In situ fabrication of Z-scheme C3N4/Ti3C2/CdS for efficient photocatalytic hydrogen peroxide production.

Photocatalysis is a potential technology to produce hydrogen peroxide with low energy consumption and no pollution. However, when using traditional photocatalysts it is hard to meet the requirements of wide visible light absorption, high carrier separation rate and sufficient active sites. Graphitic carbon nitride (g-C3N4) has great potential in the photocatalytic production of hydrogen peroxide, but its photocatalytic performance is limited by a high carrier recombination ratio. Here, we fabricated the Z-Scheme heterojunction of C3N4/Ti3C2/CdS in situ. The large specific surface area of C3N4 can provide plenty of reactive sites, and the absorption efficiency under visible light is improved with the addition of Ti3C2 and CdS. The better conductivity of Ti3C2 reduces the charge transfer resistance. With the increase of surface charge carriers, the width of the space charge region decreases and the photocurrent density increases significantly. Under visible light irradiation, the H2O2 yield of the ternary photocatalyst reaches 256 µM L-1 h-1, which is about 6 times that of pristine C3N4. After three cycles, the high photocatalytic efficiency can still be maintained. In this paper, the reaction mechanism of photocatalytic hydrogen peroxide production by the C3N4/Ti3C2/CdS composite material is proposed through an in-depth study of energy band theory, which provides a new reference for the design and preparation of high-performance materials for photocatalytic hydrogen peroxide production.

Ultraflat Langmuir-Blodgett assembled graphene oxide saturable-absorber films for pulsed near-infrared laser generation.

Large-scale production of ultraflat broadband saturable-absorber films is highly desired for passive mode-locked solid-state lasers. However, the current vapour deposition and spin coating routes for fabricating saturable absorbers (SAs) are suffering from the limited flexibility in substrate choice and complexity of mass production processes. Here, we demonstrate an ultraflat carboxyl-functionalized graphene oxide (GO-COOH) SA film via Langmuir-Blodgett (LB) assembly for solid state laser mode-locking. Hydrophilic carboxyl groups from GO sheets weaken the aggregation effect thus contribute to the uniform and stable dispersion of GO sheets in water. Such GO suspensions are made into an ultrathin large-area graphene-based SA film by a LB assembly process ensuring high surface uniformity. The room-temperature and highly repeated operation for GO LB films avoids the thermal damage of GO sheets and improves the membrane repeatability. Consequently, the ultrathin GO-COOH SA shows the modulation depth (2.3%) and low saturation intensity (24.7 KW cm-2) under 1064 nm laser irradiation. By inserting the GO SA into a Nd:GdVO4laser, both passive Q-switched (QS) and passive Q-switched mode-locked (QML) operations are also attained. The slope efficiency of QS laser is up to 35.6% and the maximum single pulse energy is 1.48µJ. In particular, the QML pulses can be achieved stably and repeatedly with an average output power of 1.33 W and a pulse energy of 13.2 nJ. Our strategy provides a new concept for improving the modulation stability of graphene-based SAs and promoting their industrial application in pulsed solid-state lasers.

One-pot synthesis of 3D Au nanoparticle clusters with tunable size and their application.

In general, the preparation of Au nanoparticle clusters (NPCs) is more challenging than that of nanoparticles. The traditional multi-step method for preparing Au NPCs is time consuming and highly sensitive to the reaction conditions. Here, we report a simple and feasible method for the rapid preparation of Au NPCs (∼30 min), in which Au (III) is reduced to Au (0) by trisodium citrate, and assembled into NPCs in the presence of a trace amount of cysteine. The surface plasmon resonance peak of the Au NPCs is tunable and ranged from visible to near-IR regions by varying the content of cysteine added. The growth process of Au NPCs was monitored by dynamic light scattering, UV-vis absorption spectroscopy and transmission electron microscopy. Their elemental composition, chemical state and molecular structure of the sample surface were measured by x-ray photoelectron spectroscopy. The proposed synthesis mechanism has guiding significance for the preparation of other NPCs. Au NPCs used as surface-enhanced Raman spectroscopy substrate has a good enhancement effect because of its unique morphology.

Stepwise Synthesis of Au@CdS-CdS Nanoflowers and Their Enhanced Photocatalytic Properties.

