Solvothermal fabrication of MoS2 anchored on ZnIn2S4 microspheres with boosted photocatalytic hydrogen evolution activity

The MoS₂/ZnIn₂S₄ composites with MoS₂ anchored on the surface of ZnIn₂S₄ microspheres were fabricated by a facile solvothermal method. To clarify the crystal phases, morphologies, chemical compositions, optical properties, and special surface areas of the obtained photocatalysts, the corresponding characterization measurements were performed. The photocatalytic H₂ evolution activities of MoS₂/ZnIn₂S₄ composites were evaluated and compared with using lactic acid as sacrificial reagents. The results showed that integrating MoS₂ with ZnIn₂S₄ could remarkably boost the photocatalytic H₂ evolution performance and the maximum H₂ evolution rate of 201 μmol h^?1 was achieved over 1 wt% MoS₂ loading on the ZnIn₂S₄, corresponding to the apparent quantum efficiency (AQE) about 3.08% at 420 nm monochromatic light. The photoelectrochemical tests and photoluminescence spectra (PL) versified that the efficient charge transfer and separation were achieved over MoS₂/ZnIn₂S₄ composite in contrast with single ZnIn₂S₄, which would significantly benefit the enhancement of photocatalytic H₂ activity. This work provides a desired strategy to design and synthesize the visible-light-response photocatalysts with MoS₂ as cocatalysts to enhance the photocatalytic activity.     

Photoelectrochemical study on charge separation mechanisms of Bi2WO6 quantum dots decorated g-C3N4

In this paper, the Bi₂WO₆ quantum dots (QDs) decorated g-C₃N₄ nanoplates were successfully synthesized via a one-step hydrothermal method. The morphology of the Bi₂WO₆ could be tuned from regular nanoplates to quantum dots. Remarkably, the Bi₂WO₆ QDs coupled with g-C₃N₄ not only prevented the aggregation, but also decreased the size of Bi₂WO₆ QDs about 3.5 nm. Meanwhile, the charge separation mechanisms of Bi₂WO₆ QDs/g-C₃N₄ photocatalyst were investigated by electrochemical impedance spectra, Mott-Schottky and linear voltammetry scans. As a result, the photoelectrochemical (PEC) experiments provided forceful evidence for the charge separation mechanism of the Bi₂WO₆ QDs/g-C₃N₄ Z-scheme. The Z-scheme system not only accelerated the separation efficiency of charge, but also improved the ability of PEC water splitting at measured 1.23 V vs. RHE.     

Unique photoelectrochemical behavior of TiO2 nanorods wrapped with novel titanium Oxy-Nitride (TiOxNy) nanoparticles

In this work, we developed novel titanium oxynitride (TiO_xN_y) nanoparticles with diameter of 25 ± 2 nm and crystalline size of ～15 nm on hydrothermally grown one-dimensional (1D) TiO₂ nanorod (TNR) arrays. Herein, the TiO_xN_y nanoparticles were synthesized by facile nitridation using TiO₂ powder at 100% NH₃ gas atmosphere. Titanium oxynitride composed of potentially energetic metal-nitrogen bonds (TiN), compared to the weaker TiO bond, becomes chemically stable in the alkaline environment, and is considered as a suitable material for photoelectrochemical (PEC) system. The PEC performance of TiO_xN_y decorated TNR (abbreviated as TiO_xN_y @TNR) films was evaluated in 0.1 M KOH solution under solar illumination condition, and achieved the potentially high photocurrent density (J) of 2.1 mA/cm² at 1.23 V versus reversible hydrogen electrode (RHE) (abbreviated as V_RHE) in the TiO_xN_y@TNR arrays, in comparison with the poor photoresponse (0.7 mA/cm² at 1.23 V_RHE) of the pristine TNR arrays. A nearly three-fold enhancement was attained in the TiO_xN_y decorated TNR arrays, attributed to the high visible light absorption and fast carrier separation, due to the hybridization with the visible active TiO_xN_y nanoparticles in the cascading band alignment between the TiO_xN_y and TNR materials. Furthermore, the introduction of TiO_xN_y layer on the TNR surface quite reduces the interfacial resistance in the solid-liquid interface region, and further, the TiO_xN_y layer contributes to the passivation of the surface states (e.g., defect, trap sites etc.) where the charge recombination reaction frequently happens, leading to the improvement of PEC performance.     

