Deep Surface Trap Filling by Photoinduced Carriers and Interparticle Electron Transport Observed in TiO2 Nanocrystalline Film with Time-Resolved Visible and Mid-IR Transient Spectroscopies
详细信息 查看全文
- 作者：Hui Zhao ; Qingli Zhang ; Yu-Xiang Weng
- 刊名：Journal of Physical Chemistry C
- 出版年：2007
- 出版时间：March 8, 2007
- 年：2007
- 卷：111
- 期：9
- 页码：3762 - 3769
- 全文大小：331K
- 年卷期：v.111,no.9(March 8, 2007)
- ISSN：1932-7455
文摘
TiO2 nanocrystal and TiO2-xSiO2 core/shell films were investigated by band gap excitation followed bydetection of the photoinduced carriers with time-resolved transient absorption spectroscopy in both the visibleand mid-IR range. The passivation of the TiO2 nanocrystal with the SiO2 shell has two goals, that is, reducingthe surface-trapped states and segregating the TiO2 nanoparticles in the sintered film to prevent interparticlecarrier migration. The results show that within the accessible mid-IR range (5-6.5 
m) the observed transientabsorptions of the TiO2 crystal and TiO2-xSiO2 core/shell films exhibit an IR spectrum typical of intrabandtransition of free electrons where the observed absorption intensity is proportional to 
hars/lambda.gif" BORDER=0 >p (hars/lambda.gif" BORDER=0 > = wavelength/m), and p was determined as 2.6 for the TiO2 nanoparticle film and 4.9 for the TiO2-xSiO2 core/shellcomposite film, respectively. Three typical kinetic phases for the transient IR absorption decay profile of theTiO2 film have been observed, that is, a rapid decay beyond the instrumental response time (~80 ns), ableaching recovery with a rising time constant around 1.8 s, and a very slow absorption decay component.The rapid decay phase is assigned as the free carrier recombination within the bulk crystal, whereas thebleaching recovery corresponds to the filling of the deep trapping sites by the shallow trapped electrons, andfinally the very slow absorption component is assigned to the interparticle electron transport and deep trappedelectron-hole recombination. For the TiO2-xSiO2 core/shell film, only the rapid decay phase can be observed.During the visible light detection, a broad-band transient absorption with a peak located at about 650 nm hasbeen observed for the TiO2 nanoparticle film, which is expected for the absorption from the shallow trappedelectrons, while the corresponding carrier relaxation kinetics can be fitted by a monoexponential decay witha time constant of about 1.8 s, correlating well to the deep trap filling process revealed by IR absorption.It is thus assigned as the filling of the deep trapped sites by the shallow trapped electrons. In contrast, noobservable transient absorption can be detected at 650 nm for the TiO2-xSiO2 core/shell film. As a result,the facts from both the IR and visible light detections indicate that the passivation of SiO2 reduces the densityof the trapped state on TiO2 nanoparticle surface substantially and blocks the interparticle carrier migrationefficiently.
m) the observed transientabsorptions of the TiO2 crystal and TiO2-xSiO2 core/shell films exhibit an IR spectrum typical of intrabandtransition of free electrons where the observed absorption intensity is proportional to hars/lambda.gif" BORDER=0 >p (hars/lambda.gif" BORDER=0 > = wavelength/m), and p was determined as 2.6 for the TiO2 nanoparticle film and 4.9 for the TiO2-xSiO2 core/shellcomposite film, respectively. Three typical kinetic phases for the transient IR absorption decay profile of theTiO2 film have been observed, that is, a rapid decay beyond the instrumental response time (~80 ns), ableaching recovery with a rising time constant around 1.8 s, and a very slow absorption decay component.The rapid decay phase is assigned as the free carrier recombination within the bulk crystal, whereas thebleaching recovery corresponds to the filling of the deep trapping sites by the shallow trapped electrons, andfinally the very slow absorption component is assigned to the interparticle electron transport and deep trappedelectron-hole recombination. For the TiO2-xSiO2 core/shell film, only the rapid decay phase can be observed.During the visible light detection, a broad-band transient absorption with a peak located at about 650 nm hasbeen observed for the TiO2 nanoparticle film, which is expected for the absorption from the shallow trappedelectrons, while the corresponding carrier relaxation kinetics can be fitted by a monoexponential decay witha time constant of about 1.8 s, correlating well to the deep trap filling process revealed by IR absorption.It is thus assigned as the filling of the deep trapped sites by the shallow trapped electrons. In contrast, noobservable transient absorption can be detected at 650 nm for the TiO2-xSiO2 core/shell film. As a result,the facts from both the IR and visible light detections indicate that the passivation of SiO2 reduces the densityof the trapped state on TiO2 nanoparticle surface substantially and blocks the interparticle carrier migrationefficiently.