Fabrication of hybrid nanostructures with complex morphologies and high photocatalytic activity is a difficult challenge because these particles require extremely high preparation skills and are not always practical. Here, hierarchical flower-like Au@CdS-CdS nanoparticles (Au@CdS-CdS nanoflowers) have been synthesized using a stepwise method. The Au@CdS-CdS nanoflowers are consisted of Au core, CdS shell, and CdS nanorods. The UV-Vis absorption range of the Au@CdS-CdS nanoflowers reaches up to 850 nm which covers the whole visible range (400-760 nm). Photoinduced charge transfer property of Au@CdS-CdS nanoflowers was demonstrated using photoluminescence (PL) spectroscopy. Compared to CdS counterparts and Au@CdS counterparts, Au@CdS-CdS nanoflowers demonstrated the highest photocatalytic degradation rate under irradiation of λ = 400-780 nm and λ = 600-780 nm, respectively. Based on structure and morphology analyses, we have proposed a possible formation mechanism of the hybrid nanostructure which can be used to guide the design of other metal-semiconductor nanostructures with complex morphologies.

Two-step phase shifting differential-recording digital holographic microscopy.

We present two-step phase-shifting differential-recording digital holographic microscopy (TPD-DH in microscopy) for phase imaging of microscopic transparent elements. Two CCDs are employed to record two interferograms at two different defocusing distances. The interferograms on the two CCD cameras are shifted for a phase retarder 0 and π via an all-optics phase shifting unit. A novel algorithm is proposed to reconstruct both amplitude and phase distributions of the object wave from the recorded interferograms. This method has the same spectrum bandwidth and measurement accuracy with those of conventional four-step phase-shifting interferometry (FS-PSI), whereas it reduces the measurement time by half.

In situ SERS monitoring of plasmonic nano-dopants during photopolymerization.

The motion of the plasmonic nano-dopant in photopolymers was monitored in situ and in real time using the surface-enhanced Raman scattering (SERS) technique. Here an Au@MBA@Ag (core-molecule-shell) nanoparticle colloid was synthesized to act as the nano-dopant and adsorbed 4-mercaptobenzoic acid (MBA) between the Au cells and Ag shells as the internal standard. The changes of the MBA signal closely reflect the motion of nanoparticles, since the MBA signal itself has time stability. Experimental data indicate that the optimized concentration of the nano-dopant can be obtained based on the peak intensity change of MBA at 1583 cm-1. This Letter provides a novel way for in situ monitoring of photophysicial and photochemical processes.

Morphology and Optical Properties of RF Sputtering Deposited Indium Nitride Layers Under Different N2/Ar Ratio.

InN thin films were prepared using reactive radio frequency sputtering on (111) Si substrates under different N2/Ar ratio. The surface morphology and optical properties of InN thin films were characterized by X-ray diffraction, atomic force microscopy, scanning electron microscope, energy dispersive spectrometer, and UV-visible-NIR spectrophotometer at room temperature. It was found that the growth rate and surface RMS roughness of InN layers all significantly decreased with the increasing of N2 content in the sputtering gas. All the InN films were wurtzite structure with the proportion of N2 increasing from 40% to 100% in the mixture gas. The highly c-axis orientation InN films could only be obtained in the composition ratios of N2 higher than 90%. The atomic percentage of N is much higher than that of In at high N2 content films, which mainly due to the chemical reaction on the surface of Si substrate. The apparent optical band gap energy is estimated at 1.83 eV. However, the InN absorption band tail is strongly influenced by the sputtering due to a change in the species of the plasma.

Effect of incorporation of different modified Al2O3 nanoparticles on holographic characteristics of PVA/AA photopolymer composites.

Al2O3 nanoparticles modified with different chemical reagents, prepared by using three chemical dispersants [high definition (HD), sodium dodecyl benzene sulfonate, and cetyl trimethyl ammonium chloride], were doped into photopolymer films in a polyvinyl alcohol/acrylamide (PVA/AA) system, respectively. A 647 nm Ar-Kr laser was used to expose and study the holographic properties of the samples. The research shows that doping Al2O3 nanoparticles into PVA/AA photopolymer film leads to different levels of improvement of the holographic characteristics. The diffraction efficiency of the sample can be raised to 93.8%, the maximum refractive index modulation increased to 2.28×10(-3), the shrinkage can be depressed to 0.8%, and the Bragg mismatch is 0.04°, while the concentration of 10 nm Al2O3 nanoparticles modified by HD dispersant is 1.02×10(-3) mol·L(-1).

Slightly off-axis interferometry for microscopy with second wavelength assistance.

Slightly off-axis interferometry for microscopy has been performed, where the dc term of the interferogram is suppressed by the object wave in another wavelength. One wavelength of the laser beam (red light) is used to generate the slightly off-axis interferogram, while the second wavelength (blue light) is employed to measure the transmittance of the specimen. Both the red light and blue light are recorded simultaneously by a color CCD camera and can be separated without cross talk via the red-green-blue components. The dc term of the slightly off-axis interferogram of red light is suppressed with the object wave of blue light. As a consequence, the requirement on the off-axis angle between the object and reference waves is relaxed as well as the requirement on the resolving power of CCD camera.

