Exploring the Effect of Ammonium Iodide Salts Employed in Multication Perovskite Solar Cells with a Carbon Electrode

Perovskite solar cells that use carbon (C) as a replacement of the typical metal electrodes, which are most commonly employed, have received growing interest over the past years, owing to their low cost, ease of fabrication and high stability under ambient conditions. Even though Power Conversion Efficiencies (PCEs) have increased over the years, there is still room for improvement, in order to compete with metal-based devices, which exceed 25% efficiency. With the scope of increasing the PCE of Carbon based Perovskite Solar Cells (C-PSCs), in this work we have employed a series of ammonium iodides (ammonium iodide, ethylammonium iodide, tetrabutyl ammonium iodide, phenethylammonium iodide and 5-ammonium valeric acid iodide) as additives in the multiple cation-mixed halide perovskite precursor solution. This has led to a significant increase in the PCE of the corresponding devices, by having a positive impact on the photocurrent values obtained, which exhibited an increase exceeding 20%, from 19.8 mA/cm², for the reference perovskite, to 24 mA/cm², for the additive-based perovskite. At the same time, the ammonium iodide salts were used in a post-treatment method. By passivating the defects, which provide charge recombination centers, an improved performance of the C-PSCs has been achieved, with enhanced FF values reaching 59%, which is a promising result for C-PSCs, and V_oc values up to 850 mV. By combining the results of these parallel investigations, C-PSCs of the triple mesoscopic structure with a PCE exceeding 10% have been achieved, while the in-depth investigation of the effects of ammonium iodides in this PSC structure provide a fruitful insight towards the optimum exploitation of interface and bulk engineering, for high efficiency and stable C-PSCs, with a structure that is favorable for large area applications.

Development of sonophotocatalytic process for degradation of acid orange 7 dye by using titanium dioxide nanoparticles/graphene oxide nanocomposite as a catalyst

In the present study, the sonophotocatalytic degradation of acid orange 7 (AO7) dye was evaluated. The catalyst used was the titanium dioxide nanoparticles/graphene oxide (TiO₂/GO) nanocomposite, which was synthesized using the Hummers and Hoffman's method and the liquid phase deposition method. TiO₂/GO nanocomposite was characterized through the analyses of transmission electron microscopy (TEM), X-ray diffraction (XRD), Energy Dispersive X-ray (EDX) spectroscopy, Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. In addition, properties of the surface area and pore size were determined by N₂ adsorption-desorption and the Barrett-Joyner-Halenda methods. After modification, the nanocomposite properties showed successful stabilization of TiO₂ on the graphene substrate and reduction of the recombinant carrier loads. By utilizing the proposed treatment, complete degradation of AO7 could be achieved under optimal operating parameters (pH = 5, initial concentration of AO7 dye = 50 mg/L, TiO₂/GO nanocomposite dose = 0.5 g/L, UV light intensity = 36 W, ultrasonic wave intensity = 35 kHz, and reaction time = 30 min). Scavenging experiments confirmed that OH and h⁺ radicals were the predominant species in the sonophotocatalytic degradation reactions of the AO7 dye. The stability study confirmed the excellent shelf life of the TiO₂/GO nanocomposite, with only a slight reduction in the degradation efficiency of the AO7 dye (<8.27%) detected, after six consecutive cycles of the sonophotocatalytic process. Studies related to the degradability of the AO7 dye and the biodegradability of the effluent from the process showed that the applied sonophotocatalytic system was able to remove the TOC concentration by 83% after a reaction time of 30 min. Moreover, the increase in the BOD₅/COD ratio was also a confirmation for the increase in biodegradability of the treated AO7 dye effluent. Finally, the toxicity test showed that the growth inhibition rate of Escherichia coli (E. coli), as a viability index, decreased to about 7.34% after a reaction time of 180 min. This result indicated the formation of compounds with low toxicity and molecular weight over the reaction time of the sonophotocatalytic process of AO7 dye.

