Investigation of the Photocatalytic Hydrogen Production of Semiconductor Nanocrystal-Based Hydrogels

Destabilization of a ligand-stabilized semiconductor nanocrystal solution with an oxidizing agent can lead to a macroscopic highly porous self-supporting nanocrystal network entitled hydrogel, with good accessibility to the surface. The previously reported charge carrier delocalization beyond a single nanocrystal building block in such gels can extend the charge carrier mobility and make a photocatalytic reaction more probable. The synthesis of ligand-stabilized nanocrystals with specific physicochemical properties is possible, thanks to the advances in colloid chemistry made in the last decades. Combining the properties of these nanocrystals with the advantages of nanocrystal-based hydrogels will lead to novel materials with optimized photocatalytic properties. This work demonstrates that CdSe quantum dots, CdS nanorods, and CdSe/CdS dot-in-rod-shaped nanorods as nanocrystal-based hydrogels can exhibit a much higher hydrogen production rate compared to their ligand-stabilized nanocrystal solutions. The gel synthesis through controlled destabilization by ligand oxidation preserves the high surface-to-volume ratio, ensures the accessible surface area even in hole-trapping solutions and facilitates photocatalytic hydrogen production without a co-catalyst. Especially with such self-supporting networks of nanocrystals, the problem of colloidal (in)stability in photocatalysis is circumvented. X-ray photoelectron spectroscopy and photoelectrochemical measurements reveal the advantageous properties of the 3D networks for application in photocatalytic hydrogen production.

Bi5O7I/g-C3N4 Heterostructures With Enhanced Visible-Light Photocatalytic Performance for Degradation of Tetracycline Hydrochloride

Bi5O7I/g-C3N4 p-n junctioned photocatalysts were synthesized by alcohol-heating and calcination in air. The structures, morphologies and optical properties of as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-Vis diffuse reflectance spectroscopy (DRS). Photocatalytic activity of the heterojunctioned composites were evaluated by degradation of Rhodamine B (RhB) and tetracycline hydrochloride (TCH) under visible light illumination. The results indicated that the composites exhibited superior efficiencies for photodegradation of RhB and TCH in comparison with pure BiOI, Bi5O7I and g-C3N4. An effective built-in electric field was formed by the interface between p-type Bi5O7I and n-type g-C3N4, which promoted the efficient separation of photoinduced electron-hole pairs. In addition, 8% Bi5O7I/g-C3N4 composite showed excellent photostability in a five-cycle photocatalytic experiment. Experiments on scavenging active intermediates revealed the roles of active species.

Improved photocatalytic activity and stability of InGaN quantum dots/C3N4heterojunction photoelectrode for CO2reduction and hydrogen production

Photocatalytic conversion of CO₂to produce fuel is considered a promising approach to reduce CO₂emissions and tackle energy crisis. GaN-based materials have been studied for CO₂reduction because of their excellent optical properties and band structure. However, low photocatalytic activity and severe photocorrosion of GaN-based photoelectrode greatly limit their applications. In this work, photocatalytic activity was improved by adopting InGaN quantum dots (QDs) combined with C₃N₄nano-sheets as photoanode, and thus the efficiency of CO₂reduction and the selectivity of hydrogen production were increased significantly. In addition, the photoelectron-chemical corrosion of photoelectrodes has been apparently controlled. InGaN QDs/C₃N₄has the highest CO and H₂productions rates of 14.69μmol mol^-1h^-1and 140μmol mol^-1h^-1which were 2.2 times and 14.5 times than that of InGaN film photoelectrode, respectively. The enhancement of photocatalytic activity is attributed to C₃N₄modification and a large electric dipole forming on the surface of InGaN QDs, which facilitate the separation and transfer of photo-generated carriers and thus promote CO₂reduction reaction. This work provides a promising strategy for the development of GaN-based photoanodes with superior stability and efficiency.

Enhancing Light Absorption and Prolonging Charge Separation in Carbon Quantum Dots via Cl-Doping for Visible-Light-Driven Photocharge-Transfer Reactions

Limited light absorption beyond the UV region and rapid photocarrier recombination are critical impediments for the improved photocatalytic performance of carbon quantum dots (CQDs) under visible-light irradiation. Herein, we demonstrate single-step microwave-assisted syntheses of O-CQDs (typical CQDs terminated by carboxylic and hydroxyl functional groups) from a sucrose precursor and Cl-doped CQDs (Cl-CQDs) from a sucralose precursor in short reaction times and without using obligatory strong acids for Cl doping. The doping of Cl into the CQDs is observed to widen the absorption range and facilitate an enhanced separation of photoexcited charge carriers, which is confirmed by the results of optical absorption, photothermal response, and pump-probe ultrafast transient absorption spectroscopy measurements of the O-CQDs and Cl-CQDs. The photoexcited charge carriers with their longer lifetimes in Cl-CQDs enabled the quick degradation of methylene blue dye, rapid conversion of Ag⁺ ions to metallic Ag nanoparticles on the CQD surfaces, and reduction of GO to a well-dispersed rGO through the photoelectron transfer reactions under visible-light irradiation. The facile Cl doping strategy, hybridization of Ag nanoparticles or rGO to CQDs, and the elevated charge separation mechanism would open up new avenues in designing CQD-based materials for futuristic applications.

