Photoinduced Reactions of Surface-Bound Species on Titania Nanotubes and Platinized Titania Nanotubes: An in Situ FTIR Study

Photoinduced conversion of surface-bound species on titania nanotubes that were first oxidized and then reduced (Ti–NT–O₂–H₂) and on platinized titania nanotubes subjected to oxidation and reduction (Pt–Ti–NT–O₂–H₂) has been investigated by means of in situ FTIR spectroscopy. Bidentate and monodentate carbonates as well as bicarbonates and carboxylates are formed subsequent to exposure of both Ti–NT–O₂–H₂ and Pt–Ti–NT–O₂–H₂ to CO₂. Formic acid was only observed on Pt–Ti–NT–O₂–H₂. UV illumination of the nanotubes led to an increase in the number of surface-bound species as a result of the further reaction with gas-phase CO₂ with a greater increase in surface species on Ti–NT–O₂–H₂ than on Pt–Ti–NT–O₂–H₂. The underlying basis of the photoinduced increase in adsorbed species is discussed for both types of nanotubes. Photoinduced reactions of surface species also take place and are remarkably different on the two types of nanotubes. UV illumination of Ti–NT–O₂–H₂ converts bidentate carbonates and bicarbonates to monodentate carbonates and carboxylates. There are less, and different, photoinduced reactions of surface species on Pt–Ti–NT–O₂–H₂: bicarbonates and monodentate carbonates convert to bidentate carbonates on the platinized titania nanotubes, and there is no obvious reaction involving carboxylates and formic acid upon irradiation of the platinized nanotubes. These differences in reactive behavior are discussed in the context of platinum acting as an efficient trap for photoelectrons which mitigates against reduction of Ti⁴⁺ to Ti³⁺, stabilizes holes, and alters the surface photochemistry taking place on the two different types of nanotubes. Photoinduced holes play an important role in photochemistry via oxidation of “structural water” and concomitant production of undercoordinated titania sites

Nanostructured PdO Thin Film from Langmuir–Blodgett Precursor for Room-Temperature H₂ Gas Sensing

Nanoparticulate thin films of PdO were prepared using the Langmuir–Blodgett (LB) technique by thermal decomposition of a multilayer film of octadecylamine (ODA)–chloropalladate complex. The stable complex formation of ODA with chloropalladate ions (present in subphase) at the air–water interface was confirmed by the surface pressure–area isotherm and Brewster angle microscopy. The formation of nanocrystalline PdO thin film after thermal decomposition of as-deposited LB film was confirmed by X-ray diffraction and Raman spectroscopy. Nanocrystalline PdO thin films were further characterized by using UV–vis and X-ray photoelectron spectroscopic (XPS) measurements. The XPS study revealed the presence of prominent Pd²⁺ with a small quantity (18%) of reduced PdO (Pd⁰) in nanocrystalline PdO thin film. From the absorption spectroscopic measurement, the band gap energy of PdO was estimated to be 2 eV, which was very close to that obtained from specular reflectance measurements. Surface morphology studies of these films using atomic force microscopy and field-emission scanning electron microscopy indicated formation of nanoparticles of size 20–30 nm. These PdO film when employed as a chemiresistive sensor showed H₂ sensitivity in the range of 30–4000 ppm at room temperature. In addition, PdO films showed photosensitivity with increase in current upon shining of visible light

Role of the Surface Lewis Acid and Base Sites in the Adsorption of CO₂ on Titania Nanotubes and Platinized Titania Nanotubes: An in Situ FT-IR Study

An understanding of the adsorption of CO₂, the first step in its photoreduction, is necessary for a full understanding of the photoreduction process. As such, the reactive adsorption of CO₂ on oxidized, reduced, and platinized TiO₂ nanotubes (Ti-NTs) was studied using infrared spectroscopy. The Ti-NTs were characterized with TEM and XRD, and XPS was used to determine the oxidation state as a function of oxidation, reduction, and platinization. The XPS data demonstrate that upon oxidation, surface O atoms become more electronegative, producing sites that can be characterized as strong Lewis bases, and the corresponding Ti becomes more electropositive producing sites that can be characterized as strong Lewis acids. Reduction of the Ti-NTs produces Ti³⁺ species, a very weak Lewis acid, along with a splitting of the Ti⁴⁺ peak, representing two sites, which correlate with O sites with a corresponding change in oxidation state. Ti³⁺ is not observed on reduction of the platinized Ti-NTs, presumably because Pt acts as an electron sink. Exposure of the treated Ti-NTs to CO₂ leads to the formation of differing amounts of bidentate and monodentate carbonates, as well as bicarbonates, where the preference for formation of a given species is rationalized in terms of surface Lewis acidity and or Lewis basicity and the availability of hydrogen. Our data suggest that one source of hydrogen is water that remains adsorbed to the Ti-NTs even after heating to 350 °C and that reduced platinized NTs can activate H₂. Carboxylates, which involve CO₂^– moieties and are similar to what would be expected for adsorbed CO₂^–, a postulated intermediate in CO₂ photoreduction, are also observed but only on the reduced Ti-NTs, which is the only surface on which Ti³⁺/O vacancy formation is observed

Effect of Mo-Incorporation in the TiO₂ Lattice: A Mechanistic Basis for Photocatalytic Dye Degradation

Photocatalytic activity of TiO₂ (anatase) is appreciably enhanced by substitutional doping of Mo in anatase lattice, in conjunction with the incorporation of nanostructured MoO₃ within the parent anatase lattice. The photocatalyst material was characterized in detail using X-ray diffraction, Raman spectroscopy, diffuse reflectance (DR-UV–Vis spectroscopy), X-ray photoelectron spectroscopy, and electron microscopy. Photocatalysis experiments were conducted using a model rhodamine-B (Rh–B) dye reaction using both UV and visible irradiation sources. The observed trends in the case of visible irradiative source can be summarized as follows: Mo-1 < Mo-2 < Mo-5 ≫ Mo-10. Attempts were made to isolate the structural factors that control photochemical behavior of these Mo–TiO₂ photocatalysts and to correlate photocatalytic activity with different structural aspects like oxidation state, band gap, surface species, etc. Mechanistic insights were acquired from ex situ ¹H NMR studies showing different intermediates and different probable routes for the Rh–B dye degradation with UV and visible radiations. The stable intermediates were formed by a direct oxidative fragmentation route, without any evidence of the initial deethylation route. The intermediates found were benzoic acid, different amines, diols, and certain acids (mostly formic and acetic acid). The adsorption of the Rh–B dye on the catalytic surface via the N-charge centers of the Rh–B was also observed

Nanocomposite of MoS₂‑RGO as Facile, Heterogeneous, Recyclable, and Highly Efficient Green Catalyst for One-Pot Synthesis of Indole Alkaloids

A nanocomposite comprised of MoS₂-RGO having unique structural features was developed by using a facile preparation strategy and demonstrated to be a highly efficient heterogeneous catalyst for the synthesis of indole alkaloids in water. The catalyst could be recycled six times without significant loss of its activity. Green chemistry matrix calculations for the reaction showed high atom economy (A.E. = 94.7%) and small <i>E</i>-factor (0.089). Using this nanocomposite as catalyst, four naturally occurring indole alkaloids, Arundine, Vibrindole A, Turbomycin B, and Trisindole, were synthesized along with their other derivatives in excellent yields