Role of Dissociatively Adsorbed Water on the Formation of Shallow Trapped Electrons in TiO₂ Photocatalysts

Abstract

The mismatch between short lifetimes of free charge carriers and slow kinetics of surface redox reactions substantially limits the efficiency of most photocatalytic systems. Hence, the knowledge of trapping and recombination of photogenerated electrons and holes at different time scales is key for a rational optimization of photocatalytic materials. In this study, we used subsecond time-resolved diffuse-reflectance FTIR spectroscopy to investigate how energy and intensity of the incident irradiation affect the dynamics of photogenerated charge carriers in TiO2 P25 photocatalysts subjected to different pretreatments and how shallow trapped electrons (STE) are formed under these conditions. Intensity-dependent measurements demonstrated that electrons and holes generated by 325 and 409 nm irradiation undergo bimolecular and trap-assisted recombination, respectively. Analysis of characteristic times of photogenerated electron absorption rise and decay indicated that the apparent charge carrier dynamics at the time scale of seconds to minutes relate to chemical trapping of photogenerated electrons and holes. The presence of dissociatively adsorbed water on the oxide surface was required for efficient STE formation. This suggests that STE form at the seconds–minutes time scale upon surface-mediated self-trapping of electrons.

Reactivity of Trapped and Accumulated Electrons in Titanium Dioxide Photocatalysis

“…i , can be experimentally confirmed by means of electron spin resonance (ESR) [38][39][40][41][42], infrared radiation (IR) [40,[42][43][44][45][46][47][48], ultraviolet-visible absorption (UV-vis) [14,42,[49][50][51], photoluminescence (PL) [52], photoacoustic [53][54][55][56], and photoelectron spectroscopies [17,18,30]. Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are powerful tools for the direct observation of surface Ti ds [17,18,30,[57][58][59].…”

“…Interestingly, the electrons Ti ds 3+ and e cb − are distinguishable by measuring IR spectra. The free electrons e cb − exhibit the typical exponential frequency-dependent spectrum that is attributed to the intra-CB transition [43,44,46,48], whereas the trapped electrons Ti ds 3+ are characterized by a broad absorption in the mid-IR region that is ascribed to a direct optical transition [46,48].…”

Photon assisted-inversion of majority charge carriers in molecular semiconductor-based organic heterojunctions

“…Such behavior is explained by the generation of additional free carriers (e or h + ), which are otherwise held in the shallow traps, upon photon exposure. 47 Accordingly, device current increases under LED exposure compared to dark current for both devices. Similar measurements on other heterojunctions and LuPc 2 resistor did not result in any significant change in the current values (I(V) curves shown in Fig.…”

In situ attenuated total reflection infrared spectroscopy study of the photocatalytic steam reforming of methanol on Pt/TiO2

“…10). This absorption shift is attributed to the formation of both photopromoted conduction band (CB) and shallow trapped (ST) electrons [8,[16][17][18]. In particular, CB electrons provoke a distinct exponential absorption growth of the IR baseline while ST electrons give rise to broad bands due to the optical excitation of electrons from the ST states into the CB.…”

“…Infrared spectroscopy has been widely used to study the interactions of methanol and water with the TiO2 surface [6][7][8][9][10] and to provide information on the photoreaction mechanism [10][11][12][13][14][15]. In addition to the detection of surface species formed during the photoreaction, time resolved FT-IR spectroscopy proved to be a powerful tool to distinguish between photopromoted free CB electrons and shallow trapped (ST) electrons and to evaluate their lifetime [8,[16][17][18].…”