However, solely relying on their intrinsic physical and material properties sometimes limits device functionalities to meet the increasing demands in versatile and complex electronic world. By leveraging the distinctive nature of the one-dimensional geometry and large surface-to-volume ratio of the nanowires, new properties can be attained through monolithic integration of conventional nanowires with other easy-synthesized functional materials. Herein, we combine high-crystal-quality III-nitride nanowires with amorphous molybdenum sulfides (a-MoS_x) to construct III-nitride/a-MoS_x core-shell nanostructures. Upon light illumination, such nanostructures exhibit striking spectrally distinctive photodetection characteristic in photoelectrochemical environment, demonstrating a negative photoresponsivity of -100.42 mA W^-1 under 254 nm illumination, and a positive photoresponsivity of 29.5 mA W^-1 under 365 nm illumination. Density functional theory calculations reveal that the successful surface modification of the nanowires via a-MoS_x decoration accelerates the reaction process at the electrolyte/nanowire interface, leading to the generation of opposite photocurrent signals under different photon illumination. Most importantly, such polarity-switchable photoconductivity can be further tuned for multiple wavelength bands photodetection by simply adjusting the surrounding environment and/or tailoring the nanowire composition, showing great promise to build light-wavelength controllable sensing devices in the future.

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.

Homogeneous Photoelectrochemical Aptasensors for Tetracycline Based on Sulfur-Doped g-C3N4/n-GaN Heterostructures Formed through Self-Assembly

The complex synthesis of photoelectric materials and the difficulty of fixing the identification elements on the photoelectrode are long-standing problems in the field of photoelectrochemical (PEC) biosensing. In this work, a simple PEC aptasensor construction strategy based on a sulfur-doped g-C₃N₄ (SCN)/n-GaN heterostructure photoelectrode was proposed. The SCN/n-GaN heterostructure can be formed through self-assembly in solution since SCN can be uniformly dispersed in solution. In addition, as a dual-function mediate, an aptamer can be fixed on an SCN substrate automatically because of the good adsorption performance of SCN. Therefore, tedious steps of PEC electrode preparation and the fixing of recognition elements were both avoided. Compared with the traditional ones, the construction difficulty and time cost of the prepared PEC aptasensors are greatly reduced. The simplified experimental process improves the stability and reproducibility of the aptasensor. Finally, tetracycline (TET) was used as a model target to verify the sensing performance of the proposed PEC strategy. TET can consume the photogenerated holes of the SCN/n-GaN heterostructure, promote carrier migration, and result in the change in the photocurrent. The linear relationship between the change in the photocurrent intensity and the TET concentration can be used to detect TET. The aptasensor has a linear range of 0.10-10.0 nmol L^-1 and the detection limit is 0.030 nmol L^-1 (3S/N). The aptasensor was applied to the detection of TET in milk samples with satisfactory results.

One-Compartment InGaN Nanowire Fuel Cell in the Light and Dark Operating Modes

A one-compartment H₂O₂ photofuel cell (PFC) with a photoanode based on InGaN nanowires (NWs) is introduced for the first time. The electrocatalytic and photoelectrocatalytic properties of the InGaN NWs are studied in detail by cyclic voltammetry, current versus time measurements, photovoltage measurements, and electrochemical impedance spectroscopy. In parallel, IrO _x (OH) _y as the co-catalyst on the InGaN NWs is evaluated to boost the catalytic activity in the dark and light. For the PFC, Ag is the best as the cathode among Ag, Pt, and glassy carbon. The PFC operates in the dark as a conventional fuel cell (FC) and under illumination with 25% increased electrical power generation at room temperature. Such dual operation is unique, combining FC and PFC technologies for the most flexible use.

