Surface Photochemical Reactions Research Articles

Temperature-programmed desorption spectrometer combining minimum gas load, fast substrate replacement, and comprehensive temperature control

With the capability of quantitative identifying surface species and measuring desorption kinetics, temperature-programmed desorption (TPD) is widely used in heterogeneous catalysis and surface science fields. Minimum gas load during adsorption, fast substrate replacement, and comprehensive temperature control are of great significance for efficient and high quality TPD experiments. Unfortunately, these requirements usually cannot be met at the same time for the existing apparatuses in surface science. In order to increase the universality, a TPD spectrometer combining minimum gas load, fast substrate replacement, and comprehensive temperature control in our laboratory has been built. By using an automatically controlled microcapillary array-based effusive molecular beam gas doser, optimizing the thermal contact at the sample stage, using liquid nitrogen transfer line and designing thermocouple connection, controllable and reproducible molecule adsorption, minimum gas load, fast substrate replacement, rapid cooling, accurate temperature measuring and excellent linear heating are achieved simultaneously. Capabilities of the TPD spectrometer, for example, determination of desorption energy and desorption order, quantitative measurements of surface species and binding sites, and investigation of surface photochemical reactions, are demonstrated by measuring the desorption of water from highly oriented pyrolytic graphite and TiO2(110) and photocatalyzed oxidation of methoxy anions on TiO2(110). The apparatus described here will contribute effectively to the high throughput measurements.

Mechanistic Insights into Graphene Oxide Driven Photocatalysis as Co-Catalyst and Sole Catalyst in Degradation of Organic Dye Pollutants

Photocatalysis is a promising route to utilize sunlight, which has been potentially used to solve energy as well as environmental problems with an emphasis on fundamental understanding and technological applications in society. Semiconductors are excellent photocatalysts but often show less efficient activities due to the fast recombination of photogenerated charge carriers and very slow kinetics of surface photochemical reactions. However, recent advancements show promising strategies to improve their photocatalytic activities, including surface modifications using suitable co-catalysts and the development of novel efficient photocatalysts. Graphene oxide (GO) is one of such nanomaterials which shows multifarious roles in photocatalysis with a great potential to act as an independent solar-driven sole photocatalyst. In this minireview, the photochemistry of GO has been discussed in view of its multifarious roles/mechanisms in improving the photocatalytic activity of metal oxide semiconductors, plasmonic nanomaterials, and also their nanocomposites. In addition, recent advancements and applications of such GO-based photocatalysts in photocatalytic degradation of organic dye pollutants, including engineering of GO as the sole photocatalyst, have been discussed. Furthermore, the challenges and future prospects for the development of GO-based photocatalysts are discussed.

Direct Heat-Induced Patterning of Inorganic Nanomaterials.

Patterning functional inorganic nanomaterials is an important process for advanced manufacturing of quantum dot (QD) electronic and optoelectronic devices. This is typically achieved by inkjet printing, microcontact printing, and photo- and e-beam lithography. Here, we investigate a different patterning approach that utilizes local heating, which can be generated by various sources, such as UV-, visible-, and IR-illumination, or by proximity heat transfer. This direct thermal lithography method, termed here heat-induced patterning of inorganic nanomaterials (HIPIN), uses colloidal nanomaterials with thermally unstable surface ligands. We designed several families of such ligands and investigated their chemical and physical transformations responsible for heat-induced changes of nanocrystal solubility. Compared to traditional photolithography using photochemical surface reactions, HIPIN extends the scope of direct optical lithography toward longer wavelengths of visible (532 nm) and infrared (10.6 μm) radiation, which is necessary for patterning optically thick layers (e.g., 1.2 μm) of light-absorbing nanomaterials. HIPIN enables patterning of features defined by the diffraction-limited beam size. Our approach can be used for direct patterning of metal, semiconductor, and dielectric nanomaterials. Patterned semiconductor QDs retain the majority of their as-synthesized photoluminescence quantum yield. This work demonstrates the generality of thermal patterning of nanomaterials and provides a new path for additive device manufacturing using diverse colloidal nanoscale building blocks.

