Particle Networks from Powder Mixtures: Generation of TiO(2)-SnO(2) Heterojunctions via Surface Charge-Induced Heteroaggregation

On the Importance of Nanoparticle Necks and Carbon Impurities for Charge Trapping in TiO2

Particle attachment and neck formation inside TiO₂ nanoparticle networks determine materials performance in sensing, photo-electrochemistry, and catalysis. Nanoparticle necks can feature point defects with potential impact on the separation and recombination of photogenerated charges. Here, we investigated with electron paramagnetic resonance a point defect that traps electrons and predominantly forms in aggregated TiO₂ nanoparticle systems. The associated paramagnetic center resonates in the g factor range between g = 2.0018 and 2.0028. Structure characterization and electron paramagnetic resonance data suggest that during materials processing, the paramagnetic electron center accumulates in the region of nanoparticle necks, where O₂ adsorption and condensation can occur at cryogenic temperatures. Complementary density functional theory calculations reveal that residual carbon atoms, which potentially originate from synthesis, can substitute oxygen ions in the anionic sublattice, where they trap one or two electrons that mainly localize at the carbon. Their emergence upon particle neck formation is explained by the synthesis- and/or processing-induced particle attachment and aggregation facilitating carbon atom incorporation into the lattice. This study represents a substantial advance in linking dopants, point defects, and their spectroscopic fingerprints to microstructural features of oxide nanomaterials.

Investigation of Photocatalyst Composites for Pollutant Degradation in a Microslit Reactor Utilizing High Throughput Screening Techniques

The high-throughput screening investigations on TiO2 based photocatalyst composites presented here have been carried out in a 60-fold parallel photoreactor. Additional catalyst testing was performed in a microslit reactor system with immobilized catalysts. For further enhancing the photocatalytic activity of TiO2 (P25), composites of P25 and, for example, Bi2 O3 , CeO2 , g-C3 N4 , WO3 or ZnO were formulated in different nominal molar ratios. The catalysts' performances were assessed by their conversion of 17α-ethinyl estradiol (EE2) in aqueous solutions, determined by LC-MS. Findings show rapid EE2 conversions in short residence times. The extensive testing of catalysts led to the conclusion that the photocatalytic conversion is rather a function of residence time than a function of the materials utilized. This makes adequate process development seem more important than material development. The novelty of this contribution lies in the unique combination of testing a wide range of composite catalysts in a unique microreactor geometry.

Self-assembled aluminum oxyhydroxide nanorices with superior suspension stability for vaccine adjuvant

The suspension stability of aluminum-based adjuvant (Alum) plays an important role in determining the Alum-antigen interaction and vaccine efficacy. Inclusion of excipients has been shown to stabilize antigens in vaccine formulations. However, there is no mechanistic study to tune the characteristics of Alum for improved suspension stability. Herein, a library of self-assembled rice-shaped aluminum oxyhydroxide nanoadjuvants i.e., nanorices (NRs), was synthesized through intrinsically controlled crystallization and atomic coupling-mediated aggregations. The NRs exhibited superior suspension stability in both water and a saline buffer. After adsorbing hepatitis B surface antigen (HBsAg) virus-like particles (VLPs), human papillomavirus virus (HPV) VLPs, or bovine serum albumin, NR-antigen complexes exhibited less sedimentation. Further mechanistic study demonstrated that the improved suspension stability was due to intraparticle aggregations that led to the reduction of the surface free energy. By using HBsAg in a murine vaccination model, NRs with higher aspect ratios elicited more potent humoral immune responses. Our study demonstrated that engineered control of particle aggregation provides a novel material design strategy to improve suspension stability for a diversity of biomedical applications.

Composite materials based on heteroaggregated particles: Fundamentals and applications

