Visible-light Reaction Research Articles

Comparing Cr, and N only doping with (Cr, N)-codoping for enhancing visible light reactivity of TiO2

The photoreactivity of titania (TiO2) nanoclusters with varying levels of N or Cr-doping, or (Cr, N)-codoping, was systematically investigated using photodegradation of methyl orange in aqueous suspensions. The shifting of the TiO2 absorption edge into the visible spectral region that is primarily attributable to band gap narrowing was found to be a reliable metric for estimating the photoreactivity of the doped nanoclusters. Compared to the weak response with undoped and N-doped TiO2, Cr-doping and (Cr, N)-codoping were found to significantly enhance photodegradation of methyl orange under visible light. The initial reaction rates increase from about 0 to above 1.6×10−2min−1 when the doping concentration of Cr in TiO2 increases from 0 to 5%. In stark contrast, under UV irradiation, doping is not only ineffective but also detrimental to the photoreactivity, and all doping including N or Cr only and (Cr, N)-codoping were found to reduce photoreactivity.

Nitrogen stabilized reactive sputtering of optimized TiO2−x photocatalysts with visible light reactivity

Photoreactive and visible light responsive nonstoichiometric mixed-phase titania was prepared by reactive direct current magnetron sputtering. Trace amounts of nitrogen were added for process stabilization without being incorporated into the films. Based on the CO2 photoreduction tests and structural and optical characterization, the influence of the trace nitrogen on the sputtered nonstoichiometric TiO2 was studied and was compared to nitrogen-doped titania.

Synthesis, characterization, and visible light activity of new nanoparticle photocatalysts based on silver, carbon, and sulfur-doped TiO 2

New nanoparticle photocatalysts based on silver, carbon, and sulfur-doped TiO 2 having only the homogeneous anatase crystalline phase and high surface area were successfully synthesized by a modified sol–gel route. The catalysts were characterized by EDX, XRD, BET, UV–vis, IR, and Raman spectroscopy. The effects of the experimental parameters on the visible light reactivity of the catalysts were evaluated for the photodegradation of gaseous acetaldehyde as a model indoor pollutant. The activity results show that the silver(I) ion, Ag +, doping significantly promotes the visible light reactivities of carbon and sulfur-doped TiO 2 catalysts without any phase transformation from anatase to rutile. Moreover, Ag/(C, S)–TiO 2 photocatalysts degrade acetaldehyde 10 times faster in visible light and 3 times faster in UV light illuminations than the accredited photocatalyst P25–TiO 2. The commendable visible photoactivities of Ag/(C, S)–TiO 2 new nanoparticle photocatalysts are predominantly attributable to an improvement in anatase crystallinity, high surface area, low band gap and nature of precursor materials used.

Photocatalytic oxidation of NO x by Pt-modified TiO 2 under visible light irradiation

Photooxidation of NO x has been studied using titanium dioxide (Ishihara ST-01) treated with H 2PtCl 6. The experimental results indicate that Pt-modified TiO 2 with different ligand structures can be synthesized according to the Pt treating procedure. TiO 2 photocatalysts with certain Pt complexes produced significantly high photocatalytic activity under visible light irradiation without decreasing photoactivity under UV light irradiation. The visible-light-induced photocatalytic activity depended on the amount of Pt, but not greatly on the coordination number of Cl ions. It was suggested that the surface complex formation associated with Ti–O–Pt chemisorption dominated the visible light reactivity.

Effect of the Anchoring Group (Carboxylate vs Phosphonate) in Ru-Complex-Sensitized TiO2 on Hydrogen Production under Visible Light

We synthesized six Ru-bipyridyl complexes having di-, tetra-, and hexacarboxylate (C2, C4, and C6) and di-, tetra-, and hexaphosphonate (P2, P4, and P6) as the anchoring group, prepared six different sensitized TiO2 samples by using them, and then systematically tested their visible light reactivity for hydrogen production in aqueous suspension (with EDTA as an electron donor) under lambda > 420 nm illumination. The properties and efficiencies of C- and P-complexes as a sensitizer depended on the number and kind of anchoring groups in very different ways. The adsorption of P-complexes on TiO2 is strong enough not to be hampered by the presence of competing adsorbates (EDTA), whereas that of C-complexes is significantly inhibited. As a result, P-TiO2 exhibited much higher activity for the hydrogen production than C-TiO2, although the visible light absorbing capabilities are comparable among C- and P-complexes. Among the six sensitizers, P2 was the most active one for the H2 production. The hydrogen production activities of C-TiO2 and P-TiO2 depended on the concentration of sensitizers and electron donors in different ways as well. How the sensitizing activity for hydrogen production is influenced by the anchoring group and the experimental conditions was investigated and discussed in detail. It is also notable that the effects of the anchoring group on the sensitized production of hydrogen were drastically different from those on the dye-sensitized solar cell we recently reported for the same set of six sensitizers.

Visible-Light-Induced Photocatalytic Degradation of 4-Chlorophenol and Phenolic Compounds in Aqueous Suspension of Pure Titania: Demonstrating the Existence of a Surface-Complex-Mediated Path

