Electron Trapping Centers Research Articles

Bioresistive Random-Access Memory with Gold Nanoparticles that Generate the Coulomb Blocking Effect Can Realize Multilevel Data Storage and Synapse Simulation.

Gold nanoparticles (Au NPs) have good biocompatibility and special quantum effects. In this Letter, we embedded Au NPs into silkworm hemolymph (SH) to improve the performance of the device and fabricated Al/SH:Au NPs/indium tin oxide (ITO)/glass resistive random access memory. The device exhibits a bipolar switching behavior with a retention time of 104 s. Compared with the Al/SH/ITO device without Au NPs, the device has a higher ON/OFF current ratio (>105) and a smaller Vreset. The improvement in device performance is attributed to the fact that Au NPs act as the electron-trapping center in the device; a Coulomb blockade occurs after electrons are trapped, thereby increasing the resistance of the device in the high-resistance state. Using optimized devices can realize multilevel data storage and can also simulate synaptic characteristics such as potentiation and depression. The device is expected to be applied to high-density, low-cost, degradable, and biocompatible storage devices and neuromorphic computing in the future.

High-resolution radiation detection using Ni/SiO2/n-4H-SiC vertical metal-oxide-semiconductor capacitor

In this article, we demonstrate the radiation detection performance of vertical metal-oxide-semiconductor (MOS) capacitors fabricated on 20 μm thick n-4H-SiC epitaxial layers with the highest energy resolution ever reported. The 100 nm SiO2 layer was achieved on the Si face of n-4H-SiC epilayers using dry oxidation in air. The Ni/SiO2/n-4H-SiC MOS detectors not only demonstrated an excellent energy resolution of 0.42% (ΔE/E×100) for 5.48 MeV alpha particles but also caused a lower enhancement in the electronic noise components of the spectrometer compared with that observed for the best high-resolution Schottky barrier detectors. The MOS detectors also exhibited a high charge collection efficiency (CCE) of 96% at the optimized operating bias despite the presence of the oxide layer. A drift-diffusion model applied to the CCE vs gate bias voltage data revealed a minority (hole) carrier diffusion length of 24 μm. Capacitance mode deep level transient spectroscopy (C-DLTS) scans in the temperature range 84–800 K were carried out to identify the resolution limiting electrically active defects. Interestingly, the C-DLTS spectra revealed both positive and negative peaks, indicating the simultaneous presence of electron (majority) and hole (minority) trap centers. It has been inferred that at the steady-state bias for the C-DLTS measurement, the MOS detector operates in the inversion mode at certain device temperatures, causing holes to populate the minority trap centers and, hence, manifests minority carrier peaks as well.

Atypical behavior of intrinsic defects and promising dopants in two-dimensional WS2

The $2\mathrm{D}\text{\ensuremath{-}}\mathrm{W}{\mathrm{S}}_{2}$ is an emerging material for next-generation electronic and optoelectronic devices. These applications are very sensitive and can be adversely affected by defects incorporated during the sample growth. Using hybrid density functional approach, we carried out a comprehensive study on intrinsic and extrinsic defects in $2\mathrm{D}\text{\ensuremath{-}}\mathrm{W}{\mathrm{S}}_{2}$. All the intrinsic defects and their complexes are found to be deep and self-compensating. S vacancy $({\mathrm{V}}_{\text{S}})$, which has been previously attributed as the source of $n$-type conductivity, turns out to be an electron trap center. We found that ${\mathrm{V}}_{\text{S}}$ gives rise to a suboptical gap, which can be the source of single-photon emitters. Interestingly, hydrogen interstitial $({\mathrm{H}}_{i})$ makes multicenter bond and acts as a shallow donor. In addition, H as adatom (H-ad) also provides shallow donor levels and is the cause of unintentional $n$-type doping. Among the extrinsic defects, halogens and transition metals are found to be promising dopants. While halogens at the S site act as shallow donors, Nb at the W site provides a reasonable shallow acceptor level with low formation energy.