Graphene Oxide Concentration Effect on the Optoelectronic Properties of ZnO/GO Nanocomposites

In this work, the effects of graphene oxide (GO) concentrations (1.5 wt.%, 2.5 wt.%, and 5 wt.%) on the structural, morphological, optical, and luminescence properties of zinc oxide nanorods (ZnO NRs)/GO nanocomposites, synthesized by a facile hydrothermal process, were investigated. X-ray diffraction (XRD) patterns of NRs revealed the hexagonal wurtzite structure for all composites with an average coherence length of about 40–60 nm. A scanning electron microscopy (SEM) study confirmed the presence of transparent and wrinkled, dense GO nanosheets among flower-like ZnO nanorods, depending on the GO amounts used in preparation. Raman spectroscopy, Fourier transform infrared spectroscopy (FTIR), ultraviolet–visible (UV–Vis) absorption spectroscopy, and photoluminescence (PL) measurements revealed the impact of GO concentration on the optical and luminescence properties of ZnO NRs/GO nanocomposites. The energy band gap of the ZnO nanorods was independent of GO concentration. Photoluminescence spectra of nanocomposites showed a significant decrease in the intensities in the visible light range and red shifted suggesting a charge transfer process. The nanocomposites’ chromaticity coordinates for CIE 1931 color space were estimated to be (0.33, 0.34), close to pure white ones. The obtained results highlight the possibility of using these nanocomposites to achieve good performance and suitability for optoelectronic applications.

Microwave-Assisted Synthesis of ZnO-rGO Core-Shell Nanorod Hybrids with Photo- and Electro-Catalytic Activity

The unique two-dimensional structure and surface chemistry of reduced graphene oxide (rGO) along with its high electrical conductivity can be exploited to modify the electrochemical properties of ZnO nanoparticles (NPs). ZnO-rGO nanohybrids can be engineered in a simple new two-step synthesis, which is both fast and energy-efficient. The resulting hybrid materials show excellent electrocatalytic and photocatalytic activity. The structure and composition of the as-prepared bare ZnO nanorods (NRs) and the ZnO-rGO hybrids have been extensively characterised and the optical properties subsequently studied by UV/Vis spectroscopy and photoluminescence (PL) spectroscopy (including decay lifetime measurements). The photocatalytic degradation of Rhodamine B (RhB) dye is enhanced using the ZnO-rGO hybrids as compared to bare ZnO NRs. Furthermore, potentiometry comparing ZnO and ZnO-rGO electrodes reveals a featureless capacitive background for an Ar-saturated solution whereas for an O₂ -saturated solution a well-defined redox peak was observed using both electrodes. The change in reduction potential and significant increase in current density demonstrates that the hybrid core-shell NRs possess remarkable electrocatalytic activity for the oxygen reduction reaction (ORR) as compared to NRs of ZnO alone.

Physical Expansion of Layered Graphene Oxide Nanosheets by Chemical Vapor Deposition of Metal-Organic Frameworks and their Thermal Conversion into Nitrogen-Doped Porous Carbons for Supercapacitor Applications

Graphene oxide (GO) nanosheets show good electrical conductivity and corrosion resistance in electrochemical devices. However, strong van der Waals attraction between adjacent nanosheets causes GO materials to collapse, reducing the exposed surfaces and limiting electron/ion transport in porous electrodes. GO nanosheets mixed with Zn₅ (OH)₈ (NO₃ )₂ ⋅2 H₂ O (ZnON) nanoplates create a layered composite structure. Exposing the resultant GO/ZnON to 2-methylimidazole vapor leads to the conversion of ZnON into the zeolitic imidazolate framework ZIF-8. The transformation of ZnON into ZIF-8 leads to a huge physical expansion of the interlayer space between the GO sheets. Annealing the material at high temperature caused the ZIF-8 to be converted into highly porous nitrogen-doped carbon, but the GO nanosheets maintained a large separation and high surface area. The morphology and porous structure of the post-annealing carbon material was sensitive to the initial ratio of ZnON to GO. The optimized sample exhibited several favorable features, including a large surface area, high degree of graphitization, and a high amount of nitrogen doping. Using chemical vapor deposition of metal-organic frameworks to physically expand nanomaterials is a novel method to increase the surface area and porosity of materials. It enabled the synthesis of nanoporous carbon electrodes with high capacitance, good rate capability, and long cyclic stability in supercapacitor devices.

