| Semiconductor photocatalysts have attracted much research attention because of their applications to solar energy conversion and environmental purification in the past decades. Among photocatalysts, TiO2as one of the most important transition metal oxides owning a favorable band edge position, nontoxicity, a strong optical absorption, and an inexpensive cost. The photocatalytic performance of TiO2has been influenced by many factors such as the phase structure, the surface area, the crystallite size, and the amount of surface hydroxyl groups, and so on. Among them, the restriction of recombination between electrons and holes is one of the key issues so as to enhance photocatalytic efficiency. So it is highly desirable to develop approaches that can validly promote charge separation in TiO2. Various methods, such as synthesizing branched structures, doping with metal or non-metal elements, post-growth hydrogen annealing, sensitizing with other small band gap semiconductor materials, and controlling its crystallite size and structure, have been studied to improve the photocatalytic properties of TiO2nano-materials. It is well-known that anatase and rutile TiO2as the two major crystalline phases are mostly used in photocatalytic reactions, and the most significant factor that influences the photocatalytic performance of TiO2is its crystal form. Anatase is generally accepted to be more efficient than rutile and the mixtures of the TiO2polymorphic phases (such as commercial Degussa P25) often show photocatalytic activities superior to those of the pure phases. The mixtures of the TiO2polymorphic phases are usually beneficial to enhance the photocatalytic activity due to the different energy levels for their corresponding conduction and valence bands. Fabrication surface-phase junctions of TiO2particles, has been demonstrated to be a valid strategy in photocatalytic performance. The report by Can Li et al. shows that the photocatalytic activity of TiO2was enhanced up significantly due to the surface-phase junction which is of great benefit to obtain efficient charge separation.Besides, for photoelectrochemical (PEC) water splitting, the main challenge is the design of photoanode materials with improved solar-to-hydrogen conversion efficiency. The traditional photoanode that is made by TiO2nanoparticles suffers from high charge recombination loss. Due to huge electron trapping/scattering at grain boundaries, the electron mobility in a nanoparticle film is only1%of that of bulk single crystal. Thus, one-dimension (ID) nanostructure such as nanorods and nanotubes (NTs) has received more attention because of their advantage in charge transport. However, most of the prepared ID nanostructure have only one crystal phase (rutile or anatase). Thus, it is highly desirable to synthesize a ID nanostructure owning the two phases, which can certainly enhance the charge separation and transportation. To date, it is still a real challenge to make such branched TiO2NRs which own the surface-phase junctions aimed at effective photoelectrochemical water splitting.In this study, branched TiO2NRs which own the surface anatase/rutile junctions were successfully synthesized via a simple modified hydrothermal method. PEC measurement shows that the photoelectrical response was increased significantly with the surface phase junction and the highest photocurrent density of1.02mA/cm2is observed at0.8V vs. RHE when the photoelectrode is illuminated by solar simulator (AM1.5G). The enhanced charge separation and transportation on surface-phase junctions of TiO2NRs contribute to the high efficient photoelectrochemical water splitting. Thus, the branched TiO2NRs owning surface-phase junctions could emerge as viable alternatives to traditional single-crystalline TiO2NRs for photoanode materials. The details are described as follows:1. We have studied the optimized conditions of synthesizing the bare TiO2NRs and the branched TiO2NRs, such as the influence of time or temperature of reaction and the amount of IV. The bare TiO2NRs owning best morphology and structure were obtained with2.5ml NaCl saturated aqueous solution and15ml HC1aqueous solution,0.4ml IV, conducted at150℃for15h. The branched TiO2NRs owning best morphology and structure were obtained under the reaction of40h.2. The bare TiO2NRs and the branched TiO2NRs were characterized by SEM、TEM、XRD and Raman, respectively. The results, such as Lattice fringes with interplanar spacing and the diffraction peaks, demonstrate the formation of new anatase-phase branches on those rutile-phase TiO2NRs.3. PEC performance of the bare TiO2NRs and branched NRs were studied, such as time-dependent and incident photon to current conversion efficiencies (IPCEs) measurement. PEC measurement shows that the photoelectrical response was increased significantly with the surface phase junction and the highest photocurrent density of1.02mA/cm2is observed at0.8V vs. RHE when the photoelectrode is illuminated by solar simulator. The branched TiO2NRs owning surface anatase/rutile junctions exhibit much better photoelectrochemcial water splitting activity than the NRs owning pure phase. The maximum IPCE for the branched TiO2NRs can reach62%at340nm, higher than that of the bare TiO2NRs which have a maximum IPCE of26%at340nm.4. To account for such high performance, light absorption properties of the branched NRs as well as the pure NRs were studied. The band gap was given by Kubelka-Munk function versus the energy of light over the bare TiO2and the branched TiO2NRs. Photogenerated electron hole pairs in the anatase branches can be separated by transferring the electron to the rutile-phase trunk and the energy barrier would suppress back electron transfer. The holes left in the valence band of anatase phase in the surface of branched TiO2NRs, efficiently increased the lifetimes of carriers and enhanced PEC performance.5. The current branched NRs also showed better performance with sensitization by quantum dots. After the sensitization with CdS-ZnS QDs, the saturation photocurrent increased to2.3mA/cm2at1V. In comparison, for the bare NRs, the saturation photocurrent just increased to0.7mA/cm2at1.5V. It should be noted that our sensitization have not been fully optimized and hopefully it can be further improved to get better PEC performance.6. Carbon quantum dots (CQDs) are successfully synthesized by a facial alkali-assisted electrochemical method. CQDs show excitation-dependent PL emission. Owing to their excellent upconversion luminescence property and high conductivity of CQDs, the highest photoconversion efficiency obtained from our CQDs/TiO2composites is-1.2%, much better than pristine TiO2. Since carbon is the large family element in the earth with low toxity, this work provides a new concept of designing sensentizer in a variety of solar energy driven applications.7. The branched TiO2NRs were synthesized by futher hydrothermal treatment after deposited by the previous method. The branched TiO2NRs were characterized by SEM、XRD and Raman, respectively. The results demonstrate the formation of new anatase-phase branches on those rutile-phase TiO2NRs. We have studied the optimized conditions of synthesizing the branched TiO2NRs, such as the influence of time or temperature of reaction. |
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