9 research outputs found

    Enhanced Photoinduced-Stability and Photocatalytic Activity of CdS by Dual Amorphous Cocatalysts: Synergistic Effect of Ti(IV)-Hole Cocatalyst and Ni(II)-Electron Cocatalyst

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    CdS is one of the most well-known and important visible-light photocatalytic materials for water splitting to produce hydrogen energy. Owing to its serious photocorrosion property (poor photoinduced stability), however, CdS photocatalyst can unavoidably be oxidized to form S<sup>0</sup> by its photogenerated holes, causing an obviously decreased photocatalytic performance. In this study, to improve the photoinduced stability of CdS photocatalyst, amorphous TiO<sub>2</sub> (referred to as Ti­(IV)) as a hole cocatalyst was successfully loaded on the CdS surface to prepare Ti­(IV)/CdS photocatalysts. It was found that the resultant Ti­(IV)/CdS photocatalyst exhibited an obviously enhanced photocatalytic stability, namely, its deactivation rate clearly decreased from 37.9% to 13.5% after five cycles of photocatalytic reactions. However, its corresponding photocatalytic activity only showed a very limited increase (ca. 37.4%) compared with the naked CdS. To further improve its photocatalytic performance, the amorphous Ni­(II) as an electron cocatalyst was subsequently modified on the Ti­(IV)/CdS surface to prepare the dual amorphous-cocatalyst modified Ti­(IV)–Ni­(II)/CdS photocatalyst. In this case, the resultant Ti­(IV)–Ni­(II)/CdS photocatalyst not only exhibited a significantly improved photocatalytic activity and stability, but also could maintain the excellent photoinduced stability of CdS surface structure. Based on the experimental results, a synergistic effect of dual amorphous Ti­(IV)–Ni­(II) cocatalysts is proposed, namely, the amorphous Ti­(IV) works as a hole-cocatalyst to rapidly capture the photogenerated holes from CdS surface, causing the less oxidation of surface lattice S<sup>2–</sup> ions in CdS, while the amorphous Ni­(II) functions as an electron-cocatalyst to rapidly transfer the photogenerated electrons and then promote their following interfacial H<sub>2</sub>-evolution reaction. Compared with the traditional noble metal cocatalysts (such as Pt and RuO<sub>2</sub>), the present amorphous Ti­(IV) and Ni­(II) cocatalysts are apparently low-cost, nontoxic, and earth-abundant, which can widely be applied in the design and development of highly efficient photocatalytic materials

    Suspensible Cubic-Phase CdS Nanocrystal Photocatalyst: Facile Synthesis and Highly Efficient H<sub>2</sub>‑Evolution Performance in a Sulfur-Rich System

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    Compared with stable-phase hexagonal CdS, the metastable cubic CdS photocatalyst usually shows a lower H<sub>2</sub>-evolution performance under visible-light irradiation. Thus, the widely reported high-performance CdS photocatalysts are mainly focused on the hexagonal phase, while the cubic-phase CdS with a high H<sub>2</sub>-evolution activity has seldom been concerned. In this study, a direct precipitation method in a sulfur-rich Na<sub>2</sub>S–Na<sub>2</sub>SO<sub>3</sub> system has been developed to prepare the suspensible cubic-phase CdS nanocrystal (<i>c</i>-CdS-NC) photocatalyst with a high H<sub>2</sub>-evolution activity. In this case, the resultant <i>c</i>-CdS-NC with a small crystal size (ca. 5 nm) and high specific surface area (>75.23 m<sup>2</sup>/g) exhibits a stable and suspensible photocatalysts due to the massive and preferential adsorption of S<sup>2–</sup>/SO<sub>3</sub><sup>2–</sup> ions on the nanocrystal surface. Photocatalytic results indicated that the suspensible <i>c</i>-CdS-NC photocatalysts clearly exhibited an obviously higher H<sub>2</sub>-evolution performance (0.36 mmol h<sup>–1</sup>) than the traditional hexagonal CdS (0.14 mmol h<sup>–1</sup>) by a factor of 2.6 times. Based on the present results, a S<sup>2–</sup>/SO<sub>3</sub><sup>2–</sup>-mediated mechanism was proposed for the enhanced H<sub>2</sub>-evolution performance of the suspensible <i>c</i>-CdS-NC, namely the massive adsorbed S<sup>2–</sup> ions on the suspensible <i>c</i>-CdS-NC surface not only promote the rapid capture of photogenerated holes but also can work as the effective active sites for H<sub>2</sub>-evolution reaction. The present work may provide important insights for developing high-performance photocatalytic materials