Properly aligned band structures in B-TiO2/MIL53(Fe)/g-C3N4 ternary nanocomposite can drastically improve its photocatalytic activity for H2 evolution: Investigations based on the experimental results

The development of new tools that could meet the demand of sustainable energy production has attracted worldwide scientific attention. Over the past few decades, significant research efforts have been carried out to efficiently reduce water to H₂ (green fuel) over semiconductor photocatalysts. Numerous semiconductor photocatalysts have been employed in photocatalysis for optimum H₂ production. All the techniques were chosen based on their flexibility, cost-effectiveness, and ease of availability. Recently, polymeric carbon nitride (g-C₃N₄) received worldwide attention in visible light photocatalysis for energy and environmental applications due to its low price, robust nature, and superior thermal stability. Nevertheless, g-C₃N₄ (CN) exhibits shortfalls such as high charge carrier's recombination rate and weak reduction ability. To overcome these drawbacks, herein, for the first time we have fabricated B-TiO₂/MIL-53(Fe)/CN ternary composite via hydrothermal and wet-chemical methods. The resultant B-TiO₂/MIL53(Fe)/CN ternary composite shows drastically improved photocatalytic activity for hydrogen evolution compared to the bare CN, B-TiO_2, and MIL53(Fe) components. The B-TiO₂/MIL53(Fe)/CN ternary composite produced approximately 166.3 and 581.2 μmol h^?1 g^?1 of hydrogen under visible light and UV–visible light irradiations, respectively, with the assistance of co-catalyst Pt. Photo-luminescence (PL) and the fluorescence (FL) spectroscopy measurements reveal that the enhanced photoactivity is due to the greatly promoted charge carrier's separation and transfer at the interfacial contact of the well-aligned three-component systems. This work will promote the design and development of efficient photocatalyst based on CN for clean energy production and environmental purification.     

Manganese doped cadmium sulfide nanocrystal for hydrogen production from water under visible light

A series of Mn²⁺ doped CdS photocatalysts were prepared by a co-precipitation method and characterized by XRD, DRS, TEM, and XPS techniques. While the band gap, crystal phase and the morphology of CdS nanocrystal were not found to be affected noticeably by Mn²⁺ doping, there was an optimal Mn²⁺ doping content of wt 0.5% where the hydrogen production was more than doubled compared to pure CdS. Calculations of density functional theory (DFT) with plane waves and pseudopotentials were used to characterize the doping effect of Mn in cubic CdS. It is assumed that Mn²⁺ serving as shallow trapping sites can separate e−/h⁺ pairs at surface of nanosized CdS, so as to greatly reduce their surface recombination and which in turn leads to improved hydrogen yield.     

Overall photocatalytic water splitting on Na2ZrxTi6−xO13 (x = 0, 1) nanobelts modified with metal oxide nanoparticles as cocatalysts

In this work, we present the preparation of Na₂Zr_xTi_6?xO₁₃ (x = 0, 1) nanobelts through a rapid solvocombustion method. The phases exhibited stable photocatalytic activity for overall water splitting under UV light. Effect of the annealing temperature on the physicochemical properties and the catalytic performance of the materials were studied. Na₂ZrTi₅O₁₃ exhibited a higher rate of H₂ evolution compared to Na₂Ti₆O₁₃, and it was attributed to the incorporation of Zr⁴⁺ in the structure, which generates a distortion in the octahedral sites of the structure. This distortion promoted an enhanced charge transport and a reduction in the recombination of the free carriers and a higher photocatalytic activity. The nanobelts were superficially modified through the deposition of metal oxide nanoparticles as cocatalyst, MO (M = Ni, Cu). The incorporation of metal oxide nanoparticles improved the charge separation process and the overall efficiency. An integral study of the structural, morphological, textural, optical and photoelectrochemical properties of the materials is presented and a charge transference mechanism on the semiconductor interface is proposed. The highest catalytic activity was obtained by Na₂ZrTi₅O₁₃ modified with CuO (2909 μmol g?¹ h^?1), and corresponds to an increase of 13.6 times the activity of the bare photocatalyst. This was attributed to an improved charge separation at the interface of n-type Na₂ZrTi₅O₁₃ and p-type CuO semiconductors. For the best of our knowledge, the activity exhibited for overall water splitting of Na₂Zr_xTi_6?xO₁₃ (x = 0, 1) nanobelts prepared by solvocombustion method and modified with the addition of MO nanoparticles in this work is higher compared to the reported in previous works.     

