Impact of surface hydroxylation in MgO-/SnO-nanocluster modified TiO_2 anatase(101) composites on visible light absorption, charge separation and reducibility

Surface modification with metal oxide nanoclusters has emerged as a candidate for the enhancement of the photocatalytic activity of titanium dioxide. An increase in visible light absorption and the suppression of charge carrier recombination are necessary to improve the efficiency. We have studied Mg4O4 and Sn4O4 nanoclusters modifying the（101） surface of anatase TiO2 using density functional theory corrected for on-site Coulomb interactions（DFT ＋ U）. Such studies typically focus on the pristine surface, free of the point defects and surface hydroxyls present in real surfaces. We have also examined the impact of partial hydroxylation of the anatase surface on a variety of outcomes such as nanocluster adsorption, light absorption, charge separation and reducibility. Our results indicate that the modifiers adsorb strongly at the surface, irrespective of the presence of hydroxyl groups, and that modification extends light absorption into the visible range while enhancing UV activity. Our model for the excited state of the heterostructures demonstrates that photoexcited electrons and holes are separated onto the TiO2 surface and metal oxide nanocluster respectively. Comparisons with bare TiO2 and other TiO2-based photocatalyst materials are presented throughout.

Surface Chemistry and Catalysis of Rare Earth Oxides I．A Study of the Reactivity of Surface Hydroxyls on CeO_2 and Pr_6O_（11） by FT－IR Spectroscopy

The surface chemical properties of CeO2 and Pr6O11 have been investigated with FT-IR spectroscopy. The reactivities of surface hydroxyls were tested through the reaction of CO. Surface formate species are formed on CeO2 and Pr6O11 under CO atmosphere at 200℃ . and the reaction becomcs more prevailing at higher temperatures especially for partially reduced samples. The surface formate species are produced via the reaction of CO with surface hydroxyls which was confirmed by the reaction of CO with D2-treated CeO2 and Pr6O11. The Surface formate can be oxidized to carbonate at temperatures exceeding 300 ℃, and the surface hydroxyls could be recovered as the formate species decompose or are oxidized to carbonate species. The roles played by the surface hydroxyls and surface active sites in the CO oxidation are discussed.

Hydroxylated graphene quantum dots as fluorescent probes for sensitive detection of metal ions

Highly sensitive methods are important for monitoring the concentration of metal ions in industrial wastewater.Here,we developed a new probe for the determination of metal ions by fluorescence quenching.The probe consists of hydroxylated graphene quantum dots(H-GQDs),prepared from GQDs by electrochemical method followed by surface hydroxylation.It is a non-reactive indicator with high sensitivity and detection limits of 0.01μM for Cu2+,0.005μM for Al3+,0.04μM for Fe3+,and 0.02μM for Cr3+.In addition,the low biotoxicity and excellent solubility of H-GQDs make them promising for application in wastewater metal ion detection.

Pt-surface oxygen vacancies coupling accelerated photo-charge extraction and activated hydrogen evolution

Semiconductors-based heterogeneous photocatalytic water splitting has been extensively studied,but it still remains challenging to accelerate the separation of electron-hole pairs and facilitate the reaction kinetics.Here we report a general strategy to fabricate highly efficient Pt/TiO_(2)photocatalyst by coupling the Pt co-catalysts and surface oxygen vacancies(VO)of TiO_(2).TiO_(2)was pre-modified with alkali or alkaline earth metals ion solutions,which produce a large number of surface hydroxyl on TiO_(2).Subsequently,the photodeposited Pt sub-nanoparticles substitute surface hydroxyl and induce surface VO on TiO_(2).The coupling of Pt and surface VO on TiO_(2)can accelerate the extraction of photo-charges through the interaction of Pt-VO-Ti bonds and reduce the hydrogen evolution barrier,thereby promoting the photocatalytic activity.The synthesized Pt-VO-TiO_(2)sample exhibits a photocatalytic hydrogen evolution activity as high as 1.5 L·g^(−1)·h^(−1),which is 2.2 times that of traditional Pt/TiO_(2).Our findings indepth understand the synergistic effect of co-catalysts and defects on photocatalysis and open up new possibilities for achieving robust photocatalytic water splitting.

Dehydroxylation action on surface of TiO_2 films restrained by nitrogen carrier gas during atomic layer deposition process

A strong influence of nitrogen gas on the content of surface hydroxyl groups of TiO2 films by atomic layer deposition（ALD） was investigated by X-ray photoelectron spectroscopy（XPS）, contact angle measuring system, and UV–Vis spectrophotometer. XPS spectra of O 1s indicate that the content of surface hydroxyl groups is varied when using N2 as carrier gas. The results of water contact angles and optical reflection spectra show that the content variation of surface hydroxyl groups influences the wetting properties and optical reflectivity of TiO2 films. A surface reaction model is suggested to explain the ALD reaction process using N2 as carrier gas.

