Surface OH Groups Research Articles

Adsorption of the herbicide 4-chloro-2-methylphenoxyacetic acid (MCPA) by goethite.

Interaction between the goethite surface and 4-chloro-2-methylphenoxyacetic acid (MCPA) herbicide was studied using density functional theory (DFT) calculations combined with molecular dynamics (MD). The important step made here lies in the use of a periodic DFT method enabling the study of a mineral surface of different protonation states, in strong contrast with previous molecular modeling studies limited to single protonation state corresponding to the point of zero charge. Different surface OH groups and MCPA proton states were used to mimic the strong effects of pH on the outer- and inner-sphere surface complexes that are theoretically possible, together with their binding energies, and their bond lengths. Modeling both a solvated and a protonated (110) goethite surface provided a major breakthrough in the acidic adsorption regime. An outer-sphere complex and a monodentate inner-sphere complex with the neutral MCPA molecule were found to be the most energetically stable adsorbate forms. MD modeling predicted that the latter forms via the sharing of the carbonyl oxygen between the MCPA carboxylate group and a singly coordinated surface hydroxyl group, releasing an H2O molecule. All the other complexes, including the bidentate inner-sphere complex, had higher relative energies and were therefore less likely. The two most likely DFT-optimized structures were used to constrain a surface complexation model applying the charge distribution multisite complexation (CD-MUSIC) approach. The adsorption constants for the complexes were successfully fitted to experimental batch equilibrium data.

Active Sites on Nanocrystalline Tin Dioxide Surface: Effect of Palladium and Ruthenium Oxides Clusters

Active sites of nanocrystalline tin dioxide materials with variable particle size, surface area, and catalytic modifiers were studied. Effect of palladium and ruthenium oxides clusters on the activity and concentration of tin dioxide surface centers was evaluated by temperature-programmed desorption techniques using probe molecules, FTIR spectroscopy, EPR, and thermogravimetric methods. The surface site concentration decrease was observed with an increase of SnO2 particle size and BET area decrease. The active sites of SnO2 were found to be selectively promoted by the additives. Accumulation of surface OH groups including hydroxyl spin centers and Broensted acid sites was characteristic for SnO2/PdOx nanocomposites as a result of water chemisorption enhancement due to proposed electronic clusters–support interaction. Ruthenium oxide was shown to increase the concentration of chemisorbed oxygen species via oxygen spillover route.

Revisiting the CO oxidation reaction on various Au/TiO2 catalysts: roles of the surface OH groups and the reaction mechanism.

This work aims to understand the influence of TiO2 surface structure in Au/TiO2 catalysts on CO oxidation. Au nanoparticles (3 wt%) in the range of 4 to 8 nm were loaded onto four kinds of TiO2 surfaces, which had different surface structures and were synthesized by calcining hydrogen titanate nanotubes at various temperatures and in different atmospheres. The Au catalyst supported on anatase nanorods exhibited the highest activity in CO oxidation at 30 degrees C among all the five Au/TiO2 catalysts including the reference catalyst of Au/TiO2-P25. X-ray photoelectron spectroscopy (XPS) and infrared emission spectra (IES) results indicate that the anatase nanorods have the most active surface on which water molecules can be strongly adsorbed and OH groups can be formed readily. Theoretical calculation indicates that the surface OH can facilitate the O2 adsorption on the anatase surface. Such active surface features are conducive to the O2 activation and CO oxidation.

Heterogeneous reaction of NO2 on α-Al2O3 in the dark and simulated sunlight.

