Effective Visible-Excited Charge Separation in Silicate-Bridged ZnO/BiVO₄ Nanocomposite and Its Contribution to Enhanced Photocatalytic Activity

It is highly desired to enhance the visible-excited charge separation of nanosized BiVO₄ for utilization in photocatalysis. Here ZnO/BiVO₄ nanocomposites in different molar-ratios are fabricated by simple wet-chemical processes, after synthesis of nanosized BiVO₄ and ZnO by hydrothermal methods. It is shown by means of atmosphere-controlled steady-state surface photovoltage spectra and transient-state surface photovoltage responses that the photogenerated charges of resulting nanocomposite shows longer lifetime and higher separation than that of BiVO₄ alone. This leads to its superior photoactivities for water oxidation to produce O₂ and for colorless pollutant degradation under visible irradiation, with about three times enhancement. Interestingly, it is suggested that the prolonged lifetime and enhanced separation of photogenerated charges in the nanocomposite is attributed to the unusual spatial transfer of visible-excited high-energy electrons, by visible radiation from BiVO₄ to ZnO on the basis of the ultralow-temperature electron paramagnetic resonance measurements and the photocurrent action spectra. Moreover, it is clearly demonstrated that the photogenerated charge separation of resulting ZnO/BiVO₄ nanocomposite could be further enhanced after introducing the silicate bridges so as to improve the visible photocatalytic activity greatly, attributed to the built bridge favorable to charge transfer. This work would provide a feasible way to enhance the solar energy utilization of visible-response semiconductor photocatalysts

Facile Synthesis of Surface-Modified Nanosized α‑Fe₂O₃ as Efficient Visible Photocatalysts and Mechanism Insight

In this study, α-Fe₂O₃ nanoparticles with high visible photocatalytic activity for degrading liquid-phase phenol and gas-phase acetaldehyde have been controllably synthesized by a simple one-pot water-organic two-phase separated hydrolysis-solvothermal (HST) method. Further, the visible photocatalytic activity is enhanced greatly after modification with a proper amount of phosphate. The enhanced activity is attributed to the increased charge separation by promoting photogenerated electrons captured by the adsorbed O₂ by means of the atmosphere-controlled surface photovoltage spectra, along with the photoelectrochemical I–V curves. On the basis of the O₂ temperature-programmed desorption measurements, it is suggested for the first time that the promotion effect results from the increase in the amount of O₂ adsorbed on the surfaces of Fe₂O₃ by the partial substitution of −Fe–OH with −Fe–O–P–OH surface ends. Expectedly, the positive strategy would be also applicable to other visible-response nanosized oxides as efficient photocatalysts. This work will provide us with a feasible route to synthesize oxide-based nanomaterials with good photocatalytic performance

Enhancement Effects of Cobalt Phosphate Modification on Activity for Photoelectrochemical Water Oxidation of TiO₂ and Mechanism Insights

Cobalt phosphate-modified nanocrystalline TiO₂ (nc-TiO₂) films were prepared by a doctor blade method using homemade nc-TiO₂ paste, followed by the post-treatments first with monometallic sodium orthophosphate solution and then with cobalt nitrate solution. The modification with an appropriate amount of cobalt phosphate could greatly enhance the activity for photoelectrochemical (PEC) water oxidation of nc-TiO₂, superior to the modification only with the phosphate anions. It is clearly demonstrated that the enhanced activity after cobalt phosphate modification is attributed to the roles of cobalt­(II) ions linked by phosphate groups with the surfaces of nc-TiO₂ mainly by means of the surface photovoltage responses in N₂ atmosphere. It is suggested that the linked cobalt­(II) ions could capture photogenerated holes effectively to produce high-valence cobalt ions, further inducing oxidation reactions with water molecules to rereturn to cobalt­(II) ions. This work is useful to explore feasible routes to improve the performance of oxide-based semiconductors for PEC water splitting to produce clean H₂ energy

Accepting Excited High-Energy-Level Electrons and Catalyzing H₂ Evolution of Dual-Functional Ag-TiO₂ Modifier for Promoting Visible-Light Photocatalytic Activities of Nanosized Oxides

