Evidencing opposite charge-transfer processes at TiO2/graphene-related materials interface through a combined EPR, photoluminescence and photocatalysis assessment

Combining TiO2 with graphene or with graphene related materials (GRMs) is an attractive strategy for enhancing the TiO2 photocatalytic activity through charge separation at the TiO2/GRMs hetero-interface. With the aim of shed light on the various factors that can affect the actual TiO2/GRMs charge transfer behavior, in the present work two series of TiO2(P25)/GRM composites containing growing amount (up to 10 wt.%) of GRMs (rGO and graphene-like layers, GL) were considered. The experimental campaign was carried out by combining standard characterization techniques, EPR analysis, excitation-resolved photoluminescence (PLE) spectroscopy and methylene blue photodegradation tests coupled with scavenging tests. Although 1% GRM load improves MB photodegradation of both P25/GRM composites, no significant MB photodegradation activity differences among the employed P25/GRM composites were highlighted by standard photocatalytic tests. Nevertheless, EPR, PLE and photodegradation analyses with scavenging agents evidenced basic differences in the two P25/GRM composites. EPR analysis showed that the spin density in rGO and GL respectively increase and decrease, as they are brought in contact with P25 in the composites. The photoluminescence (PL) spectral shape of P25 were not modified by interaction with rGO, and its intensity was uniformly and smoothly quenched as the rGO load was increased. On another hand, P25 in P25-GL composites exhibited an enhanced and spectrally-modified PL intensity, with a sharp decrease vs GL load. Hole and ROS scavenging tests evidenced that the oxidation by reactive radicals formed from photoexcited electrons is negligibly affected by the presence of the two GRMs, while the oxidation by reactive radicals formed from photoinduced holes is enhanced in P25-rGO composites and hindered in P25-GL ones. These findings suggested that two opposite charge-transfers mechanisms occur, namely electron transfer from P25 to rGO and hole transfer from P25 to GL.