Highly selective photocatalytic CO_(2) reduction to ethylene in pure water by Nb_(2)O_(5) nanoparticles with enriched surface –OH groups under simulated solar illumination

Photocatalytic CO_(2) reduction to valuable chemical compounds could be a promising approach for carbon-neutral practice.In this work,a simple and robust thermal decomposition process was developed with ammonium carbonate((NH4)2CO3)as both precipitation agent and sacrificial template to produce fine Nb_(2)O_(5) nanoparticles with the rich existence of surface hydroxyl(–OH)groups.It was found by density functional theory(DFT)calculations and experiments that the rich existence of the surface–OH groups enhanced the adsorption of both reactants(CO_(2) and H_(2)O molecules)for the photocatalytic CO_(2) reduction on these fine Nb_(2)O_(5) nanoparticles,and the highly selective conversion of CO_(2) to the high-value chemical compound of ethylene(C_(2)H_(4),~68μmol·g^(−1)·h^(−1) with~100%product selectivity)was achieved under simulated solar illumination without usage of any sacrificial agents or noble metal cocatalysts.This synthesis process may also be readily applied as a surface engineering method to enrich the existence of the surface–OH groups on various metal oxide-based photocatalysts for a broad range of technical applications.

Enhanced dielectric and ferroelectric properties of surface hydroxylated Na_(0.5)Bi_(0.5)TiO_(3)(NBT)-poly(vinylidene fluoride)(PVDF)composites

The surface hydroxylation treatment has been carried out by using hydrogen peroxide(H_(2)O_(2))to modify the surface of Na_(0.5)Bi_(0.5)TiO_(3)(NBT)particles in a ferroelectric polymer(PVDF)via solution casting technique.The FTIR study confirms the presence of hydroxyl groups on the surface of NBT.The FE-SEM analysis reveals that h-NBT particles are dispersed homogeneously within the polymer matrix.The surface hydroxylation treatment plays an important role in high dielectric constant and also reduced loss by conducting the material surface with
OH functional groups.The prepared composite with 40 wt.%of h-NBT showed enhanced dielectric constant(≈114),negligible loss(0.22)and high AC conductivity as compared to that of the unmodified NBT.Such significant enhancement in dielectric properties may be due to the strong interaction between h-NBT particles and PVDF matrix at the interface.The percolation theory is used to explain the dielectric properties of h-NBT-PVDF composite.Furthermore,the remnant polarization of the un-poled h-NBT-PVDF composites(2 Pr–1.19
C/cm^(2) for 40 wt.%of h-NBT)is also improved.The present findings give an idea of high dielectric constant and relatively low loss composite materials as a promising candidate for electronic and energy storage devices.

Thermal-, photo- and electron-induced reactivity of hydrogen species on rutile TiO_2(110) surface: Role of oxygen vacancy

Interaction of hydrogen with TiO2 plays a vital role in TiO2-based photocatalysis and thermal catalysis. In this work, we compared thermal-, photo-, and electron-induced reactivity of various types of hydrogen species on a rutile TiO2（110） surface formed by atomic H exposure at 320 and 115 K by means of thermal desorption spectroscopy, X-ray photoelectron spectroscopy and low energy electron diffraction. Atomic H interaction with rutile TiO2（110） at 115 K forms surface TiààH hydride, surface hydroxyl group, and chemisorbed water. Upon heating, surface TiààH hydride reacts to produce H2 while surface hydroxyl groups react to form both water and H2. Atomic H interaction with rutile TiO2（110） at 320 K strongly reduces TiO2 due to the continuous formation and desorption of water and forms surface hydroxyl groups and likely subsurface/bulk hydrogen species. Upon heating, hydrogen forms as the only gas-phase product and its desorption activation energy decreases with the subsurface/bulk reduction extent of rutile TiO2（110）. Surface Ti-H hydride exhibits photo-induced reactivity while both surface TiààH hydride and surface hydroxyl group exhibit electro-induced reactivity. These results have important implications for understanding the hydrogen-involved thermal and photo reactions on TiO2-based catalysts.

