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.

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 VRHE 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.

Flexible a-Si:H Solar Cells with Spontaneously Formed Parabolic Nanostructures on a Hexagonal-Pyramid Reflector.

Flexible amorphous silicon (a-Si:H) solar cells with high photoconversion efficiency (PCE) are demonstrated by embedding hexagonal pyramid nanostructures below a Ag/indium tin oxide (ITO) reflector. The nanostructures constructed by nanoimprint lithography using soft materials allow the top ITO electrode to spontaneously form parabolic nanostructures. Nanoimprint lithography using soft materials is simple, and is conducted at low temperature. The resulting structure has excellent durability under repeated bending, and thus, flexible nanostructures are successfully constructed on flexible a-Si:H solar cells on plastic film. The nanoimprinted pyramid back reflector provides a high angular light scattering with haze reflectance >98% throughout the visible spectrum. The spontaneously formed parabolic nanostructure on the top surface of the a-Si:H solar cells both reduces reflection and scatters incident light into the absorber layer, thereby elongating the optical path length. As a result, the nanopatterned a-Si:H solar cells, fabricated on polyethersulfone (PES) film, exhibit excellent mechanical flexibility and PCE increased by 48% compared with devices on a flat substrate.

Concentrated Solar Light Photoelectrochemical Water Splitting for Stable and High-Yield Hydrogen Production.

Photoelectrochemical water splitting is a promising technique for converting solar energy into low-cost and eco-friendly H2 fuel. However, the production rate of H2 is limited by the insufficient number of photogenerated charge carriers in the conventional photoelectrodes under 1 sun (100 mW cm-2 ) light. Concentrated solar light irradiation can overcome the issue of low yield, but it leads to a new challenge of stability because the accelerated reaction alters the surface chemical composition of photoelectrodes. Here, it is demonstrated that loading Pt nanoparticles (NPs) on single crystalline GaN nanowires (NWs) grown on n+ -p Si photoelectrode operates efficiently and stably under concentrated solar light. Although a large number of Pt NPs detach during the initial reaction due to H2 gas bubbling, some Pt NPs which have an epitaxial relation with GaN NWs remain stably anchored. In addition, the stability of the photoelectrode further improves by redepositing Pt NPs on the reacted Pt/GaN surface, which results in maintaining onset potential >0.5 V versus reversible hydrogen electrode and photocurrent density >60 mA cm-2 for over 1500 h. The heterointerface between Pt cocatalysts and single crystalline GaN nanostructures shows great potential in designing an efficient and stable photoelectrode for high-yield solar to H2 conversion.

Pt nanoclusters on GaN nanowires for solar-asssisted seawater hydrogen evolution.

Seawater electrolysis provides a viable method to produce clean hydrogen fuel. To date, however, the realization of high performance photocathodes for seawater hydrogen evolution reaction has remained challenging. Here, we introduce n+-p Si photocathodes with dramatically improved activity and stability for hydrogen evolution reaction in seawater, modified by Pt nanoclusters anchored on GaN nanowires. We find that Pt-Ga sites at the Pt/GaN interface promote the dissociation of water molecules and spilling H* over to neighboring Pt atoms for efficient H2 production. Pt/GaN/Si photocathodes achieve a current density of -10 mA/cm2 at 0.15 and 0.39 V vs. RHE and high applied bias photon-to-current efficiency of 1.7% and 7.9% in seawater (pH = 8.2) and phosphate-buffered seawater (pH = 7.4), respectively. We further demonstrate a record-high photocurrent density of ~169 mA/cm2 under concentrated solar light (9 suns). Moreover, Pt/GaN/Si can continuously produce H2 even under dark conditions by simply switching the electrical contact. This work provides valuable guidelines to design an efficient, stable, and energy-saving electrode for H2 generation by seawater splitting.

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.

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.

Indium Tin Oxide Branched Nanowire and Poly(3-hexylthiophene) Hybrid Structure for a Photorechargeable Supercapacitor.