Although there have been a number of publications focused on heterogeneous of NO2 on mineral particles, most of these studies were focused on β-Al2O3 and performed in the dark. Less was known about the reaction process of NO2 on α-Al2O3, especially the effect of sunlight factor. The heterogeneous reaction between NO2 and α-Al2O3 was investigated by using diffuse reflectance infrared Fourier transform spectrometry. The effects of NO2 and O2 concentrations as well as simulated sunlight were examined, and the reaction mechanism including the consumption of surface OH groups, oxidation process of nitrites, and the formation of water was also discussed in detail. It was observed that the formation rates of nitrates and nitrites were sensitive to NO2 concentrations and O2 concentrations. Nitrite was identified to be an intermediate production and disappeared very soon as [NO2] was up to 4.035 × 10(15) molecules/cm(3). Light played an important role in the changes of the electronic configuration of mineral dust, such as electronic donating ability, surface OH groups orientation, as well as the conversion efficiency between proton acid and nonproton acid, all of which could significantly enhance the heterogeneous reaction process. The reaction order for NO2 and O2 was determined to be 0.960 ± 0.111 and 0.620 ± 0.028, respectively. The uptake coefficient of NO2, which dominated the first step of the heterogeneous reaction, was calculated by the infrared absorbance with the use of ion chromatography and determined to be 9.9 × 10(-10) in the dark and varied from 2.54 to 3.33 × 10(-9) under simulated sunlight from 0.45 to 1.35 mW/cm(2). It was also found that γNO2 was independent of [NO2] and sunlight increased the uptake coefficient by three times, indicating that the heterogeneous reaction between NO2 and α-Al2O3 was enhanced under sunlight.

Effects of the Covalent Bonding Entrapment of Tetrapyrrole Macrocycles Inside Translucent Monolithic ZrO<sub>2</sub> Xerogels

While searching for adequate sol-gel methodologies for successfully trapping in monomeric and stable form either porphyrins or phthalocyanines, inside translucent monolithic silica xerogels, it was discovered that the interactions of these trapped tetrapyrrole macrocycles with Si-OH surface groups inhibit or spoil the efficient display of physicochemical, especially optical, properties of the confined species. Consequently, we have developed strategies to keep the inserted macrocycle species as far as possible from these interferences by substituting the surface-OH groups foralkylorarylgroups or trapping these species inside alternative metal oxide networks, such as ZrO2, TiO2, and Al2O3. In the present manuscript, we present, for the first time to our knowledge, a methodology for preserving the spectroscopic characteristics of metal tetrasulfophthalocyanines and cobalt tetraphenylporphyrins trapped inside the pores of ZrO2xerogels. The results obtained are contrasting with analogous silica systems and demonstrate that, in ZrO2networks, the macrocyclic species remain trapped in stable and monomeric form while keeping their original spectroscopic characteristics in a better way than when captured inside silica systems. This outcome imply a lower hydrophilic character linked to the existence of a smaller amount of surface hydroxyl groups in ZrO2networks, if compared to analogous SiO2xerogel systems. The development and study of the possibility of trapping or fixing synthetic or natural tetrapyrrole macrocycles inside inorganic networks suggest the possibility of synthesizing hybrid solid systems suitable for important applications in technological areas such as optics, catalysis, sensoring and medicine

A NEW SOURCE OF CO 2 IN THE UNIVERSE: A PHOTOACTIVATED ELEY-RIDEAL SURFACE REACTION ON WATER ICES

CO2 is one of the most abundant components of ices in the interstellar medium; however, its formation mechanism has not been clearly identified. Here we report an experimental observation of an Eley-Rideal-type reaction on a water ice surface, where CO gas molecules react by direct collisions with surface OH radicals, made by photodissociation of H2O molecules, to produce CO2 ice on the surface. The discovery of this source of CO2 provides a new mechanism to explain the high relative abundance of CO2 ice in space.

Adsorption and surfactant-mediated desorption of poly(vinylpyrrolidone) on plasma- and piranha-cleaned silica surfaces.

Optical flow cell reflectometry was used to study the adsorption of poly(vinylpyrrolidone) (PVP) to a silica surface and the subsequent surfactant adsorption and polymer desorption upon exposure to the anionic surfactant sodium dodecyl sulfate (SDS). We have studied these effects as a function of pH and surfactant concentration, but also for two different methods of silica preparation, O2 plasma and piranha cleaning. As a function of pH, a plateau in the amount adsorbed of ∼0.6 mg/m(2) is observed below a critical pH, above which the adsorption decreases to zero within 2-3 pH units. An increase in pH leads to dissociation of surface OH groups and a decreased potential for hydrogen bonding between the polymer and surface. For the plasma- and piranha-cleaned silica, the critical pH differs by 1-2 pH units, a reflection of the much larger amount of surface OH groups on piranha-cleaned silica (for a given pH). Subsequent rinsing of the adsorbed layer of PVP with an SDS solution leads to total or partial desorption of the PVP layer. Any remaining adsorbed PVP then acts as an adsorption site for SDS. A large difference between plasma- and piranha-cleaned silica is observed, with the PVP layer adsorbed to plasma-cleaned silica being much more susceptible to desorption by SDS. For a plasma-cleaned surface at pH 5.5, only 30% of the originally adsorbed PVP is remaining, while for piranha-cleaned silica, the pH can be increased to 10 before a similar reduction in the amount of adsorbed PVP is seen. For a given pH, piranha-cleaned silica has a higher surface charge, leading to a smaller amount of adsorbed SDS per PVP chain on a piranha-cleaned surface compared to a plasma-cleaned surface under identical conditions. In that way, the high negative surface charge makes desorption by negatively charged SDS more difficult. The high surface charge thus protects the neutral polymer from surfactant-mediated desorption.

