Liqiang Jing

Dynamics of photogenerated charges in the phosphate modified TiO2 and the enhanced activity for photoelectrochemical water splitting

Phosphate modified nanocrystalline TiO2 (nc-TiO2) films were prepared by a doctor blade method, followed by post-treatment with monometallic sodium orthophosphate solution. The dynamic processes of the photogenerated charges from the resulting nc-TiO2 films were thoroughly investigated by means of transient absorption spectroscopy (TAS). It is shown that photogenerated holes in the un-modified TiO2 film exhibit the same dynamic decay process as its photogenerated electrons, in oxygen-free water of pH 7. However, photogenerated holes in the phosphate modified film display a slightly faster dynamic decay process than its photogenerated electrons, and photogenerated charges of the modified film have a much longer lifetime than those of the un-modified film. These differences are attributed to the surface-carried negative charges of nc-TiO2 resulting from the phosphate groups (–Ti–O–P–O−). Interestingly, the photoelectrochemical (PEC) experiments show that modification with an appropriate amount of phosphate could improve the photocurrent density of the nc-TiO2 film electrode by about 2 times, at a voltage of 0 V in the neutral electrolyte. Based on the TAS and PEC measurements of un-modified and phosphate modified nc-TiO2 films, with different conditions, it is suggested that the prolonged lifetime of photogenerated charges can be attributed to the negative electrostatic field formed in the surface layers. It is also responsible for the increase in activity for PEC water splitting and for the reported photocatalytic degradation of pollutants. The suggested mechanism would be applicable to other oxide semiconductor photocatalysts and to modification with other inorganic anions.

Effective charge separation in the rutile TiO2 nanorod-coupled α-Fe2O3 with exceptionally high visible activities

Herein, we have fabricated rutile TiO2 nanorod-coupled α-Fe2O3 by a wet-chemical process. It is demonstrated that the visible activities for photoelectrochemical water oxidation and for degrading pollutant of α-Fe2O3 are greatly enhanced after coupling a proper amount of rutile nanorods. The enhanced activity is attributed to the prolonged lifetime and improved separation of photogenerated charges mainly by the transient surface photovoltage responses. Interestingly, the observed EPR signals (with g⊥ = 1.963 and g|| = 1.948) of Ti3+ in the fabricated TiO2-Fe2O3 nanocomposite at ultra low temperature (1.8u2005k) after visible laser excitation, along with the electrochemical impedance spectra and the normalized photocurrent action spectra, testify evidently that the spacial transfers of visible-excited high-energy electrons of α-Fe2O3 to TiO2 could happen. Moreover, it is confirmed that it is more favorable for the uncommon electron transfers of α-Fe2O3 to rutile than to anatase. This is responsible for the much obvious enhancement of visible activity of Fe2O3 after coupling with rutile TiO2, compared with anatase and phase-mixed P25 ones. This work would help us to deeply understand the uncommon photophysical processes, and also provide a feasible route to improve the photocatalytic performance of visible-response semiconductor photocatalyst for water splitting and pollutant degradation.

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.

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.

Improved visible-light activities for degrading pollutants on TiO2/g-C3N4 nanocomposites by decorating SPR Au nanoparticles and 2,4-dichlorophenol decomposition path

It has been clearly demonstrated that the visible-light photocatalytic activities of g-C3N4 (CN) for degrading 2,4-dichlorophenol (2,4-DCP) and bisphenol A (BPA) could be improved by fabricating nanocomposites with a proper amount of nanocrystalline anatase TiO2. Interestingly, the visible-light activities of the amount-optimized nanocomposite could be further improved after decorating Au nanoparticles, with 5.11- and 3.1-time improvement respectively for 2,4-DCP and BPA compared to that of CN, even much higher than that of P25 TiO2 under UV-vis irradiation. Based on the transient-state surface photovoltage responses and photoelectrochemical measurements, it is confirmed that the exceptional visible-light activities of the fabricated Au-(TiO2/g-C3N4) nanocomposites are attributed to the extended visible-light response due to the surface plasmonic resonance (SPR) of decorated Au and its catalytic function, and to the enhanced charge separation by transferring electrons from CN and SPR Au to TiO2 in the nanocomposites. The highly promoted charge separation results in the effective availability of a large number of hydroxyl radicals (OH) participating in the photocatalytic oxidation process of 2,4-DCP. Furthermore, a possible mechanism of 2,4-DCP degradation is proposed according to the detailed analyses of produced intermediates. This work provides new idea for designing Au assisted nanocomposite photocatalysts for environmental remediation.

ZnO-dotted porous ZnS cluster microspheres for high efficient, Pt-free photocatalytic hydrogen evolution

The Pt-free photocatalytic hydrogen evolution (PHE) has been the focus in the photocatalysis field. Here, the ZnO-dotted porous ZnS cluster microsphere (PCMS) is designed for high efficient, Pt-free PHE. The PCMS is designed through an easy “controlling competitive reaction” strategy by selecting the thiourea as S2− source and Zn(Ac)2·2H2O as Zn source in ethylene glycol medium. Under suitable conditions, one of the PCMS, named PCMS-1, with high SBET specific area of 194u2005m2g−1, microsphere size of 100u2005nm and grain size of 3u2005nm can be obtained. The formation of PCMS is verified by TEM, XAES, XPS, Raman and IR methods. Importantly, a series of the experiments and theoretical calculation demonstrate that the dotting of ZnO not only makes the photo-generated electrons/hole separate efficiently, but also results in the formation of the active catalytic sites for PHE. As a result, the PCMS-1 shows the promising activity up to 367u2005μmol h−1 under Pt-free condition. The PHE activity has no obvious change after addition 1u2005wt.% Pt, implying the presence of active catalytic sites for hydrogen evolution in the PCMS-1. The easy synthesis process, low preparation cost of the PCMS makes their large potential for Pt-free PHE.

