Superior Interfacial Contact Yields Efficient Electron Transfer Rate and Enhanced Solar Photocatalytic Hydrogen Generation in M/C3N4 Schottky Junctions.

Various literature studies (Table 6) have reported that dispersion of metal nanoparticles (NPs) on graphitic carbon nitride g-C3N4 (M/CN) has considerably improved the photocatalytic hydrogen yield. It is understood that metal NPs create active sites on the surface of CN and act as a cocatalyst. However, the precise changes induced by different metal NPs on the surface of CN still elude us. Here, we report a thorough understanding and comparison of the morphology, metal-support interactions, interfacial charge transfer kinetics, and band characteristics in different M/CN (M = Pt, Pd, Au, Ag, Cu) correlated with photocatalytic activity. Among all metals, Pt/CN was found to be the best performer both under sunlight and UV-visible irradiation. Under sunlight, maximum H2@ 2.7 mmol/h/g was observed over Pt/CN followed by Pd/CN > Au/CN > Ag/CN > Cu/CN ≈ CN. The present study revealed that among all metals, Pt formed superior interfacial contact with g-C3N4 as compared to other metals. The maximum Schottky barrier height (Φb,Pt) of 0.66 V was observed at Pt/CN followed by Φb,Au/CN (0.46 V) and Φb,Pd/CN (0.05 V). The presence of electron-deficient Pt in Pt-XPS, decrease in the intensity of d-DOS of Pt near the Fermi level in VB-XPS, increase in CB tail states, and cathodic shift in Vfb in MS plots sufficiently confirmed strong metal-support interactions in Pt/CN. Due to the SPR effect, Au and Ag NPs suffered from agglomeration and poor dispersion during photodeposition. Finely dispersed Pt NPs (2-4 nm, 53% dispersion) successfully competed with shallow/deep trap states and drove the photogenerated electrons to active metallic sites in a drastically reduced time period as investigated by femtosecond transient absorption spectroscopy. Typically, an interfacial electron transfer rate, KIET,avg, of 2.5 × 1010 s-1 was observed for Pt/CN, while 0.087 × 1010 s-1 was observed in Au/CN. Band alignment/potentials at M/CN Schottky junctions were derived and most favorable in Pt/CN with CB tail states much above the water reduction potential; however, in the case of Pd, these extend much below the H+/H2 potential and hence behave like deep trap states. Thus, in Pd/CN (τ0 = 4200 ps, 49%) and Ag/CN (3870 ps, 53%), electron deep trapping dominates over charge transfer to active sites. The present study will help in designing futuristic new cocatalyst-photocatalyst systems.

Synthesis, characterization and photocatalytic H2 generation over ternary indium titanate nanoparticles.

A low temperature surfactant-free solution-phase method has been successfully developed for the synthesis of ternary In2TiO5, nanoparticles using a solvothermal route. The mechanistic aspects of synthesis of In2TiO5 nanoparticles from precursors, In(acac)3 and Ti(IV) isopropoxide in benzyl alcohol at 220 degrees C under solvothermal conditions, were investigated by GC-MS and 13C{1H} NMR analysis. The N2-BET surface area of the 5-8 nm sized In2TiO5 nanoparticles was found to be 60 m2 g(-1), which decreased with increase in calcination temperature; 38 m2 g(-1) at 800 degrees C; 5 m2 g(-1) at 1200 degrees C. The High resolution transmission electron microscopy (HR-TEM) shows well-developed lattice fringes of the crystalline nanoparticles, and selected area electron diffraction (SAED), pattern was indexed to be orthorhombic In2TiO5. The nanoparticles show better photocatalytic hydrogen generation from water-methanol mixtures over bulk In2TiO5, anatase TiO2 nanoparticles prepared by identical route and commercial TiO2 photocatalyst (Degussa, P25) under UV-visible irradiation (16% UV + 84% visible). Photocatalytic properties as a function of crystallinity and surface area of indium titanate nanoparticles have also been investigated. The high photoactivity obtained is correlated with the electronic and crystal structure of In2TiO5.

Mechanism of CO + N2O reaction via transient CO3(2-) species over crystalline Fe-substituted lanthanum titanates.

Some newer mechanistic aspects investigated by in situ Fourier transform infrared (FTIR) in conjunction with catalytic activity under similar conditions over crystalline lanthanum titanates as a function of Fe substitution at the B-site for the CO + N(2)O reaction are reported for the first time in the present communication. La(2)Ti(2(1-x))Fe(2x)O(7-delta) (0.0 < or = x < or = 1.0) was synthesized by gel combustion where Fe(3+) substitution effectively enhanced the conversion rates for N(2)O reduction as compared to the pristine La(2)Ti(2)O(7) (LTOGC). Among all samples, maximum conversion over La(2)Ti(0.8)Fe(1.2)O(7-delta) [LF(0.6)GC] catalyst was observed. In situ FTIR results reveal that substitution-induced anionic vacancies/defects provide additional sites on the surface of LF(0.6)GC for CO chemisorptions, whereas a perfect stoichiometric lattice like LTOGC is devoid of such sites. Surface-adsorbed CO reacts with surface lattice oxygen in the case of nonstoichiometric LF(0.6)GC to produce carbonates (M-CO(3)(2-)) at a much lower temperature. The reaction proceeds via carbonate formation, leaving the catalytic surface oxygen deficient in LF(0.6)GC, and therefore facilitates the reduction of preadsorbed, N(2)O [N(2)O(g) + * --> N(2) + *-O) by easily adsorbing the oxygen species (*-O) generated in N(2)O reduction, which is subsequently driven away by adsorbed/gas phase CO, whereas in the case of LTOGC, progress of the reaction was sluggish in the absence of transient carbonate species. Dissociative chemisorptions of N(2)O are not facilitated on stoichiometric oxygen excess titanate, as there is no vacancy in the surface to accommodate another oxygen atom. The redox mechanism via CO(3)(2-) species is proposed for CO + N(2)O reaction over La(2)Ti(2(1-x))Fe(2x)O(7-delta), as against the associative mechanism observed in the unsubstituted sample, La(2)Ti(2)O(7), as suggested by in situ FTIR in conjunction with catalytic activity results.