Surface and catalytic properties of some γ-Al2O3 powders

The physicochemical and catalytic properties of some γ-Al2O3 samples have been investigated by XRD, SBET, porosity measurements, TEM, chemical analysis (ICP-MS) and IR spectroscopy of the surface OH groups and of adsorbed CO and pyridine. Catalytic activities in the ethanol dehydration reaction were also studied in gas phase in a tubular flow reactor from 423K to 723K at atmospheric pressure with 1.44h−1 WHSV in nitrogen as well as by ethanol temperature programmed reaction (Ethanol-TPR) in a TG/DTA/MS thermal analyzer. Even small amounts of alkali (300ppm) modify significantly the spectra in the OH stretching region and kill part of the alumina strongest Lewis acid sites with a clear decrease of catalytic activity, while 1% Na completely modifies the surface behaviour. Differences are also observed for clean samples showing slightly different morphology (lamellar versus bulky). The data showed that the catalytic activity of γ-Al2O3 is determined by a small number of sites, which are likely located on edges, corners and defects of the crystals more than on plain surfaces.

Structure–activity relationships on metal-oxides: alcohol dehydration

The Lewis-acid catalyzed dehydration of simple alcohols on TiO2, ZrO2 and γ-Al2O3 oxide-catalysts has been investigated by combining ab initio theoretical calculations with temperature programmed desorption (TPD) experiments. Both theoretical and experimental results demonstrate that γ-Al2O3 is more active in catalyzing the dehydration reactions than either TiO2 or ZrO2. The dehydration reaction occurs through an E2-elimination mechanism involving either surface O and/or OH groups of the oxides. Based on relationships between the dehydration barriers and key properties of the alcohols and the oxides, a dehydration model was developed that is able to screen the dehydration performance of various alcohols on different metal oxides and provide predictions that were in good agreement with the experimental dehydration barriers. The model accounts for the effect of surface hydration and the existence of surface OH-groups. The basicity of the surface oxygens is shown to be important in eliminating beta hydrogens of the alcohols. This work highlights the importance of surface OH-groups as active centers for the elimination of beta hydrogens of alcohols in Lewis-acid catalyzed dehydration reactions, a result different from the conventional view that surface OH-groups are associated with Bronsted acidity. Most importantly, a novel methodology is introduced that develops structure–activity relationships on oxides for the conversion of biomass derived molecules to chemicals.

Electrochemical Properties of Spinel Cobalt Ferrite Nanoparticles with Sodium Alginate as Interactive Binder

AbstractWe introduce a process of making high‐capacity and rate‐capable metal‐ferrite‐based conversion anodes for lithium‐ion batteries. Cobalt ferrite (CoFe2O4) exhibits a discharge capacity that is two‐times higher compared to the state‐of‐the‐art graphite anode, but at the same time it shows high volume change (ca. 95 %) during conversion reaction with lithium in an electrochemical environment. This large volume expansion is responsible for the particle–particle and conductive‐carbon particles–active materials detachment, which leads to cyclic instability during subsequent cycles. As observed in our earlier work, any kind of weak or strong chemical interaction between active materials and binder is necessary to achieve excellent electrochemical performance in case of conversion or alloying reactions. To compare the electrochemical activity of CoFe2O4 nanoparticles against lithium, we use conventional polyvinylidene fluoride and sodium alginate binder to fabricate electrodes. Fourier‐transform infrared measurements reveal weak hydrogen‐bond formation between surface OH groups of CoFe2O4 and COOH groups of the alginate binder. Indentation tests further confirm the increased hardness of the alginate/CoFe2O4‐based electrode films. CoFe2O4–alginate–carbon anode exhibits a high specific capacity of 890 mAh g−1 at 0.1 C rate (91.4 mA g−1) after 50 charge–discharge cycles. Even at high rate cycling with current densities such as 18280 mA g−1 (20 C), the same electrode material exhibits a specific capacity of 470 mAh g−1, which is much higher than that of conventional graphite anode at the same electrochemical conditions.