Homoaggregation of dispersed particles, i.e., aggregation of particles of the same shape, charge, size, and composition, is a well-studied field and various theoretical and experimental approaches exist to understand the major phenomena involved in such processes. Besides, heteroaggregation of particles, i.e., aggregation of particles of different shape, charge, size, or composition, has attracted widespread interest due to its relevance in various biomedical, industrial, and environmental systems. For instance, heteroaggregation of plastic contaminant particles with naturally occurring solid materials in waters (e.g., clays, silica and organic polymers) plays an important role in the decontamination technologies. Moreover, nanofabrication processes involving heteroaggregation of particles to prepare novel composite materials are widely implemented in fundamental science and in more applied disciplines. In such procedures, stable particle dispersions are mixed and the desired structure forms owing to the presence of interparticle forces of various origins, which can be tuned by performing appropriate surface functionalization as well as altering the experimental conditions. These composites are widely used in different fields from sensing through catalysis to biomedical delivery. The present review summarizes the recent progresses in the field including new findings regarding the basic principles in particle heteroaggregation, preparation strategies of heteroaggregated structures of different morphology, and the application of the obtained hybrid composites. Such information will be very helpful to those involved in the design of novel composites consisting of different nano or colloidal particles.

Vapor-phase production of nanomaterials

The synthesis of nanomaterials, with characteristic dimensions of 1 to 100 nm, is a key component of nanotechnology. Vapor-phase synthesis of nanomaterials has numerous advantages such as high product purity, high-throughput continuous operation, and scalability that have made it the dominant approach for the commercial synthesis of nanomaterials. At the same time, this class of methods has great potential for expanded use in research and development. Here, we present a broad review of progress in vapor-phase nanomaterial synthesis. We describe physically-based vapor-phase synthesis methods including inert gas condensation, spark discharge generation, and pulsed laser ablation; plasma processing methods including thermal- and non-thermal plasma processing; and chemically-based vapor-phase synthesis methods including chemical vapor condensation, flame-based aerosol synthesis, spray pyrolysis, and laser pyrolysis. In addition, we summarize the nanomaterials produced by each method, along with representative applications, and describe the synthesis of the most important materials produced by each method in greater detail.

Dissymmetric interface design of SnO2/TiO2 side-by-side bi-component nanofibers as photoanodes for dye sensitized solar cells: Facilitated electron transport and enhanced carrier separation

SnO₂/TiO₂ type II heterojunctions are often introduced to enhance the separation efficiency of photogenerated carriers in photoelectrochemical electrodes, while most of these heterojunctions are of core-shell structure, which often limits the synergistic effect from the two components. In this work, dissymmetric SnO₂/TiO₂ side-by-side bi-component nanofibers (SBNFs) with tunable composition ratios have been prepared by a novel needleless electrospinning technique with two V-shape connected conductive channels (V-channel electrospinning). Results show that this V-channel electrospinning technique is more stable, controllable and tunable for the large-scale preparation of SBNF materials compared to the traditional electrospinning using two side-by-side metal needles. And these SnO₂/TiO₂ SBNFs are dissymmetric and comprised of a tiny SnO₂ NF (tunable diameter within 20-80 nm) and a Sn-doped TiO₂ NF (diameter of ~ 250 nm) with a side-by-side structure. Moreover, the dye-sensitized solar cells (DSSCs) based these dissymmetric SnO₂/TiO₂ SBNFs show the maximum power conversion efficiency (PCE) of 8.3%, which is 2.59 times that of the ones based on the TiO₂ NFs. Series of analyses indicate that the enhancements in PCE could mainly be due to the improved electron transport via SnO₂ NFs and the enhanced carrier separation via dissymmetric SnO₂/TiO₂ heterojunction interface. This research will give some new insight in the preparation of SBNFs for high-performance photoelectrochemical devices.

Nanoparticle processing: Understanding and controlling aggregation

Nanoparticles (NPs) are commonly defined as particles with size <100 nm and are currently of considerable technological and academic interest, since they are often the starting materials for nanotechnology. Novel properties develop as a bulk material is reduced to nanodimensions and is reflected in new chemistry, physics and biology. With reduction in size, a greater function of the atoms is at the surface, and promote different interaction with its environment, as compared to the bulk material. In addition, the reduction in size alters the electronic structure of the material, resulting in novel quantum effects. Size also influences mobility, primarily controlled by Brownian motion for NPs, and relevant in biological and environmental processes. However, the small size also leads to high surface energy, and NPs tend to aggregate, thereby lowering the surface energy. In all applications, the uncontrolled aggregation of NPs can have negative effects and needs to be avoided. There are however examples of controlled aggregation of NPs which give rise to novel effects. This review article is focused on the NP features that influences aggregation. Common strategies for synthesis of NPs from the gas and liquid phases are discussed with emphasis on aggregation during and after synthesis. The theory involving Van der Waals attractive force and electrical repulsive force as the controlling features of the stability of NPs is discussed, followed by examples of how repulsive and attractive forces can be manipulated experimentally to control NP aggregation. In some applications, NPs prepared by liquid methods need to be isolated for further applications. The process of solvent removal introduces new forces such as capillary forces that promote aggregation, in many cases, irreversibly. Strategies for controlling aggregation upon drying are discussed. There are also many methods for redispersing aggregated NPs, which involve mechanical forces, as well as manipulating capillary forces and surface characteristics. We conclude this review with a discussion of aggregation relevant real-world applications of NPs. This review should be relevant for scientists and technologists interested in NPs, since emphasis has been on the practical aspects of NP-based technology, and especially, strategies relevant to controlling NP aggregation.

