Surface Photoabsorption Research Articles

Highly efficient photocatalytic antifouling activity of core-shell structural encapsulation Bi2MoO6 with visible-light responsive ZIF-67

In this work, novel Bi2MoO6/ZIF-67 S-scheme heterojunctions with core-shell structures were fabricated via in-situ enwrapping ZIF-67 nanocubes on Bi2MoO6 flower-like microspheres, which were applied in photocatalytic antifouling towards typical marine bacteria and microalgae. Experimental results indicated that Bi2MoO6/ZIF-67 displayed excellent and stable photocatalytic performances against Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli) under visible light irradiation, and almost all S. aureus and E. coli can be killed within 40 min and 50 min, respectively. Similarly, Bi2MoO6/ZIF-67 exhibited remarkable photocatalytic properties in removing microalgae, achieving the removal efficiencies of 96.0% and 84.4% towards Phaeodactylum tricornutum (P. tricornutum) and Chlorella sp. within 6 h, respectively. The introduction of ZIF-67 obviously improved the specific surface area and photoabsorption ability of Bi2MoO6, combined with the formation of S-scheme heterojunction with core-shell structure, highly enhancing the photocatalytic antifouling activity of Bi2MoO6/ZIF-67. Moreover, a possible photocatalytic mechanism was proposed, validating that h+, OH, O2–, and 1O2 played crucial roles during the photocatalytic process. The transfer direction of photoelectrons and band structures in Bi2MoO6/ZIF-67 were further verified by the density functional theory (DFT) simulated calculations, confirming the formation of S-scheme heterojunction and fast separation efficiency of photoinduced charge carriers. Furthermore, Bi2MoO6/ZIF-67 S-scheme heterojunctions were coated on dacromet and placed in actual marine environment, showing a favorable antifouling performance with fewer living microorganisms attached. This work not only provides new insights in developing novel heterojunctions with highly efficient photocatalytic activities, but also presents a promising application prospect for photocatalysts in the field of marine antifouling.

Interfacial Affinity Determined Photocatalytic Activity: A Comparison between Defective and Bulk Polymeric Carbon Nitride.

The photoexcited charge separation efficiency of photocatalysts is generally considered as the key factor for enhancement of their photocatalytic activity, and sometimes, their photoabsorption capability and interfacial reaction kinetics play a key part, but the role of interfacial affinity of photocatalysts with substrates was rarely researched systematically. Herein, nitrogen vacancy-modified polymeric carbon nitride porous nanotubes (PCNpts) were simply synthesized, using tartaric acid as a crosslinking and corrosion agent, and exhibit a remarkable increment in surface area, wettability, photoabsorption and charge separation capability, and photocatalytic activity in water splitting to produce H2, but, interestingly, exhibit substrate-dependent variation of photoactivity in contaminant degradation, compared with bulk PCN. More interestingly, the interfacial affinity of PCNpts and PCN with contaminants and H2O, rather than photoabsorption and charge separation capability, is confirmed to dominate their photoactivity.

Realizing highly efficient photoelectrochemical performance for vertically aligned 2D ZnIn2S4 array photoanode via controlled facet and phase modulation

Exploring semiconductor photoanode with low photocarrier recombination rate and high surface redox reaction rate is eternal pursuit of PEC energy conversion. Herein, intrinsic microstructure regulation strategy substitutes conventional heterostructure establishing to modulate photocarrier dynamics of layered semiconductor photoanode. Firstly, an improved in-situ growth method is designed to construct vertically aligned ZnIn2S4 nanosheet array photoanode with highly exposed surface area and prominent two-dimensional (2D) feature for optimized carrier dynamics at interface with electrolyte. Secondly, controlled phase transition from hexagonal phase ZnIn2S4 (H-ZIS) to rhombohedral phase ZnIn2S4 (R-ZIS) is realized accompanied with evidently enhanced 2D feature via regulating ethanol ratio in precursor solvent, making it highly adjustable to construct two-phase-coexisting 2D photoanode (H@R-ZIS). Introducing R-ZIS to construct homostructure with H-ZIS not merely integrates more advantageous surface activity and photo absorption of R-ZIS, but more importantly realizes highly efficient real space separation of photogenerated electron-hole pairs via directional interfacial photocarrier migration between two phases. The optimum 2D H@R-ZIS achieves excellent PEC performance with photocurrent of 3.1 mA cm−2 at 1.2 VRHE, IPCE reaching 40 % and a record ABPE of 2.65 % at 0.36 VRHE. Such phase and facet modulation strategy will inform microstructure design and optimization of other layered metal sulfide for next-generation photoanodes.