The Use of Composite TiO2/Activated Carbon Fibers as a Photocatalyst in a Sequential Adsorption/Photocatalysis Process for the Elimination of Ciprofloxacin

This work reports the performance of a sequential adsorption/photocatalysis process using activated carbon fibers with deposited TiO2 for the elimination of the antibiotic ciprofloxacin (CIP) in water. A commercial activated carbon fiber (ACF10) was selected as the support, and a TiO2 coating was synthesized using Metal Organic Chemical Vapor Deposition (MOCVD). Experiments were carried out using a photocatalytic reactor irradiated with monochromatic LEDs (365nm). Two different processes have been studied: adsorption/photolysis and adsorption/photocatalysis. The objective was to completely remove the CIP and to evaluate the efficiency of the treatment by following the formation/elimination of aromatic transformation products (ATPs), aliphatic acids, fluoride, and the TOC in the liquid phase. The adsorption kinetic of the CIP by ACF10 was rather slow (71% of CIP adsorbed by 24 h and total adsorption by 20 days). A good fit between the external diffusion limitation model and the experimental curve (kext = 0.0056 h-1) showed an external transfer limitation due to a tight weave of fibers. For the adsorption/photolysis process, a significant decrease of the concentration was achieved (95% after 6 h of irradiation), but ten different ATPs were detected in the liquid phase. To eliminate CIP, 24 h of adsorption and 6 h of irradiation were then necessary, but most of the ATPs remained in solution (total treatment duration: 72 h). With ACF10-TiO2, the same ATPs were present in solution and were eliminated after the 6 h irradiation step (total treatment duration: 30 h). At the end of the treatment, several non-toxic aliphatic acids were found to be present, showing the higher efficiency of this sequential process. The presence of a significant amount of fluorine in the liquid phase suggests some surface photochemical reactions of the adsorbed molecules (CIP and transformation products) and a partial regeneration of the composite material.

Improvement of summertime surface ozone prediction by assimilating Geostationary Operational Environmental Satellite cloud observations

Clouds play an important role in the Earth's climate system since they can affect various physical and chemical processes within the atmosphere. Misplacement of clouds is a major source of error in the numerical weather prediction (NWP) models, and it also impacts the accuracy of air quality simulations since the meteorology and air quality are directly coupled. In this study, a cloud assimilation technique was utilized to improve cloud placement within the Weather Research and Forecasting (WRF) model by assimilating Geostationary Operational Environmental Satellite (GOES)-derived cloud products. Meteorological outputs from the WRF model were then used as inputs for the Community Multiscale Air Quality (CMAQ) model. The impact of cloud assimilation on air quality was tested over the June–September 2016 period. The results indicated that, by modifying model clouds, cloud assimilation corrected surface solar radiation and photochemical reaction rates, altered light sensitive biogenic emissions, adjusted horizontal transport and vertical mixing, and finally improved the prediction of surface ozone concentration. Cloud assimilation improved daytime surface ozone prediction over most of the U.S. domain, with exceptions in California. On average, cloud assimilation improved the prediction of daytime peak ozone and reduced bias by 47% (∼1.5 ppb). The largest improvement was seen over the southeast U.S. region (∼2.6 ppb reduction in daytime peak ozone), where convective clouds are more frequent and transient and biogenic volatile organic compound (VOC) emissions are more intense than elsewhere.

Meso-erythritol-regulated BiOBr nanosheets with surface hydroxyl imprinting sites for considerably improved photocatalytic capability

BiOBr is a promising photocatalytic semiconductor material for pollutant treatment, but its photocatalytic application is limited due to its poor photo-oxidation capability. Hydroxyl radicals (OH) play a key role in surface photochemical reaction. Thus, improving the response of semiconductor catalysts to hydroxyl is a key problem to be solved through a suitable surface atomic regulation strategy. In this study, we selected a meso-erythritol (ME) containing a number of hydroxyl groups to regulate the surface of BiOBr nanosheets. The ME-regulated BiOBr sheets showed a more positive valence band and a much improved oxidation capability than the unregulated ones. The ME-regulated BiOBr nanosheets also had a positive response to OH in the photocatalytic process. The BiOBr nanosheets rapidly degraded phenol and RhB at a high concentration (0.4 mmol L−1). This strategy of surface atom regulation by using sugars provided a reliable means of obtaining highly effective photocatalytic materials.

