Faqrul Alam Chowdhury

Visible light-driven efficient overall water splitting using p -type metal-nitride nanowire arrays

Solar water splitting for hydrogen generation can be a potential source of renewable energy for the future. Here we show that efficient and stable stoichiometric dissociation of water into hydrogen and oxygen can be achieved under visible light by eradicating the potential barrier on nonpolar surfaces of indium gallium nitride nanowires through controlled p-type dopant incorporation. An apparent quantum efficiency of ∼12.3% is achieved for overall neutral (pH∼7.0) water splitting under visible light illumination (400-475 nm). Moreover, using a double-band p-type gallium nitride/indium gallium nitride nanowire heterostructure, we show a solar-to-hydrogen conversion efficiency of ∼1.8% under concentrated sunlight. The dominant effect of near-surface band structure in transforming the photocatalytic performance is elucidated. The stability and efficiency of this recyclable, wafer-level nanoscale metal-nitride photocatalyst in neutral water demonstrates their potential use for large-scale solar-fuel conversion.

Tuning the surface Fermi level on p-type gallium nitride nanowires for efficient overall water splitting

Solar water splitting is one of the key steps in artificial photosynthesis for future carbon-neutral, storable and sustainable source of energy. Here we show that one of the major obstacles for achieving efficient and stable overall water splitting over the emerging nanostructured photocatalyst is directly related to the uncontrolled surface charge properties. By tuning the Fermi level on the nonpolar surfaces of gallium nitride nanowire arrays, we demonstrate that the quantum efficiency can be enhanced by more than two orders of magnitude. The internal quantum efficiency and activity on p-type gallium nitride nanowires can reach ~51% and ~4.0 mol hydrogen h(-1) g(-1), respectively. The nanowires remain virtually unchanged after over 50,000 μmol gas (hydrogen and oxygen) is produced, which is more than 10,000 times the amount of photocatalyst itself (~4.6 μmol). The essential role of Fermi-level tuning in balancing redox reactions and in enhancing the efficiency and stability is also elucidated.

Atomic-Scale Origin of Long-Term Stability and High Performance of p-GaN Nanowire Arrays for Photocatalytic Overall Pure Water Splitting

The atomic-scale origin of the unusually high performance and long-term stability of wurtzite p-GaN oriented nanowire arrays is revealed. Nitrogen termination of both the polar (0001¯) top face and the nonpolar (101¯0) side faces of the nanowires is essential for long-term stability and high efficiency. Such a distinct atomic configuration ensures not only stability against (photo) oxidation in air and in water/electrolyte but, as importantly, also provides the necessary overall reverse crystal polarization needed for efficient hole extraction in p-GaN.

Group III-nitride nanowire structures for photocatalytic hydrogen evolution under visible light irradiation

The performance of photochemical water splitting over the emerging nanostructured photocatalysts is often constrained by their surface electronic properties, which can lead to imbalance in redox reactions, reduced efficiency, and poor stability. We have investigated the impact of surface charge properties on the photocatalytic activity of InGaN nanowires. By optimizing the surface charge properties through controlled p-type dopant (Mg) incorporation, we have demonstrated an apparent quantum efficiency of ∼17.1% and ∼12.3% for InGaN nanowire arrays under visible light irradiation (400 nm–490 nm) in aqueous methanol and in the overall neutral-pH water splitting reaction, respectively.

Defect-engineered GaN:Mg nanowire arrays for overall water splitting under violet light

We report that by engineering the intra-gap defect related energy states in GaN nanowire arrays using Mg dopants, efficient and stable overall neutral water splitting can be achieved under violet light. Overall neutral water splitting on Rh/Cr2O3 co-catalyst decorated Mg doped GaN nanowires is demonstrated with intra-gap excitation up to 450 nm. Through optimized Mg doping, the absorbed photon conversion efficiency of GaN nanowires reaches ∼43% at 375–450 nm, providing a viable approach to extend the solar absorption of oxide and non-oxide photocatalysts.

Photoelectrochemical reduction of carbon dioxide using Ge doped GaN nanowire photoanodes

We report on the direct conversion of carbon dioxide (CO2) in a photoelectrochemical cell consisting of germanium doped gallium nitride nanowire anode and copper (Cu) cathode. Various products including methane (CH4), carbon monoxide (CO), and formic acid (HCOOH) were observed under light illumination. A Faradaic efficiency of ∼10% was measured for HCOOH. Furthermore, this photoelectrochemical system showed enhanced stability for 6 h CO2 reduction reaction on low cost, large area Si substrates.