Probing particle heteroaggregation using analytical centrifugation

The controlled aggregation of colloidal particles is not only a widespread natural phenomenon but also serves as a tool to design complex building blocks with tailored shape and functionalities. However, the quantitative characterization of such heteroaggregation processes remains challenging. Here, we demonstrate the use of analytical centrifugation to characterize the heteroaggregation of silica particles and soft microgels bearing similar surface charges. We investigate the attachment as well as the stability of the formed heteroaggregates as a function of particle to microgel surface ratio, microgel size and the influence of temperature. The attachment of microgels onto the colloidal particles induces a change in the sedimentation coefficient, which is used to quantitatively identify the number of attached microgels. We corroborate the shift in sedimentation coefficient by computer simulations of the frictional properties of heteroaggregates via a modified Brownian dynamic algorithm. The comparison between theoretical investigations and experiments suggest that the microgels deform and flatten upon attachment.

Particle Consolidation and Electron Transport in Anatase TiO2 Nanocrystal Films

A sequence of chemical vapor synthesis and thermal annealing in defined gas atmospheres was used to prepare phase-pure anatase TiO₂ nanocrystal powders featuring clean surfaces and a narrow particle size distribution with a median particle diameter of 14.5 ± 0.5 nm. Random networks of these nanocrystals were immobilized from aqueous dispersions onto conducting substrates and are introduced as model systems for electronic conductivity studies. Thermal annealing of the immobilized films at 100 °C < T < 450 °C in air was performed to generate particle-particle contacts upon virtual preservation of the structural properties of the nanoparticle films. The distribution of electrochemically active electronic states as well as the dependence of the electronic conductivity on the Fermi level position in the semiconductor films was studied in aqueous electrolytes in situ using electrochemical methods. An exponential distribution of surface states is observed to remain unchanged upon sintering. However, capacitive peaks corresponding to deep electron traps in the nanoparticle films shift positive on the potential scale evidencing an increase of the trapping energy upon progressive thermal annealing. These peaks are attributed to trap states at particle-particle interfaces in the random nanocrystal network (i.e., at grain boundaries). In the potential region, where the capacitive peaks are detected, we observe an exponential conductivity variation by up to 5 orders of magnitude. The potential range featuring the exponential conductivity variation shifts positive by up to 0.15 V when increasing the sintering temperature from 100 to 450 °C. Importantly, all films approach a potential- and sintering-temperature-independent maximum conductivity of ∼10^-4 Ω^-1·cm^-1 at more negative potentials. On the basis of these results we introduce a qualitative model, which highlights the detrimental impact of electron traps located on particle-particle interfaces on the electronic conductivity in random semiconductor nanoparticle networks.

Multisensory Gas Chromatography for Field Analysis of Complex Gaseous Mixtures

A novel approach to analysis of complex gaseous mixtures is presented. The approach is based on the utilization of a compact gas chromatograph in combination with an array of highly integrated and selective metal oxide (MOX) sensors. Thanks to the implementation of a multisensory detector, the device collects multiple chromatograms in a single run. The sensors in the integrated MEMS platform are very distinct in their catalytic properties. Hence, the time separation by chromatographic column is complemented by catalytic separation by a multisensory detector. Furthermore, the device can perform the analysis in a broad range of concentrations, from ppb to hundreds of ppm. Low ppb and even sub-ppb levels of detection for some analytes were achieved. As a part of this effort, nanocomposite gas sensors were synthesized for selective detection of hydrogen sulfide, mercaptans, alcohols, ketones, and heavy hydrocarbons.