    UV- and Visible-Light Photocatalytic Activity of Simultaneously Deposited and Doped Ag/Ag(I)-TiO<sub>2</sub> Photocatalyst

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    Ag modification has been demonstrated to be an efficient strategy to improve the photocatalytic performance of TiO<sub>2</sub> photocatalysts. However, the previous studies about the Ag modification are only restricted to the surface loading of metallic Ag or Ag­(I) doping, investigations have seldom been focused on the simultaneously deposited and doped Ag/Ag­(I)-TiO<sub>2</sub> photocatalyst. In this study, Ag/Ag­(I)-TiO<sub>2</sub> photocatalyst was prepared by a facile impregnated method in combination with a calcination process (450 °C) and the photocatalytic activity was evaluated by the photocatalytic decomposition of methyl orange and phenol solutions under both UV- and visible-light irradiation, respectively. It was found that Ag­(I) doping resulted in the formation of an isolated energy level of Ag 4d in the band gap of TiO<sub>2</sub>. On the basis of band-structure analysis of Ag/Ag­(I)-TiO<sub>2</sub> photocatalyst, a possible photocatalytic mechanism was proposed to account for the different UV- and visible-light photocatalytic activities. Under visible-light irradiation, the isolated energy level of Ag 4d contributes to the visible-light absorption while the surface metallic Ag promotes the effective separation of the following photogenerated electrons and holes in the Ag/Ag­(I)-TiO<sub>2</sub> nanoparticles, resulting in a higher visible-light photocatalytic activity than the one-component Ag-modified TiO<sub>2</sub> (such as Ag­(I)-TiO<sub>2</sub> and Ag/TiO<sub>2</sub>). Under UV-light irradiation, the doping energy level of Ag­(I) ions in the band gap of TiO<sub>2</sub> acts as the recombination center of photogenerated electrons and holes, leading to a lower photocatalytic performance of Ag-doped TiO<sub>2</sub> (such as Ag/Ag­(I)-TiO<sub>2</sub> and Ag­(I)-TiO<sub>2</sub>) than the corresponding undoped photocatalysts (such as Ag/TiO<sub>2</sub> and TiO<sub>2</sub>). Considering the well controllable preparation of various Ag-modified TiO<sub>2</sub> (such as TiO<sub>2</sub>, Ag/TiO<sub>2</sub>, Ag­(I)-TiO<sub>2</sub>, and Ag/Ag­(I)-TiO<sub>2</sub>), this work may provide some insight into the smart design of novel and high-efficiency photocatalytic materials

    Phenylamine-Functionalized rGO/TiO<sub>2</sub> Photocatalysts: Spatially Separated Adsorption Sites and Tunable Photocatalytic Selectivity

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    The preferential adsorption of targeted contaminants on a photocatalyst surface is highly required to realize its photocatalytic selective decomposition in a complex system. To realize the tunable preferential adsorption, altering the surface charge or polarity property of photocatalysts has widely been reported. However, it is quite difficult for a modified photocatalyst to realize the simultaneously preferential adsorption for both cationic and anionic dyes. In this study, to realize the selective adsorption for both cationic and anionic dyes on a photocatalyst surface, the negative reduced graphene oxide (rGO) nanosheets and positive phenylamine (PhNH<sub>2</sub>) molecules are successfully loaded on the TiO<sub>2</sub> surface (PhNH<sub>2</sub>/rGO-TiO<sub>2</sub>) with spatially separated adsorption sites, where the negative rGO and positive PhNH<sub>2</sub> molecules work as the preferential adsorption sites for cationic and anionic dyes, respectively. It was interesting to find that although all the TiO<sub>2</sub> samples (including the naked TiO<sub>2</sub>, PhNH<sub>2</sub>/TiO<sub>2</sub>, rGO-TiO<sub>2</sub>, and PhNH<sub>2</sub>/rGO-TiO<sub>2</sub>) clearly showed a better adsorption performance for cationic dyes than anionic dyes, only the PhNH<sub>2</sub>/rGO-TiO<sub>2</sub> with spatially separated adsorption-active sites exhibited an opposite photocatalytic selectivity, namely, the naked TiO<sub>2</sub>, PhNH<sub>2</sub>/TiO<sub>2</sub>, and rGO-TiO<sub>2</sub> showed a preferential decomposition for cationic dyes, while the resultant PhNH<sub>2</sub>/rGO-TiO<sub>2</sub> exhibited an excellently selective decomposition for anionic dyes. In addition, the resultant PhNH<sub>2</sub>/rGO-TiO<sub>2</sub> photocatalyst not only realizes the tunable photocatalytic selectivity but also can completely and sequentially decompose the opposite cationic and anionic dyes