Site-selective characterisation of electron trapping centres in relation to chemistry, structural state and mineral phases present in single crystal alkali feldspars

Feldspars are ubiquitous natural dosimeters widely used in luminescence dating. Despite decades of research, the lattice defects as well as the mechanisms involved in luminescence production in feldspars remain poorly understood. The recently developed method of infrared photoluminescence (IRPL) excitation-emission spectroscopy has revealed the presence of two electron trapping centres emitting IRPL at 1.3 eV and 1.41 eV (IRPL1.3 and IRPL1.41 centres), and it enables detailed investigations into the ground and excited state energies of these centres. Here we make measurements of a range of single crystal alkali feldspars to understand the effects of feldspar chemical composition, crystal structure and framework disorder on the physical characteristics of IRPL1.3 and IRPL1.41 electron trapping centres. Measurements of our sample suite reveals IRPL and IR-radioluminescence (RL) emissions at 1.41 eV, 1.3 eV and, for the first time, at 1.2 eV. Our results show that whilst the IRPL1.3 trapping centre is unaffected by the M site cation occupancy, the presence of IRPL1.41 trapping centres seems to be linked to the presence of K+ ions on M sites. However, no clear trends in IRPL and IR-RL emission energies and signal intensities with chemical composition of the samples were found. Exploring the effect of framework disorder on IRPL1.3 and IRPL1.41 emissions revealed no significant changes to IRPL and IR-RL emission energies or ground state energies of the trapping centres, suggesting that the corresponding defects are not located on bridging O ions. Variations in ground state energies across the whole sample suite range from 2.04 eV to 2.20 eV for the IRPL1.3 centre and from 2.16 eV to 2.46 eV for the IRPL1.41 centre. Variations in trap depth seem to be driven by other factors than sample chemistry, degree of Al3+ disorder and number of phases present in a single crystal feldspar. Interestingly, the IR resonance peak is invariant between samples. Regarding the use of IRPL in luminescence dating, we show that optical resetting differs for the three different emissions, with the emission at ∼1.41 eV not being reset in some samples even after 18 h of solar bleaching.

Bias-modulated van der Waals heterojunction photodetector of graphene nanosheets embedded carbon film/n-Si

The breakthrough of a bias-modulated van der Waals heterojunction photodetector based on graphene nanosheets embedded carbon (GNEC) film/n-type silicon was reported. An ultra-high responsivity of 92.58 A/W and fast response time of 12.5 ns were obtained from the bias modulated GNEC film/n-Si photodiode prepared by electron-assistant sputtering system. The excellent performance comes from the high-density edges as electron trapping centers in vertically grown GNs inside GNEC film. The electron trapped at edges increases the GNEC Fermi level and reduces the recombination rate of e-h pairs during photoexcitation. Moreover, the specific detectivity of GNEC/n-Si heterojunction device is achieved of 2.93 × 1014 Jones at bias = 0 V. This work has advancements in the development and application of ultra-sensitive devices of 2D materials.

Recent progress on Ag/TiO2 photocatalysts: photocatalytic and bactericidal behaviors.

For many decades, titanium dioxide (TiO2) semiconductor has been extensively applied in several environmental applications due to its higher photocatalytic performances toward different organic pollutants, pharmaceutical compounds, and bacteria. However, its shortfall response to visible light, and the expeditious recombination rate of the photogenerated electron–hole pairs, hampers its utilization. Doping TiO2 semiconductor with silver nanoparticles is a sound strategy to (1) extend its photocatalytic activity to visible light, (2) prevent the electron/holes pairs recombination due to the formation of the Schottky barrier at the interfaces with TiO2 that act as an electron-trapping center, and (3) enhance its bactericide performances. This review focuses on the recent progress on silver-doped titanium dioxide (Ag/TiO2)-based photocatalysts. It addresses a wide range of Ag/TiO2 synthesis techniques, their physicochemical properties and discusses thoroughly the important role of silver (Ag) nanoparticles in enhancing the removal capacity and antibacterial performances of the Ag/TiO2 photocatalysts.

