Surface Passivation of GaN Nanowires for Enhanced Photoelectrochemical Water-Splitting

Interface Engineering of Monolayer MoS2/GaN Hybrid Heterostructure: Modified Band Alignment for Photocatalytic Water Splitting Application by Nitridation Treatment

Interface engineering is a key strategy to deal with the two-dimensional (2D)/three-dimensional (3D) hybrid heterostructure, since the properties of this atomic-layer-thick 2D material can easily be impacted by the substrate environment. In this work, the structural, electronic, and optical properties of the 2D/3D heterostructure of monolayer MoS₂ on wurtzite GaN surface without and with nitridation interfacial layer are systematically investigated by first-principles calculation and experimental analysis. The nitridation interfacial layer can be introduced into the 2D/3D heterostructure by remote N₂ plasma treatment to GaN sample surface prior to stacking monolayer MoS₂ on top. The calculation results reveal that the 2D/3D integrated heterostructure is energetically favorable with a negative formation energy. Both interfaces demonstrate indirect band gap, which is a benefit for longer lifetime of the photoexcited carriers. Meanwhile, the conduction band edge and valence band edge of the MoS₂ side increases after nitridation treatment. The modification to band alignment is then verified by X-ray photoelectron spectroscopy measurement on MoS₂/GaN heterostructures constructed by a modified wet-transfer technique, which indicates that the MoS₂/GaN heterostructure without nitridation shows a type-II alignment with a conduction band offset (CBO) of only 0.07 eV. However, by the deployment of interface nitridation, the band edges of MoS₂ move upward for ∼0.5 eV as a result of the nitridized substrate property. The significantly increased CBO could lead to better electron accumulation capability at the GaN side. The nitridized 2D/3D heterostructure with effective interface treatment exhibits a clean band gap and substantial optical absorption ability and could be potentially used as practical photocatalyst for hydrogen generation by water splitting using solar energy.

Highly Enhanced Visible-Light-Driven Photoelectrochemical Performance of ZnO-Modified In2S3 Nanosheet Arrays by Atomic Layer Deposition

Photoanodes based on In₂S₃/ZnO heterojunction nanosheet arrays (NSAs) have been fabricated by atomic layer deposition of ZnO over In₂S₃ NSAs, which were in situ grown on fluorine-doped tin oxide glasses via a facile solvothermal process. The as-prepared photoanodes show dramatically enhanced performance for photoelectrochemical (PEC) water splitting, compared to single semiconductor counterparts. The optical and PEC properties of In₂S₃/ZnO NSAs have been optimized by modulating the thickness of the ZnO overlayer. After pairing with ZnO, the NSAs exhibit a broadened absorption range and an increased light absorptance over a wide wavelength region of 250-850 nm. The optimized sample of In₂S₃/ZnO-50 NSAs shows a photocurrent density of 1.642 mA cm^-2 (1.5 V vs. RHE) and an incident photon-to-current efficiency of 27.64% at 380 nm (1.23 V vs. RHE), which are 70 and 116 times higher than those of the pristine In₂S₃ NSAs, respectively. A detailed energy band edge analysis reveals the type-II band alignment of the In₂S₃/ZnO heterojunction, which enables efficient separation and collection of photogenerated carriers, especially with the assistance of positive bias potential, and then results in the significantly increased PEC activity.

Nanotextured Spikes of α-Fe2O3/NiFe2O4 Composite for Efficient Photoelectrochemical Oxidation of Water

We demonstrate for the first time the application of p-NiFe₂O₄/n-Fe₂O₃ composite thin films as anode materials for light-assisted electrolysis of water. The p-NiFe₂O₄/n-Fe₂O₃ composite thin films were deposited on planar fluorinated tin oxide (FTO)-coated glass as well as on 3D array of nanospike (NSP) substrates. The effect of substrate (planar FTO and 3D-NSP) and percentage change of each component (i.e., NiFe₂O₄ and Fe₂O₃) of composite was studied on photoelectrochemical (PEC) water oxidation reaction. This work also includes the performance comparison of p-NiFe₂O₄/n-Fe₂O₃ composite (planar and NSP) devices with pure hematite for PEC water oxidation. Overall, the nanostructured p-NiFe₂O₄/n-Fe₂O₃ device with equal molar 1:1 ratio of NiFe₂O₄ and Fe₂O₃ was found to be highly efficient for PEC water oxidation as compared with pure hematite, 1:2 and 1:3 molar ratios of composite. The photocurrent density of 1:1 composite thin film on planar substrate was equal to 1.07 mA/cm² at 1.23 V_RHE, which was 1.7 times higher current density as compared with pure hematite device (0.63 mA/cm² at 1.23 V_RHE). The performance of p-NiFe₂O₄/n-Fe₂O₃ composites in PEC water oxidation was further enhanced by their deposition over 3D-NSP substrate. The highest photocurrent density of 2.1 mA/cm² at 1.23 V_RHE was obtained for the 1:1 molar ratio p-NiFe₂O₄/n-Fe₂O₃ composite on NSP (NF1-NSP), which was 3.3 times more photocurrent density than pure hematite. The measured applied bias photon-to-current efficiency (ABPE) value of NF1-NSP (0.206%) was found to be 1.87 times higher than that of NF1-P (0.11%) and 4.7 times higher than that of pure hematite deposited on FTO-coated glass (0.044%). The higher PEC water oxidation activity of p-NiFe₂O₄/n-Fe₂O₃ composite thin film as compared with pure hematite is attributed to the Z-path scheme and better separation of electrons and holes. The increased surface area and greater light absorption capabilities of 3D-NSP devices result in further improvement in catalytic activities.