    Cu(II) as a General Cocatalyst for Improved Visible-Light Photocatalytic Performance of Photosensitive Ag-Based Compounds

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    Usually, cocatalyst modification of photocatalysts is an efficient approach to enhance the photocatalytic performance by promoting effective separation of photogenerated electrons and holes. It is highly required to explore new and effective cocatalysts to further enhance the photocatalytic performance of photocatalytic materials. In the present work, Cu­(II) cocatalyst was successfully loaded on the surface of various Ag-based compounds (such as AgCl, Ag<sub>3</sub>PO<sub>4</sub>, AgBr, AgI, Ag<sub>2</sub>CO<sub>3</sub>, and Ag<sub>2</sub>O) by a simple impregnation route, and their photocatalytic activity of Cu­(II)/Ag-based photocatalysts was evaluated by the photocatalytic decolorization of methyl orange and photocatalytic decomposition of phenol solution under visible-light illumination. As one of the typical photosensitive Ag-based compounds, the photocatalytic activity of AgCl could be greatly improved by optimizing the amount of Cu­(II) cocatalyst, and the highest photocatalytic performance of the resulted Cu­(II)/AgCl was higher than that of the unmodified AgCl by a factor of 2.1. Significantly, the Cu­(II) was demonstrated to be a general and effective cocatalyst to improve the visible-light photocatalytic performance of other various photosensitive Ag-based compounds (such as AgBr, AgI, Ag<sub>3</sub>PO<sub>4</sub>, Ag<sub>2</sub>CO<sub>3</sub>, and Ag<sub>2</sub>O) in addition to the AgCl photocatalyst. Based on the present results, it is proposed that the Cu­(II) cocatalyst functions as electron scavengers to quickly capture photogenerated electrons from the excited photocatalysts and then works as reduction active sites to reduce O<sub>2</sub> effectively, resulting in an effective separation of photogenerated electrons and holes. Compared with the expensive noble metal cocatalyst (such as Pt, Au, and Pd), the present promising Cu­(II) cocatalyst can be considered to be one of the perfect cocatalysts for the smart preparation of various highly efficient photocatalysts in view of its abundance and low cost

    Facile Fabrication and Enhanced Photocatalytic Performance of Ag/AgCl/rGO Heterostructure Photocatalyst

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    Graphene/reduced graphene oxide (rGO) modification has been demonstrated to be an efficient route to improve the photocatalytic performance of various photocatalysts by promoting the effective separation of photogenerated electrons and holes. It is highly required to develop facile and environmental-friendly methods for the preparation of graphene-based photocatalytic materials. In this study, the Ag/AgCl/rGO heterostructure photocatalyst was fabricated by a mild oxidization reaction of hydrothermally prepared Ag/rGO in FeCl<sub>3</sub> solution. It was found that the reduction of graphene oxide (GO) was accompanied with the in situ formation of metallic Ag in a Ag­[(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup>-immobilized GO solution during hydrothermal treatment, while the following in situ oxidation of metallic Ag by FeCl<sub>3</sub> solution resulted in the formation of strongly coupled Ag/AgCl/rGO heterostructure photocatalyst. The photocatalytic experimental results indicated that all the resultant Ag/AgCl/rGO nanocomposite photocatalysts exhibited a much higher photocatalytic activity than the Ag/AgCl and physically mixed Ag/AgCl/rGO composite, and the Ag/AgCl/rGO (3.2 wt % rGO) showed the highest photocatalytic performance. The enhanced photocatalytic performance of Ag/AgCl/rGO heterostructures can be attributed to the cooperation effect of the effective separation of photogenerated carriers via strongly coupled rGO cocatalyst and the enrichment of organic molecules on the rGO nanosheets. Considering the facile preparation and its high photocatalytic activity, it is possible for the present Ag/AgCl/rGO nanocomposites to be widely applied in various fields such as air purification and wastewater treatment

    Visible-Light-Sensitive Photocatalysts: Nanocluster-Grafted Titanium Dioxide for Indoor Environmental Remediation