Pd@HyWO3–x Nanowires Efficiently Catalyze the CO2 Heterogeneous Reduction Reaction with a Pronounced Light Effect

The design of photocatalysts able to reduce CO2 to value-added chemicals and fuels could enable a closed carbon circular economy. A common theme running through the design of photocatalysts for CO2 reduction is the utilization of semiconductor materials with high-energy conduction bands able to generate highly reducing electrons. Far less explored in this respect are low-energy conduction band materials such as WO3. Specifically, we focus attention on the use of Pd nanocrystal decorated WO3 nanowires as a heretofore-unexplored photocatalyst for the hydrogenation of CO2. Powder X-ray diffraction, thermogravimetric analysis, ultraviolet-visible-near infrared, and in situ X-ray photoelectron spectroscopy analytical techniques elucidate the hydrogen tungsten bronze, H yWO3- x, as the catalytically active species formed via the H2 spillover effect by Pd. The existence in H yWO3- x of Brønsted acid hydroxyls OH, W(V) sites, and oxygen vacancies (VO) facilitate CO2 capture and reduction reactions. Under solar irradiation, CO2 reduction attains CO production rates as high as 3.0 mmol gcat-1 hr-1 with a selectivity exceeding 99%. A combination of reaction kinetic studies and in situ diffuse reflectance infrared Fourier transform spectroscopy measurements provide a valuable insight into thermochemical compared to photochemical surface reaction pathways, considered responsible for the hydrogenation of CO2 by Pd@H yWO3- x.

Massively Multiplexed Tip-Based Photochemical Lithography under Continuous Capillary Flow

Summary Multiplexed microarrays—where different biological probes are spatially encoded onto a surface into spots with micrometer-scale diameters—have facilitated the rapid advancement of "omics" research. Further miniaturization of feature diameters could increase the number of probes in a microarray, reduce the sample required for analysis, and decrease costs. Tip-based lithography (TBL) has gained popularity for patterning delicate, biologically active materials, but no versatile TBL-based multiplexing strategy has been devised. Here, we combine microfluidics, beam pen lithography, and photochemical surface reactions to create multiplexed arrays. For proof of concept, the thiol-ene reaction was optimized, and the reaction kinetics were analyzed. Subsequently, we created several patterns containing multiple fluorescent alkenes, where each pattern was designed to demonstrate a different capability of this instrument. This patterning strategy is a powerful approach to studying and optimizing organic reactions on surfaces and creating massively multiplexed arrays and, as such, could provide an entirely new approach for miniaturizing biochips or understanding interfacial reactivity.

Photochemically Powered AgCl Janus Micromotors as a Model System to Understand Ionic Self-Diffusiophoresis.

Micromotors are an emerging class of micromachines that could find potential applications in biomedicine, environmental remediation, and microscale self-assembly. Understanding their propulsion mechanisms holds the key to their future development. This is especially true for a popular category of micromotors that are driven by asymmetric surface photochemical reactions. Many of these micromotors release ionic species and are propelled via a mechanism termed "ionic self-diffusiophoresis". However, exactly how it operates remains vague. To address this fundamental yet important issue, we have developed a dielectric-AgCl Janus micromotor that clearly moves away from the AgCl side when exposed to UV or strong visible light. Taking advantage of numerical simulations and acoustic levitation techniques, we have provided tentative explanations for its speed decay over time as well as its directionality. In addition, photoactive AgCl micromotors demonstrate interesting gravitactic behaviors that hint at three-dimensional transport or sensing applications. The current work presents a well-controlled and easily fabricated model system to understand chemically powered micromotors, highlighting the usefulness of acoustic levitation for studying active matter free from the effect of boundaries.

Photochemical Reaction Patterns on Heterostructures of ZnO on Periodically Poled Lithium Niobate.