Selective area growth of GaN nanowires and nanofins by molecular beam epitaxy on heteroepitaxial diamond (001) substrates

GaN-on-diamond is a promising route towards reliable high-power transistor devices with outstanding performances due to better heat management, replacing common GaN-on-SiC technologies. Nevertheless, the implementation of GaN-on-diamond remains challenging. In this work, the selective area growth of GaN nanostructures on cost-efficient, large-scale available heteroepitaxial diamond (001) substrates by means of plasma-assisted molecular beam epitaxy is investigated. Additionally, we discuss the influence of an AlN buffer on the morphology of the GaN nanostructures. The nanowires and nanofins are characterized by a very high selectivity and controllable dimensions. Low temperature photoluminescence measurements are used to evaluate their structural quality. The growth of two GaN crystal domains, which are in-plane rotated against each other by 30°, is observed. The favoring of a certain domain is determined by the off-cut direction of the diamond substrates. By X-ray diffraction we show that the GaN nanostructures grow perpendicular to the diamond surface on off-cut diamond (001) substrates, which is in contrast to the growth on diamond (111), where the nanostructures are aligned with the substrate lattice. Polarity-selective wet chemical etching and Kelvin probe force microscopy reveal that the GaN nanostructures grow solely in the Ga-polar direction. This is a major advantage compared to the growth on diamond (111) and enables the application of GaN nanostructures on cost-efficient diamond for high-power/high-frequency applications.

Hot Electrons in TiO2-Noble Metal Nano-Heterojunctions: Fundamental Science and Applications in Photocatalysis

Plasmonic photocatalysis enables innovation by harnessing photonic energy across a broad swathe of the solar spectrum to drive chemical reactions. This review provides a comprehensive summary of the latest developments and issues for advanced research in plasmonic hot electron driven photocatalytic technologies focusing on TiO2-noble metal nanoparticle heterojunctions. In-depth discussions on fundamental hot electron phenomena in plasmonic photocatalysis is the focal point of this review. We summarize hot electron dynamics, elaborate on techniques to probe and measure said phenomena, and provide perspective on potential applications-photocatalytic degradation of organic pollutants, CO2 photoreduction, and photoelectrochemical water splitting-that benefit from this technology. A contentious and hitherto unexplained phenomenon is the wavelength dependence of plasmonic photocatalysis. Many published reports on noble metal-metal oxide nanostructures show action spectra where quantum yields closely follow the absorption corresponding to higher energy interband transitions, while an equal number also show quantum efficiencies that follow the optical response corresponding to the localized surface plasmon resonance (LSPR). We have provided a working hypothesis for the first time to reconcile these contradictory results and explain why photocatalytic action in certain plasmonic systems is mediated by interband transitions and in others by hot electrons produced by the decay of particle plasmons.

Coating of Phosphide Catalysts on p-Silicon by a Necking Strategy for Improved Photoelectrochemical Characteristics in Alkaline Media

The methodology of coating electrocatalysts on semiconductor substrates is critical for the catalytic performance of photoelectrochemical electrodes. A weakly bound coating leads to orders of magnitude lower efficiency and reliability compared to those required to meet the commercial demand. Herein, a facile strategy based on the hydrolysis of TiCl₄ is developed to solve the coating issue. Mesoporous tungsten phosphide (WP) particles were spin-coated and affixed onto TiO₂-protected planar p-Si by the formation of a TiO₂ necking layer between the catalyst particles and the substrates. Under 1 sun illumination, the as-prepared WP/TiO₂/Si photocathode yields a saturated current density of -35 mA cm^-2 and a durability of over 110 h with a current density over -15 mA cm^-2 at 0 V versus a reversible hydrogen electrode in a 1.0 M KOH solution, which is among the state-of-the-art performances of commercial planar Si-based photocathodes. The Kelvin probe force microscopy results suggest the successive transfer of photoelectrons from Si to TiO₂ and WP. The as-formed TiO₂ necking layer plays the key role in ensuring the surface catalytic activity and durability. This necking strategy is also applicable for coating other transition-metal phosphides, for example, MoP and FeP, thus offering a practical approach to meet the commercial requirement of low-cost, highly efficient, and durable photoelectrodes.

