TiO2 Nanocomposite Photocatalyst Research Articles

Plasmon-induced efficient solar-driven H2 production using bimetallic core-shell Cu@Ag:TiO2 nanocomposite photocatalyst

The nano-interface formed by bimetallic Cu@Ag nanoparticles (NPs) on a TiO2 surface enhances charge separation through plasmonic heterojunctions. These bimetallic Cu@Ag NPs were synthesized via the a galvanic replacement reaction (GRR) involving Ag+ in an aqueous medium and metallic Cu NPs on TiO2. During the GRR, Cuδ+ and Agδ+ charge pairs are generated by electron transfer from the catalyst surface. These charge pairs act as active sites, significantly enhancing photocatalytic hydrogen production. Additionally, the cubic crystalline phase of metallic Cu NPs is maintained after the GRR process with Ag ions, while the size of bimetallic Cu@Ag NPs remains constant. Consequently, Cu@Ag NPs supported on TiO2 (Cu@Ag:TiO2) exhibit increased donor charge density of 1.66 × 1020 cm−3 and facilitate rapid charge transfer. As a result, the photocatalytic hydrogen production rate of Cu@Ag:TiO2 is 20 times higher than that of pristine TiO2. Moreover, the visible-light responsive photocatalytic activity of Cu@Ag:TiO2 NPs reaches 3.4 mmol g−1 over 5 hours, which is 78 times higher than that of pristine TiO2. This remarkable enhancement is attributed to the formation of a plasmonic heterojunction with close contact between Cu@Ag and TiO2, enabling efficient charge carrier separation.

Unraveling the solar and visible light-induced deactivation mechanism of Pt-decorated carbon/TiO2 nanocomposite in photocatalytic hydrogen production

Photocatalytic water splitting presents an attractive and environmentally friendly method for generation of "green" hydrogen. However, the most widely used photocatalyst for hydrogen production, Pt-doped TiO2, has low stability, thus limiting its application for long term performance. In this study, the deactivation mechanism of Pt-decorated carbon TiO2 nanocomposite photocatalysts during photocatalytic hydrogen production was thoroughly investigated under solar and visible light irradiation. Solar light irradiation promoted markedly higher H2 production activity, however with lower catalyst stability. Deactivation studies using fresh catalysts/deactivated solutions and deactivated catalysts/fresh solutions point out the effect of the light used on the catalyst stability through changes on the catalyst surface as well as by the kinetics of intermediate/products formation during the reaction course. The present study revealed the importance of charge transport direction between TiO2, carbon and Pt on catalyst activity and stability and highlights the difference in visible and solar light-induced hydrogen production.

Formation of OH Radicals on BiVO4–TiO2 Nanocomposite Photocatalytic Film under Visible-Light Irradiation: Roles of Photocatalytic Reduction Channels

In this study, we investigated the effects of H2O2 addition on OH radical formation on the surfaces of visible-light-irradiated BiVO4–TiO2 nanocomposite photocatalysts. Additionally, we examined the possible roles of OH radicals formed by the reduction reaction of H2O2 on the visible-light-irradiated surfaces of photocatalytic BiVO4–TiO2 nanocomposites. The BiVO4–TiO2 nanocomposite photocatalysts were prepared by mixing a BiVO4 photocatalytic film with commercially available semiconductor particulate TiO2 photocatalysts. By removing oxygen gas from the photocatalytic reactor, the effects of oxygen molecules on OH radical formation during the visible-light irradiation of BiVO4–TiO2 nanocomposite photocatalysts were examined. During visible-light irradiation, BiVO4 and BiVO4–TiO2 photocatalysts play different roles in OH radical formation because of two characteristic reduction reaction channels: (a) the direct reduction of H2O2 on photocatalytic surfaces and (b) the indirect reduction reaction of H2O2 by superoxide radical anions (O2−).