Realizing efficient exciton dissociation in an all-organic heterojunction photocatalyst for highly improved photocatalytic H2 evolution

Although graphitic carbon nitride is a promising photocatalyst in the field of energy conversion and environmental purification, the intrinsic properties like excitonic effects and sluggish charge transfer restrict further photocatalytic applications. To circumvent these limitations, the novel all-organic heterojunction photocatalysts were constructed by anchoring organic carbon dots (O-dots) on porous graphitic carbon nitride nanosheets (O-dots/CNS). Results demonstrated that excitons can be e?ectively dissociated into electrons and holes at the interface of O-dots/CNS heterojunction, followed by holes injected to O-dots and electrons accumulated in CNS to realize efficient charge separation. Consequently, the O-dots/CNS with the optimized hydrogen (H₂) evolution performance could be reached 1564.5 μmol h^?1g^?1 under the visible light irradiation. This work not only presents new ideas for rational design photocatalytic reaction system from exciton and charge carrier, but also broaden the applications of this new kind of organic dots in the field of energy conversion.     

Wet chemistry synthesis of CuFe2O4/CdSe heterojunction for enhanced efficient photocatalytic H2 evolution under visible irradiation

A heterostructure of CuFe₂O₄/CdSe was synthesized as H₂ evolution photocatalyst under visible light. The optical absorption onset of the CuFe₂O₄/CdSe heterostructures was red-shifted to 2.30–2.48 eV, compared to that of the bare CuFe₂O₄ (2.55 eV), leading to better utilization of visible light. Furthermore, the CuFe₂O₄/CdSe samples exhibited a higher specific surface area than the bare CuFe₂O₄, due to the introduction of CdSe nanospheres. Compared to the bare CuFe₂O₄, the CuFe₂O₄/CdSe heterostructure promoted H₂ production from water splitting. The enhanced photocatalytic performance of the CuFe₂O₄/CdSe catalyst was attributed to the more efficient charge separation and lower charge transfer resistance, confirmed by fluorescence decay measurements and Nyquist plots, respectively. The band alignment between CuFe₂O₄ and CdSe resulted in an interfacial p-n junction, which directed the electron transfer from CdSe to CuFe₂O₄ and the hole transfer from CuFe₂O₄ to CdSe, achieving improved spatial separation of charge carriers.     

Photocatalytic hydrogen evolution over the isostructural titanates: Ba3Li2Ti8O20 and Na2Ti6O13 modified with metal oxide nanoparticles

Isostructural titanates exhibiting rectangular tunnel structure with the general formula Na₂Ti₆O₁₃ and Ba₃Li₂Ti₈O₂₀, were synthesized by solid state and sol-gel methods. The structural, morphological, optical, textural and electrical properties of the materials were characterized by XRD, SEM, UV-vis, BET and EIS. The photocatalytic activity for hydrogen evolution of the materials was evaluated under UV light. Na₂Ti₆O₁₃ exhibited higher activity (120 μmol/gh) than Ba₃Li₂Ti₈O₂₀ (30 μmol/gh), and it was attributed to the distortion of the octahedrons in Ba₃Li₂Ti₈O₂₀, which increases the recombination of charges in the material. Additionally, the 1D dimension obtained in Na₂Ti₆O₁₃ promotes a better charge separation, transport and utilization in this phase. The activity of the materials was enhanced with the incorporation of metal oxide nanoparticles MO (M = Cu, Ni) as cocatalysts. The activity of Ba₃Li₂Ti₈O₂₀ increased 10 times with the addition of CuO (240 μmol/gh), while the activity of Na₂Ti₆O₁₃ increased 3.5 times (416 μmol/gh). This improvement in the photocatalytic activity of the isostructural titanates was attributed to the formation of a p-n heterostructure between n-type titanates and p-type CuO, which promoted an enhanced charge separation, transference and utilization.     

Tuning of electrocatalytic activity of WO3–TiO2 nanocomposite electrode for alkaline hydrogen evolution reaction

The study reports the synthesis of mesoporous WO₃–TiO₂ nanocomposite with tuned particle size (~7 nm), pore diameter (~4.9 nm), specific surface area (S_BET = 129.112 m²/g) and pore volume (V_tot = 0.185 cm³/g) by an acid catalyzed peptization method, and its utilization for the development of stable catalytic electrode with enhanced activity towards alkaline hydrogen evolution reaction (HER). The SEM and AFM analyses confirm the formation of good quality composite electrodes with improved surface roughness through electroless deposition method. The developed WO₃–TiO₂ nanocomposite electrode exhibits low overpotential value of 120 mV with an exchange current density of 6.20 × 10^?5 mA/cm², and a low Tafel slope value of 98 mV/dec. Apart from the high HER performance, the developed WO₃–TiO₂ nanocomposite electrode exhibits competency with the state-of-the-art electrode materials for alkaline HER in industrial processes with sustained catalytic activity, tolerance behavior and long-term stability.