| With the consumption of fossilfuel, the emergence of energy crisis becomes a big problem for the human society. Solar energy, one of clean and environmentally friendly energy mode, is drawing more and more attention to meet the increasing demands in this century. As a promising method, the photoelectrochemical(PEC) water splitting into H2 and O2 on semiconductor photoanode, it offers a solution to harvest and store solar energy. It is very meaningful to develop visible-responsed photocatalyst, which is attributed to the fact that visible light occupies about 44% in solar spectrum. Hematite(α-Fe2O3) is currently considered one of the most promising photoanode materials for water splitting due to a series of desirable properties, including adequate absorption from the visible light region in solar spectrum(bandgap between 2.0 and 2.2 e V), excellent stability in an aqueous environment, and abundant existence in nature. However, it usually exhibits weak photocatalytic performance for water splitting, mainly owing to the low carrier mobility and very short excited-state lifetime. Hence, it is highly desired to enhance the photocatalytic performance of Fe2O3 for effective solar storage and utilization. The steady-state surface photovoltage spectra(SS-SPS) and transient-state surface photovoltage(TS-SPV) techniques are applied to investigate charge separation and carriers lifetime, and the results are as follows:First, we fabricated rutile Ti O2 nanorod-coupled Fe2O3 by a wet-chemical process. It is demonstrated that the visible activities for photoelectrochemical water oxidation and for degrading pollutant of Fe2O3 are greatly enhanced after coupling a proper amount of rutile nanorods. The enhancedactivity is attributed to the prolonged lifetime and improved separation of photogenerated charges mainly by the transient surface photovoltage responses. Interestingly, the observed EPR signals of Ti3+ in the fabricated Ti O2-Fe2O3 nanocomposite at ultra low temperature(1.8 k) aftervisible 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 Fe2O3 to Ti O2 could happen. Meanwhile, by controlling different phase of Ti O2, it is illustrated that the rutile with the low conduction band is beneficial for the high-energy electrons transfers compared with the anatase. Meanwhile, we have successfully constructed phosphate bridges in a Ti O2-Fe2O3 nanocomposite using wet-chemical processes. Based on FTIR, XPS and TEM measurements it is confirmed that phosphate groups form bridges that effectively connect Ti O2 and α-Fe2O3. From SS-SPS and TS-SPV measurements in N2, it is clearly demonstrated that the separation and lifetime of the photogenerated charge carriers in the Ti O2-Fe2O3 nanocomposite are greatly enhanced by the introduction of the phosphate bridges. As a consequence, the visible light photocatalytic activity in water reduction by methanol and the photoelectrochemical water oxidation are obviously improved after phosphate bridging, whichare attributed to the high-energy electron transfer from α-Fe2O3 to Ti O2, mainly on the basis of ultra-low-temperature EPR signals, EIS spectra and the normalized photocurrent action spectraSecond, we fabricated Sn O2-Fe2O3 nanocomposite by a wet-chemical process. Based on SS-SPS and TS-SPV in N2, it is demonstrated that the introduction of Sn O2 effectively promotes the charge separation and prolongs the charge carriers lifetime, leading to the high performance for water oxidation and degradiaton. Meanwhile, comparing the Ti O2 and Zr O2 with Sn O2, it is concluded that Sn O2 with the low conduction band is benefits for the high-energy electrons transfers, and high-energy electrons transfers could not happen when Zr O2 owning a higher conduction band is introduced.This work would help us to deeply understand the uncommon photophysical processes, and also provide afeasible route to improve the photocatalytic performance of visible-response semiconductor photocatalyst for water splitting and pollutant degradation. |
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