Enhancement of formaldehyde removal by graphene, S, and N doping on TiO2 nanocomposite photocatalyst

The photocatalysis process for volatile organic compounds (VOCs) such as formaldehyde is considered a potential strategy due to its high efficiency and low cost. In this study, TiO2 was co-modified by the addition of sulfur (S), nitrogen (N), and reduced graphene oxide (rGO) under a solvothermal method for its enhancement on formaldehyde degradation. Characterization analysis of the modified TiO2 displayed an average particle size of 9.37 nm and was totally composed of anatase phase. The photocatalyst of 0.1rGO/S0·05N0·1TiO2 performed efficient visible-light-driven photocatalytic activity of formaldehyde conversion under the range of 10–60% relative humidity due to lower water adsorption competition and pores blocking. The addition of rGO, S, and N increased specific surface area, light absorption, bonding of oxygen-containing functional groups, and electron transfer capacity which are critical factors in photocatalysis activity.

Silica nanoparticles derived from Arundo donax L. ash composite with Titanium dioxide nanoparticles as an efficient nanocomposite for photocatalytic degradation dye

Water pollution is a serious problem that threatens both developed and developing countries. Several methods have been used to purify contaminated water, among which the photocatalytic decomposition approach is widely used to purify contaminated water from organic pollutants. In this work, biomass derived SiO2 nanoparticles composite with TiO2 semiconductors used as an efficient photocatalyst for degradation of RhB dye molecules under UV−visible light irradiation is proclaimed. The different weight percentages of Arundo donax L. ash-derived SiO2 nanoparticles combined with TiO2 nanoparticles were prepared through the wet impregnation method. The photocatalytic degradation ability of the as-prepared samples has been scrutinized against the degradation of Rh B dye in which the pronounced photocatalytic degradation efficiency 93.7% is successfully achieved on 50 wt % SiO2-50 wt % TiO2 nanocomposite photocatalyst. The catalytic performance of the nanocomposite decreases with an increase of 50%–75% in SiO2 nanoparticles. There could have been a decrease in degradation efficiency due to an excess amount of SiO2 covering TiO2 nanoparticles, which prevented photons from reaching the nanoparticles. The efficiency of cyclic decomposition of the 50 wt% SiO2-50 wt% TiO2 composite showed only a slight change in photocatalytic capacity compared to the first cycle, which ensures the durability of the sample. However, the hydroxyl radical species play the main role in the degradation process, which has been confirmed by the scavenger test. The probable reaction mechanism is also deliberated in detail. The high photocatalytic performance of novel eco-friendly SiO2-TiO2 photocatalyst make it ideal for water purification applications.

Synthesis of tetracarboxy phthalocyanines modified TiO2 nanocomposite photocatalysts and investigation of photocatalytic decomposition of organic pollutant methylene blue under visible light

The current work reports on the synthesis and characterization of peripherally tetra-4- carboxyethylenephenoxy substituted Zn(II) (4), Co(II) (5), Cu(II) (6) and TiO(IV) (7) phthalocyanine complexes and phthalocyanines (4-7)-TiO2 photocatalysts consisting of pure TiO2 and the phthalcyanine complexes. The prepared phthalocyanine complexes were characterized via Fouirer Transform Infrared (FTIR), Ultraviolet–Visible (UV–VIS), Proton Nuclear Magnetic Resonance (1H-NMR), elemental analysis and Matrix-Assisted Laser Desorption/Ionization Time-of- Flight Mass Spectrometry (MALDI-TOF MS). Also, phthalocyanines modified TiO2 nanocomposites were synthesized via a hydrothermal method and characterized by Field Emission Gun Scanning Electron Microscope (FEG-SEM), X-ray Powder Diffraction (XRD), Ultraviolet–Diffuse Reflectance Spectroscopy (UV–DRS), Energy dispersive X-ray (EDX), FT-IR and Brunauer–Emmett–Teller (BET) techniques. The modified TiO2 photocatalysts were used for the decomposition of methylene blue (MB) dyestuff under visible light monitored by UV–VIS spectrophotometry. The photocatalysts enhanced activity for the decomposition of MB compared to pure TiO2, which is attributed to photosensitizer phthalocyanine anchored onto TiO2. The photocatalyst containing ZnPc and CoPc exhibited the highest photocatalytic decomposition activity of MB, which was completed in 110 min. Moreover, the reusability applications revealed that protect about 82% of the activity in 3 recycles.

