Golam Kibria

Wafer-level photocatalytic water splitting on GaN nanowire arrays grown by molecular beam epitaxy.

We report on the achievement of wafer-level photocatalytic overall water splitting on GaN nanowires grown by molecular beam epitaxy with the incorporation of Rh/Cr(2)O(3) core-shell nanostructures acting as cocatalysts, through which H(2) evolution is promoted by the noble metal core (Rh) while the water forming back reaction over Rh is effectively prevented by the Cr(2)O(3) shell O(2) diffusion barrier. The decomposition of pure water into H(2) and O(2) by GaN nanowires is confirmed to be a highly stable photocatalytic process, with the turnover number per unit time well exceeding the value of any previously reported GaN powder samples.

One-step overall water splitting under visible light using multiband InGaN/GaN nanowire heterostructures.

The conversion of solar energy into hydrogen via water splitting process is one of the key sustainable technologies for future clean, storable, and renewable source of energy. Therefore, development of visible light-responsive and efficient photocatalyst material has been of immense interest, but with limited success. Here, we show that overall water splitting under visible-light irradiation can be achieved using a single photocatalyst material. Multiband InGaN/GaN nanowire heterostructures, decorated with rhodium (Rh)/chromium-oxide (Cr2O3) core-shell nanoparticles can lead to stable hydrogen production from pure (pH ∼ 7.0) water splitting under ultraviolet, blue and green-light irradiation (up to ∼560 nm), the longest wavelength ever reported. At ∼440-450 nm wavelengths, the internal quantum efficiency is estimated to be ∼13%, the highest value reported in the visible spectrum. The turnover number under visible light well exceeds 73 in 12 h. Detailed analysis further confirms the stable photocatalytic activity of the nanowire heterostructures. This work establishes the use of metal-nitrides as viable photocatalyst for solar-powered artificial photosynthesis for the production of hydrogen and other solar fuels.

Remarkably enhanced photocatalytic activity of laser ablated Au nanoparticle decorated BiFeO3 nanowires under visible-light

Hybrid photocatalysts consisting of single crystalline BiFeO3 nanowires and laser ablated Au nanoparticles were synthesized by a functionalization-step-free solution process. The 1.0 wt% Au nanoparticle decorated BiFeO3 nanowires exhibit ~30 times higher photocatalytic activity for water oxidation than that exhibited by the parent wires during the first 4 h.

Breaking the Carrier Injection Bottleneck of Phosphor-Free Nanowire White Light-Emitting Diodes

We have examined the carrier injection process of axial nanowire light-emitting diode (LED) structures and identified that poor carrier injection efficiency, due to the large surface recombination, is the primary cause for the extremely low output power of phosphor-free nanowire white LEDs. We have further developed InGaN/GaN/AlGaN dot-in-a-wire core-shell white LEDs on Si substrate, which can break the carrier injection efficiency bottleneck, leading to a massive enhancement in the output power. At room temperature, the devices can exhibit an output power of ~1.5 mW, which is more than 2 orders of magnitude stronger than nanowire LEDs without shell coverage. Additionally, such phosphor-free nanowire white LEDs can deliver an unprecedentedly high color rendering index of ~92-98 in both the warm and cool white regions, with the color rendering capability approaching that of an ideal light source, i.e. a blackbody.

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.

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.

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

Phosphor-free InGaN/GaN/AlGaN core-shell dot-in-a-wire white light-emitting diodes

We have developed phosphor-free InGaN/GaN/AlGaN dot-in-a-wire core-shell white light emitting diodes, which can break the carrier injection efficiency bottleneck of conventional nanowire white light emitting diodes, leading to a dramatic enhancement of the output power. Additionally, such phosphor-free nanowire white light emitting diodes can deliver a very high color rendering index (CRI) of ~92-98.