Photoluminescence study of ZnO doped with nitrogen and arsenic

In this work, the optical properties of ZnO doped with arsenic and nitrogen were studied. The ZnO samples were grown by Metalorganic Chemical Vapor Deposition (MOCVD). The solubility of nitrogen in the ZnO films, as well as its activation upon annealing, was also investigated. Hydrogen is known as a major source for passivation of the acceptors in ZnO:N. Therefore, it is crucial to dissociate the complex(es) formed by nitrogen and hydrogen and diffuse out the hydrogen in order to prevent the reformation of such complexes. High temperatures (≥ 600 C) are required for these purposes. In order to effectively remove the hydrogen impurities from the sample, it is important to know the optical fingerprints of hydrogen and its thermal stability. Therefore, the effects of annealing and hydrogen plasma treatment on bulk ZnO (hydrothermally grown) were first studied. The use of bulk material for this purpose was motivated by the well-resolved photoluminescence (PL) lines observed for bulk ZnO, which allow the identification of the different lines related to hydrogen after plasma treatment. Annealing at 850 C was effective for the removal of most of the hydrogen related transitions in the near-band-edge emission. Also, additional transitions at ~3.364 eV and ~3.361 eV were observed after hydrogen plasma treatment, which were ascribed to hydrogen-Zn vacancy complexes. In this work, a comparative study of the annealing ambient and temperature on ZnO films grown on GaAs substrate, using diethyl zinc (DEZn) and tertiary butanol (TBOH), showed that arsenic diffuses in the ZnO films and gives a shallow level in the band gap, which is involved in an acceptor-bound exciton line at 3.35 eV. This shallow level is visible when annealing is performed in oxygen, but not when annealing is performed in nitrogen, and indeed only for annealing temperatures around 550 C. However, annealing in either ambient also causes zinc to diffuse from the ZnO films into the GaAs substrate, rendering the electrical properties deduced from Hall measurements ambiguous. For ZnO:N, NO was used as both oxygen and nitrogen sources. Monitoring the concentration of nitrogen, carbon and hydrogen in the ZnO films, the formation of different complexes from these impurities were deduced. Furthermore, an investigation of the effect of annealing on the concentrations of impurities showed that their out- diffusion was strongly dependent on the crystalline quality of the ZnO films. For porous ZnO films, obtained at low growth temperatures (≤310 C), the out-diffusion of impurities was efficient, whereas for films grown at higher temperatures, which have improved crystalline quality, the out-diffusion was practically nonexistent. The out-diffusion of unwanted impurities may activate the nitrogen dopant in the ZnO films, as was confirmed by the PL measurements on the different samples grown at different temperatures. PL transitions at ~3.24 eV and ~3.17 eV were related to substitutional NO. These transitions were more dominant in the spectra of samples grown at low temperatures. An additional transition at ~3.1 eV was assigned to a donor-acceptor pair transition involving VZn, instead of NO, as previously reported

Asymmetric supercapacitor based on activated expanded graphite and pinecone tree activated carbon with excellent stability

This work presents results obtained from the production of low-cost carbons from expanded graphite (EG) and pinecone (PC) biomass, activated in potassium hydroxide (KOH) and finally carbonized in argon and hydrogen atmosphere. A specific surface area of 808 m2 g−1 and 457 m2 g−1 were measured for activated pinecone carbon (APC) and activated expanded graphite (AEG), respectively. The electrochemical characterization of the novel materials in a 2-electrode configuration as supercapacitor electrode shows a specific capacitance of 69 F g−1 at 0.5 A g−1, high energy density of 24.6 W h kg−1 at a power density of 400 W kg−1. This asymmetric supercapacitor also exhibits outstanding stability after voltage holding at the maximum voltage for 110 h, suggesting that the asymmetric device based on different carbon materials has a huge capacity for a high-performance electrode in electrochemical applications.The National Research Foundation of South Africa (Grant Number: 61056).http://www.elsevier.com/locate/apenergy2018-12-01hj2018Electrical, Electronic and Computer Engineerin