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    Photocatalytic degradation of organic compounds requires photoexcited holes with strong oxidative power in the valence band (VB) of semiconductors. Although numerous types of doped semiconductors, such as nitrogen-doped TiO<sub>2</sub>, have been studied as visible-light-sensitive photocatalysts, the quantum yields of these materials were very low because of the limited oxidation power of holes in the nitrogen level above the VB. Recently, we developed visible-light-sensitive Cu­(II) and Fe­(III) nanocluster-grafted TiO<sub>2</sub> using a facile impregnation method and demonstrated that visible-light absorption occurs at the interface between the nanoclusters and TiO<sub>2</sub>, as electrons in the VB of TiO<sub>2</sub> are excited to the nanoclusters under visible-light irradiation. In addition, photogenerated holes in the VB of TiO<sub>2</sub> efficiently oxidize organic contaminants, and the excited electrons that accumulate in nanoclusters facilitate the multielectron reduction of oxygen. Notably, Cu­(II) and Fe­(III) nanocluster-grafted TiO<sub>2</sub> photocatalyst has the highest quantum yield among reported photocatalysts and has antiviral, self-cleaning, and air purification properties under illumination by indoor light fixtures equipped with white fluorescent bulbs or white light-emitting diodes

    Ice–Water Quenching Induced Ti<sup>3+</sup> Self-doped TiO<sub>2</sub> with Surface Lattice Distortion and the Increased Photocatalytic Activity

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    The present research reported a facile strategy to prepare Ti<sup>3+</sup> self-doped TiO<sub>2</sub> with increased photocatalytic activity. The TiO<sub>2</sub> subjected to high temperature preannealing was directly thrown into ice–water for rapid quenching. It is interesting to see that the quenched samples show pale blue color due to the absorption in visible and near-IR region. The comprehensive analyses of X-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, field-emission scanning electron microscope, and Brunauer–Emmett–Teller (BET) show that the crystallinity, the morphologies, and the specific surface area are almost unchanged after the ice–water quenching. The spectroscopic analyses of UV–vis diffusion reflectance spectra, photoluminescence spectra, and X-ray photoelectron spectra clearly show the change of electronic structure of TiO<sub>2</sub> due to presence of Ti<sup>3+</sup> ions induced by the ice–water quenching, which is further confirmed by the electron paramagnetic resonance analysis. No Ti<sup>3<b>+</b></sup> ions are generated if the preannealing temperature is below 800 °C. The energy band structure model involving the Ti<sup>3+</sup> ions and the associated oxygen defects was proposed to explain the change of UV–vis diffusion absorption. It is considered that the high concentration of oxygen defects at high preannealing temperatures can be partially frozen by the ice–water quenching, which then can denote the high concentration of excess electrons. Some excess electrons can be localized at Ti lattice sites, resulting in the presence of Ti<sup>3+</sup> ions. More interestingly, it is also seen that the rapid ice–water quenching causes the distortion of surface lattice due to the interaction between hot TiO<sub>2</sub> and water, which tends to be poly crystalline and disordered for high preannealing temperature. The surface lattice distortion is considered to be correlated with the generation of oxygen defects during the ice–water quenching. The quenched samples show obviously increased photocatalytic activity for both methylene blue degradation and hydrogen evolution under UV light illumination. Although they do not have visible activity, loading amorphous Cu­(OH)<sub><i>x</i></sub> nanoclusters can greatly increase their ability to degrade methylene blue under visible light illumination. It is also shown that the photocatalytic activity of ZnO can also be increased to some extent by the ice–water quenching. Therefore, the ice–water quenching could be a general method for increasing the photocatalytic activity of many materials

    Dye-Sensitization-Induced Visible-Light Reduction of Graphene Oxide for the Enhanced TiO<sub>2</sub> Photocatalytic Performance

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    The reduction of graphene oxide (GO) with a large-scale production has been demonstrated to be one of the key steps for the preparation of graphene-based composite materials with various potential applications. Therefore, it is highly required to develop a facile, green, and environmentally friendly route for the effective reduction of GO. In this study, a new and effective reduced method of GO nanosheets, based on the dye-sensitization-induced visible-light reduction mechanism, was developed to prepare reduced GO (rGO) and graphene-based TiO<sub>2</sub> composite in the absence of any additional reducing agents. It was found that the dye-sensitization-induced reduction process of GO was accompanied with the formation of TiO<sub>2</sub>-rGO composite nanostructure. The photocatalytic experimental results indicated that the resultant TiO<sub>2</sub>-rGO nanocomposites exhibited significantly higher photocatalytic performance than pure TiO<sub>2</sub> because of a rapid separation of photogenerated electrons and holes by the rGO cocatalyst
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