Activated carbon enhanced ozonation of oxalate attributed to HO·oxidation in bulk solution and surface oxidation: Effect of activated carbon dosage and pH

Ozonation of oxalate in aqueous phase was performed with a commercial activated carbon（AC）in this work. The effect of AC dosage and solution pH on the contribution of hydroxyl radicals（HOU） in bulk solution and oxidation on the AC surface to the removal of oxalate was studied. We found that the removal of oxalate was reduced by tert-butyl alcohol（tBA） with low dosages of AC,while it was hardly affected by tBA when the AC dosage was greater than 0.3 g/L. tBA also inhibited ozone decomposition when the AC dosage was no more than 0.05 g/L, but it did not work when the AC dosage was no less than 0.1 g/L. These observations indicate that HOUin bulk solution and oxidation on the AC surface both contribute to the removal of oxalate. HOU oxidation in bulk solution is significant when the dosage of AC is low, whereas surface oxidation is dominant when the dosage of AC is high. The oxalate removal decreased with increasing pH of the solution with an AC dosage of 0.5 g/L. The degradation of oxalate occurs mainly through surface oxidation in acid and neutral solution, but through HOUoxidation in basic bulk solution. A mechanism involving both HOUoxidation in bulk solution and surface oxidation was proposed for AC enhanced ozonation of oxalate.

Insight into phase structure-dependent soot oxidation activity of K/MnO_(2) catalyst

In the present study,two nanosized MnO_(2)with β and δ phase structures and potassium loaded MnO_(2)catalysts with varied K loading amounts (denoted as K/MnO_(2)) were prepared.Temperature programmed oxidation and isothermal reactions in loose contact modes were employed to examine the soot oxidation activity of the as-prepared catalysts.Characterization results show that as compared with β-MnO_(2),δ-MnO_(2)has larger surface area and higher content of hydroxyl groups.Upon K loading,abundant hydroxyl groups in δ-MnO_(2)effectively sequestrate K cation to form bound K species and free K species are available only at K loading above 3.0 wt.%.In contrast,the majority of K species present as free state in β-MnO_(2)even at a K loading of 1.0 wt.%due to its very low hydroxyl group content.The O_(2)temperature-programmed desorption (O_(2)-TPD) demonstrates that the catalysts with free K species exhibit strong ability in activating gaseous O_(2),whereas the catalysts only having bound K display minor O_(2)activation capability.As a result,despite of slightly lower activity of β-MnO_(2)than δ-MnO_(2),the K/β-MnO_(2)catalysts exhibit substantially higher activities than K/δ-MnO_(2)catalysts with identical K loadings.The finding in this study clearly demonstrates that for MnO_(2)based catalysts,the enhancement of catalytic activity for soot oxidation is highly K loading amount dependent and the dependency is strongly associated with the phase structure of MnO_(2).

Effects of crystal structure on the activity of MnO2 nanorods oxidase mimics

The crystal structures would directly affect the physical and chemical properties of the surface of the material,and would thus influence the catalytic activity of the material.α-MnO2,β-MnO2 andγ-MnO2 nanorods with the same morphology yet different crystal structures were prepared and tested as oxidase mimics using 3,3’,5,5’-tetramethylbenzidine(TMB)as the substrate.β-MnO2 that exhibited the highest activity had a catalytic constant of 83.75μmol·m^−2·s^−1,2.7 and 19.0 times of those ofα-MnO2 andγ-MnO2(30.91 and 4.41μmol·m^−2·s^−1),respectively.The characterization results showed that there were more surface hydroxyls as well as more Mn4+on the surface of theβ-MnO2 nanorods.The surface hydroxyls were conducive to the oxidation reaction,while Mn4+was conducive to the regeneration of surface hydroxyls.The synergistic effect of the two factors significantly improved the activity ofβ-MnO2 oxidase mimic.Usingβ-MnO2,aβ-MnO2-TMB-GSH system was established to detect the content of glutathione(GSH)rapidly and sensitively by colorimetry.This method had a wide detection range(0.11-45μM)and a low detection limit(0.1μM),and had been successfully applied to GSH quantification in human serum samples.

Hydrogen-incorporated vanadium dioxide nanosheets enable efficient uranium confinement and photoreduction

Photocatalytic reduction of U(VI)represents a novel and effective manner for the removal of U(VI)pollutant from radioactive wastewater.Herein,we successfully incorporated hydrogen into VO_(2)nanosheets,which strengthened the interaction between VO_(2)and U(VI),thereby achieving a highly active and stable photocatalyst for U(VI)reduction.With the increase of H content in hydric VO_(2)(H_(x)-VO_(2))nanosheets,the bandgap shrank from 2.29 to 1.66 eV,whereas the position of conduction bands remained more negative than the reduction potential of U(VI)/U(IV)(0.41 V vs.NHE).When irradiated by simulated sunlight,the U(VI)removal efficiency over H0.613-VO_(2)nanosheets reached up to 95.4%within 90 min,which largely outperformed 28.3%of pristine VO_(2)nanosheets.The mechanistic study demonstrated that the hydroxylated surface gave rise to the balanced O confinement sites in VO_(2)(011),leading to the stabilized adsorption configuration and increased binding strength of UO_(2)^(2+)on H_(x)-VO_(2)nanosheets.