Kinetics of photoinduced anisotropy in bacteriorhodopsin film under two pumping beams.

Photoinduced anisotropy in bacteriorhodopsin (BR) film arises from the selective bleaching of BR molecules to linearly polarized light. The kinetics of photoinduced anisotropy excited by single and two pumping beams are investigated theoretically and experimentally. Compared with a single pumping beam (650 nm), which produces comparatively small photoinduced anisotropy, dual-wavelength linearly polarized pumping beams (650 and 405 nm) can obviously change the photoinduced anisotropy. When the polarization orientation of the 405 nm pumping beam is perpendicular to that of the 650 nm pumping beam, the peak and steady values of the photoinduced anisotropy kinetic curves are remarkably enhanced. But when the two pumping beams have parallel polarization orientation, the peak and steady values are restrained. At a fixed intensity of the 650 nm pumping beam, there exists an optimal intensity for the 405 nm pumping beam to maximize the value of the photoinduced anisotropy. The photoinduced transmittance of the polarizer-BR-analyzer system is modulated by the polarization angle of the 405 nm pumping beam in an approximate-cosine form.

Influence of polarization orientation of violet light on the diffraction efficiency of bacteriorhodopsin.

When a grating is recorded in a bacteriorhodopsin film by two linear parallel polarized beams together with an auxiliary violet light, the diffraction efficiency has a dependence on the polarization orientation of the violet light as well as its intensity. A method for calculating the diffraction efficiency of gratings in bacteriorhodopsin is proposed based on the two-state photochromic model, considering the saturation effect and the polarization status of all the involved lights. It is found that the polarization orientation of the violet light produces an approximate-cosine and an approximate-sine modulation on the steady-state diffraction efficiency separately at low and high intensities, respectively. The parallel polarized violet light can improve the steady-state diffraction efficiency to a larger degree than the perpendicularly polarized violet light when both are at their optimal intensities. The optimal intensity for the parallel polarized violet light is lower than that of the perpendicular polarized one. Thus, the improvement of the steady-state diffraction efficiency is less sensitive to the intensity of perpendicular polarized violet light than to that of parallel polarized violet light.

Phase and amplitude reconstruction from a single carrier-frequency interferogram without phase unwrapping.

A rapid algorithm for phase and amplitude reconstruction from a single spatial-carrier interferogram is proposed by bringing a phase-shifting mechanism into reconstruction of a carrier-frequency interferogram. The algorithm reconstructs phase through directly obtaining and integrating its real-value derivatives, avoiding a phase unwrapping process. The proposed method is rapid and easy to implement and is made insensitive to the profile of the interferogram boundaries by choosing a suitable integrating path. Moreover, the algorithm can also be used to reconstruct the amplitude of the object wave expediently without retrieving the phase profile in advance. The feasibility of this algorithm is demonstrated by both numerical simulation and experiment.

Effect of reconstruction beam polarization on the kinetics of anisotropic gratings in bacteriorhodopsin.

Anisotropic gratings are recorded on bacteriorhodopsin films by two parallelly polarized beams, and the effect of the polarization orientation of the reconstructing beam on the diffraction efficiency kinetics is studied. A theoretical model for the diffraction efficiency kinetics of the anisotropic grating is developed by combining Jones-matrix and photochromic two-state theory. It is found that the polarization azimuth of the reconstructing beam produces a cosine modulation on the kinetics of the diffraction efficiency, being positive at the peak efficiency and negative for steady state. By adding auxiliary violet light during grating formation, the saturation of the grating can be restrained. As a result, the negative cosine modulation for the steady-state diffraction efficiency changes to a positive one. In addition, the steady-state diffraction efficiency is increased appreciably for all reconstructing polarization orientations.

[Investigation on optical limiting performance in solution of cobalt porphyrin].

The nonlinear optical effects of cobalt porphyrin have been investigated with three CW laser lines of 457.9, 488 and 514.5 nm, respectively. Three curves with peak followed by valley using the single beam z-scan technique were obtained. According to M Sheik-bahae's theory the sample has a negative nonlinear refractive index, that is, there is a thermal self-defocusing effect. Three curves of transmittance show a decrease with the increase in the incident laser power, which means that the sample has reverse saturated absorption property under the three laser wavelengths. It's well known that both thermal self-defocusing effect and reverse saturated absorption can lead to optical limiting. It was found that cobalt porphyrin has the optical limiting effect under those wavelengths, and that the critical value of optical limiting decreases with the decrease in laser wavelength. Furthermore, the effect of optical limiting is very good and the critical value is very low, so it's a new optical limiting material with a great potential application value. For that, it is possible to use the co-operation of both effects from cobalt porphyrin to produce a new kind of optical limiting device.