Hydrogen Surface Reactions and Adsorption Studied on Y2O3, YSZ, and ZrO2.

The surface reactivity of Y2O3, YSZ, and ZrO2 polycrystalline powder samples toward H2 has been comparatively studied by a pool of complementary experimental techniques, comprising volumetric methods (temperature-programmed volumetric adsorption/oxidation and thermal desorption spectrometry), spectroscopic techniques (in situ electric impedance and in situ Fourier-transform infrared spectroscopy), and eventually structural characterization methods (X-ray diffraction and scanning electron microscopy). Reduction has been observed on all three oxides to most likely follow a surface or near-surface-limited mechanism involving removal of surface OH-groups and associated formation of water without formation of a significant number of anionic oxygen vacancies. Partly reversible adsorption of H2 was proven on the basis of molecular H2 desorption. Dictated by the specific hydrophilicity of the oxide, readsorption of water eventually takes place. The inference of this surface-restricted mechanism is further corroborated by the fact that no bulk structural and/or morphological changes were observed upon reduction even at the highest reduction temperatures (1173 K). We anticipate relevant implications for the use of especially YSZ in fuel cell research, since in particular the chemical state and structure of the surface under typical reducing high-temperature conditions affects the operation of the entire cell.

Sorption of organophosphate esters by carbon nanotubes

Insights from the molecular-level mechanism of sorption of organophosphate esters (OPEs) on carbon nanotubes (CNTs) can further our understanding of the fate and transport of OPEs in the environment. The motivation for our study was to explore the sorption process of OPEs on multi-walled CNTs (MWCNTs), single-walled CNTs (SWCNTs) and their oxidized counterparts (O-MWCNTs and O-SWCNTs), and its molecular mechanism over a wide concentration range. The sorption isotherm results revealed that the hydrophobicity of OPEs dominated their affinities on a given CNT and the π–π electron donor–acceptor (EDA) interaction also played an important role in the sorption of aromatic OPEs. This π–π EDA interaction, verified with Raman and FT-IR spectroscopy, could restrict the radial vibration of SWCNTs and affect the deformation vibration γ(CH) bands of OPE molecules. The OPE surface coverage on CNTs, estimated using the nonlinear Dubinin–Ashtakhov model, indicated that the oxygen-containing functional groups on CNTs could interact with water molecules by H-bonding, resulting in a decrease in effective sorption sites. In addition, FTIR analysis also confirmed the occurrence of Brønsted acid–base interactions between OPEs and surface OH groups of SWCNTs. Our results should provide mechanistic insights into the sorption mechanism of OPE contaminants on CNTs.

Structure of Mo2Cx and Mo4Cx Molybdenum Carbide Nanoparticles and Their Anchoring Sites on ZSM-5 Zeolites

Mo carbide nanoparticles supported on ZSM-5 zeolites are promising catalysts for methane dehydroaromatization. For this and other applications, it is important to identify the structure and anchoring sites of Mo carbide nanoparticles. In this work, structures of Mo2Cx (x = 1, 2, 3, 4, and 6) and Mo4Cx (x = 2, 4, 6, and 8) nanoparticles are identified using a genetic algorithm with density functional theory (DFT) calculations. The ZSM-5 anchoring sites are determined by evaluating infrared vibrational spectra for surface OH groups before and after Mo deposition. The spectroscopic results demonstrate that initial Mo oxide species preferentially anchors on framework Al sites and partially on Si sites on the external surface of the zeolite. In addition, Mo oxide deposition causes some dealumination, and a small fraction of Mo oxide species anchor on extraframework Al sites. Anchoring modes of Mo carbide nanoparticles are evaluated with DFT cluster calculations and with hybrid quantum mechanical and molecular ...