Effect of Ce-Doping on Microstructure and Adsorption- Photodegradation Behaviors of the Hydrothermally-Synthesized TiO2 Nanotubes

Ce-doped TiO2 nanotubes (Ce-TNTs) were synthesized by a hydrothermal method to obtain high-efficiency adsorption and photodegradation abilities for methylene blue (MB) dye. The transmission electron microscope (TEM), X-ray diffractimeter (XRD), nitrogen adsorption, X-ray photoelectron spectrum (XPS), and photodegradation tests were used to characterize the morphology, microstructure, ionic valance, and degradation behaviors of the TNTs. Results show that the Ce-doped TNTs are composed of anatase tubulars with an inner-hole diameter of 5 nm, outer diameter of 10–15 nm, length of several hundred nanometers, and a small amount of CeO2 nanoparticles. Ce3+ and Ce4+are dissolved in the anatase crystals, the ratio of which increases with an increase in Ce addition. Ce-doping yields a higher amount of surface oxygen, which results in a strong physical and chemical adsorption to the cationoid MB. 2.5 mol% Ce-doping produces the largest specific surface area, porosity, and photoabsorption threshold and the lowest Zeta potential, yielding the highest adsorption efficiency and photocatalytic ability even under sunlight irradiation.

Synthesis of Vitamin B3 through a Heterogeneous Photocatalytic Approach Using Metal-Free Carbon Nitride-Based Catalysts.

Vitamin B3 (nicotinic acid, VB3) was synthesized through the photocatalytic oxidation of 3-pyridinemethanol (3PM) under visible-light-emitting diode (LED) irradiation using metal-free graphitic carbon nitride (GCN) - based materials. A bulk (GCN) material was prepared by a simple thermal treatment using dicyandiamide as the precursor. A post-thermal treatment under static air and nitrogen flow was employed to obtain the GCN-T and GCN-T-N materials, respectively. The conditions adopted during the post-treatment revealed differences in the resulting materials’ morphological, electronic, and optical properties. The post-treated photocatalysts revealed an enhanced efficiency in the oxidation of 3PM into VB3, with the GCN-T-N photocatalyst being the best-performing material. The defective surface, reduced crystallinity, and superior photoabsorption of GCN-T-N account for this material’s improved performance in the production of VB3. Nevertheless, the presence of nitrogen vacancies in the carbon nitride structure and, consequently, the creation of mid-gap states also accounts to its highly oxidative ability. The immobilization of GCN-T-N in sodium alginate hydrogel was revealed as a promising strategy to produce VB3, avoiding the need for the photocatalyst separation step. Concerning the mechanism of synthesis of VB3 through the photocatalytic oxidation of 3PM, it was possible to identify the presence of 3-pyridinecarboxaldehyde (3PC) as the intermediary product.

The Research on the Construction and the Photocatalytic Performance of BiOI/NH2-MIL-125(Ti) Composite

The BiOI/NH2-MIL-125(Ti) composite photocatalyst with excellent photocatalytic performance was prepared by the solvothermal method. For the BiOI/NH2-MIL-125(Ti) (BNMT) system, the contents of NH2-MIL-125(Ti) in BNMT-4, BNMT-5, BNMT-7, BNMT-9, and BNMT-10 were 4 wt %, 5 wt %, 7 wt %, 9 wt %, and 10 wt %, respectively. XRD, XPS, SEM, and TEM characterizations indicated that BiOI/NH2-MIL-125(Ti) was successfully prepared. Brunauer, Emmett, and Teller (BET) and UV–vis diffuse reflectance spectra photoelectrochemical analysis indicated that BNMT-9 can make the specific surface area and photo absorption region larger than BiOI. In addition, the separation efficiency of photogenerated carriers was improved, and the recombination efficiency was reduced. The degradation percentages of Rhodamine B (RhB) and p-chlorophenol (P-CP) reached 99% and 90% over BNMT-9 under visible light irradiation. Additionally, the catalysts had high stability. The results of the active spices trapping experiments test indicated that h+ was the main active species. The possible degradation mechanism was proposed.

