Wafer-Level Artificial Photosynthesis for CO₂ Reduction into CH₄ and CO Using GaN Nanowires

We report on the first demonstration of high-conversion-rate photochemical reduction of carbon dioxide (CO₂) on gallium nitride (GaN) nanowire arrays into methane (CH₄) and carbon monoxide (CO). It was observed that the reduction of CO₂ to CO dominates on as-grown GaN nanowires under ultraviolet light irradiation. However, the production of CH₄ is significantly increased by using the Rh/Cr₂O₃ core/shell cocatalyst, with an average rate of ∼3.5 μmol g_cat^–1 h^–1 in 24 h. In this process, the rate of CO₂ to CO conversion is suppressed by nearly an order of magnitude. The rate of photoreduction of CO₂ to CH₄ can be further enhanced and can reach ∼14.8 μmol g_cat^–1 h^–1 by promoting Pt nanoparticles on the lateral <i>m</i>-plane surfaces of GaN nanowires, which is nearly an order of magnitude higher than that measured on as-grown GaN nanowire arrays. This work establishes the potential use of metal-nitride nanowire arrays as a highly efficient photocatalyst for the direct photoreduction of CO₂ into chemical fuels. It also reveals the potential of engineered core/shell cocatalysts in improving the selectivity toward more valuable fuels

Photoinduced Conversion of Methane into Benzene over GaN Nanowires

As a class of key building blocks in the chemical industry, aromatic compounds are mainly derived from the catalytic reforming of petroleum-based long chain hydrocarbons. The dehydroaromatization of methane can also be achieved by using zeolitic catalysts under relatively high temperature. Herein we demonstrate that Si-doped GaN nanowires (NWs) with a 97% rationally constructed <i>m</i>-plane can directly convert methane into benzene and molecular hydrogen under ultraviolet (UV) illumination at rt. Mechanistic studies suggest that the exposed <i>m</i>-plane of GaN exhibited particularly high activity toward methane C–H bond activation and the quantum efficiency increased linearly as a function of light intensity. The incorporation of a Si-donor or Mg-acceptor dopants into GaN also has a large influence on the photocatalytic performance

High Efficiency Solar-to-Hydrogen Conversion on a Monolithically Integrated InGaN/GaN/Si Adaptive Tunnel Junction Photocathode

H₂ generation under sunlight offers great potential for a sustainable fuel production system. To achieve high efficiency solar-to-hydrogen conversion, multijunction photoelectrodes have been commonly employed to absorb a large portion of the solar spectrum and to provide energetic charge carriers for water splitting. However, the design and performance of such tandem devices has been fundamentally limited by the current matching between various absorbing layers. Here, by exploiting the lateral carrier extraction scheme of one-dimensional nanowire structures, we have demonstrated that a dual absorber photocathode, consisting of p-InGaN/tunnel junction/n-GaN nanowire arrays and a Si solar cell wafer, can operate efficiently without the strict current matching requirement. The monolithically integrated photocathode exhibits an applied bias photon-to-current efficiency of 8.7% at a potential of 0.33 V versus normal hydrogen electrode and nearly unity Faradaic efficiency for H₂ generation. Such an adaptive multijunction architecture can surpass the design and performance restrictions of conventional tandem photoelectrodes

15.3%-Efficient GaAsP Solar Cells on GaP/Si Templates

As single-junction Si solar cells approach their practical efficiency limits, a new pathway is necessary to increase efficiency in order to realize more cost-effective photovoltaics. Integrating III–V cells onto Si in a multijunction architecture is a promising approach that can achieve high efficiency while leveraging the infrastructure already in place for Si and III–V technology. In this Letter, we demonstrate a record 15.3%-efficient 1.7 eV GaAsP top cell on GaP/Si, enabled by recent advances in material quality in conjunction with an improved device design and a high-performance antireflection coating. We further present a separate Si bottom cell with a 1.7 eV GaAsP optical filter to absorb most of the visible light with an efficiency of 6.3%, showing the feasibility of monolithic III–V/Si tandems with >20% efficiency. Through spectral efficiency analysis, we compare our results to previously published GaAsP and Si devices, projecting tandem GaAsP/Si efficiencies of up to 25.6% based on current state-of-the-art individual subcells. With the aid of modeling, we further illustrate a realistic path toward 30% GaAsP/Si tandems for high-efficiency, monolithically integrated photovoltaics