Facet-Dependent Photocatalytic Activity and Facet-Selective Etching of Silver(I) Oxide Crystals with Controlled Morphology

Growth of Size-Tunable Ag2O Polyhedra and Revelation of Their Bulk and Surface Lattices

By primarily adjusting the reagent amounts, particularly the volume of AgNO3 solution introduced, Ag2O cubes with decreasing sizes from 440 to 79 nm, octahedra from 714 to 106 nm, and rhombic dodecahedra from 644 to 168 nm are synthesized. 733 nm cuboctahedra are also prepared for structural analysis. With in-house X-ray diffraction (XRD) peak calibration, shape-related peak shifts are recognizable. Synchrotron XRD measurements at 100 K reveal the presence of bulk and surface layer lattices. Bulk cell constants also deviate slightly. They show a negative thermal expansion behavior with shrinking cell constants at higher temperatures. The Ag2O crystals exhibit size- and facet-dependent optical properties. Bandgaps red-shift continuously with increasing particle sizes. Optical facet effect is also observable. Moreover, synchrotron XRD peaks of a mixture of Cu2O rhombicuboctahedra and edge- and corner-truncated cubes exposing all three crystal faces can be deconvoluted into three components with the bulk and the [111] microstrain phase as the major component. Interestingly, while the unheated Cu2O sample shows clear diffraction peak asymmetry, annealing the sample to 450 K yields nearly symmetric peaks even when returning the sample to room temperature, meaning even moderately high temperatures can permanently change the crystal lattice.

Formation of Silver Rhombic Dodecahedra, Octahedra, and Cubes through Pseudomorphic Conversion of Ag2O Crystals with Nitroarene Reduction Activity

Using ethanol as a co-solvent, relatively small-sized Ag₂O octahedra (∼645 nm in opposite corner distance) and rhombic dodecahedra (∼540 and 655 nm in opposite face distance) were synthesized in aqueous solutions. Ag₂O cubes synthesized in an aqueous solution have an edge length of ∼425 nm. Band gaps of these crystals have been obtained, revealing the presence of facet-dependent light absorption properties. The Ag₂O rhombic dodecahedra, octahedra, and cubes were treated with ammonia borane in ethanol at 50 °C for just 10 min to pseudomorphically convert to Ag polyhedra of the corresponding morphologies. Transmission electron microscopy characterization confirms that the Ag cubes, octahedra, and rhombic dodecahedra are bound by the {100}, {111}, and {110} faces, respectively. The Ag rhombic dodecahedra, available for the first time, showed more superior catalytic activity toward 4-nitroaniline reduction at 50 °C than Ag octahedra and cubes, and gave 100% product yield after 1 h of reaction. This work demonstrates the value of forming Ag rhombic dodecahedra, exposing {110} surfaces that may be useful in other organic transformations.

β-SnWO4 with Morphology-Controlled Synthesis and Facet-Depending Photocatalysis

Faceted β-SnWO₄ microcrystals are prepared with different morphologies including tetrahedra, truncated tetrahedra, truncated octahedra, and short-spiked and long-spiked spikecubes. All of these morphologies are prepared with comparable experimental conditions via microwave-assisted synthesis of high-boiling alcohols (the so-called polyol method). The decisive parameters for controlled formation of one or the other morphology of faceted β-SnWO₄ microcrystals are studied and discussed, including microwave-assisted heating, Sn(OH)₂ as the Sn²⁺ reservoir, the temperature of particle nucleation, the temperature of particle growth, and the concentration of the starting materials. Morphology and crystallinity are characterized by scanning electron microscopy, X-ray powder diffraction, UV-vis, and Fourier-transform infrared spectroscopy. Finally, the photocatalytic properties of all obtained faceted microcrystals-tetrahedra, truncated tetrahedra, truncated octahedra, cubes, and short-spiked and long-spiked spikecubes-are exemplarily compared with regard to the photocatalytic decomposition of rhodamine B and the influence of the respective surface crystal planes.