Improving electrochemical performance of reduced graphene oxide by counteracting its aggregation through intercalation of nanoparticles

Herein, we report the fabrication and characterization of hybrid electrode material for supercapacitor applications. CaCO₃ nanoparticles (Nps) are used as intercalator to avoid the restacking behavior of reduced graphene oxide (rGO) nanosheets. CaCO₃ Nps and rGO sheets are fabricated employing precipitation technique and microwave irradiation method, respectively. The intercalation process is performed by magnetic stirring followed by ultra-sonication technique. As prepared CaCO₃ Nps, rGO and rGO intercalated with 2.5 wt.% and 5 wt.% CaCO₃ Nps are characterized using X-ray diffraction analysis, Fourier transform infrared spectroscopy and field emission scanning electron microscopy to evaluate the crystalline characteristics, molecular vibrations, and morphology, respectively. The prepared electrode materials are coated separately on the glassy carbon electrode and their electrochemical performance displayed remarkable capacitance values for rGO nanosheets intercalated with 2.5 wt.% and 5 wt.% CaCO₃ Nps. From the obtained results, it is clear that the specific capacitance of 2.5 wt.% CaCO₃ intercalated rGO displays higher specific capacitance of 84.5 F/g at 5 mV/s with high retention stability. The mechanism behind the improvement in the electrochemical behavior is due to the increase in active surface area which is explained via Brunauer-Emmett-Teller analysis and energy-dispersive X-ray spectroscopic analysis.

Ag-doped ZnO nanorods embedded reduced graphene oxide nanocomposite for photo-electrochemical applications

In this paper, the Ag-doped zinc oxide nanorods embedded reduced graphene oxide (ZnO:Ag/rGO) nanocomposite was synthesized for photocatalytic degradation of methyl orange (MO) in the water. The microstructural results confirmed the successful decoration of Ag-doped ZnO nanorods on rGO matrix. The photocatalytic properties, including photocatalytic degradation, charge transfer kinetics and photocurrent generation, are systematically investigated using electrochemical impedance spectroscopy (EIS), photocurrent transient response (PCTR) and open circuit voltage decay (OCVD). The results of photocatalytic dye degradation measurements indicated that ZnO:Ag/rGO nanocomposite is more effective than pristine ZnO to degrade the MO dye, and the degradation rate reached 40.6% in 30 min. The decomposition of MO with ZnO:Ag/rGO nanostructure followed first-order reaction kinetics with a reaction rate constant (K a) of 0.01746 min-1. The EIS, PCTR and OCVD measurements revealed that the Ag doping and incorporation of rGO could suppress the recombination probability in ZnO by the separation of photo-generated electron-hole pairs, which leads to the enhanced photocurrent generation and photocatalytic activity. The photocurrent density of ZnO:Ag/rGO, ZnO/rGO and pristine ZnO are 206, 121.4 and 88.8 nA cm-2, respectively.

Large magnetization modulation in ZnO-based memory devices with embedded graphene quantum dots

Magnetization modulation in oxide-based resistive random-access memories facilitates their application in multifunctional memory devices and spintronics. However, the small magnetization modulation in oxide films hinders their practical applications. In this paper, we report a significant enhancement in the magnetization modulation of ZnO films upon embedding graphene quantum dots (GQDs). The magnetization-modulation ratio is greater than 500% in the ZnO-GQD hybrid films under applied biases of only 0.23/-0.20 V. This magnetization-modulation ratio is the highest value reported to date in pure or magnetic-ion-doped metal-oxide films. Further analyses indicate that the exchange of oxygen between the GQDs and ZnO, under a reversible electric field, plays an important role in enhancing the magnetization modulation. This work provides a new direction for the application of GQDs.

Resistive switching device with highly asymmetric current-voltage characteristics: a solution to backward sneak current in passive crossbar arrays

With the view towards future non-volatile random access memories that can be integrated at a large scale, extensive study on resistive switching (RS) devices arranged in a crossbar array is currently underway. Although the crossbar array architecture offers relatively simple and acceptable scalability, the presence of sneak current is recognized as a critical issue that needs to be resolved at device level. In addressing this issue, we demonstrate a new type of RS device fabricated by combining graphene oxide (G-O) and zinc oxide (ZnO) with highly asymmetric current-voltage (I-V) characteristics depending on the polarity of bias voltage. The distinctive highly asymmetric I-V characteristics result from the presence of a hetero-junction interface formed between the G-O and ZnO layers. This hetero-junction manifests resistance in the range of GΩ under both forward and reverse bias voltage when the device is in the OFF state, in contrast, when the device is in the ON state, it exhibits resistance in the range of MΩ or kΩ under forward bias and GΩ under reverse bias. We propose to employ demonstrated RS devices with highly asymmetric I-V characteristics to mitigate adverse effects of the sneak current.