The internal electric field in LiNbO3 provides a driving force for heterogeneous photocatalytic reactions, where photoexcited holes or electrons can participate in redox reactions on positive (+c) and negative (-c) domain surfaces and at the domain boundaries. One method to characterize the surface chemical reactivity is to measure photoinduced Ag deposition by immersing the LiNbO3 in an aqueous AgNO3 solution and illuminating with above bandgap light. Reduction of Ag+ ions leads to the formation of Ag nanoparticles at the surface, and a high density of Ag nanoparticles indicates enhanced surface photochemical reactions. In this study, an n-type semiconducting ZnO layer is deposited on periodically poled LiNbO3 (PPLN) to modulate the surface electronic properties and impact the surface redox reactions. After plasma enhanced atomic layer deposition (PEALD) of 1, 2, 4, and 10 nm ZnO thin films on PPLN substrates, the substrates were immersed in aqueous AgNO3 and illuminated with above band gap UV light. The Ag nanoparticle density increased for 1 and 2 nm ZnO/PPLN heterostructures, indicating an enhanced electron density at the ZnO/PPLN surface. However, increasing the ZnO thickness beyond 2 nm resulted in a decrease in the Ag nanoparticle density. The increase in nanoparticle density is related to the photoexcited charge density at the ZnO/PPLN interface and the presence of a weakly adsorbed Stern layer at the ZnO surface. The decrease in the nanoparticle density for thicker ZnO is attributed to photoexcited electron screening in the ZnO layer that suppresses electron flow from the LiNbO3 to ZnO surface.

Enhanced Visible‐Light Photoelectrocatalytic Activity of {001}TiO2 Electrodes Assisted with Carbon Quantum Dots

AbstractAiming at further developing the application of highly reactive {001}TiO2 in photoelectrocatalytic oxidation, which is only limited by its response in UV light, carbon quantum dots (CQDs) are introduced to provide the excitation sources for {001}TiO2 through upconversion from visible to UV light. The CQDs/{001}TiO2 photoelectrode is prepared by loading the CQDs on truncated square pyramid‐shaped TiO2 spheres exposed to high‐activity {001} facets, using a simple impregnation–drying method. The results indicate that the CQDs/{001}TiO2 possess enhanced photoelectrocatalytic ability with an increased photocurrent of 12 mA cm−2 and photocurrent decay time of 1.75 s. Compared with pure {001}TiO2, the surface photochemical reaction resistance of CQDs/{001}TiO2 decreases, whereas the number of photogenerated carriers increase greatly. This visible‐light photoelectrocatalytic activity results from the synergistic effect of CQDs and the {001} facet of anatase TiO2.

Optimal control of open quantum systems: A combined surrogate Hamiltonian optimal control theory approach applied to photochemistry on surfaces

In this paper, control of open quantum systems with emphasis on the control of surface photochemical reactions is presented. A quantum system in a condensed phase undergoes strong dissipative processes. From a theoretical viewpoint, it is important to model such processes in a rigorous way. In this work, the description of open quantum systems is realized within the surrogate hamiltonian approach [R. Baer and R. Kosloff, J. Chem. Phys. 106, 8862 (1997)]. An efficient and accurate method to find control fields is optimal control theory (OCT) [W. Zhu, J. Botina, and H. Rabitz, J. Chem. Phys. 108, 1953 (1998); Y. Ohtsuki, G. Turinici, and H. Rabitz, J. Chem. Phys. 120, 5509 (2004)]. To gain control of open quantum systems, the surrogate hamiltonian approach and OCT, with time-dependent targets, are combined. Three open quantum systems are investigated by the combined method, a harmonic oscillator immersed in an ohmic bath, CO adsorbed on a platinum surface, and NO adsorbed on a nickel oxide surface. Throughout this paper, atomic units, i.e., ℏ = m(e) = e = a(0) = 1, have been used unless otherwise stated.