Flexible CuS/TiO2 based composites made with recycled bags and polystyrene for the efficient removal of the 4-CP pesticide from drinking water

• Recycled Bags from supermarket and polystyrene from food packaging were employed to make flexible photocatalytic composites. • The flexible composites degraded 4-CP under UV–VIS irradiation. • The recycled materials were coated by CuS/TiO 2 nanoparticles (CuST NPs) for the photocatalytic degradation of 4-CP. • A total degradation of 4-CP was obtained by using the composite made with recycled bags and CuST NPs. • The CuST powder and the composite made with recycled polystyrene produced maximum degradation of 92 and 89% after 4 h. We report the use of flexible photocatalytic composites for the degradation of the 4-chlorophenol (4-CP) contaminant from the drinking water. These composites were made of recycled bags (RBag) or recycled polystyrene (RPS) from food packaging and CuS/TiO 2 (CuST) nanoparticles. The structural and morphological characterization indicate that the CuST nanoparticles have an average size of size of 41 ± 2 nm and present a mixture of anatase/brookite phases. The CuS/TiO 2 powders were dispersed in water contaminated with 4-CP and produced a maximum degradation percentage of 92% after 4 h under UV–VIS irradiation. In the case of the RBag-CuST and RPS-CuST composites, they floated on the contaminated water and produced maximum degradation percentages of 89 and 100%, respectively. After 3 cycles of continuous use, the degradation percentages decreased from 92 to 84%, from 89 to 81% and from 100 to 95% for the CuS/TiO 2 powders, RPS-CuST and RBag-CuST composites, respectively. Scavenger experiments were carried out and found that the main oxidizing agent for the degradation of 4-CP was the OH radical. The production of such radical decreased in the order RBag-CuST > CuS/TiO 2 -powders > RPS-CuST. The RBag-CuST was more efficient for the degradation of 4-CP because its production of oxidizing agents was the highest and it contained more oxygen vacancies defects (electron trapping centers) than the other photocatalysts, which was confirmed by the absorbance measurements. In general, we demonstrated that making flexible photocatalytic materials from recycled bags/polystyrene is a feasible and low cost option to remove pesticide contaminants from the drinking water. Those composites can be removed manually from the cleaned water, which is not possible with the conventional photocatalytic powders.

Fabrication, characterization of O doped g-C3N4 materials via a green ascorbic acid-assisted calcination route

Using melamine as raw material, metal-free g-C3N4 photocatalyst was synthesized by thermal polymerization method. After grinding the produced g-C3N4 and ascorbic acid (Vitamin C, Vc), the mixture was calcined at 300 °C to obtain rich O doped g-C3N4 (O-g-C3N4) materials. Some physical chemistry technologies, e.g. XRD, TG-DSC, FT-IR, Raman, XPS, SEM, TEM, BET, UV–vis DRS, PL, and photoelectrochemical test were used to characterize the catalyst. The results show that O-g-C3N4 possesses high O content (~10 wt%) and displays no obvious morphological change compared to g-C3N4. However, after O doping, its band gap energy was significantly reduced and its visible light absorption ability was largely boosted. In addition, the CO bond generated by O doping can act as a strong electron trap centers and effectively promote the separation of photo-generated electrons and holes pair. The prepared O-g-C3N4 displayed high performance for aqueous Cr(VI) removal. Under xenon lamp illumination for 150 min, over 90%Cr(VI) can be reduced over O-g-C3N4. Moreover, in Cr(VI) removal, O-g-C3N4 appeared high photocatalytic stability and high compatibility with common anions (SO42−, CO32−, Cl−) and cations (Ca2+, Na+, K+) in water.