In Vitro Cytotoxicity and Morphological Assessments of GO-ZnO against the MCF-7 Cells: Determination of Singlet Oxygen by Chemical Trapping

Graphene-based materials have attracted considerable interest owing to their distinctive characteristics, such as their biocompatibility in terms of both their physical and intrinsic chemical properties. The use of nanomaterials with graphene as a biocompatible agent has increased due to an uptick in dedication from biomedical investigators. Here, GO-ZnO was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), ultraviolet-visible (UV-Vis) spectroscopy, energy dispersive X-ray analysis (EDAX), and Raman spectroscopy for structural, morphological, and elemental analysis. The toxic extent of GO-ZnO was noted by a methyl-thiazole-tetrazolium (MTT), while cellular morphology was observed towards the MCF-7 cells using an inverted microscope at magnification 40×. The cytotoxic effect of GO-ZnO investigated the cell viability reduction in a dose-dependent manner, as well as prompted the cell demise/destruction in an apoptotic way. Moreover, statistical analysis was performed on the experimental outcomes, with p-values < 0.05 kept as significant to elucidate the results. The generation of reactive oxygen species (ROS) demonstrated the potential applicability of graphene in tumor treatment. These key results attest to the efficacy of GO-ZnO nanocomposites as a substantial candidate for breast malignancy treatment.

Efficient Photocatalytic Degradation of Malachite Green in Seawater by the Hybrid of Zinc-Oxide Nanorods Grown on Three-Dimensional (3D) Reduced Graphene Oxide(RGO)/Ni Foam

A hybrid of ZnO nanorods grown onto three-dimensional (3D) reduced graphene oxide (RGO)@Ni foam (ZnO/RGO@NF) is synthesized by a facile hydrothermal method. The as-prepared hybrid material is physically characterized by SEM, XRD, Raman, and X-ray photoelectron spectroscopy (XPS). When the as-prepared 3D hybrid is investigated as a photocatalyst, it demonstrates significant high photocatalytic activity for the degradation of methylene blue (MB), rhodamine (RhB), and mixed MB/RhB as organic dye pollutants. In addition, the practical application and the durability of the as-prepared catalyst to degradation of malachite green (MG) in seawater are firstly assessed in a continuous flow system. The catalyst shows a high degradation efficiency and stable photocatalytic activity for 5 h continuous operation, which should be a promising catalyst for the degradation of organic dyes in seawater.

Organic semiconductor/graphene oxide composites as a photo-anode for photo-electrochemical applications

An intimate physical mixture of graphene oxide (GO) and semiconducting organic molecules like bromophenathrene (BrPh) and bromopyrene (BrPy) was prepared by using a ball milling technique. The structural, microstructural, physical and chemical properties of the mixtures (20 wt% of GO) were analyzed by X-ray diffraction, SEM, FT-IR, TGA and TCSPC studies. Furthermore, the electrochemical properties like AC electrical conductivity, transient photocurrent response (PCTR) and open circuit voltage (OCVD) of the samples were analyzed. It has been observed from TCSPC and OCVD measurements that 20 wt% of GO in the semiconductor composite leads to an enhanced life-time of photo-generated charge carriers. The physical mixture composites exhibit a higher photocurrent than pure BrPh and BrPy.

New organic materials with longer life-times of the charge carrier and enhancement of photocurrent generation were developed.