We report a photorechargeable supercapacitor that can convert solar energy to chemical energy and store it. The supercapacitor is composed of indium tin oxide branched nanowires (ITO BRs) and poly(3-hexylthiophene) (P3HT) semiconducting polymers. ITO BRs showed electrical double layer capacitive characteristics that originated from the unique porous and self-connected network structure. The hybrid structure of ITO BR/P3HT exhibited spontaneous light harvesting, energy conversion, and charge storage. As a result, photocharging/discharging of ITO BR/P3HT showed an areal capacitance of 2.44 mF/cm2 at a current density of 0.02 mA/cm2. The proof-of-concept photorechargeable device, composed of ITO BRs, ITO BR/P3HT, and Na2SO4/polyvinyl acetate gel electrolyte, generated a photovoltage as high as 0.28 V and stored charge effectively for tens of seconds. The combination of dual functions in a single hybrid material may achieve breakthrough advances.

Grain Boundary Engineering of Cu-Ag Thin-Film Catalysts for Selective (Photo)Electrochemical CO2 Reduction to CO and CH4.

We investigated the relationship between grain boundary (GB) oxidation of Cu-Ag thin-film catalysts and selectivity of the (photo)electrochemical CO2 reduction reaction (CO2 RR). The change in the thickness of the Cu thin film accompanies the variation of GB density, and the Ag layer (3 nm) has an island-like morphology on the Cu thin film. Therefore, oxygen from ambient air penetrates into the Cu thin film through the GB of Cu and binds with it because the uncoordinated Cu atoms at the GBs are unstable. It was found that the Cu thin film with a small grain size was susceptible to spontaneous oxidation and degraded the faradaic efficiency (FE) of CO and CH4. However, a relatively thick (≥80 nm) Cu layer was effective in preventing the GB oxidation and realized catalytic properties similar to those of bulk Cu-Ag catalysts. The optimized Cu (100 nm)-Ag (3 nm) thin film exhibited a unique bifunctional characteristic, which enables selective production of both CO (FECO = 79.8%) and CH4 (FECH4 = 59.3%) at a reductive potential of -1.0 and -1.4 VRHE, respectively. Moreover, the Cu-Ag thin film was used as a cocatalyst for photo-electrochemical CO2 reduction by patterning the Cu-Ag thin film and a SiO2 passivation layer on a p-type Si photocathode. This novel architecture improved the selectivity of CO and CH4 under light illumination (100 mW/cm2).

Abnormal dewetting of Ag layer on three-dimensional ITO branches to form spatial plasmonic nanoparticles for organic solar cells.

Three-dimensional (3D) plasmonic structures have attracted great attention because abnormal wetting behavior of plasmonic nanoparticles (NPs) on 3D nanostructure can enhance the localized surface plasmons (LSPs). However, previous 3D plasmonic nanostructures inherently had weak plasmonic light absorption, low electrical conductivity, and optical transmittance. Here, we fabricated a novel 3D plasmonic nanostructure composed of Ag NPs as the metal for strong LSPs and 3D nano-branched indium tin oxide (ITO BRs) as a transparent and conductive framework. The Ag NPs formed on the ITO BRs have a more dewetted behavior than those formed on the ITO films. We experimentally investigated the reasons for the dewetting behavior of Ag NPs concerning the geometry of ITO BRs. The spherical Ag NPs are spatially separated and have high density, thereby resulting in strong LSPs. Finite-domain time-difference simulation evidenced that spatially-separated, high-density and spherical Ag NPs formed on ITO BRs dramatically boost the localized electric field in the active layer of organic solar cells (OSCs). Photocurrent of PTB7:PCBM OSCs with the ITO BRs/Ag NPs increased by 14%.

Subwavelength-scale nanorods implemented hexagonal pyramids structure as efficient light-extraction in Light-emitting diodes.