G-C3N4/TiO2 nanocomposite photocatalyst for methylene blue photodegradation under visible light

g-C3N4/TiO2 nanocomposite photocatalyst for methylene blue photodegradation under visible light

Synthesis of Nanocomposite and Study Degradation of Phenol Red Dye

This study aims to investigate the capability of semiconductors such as titanium dioxide, copper oxide and zinc oxide used to remove a hazardous Phenol Red organic dye texture from an aqueous solution. In this sheet, (ZnO), (CuO) and (TiO2) nanoparticles were synthesised utilise a simple chemical process. Nanocomposite has been synthesised by physical process. The purpose of the research was to investigate semiconductors’ capability, such as titanium dioxide, copper oxide, and zinc oxide, to take off a hazardous Phenol Red, a texture dye from an aqueous solution. These nanoparticles and composites’ structural properties described using (XRD) and Field Emission Scanning Electron Microscope (FE-SEM). This paper describes photocatalytic degradation of Phenol Red dye solution using composite CuO/ZnO /TiO2 nanocomposite photocatalysts, in the form of CuO/ ZnO/TiO2 composite as a paint on the Stainless Steel cell, under ultraviolet (UV) irradiations. The effect of factors affecting the reaction, such as the dye’s primary concentration, the effect of temperature, and the value of the acidic function, were studied. The experiential results show that the CuO/ ZnO/TiO2 composite can remove the Phenol Red from wastewater by X-ray diffraction (XRD) and an average volume of copper oxide molecules. The formula of Debar Shearer found that it is equal to 21.33 nm. For zinc oxide, the particle size constructed to be 18.13 nm and titanium oxide was found to be 42.63, and the particle size of the Nano chemical mixture was 20.48 nm. And CuO/ZnO /TiO2 Nanocomposite 20.48, respectively.

Efficient photocatalytic degradation of phenol by Ag-doped TiO2 nanocomposite photocatalysts under visible light irradiation in a three-phase fluidized bed reactor

Efficient photocatalytic degradation of phenol by Ag-doped TiO2 nanocomposite photocatalysts under visible light irradiation in a three-phase fluidized bed reactor

Multiple Modification of Titanium Dioxide to Enhance Its Photocatalytic Performance

AbstractThe TiO2 nanocomposite photocatalyst P25/fluorinated graphene (P25/FG) was synthesized using the one‐step hydrothermal method. During hydrothermal process, the partial defluorination of FG, the surface fluorine doping of P25 as well as the deposition of P25 on graphene nanosheet were achieved. The P25/FG photocatalyst behaved excellent photocatalytic activity in photodegradation of methyl orange (MO) with degradation rate constant of 0.169 min−1, 5.6 times of that for P25 and 2.2 times for P25‐graphene catalyst under same conditions. It was demonstrated that the enhancement of photocatalytic activity of P25/FG can be attributed to the multiple modification of P25 brought by FG, including the effect of fluorine‐doped graphene, surface fluorine doping of P25 as well as dissociative fluorine ions in solution generated from UV‐assisted photocatalytic defluorination.

Hydrothermal preparation of B–TiO2-graphene oxide ternary nanocomposite, characterization and photocatalytic degradation of bisphenol A under simulated solar irradiation

Boron-doped TiO2 decorated on graphene oxide photocatalysts (denoted as GO@B–TiO2) were prepared for the effective photocatalytic treatment of bisphenol A (BPA) from water under simulated solar light. To prepare GO@B–TiO2 nanocomposites, B–TiO2 was initially synthesized via a sol-gel procedure and subsequently decorated on graphene oxide at 70 °C, by a facile hydrothermal method. Characterization results show that B–TiO2 particles are anchored on the surface of graphene oxide successfully. Their photocatalytic efficiency could also be adjusted by changing the ratios between B–TiO2 and GO. The optimum GO loading is 2 wt%, at which GO@B–TiO2 nanocomposite photocatalyst could achieve the best BPA degradation percentage of 47.66% compared with B–TiO2 materials (18.78%) after 240 min of solar irradiation. The dominant reactive oxygen species (ROS) formed during the photocatalytic abatement of BPA were determined to be O2.- and .OH. The presence of carbonate and chloride induced moderate inhibitory effect on GO@B–TiO2 (2 wt% GO), while bicarbonate and nitrate displayed only minor effect towards photocatalytic degradation of BPA. Based on these results, the as-prepared nanocomposites may be used as efficient and promising photocatalysts in secondary treatments to degrade recalcitrant contaminants in water exploiting the visible-light region of the full solar spectrum.