Remote Excitation Polarization-Dependent Surface Photochemical Reaction by Plasmonic Waveguide

For the first time, we report remote excitation polarization-dependent surface photochemical reaction by plasmonic waveguide. Remote excitation polarization-dependent surface-enhanced Raman scattering (SERS) spectra indicate a surface photochemical reaction that p-aminothiophenol is converted to p,p′-dimercaptoazobenzene (DMAB) induced by the plasmonic waveguide. Surface plasmon polaritons generated at the end of a silver nanowire can propagate efficiently along the nanowire, and be coupled by nanoparticles near the nanowire as a nanoantenna. Massive electromagnetic enhancement is generated in the nanogap between the nanowire and the nanoparticles. The remote excitation polarization-dependent SERS spectra can be obtained experimentally in the nanogaps; furthermore, the remote excitation polarization-dependent SERS spectra of DMAB reveal the occurrence of this surface catalytic reaction. Theoretical simulations using finite-difference time-domain methods strongly support our experimental results.

Emission-photoactivity cross-processing of mesoporous interfacial charge transfer in Eu3+ doped titania

Periodic mesoporous Eu(3+) doped titania materials were obtained through the EISA (Evaporation Induced Self Assembly) process. Eu(3+) ions, entrapped within the semi-crystalline walls of the highly porous framework, appear to be advantageous during the probing of surface photochemical reactions. Its emission intensity is very sensitive to the presence of physisorbed molecules, in gas or liquid phase, that reside within the pores. In particular, strong fluctuations in intensity of the (5)D(0)→(7)F(2) transition were observed under UV light exposure on the time scale of tens of seconds. The emission modulation dynamics show a strong correlation with the crystallinity of the titania matrix. Correlation of the emission with the photocatalytic activity of the semiconductor for photodegradation of an organic molecule is observed. A model is proposed to describe the involved mechanisms.

Quantum chemical calculations on solvation effects for selected photoreactive aromatic organic molecules of atmospheric relevance

The application of computational methods to studies in atmospheric chemistry is instrumental in understanding reaction mechanisms, dynamics, kinetics, particle nucleation and growth, and heterogeneous reactivity. There are a limited number of theoretical and experimental studies on the impact of surface photochemical reactions on water solubility and electronic properties of atmospheric aerosols, particularly those containing organic matter. We report computational results using density functional theory (DFT/B3LYP) on geometrical and electronic properties of selected aromatic organic molecules of interest to heterogeneous photochemistry in atmospheric aerosols. The impact of simulated explicit and implicit solvation on the aforementioned properties was examined for a number of organic compounds. Electronic absorption spectra calculated using time-dependent DFT with the B3LYP functional and 6-31G∗ basis set showed excellent agreement with experimental UV–visible spectra collected for these organic compounds. Results are discussed in light of the relative importance of implicit and explicit solvation for calculating the energies of electronic transitions, correlation between water solubility and hydrogen bonding between explicit water molecules and organic molecules.

Development of ultra-short pulse VUV laser system for nanoscale processing

We have developed intense vacuum ultraviolet (VUV) radiation sources for advanced material processing, such as photochemical surface reactions and precise processing on a nanometer scale. We have constructed a new VUV laser system to generate sub-picosecond pulses at the wavelength of 126 nm. A seed VUV pulse was generated in Xe as the 7th harmonic of a 882-nm Ti:sapphire laser. The optimum conversion was achieved at the pressure of 1.2 Torr. The seed pulse will be amplified by the $\mathrm{Ar}_{2}^{*}$ media generated by an optical-field-induced ionization Ar plasma produced by the Ti:sapphire laser. We have obtained a gain coefficient of g=0.16 cm−1. Our developing system will provide VUV ultra-short pulses with sub-μJ energy at a repetition rate of 1 kHz.

