This study systematically re-examines the titania-catalysed photo-oxidation of methylene blue (MB) in aqueous solution at 20°C, placing particular emphasis on the effects of TiO 2 crystallite size, TiO 2 polymorph (anatase, brookite, rutile and combinations thereof) and experimental test conditions on the rate of MB photo-oxidation. For all TiO 2 samples tested, the highest rate of MB photo-oxidation was observed at pH 6, slightly above the isoelectric point of TiO 2 (*5.8 for P25 TiO 2 ). Increasing the ionic strength at pH 6 induced MB dimer formation in solution, and lowered the rate of MB photo-oxidation by TiO 2 . For all TiO 2 polymorphs, the surface area normalised rate increased with crystallite size reflecting the corresponding reduction in surface and bulk defects (electronhole pair recombination sites). The optimum crystallite sizes were *20-25 nm for anatase and *50 nm for brookite. The photocatalytic activity of the different TiO 2 powders followed the general order P25 [ anatase [ brookite ) rutile, with the high activity of P25 TiO 2 providing strong evidence that anatase-rutile heterojunctions act as ''hotspots'' for MB photo-oxidation. Mixed phase anatase-rutile or brookite-rutile powders, each containing *5 wt% rutile, demonstrated superior area normalized photocatalytic activities for MB photo-oxidation compared to pure phase anatase or brookite powders of comparable crystallite size. Finally, deposition of Pd, Pt or Au nanoparticles decreased the activity of P25 TiO 2 for MB photo-oxidation. This paper clarifies long-standing confusion in the scientific literature about the photo-oxidation of aqueous MB over TiO 2 and M/TiO 2 (M = Pd, Pt and Au) photocatalysts.
The films of ZnO–SnO2 system were deposited on glass substrates by simultaneous dc magnetron sputtering apparatus, in which ZnO and SnO2:Sb (Sb2O5 3 wt % doped) targets faced each other. The substrate temperatures were maintained at 150, 250, and 350 °C, respectively. As an experimental parameter, current ratio δ=IZn/(IZn+ISn), which corresponds to ZnO target current (IZn) divided by the sum of ZnO and SnO2:Sb target currents (IZn+ISn), was adopted. Amorphous transparent films appeared for 0.50⩽δ⩽0.73, which could be correlated to compositions as [Zn]/([Sn]+[Zn])=0.33–0.67 by x-ray fluorescent analysis. At [Zn]/([Sn]+[Zn])=1/2 (δ=0.62), 2/3 (δ=0.73) and all other ratios in as-deposited films, neither crystalline ZnSnO3 nor Zn2SnO4 was obtained. Minimum resistivity of 4–6×10−2 Ω cm was found at δ=0.50, whose composition was approximately SnO2⋅ZnSnO3. Resistivity increased linearly with an increase of the current ratio, until the composition reached Zn2SnO4. The amorphous phase showed a constant Hall mobility of ∼10 cm2/V s and a linear decrease in carrier concentration with increasing Zn content.
Room
temperature (RT) bistable switching materials continue to
fascinate the scientists since they can be utilized for a new class
of molecular-based switches or memories. While the spin crossover
(SCO) compound is categorized into these attractive materials, designing
of a SCO system showing desirable bistability (i.e., wide hysteresis
loop spanning RT) in a rational way is still a very challenging issue.
We report herein a new family of neutral molecular iron(II) complexes
showing hysteretic SCO in a wide range of switching temperatures (239–409
K) and hysteresis widths (1–31 K) spanning RT. These materials
were obtained as single crystals of two solvent-free compounds [FeII(ptm2-dmpn)(NCS)2] (1; ptm2-dmpn = N,N′-bis[(1-phenyl-1H-1,2,3-triazol-4-yl)methylene]-2,2-dimethylpropane-1,3-diamine)
and [FeII(p-ttm2-dmpn)(NCS)2] (2; p-ttm2-dmpn = N,N′-bis[(1-p-tolyl-1H-1,2,3-triazol-4-yl)methylene]-2,2-dimethylpropane-1,3-diamine),
and two solvatomorphs [FeII(p-ttm2-etpn)(NCS)2]·solvent (p-ttm2-etpn = N,N′-bis[(1-p-tolyl-1H-1,2,3-triazol-4-yl)methylene]-1-ethylpropane-1,3-diamine,
and solvent = 0.5H2O and 0.5MeCN·0.5MeOH·H2O for 3a and 3b, respectively).
All compounds are constructed of a three-dimensional (3D) flexible
supramolecular network by multiple weak CH···S hydrogen
bonds incorporating an additional orderly dimensional structure by
aromatic interactions (i.e., π–π and CH···π
interactions) between adjacent aromatic rings of tetradentate ligands.
These 3D networks can accommodate a number of molecular motions such
as (1) NCS bending, (2) tetradentate ligand biting, (3) aromatic ring
rotation, and (4) propylene fragment oscillation to various degrees
depending on a slight modification of small alkyl substituents and
lattice solvents. The present crystal engineering approach of introducing
concerted multimolecular motions into a 3D flexible network can be
a new designing concept for controlling switching temperature and
cooperativity in a systematic way toward the discovery of unprecedented
RT bistable SCO materials.
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