Controlled crystal growth determines the shape, size, and exposed facets of a crystal, which usually has different surface physicochemical properties. Herein we report the size and facet control synthesis of anatase TiO2 nanocrystals (NCs). The exposed facets are found to play a crucial role in the photocatalytic activity of TiO2 NCs. This is due to the known preferential flow of photogenerated carriers to the specific facets. Although, in recent years, the main focus has been on increasing the surface area of high-energy exposed facets such as {001} and {100} to improve the photocatalytic activity, here we demonstrate that the presence of both the high-energy {001} oxidative and low-energy {101} reductive facets in an optimum ratio is necessary to reduce the charge recombination and thereby enhance photocatalytic activity of TiO2 NCs.

Cu-Cu2O core-shell nanoparticles (NPs) of different shapes over an extended nanosize regime of 5-400 nm have been deposited on a H-terminated Si(100) substrate by using a simple, one-step, templateless, and capping-agent-free electrochemical method. By precisely controlling the electrolyte concentration [CuSO4 x 5H2O] below their respective critical values, we can obtain cubic, cuboctahedral, and octahedral NPs of different average size and number density by varying the deposition time under a few seconds (<6 s). Combined glancing-incidence X-ray diffraction and depth-profiling X-ray photoelectron spectroscopy studies show that these NPs have a crystalline core-shell structure, with a face-centered cubic metallic Cu core and a simple cubic Cu2O shell with a CuO outerlayer. The shape control of Cu-Cu2O core-shell NPs can be understood in terms of a diffusion-limited progressive growth model under different kinetic conditions as dictated by different [CuSO4 x 5H2O] concentration regimes.

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Electrochemical oxygen evolution reaction (OER) involves high overpotential at oxygen evolving electrode and thereby suffers significant energy loss in the proton exchange membrane water electrolyzer. In order to reduce the OER overpotential, precious ruthenium and iridium oxides are most commonly used as anode electrocatalyst. Here we report marked reduction in overpotential for the OER using transition metal (TM) doped TiO 2 nanocrystals (NCs). This reduction in overpotential is attributed to d-orbitals splitting of the doped TMs in the TM-doped TiO 2 NCs and their interactions with the oxyradicals (intermediates of OER) facilitating the OER. The d-orbital spitting of TMs in TM-doped TiO 2 NCs is evident from the change in original pearl white color of undoped TiO 2 NCs and UV-vis absorption spectra. Hu et al. reported improved OER activity of IrO 2 /Nb−TiO 2 catalyst as compared to unsupported IrO 2 catalyst. 11 This suggests potential of TiO 2 as electrocatalyst in addition to its advantageous properties of high thermal and chemical stability, lost cost, and commercial availability. Recently, Cai et al. reported enhanced OER with Co-doped TiO 2 nanowires synthesized by sol-flame process. 20 A few other recent reports also confirm the reduction of OER overpotential with TM-doped TiO 2 . 21,22 However, the exact rationality behind such improved performance remains unclear. Here we report reduced overpotential of OER using TM-doped TiO 2 nanocrystals (NCs) synthesized by a facile low temperature hydrothermal technique. This reduced overpotential is attributed to d-orbital splitting of TMs in TiO 2 NCs producing mid gap energy states as confirmed from the UV-vis absorption spectroscopy. The d-orbital splitting of three TMs (Fe, Co, and Cu) and their role in OER overpotential is discussed in the present report. 2. EXPERIMENTAL DETAILS 2.1 Materials. All the chemicals were analytical grade and used as received without further purification. Titanium tetraisopropoxide (TTIP) [99.999%], Cu(NO 3 ) 2 .3H 2 O, Fe(SO 4 ).7H 2 O, Co(NO 3 ) 2 .6H 2 O, tetrabutyl ammonium hydroxide (TBAH) [(C 4 H 9 ) 4 NOH in 0.1 N aqueous], and diethanolamine (DEA) were purchased from Merck. Synthesis of TM doped TiO 2 NCs.In a typical synthesis, 3 mmol of TTIP was added in a mixture of TBAH (40 mmol) and DEA (160 mmol), and stirred for 5 min at room temperature. Then the viscous solution was transferred to a Teflon-lined stainless steel autoclave and heated at 225 °C for 24 h. After the heat treatment, the autoclave was allowed to cool down to room temperature naturally and the product was collected by centrifuge and washed with water and ethanol several times. The final product was dried in air at 60 °C for 24 h. The overall yield of the product was 90−95 %. TM-doped TiO 2 NCs was synthesized by

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The exposed facets of a crystal are known to be one of the key factors to its physical, chemical and electronic properties. Herein, we demonstrate the role of amines on the controlled synthesis of TiO2 nanocrystals (NCs) with diverse shapes and different exposed facets. The chemical, physical and electronic properties of the as-synthesized TiO2 NCs were evaluated and their photoredox activity was tested. It was found that the intrinsic photoredox activity of TiO2 NCs can be enhanced by controlling the chemical environment of the surface, i.e.; through morphology evolution. In particular, the rod shape TiO2 NCs with ∼25% of {101} and ∼75% of {100}/{010} exposed facets show 3.7 and 3.1 times higher photocatalytic activity than that of commercial Degussa P25 TiO2 toward the degradation of methyl orange and methylene blue, respectively. The higher activity of the rod shape TiO2 NCs is ascribed to the facetsphilic nature of the photogenerated carriers within the NCs. The photocatalytic activity of TiO2 NCs are found to be in the order of {101}+{100}/{010} (nanorods) > {101}+{001}+{100}/{010} (nanocuboids and nanocapsules) > {101} (nanoellipsoids) > {001} (nanosheets) providing the direct evidence of exposed facets-depended photocatalytic activity.

Adsorption and UV/visible photocatalytic activity of echinoid-like Ag and Ti-loaded BiOI were tested for methyl orange, Rhodamine B and methylene blue.