Trioctylphosphine Oxide (TOPO)-Assisted Facile Fabrication of Phosphorus-Incorporated Nanostructured Carbon Nitride Toward Photoelectrochemical Water Splitting with Enhanced Activity.

Designing nanostructured arrays of two-dimensional surfaces and interfaces is a versatile approach to increasing their photoelectrochemical activity. Here, phosphorus (P)-incorporated nanostructured carbon nitride (h-PCN) with an enlarged surface area is fabricated by employing trioctylphosphine oxide (TOPO) as a dopant precursor for visible-light-driven photoelectrochemical water splitting to produce hydrogen. The structural, morphological, and electronic properties of the photocatalyst have been characterized through various physicochemical techniques. We show that the incorporation of P into the g-C3N4 framework enhances light absorption over broad regimes, charge separation, and migration, as well as the specific surface area, showing excellent photocurrent enhancement (5.4 folds) in the cathodic direction as compared to bulk g-C3N4. Moreover, the photocathode shows 3.3-fold enhancement in current at zero biased potential. Without using any cocatalyst, the photoelectrodes produced 27 µmol h-1 of H2 and 13 µmol h-1of O2 with 95% faradic efficiency. The excellent photoelectrochemical behavior toward water-splitting reactions by the photoelectrode is attributed to the synergistic effect of P incorporation and active sites emerging from the nanostructured architecture of the material. This work demonstrates the facile fabrication of nanostructured P-incorporated g-C3N4 toward water-splitting reactions to produce hydrogen without using a cocatalyst in a simple and cost-effective way.

Prominence of Cu in a plasmonic Cu-Ag alloy decorated SiO2@S-doped C3N4 core-shell nanostructured photocatalyst towards enhanced visible light activity.

A series of Cu-Ag bimetal alloys decorated on SiO2 and the fabrication of few-layer S-doped graphitic carbon nitride (SC) warped over it to form a core-shell nanostructured morphology have been demonstrated and well characterized through various physiochemical techniques. HRTEM data confirmed the formation of a compact nanojunction between the SiO2 and SC, where Cu-Ag is embedded uniformly with an average particle size of 1.3 nm. The Ag : Cu (1 : 3) between SiO2 and SC produces 1730 µmol h-1 g-1 of H2 under visible light illumination. Moreover, 6.2-fold current enhancement in the case of Ag : Cu (1 : 3) as compared to the Ag-loaded core-shell nanostructured photocatalyst indicates higher electron-hole-pair separation. The excellent activity was due to the synergistic alloying and plasmonic effect of Ag and Cu. DFT studies reveal that the Cu atom in the Cu-Ag bimetal alloy plays a pivotal role in the generation of H2, and the reaction proceeds via a 4-membered transition state. The mechanistic insight proceeds from the generation of hot electrons due to the LSPR effect and their transfer to the SC layer via a compact nanojunction.

Black titania an emerging photocatalyst: review highlighting the synthesis techniques and photocatalytic activity for hydrogen generation.

The TiO2 semiconductor photocatalyst is in the limelight of sustainable energy research in recent years because of its beneficial properties. However, its wide band-gap and rapid exciton recombination rate makes it a lame horse, and reduces its photocatalytic efficiency. Recently, researchers have developed facile methods for lowering the band-gap, so that it captures a wide range of solar spectrum, but the efficiency is still way behind the target value. After the discovery of black titania (B-TiO2), the associated drawbacks of white TiO2 and its modified forms were addressed to a large extent because it not only absorbs photons in a broad spectral range (UV to IR region), but also modifies the structural and morphological features, along with the electronic properties of the material, significantly boosting the catalytic performance. Hence, B-TiO2 effectively converts solar energy into renewable chemical energy i.e. green fuel H2 that can ultimately satisfy the energy crisis and environmental pollution. However, the synthesis techniques involved are quite tedious and challenging. Hence, this review summarizes various preparation methods of B-TiO2 and the involved characterization techniques. It also discusses the different modification strategies adopted to improve the H2 evolution activity, and hopes that this review acts as a guiding tool for researchers working in this field.

Cu-Ag Bimetal Alloy Decorated SiO2@TiO2 Hybrid Photocatalyst for Enhanced H2 Evolution and Phenol Oxidation under Visible Light.

