Thomas E. Mallouk

The dynamics and efficiency of photoinduced electron transfer were measured at dye-sensitized photoanodes in aqueous (acetate buffer), nonaqueous (acetonitrile), and mixed solvent electrolytes by nanosecond transient spectroscopy (TAS) and ultrafast optical-pump terahertz-probe spectroscopy (OPTP). Higher injection efficiencies were found in mixed solvent electrolytes for dye-sensitized SnO2/TiO2 core/shell electrodes, whereas the injection efficiency of dye-sensitized TiO2 electrodes decreased with the increasing acetonitrile concentration. The trend in injection efficiency for the TiO2 electrodes was consistent with the solvent-dependent trend in the semiconductor flat-band potential. Photoinduced electron injection in core-shell electrodes has been understood as a two-step process involving ultrafast electron trapping in the TiO2 shell followed by slower electron transfer to the SnO2 core. The driving force for shell-to-core electron transfer increases as the flat band potential of TiO2 shifts negatively with increasing concentration of acetonitrile. In acetonitrile-rich electrolytes, despite the larger driving force, electron injection is suppressed. Interestingly, a net negative photoconductivity in the SnO2 core is observed in mixed solvent electrolytes by OPTP. We hypothesize that an electric field is formed across the TiO2 shell from the oxidized dye molecules after injection. The intrinsic conduction band electrons in SnO2 are trapped at the core-shell interface by the electric field, resulting in a negative photoconductivity transient. The overall electron injection efficiency of the dye- sensitized SnO2/TiO2 core/shell photoanodes, measured on longer …

Efficient and stable photoelectrochemical reduction of CO2 into highly reduced liquid fuels remains a formidable challenge, which requires an innovative semiconductor/catalyst interface to tackle. In this study, we introduce a strategy involving the fabrication of a silicon micropillar array structure coated with a superhydrophobic fluorinated carbon layer for the photoelectrochemical conversion of CO2 into methanol. The pillars increase the electrode surface area, improve catalyst loading and adhesion without compromising light absorption, and help confine gaseous intermediates near the catalyst surface. The superhydrophobic coating passivates parasitic side reactions and further enhances local accumulation of reaction intermediates. Upon one-electron reduction of the molecular catalyst, the semiconductor–catalyst interface changes from adaptive to buried junctions, providing a sufficient thermodynamic driving …

Photoelectrodes consisting of metal–insulator–semiconductor (MIS) junctions are a promising candidate architecture for water splitting and for the CO2 reduction reaction (CO2RR). The photovoltage is an essential indicator of the driving force that a photoelectrode can provide for surface catalytic reactions. However, for MIS photoelectrodes that contain metal nanoparticles, direct photovoltage measurements at the metal sites under operational conditions remain challenging. Herein, we report a new in situ spectroscopic approach to probe the quasi-Fermi level of metal catalyst sites in heterogeneous MIS photoelectrodes via surface-enhanced Raman spectroscopy. Using a CO2RR photocathode, nanoporous p-type Si modified with Ag nanoparticles, as a prototype, we demonstrate a selective probe of the photovoltage of ∼0.59 V generated at the Si/SiOx/Ag junctions. Because it can directly probe the photovoltage …

Micro- and nanoscopic particles that swim autonomously and self-assemble under the influence of chemical fuels and external fields show promise for realizing systems capable of carrying out large-scale, predetermined tasks. Different behaviors can be realized by tuning swimmer interactions at the individual level in a manner analogous to the emergent collective behavior of bacteria and mammalian cells. However, the limited toolbox of weak forces with which to drive these systems has made it difficult to achieve useful collective functions. Here, we review recent research on driving swimming and particle self-organization using acoustic fields, which offers capabilities complementary to those of the other methods used to power microswimmers. With either chemical or acoustic propulsion (or a combination of the two), understanding individual swimming mechanisms and the forces that arise between individual …

We report a precious‐metal‐free molecular catalyst‐based photocathode that is active for aqueous CO2 reduction to CO and methanol. The photoelectrode is composed of cobalt phthalocyanine molecules anchored on graphene oxide which is integrated via a (3‐aminopropyl)triethoxysilane linker to p‐type silicon protected by a thin film of titanium dioxide. The photocathode reduces CO2 to CO with high selectivity at potentials as mild as 0 V versus the reversible hydrogen electrode (vs RHE). Methanol production is observed at an onset potential of −0.36 V vs RHE, and reaches a peak turnover frequency of 0.18 s−1. To date, this is the only molecular catalyst‐based photoelectrode that is active for the six‐electron reduction of CO2 to methanol. This work puts forth a strategy for interfacing molecular catalysts to p‐type semiconductors and demonstrates state‐of‐the‐art performance for photoelectrochemical CO2 …

With the goal of achieving large-scale H2 production from renewable resources, water splitting into H2 and O2 using semiconductor photocatalysts (sometimes called artificial photosynthesis) has been studied for five decades. Unfortunately, the lack of rigour and reproducibility in the data collection and analysis of experimental results has hindered progress in the field. This Primer provides a comprehensive overview of proper characterization and evaluation of photocatalysts for overall water splitting. In particular, the Primer covers various pitfalls in photocatalysis research, best practices for reproducibility and reliable methods for conducting rigorous experiments. The recommendations are intended to reduce false positives in the literature and to promote progress towards a practical technology for producing H2 from water by using sunlight.

Understanding topological spin textures is important because of scientific interests and technological applications. However, observing nanoscale magnetization and mapping out their interactions in 3D have been challenging–due to the lack of nondestructive vector nanoimaging techniques that penetrate thick samples. Recently, we developed a new characterization technique, soft x-ray vector ptycho-tomography, to image spin textures with a 3D vector spatial resolution of 10 nm. Using 3D magnetic metamaterial as an example, we demonstrated the creation and observation of topological magnetic monopoles and their interactions. We expect this method to be applied broadly to image vector fields in magnetic samples and beyond.

Nanostructured semiconductors can exhibit thermal properties unachievable in bulk systems, and will play a crucial role in next-generation nanoelectronics and energy efficient devices. However, first principles models are too computationally challenging for 3D nanostructured geometries, while mechanistic approaches make overly-simplistic assumptions about the nature of phonon-boundary interactions. Here, we study heat flow in a 3D nanostructured silicon metalattice, using an infrared pump laser to excite the sample and an extreme ultraviolet probe to monitor its relaxation. Analyzing surface acoustic waves launched by laser-excited metallic gratings, we first nondestructively extract porosity and elastic properties, and validate using electron tomography images. We then model the heat flow dynamics, which obey a Fourier-like relation with an ultra-low apparent thermal conductivity. Through an analogy to …