| With the increasing in consumption of fossil energy, energy crisis andenvironmental issues are becoming serious threats to the long-term development ofhuman society. Governments and scientists are trying to find green technologies assustainable ways to address these concerns. Among potential solutions,semiconductor-based materials has emerged as one of the most attractive techniquesbecause it is considered as an economic, renewable, and clean technique, whichrequires only the inexhaustible and efficient solar light as a driving force, and asuitable semiconductor can conduct reactions for a variety of applications, such ashydrogen production from water splitting, decomposition of organic pollutants andsolar cells etc.. In recent years, great and fruitful efforts have been made in the fieldof semiconductor applications, but the efficiency of the semiconductor photoelectricmaterials in various systems is much less than the standard of industrial application.One of the reason for this could be ascribed to the fact that mechanism of thephotogenerated carriers behavior of semiconductor materials is still unclear, which isunfavorable for the design a highly efficient photoelectric materials. It is well knownthat a semiconductor material absorbs light of energy greater than or equal to its bandgap, causing the formation of photogenerated charges which can participate in a solarcell reaction or can further generate free radicals in the system for redox of thesubstrate. However, photogenerated charges need to reach the surface of materials to participate in the reaction. Therefore, the researches on photogenerated chargebehavior of semiconductor surface/interface states play an important role in thephoto(electric)catalysis and photoelectric conversion process.Although various approaches have been taken to understand the properties ofsemiconductor surfaces/interfaces, to the best our knowledge, the mechanism ofcharges transfer at the semiconductor surface/interface is still vague relatively. Themain reason is attributed to the lack of effective detection tools for photogeneratedcharge behavior at the surface/interface states. Therefore, to study photoelectricproperties of semiconductor surface/interface states, we present a new method basedon the surface photovoltage technique, and then in the combination of other detectiontechniques to obtain the information on photogenerated charge behavior atsurface/interface states.Details as follows:(1) We present a new method surface photovoltage electric field scanningtechnique to assist understanding surface/interface states photoelectric properties,which is a kind of surface photovoltage application. Due to the small capturecross-section for light, the density of photogenerated carriers is too low to analyze thebehaviors of photogenerated electrons and holes at surface states. Therefore, anexternal electric field is applied to the two sides of the sample to vary the mobiledirection and diffusive distance of photogenerated charge carriers. Thus, thephotogenerated charge carriers transfer process can be detected clearly. In addition,once an external electric field was applied, charge population of the surface states willbe changed, and characteristic that related to the surface state charges energy can berevealed. Surface photovoltage electric field scanning tests for TiO2(P25) and CdSwere conducted at410nm and540nm, respectively. It was confirmed that the surfacestate of TiO2is a donor state and the surface state of CdS is an acceptor state. P-n typeCu2O was also tested by means of surface photovoltage electric field scanningtechnique, and then the direction and strength of the interface electric field(approximately200mV) were obtained. Based on the above results, we believed thatsurface photovoltage electric field scanning technique is an effective tool to investigate the characteristics of photogenerated carriers related to surface/interfacestates.(2) In this work, the Au/TiO2and Pt/TiO2nanomaterials were prepared, and theinterface-charge-transfer processes on Au/TiO2and Pt/TiO2were analyzed by surfacephotovoltage, field-induced photovoltage and surface photovoltage electric fieldscanning techniques, respectively. It was found that Au or Pt nanoparticles loadingaffect the band structure of TiO2. Due to a difference in the work function between Auand Pt (WPtis higher than WAu), the effect of noble metal loading on the intrinsicband of TiO2is different to some degree, resulting in competition of two kinds ofphotogenerated charge migration: photogenerated electrons tansfer towardsthe surface are detected as a negative signal of SPV; photogenerated holes migrate tothe surface are detected as a positive signal of SPV.(3) Ti3+self-doped TiO2nanoparticles were synthesized under N2calcination.The photogenerated charge behavior of Ti3+surface doping TiO2was detected bymeans of the surface photovoltage techniques. It was found that the doped Ti3+ionformed an acceptor surface state on the surface of TiO2. The acceptor state cancapture the photogenerated electrons, and then prolong the life of the photogeneratedcarriers. To further study the effect of doping sites on the photoelectric properties ofTiO2nanoparticles, Ti3+surface doping TiO2(S-TiO2) and Ti3+bulk doping TiO2(B-TiO2) nanoparticles were tested. The results showed that S-TiO2and B-TiO2havedifferent ways of electron capture, and the rate of electron capture in B-TiO2is faster,resulting in enhanced photocatalytic H2production activity. |
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