Enhancing photocatalytic H2O2 production with Au co-catalysts through electronic structure modification.

Gold-based co-catalysts are a promising class of materials with potential applications in photocatalytic H2O2 production. However, current approaches with Au co-catalysts show limited H2O2 production due to intrinsically weak O2 adsorption at the Au site. We report an approach to strengthen O2 adsorption at Au sites, and to improve H2O2 production, through the formation of electron-deficient Auδ+ sites by modifying the electronic structure. In this case, we report the synthesis of TiO2/MoSx-Au, following selective deposition of Au onto a MoSx surface which is then further anchored onto TiO2. We further show that the catalyst achieves a significantly increased H2O2 production rate of 30.44 mmol g-1 h-1 in O2-saturated solution containing ethanol. Density functional theory calculations and X-ray photoelectron spectroscopy analysis reveal that the MoSx mediator induces the formation of electron-deficient Auδ+ sites thereby decreasing the antibonding-orbital occupancy of Au-Oads and subsequently enhancing O2 adsorption. This strategy may be useful for rationally designing the electronic structure of catalyst surfaces to facilitate artificial photosynthesis.

Free-Electron Inversive Modulation to Charge Antibonding Orbital of ReS2 Cocatalyst for Efficient Photocatalytic Hydrogen Generation.

The free electron transfer between cocatalyst and photocatalyst has a great effect on the bonding strength between the active site and adsorbed hydrogen atom (Hads), but there is still a lack of effective means to purposely manipulate the electron transfer in a beneficial direction of H adsorption/desorption activity. Herein, when ReSx cocatalyst is loaded on TiO2 surface, a spontaneous free-electron transfer from ReSx to TiO2 happens due to the smaller work function of ReSx, causing an over-strong S-Hads bond. To prevent the over-strong S-Hads bonds of ReSx in the ReSx/TiO2 , a free-electron reversal transfer strategy is developed to weaken the strong S-Hads bonds via increasing the work function of ReSx by incorporating O to produce ReOSx cocatalyst. Research results attest that a larger work function of ReOSx than that of TiO2 can induce reversal transfer of electrons from TiO2 to ReOSx to produce electron-rich S(2+δ)-, causing the increased antibonding-orbital occupancy of S-Hads in ReOSx/TiO2 . Accordingly, the stability of adsorbed H on S sites is availably decreased, thus weakening the S-Hads of ReOSx. In this case, an electron-rich S(2+δ)--mediated "capture-hybridization-conversion" mechanism is raised . Benefiting from such property, the resultant ReOSx/TiO2 photocatalyst exhibits a superior H2-evolution rate of 7168 µmol h-1 g-1 .

Reversing Free-Electron Transfer of MoS2+x Cocatalyst for Optimizing Antibonding-Orbital Occupancy Enables High Photocatalytic H2 Evolution.

The interaction between a co-catalyst and photocatalyst usually induces spontaneous free-electron transfer between them, but the effect and regulation of the transfer direction on the hydrogen-adsorption energy of the active sites have not received attention. Herein, to steer the free-electron transfer in a favorable direction for weakening S-Hads bonds of sulfur-rich MoS2+x , an electron-reversal strategy is proposed for the first time. The core-shell Au@MoS2+x cocatalyst was constructed on TiO2 to optimize the antibonding-orbital occupancy. Research results reveal that the embedded Au can reverse the electron transfer to MoS2+x to generate electron-rich S(2+δ)- active sites, thus increasing the antibonding-orbital occupancy of S-Hads in the Au@MoS2+x cocatalyst. Consequently, the increase in the antibonding-orbital occupancy effectively destabilizes the H 1s-p antibonding orbital and weakens the S-Hads bond, realizing the expedited desorption of Hads to rapidly generate a lot of visible H2 bubbles. This work delves deep into the latent effect of the photocatalyst carrier on cocatalytic activity.

Cyano group-enriched crystalline graphitic carbon nitride photocatalyst: Ethyl acetate-induced improved ordered structure and efficient hydrogen-evolution activity.