TiO2 as a Photocatalyst for Water Splitting-An Experimental and Theoretical Review

Hydrogen produced from water using photocatalysts driven by sunlight is a sustainable way to overcome the intermittency issues of solar power and provide a green alternative to fossil fuels. TiO₂ has been used as a photocatalyst since the 1970s due to its low cost, earth abundance, and stability. There has been a wide range of research activities in order to enhance the use of TiO₂ as a photocatalyst using dopants, modifying the surface, or depositing noble metals. However, the issues such as wide bandgap, high electron-hole recombination time, and a large overpotential for the hydrogen evolution reaction (HER) persist as a challenge. Here, we review state-of-the-art experimental and theoretical research on TiO₂ based photocatalysts and identify challenges that have to be focused on to drive the field further. We conclude with a discussion of four challenges for TiO₂ photocatalysts-non-standardized presentation of results, bandgap in the ultraviolet (UV) region, lack of collaboration between experimental and theoretical work, and lack of large/small scale production facilities. We also highlight the importance of combining computational modeling with experimental work to make further advances in this exciting field.

External Control of GaN Band Bending Using Phosphonate Self-Assembled Monolayers

We report on the optoelectronic properties of GaN(0001) and (11̅00) surfaces after their functionalization with phosphonic acid derivatives. To analyze the possible correlation between the acid's electronegativity and the GaN surface band bending, two types of phosphonic acids, n-octylphosphonic acid (OPA) and 1H,1H,2H,2H-perfluorooctanephosphonic acid (PFOPA), are grafted on oxidized GaN(0001) and GaN(11̅00) layers as well as on GaN nanowires. The resulting hybrid inorganic/organic heterostructures are investigated by X-ray photoemission and photoluminescence spectroscopy. The GaN work function is changed significantly by the grafting of phosphonic acids, evidencing the formation of dense self-assembled monolayers. Regardless of the GaN surface orientation, both types of phosphonic acids significantly impact the GaN surface band bending. A dependence on the acids' electronegativity is, however, only observed for the oxidized GaN(11̅00) surface, indicating a relatively low density of surface states and a favorable band alignment between the surface oxide and acids' electronic states. Regarding the optical properties, the covalent bonding of PFOPA and OPA on oxidized GaN layers and nanowires significantly affects their internal quantum efficiency, especially in the nanowire case due to the large surface-to-volume ratio. The variation in the internal quantum efficiency is related to the modification of both the internal electric fields and surface states. These results demonstrate the potential of phosphonate chemistry for the surface functionalization of GaN, which could be exploited for selective sensing applications.

Pt/AlGaN Nanoarchitecture: Toward High Responsivity, Self-Powered Ultraviolet-Sensitive Photodetection

Energy-saving photodetectors are the key components in future photonic systems. Particularly, self-powered photoelectrochemical-type photodetectors (PEC-PDs), which depart completely from the classical solid-state junction device, have lately intrigued intensive interest to meet next-generation power-independent and environment-sensitive photodetection. Herein, we construct, for the first time, solar-blind PEC PDs based on self-assembled AlGaN nanostructures on silicon. Importantly, with the proper surface platinum (Pt) decoration, a significant boost of photon responsivity by more than an order of magnitude was achieved in the newly built Pt/AlGaN nanoarchitectures, demonstrating strikingly high responsivity of 45 mA/W and record fast response/recovery time of 47/20 ms without external power source. Such high solar-blind photodetection originates from the unparalleled material quality, fast interfacial kinetics, as well as high carrier separation efficiency which suggests that embracement of defect-free wide-bandgap semiconductor nanostructures with appropriate surface decoration offers an unprecedented opportunity for designing future energy-efficient and large-scale optoelectronic systems on a silicon platform.