Morphological characterization and impedance spectroscopy study of porous 3D carbons based on graphene foam-PVA/phenol-formaldehyde resin composite as an electrode material for supercapacitors

The design and fabrication of porous electrode materials is highly desirable for improving the performance of electrochemical supercapacitors (ECs) and thus, it is important to produce such porous materials in large quantities. In this study, we used a microwave method to produce porous carbonaceous materials designated as graphene foam/polyvinyl alcohol/formaldehyde (GF/PVA/F) and graphene foam-polyvinyl alcohol/phenol-formaldehyde (GF/PVA/PF) from graphene foam, phenol formaldehyde and polyvinyl alcohol (PVA). Scanning electron microscopy (SEM), Raman spectroscopy and Fourier-Transform Infrared Spectroscopy (FTIR) were used to characterize the surface morphology, structural defects and functional groups of the materials respectively. Based on these porous materials, the two symmetrical ECs fabricated exhibited a specific capacitance in the range of 0.62–1.92 F cm 2, phase angles of 81 and 84 and resistor–capacitor (RC) relaxation time constants of 4 and 14 seconds. The physicochemical properties of the electrolyte ion (diffusion) and its influence on the capacitive behavior of the porous materials were elucidated. These encouraging results demonstrate the versatile potential of these porous materials (GF/PVA/F and GF/PVA/PF) in developing high energy storage devices.South African Research Chairs Initiative of the South African Department of Science and Technology (SARCHi-DST), the National Research Foundation (NRF) and the University of Pretoria.http://www.rsc.org/advanceshb201

P3HT:PCBM/nickel-aluminum layered double hydroxide-graphene foam composites for supercapacitor electrodes

In this paper, a simple dip-coating technique is used to deposit a P3HT:PCBM/Nickel Aluminum layered double hydroxide-graphene foam (NiAl-LDH-GF) composite onto a nickel foam (NF) serving as a current collector. A self-organization of the polymer chains is assumed on the Ni-foam grid network during the slow “dark” drying process in normal air. Electrochemical cyclic voltammetry (CV) and constant charge-discharge (CD) measurements show an improvement in the supercapacitive behavior of the pristine P3HT:PCBM by an order of magnitude from 0.29 F cm-2 (P3HT:PCBM nanostructures) to 1.22 F cm-2 (P3HT:PCBM/NiAl-LDH-GF composite structure) resulting from the addition of NiAl-LDH-GF material at a current density of 2 mA cm-2. This capacitance retention after cycling at 10 mA cm-2 also demonstrates the electrode material’s potential for supercapacitor applications.South African Research Chairs Initiative (SARChI) of the Department of Science and Technology (DST) and the National Research Foundation (NRF).http://link.springer.com/journal/100082015-08-30hb201

Functionalized graphene foam as electrode for improved electrochemical storage

We report on a non-covalent functionalization of graphene foam (GF) synthesized via chemical vapour deposition (CVD). The GF was treated with pyrene carboxylic acid (PCA) which acted as a source of oxygen and/or hydroxyl groups attached to the surface of the graphene foam for its electrochemical performance improvement. The modified graphene surface enabled a high pseudocapacitive effect on the GF. A specific capacitance of 133.3 F g−1, power density ∼ 145.3 kW kg−1 and energy density ∼ 4.7 W h kg−1 were achieved based on the functionalized foam in 6 M KOH aqueous electrolyte. The results suggest that non-covalent functionalization might be an effective approach to overcome the restacking problem associated with graphene electrodes and also signify the importance of surface functionalities in graphene-based electrode materials.South African Research Chairs Initiative of the Department of Science and Technology (SARCHi-DST), the National Research Foundation (NRF) and University of Pretoria.http://link.springer.com/journal/100082015-09-30hb201

Renewable pine cone biomass derived carbon materials for supercapacitor application

