Improvement in Photoelectrochemical Water Splitting Performance of GaN-nanowire Photoanode Using MXene.

The photoelectrochemical water splitting (PEC-WS) performance of a photoanode consisting of GaN nanowires (NWs) is significantly improved using a Ti3C2-MXene coating as an intermediate layer to promote carrier transfer toward the electrolyte. The maximum current density and applied-bias photon-to-current efficiency of the photoanode comprising GaN NWs coated with Ti3C2-MXene (MGNWs) are measured to be 34.24 mA/cm2 and 14.47% at 1.2 and 0.4 V versus a reversible hydrogen electrode (RHE), respectively. These values are much higher than those of the GaN-NW photoanode without Ti3C2-MXene (4.04 mA/cm2 and 1.95%) and also markedly exceed those of previously reported photoanodes. After 8 days of PEC-WS, the current density was measured to be 31.07 mA/cm2, which corresponds to 97.58% of that measured immediately after the reaction started. Based on the time dependence of the current density, the hydrogen evolution rate over the reaction time is calculated to be 0.58 mmol/cm2·h. The results confirm that the PEC-WS performance of the optimized MGNW photoanode is superior to and more stable than those of previously reported photoanodes.

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.

Flexible 1.3 µm photodetector fabricated with InN nanowires and graphene on overhead projector transparency sheet.

We report the first demonstration of flexible photodetectors, operating at the wavelength window of 1.3 µm, fabricated with InN nanowires (NWs) and graphene on an overhead projector transparency (OHP) sheet. The InN NWs, used as an absorption medium for the device, were formed on a Si substrate and exhibited strong emission with a peak wavelength of 1.3 µm at room temperature. They were randomly and horizontally embedded in the graphene sandwich structure functioned as a carrier channel. The photocurrent and photoresponsivity of the flexible photodetector were found to be 1.17 mA and 0.48 A W-1, respectively, at a voltage of 1 V and a light intensity of 60 mW cm-2 of a xenon lamp. The photocurrent measured when the photodetector was bent under a strain of 3% was 1.15 mA, which corresponds to 98.3% compared to that before bending. Moreover, the photocurrent and photoresponsivity of the flexible photodetector measured after the 200 cyclic-bending tests are comparable to those measured before bending.

Room-temperature operation of light-assisted NO2gas sensor based on GaN nanowires and graphene.

We report the successful demonstration of a light-assisted NO2gas sensor that operates at room temperature with high response. The gas sensor was fabricated with high-crystalline undoped-GaN nanowires (NWs) and graphene functioning as the light-absorbing medium and carrier channel, respectively. Exposure of the gas sensor to the NO2concentration of 100 ppm at a light intensity of 1 mW cm-2of a xenon lamp delivered a response of 16% at room temperature, which increased to 23% when the light intensity increased to 100 mW cm-2. This value is higher than those previously reported for GaN-based NO2gas sensors operating at room temperature. The room-temperature response of the gas sensor measured after six months was calculated to be 21.9%, which corresponds to 95% compared to the value obtained immediately after fabricating the devices. The response of the gas sensor after independently injecting NO2, H2S, H2, CO, and CH3CHO gases were measured to be 23, 5, 2.6, 2.2, and 1.7%, respectively. These results indicate that the gas sensor using GaN NWs and graphene provides high response, long-term stability, and good selectivity to NO2gas at room temperature. In addition, the use of undoped-GaN NWs without using additional catalysts makes it possible to fabricate gas sensors that operate at room temperature simpler and better than conventional technologies.

Stretchable Inorganic GaN-Nanowire Photosensor with High Photocurrent and Photoresponsivity.

To effectively implement wearable systems, their constituent components should be made stretchable. We successfully fabricated highly efficient stretchable photosensors made of inorganic GaN nanowires (NWs) as light-absorbing media and graphene as a carrier channel on polyurethane substrates using the pre-strain method. When a GaN-NW photosensor was stretched at a strain level of 50%, the photocurrent was measured to be 0.91 mA, corresponding to 87.5% of that (1.04 mA) obtained in the released state, and the photoresponsivity was calculated to be 11.38 A/W. These photosensors showed photocurrent and photoresponsivity levels much higher than those previously reported for any stretchable semiconductor-containing photosensor. To explain the superior performances of the stretchable GaN-NW photosensor, it was approximated as an equivalent circuit with resistances and capacitances, and in this way, we analyzed the behavior of the photogenerated carriers, particularly at the NW-graphene interface. In addition, the buckling phenomenon typically observed in organic-based stretchable devices fabricated using the pre-strain method was not observed in our photosensors. After a 1000-cycle stretching test with a strain level of 50%, the photocurrent and photoresponsivity of the GaN-NW photosensor were measured to be 0.96 mA and 11.96 A/W, respectively, comparable to those measured before the stretching test. To evaluate the potential of our stretchable devices in practical applications, the GaN-NW photosensors were attached to the proximal interphalangeal joint of the index finger and to the back of the wrist. Photocurrents of these photosensors were monitored during movements made about these joints.

Highly Efficient Photoelectrochemical Water Splitting Using GaN-Nanowire Photoanode with Tungsten Sulfides.

In the present study, we have achieved high-performance photoelectrochemical water splitting (PEC-WS) using GaN nanowires (NWs) coated with tungsten sulfide (WxS1-x) (GaN-NW-WxS1-x) as a photoanode. The measured current density and applied-bias photon-to-current efficiency were 20.38 mA/cm2 and 13.76%, respectively. These values were much higher than those reported previously for photoanodes with any kind of III-nitride nanostructure. The amount of hydrogen gas formed was 1.01 mmol/cm2 from 7 h PEC-WS, which was also much higher than the previously reported values. The drastic improvement in the PEC-WS performance using the GaN-NW-WxS1-x photoanode was attributed to an increase in the number of photogenerated carriers due to the highly crystalline GaN NWs, and acceleration of separation of photogenerated carriers and consequent suppression of charge recombination because of nitrogen-terminated surfaces of NWs, sulfur vacancies in WxS1-x, and type-II band alignment between NW and WxS1-x. The degree of impedance matching, evaluated from Nyquist plots, was considered to analyze charge transfer characteristics at the interface between the GaN-NW-WxS1-x photoanode and 0.5-M H2SO4 electrolyte. Considering the material system and scheme for the PEC-WS, our approach provides an efficient way to improve hydrogen evolution reaction.