Design and assembly of ternary Pt/Re/SnO2 NPs by controlling the zeta potential of individual Pt, Re, and SnO2 NPs

In this study Pt, Re, and SnO₂ nanoparticles (NPs) were combined in a controlled manner into binary and ternary combinations for a possible application for ethanol oxidation. For this purpose, zeta potentials as a function of the pH of the individual NPs solutions were measured. In order to successfully combine the NPs into Pt/SnO₂ and Re/SnO₂ NPs, the solutions were mixed together at a pH guaranteeing opposite zeta potentials of the metal and oxide NPs. The individually synthesized NPs and their binary/ternary combinations were characterized by Fourier transform infrared spectroscopy (FTIR) and scanning transmission electron microscopy (STEM) combined with energy dispersive X-ray spectroscopy (EDS) analysis. FTIR and XPS spectroscopy showed that the individually synthesized Pt and Re NPs are metallic and the Sn component was oxidized to SnO₂. STEM showed that all NPs are well crystallized and the sizes of the Pt, Re, and SnO₂ NPs were 2.2, 1.0, and 3.4 nm, respectively. Moreover, EDS analysis confirmed the successful formation of binary Pt/SnO₂ and Re/SnO₂ NP, as well as ternary Pt/Re/SnO₂ NP combinations. This study shows that by controlling the zeta potential of individual metal and oxide NPs, it is possible to assemble them into binary and ternary combinations. Graphical abstractᅟ.

An enhanced degree of charge transfer in dye-sensitized solar cells with a ZnO-TiO2/N3/Ag structure as revealed by surface-enhanced Raman scattering

A number of recent studies have focused on improving the performance of dye-sensitized solar cells (DSSCs). Cells with a ZnO-TiO₂/N3/Ag structure have attracted particular attention because of their excellent power conversion efficiencies. Using a dendritic crystal ZnO-TiO₂ composite semiconductor and Ag in conjunction leads to different charge-transfer (CT) processes, and this is the main theoretical basis for the improvement of DSSC performances. Thus, in the present study, TiO₂/N3, ZnO/N3, ZnO-TiO₂/N3, TiO₂/N3/Ag, ZnO/N3/Ag, and ZnO-TiO₂/N3/Ag assemblies have been fabricated and their CT processes have been monitored by using surface-enhanced Raman scattering (SERS) spectra, with particular focus on the differences caused by the synergistic effect of the ZnO-TiO₂ component. The dye loading capacity of the dendritic crystal ZnO-TiO₂ is much larger than that of TiO₂. There are extra enhancements in the SERS intensity and degree of CT (ρ_CT) in ZnO-TiO₂/N3 compared to ZnO + TiO₂/N3 (based on a simulation curve for the physically mixed TiO₂ and ZnO semiconductors) with 476.5 nm excitation due to the synergistic effect of the ZnO-TiO₂ component. And these enhancements in ZnO-TiO₂/N3/Ag compared to ZnO + TiO₂/N3/Ag appear with 476.5 and 532 nm excitation, which are particularly large with 532 nm excitation. Accordingly, the participation of Ag in this synergistic effect can reduce its energy threshold, which will make it easier to appear. Finally, to rationalize these extra enhancements, the models describing the CT mechanism have been proposed. Thus, the use of the dendritic crystal ZnO-TiO₂ composite semiconductor in the semiconductor/N3/Ag system can improve the adsorption capacity of N3 compared to that with TiO₂. Meanwhile, the synergistic effect of ZnO-TiO₂ and Ag can promote the CT process, demonstrating that ZnO-TiO₂/N3/Ag is an excellent structure for DSSCs.

Bovine Serum Albumin Adsorption on TiO2 Colloids: The Effect of Particle Agglomeration and Surface Composition