Hydrogen passivation: a proficient strategy to enhance the optical and photoelectrochemical performance of InGaN/GaN single-quantum-well nanorods

Recently, III-nitride semiconductor nanostructures, especially InGaN/GaN quantum well nanorods (NRs), have been established as a promising material of choice for nanoscale optoelectronics and photoelectrochemical (PEC) water-splitting applications. Due to the large number of surface states, III-nitride NRs suffer from low quantum efficiency. Therefore, control of the surface states is necessary to improve device performance in real-time applications. In this work, we investigated the effect of hydrogen plasma treatment on the optical properties of InGaN/GaN single-quantum-well (SQW) NRs. The low-temperature photoluminescence (PL) studies revealed that yellow and green emissions overlapped and the yellow band is more dominant in the pristine InGaN/GaN SQW NRs. However, the emission corresponding to yellow luminescence was strongly suppressed and the green emission is more intensified in hydrogenated InGaN/GaN SQW NRs. Furthermore, the time-resolved PL spectroscopy studies revealed that the carrier lifetimes of hydrogenated InGaN/GaN SQW NRs are relatively short compared to the pristine InGaN/GaN SQW, indicating the effective reduction of non-radiative centers. From the PEC measurement, the photocurrent density of hydrogenated InGaN/GaN SQW NRs in the H₂SO₄ solution is found to be 5 mA cm^-2 at -0.48 V versus reversible hydrogen electrode, which is 3.5-fold larger than that of pristine ones. These findings shed new light on the significance of surface treatment on the optical properties and thus nanostructured photoelectrodes for PEC applications.

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.

InGaN Nanorods Decorated with Au Nanoparticles for Enhanced Water Splitting Based on Surface Plasmon Resonance Effects

Photoelectrochemical (PEC) water splitting has great application potential in converting solar energy into hydrogen energy. However, what stands in the way of the practical application of this technology is the low conversion efficiency, which can be promoted by optimizing the material structure and device design for surface functionalization. In this work, we deposited gold nanoparticles (Au NPs) with different loading densities on the surface of InGaN nanorod (NR) arrays through a chemical solvent route to obtain a composite PEC water splitting system. Enhanced photocatalytic activity, which can be demonstrated by the surface plasmon resonance (SPR) effect induced by Au NPs, occurred and was further confirmed to be associated with the different loading densities of Au NPs. These discoveries use solar water splitting as a platform and provide ideas for exploring the mechanism of SPR enhancement.

Understanding Surface Modulation to Improve the Photo/Electrocatalysts for Water Oxidation/Reduction

Water oxidation and reduction reactions play vital roles in highly efficient hydrogen production conducted by an electrolyzer, in which the enhanced efficiency of the system is apparently accompanied by the development of active electrocatalysts. Solar energy, a sustainable and clean energy source, can supply the kinetic energy to increase the rates of catalytic reactions. In this regard, understanding of the underlying fundamental mechanisms of the photo/electrochemical process is critical for future development. Combining light-absorbing materials with catalysts has become essential to maximizing the efficiency of hydrogen production. To fabricate an efficient absorber-catalysts system, it is imperative to fully understand the vital role of surface/interface modulation for enhanced charge transfer/separation and catalytic activity for a specific reaction. The electronic and chemical structures at the interface are directly correlated to charge carrier movements and subsequent chemical adsorption and reaction of the reactants. Therefore, rational surface modulation can indeed enhance the catalytic efficiency by preventing charge recombination and prompting transfer, increasing the reactant concentration, and ultimately boosting the catalytic reaction. Herein, the authors review recent progress on the surface modification of nanomaterials as photo/electrochemical catalysts for water reduction and oxidation, considering two successive photogenerated charge transfer/separation and catalytic chemical reactions. It is expected that this review paper will be helpful for the future development of photo/electrocatalysts.

