Comparison of Ga2O3 and TiO2 Nanostructures for Photocatalytic Degradation of Volatile Organic Compounds

Synthesis of Petitjeanite Bi3O(OH)(PO4)2 Photocatalytic Microparticles: Effect of Synthetic Conditions on the Crystal Structure and Activity toward Degradation of Aqueous Perfluorooctanoic Acid (PFOA)

The discovery of synthetic Bi₃O(OH)(PO₄)₂ [BOHP] and its application toward photocatalytic oxidation of the water contaminant perfluorooctanoic acid (PFOA) have prompted further interest in development. Despite its high activity toward PFOA degradation, the scarce appearance in the literature and lack of research have left a knowledge gap in the understanding of BOHP synthesis, formation, and photocatalytic activity. Herein, we explore the crystallization of BOHP microparticles via hydrothermal syntheses, focusing on the influence of ions and organics present in the reaction solution when using different hydroxide amendments (NaOH, NH₄OH, NMe₄OH, and NEt₄OH). To better understand the unique structure-activity aspects of BOHP, the related bismuth oxy phosphate (BOP) structural family was also explored, including A-BOP (A = Na⁺ and K⁺) and M-BOP derivatives (M = Ca²⁺, Sr²⁺, and Pb²⁺). Results from materials characterization, including X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy, indicated that the crystal structure, morphology, and atomic composition were significantly influenced by solution pH, inorganic metal cations (Na⁺, K⁺, Ca²⁺, Sr²⁺, and Pb²⁺), and organic amines. Experiments involving ultraviolet photocatalytic destruction of PFOA by a BOHP suspension revealed that catalytic activity was influenced by the choice of reagents and their variable effect on surface facet growth and crystal defects in the resulting microparticles. Together, this work provides a strategy for crystal facet and surface defect engineering with the potential to expand to other metal oxides within the hydrothermal system.

Impact of Al doping on a hydrothermally synthesized β-Ga2O3 nanostructure for photocatalysis applications

Aluminum (Al)-doped beta-phase gallium oxide (β-Ga₂O₃) nanostructures with different Al concentrations (0 to 3.2 at%) are synthesized using a hydrothermal method. The single phase of the β-Ga₂O₃ is maintained without intermediate phases up to Al 3.2 at% doping. As the Al concentration in the β-Ga₂O₃ nanostructures increases, the optical bandgap of the β-Ga₂O₃ increases from 4.69 (Al 0%) to 4.8 (Al 3.2%). The physical, chemical, and optical properties of the Al-doped β-Ga₂O₃ nanostructures are correlated with photocatalytic activity via the degradation of a methylene blue solution under ultraviolet light (254 nm) irradiation. The photocatalytic activity is enhanced by doping a small amount of substitutional Al atoms (0.6 at%) that presumably create shallow level traps in the band gap. These shallow traps retard the recombination process by separating photogenerated electron–hole pairs. On the other hand, once the Al concentration in the Ga₂O₃ exceeds 0.6 at%, the crystallographic disorder, oxygen vacancy, and grain boundary-related defects increase as the Al concentration increases. These defect-related energy levels are broadly distributed within the bandgap, which act as carrier recombination centers and thereby degrade the photocatalytic activity. The results of this work provide new opportunities for the synthesis of highly effective β-Ga₂O₃-based photocatalysts that can generate hydrogen gas and remove harmful volatile organic compounds.

The photocatalytic activity is correlated with different parameters affecting the photocatalytic reactions; redox potential (RP), surface area (SA), crystal defect (CD), oxygen defect (OD), and grain-boundary induced defect (GD).

Fast-Response Colorimetric UVC Sensor Made of a Ga2O3 Photocatalyst with a Hole Scavenger

A fast-response colorimetric ultraviolet-C (UVC) sensor was demonstrated using a gallium oxide (Ga₂O₃) photocatalyst with small amounts of triethanolamine (TEOA) in methylene blue (MB) solutions and a conventional RGB photodetector. The color of the MB solution changed upon UVC exposure, which was observed using an in situ RGB photodetector. Thereby, the UVC exposure was numerically quantified as an MB reduction rate with the R value of the photodetector, which was linearly correlated with the measured spectral absorbance using a UV-Vis spectrophotometer. Small amount of TEOA in the MB solution served as a hole scavenger, which resulted in fast MB color changes due to the enhanced charge separation. However, excessive TEOA over 5 wt.% started to block the catalytical active site on the surface of Ga₂O₃, prohibiting the chemical reaction between the MB molecules and catalytic sites. The proposed colorimetric UVC sensor could monitor the detrimental UVC radiation with high responsivity at a low cost.

Synthesis and Photocatalytic Activity of β-Ga2O3 Nanostructures for Decomposition of Formaldehyde under Deep Ultraviolet Irradiation

An attempt to degrade volatile organic compounds (VOCs) and sterilize air simultaneously is highly desirable to improve indoor air quality. With the help of deep ultraviolet (UVC) lighting, harmful bacteria that exists in the air can be destroyed. Thus, a new photocatalytic substance that can break down VOCs under UVC irradiation is required. Here, we demonstrate the photocatalytic activity of β-Ga2O3 nanostructures, synthesized via the solvothermal method for removing formaldehyde (HCHO) under deep ultraviolet irradiation. The Raman and XRD results indicated that as-synthesized nanostructures showed β-crystalline phase with a monoclinic structure. The photoluminescence spectrum exhibited a broad and strong green emission peak at 510 nm, which was likely attributed to a considerable amount of oxygen and gallium vacancies formed during the calcinating process. The photocatalytic efficiency for decomposing HCHO at room temperature under deep ultraviolet irradiation (278 nm) of the synthesized β-Ga2O3 nanoparticles is higher than that of the β-Ga2O3 nanorods. Both nanoparticles and nanorods obeyed the pseudo-first-order Langmuir-Hinshelwood kinetic model with a degradation rate constant of 0.057 and 0.033 min−1, corresponding to the efficiency of 82% and 62% in the formaldehyde removal, respectively.