Removal of ammonium ion from water by Na-rich birnessite:Performance and mechanisms

Na-rich birnessite（NRB） was synthesized by a simple synthesis method and used as a high-efficiency adsorbent for the removal of ammonium ion（NH＋4） from aqueous solution.In order to demonstrate the adsorption performance of the synthesized material,the effects of contact time,pH,initial ammonium ion concentration,and temperature were investigated.Adsorption kinetics showed that the adsorption behavior followed the pseudo second-order kinetic model.The equilibrium adsorption data were fitted to Langmuir and Freundlich adsorption models and the model parameters were evaluated.The monolayer adsorption capacity of the adsorbent,as obtained from the Langmuir isotherm,was 22.61 mg NH＋4-N/g at283 K.Thermodynamic analyses showed that the adsorption was spontaneous and that it was also a physisorption process.Our data revealed that the higher NH＋4adsorption capacity could be primarily attributed to the water absorption process and electrostatic interaction.Particularly,the high surface hydroxyl-content of NRB enables strong interactions with ammonium ion.The results obtained in this study illustrate that the NRB is expected to be an effective and economically viable adsorbent for ammonium ion removal from aqueous system.

Unique adsorption behaviors of NO and O_2 at hydrogenated anatase TiO_2(101)

Titanium dioxide（TiO2） is one of the most widely studied transition metal oxides, especially for its unique performances in heterogeneous photocatalysis. Different phases of TiO2 have been found to exhibit different photo-activities, though the origins are still not fully understood. In this work, we use the density functional theory（DFT） calculations, corrected by on-site Coulomb and long-range dispersion interactions, to study the adsorptions of nitric oxide（NO） and oxygen（O2） molecules on the clean and hydrogenated anatase TiO2（101） surfaces. We also compare the detailed calculated results regarding their structural, energetic and electronic properties with those obtained at rutile TiO2（110）. It has been found that the behaviors of the surface localized electrons being transferred from adsorbed H, as well as the adsorption behaviors of NO and O2 are quite different at the two surfaces, which can be attributed to their characteristic local bonding structures around the surface hydroxyl. These results may also help explain the different photocatalytic activities of these two main facets of anatase and rutile TiO2

How interfacial electron-donating defects influence the structure and charge of gold nanoparticles on TiO_(2)support

The reduction degree of TiO_(2)support is critical to the performances of metal catalysts.In many previous theoretical calculations,only the bridge oxygen vacancy(Ov)was considered as the electron-donating defect on reduced rutile TiO_(2)(r-TiO_(2−x))supports.However,titanium adatoms(Tiad.),oxidized titanium islands(Tiad.On),and acid hydroxyls(ObrH)also exist at the metal/support interface.By conducting density functional theory(DFT)calculations and ab initio molecular dynamics(AIMD)simulations,we compared r-TiO_(2−x)surfaces with Ov,Tiad.,Tiad.On,and ObrH sites loaded with Au nanoparticles(NPs).The results showed the Au NPs were oxygen-phobic but titanium-philic,resulting in wetting of Ov and Tiad.but short contact with Tiad.On and ObrH.The Bader charges of Au NPs(QM)showed a good linear relationship with the ideal number of donating electrons(Ne)from the defective sites(QM=−KeNe+QM,S),demonstrating the intrinsic electron allocation at the interface.The Ov,Tiad.,and Tiad.On exhibited similar slopes(Ke),relatively steeper than that of ObrH.That means in the scope of Au NP charge state,the Tiad.and Tiad.On have a close electron-donating ability with Ov,but the ObrH donates relatively fewer electrons.This linear relationship can be extended approximately to other metals.The higher the metal work function,the steeper the Ke for easier electron donation from defective sites.The stronger the metal oxygen affinity,the more positive the intercept(QM,S).That explains the easy generation of metallic or negative Pt and Au NPs on r-TiO_(2−x),but hard for Cu and Zn in experiment.That provides theoretical guidance for regulating the charge of metal NPs over TiO_(2−x)supports.