Globally increasing energy demands and environmental concerns related to the use of fossil fuels have stimulated extensive research to identify new energy systems and economies that are sustainable, clean, low cost, and environmentally benign. Hydrogen generation from solar-driven water splitting is a promising strategy to store solar energy in chemical bonds. The subsequent combustion of hydrogen in fuel cells produces electric energy, and the only exhaust is water. These two reactions compose an ideal process to provide clean and sustainable energy. In such a process, a hydrogen evolution reaction (HER), an oxygen evolution reaction (OER) during water splitting, and an oxygen reduction reaction (ORR) as a fuel cell cathodic reaction are key steps that affect the efficiency of the overall energy conversion. Catalysts play key roles in this process by improving the kinetics of these reactions. Porphyrin-based and corrole-based systems are versatile and can efficiently catalyze the ORR, OER, and HER. Becau...

Energy-Related Small Molecule Activation Reactions: Oxygen Reduction and Hydrogen and Oxygen Evolution Reactions Catalyzed by Porphyrin- and Corrole-Based Systems

A novel graphene-bridged Ag3PO4/Ag/BiVO4 (040) Z-scheme heterojunction with excellent visible-light-driven photocatalytic performance was fabricated using a facile in situ deposition method followed by photo-reduction. The as-obtained nanocomposite was employed to degrade tetracycline (TC) in water under visible light irradiation. Compared to pure BiVO4, Ag3PO4 and other nanocomposites, Ag/Ag3PO4/BiVO4/RGO displayed more superior photodegradation efficiency with 94.96% removal of TC (10 mg/L) in 60 min, where the optimal conditions was catalysis dosage 0.50 g/L and initial pH at ca. 6.75. The influences of TC concentrations, light irradiation condition, coexistence ions and water sources were also investigated in details. The enhanced photocatalytic activities could be attributed to the suppression of charge recombination, high specific surface area and desirable absorption capability of Ag/Ag3PO4/BiVO4/RGO, which were in sequence confirmed by PL, PC, EIS, BET and DRS tests. The synergistic effects of RGO and Ag/Ag3PO4 in the hybrid could also contribute to the improved photo-stability and recyclability towards TC decomposition. In addition, radical trapping experiments and ESR measurement revealed that the photo-induced active species superoxide radical ( O2−) and holes (h+) were the predominant active species in the photocatalytic system. The Ag/Ag3PO4/BiVO4/RGO nanocomposite also possessed desirable photocatalytic performance on the degradation of TC from real wastewater, further verifying its potential in practical industries. This work provides a promising approach to construct visible-light response and more stabilized nanocomposite photocatalysts applied in efficient treatment of persistent pollutants in wastewater.

Hierarchical assembly of graphene-bridged Ag3PO4/Ag/BiVO4 (040) Z-scheme photocatalyst: An efficient, sustainable and heterogeneous catalyst with enhanced visible-light photoactivity towards tetracycline degradation under visible light irradiation

In this review, for a variety of metals and semiconductors, an attempt is made to generalize observations in the literature on the effect of process conditions applied during photodeposition on (i) particle size distributions, (ii) oxidation states of the metals obtained, and (iii) consequences for photocatalytic activities. Process parameters include presence or absence of (organic) sacrificial agents, applied pH, presence or absence of an air/inert atmosphere, metal precursor type and concentration, and temperature. Most intensively reviewed are studies concerning (i) TiO2; (ii) ZnO, focusing on Ag deposition; (iii) WO3, with a strong emphasis on the photodeposition of Pt; and (iv) CdS, again with a focus on deposition of Pt. Furthermore, a detailed overview is given of achievements in structure-directed photodeposition, which could ultimately be employed to obtain highly effective photocatalytic materials. Finally, we provide suggestions for improvements in description of the photodeposition methods applied when included in scientific papers.

Methods, Mechanism, and Applications of Photodeposition in Photocatalysis: A Review.

Bismuth vanadate (BiVO4) is a promising visible-light driven semiconductor photocatalyst with various benefits such as low production cost, low toxicity, high photostability, resistance to photo-corrosion and narrow band gap with a good response to visible-light excite. However, the fast recombination of photoinduced charge carriers restricts their photocatalytic activity. In the past decades, many attempts were adopted to enhance the photocatalytic activity of BiVO4. Significant advances in understanding the fundamental issues and the development of an efficient photocatalyst have been made in current years. In this review, we have provided a comprehensive overview of the latest progress on the morphology control and growth mechanism of BiVO4 micro/nano-structures, doping with metal and non-metal elements and semiconductor coupling along with some highlights in the photodegradation of organic pollutants under visible-light illumination. This review may benefit the researchers and engineers in the arena of material chemistry for designing new BiVO4 based photocatalysts with low production cost and high efficiency.

A review on BiVO4 photocatalyst: Activity enhancement methods for solar photocatalytic applications

Bismuth-based semiconductors are a unique and promising group of recently developed advanced photocatalytic materials. They have been widely applied in several areas including the generation of H2 by splitting water, decomposition of organic and inorganic pollutants in both wastewater and polluted air, and organic synthesis through harvesting the energy of light. The electronic structure of bismuth-based semiconductors confers them with a suitable band gap for visible-light response and a well-dispersed valence band composed of hybrid orbitals of Bi6s and O2p, making them a promising candidate when compared to other metal oxide semiconductors. In addition, they are simple to operate and prepare with controlled morphologies, making them attractive as potential photocatalysts. The purposes of this review are (1) summarization of advanced bismuth-based compounds that have been applied to date, (2) statement of challenges facing this material and possible approaches to overcome them, and (3) suggestions for future work.