The visible-light-induced degradation reaction of 4-chlorophenol (4-CP) was investigated in aqueous suspension of pure TiO2. Contrary to common expectations, 4-CP could be degraded under visible illumination (lambda > 420 nm), generating chlorides and CO2 concomitantly. The observed visible reactivity was not due to the presence of trace UV light since the visible-light-induced reactions exhibited behaviors distinguished from those of UV-induced reactions. Dichloroacetate could not be degraded under visible light, whereas it degraded with a much faster rate than 4-CP under UV irradiation. The addition of tert-butyl alcohol, a common OH radical scavenger, did not affect the visible reactivity of 4-CP, which indicates that OH radicals are not involved. Other phenolic compounds such as phenol and 2,4-dichlorophenol were similarly degraded under visible light. The surface complexation between phenolic compounds and TiO2 appears to be responsible for the visible light reactivity. Diffuse reflectance UV-vis spectra showed that 4-CP adsorbed on TiO2 powder induced visible light absorption. The visible light reactivity among several TiO2 samples was apparently correlated with the surface area of TiO2. The visible-light-induced photocurrents on a TiO2 electrode could be obtained only in the presence of 4-CP. It is proposed that a direct electron transfer from surface-complexed phenol to the conduction band of TiO2 upon absorbing visible light (through ligand-to-metal charge transfer) initiates the oxidative degradation of phenolic compounds. When the surface complex formation was hindered by surface fluorination, surface platinization, and high pH, the visible-light-induced degradation of 4-CP was inhibited. The evidence of visible-light-induced reactions and the experimental conditions affecting the visible reactivity were discussed in detail.

Simultaneous and Synergistic Conversion of Dyes and Heavy Metal Ions in Aqueous TiO2 Suspensions under Visible-Light Illumination

This study reports synergistic effects in the simultaneous conversion of dyes and heavy metal ions in aqueous TiO2/dye/metal ion systems (ternary components) under visible light (lambda > 420 nm). TiO2/Cr(VI)/Acid Orange 7 (AO7), TiO2/Cr(VI)/Rhodamine B (RhB), TiO2/Ag+/AO7, and TiO2/Ag+/RhB were chosen as test systems. Although dyes can be degraded in TiO2 suspensions under visible light, their removal rates were markedly enhanced in the presence of metal ions. Similarly, the reduction rates of metal ions in visible-light-illuminated TiO2 suspensions were negligible, but they were highly accelerated with dyes present. In particular, the synergistic effect in the ternary system of TiO2/Cr(VI)/AO7 was outstanding. The presence of dissolved oxygen increased the photoreduction rate of Cr(VI) despite the fact that Cr(VI) and O2 are competing electron acceptors. This is ascribed to in-situ photogenerated H2O2 from O2, which acts as a reductant of Cr(VI). RhB and Ag+ ions could be also converted simultaneously under visible light both in the presence and absence of TiO2. The visible-light-induced reduction of Ag+ did not occur at all in TiO2/Ag+ system, but it was enabled in both TiO2/Ag+/ RhB and TiO2/Ag+/AO7 to generate Ag particles. On the other hand, the binary systems of Cr(VI)/AO7, Ag+/AO7, and Ag+/RhB show significant visible-light activities for the conversion of both dye and metal ion. In this case, metal ions and dyes seem to form complexes that induce intracomplex electron transfers upon visible-light absorption. The Cr(VI)/RhB system, however, exhibited insignificant visible-light reactivity.

Effects of Surface Anchoring Groups (Carboxylate vs Phosphonate) in Ruthenium-Complex-Sensitized TiO2 on Visible Light Reactivity in Aqueous Suspensions

This study compares the visible light reactivities and properties of two sensitized Pt/TiO2 photocatalysts (Pt/TiO2/RuL3) on which ruthenium bipyridyl complexes (RuL3) are anchored through carboxylic (c-RuL3) or phosphonic (p-RuL3) acid groups. The photoreductive dechlorination of CCl4 and trichloroacetate (TCA) and the hydrogen production in aqueous suspensions were used as probe reactions for the visible light reactivity. The molar absorptivity of p-RuL3 was lower than that of c-RuL3, but Pt/TiO2/p-RuL3 exhibited higher visible light activities than Pt/TiO2/c-RuL3 for all tested reactions. On the other hand, the steady-state photocurrent obtained with the Pt/TiO2/p-RuL3 (or TiO2/p-RuL3) electrode was lower than that obtained with the Pt/TiO2/c-RuL3 (or TiO2/c-RuL3) electrode. As for the sensitizer adsorption, more p-RuL3 than c-RuL3 adsorbed on a TiO2 surface over a wide pH range: the adsorption of p-RuL3 decreased above pH 7, whereas that of c-RuL3 was reduced even at pH >4. Therefore, the photoreacti...

Highly enhanced photoreductive degradation of perchlorinated compounds on dye-sensitized metal/TiO2 under visible light.

This study reports an example of visible-light photocatalyst based on TiO2 modified by ruthenium-complex sensitizers and noble metal deposits. The photodegradation of trichloroacetate (TCA) and carbon tetrachloride was used as a probe reaction for evaluating the visible light activity of the photocatalyst under the illumination of lambda > 420 nm. Photodeposition of platinum nanoparticles on dye-sensitized TiO2 (Pt/TiO2/Ru(II)L3) drastically enhanced the degradation rate of TCA and CCl4. The visible light reactivity of Pt/TiO2/Ru(II)L3 was optimal with [Ru(II)L3] = 10 microM, [TiO2] = 0.5 g/L, and Pt loading of about 0.2 wt %. Although no electron donors to regenerate the oxidized Ru-sensitizers were added in the aqueous suspension, the photoreductive dechlorination of perchlorinated compounds proceeded far beyond the stoichiometric limit of the initial sensitizer concentration. Water acted as an electron donor to regenerate the sensitizer with a concurrent production of dioxygen. On the other hand, Pt/TiO2/Ru(II)L3 was completely inactive in the presence of dissolved oxygen and the in-situ generated dioxygen gradually decelerated the dechlorination rate. Conduction band electrons transferred to O2 in preference to CCl4 and TCA on Pt deposits. Other noble metals (Ag, Au, and Pd) deposited on TiO2 showed a better oxygen-tolerance but less visible-light reactivity than PtTiO2/Ru(II)L3. Effects of metal loading on the visible light activity and its implications for the efficientvisible-light photocatalyst development are discussed.