Modification Strategies with Inorganic Acids for Efficient Photocatalysts by Promoting the Adsorption of O2

Efficient photocatalysis for degrading environmental organic pollutants on semiconductors requires photogenerated charge carrier separation to drive the photochemical processes. To ensure charge separation, it is indispensable to make charges captured effectively. Generally, the step for capturing the photogenerated electrons by the surface adsorbed O2 is relatively slow as compared to that for capturing holes by the surface adsorbed hydroxyl groups so that it is taken as the rate-determining step. However, it is frequently neglected. Thus, it is greatly desired to develop feasible strategies to promote the adsorption of O2 for efficient photocatalysts. In this paper, we have mainly discussed surface modification with inorganic acids, such as H3PO4, HF, and H3BO3, to enhance photogenerated charge carrier separation based on oxygen adsorption promotion for photocatalytic degradation of environmental pollutants. Among these acids, the function and mechanism of H3PO4 are highlighted because of its good performance and universality. Several important photocatalyst systems, mainly including TiO2, α-Fe2O3, and g-C3N4, along with the nanostructured carbons as electron acceptors in nanocomposites, are addressed to improve the ability to adsorb O2. A key consideration in this review is the development of a strategy for the promotion of adsorbed O2 for efficient photocatalysts, along with the process mechanisms by revealing the relationships among the adsorbed O2, photogenerated charge carrier separation, and photocatalytic performance. Interestingly, it is suggested that the enrichment in surface acidity be favorable for promotion of O2 adsorption, leading to the improved charge carrier separation and then to the enhanced photoactivities of various semiconductor photocatalysts. Moreover, several outlooks are put forward.

Exceptional performance of photoelectrochemical water oxidation of single-crystal rutile TiO2 nanorods dependent on the hole trapping of modified chloride

It is highly desired to effectively trap photogenerated holes for efficient photoelectrochemical (PEC) water oxidation to evolve O2 on oxide semiconductors. Herein, it is found for the first time mainly based on the time-resolved- and atmosphere-controlled- surface photovoltage responses that the modified chloride would effectively trap photogenerated holes so as to prolong the charge lifetime and hence promote charge separation of single-crystal rutile TiO2 nanorods. Its strong capacity to trap holes, comparable to the widely-used methanol and Co(II) phosphate, is well responsible for the exceptional photoactivities for PEC water oxidation to evolve O2 on rutile nanorods with a proper amount of chloride modified, about 2.5-time high as that on the resulting anatase nanoparticles, even 10-time if the surface area is considered. Moreover, it is suggested that the hole trapping role of chemically-adsorbed chloride is related to its lonely-pair electrons, and to the subsequently-produced intermediate Cl atoms with proper electronegativity for evolving O2. Interestingly, this finding is also applicable to the chloride-modified anatase TiO2. This work will provide a feasible strategy to design high-activity nanostructured semiconductor photoanodes for PEC water oxidation, even for overall water splitting.

Synthesis of silicate-bridged ZnO/g-C3N4 nanocomposites as efficient photocatalysts and its mechanism

In this study, silicate-capped ZnO/g-C3N4 nanocomposites were successfully fabricated by a simple wet chemical process. The photocatalytic activities of g-C3N4 for the degradation of phenol and the production of H2 are greatly enhanced after coupling with an appropriate amount of nanocrystalline ZnO. This is attributed to the prolonged lifetime and increased separation of the photogenerated charges, which are mainly based on atmosphere-controlled steady-state surface photovoltage spectra and time-resolved surface photovoltage responses. Interestingly, the lifetime and separation of photogenerated charges are further improved after the introduction of silicate groups as linkers between ZnO and g-C3N4, which consequently lead to enhanced photocatalytic activities, as much as 4 times higher compared to g-C3N4. Evidently, the built silicate linkers, which act as bridges, are highly favorable for charge transfer and separation in the fabricated heterojunctional nanocomposites and also for efficient photocatalysis. The present work provides a simple and feasible idea to enhance photogenerated charge separation so as to improve the photoactivities of nanocomposites.

Coupling of Nanocrystalline Anatase TiO2 to Porous Nanosized LaFeO3 for Efficient Visible-Light Photocatalytic Degradation of Pollutants

In this work we have successfully fabricated nanocrystalline anatase TiO2/perovskite-type porous nanosized LaFeO3 (T/P-LFO) nanocomposites using a simple wet chemical method. It is clearly demonstrated by means of atmosphere-controlled steady-state surface photovoltage spectroscopy (SPS) responses, photoluminescence spectra, and fluorescence spectra related to the formed OH− radical amount that the photogenerated charge carriers in the resultant T/P-LFO nanocomposites with a proper mole ratio percentage of TiO2 display much higher separation in comparison to the P-LFO alone. This is highly responsible for the improved visible-light activities of T/P-LFO nanocomposites for photocatalytic degradation of gas-phase acetaldehyde and liquid-phase phenol. This work will provide a feasible route to synthesize visible-light responsive nano-photocatalysts for efficient solar energy utilization.