The molten salt-assisted route is one of the most important methods to improve the crystallinity of conventionally disordered bulk graphitic carbon nitride (g-C3N4). However, the residual potassium ions from potassium chloride/lithium chloride molten salt can greatly impact the ordered structure of g-C3N4 and serve as the recombination centers of photoinduced carriers, causing limited photocatalytic hydrogen-evolution performance. In this article, the ethyl acetate-mediated method is first developed to not only further improve the ordered structure of traditional crystalline g-C3N4, but also produce more cyano groups for preparing highly efficient g-C3N4 photocatalysts. Herein, the ethyl acetate can gradually hydrolyze to produce hydrogen ions, which can promote the more ordered sheet-like structure and more cyano groups by effective removal of residual potassium ions in the traditional crystalline g-C3N4, leading to the formation of cyano group-enriched crystalline g-C3N4 photocatalysts (CC-CN). As a result, the resultant CC-CN displays the remarkably enhanced photocatalytic hydrogen-evolution performance (295.30 µmol h-1 with an apparent quantum efficiency about 12.61%), in comparison to the bulk g-C3N4 (14.97 µmol h-1) and traditional crystalline g-C3N4 (24.60 µmol h-1). The great improvement of photocatalytic performance can mainly be ascribed to the synergism of improved ordered structure and abundant cyano groups, namely, the efficient transfer and separation of photoinduced charges as well as excellent interfacial hydrogen-generation reaction, respectively. The present work may deliver new strategies to prepare other high-crystalline photocatalysts with great efficiency.

Optimizing Atomic Hydrogen Desorption of Sulfur-Rich NiS1+ x Cocatalyst for Boosting Photocatalytic H2 Evolution.

Low-cost transition-metal chalcogenides (MSx ) are demonstrated to be potential candidate cocatalyst for photocatalytic H2 generation. However, their H2 -generation performance is limited by insufficient quantities of exposed sulfur (S) sites and their strong bonding with adsorbed hydrogen atoms (SHads ). To address these issues, an efficient coupling strategy of active-site-enriched regulation and electronic structure modification of active S sites is developed by rational design of core-shell Au@NiS1+ x nanostructured cocatalyst. In this case, the Au@NiS1+ x cocatalyst can be skillfully fabricated to synthesize the Au@NiS1+ x modified TiO2 (denoted as TiO2 /Au@NiS1+ x ) by a two-step route. Photocatalytic experiments exhibit that the resulting TiO2 /Au@NiS1+ x (1.7:1.3) displays a boosted H2 -generation rate of 9616 µmol h-1 g-1 with an apparent quantum efficiency of 46.0%  at 365 nm, which is 2.9 and 1.7 times the rate over TiO2 /NiS1+ x and TiO2 /Au, respectively. In situ/ex situ XPS characterization and density functional theory calculations reveal that the free-electrons of Au can transfer to sulfur-enriched NiS1+ x to induce the generation of electron-enriched Sδ - active centers, which boosts the desorption of Hads for rapid hydrogen formation via weakening the strong SHads bonds. Hence, an electron-enriched Sδ - -mediated mechanism is proposed. This work delivers a universal strategy for simultaneously increasing the active site number and optimizing the binding strength between the active sites and hydrogen adsorbates.

Hydroxyl-enriched highly crystalline TiO2 suspensible photocatalyst: facile synthesis and superior H2-generation activity.

A hydroxyl-enriched highly crystalline TiO2 suspensible photocatalyst was synthesized via a facile ethanol-controlled hydrolysis and following an in situ crystallization method. In the absence of any cocatalysts, the resultant photocatalyst displayed a clearly higher H2-evolution rate (622 µmol h-1 g-1) than the well-known commercial P25 TiO2 (190 µmol h-1 g-1).

High-yield lactic acid-mediated route for a g-C3N4 nanosheet photocatalyst with enhanced H2-evolution performance.

Facile and novel strategies to prepare g-C3N4 nanosheets are required to greatly improve their photocatalytic H2-production activity. In this study, a lactic acid-mediated synthesis route has been developed to prepare g-C3N4 nanosheets, which includes the preassembled formation of lactic acid-melamine co-monomers, followed by direct high-temperature calcination. In this case, it is found that during high-temperature calcination, the lactic acid molecules can greatly prevent the serious polymerization of melamine molecules, resulting in the formation of g-C3N4 nanosheets. Moreover, owing to the strong coupling with melamine molecules, lactic acid can also significantly increase the production rate (ca. 35.16 wt%) of g-C3N4 nanosheets from the melamine precursor via preventing the rapid sublimation of melamine and its intermediates during the calcination progress compared with the well-known two-step calcination method. Photocatalytic experimental data reveal that the resultant g-C3N4 nanosheet photocatalysts show a greatly improved H2-production rate, and the g-C3N4 (500 µL) sample exhibits the best photocatalytic performance, which is obviously two times higher than that of the conventional bulk g-C3N4. In addition to lactic acid, it is very interesting to find that acetic acid can also be used to prepare g-C3N4 nanosheets via a similar formation mechanism, strongly suggesting the universality and versatility of the present lactic acid-mediated synthesis route. The present synthesis strategy may broaden the horizons for the synthesis of high-efficiency photocatalysts.