Bismuth-based photocatalytic semiconductors: Introduction, challenges and possible approaches

Recently, photocatalytic processes have been shown to be a promising low-cost and sustainable alternative for water and wastewater treatment. In this study Ag/BiVO4 composites were fabricated and their photocatalytic disinfection activity was tested against Escherichia coli under visible light (λ > 420 nm). Deposition of silver nanoparticles on the surface of BiVO4 led to a significant improvement of the photocatalytic activity. Irradiation of the suspension of Escherichia coli (107 CFU mL−1) in the presence of Ag/BiVO4 resulted in the total disinfection of the cells within 3 h. The photocatalytic activity of the composite was stable in repeated runs. The disinfection study was also conducted under the tropical afternoon sun in Singapore. The significant enhancement in the photocatalytic activity of Ag/BiVO4 can be ascribed to the effect of metallic silver nanoparticles, which act as an electron trap on the surface of BiVO4 and promote the separation of photo-induced electron/hole pairs for the generation of reactive oxygen species.

Effect of depositing silver nanoparticles on BiVO4 in enhancing visible light photocatalytic inactivation of bacteria in water

The development of artificial photosynthesis systems that can efficiently catalyze water oxidation to generate oxygen remains one of the most important challenges in solar energy conversion to chemical energy. In photosystem II (PSII), the Mn4CaO5 cluster adopts a distorted coordination geometry and every two octahedra are linked by di-μ-oxo (edge-shared) or mono-μ-oxo (corner-shared) bridges, which is recognized as a critical structure motif for catalytic water oxidation. These structural features provide guidance on the design and synthesis of new water oxidation catalysts. Herein we synthesized a new layered organic cobalt phosphonate crystal, Co3(O3PCH2–NC4H7–CO2)2·4H2O (1) and demonstrate it as a heterogeneous catalyst for water oxidation. Its catalytic activity was compared to those of cobalt phosphonates with different structures (2–4) in terms of O2 evolution rate and O2 yield under the same reaction conditions. The compound with both mono- and di-μ-oxo bridged octahedral cobalt displays superior catalytic activity. In contrast, the presence of only mono-μ-oxo bridged cobalt in the structure results in lower O2 yield and O2 evolution rate. Further structural analysis reveals that the presence of a longer Co–N bond induces a distorted dissymmetry coordination geometry, and consequently facilitates water oxidation. These results provide important insight into the design of water oxidation catalysts.

Bio-inspired organic cobalt(II) phosphonates toward water oxidation

We introduced photon-trapping microstructures into GeSn-based photodetectors for the first time, and achieved high-efficiency photo detection at 2 µm with a responsivity of 0.11 A/W. The demonstration was realized by a GeSn/Ge multiple-quantum-well (MQW) p-i-n photodiode on a GeOI architecture. Compared with the non-photon-trapping counterparts, the patterning and etching of photon-trapping microstructure can be processed in the same step with mesa structure at no additional cost. A four-fold enhancement of photo response was achieved at 2 µm. Although the incorporation of photo-trapping microstructure degrades the dark current density which increases from 31.5 to 45.2 mA/cm2 at -1 V, it benefits an improved 3-dB bandwidth of 2.7 GHz at bias voltage at -5 V. The optical performance of GeSn/Ge MQW photon-trapping photodetector manifests its great potential as a candidate for efficient 2 µm communication. Additionally, the underlying GeOI platform enables its feasibility of monolithic integration with other photonic components such as waveguide, modulator and (de)multiplexer for optoelectronic integrated circuits (OEICs) operating at 2 µm.

High-efficiency GeSn/Ge multiple-quantum-well photodetectors with photon-trapping microstructures operating at 2 µm.

Spiral waveguides on a new germanium-on-silicon nitride (GON) platform with a wide transparency and a large core-clad index contrast for mid-infrared (mid-IR) sensing applications are demonstrated. Spiral waveguide sensors with a low bending loss on this platform enable compact sensors for mid-IR absorption spectroscopy. A minimum volumetric concentration of 5% isopropanol (IPA) in an IPA–acetone mixture is measured. This detection limit is three times lower than the counterpart waveguide, fabricated on the regular germanium-on-silicon platform with similar propagation loss at 3.73 μm wavelength. This silicon-compatible GON sensor is promising for applications such as environmental studies, industrial leak detection, process control, medical breath analysis, and many more.

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Simon Chun Kiat Goh

Papers

Effect of depositing silver nanoparticles on BiVO4 in enhancing visible light photocatalytic inactivation of bacteria in water

Bio-inspired organic cobalt(II) phosphonates toward water oxidation

High-efficiency GeSn/Ge multiple-quantum-well photodetectors with photon-trapping microstructures operating at 2 µm.

Spiral Waveguides on Germanium-on-Silicon Nitride Platform for Mid-IR Sensing Applications

Ar/N2 Plasma Induced Metastable CuxNy for Cu-Cu Direct Bonding