Improving the visible-light photocatalytic activity of Bi2O4 by the combination of the p–n junction and oxygen vacancy strategy for the degradation of organic pollutants

Constructing p–n heterojunction has been recognized as an effective solution to facilitate the separation of photogenerated carriers through the internal electric field. The introduction of oxygen vacancies (OVs) on the surface of photocatalyst can produce an intermediate band (IB) below the conduction band (CB) of photocatalyst and act as electron trap centers. The catalytic activity of photocatalyst can be improved by adopting p–n heterojunction or OVs strategy, respectively. In this paper, a novel high-performance BiOCl-OVs/Bi2O4 p–n heterostructure, combining the advantages of the above two strategies, was successfully synthesized via a facile mechanical stirring method. The structure, morphology, and photoelectrochemical properties of the samples were studied in detail. The obtained BiOCl-OVs/Bi2O4 catalysts show enhanced visible-light photocatalytic activity compared with pure Bi2O4. The apparent rate constant of the optimal BiOCl-OVs/Bi2O4-2/5 sample is 1.66 and 2.71 times higher than that of Bi2O4 for the degradation of methyl orange (MO) and bisphenol A (BPA), respectively. The improved photocatalytic performance of BiOCl-OVs/Bi2O4 catalyst is mainly attributed to the combined p–n heterojunction effect and OVs formed on the surface of BiOCl. Finally, a probable photocatalytic reaction mechanism was proposed.

Effect of morphology on the hydrogen production of V2O5 nano/micro-particles synthesized by a biodegradable template

We report in this work the morphological, structural and hydrogen generation properties of V2O5 (VO) synthesized by a wet chemical method. We obtained morphologies of nanoplates and nanopillars by using sunflowers’ petals and the center of sunflower (disk florets) as biodegradable templates during the synthesis. A morphology of microbelt is obtained in absence of a biodegradable template in the synthesis. According to the X-ray patterns, the microbelts presented a pure orthorhombic phase, while the nanoplates and nanopillars had a mixture of monoclinic/orthorhombic phases. The monoclinic phase corresponded to VO2 and its content was below 3%. The performance of the three morphologies was evaluated for hydrogen generation under simulated solar light and obtained a maximum hydrogen generation rate of 65.5 μmol g−1 h−1 for the nanopillars. In general, the hydrogen generation decreased in the order: VO-nanopillars > VO-nanoplates > VO-microbelts. The morphology on nanopillars presented the highest hydrogen generation because it had the highest absorption of light as well as the highest content of defects (oxygen vacancies), which acted as electron trapping centers that delayed the electron-hole recombination. The presence of such defects in the VO with different morphologies was confirmed by photoluminescence measurements, since a red emission at 650 nm was observed. Hence, this study demonstrates that finding the optimum morphology for VO with the highest hydrogen production is relevant, since it can be employed to make nanocomposites with several heterojunctions, leading to higher hydrogen production rates.

Preparation of S-doped g-C3N4 with C vacancies using the desulfurized waste liquid extracting salt and its application for NOx removal

In this study, graphitized carbon nitride photocatalysts were successfully prepared by using the desulfurized waste liquid extracting salt from coking plants. The NOx removal efficiency of g-C3N4 synthesized using ammonium thiocyanate (NH4SCN), ammonium thiosulfate ((NH4)2S2O3) and ammonium sulfate ((NH4)2SO4) with different mass ratios were evaluated. The results showed that due to the addition of (NH4)2S2O3 and (NH4)2SO4 a large amount of sulfur-containing gas was released during pyrolysis, which significantly increased the specific surface area and pore volume of the photocatalysts. The release of sulfur-containing gas also resulted in the incorporation of sulfur, and due to this doping process the band gap of the photocatalyst was widened to 2.94 eV, thus enhancing its visible light response. C vacancies were formed in the hepazine ring, which increased the electron density around the N–(C)3 bridge bond and formed an electron trapping center, thus inhibiting the recombination of charge carriers. The photocatalysts had increased O2 adsorption capacity and increased number of amino groups on their surface. This facilitated the rapid generation and consumption of reactive oxygen species, inhibited the accumulation of NO2 on the surface, reduced the production of NO2, thus improving the NOx removal efficiency of the photocatalysts. The principle of using “waste for waste control” was achieved, and a new method for nitrogen oxide removal was provided.