Significant enhancement of photocatalytic activity over bifunctional ZnO–TiO2 catalysts for 4-chlorophenol degradation

In this study, ZnO–TiO2 binary oxide catalysts were successfully prepared by a solid-state dispersion. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, diffuse reflectance spectroscopy, transmission electron microscopy, Brunauer–Emmett–Teller and photoluminescence techniques. It was observed that the interaction between ZnO and TiO2 affected the photocatalytic efficiency of binary oxide catalysts. 20wt% ZnO–TiO2 photocatalyst exhibited much higher photocatalytic activity than pure TiO2, ZnO and P-25 in the degradation of 4-chlorophenol under low UV irradiation. The chemical interaction between ZnO and TiO2 in the form of the Zn–O–Ti– bond may affect the photoefficiency of binary oxide catalysts. The profound effect of binary oxide catalyst for 4-chlorophenol degradation was generally considered due to small particle size, lower band gap energy and the presence of more surface OH groups than that of the pure TiO2 and ZnO photocatalyst. It was concluded that in the case of 20wt% ZnO–TiO2 catalyst, the main intermediates of degradation were phenol, 2-chlorophenol, hydroquinone and catechol, while with P-25 the main intermediates were ring-opening products, suggesting that different reaction mechanisms may have taken place depending on the nature of the catalyst.

PH-responsive ibuprofen delivery in silane-modified poly(methylacrylic acid) coated bimodal mesoporous silicas

A facile grafting-onto strategy has been exploited to prepare a smart pH-controlled drug delivery system (P–B) via surface-grafting of the preformed silane modified with PMAA [poly(methylacrylic acid)] onto the surface of BMMs (bimodal mesoporous silicas). Accordingly, the PMAA acted as the pH-sensitive component, and the silane provided an anchor with the surface OH group of BMMs. Thereby, a series of P–B with different mass ratio of BMMs to PMAA have been synthesized. Meanwhile, ibuprofen (IBU) was applied as a model drug to investigate the influences of different pH values on drug release. The results showed that the obtained P–B presented a flexible control over drug release by adjusting the grafted amount of PMAA. Moreover, the drug release amount of the drug-loaded samples was pH dependent and increased with the increasing of pH value, implying that the P–B are promising drug-carriers in application of pH-responsive controlled drug delivery systems.

Characterization of the surface modification process of Al2O3, TiO2 and ZrO2 powders by PFAS molecules

Four types of perfluoroalkylsilanes (PFAS) molecules with various lengths of fluorocarbon chains were used to modify Al2O3, TiO2 and ZrO2 powders. The surface grafting process was examined by 29Si NMR, TGA and FT-MIR techniques. Results reveal three possible types of bonding of the hydrophobic chains to the metal oxide surface. Moreover, it was established that the type of PFAS molecules and the type of modified surface have a strong influence on the mode and amount of hydrophobic molecules anchored to the surface. It was observed that alumina oxide is modified with the highest efficiency. The fraction of OH surface groups utilized during grafting of Al2O3 by C6, C8 and C10 are similar (64–68%), but for samples grafted by C12 are much lower (40%). The PFAS molecules with shorter fluorocarbon chains (C6 and C8) are attached mainly by siloxane (T3) and geminal silanol bonds (T2), whereas longer chains (C10 and C12) are attached generally by single (T1) and geminal silanol (T2) bonds. The hindrance steric effect is observed for powder samples grafted by C12 molecules.Additionally the influence of contact with water and butanol on the conformational orientation of PFAS was evaluated. It was found, that in contact with water molecules, PFAS chains are in a tangled form, whereas in contact with butanol molecules the grafted chains are straightened.

H2O/D2O Exchange on SnO2 Materials in the Presence of CO: Operando Spectroscopic and Electric Resistance Measurements

Water isotope exchange in the presence of CO on two undoped tin dioxides has been studied using modulation excitation diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and resistance measurements at 300 °C. Our results reveal that the material synthesized from tin tetrachloride (SnO2Cl) manifests higher affinity to chemisorbed water than that made from tin hydroxide acetate (SnO2Ac). The latter was shown to exhibit a strong correlation between the evolution of surface OH groups (bridging type, involved in hydrogen bonding) and electric resistance upon increasing concentration of CO. Water desorption kinetics, being independent of CO concentration for both materials, was found to be slower for SnO2Cl by ca. 30% with respect to SnO2Ac. High affinity to water as well as low sensor signals to CO in humid air reported for SnO2Cl were proposed to originate from traces of Cl ions (about 0.15 wt % for SnO2Cl and 0.03 wt % for SnO2Ac) and not microstructure, which has been confirmed to be simila...