Modified p-GaN Microwells with Vertically Aligned 2D-MoS2 for Enhanced Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting has been considered as the future technology for storing solar energy in the chemical bonds. However, due to the search of ideal heterostructured materials for photoanode/cathode, the full potential of this technology has not been realized yet. Herein we present, the nanotextured hexagonal microwell of p-GaN [p-GaN(Et)] synthesized via wet chemical etching route as a photocathode (PC) for PEC water splitting. The p-GaN(Et) was further modified by interconnected nanowall network of two-dimensional (2D) transition metal dichalcogenide (MoS₂) [2D-MoS₂/p-GaN(Et)]. Both PCs were characterized for their morphology, structures, and optical and electronic properties. The overall PEC performance was validated through photocurrent values followed by the amount of hydrogen and oxygen evolution. This combination of 2D-MoS₂/p-GaN(Et) outplayed pristine p-GaN(Et) by several orders of magnitude in overall PEC performance. The extraordinary stability under a continuous operating condition with 1 sun illumination (100 mW/cm²) provides the much-needed flavor of an efficient photocathode. The optimized photocathode [2D-MoS₂/p-GaN(Et)] shows the highest applied bias photon-to-current conversion efficiency of ∼3.18% with hydrogen evolution rate of 89.56 μmol/h at -0.3 V vs RHE. This wafer-level cost-effective synthesis of 2D-MoS₂/GaN heterostructure based PCs opens a new way for large-scale solar-fuel conversion.

A homogeneous photoelectrochemical hydrogen sulfide sensor based on the electronic transfer mediated by tetrasulfophthalocyanine

A homogeneous photoelectrochemical sensor for H₂S detection based on the electronic transfer mediated by [Fe(iii)PcS₄]⁺was developed with an un-modified photoelectrode.

Growth of Doped SrTiO3 Ferroelectric Nanoporous Thin Films and Tuning of Photoelectrochemical Properties with Switchable Ferroelectric Polarization

Ferroelectric polarization is an intriguing physical phenomenon for tuning charge-transport properties and finds application in a wide range of optoelectronic devices. So far, ferroelectric materials in a planar geometry or chemically grown nanostructures have been used. However, these structural architectures possess serious disadvantages such as small surface areas and structural defects, respectively, leading to reduced performance. Herein, the growth of room-temperature ferroelectric nanoporous/nanocolumnar structure of Ag,Nb-codoped SrTiO₃ (Ag/Nb:STO) using pulsed laser deposition is reported and demonstrated to have enhanced photoelectrochemical (PEC) properties using ferroelectric polarization. By manipulating the external electrical bias, ∼3-fold enhancement in the photocurrent from 40 to 130 μA·cm^-2 of film area is obtained. Concurrently, the flat-band potential is decreased from -0.55 to -1.13 V, revealing a giant ferroelectric tuning of the band alignment at the semiconductor surface and enhanced charge transfer. In addition, an electrochemical impedance spectroscopy study confirmed the tuning of the charge transfer with ferroelectric polarization. Our nanoporous ferroelectric-semiconductor approach offers a new platform with great potential for achieving highly efficient PEC devices for renewable energy applications.

Highly Efficient and Stable Photoelectrochemical Hydrogen Evolution with 2D-NbS2/Si Nanowire Heterojunction

In recent days, 2-dimensional (2D) niobium disulfide (NbS₂) with near-zero Gibbs free energy and superlative acid electrolyte stability has provoked a great deal of interest toward hydrogen evolution reaction (HER) electrocatalyst due to its active basal and edge sulfur sites. Herein, we developed a single step method for the direct deposition of 2D-NbS₂ on high-aspect-ratio topographies of silicon nanowires (NWs) by chemical vapor deposition for the applications in HER electrocatalyst. The resultant 2D-NbS₂ electrocatalyst demonstrates the HER overpotential of ∼74 mV vs RHE (reversible hydrogen electrode) @ 1 mA/cm² under acidic conditions and stable for more than 20 h. More importantly, we developed the Si NWs array based photoelectrochemical water-splitting system with the direct deposition of 2D-NbS₂ as HER catalyst. The resultant 2D-NbS₂-Si NWs photocathode system demonstrates improved charge transfer characteristics at the Si-NbS₂ interfaces that leads to an enhanced turn on potential (from 0.06 to 0.34 V vs RHE) with the current density of -28 mA/cm² at the 0 V vs RHE. The results evidence the synergistic effect of 2D-NbS₂ electrocatalysts that addresses poor surface kinetics of Si toward solar water electrolysis. Our comprehensive studies reveal NbS₂ as a new class of photoelectrochemical cocatalyst for efficient solar HER performance by promoting the charge transfer process with prolonged acid stability.