To improve the photocatalytic activities of narrow band gap oxide semiconductors for H₂ evolution under solar irradiation, it is highly desired to develop effective acceptors for visible light-excited high-energy-level electrons. Herein, we have successfully fabricated Ag-modified TiO₂/BiVO₄ nanocomposites by putting nanosized BiVO₄ into the Ag modified TiO₂ sol. Both steady-state and transient-state- surface photovoltage spectra demonstrate that photogenerated charge separation and lifetime of nanosized BiVO₄ is promoted when coupling with TiO₂ and modifying an appropriate amount of Ag, while the lifetime of photogenerated electrons got prolonged. Interestingly, the resulting Ag-TiO₂/BiVO₄ nanocomposites exhibit excellent visible light activities for H₂ evolution, although the visible light activities of TiO₂/BiVO₄ one, Ag/BiVO₄ and bare BiVO₄ are neglectable, indicating that Ag-TiO₂ could be utilized as effective acceptors for hydrogen production. It is suggested based on the experimental data that the effective acceptors be attributed to the used TiO₂ for accepting the high-energy-level electrons generated from BiVO₄ and to the modified Ag for being reduced then to catalyze H₂-evolution reactions. The developed strategy is versatile for other narrow band gap semiconductors, like WO₃ and Fe₂O₃

Exceptional Photocatalytic Activity of 001-Facet-Exposed TiO₂ Mainly Depending on Enhanced Adsorbed Oxygen by Residual Hydrogen Fluoride

Is it true that the exceptional photocatalytic activity of 001-facet-exposed TiO₂ is attributed to its high-energy surfaces? In this work, nanocrystalline anatase TiO₂ with different percentages of the exposed (001) facet has been controllably synthesized with a hydrothermal process using hydrofluoric acid as a morphology-directing agent. It is shown that the percentage of (001)-facet exposure is tuned from 6 to 73% by increasing the amount of used hydrofluoric acid, and meanwhile the amount of residual fluoride in the as-prepared TiO₂ is gradually increased. As the percentage of (001) facet is increased, the corresponding TiO₂ gradually exhibits much high photocatalytic activity for degrading gas-phase acetaldehyde and liquid-phase phenol. It was unexpected that the photocatalytic activity would obviously decrease when the residual fluoride was washed off with NaOH solution. By comparing F-free 001-facet-exposed TiO₂ with the F-residual one, it is concluded that the exceptional photocatalytic activity of the as-prepared 001-facet-exposed TiO₂ depends mainly on the residual hydrogen fluoride linked to the surfaces of TiO₂ via the coordination bonds between Ti⁴⁺ and F^–, as well as slightly on the high-energy 001-facet exposure, by means of the temperature-programmed desorption (TPD) measurements, the atmosphere-controlled surface photovoltage spectra, and the isoelectric point change. On the basis of the O₂-TPD tests, theoretical calculations, and O₂ electrochemical reduction behaviors, it is further suggested for the first time that the residual hydrogen fluoride as the form of −Ti:F–H could greatly enhance the adsorption of O₂ so as to promote the photogenerated electrons captured by the adsorbed O₂, leading to the great increase in the charge separation and then in the photocatalytic activity. This work would clarify the high-activity mechanism of widely investigated TiO₂ with high-energy 001-facet exposure and also provide feasible routes to further improve photocatalytic activity of TiO₂ and other oxides

Exceptional Visible-Light Activities of TiO₂‑Coupled N‑Doped Porous Perovskite LaFeO₃ for 2,4-Dichlorophenol Decomposition and CO₂ Conversion

In this work, TiO₂-coupled N-doped porous perovskite-type LaFeO₃ nanocomposites as highly efficient, cheap, stable, and visible-light photocatalysts have successfully been prepared via wet chemical processes. It is shown that the amount-optimized nanocomposite exhibits exceptional visible-light photocatalytic activities for 2,4-dichlorophenol (2,4-DCP) degradation by ∼3-time enhancement and for CO₂ conversion to fuels by ∼4-time enhancement, compared to the resulting porous LaFeO₃ with rather high photoactivity due to its large surface area. It is clearly demonstrated, by means of various experimental data, especially for the ·OH amount evaluation, that the obviously enhanced photoactivities are attributed to the increased specific surface area by introducing pores, to the extended visible-light absorption by doping N to create surface states, and to the promoted charge transfer and separation by coupling TiO₂. Moreover, it is confirmed from radical trapping experiments that the photogenerated holes are the predominant oxidants in the photocatalytic degradation of 2,4-DCP. Furthermore, a possible photocatalytic degradation mechanism for 2,4-DCP is proposed mainly based on the resultant crucial intermediate, 2-chlorosuccinic acid with <i>m</i>/<i>z</i> = 153, that readily transform into CO₂ and H₂O. This work opens up a new feasible route to synthesize visible-light-responsive high-activity perovskite-type nanophotocatalysts for efficient environmental remediation and energy production

Synthesis of Efficient Nanosized Rutile TiO₂ and Its Main Factors Determining Its Photodegradation Activity: Roles of Residual Chloride and Adsorbed Oxygen