Vertically aligned ZnO@CdS nanorod heterostructures for visible light photoinactivation of bacteria

Vertically aligned ZnO@CdS nanorod heterostructure films with various loadings of CdS nanoparticle shell were synthesized and applied in photoinactivation of Escherichia coli bacteria under visible light irradiation. While neither the bare ZnO nanorods (with band-gap energy (Eg) of ∼3.28eV) under visible light irradiation nor the nanorod heterostructures in dark exhibited any significant antibacterial activity, the ZnO@CdS nanorod heterostructures (with Eg ∼2.5–2.6eV) could successfully inactivate the bacteria under visible light irradiation. Furthermore, it was found that an optimum loading of the CdS shell (corresponding to the effective thickness less than ∼15nm) is required to achieve a complete inactivation of the bacteria after 24h visible light irradiation at room temperature. Using X-ray photoelectron spectroscopy, the lower photoinactivation efficiencies of the ZnO@CdS nanorod heterostructure at CdS thicknesses lower and higher than the optimum one were assigned to lower amounts of CdS nanoparticles and OH bonds (substantially existed on the hydrothermally synthesized ZnO nanorods) which are responsible for absorption of the visible light and production of hydroxyl radicals, respectively. Water contact angle measurements showed that the sample with more surface OH groups has a more hydrophilic surface and so more antibacterial activity.

A study of commercial transition aluminas and of their catalytic activity in the dehydration of ethanol

Conversion of ethanol was investigated over four commercial aluminas prepared by different industrial procedures and one commercial silica-alumina. Characterization was performed by TEM, XRD, SBET and porosity measurements, and IR spectroscopy of the surface OH groups and of adsorbed CO and pyridine. Different features are attributed to different phases (γ-, δ-, θ-Al2O3) and different impurities (Na+, Cl−). Total conversion of ethanol with >99% selectivity to ethylene is achieved at 623K over the purer Al2O3 catalyst (Na<0.002wt%). The most active sites are believed to be Lewis acidic Al3+ sites in a tetrahedral environment located on edges and corners of the nanocrystals. Ethanol adsorbs dissociatively on Lewis acid-base pair sites but may also displace water and/or hydroxyl groups from Lewis acidic Al3+ sites forming the active intermediate ethoxy species. Surface ethoxy groups are supposed to be intermediate species for both diethyl ether and ethylene production. Silica-alumina also works as a Lewis acid catalyst. The slightly lower activity on surface area basis of silica-alumina than aluminas attributed to the lower density of Lewis acid sites and the absence of significant basicity.

Surface-Modified TiO2 Photocatalysts Prepared by a Photosynthetic Route: Mechanism, Enhancement, and Limits.

Surface-modified TiO2 photocatalysts were synthesized by a photosynthetic route involving visible-light-induced (λ>455 nm) activation of benzene and toluene at the surface of TiO2 leading to the formation of carbonaceous polymeric deposits. IR spectroscopic and photoelectrochemical experiments showed that the mechanism of the photosynthetic reactions involves intra-bandgap surface states at TiO2 related to surface OH groups interacting with adsorbed aromatic molecules. The photosynthesized surface-modified TiO2 materials exhibited enhanced activity, relative to pristine TiO2 , in photocatalytic degradation (and complete mineralization) of 4-chlorophenol. The improvement was pronounced particularly under visible-light (λ>455 nm) irradiation with the relative initial photodegradation rate enhanced by a factor of four. The surface-modified photocatalysts exhibited good stability under the operating conditions, and the optimum carbon content was approximately 0.5 wt %. Mechanistic studies showed that the enhanced visible-light photodegradation of 4-chlorophenol is due to modified surface-adsorption properties that facilitate formation of a surface complex between titania and 4-chlorophenol, rather than due to any sensitizing effect of the carbonaceous deposits. The study highlights the importance of considering the interaction between pollutant molecules and the photocatalyst surface in heterogeneous photocatalysis, and possibly opens up a route for photosynthesis of further surface-modified photocatalysts with tuned surface properties.