Perovskite Oxide Based Electrodes for High-Performance Photoelectrochemical Water Splitting

Photoelectrochemical (PEC) water splitting is an attractive strategy for the large-scale production of renewable hydrogen from water. Developing cost-effective, active and stable semiconducting photoelectrodes is extremely important for achieving PEC water splitting with high solar-to-hydrogen efficiency. Perovskite oxides as a large family of semiconducting metal oxides are extensively investigated as electrodes in PEC water splitting owing to their abundance, high (photo)electrochemical stability, compositional and structural flexibility allowing the achievement of high electrocatalytic activity, superior sunlight absorption capability and precise control and tuning of band gaps and band edges. In this review, the research progress in the design, development, and application of perovskite oxides in PEC water splitting is summarized, with a special emphasis placed on understanding the relationship between the composition/structure and (photo)electrochemical activity.

Intrinsic Properties of Macroscopically Tuned Gallium Nitride Single-Crystalline Facets for Electrocatalytic Hydrogen Evolution

The anisotropy of crystalline materials results in different physical and chemical properties on different facets, which warrants an in-depth investigation. Macroscopically facet-tuned, high-purity gallium nitride (GaN) single crystals were synthesised and machined, and the electrocatalytic hydrogen evolution reaction (HER) was used as the model reaction to show the differences among the facets. DFT calculations revealed that the Ga and N sites of GaN (100) had a considerably smaller ΔG_H* value than those of the metal Ga site of GaN (001) or N site of GaN (00-1), thereby indicating that GaN (100) should be more catalytically active for the HER on account of its nonpolar facet. Subsequent experiments testified that the electrocatalytic performance of GaN (100) was considerably more efficient than that of other facets for both acidic and alkaline HERs. Moreover, the GaN crystal with a preferentially (100) active facet had an excellently durable alkaline electrocatalytic HER for more than 10 days. This work provides fundamental insights into the exploration of the intrinsic properties of materials and designing advanced materials for physicochemical applications.

High performance III-V photoelectrodes for solar water splitting via synergistically tailored structure and stoichiometry

Catalytic interface of semiconductor photoelectrodes is critical for high-performance photoelectrochemical solar water splitting because of its multiple roles in light absorption, electrocatalysis, and corrosion protection. Nevertheless, simultaneously optimizing each of these processes represents a materials conundrum owing to conflicting requirements of materials attributes at the electrode surface. Here we show an approach that can circumvent these challenges by collaboratively exploiting corrosion-resistant surface stoichiometry and structurally-tailored reactive interface. Nanoporous, density-graded surface of 'black' gallium indium phosphide (GaInP₂), when combined with ammonium-sulfide-based surface passivation, effectively reduces reflection and surface recombination of photogenerated carriers for high efficiency photocatalysis in the hydrogen evolution half-reaction, but also augments electrochemical durability with lifetime over 124 h via strongly suppressed kinetics of corrosion. Such synergistic control of stoichiometry and structure at the reactive interface provides a practical pathway to concurrently enhance efficiency and durability of semiconductor photoelectrodes without solely relying on the development of new protective materials.