Easy preparation of recyclable thermally stable visible-light-active graphitic-C3N4/TiO2 nanocomposite photocatalyst for efficient decomposition of hazardous organic industrial pollutants in aqueous medium

Graphitic-C3N4/TiO2 nanocomposite was prepared as a photocatalyst (PC) active under visible light (λ ≥ 420 nm) by preparation of graphitic carbon nitride (g-C3N4) from melamine followed by an effective easy impregnation method. Several g-C3N4/TiO2 composites containing 1 to 12 wt% g-C3N4 were synthesized and characterized using X-ray diffraction (XRD) analysis, scanning electron microscopy (SEM), thermogravimetric analysis (TGA), differential thermal analysis (DTA), photoluminescence (PL) spectroscopy, diffusion reflectance spectroscopy (DRS), and Brunauer–Emmett–Teller (BET) measurements. A photocatalytic mechanism is proposed based on the relative positions of the energy bands of the two constituents. Compared with its individual components, g-C3N4/TiO2 demonstrated unusually high photocatalytic activity for phenol decomposition in aqueous phase under visible-light irradiation. The heterojunction was optimized in the 5 wt% g-C3N4/TiO2 nanocomposite due to the well-matched bandgap structure (optimum loading) and excellent electron–hole pair separation in the conduction and valence band of TiO2 and g-C3N4, respectively. After 2 h of visible-light irradiation, 68 % degradation was observed when using this optimum composition. The performance was slightly decreased (to 66 %) after recycling of the catalyst four times (used a total of five times), but remained reliable for industrial applications considering other factors. In this system, TiO2 (Degussa P25) seems to play the principal PC role, while g-C3N4 acts as a sensitizer for absorption of visible light. Due to the enhanced visible-light absorption ability enabled by g-C3N4 in the composite, stable electron–hole (e−–h+) pairs produced at the interface of the heterojunction lead to generation of highly reactive free radicals (·O2, ·OH, etc.) which together initiate degradation of phenol but individually suffer from some limitation that must be overcome. The thermal stability and recycling efficiency of this PC will enable its use in industrial applications as a cost-effective sustainable cleanup candidate. The prepared g-C3N4/TiO2 exhibits stable electron–hole (e−–h+) pair separation at the heterojunction under visible light for enhanced degradation of organic pollutants via a redox mechanism. The g-C3N4 loading affects the photocatalytic activity, with the 5 wt% g-C3N4/TiO2 composite exhibiting the highest degradation, with recycling.

Pristine graphene: functionalization, fabrication, and nanocomposite materials

Graphene has a wide range of unique properties including high carrier mobility, optical transparency, and exceptional electrical and thermal conductivity. The interest in graphene has been largely fueled by its potential to revolutionize a number of technologically important areas including materials science, biomedical science, and energy. Despite its high potentials, major challenges still remain. These include poor solubility, intrinsic zero band gap energy, low reactivity and the availability of well-defined pristine graphene in large quantity, which have hampered the rapid development of graphene-based functional devices. Many of these challenges can be potentially addressed through chemical functionalization of the material. Covalent functionalization can enhance graphene’s properties including opening its band gap, tuning conductivity, improving solubility, and enhancing the properties of graphene-based composite materials. This paper discusses our work on the chemical functionalization of pristine graphene, and synthesis of graphene-nanoparticle composite materials and three-dimensional graphene (3DG)-TiO2 nanocomposite photocatalyst. In photocatalytic CO2 reduction, the 3DG-TiO2 nanocomposite demonstrated excellent activity, about 11 times higher than TiO2 nanoparticles.

Enhanced photocatalytic degradation of dyes over graphene/Pd/TiO2 nanocomposites: TiO2 nanowires versus TiO2 nanoparticles

