Probing Ultrafast Photoinduced Electron Transfer to TiO₂ from CdS Nanocrystals of Varying Crystallographic Phase Content

Abstract

The photoinduced electron transfer processes in a nano-heterostructured semiconductor assembly are complex and depend on various parameters of the constituents of the assembly. We present here the ultrafast electron transfer characteristics of an assembly comprising of a wide band semiconductor, titanium dioxide (TiO 2 ) attached to light harvesting cadmium sulfide (CdS) nanocrystals of varying crystallographic phase content. Quantitative analysis of synchrotron high resolution X-ray diffraction data of CdS nanocrystals precisely reveal the presence of both wurtzite and zinc blende phases in varying amounts. The estimated content of crystal phases is observed to be strongly dependent on an important synthesis parameter viz. the ratio of the two solvents. The biphasic nature of CdS influences directly the shape of the nanocrystal at long reaction times as well as the transfer of the photo-excited electrons from the CdS to TiO 2 as obtained from transient absorption spectroscopy. Higher amount of zinc blende phase is observed to be beneficial for fast electron transfer across the CdS-TiO 2 interface. The electron transfer rate constant differs by one order in magnitude between the CdS nanocrystals and varies linearly with the fraction of the phases.

Persistent Quantum Coherence and Strong Coupling Enable Fast Electron Transfer across the CdS/TiO2 Interface: A Time-Domain ab Initio Simulation

“…The time constants of the dynamics are obtained by fitting to the exponential decay f(t) = exp(−t/τ). The calculated subpico-second transfer time, 260 fs, is consistent with transfer times measured by TA spectroscopy, 300−600 fs, 30 for the CdS/ TiO2 heterostructure. The same fitting has been used for time evolution of the energies of the excited electrons transferring into TiO2 (see Figure S4).…”

“…Two possible electron–hole recombination processes within CdS and across the CdS/TiO 2 interface have been resolved to take place over longer time periods, and these are in very good agreement with reported time scales from TA measurements. − The slow recombination across the junction is due to very weak nonadiabatic coupling (NAC) between associated two edge electronic states as well as extremely rapid loss of the quantum coherence between these states.…”

“…This enables an effective charge separation across the CdS/TiO 2 junction as a result of the thermally favorable injection of visible-light-excited electrons from the conduction band (CB) of CdS into the CB of TiO 2 . The electron transfer has been measured to be faster than the recombination of the photoelectrons and holes. − This fast transfer could reduce the recombination of the photoinduced charges, thus resulting in longer-lived photoinduced electrons, which is desirable for postseparation photoreduction processes. , However, the electron transfer efficiency could be reduced by other competitive electron loss pathways, such as electron–hole recombination across the interface or charge trapping on the surface of the semiconductor electron donor. , …”

Interfacial Charge Transfer in Photoelectrochemical Processes

“…The study revealed that the photoexcited electrons gather in the graphene layer and photogenerated holes accumulate in the anatase TiO 2 (001) facets due to interfacial electron transfer and is indicated by the enhanced absorption intensity in the visible region due to red shift of the absorption spectra is observed and which lead to the enhancement of the photocatalytic efficiency. Similarly, transfer of photogenerated excitons at the interface has been established in CdS‐TiO 2 composites by transient absorption spectroscopy . The interfacial charge transfer time scale at semiconductor–semiconductor interface is within the picosecond‐microsecond range .…”

Construction of immobilized CuS/TiO2 nanobelts heterojunction photocatalyst for photocatalytic degradation of enrofloxacin: synthesis, characterization, influencing factors and mechanism insight

“…Mazumdar et al . synthesized the nanoheterostructured semiconductor CdS‐TiO 2 , and found an ultrafast electron transfer characteristic across the CdS–TiO 2 interface . Therefore, the construction of a TiO 2 ‐based heterojunction photocatalyst may be a good strategy to enhance the photocatalytic activity.…”