Protein adsorption at nanostructured oxides strongly depends on the synthesis conditions and sample history of the material investigated. We measured the adsorption of bovine serum albumin (BSA) to commercial Aeroxide TiO₂ P25 nanoparticles in aqueous dispersions. Significant changes in the adsorption capacity were induced by mild sample washing procedures and attributed to the structural modification of adsorbed water and surface hydroxyls. Motivated by the lack of information about the sample history of commercial TiO₂ nanoparticle samples, we used vapor-phase-grown TiO₂ nanoparticles, a well-established model system for adsorption and photocatalysis studies, and performed on this material for the first time a systematic and quantitative BSA adsorption study. After alternating vacuum and oxygen treatment of the nanoparticle powders at elevated temperatures for surface purification, we determined size distributions covering both the size of the individualized nanoparticles and nanoparticle agglomerates using transmission electron microscopy (TEM), X-ray diffraction (XRD), and dynamic light scattering (DLS) in an aqueous dispersion. Quantitative BSA adsorption measurements at different pH values and thus variable combinations of surface-charged proteins and TiO₂ nanoparticles revealed a consistent picture: BSA adsorbs only at the outer agglomerate surfaces without penetrating the interior of the agglomerates. This process levels at coverages of single monolayers, which resist consecutive simple washing procedures. A detailed analysis of the protein-specific IR amide bands reveals that the adsorption-induced protein conformational change is associated with a decrease in the helical content. This study underlines that robust qualitative and quantitative statements about protein adsorption and corona formation require well-documented and controllable surface properties of the nanomaterials involved.

Extending charge separation lifetime and distance in patterned dye-sensitized SnO2–TiO2 μm-thin films

Charge separation in dye sensitized SnO₂–TiO₂ μm-thin films was substantially extended by an innovative patterning design strategy.

Charge Transport in Two-Photon Semiconducting Structures for Solar Fuels

Semiconducting heterostructures are emerging as promising light absorbers and offer effective electron-hole separation to drive solar chemistry. This technology relies on semiconductor composites or photoelectrodes that work in the presence of a redox mediator and that create cascade junctions to promote surface catalytic reactions. Rational tuning of their structures and compositions is crucial to fully exploit their functionality. In this review, we describe the possibilities of applying the two-photon concept to the field of solar fuels. A wide range of strategies including the indirect combination of two semiconductors by a redox couple, direct coupling of two semiconductors, multicomponent structures with a conductive mediator, related photoelectrodes, as well as two-photon cells are discussed for light energy harvesting and charge transport. Examples of charge extraction models from the literature are summarized to understand the mechanism of interfacial carrier dynamics and to rationalize experimental observations. We focus on a working principle of the constituent components and linking the photosynthetic activity with the proposed models. This work gives a new perspective on artificial photosynthesis by taking simultaneous advantages of photon absorption and charge transfer, outlining an encouraging roadmap towards solar fuels.

Comparison Study on the Stability of Copper Nanowires and Their Oxidation Kinetics in Gas and Liquid

The unsaturated "dangling" bonds on the surface of nanomaterials are extremely sensitive to the external environment, which gives nanomaterials a dual nature, i.e., high reactivity and poor stability. However, studies on the long-term effects of stability and reactivity of nanomaterials under practical conditions are rarely found in the literature and lag far behind other research. Furthermore, the long-term effects on the stability and reactivity of a nanomaterial without coating under practical conditions are seriously long-neglected. Herein, by choosing copper nanowire as an example, we systematically study the stability of copper nanowires (CuNWs) in the liquid and gas phase by monitoring the change of morphology, phase, and valence state of CuNWs during storage. CuNWs exhibit good dispersibility and durable chemical stability in polar organic solvents, while CuNWs stored in water or nonpolar organic solvents evolve into a mace-like structure. Additionally, fresh CuNWs are oxidized into CuO nanotubes with thin shells by heating in air. The activation energies of oxidation of CuNWs in the gas phase are determined by the Kissinger method. More importantly, the different oxidation pathways have significant effects on the final morphology, surface area, phase, optical absorption, band gap, and vibrational property of the oxidation products. Understanding the stability and reactivity of Cu nanostructures will add value to their storage and applications. This work emphasizes the significant issue on the stability of nanostructures, which should be taken into account from the viewpoint of their practical application.

Unprecedentedly enhanced solar photocatalytic activity of a layered titanate simply integrated with TiO2 nanoparticles

We report a simple, low-cost methodology for unprecedentedly enhancing the photocatalytic activity of layered inorganic semiconductors.

Construction of a 2D/2D g-C3N4/rGO hybrid heterojunction catalyst with outstanding charge separation ability and nitrogen photofixation performance via a surface protonation process

Heterojunction catalyst PCN/rGO with effective interfacial contact by incorporating reduced graphene oxide and protonated g-C₃N₄ and outstanding nitrogen photofixation ability was synthesized via a electrostatic self-assembly strategy.