Oxygen vacancy induced electron traps in tungsten doped Bi2MoO6 for enhanced photocatalytic performance

As the concentration of the W dopant increased in the Bi2Mo1−xWxO6 nanosheets, the density of the oxygen vacancies became higher, which served as electron trap centers to lower the recombination rate and enhance the photocatalytic performance.

Enhancing the hydrogen generation of TiO2 nanoparticles by decorating its surface with BiI3 and PbI2 quantum dots

This work reports the performance of TiO2/BiI3 and TiO2/PbI2 nanocomposites for hydrogen generation. BiI3 and PbI2 quantum dots (QDs) were grown on TiO2 (P25 Degussa) using a fast injection method. According to the analysis by X-ray diffraction, the nanocomposites have a mixture of anatase, rutile and cubic phases from TiO2, BiI3 and PbI2. The images obtained from transmission electron microscopy revealed that the TiO2 support have sizes in the range of 70–220 nm while the QDs of BiI3 and PbI2 (co-catalysts) grown on TiO2 have sizes in the range of 12–17 nm. The presence of these iodides on TiO2 created oxygen vacancies defects (confirmed by photoluminescence measurements) that extended the light absorption of TiO2 from the UV to the VIS range. According to the results from the photocatalytic experiments for hydrogen generation (achieved using pure water and UV-VIS light), the hydrogen generation rates produced by the TiO2/BiI3 and TiO2/PbI2 nanocomposites were 437–580 times, 81–108 times and 21–30 times, higher than these for pure TiO2, PbI2 and BiI3, respectively. The maximum hydrogen generation rates of the TiO2/BiI3 and TiO2/PbI2 nanocomposites were 290.7 and 219.2 μmol h−1 g−1, respectively. In addition, the TiO2/BiI3 and TiO2/PbI2 nanocomposites contained defects that acted as electron trapping centers, which in turn, delayed the electron-hole recombination and this favored the photocatalytic generation of H2. Moreover, the heterojunction formed between the TiO2 and the iodides allowed the transfer of electrons from the conduction band of TiO2 toward the conduction band of the iodides, creating a “sink” for the electrons which delayed the electron hole recombination. The results presented here demonstrated that the deposition of iodide co-catalyst on TiO2 is a feasible option to enhance the hydrogen generation.

Insight into enhanced thermoluminescence property of (Mg, Cu, Ag)-Doped LiF:A DFT study

Thermoluminescence dosimeter (TLD) has been shown to be one of the most popular devices to measure absorbed radiation dose. Increasing applications of radiation in our day-to-day life motivates to develop efficient phosphor materials for TLD. LiF based TLD has attracted immense interest in the area of medical and industrial applications of radiation. Present study explored the origin of recent experimental observation of improvement of TL sensitivity of LiF due to doping with Mg, Cu and Ag. For this purpose, we have systematically investigated the geometry and electronic structure of LiF in absence and presence of individual dopant element as well as in combination using density functional theory as a tool. Present study indicates that the relative defect formation energy for doping with individual element and in combination is strongly dependent on the chemical environment. Our study reveals that the requirement of a good TLD material, i.e., electron trapping, hole trapping, and luminescent centers is not satisfied in presence of only individual dopant element. Interestingly, this condition is nicely fulfilled by simultaneous presence of all the three dopant elements. Each dopant element is found to play specific role in creating different types of trapping centers. Present study also explored the underlying mechanism of the TL process in (Mg, Cu, Ag)-doped LiF. To explain the origin of experimentally observed absorption and emission spectrum, we have calculated thermodynamic transition levels (TTLs), and optical transition levels for different doped systems. Finally, the present study predicts the emission range of (Mg, Cu, Ag)-doped LiF, which is known to be very much crucial for selecting the appropriate light detection system.