In this study, at first, TiO2 nanowire was prepared by an alkaline hydrothermal process. In the following, Gr/Pd/TiO2-NPs and Gr/Pd/TiO2-NWs were synthesized by a combination of hydrothermal and photodeposition methods. The properties of as prepared products were characterized using XRD, FT-IR, SEM, DRS, TEM, ICP-OES, EDS and TGA analysis. SEM results confirmed nanodimension structure for all samples. Also the band gap values obtained using DRS technique suggests that all the samples have semiconductor behavior. Using TGA analysis, the amount of graphene loaded onto the powders was confirmed. Photocatalytic degradation of rhodamine B by TiO2-NWs, Gr/Pd/TiO2-NPs and Gr/Pd/TiO2-NWs nanocomposites was compared under ultraviolet light irradiation. Results confirmed that the Gr/Pd/TiO2-NWs composite show the highest photocatalytic activity due to much higher available surface area of TiO2 substrate in nanowire structure. It is expected that the synthesis of the high surface area TiO2 nanowires, facile photodeposition of palladium into its texture, and simple conversion of GO to graphene during hydrothermal process without using strong reducing agents, could be a suitable rote for preparing different types of carbon based TiO2 nanocomposite photocatalysts.

Enhancement of photodegradation of Ce, N, and P tri-doped TiO2/AC by microwave radiation with visible light response for naphthalene

This study focused on the microwave-assisted one-step synthesis and evaluation of visible light-sensitized Ce, N, and P tri-doped titanium dioxide nanoparticles loaded on activated carbon (TiO2/AC) photocatalyst for the photocatalytic degradation of naphthalene. X-ray photoelectron spectroscopy (XPS) of the samples showed the presence of both Ce3+ and Ce4+ oxidation states. A red shift of the absorption edge in the UV–visible spectra was observed when Ce, N, or P was doped in the band gap of TiO2. Ce, N, and P tri-doped TiO2/AC exhibited the highest photocatalytic activity (97.9%) and rate constant under visible-light irradiation. The photocatalyst could be applied in environmental purification as indicated by its excellent performance. The possible synergetic mechanism for the photodegradation was also discussed. This study provides an effective strategy for synthesizing novel high-performance TiO2 nanocomposite photocatalyst to degrade naphthalene.

Efficient photocatalytic degradation of organic pollutants by magnetically recoverable nitrogen-doped TiO2 nanocomposite photocatalysts under visible light irradiation.

Preparation of novel nanocomposite particles (NCPs) with high visible-light-driven photocatalytic activity and possessing recovery potential after advanced oxidation process (AOP) is much desired. In this study, pure anatase phase titania (TiO2) nanoparticles (NPs) as well as three types of NCPs including nitrogen-doped titania (TiO2-N), titania-coated magnetic silica (Fe3O4 cluster@SiO2@TiO2 (FST)), and a novel magnetically recoverable TiO2 nanocomposite photocatalyst containing nitrogen element (Fe3O4 cluster@SiO2@TiO2-N (FST-N)) were successfully synthesized via a sol-gel process. The photocatalysts were characterized by X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field emission scanning electron microscopy (FE-SEM) with an energy-dispersive X-ray (EDX) spectroscopy analysis, X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy (DRS), and vibrating sample magnetometer (VSM). The photocatalytic activity of as-prepared samples was further investigated and compared with each other by degradation of phenol, as a model for the organic pollutants, in deionized (DI) water under visible light irradiation. The TiO2-N (55 ± 1.5%) and FST-N (46 ± 1.5%) samples exhibited efficient photocatalytic activity in terms of phenol degradation under visible light irradiation, while undoped samples were almost inactive under same operating conditions. Moreover, the effects of key operational parameters, the optimum sample calcination temperature, and reusability of FST-N NCPs were evaluated. Under optimum conditions (calcination temperature of 400 °C and near-neutral reaction medium), the obtained results revealed efficient degradation of phenol for FST-N NCPs under visible light irradiation (46 ± 1.5%), high yield magnetic separation and efficient reusability of FST-N NCPs (88.88% of its initial value) over 10 times reuse.

Detection of reactive oxygen species (ROS) and investigation of efficient visible-light-responsive photocatalysis via nanoscale PbSe sensitized TiO2

In this study, a novel visible-light-responsive PbSe–TiO2 nanocomposite photocatalysts were successfully synthesized through a facile sonochemical-assisted method, and characterized by XRD, SEM–EDX, TEM, and DRS to confirm its structure, morphology, composition, and optical property. In decolorization of rhodamine B, a significant enhancement in the reaction rate was observed with PbSe–TiO2 composite, compared to the pure TiO2. The generation of reactive oxygen species was detected through the oxidation reaction from 1,5-diphenyl carbazide (DPCI) to 1,5-diphenyl carbazone (DPCO). The high photocatalytic activity can be attributed to the synergetic effect of PbSe nanoparticles and TiO2 by extending the spectral response of TiO2 to visible region and also help to reduce the charge recombination. Finally, the possible mechanism for generation of reactive oxygen species under visible light irradiation was proposed as well.