The environmental degradation and hazard to human life caused by the depletion of fossils fuels and the urgent need for sustainable energy sources to meet the rising demand in energy has led to the exploration of novel materials that are environmentally friendly, low cost and less hazardous to human life for energy storage application using the green chemistry approach. Herein, we report on the transformation of the readily abundant pine cone biomass into porous carbon via KOH activation and carbonization at 800 C as electrode materials for supercapacitors. The porous carbon material exhibited a mesoporous framework with a specific surface area of 1515 m2 g 1, a high voltage window of 2.0 V, a gravimetric capacitance of 137 F g 1, energy density of 19 W h kg 1 and excellent cyclability in neutral 1 M Na2SO4 electrolyte for a symmetric carbon/carbon electrode cell. The result shows that the material is robust and shows great promise with neutral electrolytes in high-performance energystorage devices.South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation of South Africa (Grant No. 97994).http://www.rsc.orgadvances2017-07-31hb2016Institute of Applied MaterialsPhysic

Symmetric supercapacitors based on porous 3D interconnected carbon framework

The construction and design of novel porous carbons for electric double-layer capacitors (EDLCs) application to meet the increasing demand and supply of energy is eminent. This is important because the pore volume (PV)/micropore volume (MV) in the porous network architecture of the carbon is mostly responsible for the ion traps in energy storage. Three dimensional carbon materials based on graphene materials with relatively high specific surface area (SSA) represents a promising material candidate for EDLCs applications. In this work, we synthesized highly porous carbon from graphene foam (GF) and polyvinyl alcohol PVA as a sacrificial template, and investigate their performance as electrodes for EDLCs applications. The as-produced carbons present a fairly large surface area (502 m2 g−1), and a highly porous interconnected framework with mesopore walls and micropore texture which are suitable as electrode for energy storage. As electrode material in a symmetric configuration the activated graphene foam (AGF) showed a specific capacitance of 65 F g−1, energy density of 12 Wh kg−1, power density of 0.4 kW kg−1, good rate performance and excellent long term stability in 1 M Na2SO4 aqueous with no capacitance loss after 3000 cycles.A. Bello acknowledges University of Pretoria and NRF financial support for his Postdoc fellowship, while D. Y. Momodu and F. Barzegar acknowledge financial support from University of Pretoria and the NRF for PhD bursaries.http://www.elsevier.com/locate/electactahj201

Electrochemical analysis of Co3(PO4)2·4H2O/graphene foam composite for enhanced capacity and long cycle life hybrid asymmetric capacitors

In this paper, we explore the successful hydrothermal approach to make Co3(PO4)2·4H2O/GF micro-flakes composite material. The unique sheet-like structure of the graphene foam (GF) significantly improved the conductivity of the pristine Co-based material, which is a key limitation in supercapacitors application. The composite electrode material exhibited superior capacitive conduct in 6 M KOH aqueous electrolyte in a 3-electrode set-up as compared to the pristine cobalt phosphate material. The material was subsequently adopted as a cathode in an asymmetric cell configuration with carbonization of Fe cations adsorbed onto polyaniline (PANI) (C-Fe/PANI), as the anode. The Co3(PO4)2·4H2O/GF//C-FP hybrid device showed outstanding long life cycling stability of approximately 99% without degradation up to 10000 cycles. A specific energy density as high as 24 W h kg−1, with a corresponding power density of 468 W kg−1 was achieved for the device. The results demonstrated the efficient utilization of the faradic-type Co3(PO4)2·4H2O/GF composite along with a functionalized carbonaceous electric double layer (EDL)-type material to produce a hybrid device with promising features suitable for energy storage applications.The South African Research Chairs Initiative (SARChI) of the Department of Science and Technology and the National Research Foundation (NRF) of South Africa (Grant No. 61056). Abdulmajid A. Mirghni acknowledges the financial support from University of Pretoria, the NRF through the SARChI in Carbon Technology and Materials, and also Al Fashir University, Sudan.http://www.elsevier.com/locate/electacta2019-09-01hj2018Physic