Nanosized WO 3 thin film as a multifunctional hydrogen material for achieving photolysis in CuCl films via hydrogen photosensitization

The paper highlights less-common properties of nanosized WO 3 thin films which make it possible to achieve photolysis in CuCl films via the use of a CuCl–WO 3 double-layer structure. The WO 3 film provides first photoinduced detachment of hydrogen atoms from organic molecules adsorbed on the oxide surface; the hydrogen atoms, being transferred into WO 3, turn into electrons and protons which migrate then to the CuCl film forming sensitizing centres on the halide surface being illuminated simultaneously with the illumination of the WO 3 surface. This in turn gives rise to photolysis in the CuCl films, which does not occur without the hydrogen sensitization. The nanosized WO 3 film makes it possible to trigger two famous surface photochemical reactions, first proton-coupled electron transfer between the hydrogen-donor molecules and the oxide surface, and then the photolysis of the halide, which radically changes the optical parameters of both the WO 3 and the CuCl films.

Shedding light on surfaces—using photons to transform and pattern material surfaces

The ultimate goal of surface modification is to quantitatively control surface properties by precise manipulation of surface chemical structure at the molecular level. Advances in the understanding of molecular design principles for soft matter surfaces can be combined with the available arsenal of interesting photochemical reactions to create an exciting paradigm for surface modification: the use of photons to both transform and pattern chemical functionality at soft matter surfaces. The success of the paradigm is predicated on the ability to design and synthesize “photochemical surface delivery vehicles”, complex photoactive molecules that form stable surface monolayers and subsequently deliver photoactive moieties to the surface. Shedding light onto these smart, modified surfaces brings about a wide variety of precise photochemical reactions that are preprogrammed within the surface delivery vehicle. Surface chemical patterns are formed by exposure through a mask. Some photochemical surface transformation can be considered as “green” chemistry since only photons are required as reagents. In this review, we provide a brief tutorial on photochemistry fundamentals to illustrate the nature of possible photochemical surface reactions and discuss the principles of design for photochemical surface delivery vehicles. Applications of the paradigm drawn from a variety of fields emphasize the tremendous potential for photochemical surface transformation and patterning on both hard and soft substrates.

Annual atmospheric mercury species in Downtown Toronto, Canada

Real-time concentrations of atmospheric gaseous elemental mercury (GEM), reactive gaseous mercury (RGM), and mercury associated with particles having sizes <2.5 microm (P-Hg < 2.5) were simultaneously measured in downtown Toronto from December 2003 to November 2004. The annual mean concentrations of GEM, P-Hg < 2.5 and RGM were 4.5 +/- 3.1 ng m(-3) (99.2%), 21.5 +/- 16.4 pg m(-3) (0.5%) and 14.2 +/- 13.2 pg m(-3) (0.3%), respectively. The concentrations for all the measured Hg species were highly variable throughout the year and were lower in winter than in the other three seasons. The maximum concentrations of Hg species were observed in June and were a result of the high number of Hg spikes (using [GEM] >10 ng m(-3) as an indicator) that occurred in the month. Nighttime (between 9pm-6am) concentrations of Hg species were higher than those of daytime. The results revealed: (1) an urban area is a continuous source of Hg species that have the potential to pose impacts on local, regional and global scales; (2) local/regional anthropogenic sources contributed significantly to the levels and the distributions of the Hg species in the urban atmosphere. More studies are needed to identify and quantify the anthropogenic sources of Hg and the Hg species emitted from these sources; (3) surface emission and photochemical reactions (including the reactions involving ozone) did not have significant influence on the levels of Hg species and their distribution in the urban atmosphere.

Photo-stimulated proton-coupled electron transfer in quasi-amorphous WO3 and MoO3 thin films

WO3 and MoO3 are important hydrogen storage and transport materials capable of accommodating large quantities of hydrogen. Upon insertion of hydrogen, their properties change dramatically, giving rise to electrochromic and photochromic transformations that have been intensively investigated. The materials are also effective catalysts for hydrogen production via photochemical surface reactions. The kinetics was investigated of the photo-injection of hydrogen into highly disordered WO3 and MoO3 thin films over a wide temperature range. A low-temperature non-zero reaction rate limit was discovered for photo-stimulated hydrogen atom transfer between adsorbed molecules and the oxide surface, which provides unambiguous evidence for a quantum mechanism reaction. The reaction model exhibits a similarity to proton-coupled electron transfer, which is a basic mechanism of bioenergetics conversion. A detailed description of the experiments is provided, together with a comprehensive discussion.