Facet-Engineered Surface and Interface Design of Photocatalytic Materials

The facet-engineered surface and interface design for photocatalytic materials has been proven as a versatile approach to enhance their photocatalytic performance. This review article encompasses some recent advances in the facet engineering that has been performed to control the surface of mono-component semiconductor systems and to design the surface and interface structures of multi-component heterostructures toward photocatalytic applications. The review begins with some key points which should receive attention in the facet engineering on photocatalytic materials. We then discuss the synthetic approaches to achieve the facet control associated with the surface and interface design. In the following section, the facet-engineered surface design on mono-component photocatalytic materials is introduced, which forms a basis for the discussion on more complex systems. Subsequently, we elucidate the facet-engineered surface and interface design of multi-component photocatalytic materials. Finally, the existing challenges and future prospects are discussed.

Stretchable Biofuel Cells as Wearable Textile-based Self-Powered Sensors

Highly stretchable textile-based biofuel cells (BFCs), acting as effective self-powered sensors, have been fabricated using screen-printing of customized stress-enduring inks. Due to synergistic effects of nanomaterial-based engineered inks and the serpentine designs, these printable bioelectronic devices endure severe mechanical deformations, e.g., stretching, indentation, or torsional twisting. Glucose and lactate BFCs with the single enzyme and membrane-free configurations generated the maximum power density of 160 and 250 µW cm^-2 with the open circuit voltages of 0.44 and 0.46 V, respectively. The textile-BFCs were able to withstand repeated severe mechanical deformations with minimal impact on its structural integrity, as was indicated from their stable power output after 100 cycles of 100% stretching. By providing power signals proportional to the sweat fuel concentration, these stretchable devices act as highly selective and stable self-powered textile sensors. Applicability to sock-based BFC and self-powered biosensor and mechanically compliant operations was demonstrated on human subjects. These stretchable skin-worn "scavenge-sense-display" devices are expected to contribute to the development of skin-worn energy harvesting systems, advanced non-invasive self-powered sensors and wearable electronics on a stretchable garment.

Synthesis of Diverse Ag2O Crystals and Their Facet-Dependent Photocatalytic Activity Examination

Sub- to micrometer-sized Ag2O cubes, great rhombicuboctahedra, cuboctahedra, corner-truncated octahedra, octahedra, and rhombic dodecahedra have been synthesized at room temperature using simple molar ratios of NH4NO3, NaOH, and AgNO3 solutions with a short reaction time. In addition, tuning the concentration of NH3 in the solution can provide more particle morphologies including edge- and corner-truncated cubes, small rhombicuboctahedra, and edge-truncated octahedra to enrich Ag2O shape diversity. X-ray photoelectron spectroscopy (XPS) spectra indicate surface composition of various crystals as pure Ag2O. Diffuse reflectance spectra have been used to determine the band gap of Ag2O cubes. Ag2O cubes, octahedra, and rhombic dodecahedra having the same total particle surface area were used for facet-dependent photocatalytic activity comparison in the degradation of methyl orange. Cubes are comparably highly active for this reaction, while octahedra and rhombic dodecahedra give moderate and low catalytic activities, respectively. Electron paramagnetic resonance (EPR) measurements confirm this reactivity order. Although all Ag2O samples show significant etching during photocatalysis, metallic silver is not produced.

Shape control of inorganic nanoparticles from solution

Inorganic materials with controllable shapes have been an intensely studied subject in nanoscience over the past decades. Control over novel and anisotropic shapes of inorganic nanomaterials differing from those of bulk materials leads to unique and tunable properties for widespread applications such as biomedicine, catalysis, fuels or solar cells and magnetic data storage. This review presents a comprehensive overview of shape-controlled inorganic nanomaterials via nucleation and growth theory and the control of experimental conditions (including supersaturation, temperature, surfactants and secondary nucleation), providing a brief account of the shape control of inorganic nanoparticles during wet-chemistry synthetic processes. Subsequently, typical mechanisms for shape-controlled inorganic nanoparticles and the general shape of the nanoparticles formed by each mechanism are also expounded. Furthermore, the differences between similar mechanisms for the shape control of inorganic nanoparticles are also clearly described. The authors envision that this review will provide valuable guidance on experimental conditions and process control for the synthesis of inorganic nanoparticles with tunable shapes in the solution state.