Subwavelength-scale nanorods were implemented on the hexagonal pyramid of photochemically etched light-emitting diodes (LEDs) to improve light extraction efficiency (LEE). Sequential processes of Ag deposition and inductively coupled plasma etching successfully produce nanorods on both locally unetched flat surface and sidewall of hexagonal pyramids. The subwavelength-scale structures on flat surface offer gradually changed refractive index, and the structures on side wall of hexagonal pyramid reduce backward reflection, thereby enhancing further enhancement of the light extraction efficiency. Consequently, the nanorods implemented LED shows a remarkable enhancement in the light output power by 14% compared with that of the photochemically etched LEDs which is known to exhibit the highest light output power. Theoretical calculations using a rigorous coupled wave analysis method reveal that the subwavelength-scale nanorods are very effective in the elimination of TIR as well as backward reflections, thereby further enhancing LEE of the LEDs.

Spontaneously Formed CuSx Catalysts for Selective and Stable Electrochemical Reduction of Industrial CO2 Gas to Formate.

The electrochemical CO2 reduction in aqueous media is a promising method for both the mitigation of climate changes and the generation of value-added fuels. Although many researchers have demonstrated selective and stable catalysts for electrochemical reduction of pure CO2 gas, the conversion of industrial CO2 gas has been limited. Here, we fabricated the copper sulfide catalysts (CuSx), which were spontaneously formed by dipping a Cu foil into a laboratory-prepared industrial CO2-purged 0.1 M KHCO3 electrolyte. Because industrial CO2 contains H2S gas, sulfur species dissolved in the electrolyte can easily react with the Cu foil. As the concentration of dissolved sulfur species increased, the reaction between the Cu foil and sulfur enhanced. As a result, the average size and surface density of CuSx nanoparticles (NPs) increased to 133.2 ± 33.1 nm and 86.2 ± 3.3%, respectively. Because of the larger amount of sulfur content and the enlarged electrochemical surface area of CuSx NPs, the Faradaic efficiency (FE) of formate was improved from 22.7 to 72.0% at -0.6 VRHE. Additionally, CuSx catalysts showed excellent stability in reducing industrial CO2 to formate. The change in FE was hardly observed even after long-term (72 h) operation. This study experimentally demonstrated that spontaneously formed CuSx catalysts are efficient and stable for reducing the industrial CO2 gas to formate.

Ultrafast and Chemically Stable Transfer of Au Nanomembrane Using a Water-Soluble NaCl Sacrificial Layer for Flexible Solar Cells.

Large-scale industrial application of flexible device has called for development of transfer methods that deliver high yield and stability. Here, we show an ultrafast and chemically stable transfer method by using a water-soluble NaCl sacrificial layer. Extremely thin (10 nm) and large-area (4 in. wafer) free-standing Au nanomembranes (NMs) prepared on silicon substrate were successfully transferred to flexible PDMS substrate by dissolving the NaCl sacrificial layer. This transfer method enables highly transparent and electrically conductive Au NMs on PDMS substrate. To transfer a multilayered optoelectronic device, we fabricated flexible hydrogenated amorphous silicon (a-Si:H) solar cell on a glass substrate and transferred it to a PDMS substrate. There was no degradation of the electrical characteristic of the solar cell after the transfer. This approach enables the integration of high-temperature-processed a-Si:H solar cell onto low-temperature tolerant flexible polymer substrate without chemical contamination or damage.

Extremely flat metal films implemented by surface roughness transfer for flexible electronics.

We present an innovative approach to fabricate an extremely flat (EF) metal film which was done by depositing metal on an extremely flat mother substrate, then detaching the metal from the substrate. The detached flexible metal films had a roughness that was within 2% of the roughness of the mother substrate, so EFs with R a < 1 nm could be fabricated using the surface roughness transfer method. With quantitative analysis using in situ synchrotron XPS, it was concluded that the chemical reaction of oxygen atoms with the metal film played a critical role in designing a peel-off system to get extremely flat metal films from the mother substrate. The OLED was successfully implemented on the metal film. The OLED's luminance could be increased from 15 142 to 17 100 cd m-2 at 25 mA m-2 by replacing the glass substrate with an EF copper (Cu) substrate, due to the enhanced heat dissipation during the operation. This novel method can be very useful for mass production of large scale, low-cost and high quality metal films using roll-to-roll process.

