has distinguished itself due to its visiblelight absorption capability and its suitable band edge positions which comfortably accommodate oxygen generation potential. [8] The narrow band gap of BiVO 4 (≈2.4 eV) facilitates the light absorption up to 11% of the solar spectrum, moderately higher than the ≈4% by TiO 2 . [9] However, the inherently poor hole transfer at BiVO 4 /water interface leads to the sluggish water oxidation kinetics and the fast electron-hole recombination, and thus results in a unsatisfactory overall quantum efficiency. [10] The conventional strategies to overcome these limitations of BiVO 4 include morphology control [11] and doping with foreign atoms (metals [12,13] and nonmetals). [14,15] However, another strategy that has been getting popular nowadays is modification of BiVO 4 with OER electrocatalyst, [16,17] which are referred to as cocatalyst in photocatalysis. The roles of cocatalyst in photocatalytic OER are as follows:(1) cocatalyst lowers overpotential by serving as favorable active site for O 2 generation; [18] (2) cocatalyst provides suitable trapping sites for the photo generated charges and thus improves electron-hole separation; (3) suitable cocatalyst, by selectively and timely removing the photo generated charges, particularly the holes, decreases the photo corrosion, particularly oxidation of some unstable photocatalysts, for example MoS 2 as cocatalyst for Cu 2 O, [19] cobalt phosphate (Co-Pi) as cocatalyst for CdS. [20] Thus rational selection of an appropriate electrocatalyst in photocatalytic OER can be vital for an effective water splitting where OER half reaction is the limiting factor.In literature, several electrocatalysts (e.g., FeOOH, [21] FeOOH/NiOOH, [22] Co 3 O 4 , [23] layered double hydroxide, [24] and FeSe 2 ) [25] have been investigated as cocatalysts along with BiVO 4 in photocatalytic OER (Table S1, Supporting Information). In particular, Co-Pi, first reported by Nocera et al. in 2008, is considered as the first generation photocatalytic OER cocatalyst for BiVO 4 . [26][27][28] However, the dissolution of Co-Pi in phosphate buffer (pH-7) seriously dampers its application perspective. [29] Modification of BiVO 4 with individual NiO or Co 3 O 4 layer has recently shown improved photocatalytic OER performance. [29] Nowadays, multimetal oxides (e.g., binary/tertiary metal oxides) are being intensively investigated in various fields. MoreHere, a simple and efficient preparation of NiCoO 2 nanoparticle modified nanoporous bismuth vanadate (BiVO 4 ) thin film and its application in photoelectrocatalytic (PEC) oxygen evolution reaction (OER) is demonstrated. The role of NiCoO 2 in the composite electrode (BiVO 4 /NiCoO 2 ) is twofold: OER cocatalyst and band structure modifier. It improves surface reaction kinetics for PEC OER and enhances charge separation efficiency simultaneously, which is believed to be a determining factor for the unprecedentedly high PEC OER performance of this BiVO 4 /NiCoO 2 nanocomposite. The photocurrent density of 3.6 mA cm −2 at 1.2...

Face-centered cubic (fcc) lattice is the only known crystal structure of bulk gold. In the present work, we report the presence of body-centered tetragonal (bct) and body-centered orthorhombic (bco) phases in bipyramidal Au microcrystals with penta-twinned tips. These microcrystals have been obtained by thermolysis of (AuCl4)(-) stabilized with tetraoctylammonium bromide (ToABr) in air at about 220 °C for 30 min. Using a laboratory monochromatic X-ray source, the non-fcc phases could be readily detected. The remarkable occurrence of non-fcc phases of Au grown in the temperature window of 200-250 °C results from the geometrically induced strains in the bipyramids. Having derived first-principles theoretical support for the temperature-dependent stability of non-fcc Au structures under stress, we identify its origin in soft modes. Annealing at high temperatures relieves the stress, thus destabilizing the non-fcc phases.

No abstract

Although the stability of Au in the face-centered cubic (FCC) phase at high temperatures and pressures has been well studied, the stability in other lattice phases rarely encountered in crystallite domains in microscopy studies has not been explored much because of their nanometric extensions. A recent report on Au microcrystallites crystallized in body-centered tetragonal (BCT) and body-centered orthorhombic (BCO) phases prompted the work presented here, in which we have investigated for the first time the structural stability of the BCT and BCO phases at high temperatures and separately at high pressures using high-energy synchrotron X-ray diffraction. A reversible phase transition was observed for pressures of up to ∼40 GPa, indicating unusual stability of the non-FCC Au phases. However, during a high-temperature treatment at ∼700 °C, the transformation to FCC was irreversible.

Focusing light at sub-wavelength region opens up interesting applications in optical sensing and imaging beyond the diffraction limit. In the past, tapered Au wires with carved gratings have been employed to achieve nanofocusing. The fabrication process however, is expensive and the obtained wires are polycrystalline with high surface roughness. A chemical synthetic method overcoming these hurdles should be an attractive alternative. Here, we report a method to chemically synthesize Au microcrystals (~10 μm) bearing pentagonal bipyramidal morphology with surface corrugations assignable to high index planes. The method is a single step solid state synthesis at a temperature amenable to common substrates. The microcrystals are tapered at both ends forming sharp tips (~55 nm). Individual microcrystals have been used as pick and probe SERS substrates for a dye embedded in a polymer matrix. The unique geometry of the microcrystal also enables light propagation across its length.