Enhanced water splitting performance of GaN nanowires fabricated using anode aluminum oxide templates

Highly ordered GaN nanowires were fabricated using an anodic aluminum oxide (AAO) template. Compared to planar GaN, the GaN nanowires significantly increased the light absorption, and the saturated photocurrent increased by a factor of 5 from 0.075 to 0.38 mA cm⁻². The photocurrent increase with the GaN nanowires is not only due to their increased surface to volume ratio and reduction in the distance for photo-generated carriers to reach the electrolyte, but also the built-in electric field, which mainly contribute to the enhancement in their water splitting ability. The GaN nanowires can lead to band bending due to their surface states and the formation of a polarized electric field to accelerate the separation of photo-generated carriers. We also established a theoretic model to simulate the band bending in the nanowires. The results showed that when the nanowire diameters are equal or bigger than the full width of depletion region, the nanowires have the maximum electric field, which improves their water splitting performance significantly. These results provide a cost-effective way for highly efficient water splitting.

Highly ordered GaN nanowires were fabricated using an anodic aluminum oxide (AAO) template.

First-principles modeling of GaN(0001)/water interface: Effect of surface charging

The accumulation properties of photogenerated carriers at the semiconductor surface determine the performance of photoelectrodes. However, to the best of our knowledge, there are no computational studies that methodically examine the effect of "surface charging" on photocatalytic activities. In this work, the effect of excess carriers at the semiconductor surface on the geometric and electronic structures of the semiconductor/electrolyte interface is studied systematically with the aid of first-principles calculations. We found that the number of water molecules that can be dissociated follows the "extended" electron counting rule; the dissociation limit is smaller than that predicted by the standard electron counting rule (0.375 ML) by the number of excess holes at the interface. When the geometric structure of the GaN/water interface obeys the extended electron counting rule, the Ga-originated surface states are removed from the bandgap due to the excess holes and adsorbates, and correspondingly, the Fermi level becomes free from pinning. Clearly, the excess charge has a great impact on the interface structure and most likely on the chemical reactions. This study serves as a basis for further studies on the semiconductor/electrolyte interface under working conditions.

Plasmonic enhanced Cu2O-Au-BFO photocathodes for solar hydrogen production

A novel Cu₂O-Au-BFO heterostructure photocathode was constructed which significantly improved the efficiency of photo-generated carrier transfer for solar hydrogen production. A BiFeO₃ (BFO) ferroelectric film was synthesized on top of a Cu₂O layer by a sputtering process. The BFO layer acted to protect the Cu₂O layer from photochemical corrosion, increasing photoelectrochemical (PEC) stability. The p–n heterojunction between Cu₂O and BFO layers enhanced the PEC properties by suppressing charge recombination and improved interfacial charge transfer efficiency. When Cu₂O and BFO are interfaced by Au Nanoparticles (NPs) the PEC performance was further enhanced, due to hot-electron transfer at the plasmonic resonance. After positive poling, the depolarization field across the whole volume of BFO film drove electrons into the electrolyte solution, inducing a significant anodic shift, V_op of 1.01 V vs. RHE, together with a significantly enhanced photocurrent density of −91 μA/cm² at 0 V vs. RHE under 100 mW/cm² illumination. The mechanism was investigated through experimental and theoretivcal calculations.

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.

Twofold Porosity and Surface Functionalization Effect on Pt-Porous GaN for High-Performance H2-Gas Sensors at Room Temperature

The achievement of H2 detection, up to 25 ppm, at room temperature using sulfur-treated, platinum (Pt)-decorated porous GaN is reported in this study. This achievement is attributed to the large lateral pore size, Pt catalyst, and surface treatment using organic sulfide. The performance of H2-gas sensors is studied as a function of the operating temperature by providing an adsorption activation energy of 22 meV at 30 ppm H2, confirming the higher sensitivity of the sulfide-treated Pt-porous GaN sensor. Furthermore, the sensing response of the sulfide-treated Pt-porous GaN gas sensor increases with the increase in porosity (surface-to-volume ratio) and pore radii. Using the Knudsen diffusion-surface reaction equation, the H2 gas concentration profile is simulated and fitted within the porous GaN layer, revealing that H2 diffusion is limited by small pore radii because of its low diffusion rate. The simulated gas sensor responses to H2 versus the pore diameter show the same trend as observed for the experimental data. The sulfide-treated Pt-porous GaN sensor achieves ultrasensitive H2 detection at room temperature for 125 nm pore radii.