Nanosized TiO₂ containing different contents of rutile phase was controllably synthesized by a hydrochloric acid-modified hydrothermal process. It is demonstrated that the formation of rutile phase in TiO₂ mainly depends on the role of chlorine anions in the synthesis, and a certain amount of residual chloride would exist on the surfaces of the resulting nanocrystalline rutile TiO₂. Interestingly, the as-prepared rutile shows high activity for photodegradation of rhodamine B dye compared with the as-prepared anatase, even superior to the P25 TiO₂. It is mainly attributed to the residual chloride that could promote the dye adsorbed on the surfaces of TiO₂, consequently accelerating the photosensitization oxidation reactions of the dye molecules. In the photodegradation of liquid-phase phenol and gas-phase aldehyde, the as-prepared rutile TiO₂ samples display low activity, which is attributed to the photogenerated electrons weakly captured by the adsorbed oxygen, since the residual chloride could effectively capture photoinduced holes based on the atmosphere-controlled surface photovoltage spectroscopy results. Further, the photoactivity of resulting rutile for degrading phenol and aldehyde is greatly enhanced by modifying a proper amount of phosphoric acids to increase the adsorption of O₂, even higher than that of the P25 TiO₂. This work would explore feasible routes to synthesize efficient nanosized rutile TiO₂-based photocatalysts for degrading colored and colorless organic pollutants by investigating the rate-determining factors in the photodegradation processes

Enhanced Cocatalyst-Free Visible-Light Activities for Photocatalytic Fuel Production of g‑C₃N₄ by Trapping Holes and Transferring Electrons

We have successfully synthesized boron-doped g-C₃N₄ nanosheets (B-CN) and its nanocomposites with nanocrystalline anatase TiO₂ (T/B-CN). The as-prepared T/B-CN nanocomposites with the proper amounts of boron and TiO₂ exhibit rather high cocatalyst-free photoactivities for producing H₂ from CH₃OH solution (∼29× higher) and CH₄ from CO₂-containing water (∼16× higher) under visible-light irradiation, compared to those of bare g-C₃N₄. This is attributed to the greatly enhanced photogenerated charge separation after doping boron and subsequent coupling with TiO₂, mainly based on the measurements of atmosphere-controlled steady-state surface photovoltage spectra, transient-state surface photovoltage responses, photoluminescence spectra, and fluorescence spectra related to the produced hydroxyl radical amount. It is suggested for the first time that the great charge separation enhancement results from the B-induced surface states near the valence band top to trap holes and the formed heterojunctions to transfer electrons from B-CN to TiO₂. Moreover, the created surface states are also responsible for the visible-light extension from 450 nm of g-C₃N₄ to 500 nm of B-CN (T/B-CN) for solar fuel production. Interestingly, the obtained 6T/6B-CN exhibits much larger quantum efficiencies, which are 3.08% for hydrogen evolution and 1.68% for CH₄ production at λ = 420 nm, respectively, with 5.1× and 7.6× enhancement as compared to CN, even superior to other works. This work will provide feasible routes to synthesize g-C₃N₄-based nanophotocatalysts for efficient solar fuel production

Controlled Synthesis of Nitro-Terminated Oligothiophene/Crystallinity-Improved g‑C₃N₄ Heterojunctions for Enhanced Visible-Light Catalytic H₂ Production

It is highly desired to explore closely contacted polymer semiconductor/g-C3N4 heterojunction photocatalysts with promoted photogenerated-carrier separation and extended visible-light response for efficient visible-light-driven H2 production. Here, we first synthesized the nitro-terminated oligothiophene (OTh) by the controlled copolymerization of thiophene and 2-nitrothiophene monomers, then constructed the nitro-terminated oligothiophene/crystallinity-improved g-C3N4 (OTh/g-C3N4) heterojunctions by a grinding-induced combination strategy. The ratio-optimized 20OTh5/g-C3N4 shows highly efficient H2 production activity up to 3.63 mmol h–1 g–1 under visible-light irradiation, with ∼25.9-time enhancement compared to that of g-C3N4. As verified by time-resolved photoluminescence spectra, surface photovoltage spectra, and the fluorescence spectra related to •OH amounts, the improved photocatalytic activity is due to the promoted photogenerated-carrier transfer and separation in the heterojunctions and the expanded visible-light response. It is also confirmed that the controlled OTh chain length, improved g-C3N4 crystallinity, and tight interface contact dependent on the hydrogen bonds and N···S interactions between OTh and g-C3N4 are reasonable for enhanced photogenerated-carrier separation with the electron transfer from OTh to g-C3N4. This work illustrates a feasible strategy to construct efficient polymer semiconductor/g-C3N4 heterojunction photocatalysts for solar-light-driven H2 production