Anchoring Ni single atoms on sulfur-vacancy-enriched ZnIn2S4 nanosheets for boosting photocatalytic hydrogen evolution

The photocatalytic H 2 evolution performance of ZnIn 2 S 4 are significantly enhanced by the synergistic effect of sulfur vacancy and Ni single atom. Structure manipulation of photocatalysts at an atomic scale is a promising way to improve its photocatalytic performance. Herein, we realize the anchoring of single Ni atoms on the ZnIn 2 S 4 nanosheets with rich sulfur vacancies. Experimental results demonstrate that single Ni atoms induce the formation of Ni-O-M (Zn/In) atomic interface, which can efficiently promote the carriers separation and prolong the carrier life time. In addition, in situ electron spin resonance spectroscopy (ESR) confirms that the single Ni atoms act as an electron trapping center for protons reduction. As a result, the single Ni atoms decorated ZnIn 2 S 4 nanosheets with rich sulfur vacancies (Ni/ZnIn 2 S 4 -RV s ) shows a hydrogen evolution rate up to 89.4 μmol h −1 , almost 5.7 and 2.3 times higher compared to that of ZnIn 2 S 4 nanosheets with poor sulfur vacancies and rich sulfur vacancies (denoted as ZnIn 2 S 4 -PV s and ZnIn 2 S 4 -RV s ). This work opens up a new perspective manipulating the single-atom cocatalyst and sulfur vacancy on sulfide supports for improving photocatalytic hydrogen evolution.

A one-pot sealed ammonia self-etching strategy to synthesis of N-defective g-C3N4 for enhanced visible-light photocatalytic hydrogen

Photocatalytic hydrogen (H2) evolution from water is considered as a prospective approach, which can convert inexhaustible solar energy into chemical energy to alleviate energy crisis and environmental problems. Herein, the N-defective g-C3N4 with porous structure was firstly synthesized in a sealed crucible by one-step thermal polymerization method. The experimental data showed that the yield of the catalyst was obviously increased under sealing condition. Moreover, the N-defective g-C3N4 prepared from urea precursor under sealed condition reached an optimum photocatalytic H2 production rate of 597.4 μmol/h and an apparent quantum efficiency of 15.6% at wavelength of 420 nm. This enhanced photocatalytic H2 production performance is mainly ascribed to the introduction of N-defects, which not only extended of the visible light absorption, but also acted as the electron trap centers to suppress the recombination of the photogenerated electron and hole pairs. This work offers one-step facile strategy for the introduction of N-defects to prepare N-defective g-C3N4 with superior photocatalytic activity, which is also a great substitute for the high-energy consuming and complicated synthetic routes.

Solar active Cu2+-ZnS Photocatalyst for efficient photodegradation of 4-chlorophenol: effective cation doping effect

Cu2+ doped ZnS nanoparticles (Cu/ZnS) were prepared through a modified chemical precipitation method. The nanoparticles were characterized using UV–vis, PL, XRD, XPS, N2 sorpometry and TEM techniques. The nanoparticles morphology was spherical in shape with a particle size of ∼11.8 nm and ∼13.4 nm for bare and Cu2+ doped ZnS, respectively. Upon doping Cu2+ (0–5 mol %) into ZnS, tuning of band gap from 3.93 to 2.72 eV along with a remarkable enhancement in surface area from 89.5–152.7 m2. g−1 were successfully achieved. Moreover, Cu2+ act as electron trapping centers, which hinder the recombination of the photogenerated electrons and holes as evidenced from the PL intensity behavior. The photocatalytic experiment by degrading 4-chlorophenol (4-CP) under natural solar radiation indicated that the photodegradation efficiency of Cu2+ doped ZnS nanoparticles increased to 91.2 % compared to 22 % for undoped ZnS nanoparticles. Total organic carbon (TOC) analysis was performed to assess the mineralization ability of the Cu2+ doped ZnS nanoparticles which gave 91.5 % TOC removal. For in-depth understanding of the reasons behind Cu2+ doped ZnS nanoparticles photoreactivity, the revealing of reactive oxidative species (ROS) was achieved through scavenging trapping experiments.

