Interactions of molecules with the surface of TiO particles are of fundamental and technological importance. One example is that the adsorption density and energy of the dye molecules on TiO particles affect the efficiency of dye-sensitized solar cells (DSSC). In this work, we present measurements characterizing the adsorption of the two isomers, para-ethyl red (p-ER) and ortho-ethyl red (o-ER), of a dye molecule potentially applicable for DSSC onto TiO particles by second harmonic scattering (SHS). It is found that while at the wavelengths used here o-ER has a much bigger molecular hyperpolarizability, p-ER exhibits strong SHS responses but o-ER gives no detectable SHS when the dyes are added to the TiO colloids, respectively. This observation indicates that o-ER does not adsorb onto TiO, likely due to steric hindrance. Furthermore, we investigate how solvents affect the surface adsorption strength and density of p-ER onto TiO in four aprotic solvents with varying polarity. The absolute magnitude of the adsorption free energy was found to increase with the specific solvation energy that represents the ability of accepting electrons and solvent polarity. It is likely that resolvation of the solvent molecules displaced by the adsorption of the dye molecule at the surface in stronger electron-accepting and more polar solvents results in a larger adsorption free energy for the dye adsorption.

Metal−semiconductor heterostructures are believed to improve hot-electron injection efficiency and influence the photocatalytic performance. Understanding the carrier dynamics at the heterostructure is essential for designing more efficient photocatalysts. Herein, we fabricated a Schottky heterostructure using two-dimensional (2D) titanium carbide MXene (Ti 3 C 2 T x , where T x stands for surface terminations, such as O or OH) and a TiO 2 semiconductor and examined the carrier dynamics at the heterostructure using time-resolved infrared techniques. MXene exhibits robust metallic properties in terms of photoconductivity comparable to those of high-quality 2D graphene materials. The photoexcitation of MXene greatly increases the scattering rate and leads to a decreased photoconductivity. When Ti 3 C 2 T x comes in close contact with the TiO 2 semiconductor, band bending leads to the formation of a Schottky barrier at the contact junction. In this plasmonic TiO 2 /Ti 3 C 2 T x heterostructure, hot electrons are excited only from MXene upon photon absorption at wavelengths far below the TiO 2 band gap. Under these conditions, the Ti 3 C 2 T x -generated plasmonic electrons are transferred into the conduction band of the TiO 2 semiconductor over the Schottky barrier with a fast time constant of 180 fs. The strong electronic coupling between oxygen-terminated Ti 3 C 2 T x and TiO 2 is due to their proximity, and the resulting interactions are likely responsible for the fast electron transfer in the composites. Our results demonstrate a potential of 2D MXene materials in plasmonic applications and provide new insights into the design of MXene-based photocatalysts.

Adsorption of the carboxylic anchoring dye, p-ethyl red (p-ER), onto TiO2 nanoparticles in protic vs aprotic solvents was studied in situ using the surface-specific technique, second harmonic light scattering (SHS). Two different adsorption schemes were proposed to account for p-ER interactions with TiO2 under different solvent environments. In aprotic solvents, p-ER adsorbs directly onto TiO2. Conversely, in protic solvents, in which solvent molecules bind stronger than p-ER with TiO2, the dye molecules adsorb onto the solvent shell around the particle but not directly to the TiO2 surface. In addition, a portion of the p-ER molecules form hydrogen bonds with the protic solvent molecules. The two different adsorption models reproduce the measured adsorption isotherms detected by SHS. Specifically, the p-ER molecules adsorb with a smaller free energy change and a larger density in protic solvents than in aprotic solvents. Our results indicate that protic solvents are undesirable for administering adsorption of carboxylic dyes in dye-sensitized solar cell applications as the dye molecules do not directly adsorb onto the TiO2 particle.

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Among the world’s most deadly toxins are a class of organophosphates that are used as chemical warfare agents (CWAs). It is imperative to continue to develop novel means for mitigation and protection against these chemical threats. Sensitizing the surface of metal oxide semiconductors with plasmonic nanoparticles for photocatalytic degradation of chemical threats has been a prominent area of research in recent years. Anisotropic silver nanoplateles were purposefully grown on the surface of TiO2 fibers, in order to determine the impact of silver nanoparticle shape on (1) the generation of hot electrons by the silver, (2) the subsequent transfer of those electrons from the silver into the TiO2, and (3) the photocatalytic behavior of the Ag–TiO2 composite. To elucidate the charge injection properties of the composites, transient absorption experiments (pump–probe experiments) were undertaken. These involved pumping the composite samples with a range of discrete visible wavelengths and probing the composite within the intraband transitions of the TiO2. As a complement to these experiments, the photocatalytic properties of the Ag–TiO2 composite fibers were studied via the photocatalytic hydrolysis of methyl paraoxon, a chemical warfare agent simulant. This involved exposure of the methyl paraoxon to either red, green, blue, or white LED illumination. For both the transient absorption and photocatalytic experiments, maximum efficiency was observed for those scenarios in which the resonance of the silver platelets most closely matched the wavelength of incident radiation. Furthermore, the composite with silver nanoplatelets clearly outperformed its counterpart with silver nanospheres, in terms of both charge injection and photocatalytic behavior. We believe these results shall serve as a basis for future catalytic research in which the resonance of anisotropic plasmonic nanoparticles (in a given composite) shall be designed to match the wavelength of incident radiation.