Synthesis and photocatalytic activity of dumbbell-like mesoporous ZnO micro-nanomaterials with tetrasulfonatomethyl-n-hexyl calix [4]resorcinarene tetrasodium salt as hard-soft template

Dumbbell-like ZnO micro-nanomaterials with uniform morphology were prepared with tetrasulfonatomethyl-n-hexyl calix [4]resorcinarene tetrasodium salt (THCRT) by a simple reflux method. The ZnO samples were characterized by X-ray diffraction, field emission scanning electron microscope, transmission electron microscopy, high-resolution transmission electron microscopy, thermogravimetric analysis, Brunauer-Emmett-Teller technique, Fourier transform infrared spectroscopy, ultraviolet–visible diffuse reflectance spectra, and Photoluminescence. The as-prepared ZnO micro-nanomaterials with a hexagonal wurtzite structure exhibited dumbbell-like morphology with a length of 700–900 nm and a hemisphere diameter of 500–700 nm. The THCRT was involved in the formation of ZnO nanostructure framework during the assembly and stacking of ZnO nanoparticles. The as-obtained dumbbell-like micro-nanomaterials possessed obvious mesoporous structure after the calcination at 500 °C and 650 °C, which is due to the rigid hydrophobic cavity of THCRT as hard template. The effects of the zinc sources, amount of THCRT, surfactants and amount of ammonia on the size and morphology of the ZnO samples were studied. The results indicate that THCRT with multi-head and multi-tail as soft template has a good protective effect on the growth of ZnO crystals. This study shows that the as-prepared ZnO product has a dumbbell-like mesoporous micro-nanostructure due to the special structure of THCRT which consists of rigid cup cavity, hydrophilic heads and hydrophobic tails. The evolution process of the dumbbell-like ZnO micro-nanostructures was investigated by FE-SEM, and a possible formation mechanism was proposed. The as-prepared ZnO micro-nanomaterials exhibited good adsorption capacity without calcination and excellent photocatalytic activity for degradation of methyl violet after calcination. Due to the excellent properties of micro-nanoscale, suitable pore size, high specific surface and good photoabsorption, the as-prepared dumbbell-like mesoporous ZnO micro-nanomaterials exhibit potential application as photocatalyst.

Characteristic lengths for three-carrier transport with spin-flip and electron-hole recombination

The exact solution of the linearized, steady-state transport equation for three-carrier systems, such as can occur for semiconductors and ionic conductors, is constructed starting from the near-equilibrium entropy-production requirements of irreversible thermodynamics. Three characteristic modes are found, one associated with electrostatic screening (which is often neglected), and two modes associated with diffusion and ``reactions.'' For a spintronics model with up and down electrons and unpolarized holes, the ``reactions'' are spin-flip and electron-hole recombination. We discuss how the variations in carrier density, diffusivity, recombination rate, and spin relaxation time affect the characteristic lengths. We apply these modes to study spin-polarized surface photoabsorption.

In situ Observation of Formation Process of Negative Electron Affinity Surface of GaAs by Surface Photo-Absorption

We have used surface photo-absorption (SPA) to investigate the formation of negative electron affinity (NEA) surfaces on p-GaAs during the Yo-Yo method, under an alternating supply of Cs and O2. The SPA spectra showed that the surface during the first Cs step was different from those in the following Cs and O2 steps. This suggests that the surface structure did not change after the initial surface was formed, indicating that there could be two Cs adsorption sites on the GaAs surface, which is different from previously proposed models.

Pure water splitting to generate H2 over strontium niobate and tantalate: A comparative study

To clarify the nature of different photocatalytic activities between niobate and tantalate, a comparative study of the water splitting into H2 over strontium niobate and tantalate was performed. These isostructural photocatalysts were prepared by a facile and simple hydrothermal reaction and characterized by powder X-ray diffraction (XRD), N2-sorption, UV-Vis diffuse reflectance spectra (UV-Vis DRS), and transmission electron microscopy (TEM). The effects of crystal structure, Brunauer-Emmett-Teller (BET) surface area, photoabsorption performance, morphology, and electronic structure are discussed.

The use of nitrogen to disorder GaInP

Significant changes in microstructure, surface structure, and alloy composition have been observed in GaInP with the addition of nitrogen. These effects occur due to surface changes induced by small concentrations of nitrogen. Transmission electron microscopy and photoluminescence experiments indicate that the use of the surfactant N nearly eliminates the CuPtB ordered structure typically seen in organometallic vapor phase epitaxially grown Ga0.5In0.5P lattice matched to GaAs. In situ surface photoabsorption measurements show a large change in the surface structure indicating that N reduces the concentration of P dimers on the surface, decreasing the driving force for ordering. Nitrogen also has a significant effect on the Ga/In ratio in the bulk. The indium content in the solid is decreased by 71% with a DMHy/III ratio of 60 (DMHy—1,1 dimethyl hydrazine). Nitrogen roughens the surface causing the formation of hillocks and small rectangular features that can be up to 50 nm tall.