Pb-Free Cs3Bi2I9 Perovskite as a Visible-Light-Active Photocatalyst for Organic Pollutant Degradation

In our work, we employed Cs₃Bi₂I₉ as a visible-light-active photocatalyst, synthesized with a low-temperature solvothermal method. The morphological and structural properties of the as-prepared perovskite were investigated, and the results were compared to previous studies to confirm its nature and the quality of the synthesis procedure. Transient absorption spectroscopy was applied in order to investigate the generation and lifetime of photogenerated charge carriers, revealing their formation after visible light excitation. The potential photocatalytic activity of the as-prepared metal halide perovskite was applied for the removal of Rhodamine B in aqueous solution, demonstrating an excellent activity of 93% after 180 min under visible-light irradiation. The current research aims to provide insights into the design of a new visible-light-active photocatalyst, Cs₃Bi₂I₉, selected for its high application value in the field of advanced materials for light harvesting.

Amine Functionalized Metal-Organic Framework Coordinated with Transition Metal Ions: d-d Transition Enhanced Optical Absorption and Role of Transition Metal Sites on Solar Light Driven H2 Production

Design and development of efficient photocatalysts for H₂ production from water and sunlight have gained significant attention as the solar assisted approach is considered to be a promising approach for the generation of clean fuel. However, the poor charge carrier separation and light harvesting ability of existing photocatalysts limits the efficiency of photoconversion of water. In this work, the synthesis of transition metal ions (M²⁺ = Co²⁺ , Cu²⁺ , and Ni²⁺ ) coordinated with Ti-metal organic frameworks (Ti-MOFs) through a simple post-synthetic coordination method for efficient solar light-driven H₂ production is reported. Notably, coordination of M²⁺ ions with Ti-MOF significantly improves the optical absorption by d-d transitions and the multimetal sites facilitate the fast charge carrier separation, thereby enhancing the solar light-driven H₂ production activity. Very interestingly, the rate of solar light-driven H₂ production is varied with respect to different metal ions coordination due to the position of d-d bands absorption in the solar spectrum, and the complexing tendency of M²⁺ ions with sacrificial electron donors. A maximum solar H₂ production rate of 1583.55 µmol h^-1 g^-1 is achieved with a Cu²⁺ coordinated Ti-MOF, which is ≈13 fold higher than that of the pristine Ti-MOF.

Conformal BiVO4-Layer/WO3-Nanoplate-Array Heterojunction Photoanode Modified with Cobalt Phosphate Cocatalyst for Significantly Enhanced Photoelectrochemical Performances

Constructing semiconductor heterojunctions via surface/interface engineering is an effective way to enhance the charge carrier separation/transport ability and thus the photoelectrochemical (PEC) properties of a photoelectrode. Herein, we report a conformal BiVO₄-layer/WO₃-nanoplate-array heterojunction photoanode modified with cobalt phosphate (Co-Pi) as oxygen evolution cocatalyst (OEC) for significant enhancement in PEC performances. The BiVO₄/WO₃ nanocomposite is fabricated by coating a thin conformal BiVO₄ layer on the surface of presynthesized WO₃ nanoplate arrays (NPAs) via stepwise spin-coating, and the decoration of Co-Pi OEC is realized by photoassisted electrodeposition method. The optimized Co-Pi@BiVO₄/WO₃ heterojunction photoanode shows a maximum photocurrent of 1.8 mA/cm² at 1.23 V vs RHE in a phosphate buffer electrolyte under an AM1.5G solar simulator, which is 5 and 12 times higher than those of bare WO₃ and BiVO₄ photoanode, respectively. Measurements of UV-vis absorption spectra, electrochemical impedance spectra (EIS) and photoluminescence (PL) spectra reveal that the enhanced PEC performances can be attributed to the increased charge carrier separation/transport benefited from the type II nature of BiVO₄/WO₃ heterojunction and the promoted water oxidation kinetics and photostability owing to the decoration of Co-Pi cocatalyst.

β-Cyclodextrin as a Precursor to Holey C-Doped g-C3 N4 Nanosheets for Photocatalytic Hydrogen Generation

A green, template-free and easy-to-implement strategy was developed to access holey g-C₃ N₄ (GCN) nanosheets doped with carbon. The protocol involves heating dicyandiamide with β-cyclodextrin (βCD) prior to polymerization. The local symmetry of the GCN skeleton is broken, yielding CxGCN (x corresponds to the initial amount of βCD used) with pores and a distorted structure. The electronic, emission, optical and textural properties of the best-performing material, C2GCN, were significantly modified as compared to bulk GCN. The spectroscopic and luminescent features of C2GCN show the characteristic π-π* electronic transition of GCN, accompanied by much stronger n-π* electronic transitions owing to the porous and distorted network. These new electronic transitions, along with the presence of additional carbon synergistically contributed to enhanced visible light absorption and restrained recombination of electron-hole pairs. Steady-state and time-resolved photoluminescence showed an effective quench of the fluorescence emission, accompanied by a decrease of fluorescence lifetime of C2GCN (2.20 ns) in comparison with GCN (5.85 ns), owing to the delocalization of electron and holes to new recombination centers. The photocatalytic activity of C2GCN was attributed to efficient charge carrier separation and improved visible-light absorbing ability. As result, C2GCN exhibited ∼5 times higher photocatalytic H₂ generation under visible light than bulk GCN.