The solar‐to‐chemical energy conversion of greenhouse gas CO2 into carbon‐based fuels is a very important research challenge, with implications for both climate change and energy security. Herein, the key attributes of hydroxides and oxygen vacancies are experimentally identified in non‐stoichiometric indium oxide nanoparticles, In2O3‐x(OH)y, that function in concert to reduce CO2 to CO under simulated solar irradiation.

Fabrication of anisotropic hydrogels exhibiting direction-dependent structure and properties have attracted great interest in biomimicking, tissue engineering and bioseparation. Herein, we report a single-step freeze casting-based fabrication of structurally and mechanically anisotropic aerogels and hydrogels composed of hydrazone cross-linked poly(oligoethylene glycol methacrylate) (POEGMA) and cellulose nanocrystals (CNCs). We show that by controlling the composition of the CNC/POEGMA dispersion and the freeze casting temperature, aerogels with fibrillar, columnar, or lamellar morphologies can be produced. Small-angle X-ray scattering experiments show that the anisotropy of the structure originates from the alignment of the mesostructures, rather than the CNC building blocks. The composite hydrogels show high structural and mechanical integrity and a strong variation in Young's moduli in orthogonal directions. The controllable morphology and hydrogel anisotropy, coupled with hydrazone cross-linking and biocompatibility of CNCs and POEGMA, provide a versatile platform for the preparation of anisotropic hydrogels.

Silicon constitutes 28% of the earth's mass. Its high abundance, lack of toxicity and low cost coupled with its electrical and optical properties, make silicon unique among the semiconductors for converting sunlight into electricity. In the quest for semiconductors that can make chemicals and fuels from sunlight and carbon dioxide, unfortunately the best performers are invariably made from rare and expensive elements. Here we report the observation that hydride-terminated silicon nanocrystals with average diameter 3.5 nm, denoted ncSi:H, can function as a single component heterogeneous reducing agent for converting gaseous carbon dioxide selectively to carbon monoxide, at a rate of hundreds of μmol h−1 g−1. The large surface area, broadband visible to near infrared light harvesting and reducing power of SiH surface sites of ncSi:H, together play key roles in this conversion. Making use of the reducing power of nanostructured hydrides towards gaseous carbon dioxide is a conceptually distinct and commercially interesting strategy for making fuels directly from sunlight.

The development of strategies for increasing the lifetime of photoexcited charge carriers in nanostructured metal oxide semiconductors is important for enhancing their photocatalytic activity. Intensive efforts have been made in tailoring the properties of the nanostructured photocatalysts through different ways, mainly including band-structure engineering, doping, catalyst-support interaction, and loading cocatalysts. In liquid-phase photocatalytic dye degradation and water splitting, it was recently found that nanocrystal superstructure based semiconductors exhibited improved spatial separation of photoexcited charge carriers and enhanced photocatalytic performance. Nevertheless, it remains unknown whether this strategy is applicable in gas-phase photocatalysis. Using porous indium oxide nanorods in catalyzing the reverse water-gas shift reaction as a model system, we demonstrate here that assembling semiconductor nanocrystals into superstructures can also promote gas-phase photocatalytic processes. Transient absorption studies prove that the improved activity is a result of prolonged photoexcited charge carrier lifetimes due to the charge transfer within the nanocrystal network comprising the nanorods. Our study reveals that the spatial charge separation within the nanocrystal networks could also benefit gas-phase photocatalysis and sheds light on the design principles of efficient nanocrystal superstructure based photocatalysts.

In 2 O 3-x (OH) y nanoparticles have been shown to function as an effective gas-phase photocatalyst for the reduction of CO 2 to CO via the reverse water-gas shift reaction. Their photocatalytic activity is strongly correlated to the number of oxygen vacancy and hydroxide defects present in the system. To better understand how such defects interact with photogenerated electrons and holes in these materials, we have studied the relaxation dynamics of In 2 O 3-x (OH) y nanoparticles with varying concentration of defects using two different excitation energies corresponding to above-band-gap (318-nm) and near-band-gap (405-nm) excitations. Our results demonstrate that defects play a significant role in the excited-state, charge relaxation pathways. Higher defect concentrations result in longer excited-state lifetimes, which are attributed to improved charge separation. This correlates well with the observed trends in the photocatalytic activity. These results are further supported by density-functional theory calculations, which confirm the positions of oxygen vacancy and hydroxide defect states within the optical band gap of indium oxide. This enhanced understanding of the role these defects play in determining the optoelectronic properties and charge carrier dynamics can provide valuable insight toward the rational development of more efficient photocatalytic materials for CO 2 reduction.indium oxide | solar fuels | CO 2 hydrogenation | transient absorption | surface defects C oncerns over climate change and the projected rise in global energy demand have motivated researchers to develop alternative, more sustainable ways to generate energy from naturally abundant and renewable sources (1-3). An important challenge associated with using renewable energy sources, such as solar, is their inherent intermittent nature (4-6). The emerging field of solar fuels seeks to address this issue by storing radiant solar energy in chemical bonds, which can then be released on demand and act as a drop-in replacement for traditional fossil fuels (7-11). By using the greenhouse gas, CO 2 , currently regarded as a waste product, as a feedstock and converting it into valuable products such as solar fuels or platform chemicals, we could simultaneously address concerns over climate change and energy security while creating significant economic benefits (12)(13)(14). However, despite recent advances in the development of active materials that can drive the photocatalytic reduction of CO 2 into useful chemical species, much remains unknown about the fundamental physical properties that control the activity of a photocatalyst. To facilitate the rational development and improvement of photocatalytic materials, a detailed understanding of the complex interplay between chemical, optical, and electronic processes is needed.Indium oxide has many favorable optical, electronic, and surface properties that make it a compelling choice as a photocatalyst for CO 2 reduction. It has a relatively high conduction band, and common defects such as oxyge...