25aC03 As desorption process on GaAs(111)A surface by surface photo absorption(NCCG-34)

25aC03 As desorption process on GaAs(111)A surface by surface photo absorption(NCCG-34)

Two-step As-desorption from (001) InP observed by surface photoabsorption

The investigation of As desorption from the (001) InP during metalorganic chemical vapor deposition was performed using surface photoabsorption (SPA). The behavior of the monochromatic SPA signal showed a clear existence of a metastable state after the AsH3 was turned off. SPA spectra at each stable surface were taken to confirm the interpretation. This result indicates that the previous criterion to measure the amount of As/P exchange reaction is not necessarily correct and that the As-desorption process should be understood as a two-step process.

NESSPIOM—Network for enhanced semiconductor surface processing through in situ optical monitoring

AbstractThe activities of the Network for Enhanced Semiconductor Surface Processing through In situ Monitoring (NESSPIOM), based in the UK and funded through the Engineering and Physical Sciences Research Council, are described. This group, consisting of academics and industrialists, is evaluating the potential benefits that may accrue from the use of optical monitoring methods during semiconductor surface modification, both from a fundamental and an applied perspective. The findings of the network to date are presented. Interferometry, light scattering techniques and diode laser spectroscopy are highlighted as methods that may result in improved materials or processes, whereas other techniques such as reflectance anisotropy spectroscopy, spectroscopic ellipsometry, surface photoabsorption and second harmonic generation are found to provide more fundamental information. The future for the application of methods such as this within both the academic and industrial communities is discussed. Copyright © 2001 John Wiley & Sons, Ltd.

Time dependent surfactant effects on growth of GaInP heterostructures by organometallic vapor phase epitaxy

It has been shown that adding the surfactant Sb during growth eliminates ordering in GaInP by destroying the P dimers on the reconstructed surface. Consequently, the understanding of ordering can provide fundamental information about the surface during growth. This paper reports on the use of the surfactant Sb to modulate the order parameter of GaInP to produce disorder-on-order-on-disorder (D/O/D) heterostructures. It was found that heterostructures grown without interruptions have graded interfaces, but heterostructures grown with interruptions have abrupt interfaces. Secondary-ion mass spectroscopy (SIMS) measurements on the heterostructure grown without interruptions indirectly suggests that the Sb surface concentration changed significantly during growth as triethylantimony was added and removed from the system. Consequently, time dependent surface photoabsorption (SPA) measurements were made to determine the change in the P dimer concentration with the addition of triethylantimony. Comparison of the transient time constants with the Langmuir model suggests that relatively high concentrations of Sb accumulate on the surface during growth of the disordered layers. In addition, the results suggest that a critical concentration of Sb on the surface is necessary before the material disorders. Using the surfactant Sb with interruption results in a powerful tool for controlling the surface during growth. This technique was used to produce a GaInP D/O/D heterostructure with a very thin (67 Å) ordered layer, sharp interfaces and good photoluminescence. The ability of this tool to produce advanced quantum well devices appears feasible.

Kinetics of native oxide film growth on epiready GaAs

The evolution of the native oxide layer on polished epiready GaAs wafers has been studied over a period of almost 2 years using grazing incidence X-ray reflectivity and surface photoabsorption. Using computer-simulated reflectivity profiles to fit the X-ray data, the thickness, density and effective roughness of the oxide layer was measured as a function of time. Surface photoabsorption measurements within a metal-organic vapour phase epitaxy reactor indicated that, after about 5 months, oxide species appeared that required higher temperatures to desorb. There was no difference in the oxide species present for wafers stored in air or in an inert atmosphere. The data suggest that the thickening occurs by reaction of the oxide with the bulk semiconductor material.

Monitoring of Silicon Nano-Crystal Dots Formation on SiO2 and on Si3N4 in an UHV-CVD System

AbstractApplicability of SPA (Surface Photo Absorption) as an in-situ monitor of Si nanocrystal formation was comparatively studied to a monitoring with pyrometer.Although both SPA and pyrometer provide a signature of the Si nanocrystals nucleation and growth process as a function of irradiation time, SPA was found to be more sensitive to detect formation of desired nanocrystals on both SiO2 and Si3N4 surfaces. By using SPA as an in-situ monitor of nanocrystal formation, good repeatability of both nanocrystals' density and size was obtained on SiO2 and Si3N4 surfaces.