Enhanced photocatalytic activity of nitrogen and indium co-doped mesoporous TiO2 nanocomposites for the degradation of 2,4-dinitrophenol under visible light

Mesoporous N/In2O3–TiO2 nanocomposite photocatalysts were synthesized by sol-gel route using Pluronic P123 as the structure directing template. The synthesized composite materials were successfully characterized by X-ray powder diffraction, high resolution transmission electron microscopy, N2 adsorption–desorption studies, X-ray photoelectron spectroscopy, diffuse reflectance UV–vis spectroscopy, Fourier transform infrared spectroscopy and photoluminescence spectroscopy. The photocatalytic activities of all the synthesized catalysts were evaluated for the degradation of 2,4-dinitrophenol under visible light irradiation. The results demonstrated that the mesoporous N/In2O3–TiO2 showed higher efficiency than meso TiO2, N-TiO2 and In2O3–TiO2 under visible light irradiation and the optimum molar ratio of N and In to Ti is 0.3wt%. DRUV–vis revealed that the substitution of N− and In3+ dopants on TiO2 lattice shifted the light absorption to the longer wavelength and reduced the band gap energy. The enhanced •OH radicals formation during the photocatalytic reaction was revealed by photoluminescence spectra. The photoluminescence spectra of synthesized catalysts revealed that the efficient charge separation of photo induced charge carriers for 0.3wt% N/In2O3–TiO2 nanocomposite. The enhanced surface area, large pore volume and large pore diameter for 0.3wt% N/In2O3–TiO2 improved the photocatalytic efficiency. In3+ ions can easily trap and transfer the excited electrons to the adsorbed O2 molecules, hence efficiently extending the life time of electron–hole pair.

Environmental remediation of aqueous cyanide by photocatalytic oxidation using a NiFe2O4/TiO2–SiO2 core–shell nanocomposite

A core–shell NiFe2O4/SiO2–TiO2 nanocomposite photocatalyst was prepared in a three-stage manner. The NiFe2O4 core was prepared by applying an organic precursor method. This core was coated with SiO2 and then with TiO2. The optimum preparation conditions were determined by examining various molar ratios of Si, Ti, ethanol, and ammonia. X-ray powder diffraction, DR-UV, TEM, and magnetization techniques were used to characterize the nanocomposite. Using molar ratios of SiO2/NiFe2O4 = 0.03, ethanol/NiFe2O4 = 20, ammonia/NiFe2O4 = 1, and Ti/ethanol = 0.8, a magnetic photocatalyst of enhanced properties was synthesized. A surface area of 520 m2/g, saturation magnetization value of 53.2 amu/g, coercivity of 500.0 Oe, and a band gap of 2.54 eV were observed for the synthesized NiFe2O4/SiO2–TiO2 nanocomposite photocatalyst. These characteristics allowed excellent photodegradation of the toxic cyanide ion. In addition, the strong magnetic properties allow for the efficient reuse of the catalyst.

Graphene–TiO2 nanocomposite photocatalysts for selective organic synthesis in water under simulated solar light irradiation

Heterogeneous photocatalysis offers a promising route to realize green oxidation processes in organic synthesis because such selective organic transformation processes can be enabled under solar light irradiation with ambient conditions. In this research, graphene–TiO2 nanocomposites have been utilized for aerobic selective oxidation of benzylic alcohols to corresponding aldehydes and acids in water under simulated solar light irradiation. The photocatalytic performance and related properties of solvent exfoliated graphene–TiO2 (SEG–TiO2) and reduced graphene oxide–TiO2 (RGO–TiO2) have been comparatively studied. It has been found that decreasing the defect density of graphene, by using SEG instead of graphene oxide (GO) as the precursor of RGO, can enhance the photoactivity of graphene–TiO2 nanocomposites due to the improved electron conductivity of SEG as compared to that of RGO. It is hoped that this work would stimulate further interest in utilizing graphene–semiconductor nanocomposite photocatalysts in the field of green-chemistry-oriented photocatalytic selective organic transformation.