Heteroaggregation of Cerium Oxide Nanoparticles and Nanoparticles of Pyrolyzed Biomass

Heteroaggregation with indigenous particles is critical to the environmental mobility of engineered nanomaterials (ENM). We studied heteroaggregation of ceria nanoparticles (n-CeO2), as a model for metal oxide ENM, with nanoparticles of pyrogenic carbonaceous material (n-PCM) derived from pecan shell biochar, a model for natural chars and human-made chars used in soil remediation and agriculture. The TEM and STEM images of n-PCM identify both hard and soft particles, both C-rich and C,O,Ca-containing particles (with CaCO3 crystals), both amorphous and "onion-skin" C-rich particles, and traces of nanotubes. Heteroaggregation was evaluated at constant n-CeO2, variable n-PCM concentration by monitoring hydrodynamic diameter by dynamic light scattering and ζ-potential under conditions where n-PCM is "invisible". At pH 5.3, where n-CeO2 and n-PCM are positively and negatively charged, respectively, and each stable to homoaggregation, heteroaggregation is favorable and occurs by a charge neutralization-charge reversal mechanism (CNCR): in this mechanism, primary heteroaggregates that form in the initial stage are stable at low or high n-PCM concentration due to electrostatic repulsion, but unstable at intermediate n-PCM concentration, leading to secondary heteroaggregation. The greatest instability coincides with full charge neutralization. At pH 7.1, where n-CeO2 is neutral and unstable alone, and n-PCM is negative and stable alone, heteroaggregation occurs by a charge-accumulation, core-shell stabilization (CACS) mechanism: n-PCM binds to and forms a negatively charged shell on the neutral surface of the nascent n-CeO2 core, stabilizing the core-shell heteraggregate at a size that decreases with n-PCM concentration. The CNCR and CACS mechanisms give fundamental insight into heteroaggregation between oppositely charged, and between neutral and charged nanoparticles.

Enhanced visible-light photocatalytic activity of g-C3N4/Zn2GeO4 heterojunctions with effective interfaces based on band match

Fabricating heterojunction photocatalysts is an important strategy for speeding up the separation rate of photogenerated charge carriers, which is attracting greater interest. However, the choice of three factors, individual materials, band offsets, and effective interfaces, is still important for fabricating efficient heterojunction photocatalysts. Herein, efficient g-C3N4/Zn2GeO4 photocatalysts with effective interfaces were designed by controlling the surface charges of the two individual materials inside the same aqueous dispersion medium, making use of the electrostatic attraction between oppositely charged particles. The g-C3N4/Zn2GeO4 heterojunction with opposite surface charge (OSC) showed higher visible-light photocatalytic activity for degradation of methylene blue than those of pure g-C3N4, pure Zn2GeO4, and the g-C3N4/Zn2GeO4 with identical surface charge (ISC). The investigation of the light absorption spectrum, adsorption ability, and photocurrent responses revealed that the improved separation of photogenerated carriers was the main reason for the enhancement of the OSC g-C3N4/Zn2GeO4 sample's photocatalytic activity. By combining with theoretical calculations, we investigated the microscopic mechanisms of interface interaction and charge transfer between g-C3N4 and Zn2GeO4. The photogenerated electrons in the g-C3N4 N 2p states directly excited into the Zn 4s and Ge 4s hybrid states of Zn2GeO4. The strategy of designing and preparing a g-C3N4/Zn2GeO4 composite catalyst in this work is very useful for fabricating other efficient heterojunction photocatalysts.

Visible light-driven decomposition of gaseous benzene on robust Sn2+-doped anatase TiO2 nanoparticles

The Sn²⁺-doped TiO₂ nanoparticles are excellent and promising visible-light photocatalysts for the decomposition of benzene with robust photostability.

Synthesis of F-TiO2 Nanosheets and its Photocatalytic Oxidation of Benzene to Phenol

F-TiO2 nanosheets were synthesized by alcohothermal method. Surface fluorination enhances the absorption of TiO2 in the visible region. Meanwhile, the strongest electronegativity of the fluorine reduces recombination of electrons and holes. Thus compared with the naked TiO2, the F-TiO2 has excellent photocatalytic properties in oxidation of benzene to phenol. The results suggest that benzene conversion and phenol selectivity are up to 35.6% and 93.8%, respectively.