With a broader objective to replace visible light driven Pt-based photoelectrochemical/catalytic hydrogen evolution, a series of cost-effective bimetallic nanoalloys of Cu-Ag have been deposited on core-shell nanostructured SiO2@TiO2 through a facile reduction route. The physicochemical properties, i.e. crystal structure, morphology, chemical environment, and optical properties of Cu-Ag bimetal alloy decorated SiO2@TiO2 hybrid photocatalyst, have been thoroughly investigated through X-ray diffraction, high resolution transmission electron microscopy, field emission scanning electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse reflectance spectroscopy, and photoluminescence spectroscopy, respectively. TEM study confirms the coating of an ultrathin layer of TiO2 shell on 100 nm sized SiO2 core, and about 4.5 nm of Ag-Cu nanoalloys are uniformly distributed on the core-shell nanostructure. The higher light absorption throughout the visible range and better separation of charge carrier by Ag-Cu (1:3) deposited SiO2@TiO2 hybrid compared to other counterparts is confirmed from UV-vis, diffuse reflectance spectroscopy, photoluminescence, and electrochemical impedance studies. Eightfold higher photocurrent enhancements, threefold enhanced photocatalytic hydrogen generation, and twofold higher phenol oxidation activities of Ag-Cu (1:3) deposited SiO2@TiO2 hybrid compared to those of the monometallic plasmonic catalyst may be attributed to the synergetic effect of enriched light harvesting and surface plasmon induced hot electron transfer from the nanoalloy to the TiO2 interface, resulting in efficient charge transfer.

Serendipitous Assembly of Mixed Phase BiVO4 on B-Doped g-C3N4: An Appropriate p-n Heterojunction for Photocatalytic O2 evolution and Cr(VI) reduction.

Type II p-n heterojunction B-doped g-C3N4/BiVO4 moieties have been fabricated by depositing n-type BiVO4 on the surface of p-type 1 wt % B-doped g-C3N4 for the first time. The materials were characterized by PXRD, XPS, UV-vis DRS, IR, PL, and Raman analysis. The photocatalytic activities of as synthesized samples were studied toward reduction of Cr(VI) and water splitting reaction to generate oxygen. The results reveal that Type II p-n heterojunction considerably enhance the photocatalytic activity as compared to neat n-type BiVO4 and p-type B-doped g-C3N4. 50% BiVO4/B-doped g-C3N4 heterostructure exhibited the best photocatalytic activity, which is 7.9-fold higher than that of BiVO4 and followed by a pseudo first order kinetics with apparent first order rate constant of 0.063 min-1. Again the heterojunction is able to produce 4.2 times higher oxygen evolution value as related to pristine BiVO4. The superior photocatalytic activity is attributed to higher visible light utilization and lower recombination of electron-hole pairs by creating a p-n junction. PXRD and HRTEM data suggest the formation of mixed phase monoclinic and tetragonal BiVO4 thereby creating a heterojunction for the improvement of the photocatalytic performances. The formation of mixed-phase BiVO4 is attributed to the high temperature calcination as well as surface energy of B-doped g-C3N4. The oxygen vacancy in the system is confirmed through XPS and Raman analysis. Moreover, the excellent photocurrent response by the designed photocatalyst at lower overpotential and decrease in carrier recombination as compared to bulk one, studied from LSV and electrochemical impedance spectroscopy, validate the unique photocatalytic activity of the catalyst. The formation of the p-n heterojunction is confirmed from a Mott-Schottky analysis. The work shed new light on the assembly of the p-n heterojunction, showing excellent photocatalytic properties in a simple way.

Surface-Plasmon-Resonance-Induced Photocatalysis by Core-Shell SiO2@Ag NCs@Ag3PO4 toward Water-Splitting and Phenol Oxidation Reactions.

A series of core-shell-structured SiO2@Ag NCs@Ag3PO4 photocatalysts with varying percentages of silver nanoclusters (Ag NCs) have been synthesized by using SiO2 as a core material. The crystal structure, morphology, chemical composition, and photophysical properties of as-synthesized materials have been thoroughly analyzed through powder X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-vis diffuse-reflectance spectroscopy, and photoluminescence (PL) spectroscopy techniques. The introduction of Ag NCs has effectively reduced the photogenerated electron-hole recombination rate, as evidenced from PL and Nyquist plots. The electrochemical properties of the photocatalysts were studied through photocurrent measurement, and 23-fold current enhancements in the cathodic direction are observed. The excellent current enhancement by the photocatalyst is attributed to the presence of Ag NCs. The effectiveness of the photocatalysts toward photocatalytic water splitting was studied and produced 2460 µmol h-1 g-1 hydrogen and 1236 µmol h-1 g-1 oxygen by 2 wt % loaded Ag NCs. Again the photocatalytic phenol oxidation has been explored, and the best catalyst is able to oxidize 91% phenol upon visible-light irradiation. The photocatalyst having 2 wt % Ag NCs shows better activity toward both water splitting and phenol oxidation compared to others, which is attributed to better visible-light absorption efficiency, lower electron-hole recombination rate, and low interfacial charge-transfer resistance.