Monolithic Nanoporous In-Sn Alloy for Electrochemical Reduction of Carbon Dioxide.

Nanostructured metal catalysts to convert CO2 to formate, which have been extensively studied over decades, have many problems such as durability, lifetime, high process temperature, and difficulty in controlling the morphology of nanostructures. Here, we report a facile method to fabricate monolithic nanoporous In-Sn alloy, a network of nanopores, induced by electroreduction of indium tin oxide nanobranches (ITO BRs). The electroreduction process concentrated a local electric field at the tip of the nanostructure, leading to current-assisted joule-heating to form a nanoporous In-Sn alloy. Scanning electron microscopy images showed that the nanopore size of In-Sn alloy could be controlled from 1176 to 65 nm by tuning the electroreduction condition: the applied potential and the time. As a result, formate Faradaic efficiency could be improved from 42.4% to 78.6%. Also, current density was increased from -6.6 to -9.6 mA/cm2 at -1.2 VRHE, thereby resulting in the highest HCOO- production rate of 75.9 µmol/(h cm2). Detachment of catalysts from the substrate was not observed even after a long-term (12 h) electrochemical measurement at high potential (-1.2 VRHE). This work provides a design rule to fabricate highly efficient and stable oxide-derived electrocatalysts.

Wavelength-Scale Structures as Extremely High Haze Films for Efficient Polymer Solar Cells.

Wavelength-scale inverted pyramid structures with low reflectance and excellent haze have been designed for application to polymer solar cells (PSCs). The wavelength-scale structured haze films are fabricated on the back surface of glass without damages to organic active layer by using a soft lithographic technique with etched GaN molds. With a rigorous coupled-wave analysis of optical modeling, we find the shift of resonance peaks with the increase of pattern's diameter. Wavelength-scale structures could provide the number of resonances at the long wavelength spectrum (λ = 650-800 nm), yielding enhancement of power conversion efficiency (PCE) in the PSCs. Compared with a flat device (PCE = 7.12%, Jsc = 15.6 mA/cm(2)), improved PCE of 8.41% is achieved in a haze film, which is mainly due to the increased short circuit current density (Jsc) of 17.5 mA/cm(2). Hence, it opens up exciting opportunities for a variety of PSCs with wavelength-scale structures to further improve performance, simplify complicated process, and reduce costs.

A Challenge Beyond Bottom Cells: Top-Illuminated Flexible Organic Solar Cells with Nanostructured Dielectric/Metal/Polymer (DMP) Films.

Top-illuminated flexible organic solar cells with a high power conversion efficiency (≈6.75%) are fabricated using a dielectric/metal/polymer (DMP) electrode. Employing a polymer layer (n = 1.49) makes it possible to show the high transmittance, which is insensitive to film thickness, and the excellent haze induced by well-ordered nanopatterns on the DMP electrode, leading to a 28% of enhancement in efficiency compared to bottom cells.

Three-dimensional nanobranched indium-tin-oxide anode for organic solar cells.

A nanostructured three-dimensional (3D) electrode using transparent conducting oxide (TCO) is an effective approach for increasing the efficiency of optoelectronic devices used in daily life. Tin-doped indium oxide (ITO) is a representative TCO with high conductivity and a high work function for anode applications. This paper reports the fabrication of a large-area ITO nanostructure with a branch shape using an electron beam evaporation process at temperatures as low as 80 °C, which was free of any carrier gas and catalyst. The large surface to volume ratio in the anode by the ITO nanobranches increases both the hole mobility by a 3D pathway and light absorbance by scattering, resulting in organic solar cells with a 12% increase in photocurrent and 20% photoconversion efficiency based on the bulk heterojunction of P3HT [region-regular poly(3-hexylthiophene)] and PCBM [phenyl-C61-butyric acid methyl ester].