Light Modulation and Water Splitting Enhancement Using a Composite Porous GaN Structure

On the basis of the laterally porous GaN, we designed and fabricated a composite porous GaN structure with both well-ordered lateral and vertical holes. Compared to the plane GaN, the composite porous GaN structure with the combination of the vertical holes can help to reduce UV reflectance and increase the saturation photocurrent during water splitting by a factor of ∼4.5. Furthermore, we investigated the underlying mechanism for the enhancement of the water splitting performance using a finite-difference time-domain method. The results show that the well-ordered vertical holes can not only help to open the embedded pore channels to the electrolyte at both sides and reduce the migration distance of the gas bubbles during the water splitting reactions but also help to modulate the light field. Using this composite porous GaN structure, most of the incident light can be modulated and trapped into the nanoholes, and thus the electric fields localized in the lateral pores can increase dramatically as a result of the strong optical coupling. Our findings pave a new way to develop GaN photoelectrodes for highly efficient solar water splitting.

Stable and Reversible Lithium Storage with High Pseudocapacitance in GaN Nanowires

In this work, gallium nitride (GaN) nanowires (NWs) were synthesized by chemical vapor deposition (CVD) process. The hybrid electrode showed the capacity up to 486 mAh g^-1 after 400 cycles at 0.1 A g^-1. Even at 10 A g^-1, the reversible capacity can stabilize at 75 mAh g^-1 (after 1000 cycles). Pseudocapacitive capacity was defined by kinetics analysis. The dynamics analysis and electrochemical reaction mechanism of GaN with Li⁺ was also analyzed by ex situ XRD, HRTEM, and XPS results. These results not only cast new light on pseudocapacitance enhanced high-rate energy storage devices by self-assembled nanoengineering but also extend the application range of traditional binary III/V semiconductors.

11.5% efficiency of TiO2 protected and Pt catalyzed n+np+-Si photocathodes for photoelectrochemical water splitting: manipulating the Pt distribution and Pt/Si contact

A combination of hydrogen passivation, electroless deposition of a Pt catalyst and coating a TiO₂ layer is very effective to get highly efficient and stable n⁺np⁺-Si photocathodes.

Promoter-free synthesis of monolayer MoS2 by chemical vapour deposition

Piranha treatment provides an ideal platform for the controlled growth of large-scale monolayer MoS₂ on dielectric and semiconductor substrates for device applications.

Emission dynamics of hybrid plasmonic gold/organic GaN nanorods

We studied the emission of bare and aluminum quinoline (Alq₃)/gold coated wurtzite GaN nanorods by temperature- and intensity-dependent time-integrated and time-resolved photoluminescence (PL). The GaN nanorods of ∼1.5 μm length and ∼250 nm diameter were grown by plasma-assisted molecular beam epitaxy. Gold/Alq₃ coated GaN nanorods were synthesized by organic molecular beam deposition. The near band-edge and donor-acceptor pair luminescence was investigated in bare GaN nanorods and compared with multilevel model calculations providing the dynamical parameters for electron-hole pairs, excitons, impurity bound excitons, donors and acceptors. Subsequently, the influence of a 10 nm gold coating without and with an Alq₃ spacer layer was studied and the experimental results were analyzed with the multilevel model. Without a spacer layer, a significant PL quenching and lifetime reduction of the near band-edge emission is found. The behavior is attributed to surface band-bending and Förster energy transfer from excitons to surface plasmons in the gold layer. Inserting a 5 nm Alq₃ spacer layer reduces the PL quenching and lifetime reduction which is consistent with a reduced band-bending and Förster energy transfer. Increasing the spacer layer to 30 nm results in lifetimes which are similar to uncoated structures, showing a significantly decreased influence of the gold coating on the excitonic dynamics.

Stability and Performance of Sulfide-, Nitride-, and Phosphide-Based Electrodes for Photocatalytic Solar Water Splitting

With the past decade of worldwide sustained efforts on artificial photosynthesis for photocatalytic solar water splitting and clean hydrogen generation by dedicated researchers and engineers from different disciplines, substantial progress has been achieved in raising its overall efficiency along with finding new photocatalysts. Various materials, systems, devices, and better fundamental understandings of the interplay between interfacial chemistry, electronic structure, and photogenerated charge dynamics involved have been developed. Nevertheless, the overall photocatalytic performance is yet to achieve its maximum theoretical limit. Moreover, the stability of well-known semiconductors (as well as novel ones) remains the biggest challenge that scientists are facing to develop durable industrial-scale devices for large-scale water oxidation and overall solar water splitting. In this Perspective, we summarize the major achievements and the different approaches carried out to improve the stability and performance of photoelectrodes based on sulfide, nitride, and phosphide semiconductors.

Graphene-Oxide-Assisted Synthesis of GaN Nanosheets as a New Anode Material for Lithium-Ion Battery

As the most-studied III-nitride, theoretical researches have predicted the presence of gallium nitride (GaN) nanosheets (NSs). Herein, a facile synthesis approach is reported to prepare GaN NSs using graphene oxide (GO) as sacrificial template. As a new anode material of Li-ion battery (LIBs), GaN NSs anodes deliver the reversible discharge capacity above 600 mA h g^-1 at 1.0 A g^-1 after 1000 cycles, and excellent rate performance at current rates from 0.1 to 10 A g^-1. These results not only extend the family of 2D materials but also facilitate their use in energy storage and other applications.