Graphitic carbon nitride (g-C3N4) is a robust organic semiconductor photocatalyst with proven H2 evolution ability. However, its application in a photoelectrochemical system as a photocathode for H2 production is extremely challenging with the majority of reports representing it as a photoanode. Despite research into constructing g-C3N4 photocathodes in recent years, factors affecting an n-type semiconductor’s properties as a photocathode are still not well-understood. The current work demonstrates an effective strategy to transform an n-type g-C3N4 photoanode material into an efficient photocathode through introducing electron trap states associated with both N-defects and C–OH terminal groups. As compared to the g-C3N4 photoelectrode, this strategy develops 2 orders of magnitude higher conductivity and 3 orders of magnitude longer-lived shallow-trapped charges. Furthermore, the average OCVD lifetime observed for def-g-C3N4 is 5 times longer than that observed for g-C3N4. Thus, clear photocathode behavior has been observed with negative photocurrent densities of around −10 μA/cm2 at 0 V vs RHE. Open circuit photovoltage decay (OCVD), Mott–Schottky (MS) plot, and transient absorption spectroscopy (TAS) provide consistent evidence that long-lived shallow-trapped electrons that exist at about the microsecond time scale after photoexcitation are key to the photocathode behavior observed for defect-rich g-C3N4, thus further demonstrating g-C3N4 can be both a photoanode and a photocathode candidate.

Photocatalysis is a promising sustainable method to generate solar fuels for the future, as well as having other applications such as water/air purification. However, the performance of photocatalysts is often limited by poor charge carrier dynamics. To improve charge carrier dynamics, it is necessary to characterize and understand charge carrier behavior in photocatalytic systems. This critical review will present Transient Absorption Spectroscopy (TAS) as a useful technique for understanding the behavior of photoexcited charges in semiconductor photocatalysts. The role of TAS amongst other techniques for characterizing charge carrier behavior will be outlined. Basic principles behind TAS will be introduced, and interpretation of TAS spectra and kinetics will be discussed in the context of exemplar literature. It will be demonstrated that TAS is a powerful technique to obtain fundamental understanding of the behavior of photoexcited charges.

Time-resolved absorption and IR spectroscopies can explore the charge dynamics and kinetics of heterogeneous photocatalytic systems and elucidate the correlation between materials design, charge carrier behavior, and photocatalytic activity.

Methanol with the 12.5wt% H2 content is widely considered as a liquid hydrogen medium. Coupling with water with the 11%% H2 content, liquid water reforming of methanol for H2 synthesis is a promising way for on-demand hydrogen production 1 . We demonstrated an atomic-level catalyst design strategy, using synergy between single atoms and nanodots for H2 production. The PtCu-TiO2 sandwich photocatalyst achieved a remarkable H2 yield (2383.9 µmol h -1 or 476.8 mmol g -1 h -1 ) with a high apparent quantum efficiency (99.2%). Furthermore, the oxidation product is high-value chemical formaldehyde with 98.6% selectivity, instead of CO2, leading to a nearly zero-carbon-emission process. Detailed investigations indicated a dual-role of copper atoms: an electron acceptor to facilitate photoelectron transfer to Pt, a hole acceptor for the selective oxidation of methanol to formaldehyde, thus avoiding overoxidation to CO2. The synergy between Pt nanodots and Cu single atoms together reduced the activation energy of this process to 13.2 kJ/mol.Hydrogen as one of the most critical energy vectors can be derived from diverse resources, including natural gas, biomass/bioalcohols and water driven by primary energy sources 2 . However, this secondary energy source suffers from transportation and storage issues. On-site synthesis of hydrogen has been reported as a promising technology to meet the requirement of the end user and to bypass such transportation and storage challenge. Among various hydrogen source feedstocks, methanol has widely been considered as the most crucial candidate due to its high mass and volume hydrogen density as well as easy transportation and storage using the

The intrinsic behavior of photogenerated charges and reactions with chemicals are key for a photocatalytic process. To observe these basic steps is of great importance. Here we present a reliable and robust system to monitor these basic steps in powder photocatalysts, and more importantly to elucidate the key issue in photocatalytic methane conversion over the benchmark catalyst TiO 2 . Under constant excitation, the absorption signal across the NIR region was demonstrated to be dominated by photoexcited electrons, the absorption of photoexcited holes increases toward shorter wavelengths in the visible region, and the overall shapes of the photoinduced absorption spectra obtained using the system demonstrated in the present work are consistent with widely accepted transient absorption results. Next, in situ measurements provide direct experimental evidence that the initial step of methane activation over TiO 2 involves oxidation by photoexcited holes. It is calculated that 90 ± 6% of photoexcited electrons are scavenged by O 2 (in dry air), 61 ± 9% of photoexcited holes are scavenged by methane (10% in argon), and a similar amount of photoexcited electrons can be scavenged by O 2 even when the O 2 concentration is reduced by a factor of 10. The present results suggest that O 2 is much more easily activated in comparison to methane over anatase TiO 2 , which rationalizes the much higher methane/O 2 ratio frequently used in practice in comparison to that required stoichiometrically for photocatalytic production of value-added chemicals via methane oxidation with oxygen. In addition, methanol (a preferable product of methane oxidation) is much more readily oxidized than methane over anatase TiO 2 .