Vertically aligned MoS2 nanosheets on graphene for highly stable electrocatalytic hydrogen evolution reactions

Conducting an efficient hydrogen evolution reaction (HER) using two-dimensional molybdenum disulphide as electrocatalysts remains a challenging task due to the insufficient active edge sites. In this regard, herein, molybdenum disulphide nanosheets with rich active sulphur sites were vertically grown on the graphene surface via a chemical vapour deposition process. The direct integration of vertically aligned MoS2 nanosheets on graphene forms a van der Waals (vdW) heterojunction, which facilitates a barrier-free charge transport towards the electrolyte as a result of unique and well-matched energy band alignment at the interface. The prospective combination of Ohmic graphene/MoS2 heterostructure and the high electrocatalytic edge activity of sulphur delivers an incredibly and small turn-on potential of 0.14 V vs. RHE in the acid electrolyte solution. Most importantly, the use of a vertical vdW device architecture exhibits nearly 8× improvement in HER than that of its layered counterpart. Moreover, the HER reaction is highly stable over 50 hours of continuous operation even after 150 days. The combined analysis of our study makes it certain that the graphene/MoS2 heterostructure will be an efficient alternative electrode for low-cost and large-scale electrochemical applications.

Self-Assembled Single-Crystalline GaN Having a Bimodal Meso/Macropore Structure To Enhance Photoabsorption and Photocatalytic Reactions

This paper describes the self-assembled fabrication of single-crystal GaN with a bimodal pore (meso/macropore) size distribution (BiPS-GaN). A 4.7 μm-thick BiPS-GaN layer was grown spontaneously using halogen-free vapor phase epitaxy in conjunction with boron impurity doping (>1 × 10¹⁹ atoms/cm³) on a GaN template fabricated via metalorganic chemical vapor deposition (MOCVD-GaN). The boron impurity acted as a surfactant, and its segregation generated a dense (>1 × 10¹⁰ cm^-2), homogeneous distribution of mesopores with sizes of 30-40 nm in GaN during growth. In addition, macropores with sizes of 0.1-2 μm were produced by the fusion of mesopores in close proximity to one another. As a result, BiPS-GaN exhibited a high density of both meso- and macropores, all aligned in the vertical direction (that is, along the c axis). BiPS-GaN showed good electroconductivity and almost the same high degree of crystallinity as the MOCVD-GaN template. Furthermore, the hybrid meso/macropore structure of BiPS-GaN imparted excellent photoabsorption properties and allowed this material to work as an efficient support for a nanosized IrO _x catalyst. The photocurrent density in BiPS-GaN was enhanced by as much as a factor of 5 compared to planar GaN by effective absorption due to the hybrid meso/macropore structure of BiPS-GaN. Moreover, the oxygen generation efficiency of BiPS-GaN with the IrO _x catalyst was approximately doubled, compared to that of BiPS-GaN without IrO _x, while maintaining long-term stability. These results demonstrate that BiPS-GaN fabricated in this facile manner has significant potential in applications such as photoelectrochemical reactions and catalysis.

Construction, characterization, and growth mechanism of high-density jellyfish-like GaN/SiOxNy nanomaterials on p-Si substrate by Au-assisted chemical vapor deposition approach

High-density GaN/SiO_xN_y jellyfish-like nanomaterials are synthesized on Au-coated p-type Si substrates by a chemical vapor deposition approach.

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.