Pulsed laser deposited Cr2O3 nanostructured thin film on graphene as anode material for lithium-ion batteries

Pulsed laser deposition technique was used to deposit Cr2O3 nanostructured thin film on a chemical vapour deposited few-layer graphene (FLG) on nickel (Ni) substrate for application as anode material for lithium-ion batteries. The experimental results show that graphene can effectively enhance the electrochemical property of Cr2O3. For Cr2O3 thin film deposited on Ni (Cr2O3/Ni), a discharge capacity of 747.8 mA h g-1 can be delivered during the first lithiation process. After growing Cr2O3 thin film on FLG/Ni, the initial discharge capacity of Cr2O3/FLG/Ni was improved to 1234.5 mA h g-1. The reversible lithium storage capacity of the as-grown material is 692.2 mA h g-1 after 100 cycles, which is much higher than that of Cr2O3/Ni (111.3 mA h g-1). This study reveals the differences between the two material systems and emphasizes the role of the graphene layers in improving the electrochemical stability of the Cr2O3 nanostructured thin film.This work was sponsored within the framework of the UNESCO UNISA Africa Chair in Nanosciences & Nanotechnology and the Nanosciences African Network (NANOAFNET) by the National Research Foundation of South Africa, the African Laser Centre (ALC), the University of South Arica (UNISA) in collaboration with the Vice-Chancellor of the University of Pretoria, the National Research Foundation (NRF) of South Africa, iThemba LABS and the Abdus Salam ICTP-Trieste, Italy.http://www.elsevier.com/locate/jalcom2016-07-31hb2016Physic

Preparation and characterization of porous carbon from expanded graphite for high energy density supercapacitor in aqueous electrolyte

In this work, we present the synthesis of low cost carbon nanosheets derived from expanded graphite dispersed in Polyvinylpyrrolidone, subsequently activated in KOH and finally carbonized in Ar atmosphere. Interconnected sheet-like structure with low concentration of oxygen (9.0 at.%) and a specific surface area of 457 m2 g-1 was obtained. The electrochemical characterization of the carbon material as supercapacitor electrode in a 2-electrode configuration shows high specific capacitance of 337 F g-1 at a current density of 0.5 A g-1 as well as high energy density of 37.9 Wh kg-1 at a power density of 450 W kg-1.This electrical double layer capacitor electrode also exhibits excellent stability after floating test for 120 h in 6 M KOH aqueous electrolyte. These results suggest that this activated expanded graphite (AEG) material has great potential for high performance electrode in energy storage applications.South African Research Chairs Initiative (SARChi) in Carbon Technology and Materials of the Department of Science and Technology (DST) and the National Research Foundation (NRF).http://www.elsevier.com/locate/cose2017-03-31hb2016Institute of Applied MaterialsInsurance and Actuarial ScienceInvestment and Trade Policy Centre (ITPC)JournalismJurisprudenceLaboratory for Microscopy and MicroanalysisLaw ClinicLaw Student AffairsLife Cycle EngineeringMammal Research InstituteMapungubwe museumMarketing and Communication ManagementMarketing ManagementMaterials Science and Metallurgical EngineeringMathematics and Applied MathematicsMaxillo-Facial and Oral SurgeryMechanical and Aeronautical EngineeringMedical MicrobiologyMedical OncologyMedical VirologyMercantile LawMicrobiology and Plant PathologyMining EngineeringModern European LanguagesMusicNeurologyNeuroscience ResearchNew Testament StudiesNuclear MedicineNursing ScienceObstetrics and GynaecologyOccupational TherapyOdontologyOld Testament StudiesOral Pathology and Oral BiologyOrthodonticsOrthopaedic SurgeryOtorhinolaryngologyPaediatrics and Child HealthParaclinical SciencesPeriodontics and Oral MedicinePharmacologyPhilosophyPhilosophy and Ethics of Mental HealthPhysic