Synthesis of graphene-transition metal oxide hybrid nanoparticles and their application in various fields

Single layer graphite, known as graphene, is an important material because of its unique two-dimensional structure, high conductivity, excellent electron mobility and high surface area. To explore the more prospective properties of graphene, graphene hybrids have been synthesised, where graphene has been integrated with other important nanoparticles (NPs). These graphene-NP hybrid structures are particularly interesting because after hybridisation they not only display the individual properties of graphene and the NPs, but also they exhibit further synergistic properties. Reduced graphene oxide (rGO), a graphene-like material, can be easily prepared by reduction of graphene oxide (GO) and therefore offers the possibility to fabricate a large variety of graphene-transition metal oxide (TMO) NP hybrids. These hybrid materials are promising alternatives to reduce the drawbacks of using only TMO NPs in various applications, such as anode materials in lithium ion batteries (LIBs), sensors, photocatalysts, removal of organic pollutants, etc. Recent studies have shown that a single graphene sheet (GS) has extraordinary electronic transport properties. One possible route to connecting those properties for application in electronics would be to prepare graphene-wrapped TMO NPs. In this critical review, we discuss the development of graphene-TMO hybrids with the detailed account of their synthesis. In addition, attention is given to the wide range of applications. This review covers the details of graphene-TMO hybrid materials and ends with a summary where an outlook on future perspectives to improve the properties of the hybrid materials in view of applications are outlined.

One-pot synthesis of manganese porphyrin covalently functionalized graphene oxide for enhanced photocatalytic hydrogen evolution

In this paper, graphene oxide (GO) sheets covalently functionalized with (5,10,15,20-tetraphenyl) porphinato manganese(III) (MnTPP) has been successfully synthesized and tested as a photocatalyst for hydrogen evolution from water under UV-vis light irradiation. The obtained sample was systematically characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-vis, Fourier transform infrared (FTIR), and Raman spectroscopy. The results show that the MnTPP moiety has been successfully grafted on the graphene oxide surface to form MnTPP modified GO (GO-MnTPP). The fluorescence quenching and photocurrent enhancement of GO-MnTPP confirm that the rapid electrons transfer from photoexcited the MnTPP moiety to the GO sheets. The platinized GO-MnTPP exhibits enhanced photocatalytic activity for water reduction to produce hydrogen. Moreover, with the assistance of polyvinyl pyrrolidone (PVP), the photocatalytic activity is further improved because of aggregation prevention of the GO-MnTPP nanocomposite. This study provides a facile method to build porphyrin-graphene-based photocatalysts for solar energy conversion.

Facile synthesis of uniformly dispersed ZnO nanoparticles on a polystyrene/rGO matrix and its superior electrical conductivity and photocurrent generation

Herein, a ZnO/PS/rGO composite was prepared via a simple reflex method and its microstructural and physical properties were characterized using XRD, SEM, HRTEM, TGA, FTIR, UV-visible, PL spectroscopy, PCTR and OCVD measurements.

Charge transfer and surface defect healing within ZnO nanoparticle decorated graphene hybrid materials

To harness the unique properties of graphene and ZnO nanoparticles (NPs) for novel applications, the development of graphene-ZnO nanoparticle hybrid materials has attracted great attention and is the subject of ongoing research. For this contribution, graphene-oxide-ZnO (GO-ZnO) and thiol-functionalized reduced graphene oxide-ZnO (TrGO-ZnO) nanohybrid materials were prepared by novel self-assembly processes. Based on electron paramagnetic resonance (EPR) and photoluminescence (PL) investigations on bare ZnO NPs, GO-ZnO and TrGO-ZnO hybrid materials, we found that several physical phenomena were occurring when ZnO NPs were hybridized with GO and TrGO. The electrons trapped in Zn vacancy defects (VZn(-)) within the core of ZnO NPs vanished by transfer to GO and TrGO in the hybrid materials, thus leading to the disappearance of the core signals in the EPR spectra of ZnO NPs. The thiol groups of TrGO and sulfur can effectively "heal" the oxygen vacancy (VO(+)) related surface defects of ZnO NPs while oxygen-containing functionalities have low healing ability at a synthesis temperature of 100 °C. Photoexcited electron transfer from the conduction band of ZnO NPs to graphene leads to photoluminescence (PL) quenching of near band gap emission (NBE) of both GO-ZnO and TrGO-ZnO. Simultaneously, electron transfer from graphene to defect states of ZnO NPs is the origin of enhanced green defect emission from GO-ZnO. This observation is consistent with the energy level diagram model of hybrid materials.

Enhanced UV detection by transparent graphene oxide/ZnO composite thin films

Highly transparent graphene oxide–ZnO composite films synthesized by simple chemical method inhibit electron–hole recombination, modulate carrier transport and enhance UV detection capability.