Surface processes in OMVPE – the frontiers

Surface processes have long been known to be an important part of any epitaxial growth process. These processes are closely linked to the surface structure. However, until recently, the surface structure and the surface processes were difficult to study experimentally for conventional vapor-phase epitaxy (VPE) and liquid-phase epitaxy. Recently, optical techniques such as surface photo absorption (SPA) have been developed to the point that they give useful information about the surface reconstruction in situ during organometallic vapor-phase epitaxial (OMVPE) growth. Thus, they can in many cases be used to monitor the surface processes. A powerful method for controlling the surface structure during epitaxial growth using surfactants has recently emerged. This work describes the use of the surfactants Te, a donor, and As, Sb, and Bi, elements that are isoelectronic with P, on the properties of GaInP grown by OMVPE. These surfactants are found to significantly affect the microscopic arrangement of Ga and In atoms in the bulk solid by effecting a change in the surface structure. CuPt ordering is ubiquitous in III/V semiconductor alloys. It is significant because of the dependence of bandgap energy on the degree of order. The CuPt structure is formed due to the strain induced by the formation of [ 1 ̄ 1 0] P dimers on the surface. Each of the surfactants studied is found to result in disordering for layers grown using conditions that would otherwise produce highly ordered GaInP. Te yields disordered material with no change in the SPA spectra. However, the step velocity is found to increased markedly. Thus, the effect appears to be kinetic. Sb causes disordering due to a replacement of [ 1 ̄ 1 0] P dimers on the nominally (0 0 1) surface by larger Sb dimers, which reduces the strain-induced driving force for CuPt ordering at the surface. Thus, the effect is due to surface thermodynamics. For high Sb concentrations in the vapor, a triple-period ordered structure is formed. The appearance of this phase coincides with a distinct change in the surface reconstruction as indicated by SPA spectroscopy. Modulation of the TESb flow rate during growth was used to produce an abrupt order/disorder heterostructure with a bandgap energy difference of 135 meV with no significant change in solid composition at the interface. SPA results show that addition of As during growth also reduces the degree of order by displacing some of the [ 1 ̄ 1 0] P dimers on the surface. In this case, significant As concentrations in the solid of a few percent are observed. Thus, As is not an effective surfactant. Addition of Bi during growth results in a change in the surface reconstruction, as indicated by SPA spectroscopy, for Bi concentrations producing disordered GaInP. Unlike Sb and As, the Bi also causes a marked increase in the step velocity coincident with the loss of order. For singular (001) substrates, island formation is suppressed by Bi, resulting in the growth of much smoother layers. Modulation of the TMBi concentration during growth has been used to produce disorder/order heterostructures. The use of isoelectronic surfactants during growth to influence the properties of a semiconducting solid is a new and exciting development in control of the OMVPE growth process. It is expected that the use of isoelectronic surfactants to determine the surface reconstruction will find application in the growth of complex device structures. It also appears likely that this will be useful for controlling other characteristics of the growth process and the properties of the resultant semiconductor materials.

Investigation of Hydrogen Chemisorption on GaAs (111)A Ga Surface by In Situ Monitoring and Ab Initio Calculation

Hydrogen chemisorption on the GaAs (111)A Ga surface is investigated under atmospheric pressure using an in situ optical monitoring system which consists of GaAs halogen transport atomic layer epitaxy (ALE) and surface photoabsorption (SPA) systems. The results of the in situ monitoring indicate the existence of a stable hydrogen-terminated surface. It is shown that hydrogen in the carrier gas reacts dissociatively with the (111)A Ga surface, and the relationship between the surface hydrogen coverage and hydrogen partial pressure is explained well by the Langmuir isotherm with dissociation. Atomic configuration on the surface is examined further by means of ab initio molecular dynamics calculation. The total energy change due to the calculated process is very close to the standard enthalpy of adsorption obtained by the experiment.

Bi surfactant control of ordering and surface structure in GaInP grown by organometallic vapor phase epitaxy

The surfactant Bi has been added during organometallic vapor phase epitaxial growth (OMVPE) of GaInP using the precursor trimethylbismuth. The addition of a small amount of Bi during growth results in disordered material using conditions that would otherwise produce highly ordered GaInP. Significant changes in the surface structure are observed to accompany the disordering. Atomic force microscopy measurements show that Bi causes an order of magnitude increase in step velocity, leading to the complete elimination of three-dimensional islands for growth on singular (001) GaAs substrates, and a significant reduction in surface roughness. Surface photoabsorption measurements indicate that Bi reduces the number of [1̄10] P dimers on the surface. Secondary ion mass spectroscopy measurements reveal that the Bi is rejected from the bulk, even though it changes the surface reconstruction. Clearly, Bi acts as a surfactant during OMVPE growth of GaInP. The difference in band gap energy caused by the reduction in order parameter during growth is measured using photoluminescence to be about 110 meV for layers grown on singular substrates. Disorder/order/disorder heterostructures were successfully produced in GaInP with a constant solid composition by modulating the TMBi flow rate during growth.