Photoinduced electron-transfer (ET) dynamics in Fe(II)(CN) 6 4sensitized TiO 2 nanoparticles in D 2 O solution are studied by subpicosecond tunable laser spectroscopy in the mid-infrared and visible region. The dynamics of the injected electrons are monitored by the mid-IR absorption of electrons in the semiconductor, and the corresponding dynamics of the adsorbate are monitored by the vibrational spectra of the CN stretching mode region and electronic absorption in the visible. After 400 nm excitation, the forward electron injection time from Fe(II)(CN) 6 4to TiO 2 occurs in <50 fs, indicating a direct photoinduced charge-transfer process. The back ET from TiO 2 to Fe(III)(CN) 6 3in the <1 ns time scale is found to be a non-single-exponential process. The best three-exponential fit to the data yields back ET time constants of 3 ps (35%), 40 ps (30%), and >1 ns (35%). Combining with previous measurements in the nanosecond to microsecond time scale (Lu et al.
The design of efficient and stable photocatalysts for robust CO 2 reduction without sacrifice reagent or extra photosensitizer is still challenging. Herein, a single-atom catalyst of isolated single atom cobalt incorporated into Bi 3 O 4 Br atomic layers is successfully prepared. The cobalt single atoms in the Bi 3 O 4 Br favors the charge transition, carrier separation, CO 2 adsorption and activation. It can lower the CO 2 activation energy barrier through stabilizing the COOH* intermediates and tune the rate-limiting step from the formation of adsorbed intermediate COOH* to be CO* desorption. Taking advantage of cobalt single atoms and two-dimensional ultrathin Bi 3 O 4 Br atomic layers, the optimized catalyst can perform light-driven CO 2 reduction with a selective CO formation rate of 107.1 µmol g −1 h −1 , roughly 4 and 32 times higher than that of atomic layer Bi 3 O 4 Br and bulk Bi 3 O 4 Br, respectively.
Photocatalytic hydrogen evolution from pure water is successfully realized by using interstitial P-doped CdS with rich S vacancies (CdS-P) as the photocatalyst in the absence of any electron sacrificial agents. Through interstitial P doping, the impurity level of S vacancies is located near the Fermi level and becomes an effective electron trap level in CdS-P, which can change dynamic properties of photogenerated electrons and thus prolong their lifetimes. The long-lived photogenerated electrons are able to reach the surface active sites to initiate an efficient photocatalytic redox reaction. Moreover, the photocatalytic activity of CdS-P can be further improved through the loading of CoP as a cocatalyst.
Solar hydrogen production using photocatalytic water splitting is regarded as a promising strategy for harnessing solar energy to supply hydrogen energy. 1,2 Titanium dioxide (TiO2) is a popular and standard semiconductor used in photocatalysis, and exists in three common crystalline structures, anatase, rutile and brookite, that have been extensively investigated. Generally, anatase TiO2 is recognized as the most active phase in photocatalysts for environmental applications, while rutile and brookite TiO2 are seldom considered. [3][4][5][6] In the past few decades, almost all the researches on TiO2 can only obtain H2 but no O2 was detected during photocatalytic overall water splitting although it has thermodynamic feasible band structure. In the photocatalytic overall water splitting reaction (POWS, 2 2 2 22 H O H O ), H2 and O2 should be produced simultaneously with H2/O2 stoichiometric ratio of 2.0, which has been achieved in photoelectrochemical (PEC) system using TiO2 photoanode as early as 1972. 7 However, it has seldom been achieved on TiO2-based nanoparticulate photocatalyst. This challenge persists despite the fact that TiO2 has suitable band structure for both proton reduction and water oxidation under UV light irradiation. Besides, a similar phenomenon has been observed for other popular photocatalysts (e.g., Ta3N5, TaON). That is, some photocatalysts have suitable band structures that are thermodynamically feasible for POWS, yet they fail to catalyze POWS reaction.Immense efforts have been made to achieve POWS on TiO2 previously. The introduction of some inorganic ions (e.g., Cl -, CO3 2-) has been reported to somewhat improve the stoichiometric production of H2 and O2, which might be attributed to the intermediates involving the ions (e.g., C2O4 2-, ClO -) that are formed in these systems. 8,9 produce O2 on rutile. However, for anatase and brookite TiO2, the formed · OH radical may be strongly absorbed on the surface and coupled to evolve O2 after saturation of absorption. The different oxygen-containing intermediates formed on anatase and rutile TiO2 can also be demonstrated by the diversity of surface hydroxyl oxygen of TiO2 before and after the reaction 29,30 (Figures S6 and S7). It was found that the proportion of hydroxyl oxygen for anatase TiO2 was obviously increased after UV light irradiation (from 5.0% to 9.7%) but remained almost unchanged for rutile TiO2. Thus, the peroxy species are the most likely oxygen-containing intermediate derived from water oxidation on rutile TiO2 while · OH radical species are preferring to prevail from water oxidation on anatase TiO2. Our EPR results suggest that different intermediates are really formed during the photocatalytic water splitting for three kinds of TiO2 samples (peroxy species for rutile TiO2, · OH radical for anatase and brookite TiO2). The different intermediates resulted in different surface reaction processes, which mainly contribute to the kinetics for POWS on TiO2-based photocatalysts. EPR experiments without electron trapping agent were...
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