Understanding the metastable states in K-Na aluminosilicates using novel site-selective excitation-emission spectroscopy

Optically stimulated luminescence (OSL) and thermoluminescence (TL) from the metastable states in solids are widely used in luminescent phosphors, dosimetry, geochronology, and thermo- and photo-chronometry. OSL and TL result from a combination of three different processes (charge detrapping, transport, and recombination) and are, therefore, not ideal for characterizing the charge trapping states. Therefore, despite many decades of research, the OSL and TL kinetics and the associated defect systems remain poorly understood in natural minerals. Recently, a radio-photoluminescence (RPL) signal has been discovered in feldspar (K-Na-Ca aluminosilicates occupying > 50% of Earth’s crust) which helps overcome this limitation. This site-selective signal termed infrared photoluminescence (IRPL) arises from radiative relaxation of the excited state of the main electron trapping center (principal trap).In this study, IRPL excitation and emission spectroscopy at cryogenic temperatures reveals two distinct electron-trapping centers (i.e. two principal traps) in feldspar, and helps to determine their trap depths and the excited-state energies. The two trapping centers show the same electron capture cross-sections and the excited-state relaxation lifetimes, but different ground- and excited-state energies. Based on this peculiar combination of trap characteristics, we conclude that that the principal traps consist of the same defect residing at two different crystal sites. The differences in the energy levels of the two principal traps explain their distinct optical and thermal bleaching behavior.

Luminescence dynamics of MgGa2O4 prepared by solution combustion synthesis

Magnesium gallate (MgGa2O4) was synthesized by solution combustion synthesis method at 500 °C. The X-ray diffraction (XRD) pattern indicates that the prepared compound crystalized into a single phase, and a porous morphology associated with combustion was observed from the image of the scanning electron microscopy (SEM). A diffuse reflectance spectrum was recorded using the UV–Vis–NIR absorption spectrometer, which revealed the band-to-band absorption edge, from which the band gap of the material was approximated using the Kubelka-Munk relation. The photoluminescence (PL) emission displayed a broad blue emission from 350 to 600 nm, when the sample was excited with a 235 nm beam. The thermoluminescence (TL) spectroscopy was used to prove that the associated kinetics in trapping and de-trapping of electrons by the electron trapping centres were of general order. The corresponding depths of the dominant electron trapping centres were approximated to 0.65 ± 0.01eV to 1.23 ± 0.01eV. Using the general gradient approximation (GGA-DFT) as implemented within the CASTEP code, we found that the MgGa2O4 compound has energy levels close to the conduction band, attributed to oxygen vacancies.

Effect of Pt loading on the hydrogen production of CNT/Pt composites functionalized with carboxylic groups

This work reports the morphological and photocatalytic hydrogen generation properties of CNT/Pt composites with and without functionalization by carboxylic/oxygen groups. The composites with and without functionalization were named f-CNT/Pt and CNT/Pt, respectively. Several f-CNT/Pt and CNT/Pt composites with different content of Pt NPs (from 0 to 30 wt%) were synthesized and analyzed by scanning electron microscopy (SEM). Those images revealed that the composites without functionalization presented higher agglomerations of Pt nanoparticles (NPs). Furthermore, the average sizes of the Pt NPs in the named f-CNT/Pt composites (2.3–2.9 nm) were lower than these in the CNT/Pt composites (2.5–3.1 nm). The hydrogen generation rates were also calculated from the decomposition of pure water under UV irradiation (365 nm) and found maximum values of 45.4 and 193.9 μmol·h−1 g−1 for the CNT/Pt and f-CNT/Pt composites (they contained 20 wt% of Pt NPs), respectively. Additional experiments for hydrogen generation were achieved using sodium sulfite as sacrificial agent; in this case, a maximum value of 13850 μmol·h−1 g−1 was obtained for the f-CNT/Pt composite. The f-CNT/Pt composites produced more hydrogen than the CNT/Pt composites because they presented higher content of defects; this was confirmed by the Raman spectra. We also showed that the Pt NPs acted as electron trap centers, which delayed the recombination of the photogenerated electrons and holes, this in turn, enhanced the hydrogen generation rates of the composites (the hydrogen generation was maximized by varying the content of Pt NPs deposited on the CNTs). The CNT/Pt composites presented here were simpler and easier to synthesize than the previous published ternary systems based on TiO2, CNTs and Pt NPs.