Dye-sensitized InGaN nanowire arrays for efficient hydrogen production under visible light irradiation

Solar water splitting is a key sustainable energy technology for clean, storable and renewable source of energy in the future. Here we report that Merocyanine-540 dye-sensitized and Rh nanoparticle-decorated molecular beam epitaxially grown In0.25Ga0.75N nanowire arrays have produced hydrogen from ethylenediaminetetraacetic acid (EDTA) and acetonitrile mixture solution under green, yellow and orange solar spectra (up to 610 nm) for the first time. An apparent quantum efficiency of 0.3% is demonstrated for wavelengths 525-600 nm, providing a viable approach to harness deep-visible and near-infrared solar energy for efficient and stable water splitting.

A photochemical diode artificial photosynthesis system for unassisted high efficiency overall pure water splitting

The conversion of solar energy into chemical fuels can potentially address many of the energy and environment related challenges we face today. In this study, we have demonstrated a photochemical diode artificial photosynthesis system that can enable efficient, unassisted overall pure water splitting without using any sacrificial reagent. By precisely controlling charge carrier flow at the nanoscale, the wafer-level photochemical diode arrays exhibited solar-to-hydrogen efficiency ~3.3% in neutral (pH ~ 7.0) overall water splitting reaction. In part of the visible spectrum (400–485 nm), the energy conversion efficiency and apparent quantum yield reaches ~8.75% and ~20%, respectively, which are the highest values ever reported for one-step visible-light driven photocatalytic overall pure water splitting. The effective manipulation and control of charge carrier flow in nanostructured photocatalysts provides critical insight in achieving high efficiency artificial photosynthesis, including the efficient and selective reduction of CO2 to hydrocarbon fuels.A major challenge facing solar-to-fuel technologies is the integration of light-absorbing and catalytic components into efficient water-splitting devices. Here, the authors construct a photochemical diode array to harvest visible light and split pure water at high solar-to-hydrogen efficiencies.

Optically active dilute-antimonide III-nitride nanostructures for optoelectronic devices

We have studied the epitaxy, energy bandgap, and structural and optical properties of GaSbN nanostructures in the dilute antimony (Sb) limit (Sb concentration < 1%). GaSbN nanowire structures are grown on a Si substrate by plasma-assisted molecular beam epitaxy. It is observed, both theoretically and experimentally, that the incorporation of a very small amount of Sb (<1%) in GaN can substantially reduce the energy bandgap of GaN from 3.4 eV to ∼2 eV. We have further demonstrated that emission wavelengths of GaSbN nanowires can be tuned from ∼365 nm to 600 nm at room-temperature by varying the Sb incorporation. Functional GaSbN nanowire light-emitting diodes are also demonstrated, which exhibit strong emission in the deep-visible spectral range.

Achieving artificial photosynthesis with nanowires

For decades, researchers have been striving to develop an efficient, stable, and cost-effective photocatalyst that can decompose water into its constituents (i.e., hydrogen and oxygen) by employing solar energy. This process, known as artificial photosynthesis, promises to be one of the key sustainable energy technologies of the future, enabling clean, storable, and affordable energy (i.e., hydrogen and other fuels) from just sunlight and water. Although significant progress has been made over the last decade, low yield, photocatalyst instability, and ineffective use of the solar spectrum represent the key bottlenecks standing in the way of the translation of this technology from research and development to the marketplace. Researchers have thus far been largely focused on metal oxide-based photocatalysts, due to their photostability in aqueous solution. However, because of their large bandgap and poor optoelectronic properties, most metal oxides are incapable of harnessing visible solar photons, which comprise 43% of the solar spectrum. To overcome this issue, we have been working on the development of group III-nitride photocatalysts, which due to their tunable bandgaps are capable of harnessing the majority of the solar spectrum (200–2000nm). Additionally, IIInitride possesses near-perfect band-edge positions (its bandgap can straddle the redox potential of water) and extreme stability against photocorrosion. To enhance the effective reaction surface area, we have grown nearly defect-free 1D gallium nitride (GaN) and indium GaN (InGaN) nanowires with high surfaceto-volume ratio by plasma-assisted molecular beam epitaxy (see Figure 1). Although III-nitride materials have many ideal characteristics for solar water-splitting applications, they also possess drawbacks. Due to the presence of surface states and/or defects, the conduction and valence bands at the surface of these materials can be subject to upward or downward bending, caused by the Figure 1. Scanning electron microscopy (SEM) image of as-grown p-type gallium nitride (p-GaN) nanowire arrays on a silicon (111) substrate.3