Microwave-assisted Chemical Reduction Research Articles

Tailoring iridium-palladium nanoparticles with Ir-rich skin: a highly durable anode electrocatalyst for acidic water electrolysis via a facile microwave-assisted chemical reduction method.

Electrochemical water splitting under acidic conditions is a clean way towards producing hydrogen fuels. The slow kinetics of the oxygen evolution reaction (OER) at the anode is currently a bottleneck for commercial acceptance of this technology. Therefore, arriving at more efficient and sustainable OER electrocatalysts is highly desirable. We here demonstrate the synthesis of iridium-palladium (IrPd) alloy nanoparticles (2-5 nm) with variable average composition (Ir : Pd = 1 : 0, 1 : 1, 1 : 3, 1 : 6, 1 : 9 and 0 : 1) using a facile one-pot microwave-assisted chemical reduction method. The IrPd nanoparticles show structure- and composition-dependent OER performance in acidic media. Utilizing different reduction strengths and precursor ratios, successful alloy catalysts were prepared with Ir-rich skin and sublayers of different Pd compositions. Their structures were revealed using high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, and hydrogen underpotential deposition (Hupd) studies. It turned out that (1) the alloy OER catalyst also has a high electrochemically active surface area for hydrogen adsorption/desorption, (2) the OER performance is strongly dependent on the surface Ir contribution and (3) the intact Ir skin is essential for electrocatalyst stability.

Scalable approach to fabricate paper-based biomass reduced graphene sensor for the detection of exhaled diabetic breath

Herein, we demonstrate a microwave-assisted chemical reduction technique to exfoliate a few layers of graphene from the natural waste material, ‘coconut shell’. The microwave irradiation coconut shell is subjected to structural, morphological and functional groups characterization methods including SEM, Raman, FTIR and XPS spectroscopic analyses. The formation of biomass reduced graphene (BRG) has been confirmed through Raman and FTIR spectroscopic analyzes with the presence of D, G and 2D and other functional spectral bands, respectively. The surface topography of the BRG exhibits two-dimensional mat structures with wrinkle topography, imaged by electron microscopic techniques. The metallic behaviour of the BRG is evaluated by band structure calculation using density functional theory. The synthesized nanostructure has been evaluated for exhaled diabetic breath sensing application by fabricating sensor device on the paper-based substrate by roll-to-roll coating technique. The BRG sensor exhibited enhanced sensing response at a very lower concentration of diabetic biomarker with long term stability and rapid response/recovery time of 1.11 s/41.25 s, respectively. Based on our findings, the microwave-assisted BRG is a potential candidate for fabricating highly scalable, inherently safe, economically viable and excellent sensing performance to detect exhaled diabetic breath at room temperature.

Electrooxidation reactions of methanol and ethanol on Pt–MoO3 for dual fuel cell applications

Pt–MoO3 was synthesized by microwave-assisted chemical reduction. The physicochemical characterization showed that the electrocatalyst contained nanoparticles of Pt and clusters of MoO3. The average particle size of the catalytic material was 2.5 nm. The electrochemical results showed that the Pt–MoO3/C was suitable to carry out the electrooxidation reactions of ethanol and methanol indistinctly, avoiding CO poisoning. It was possible to compare the results with commercial Pt/C. The synthesized material showed a better electrochemical performance. Different simulations were performed using the Nernst equation to evaluate the influence of temperature, internal resistance, and the current density losses as a function of the fuel used. The theoretical results indicated that the electrical power of the mono-cell improves by 21.5% when the energy vector is changed from methanol to ethanol at the maximum power point, obtaining an electrical potential change ΔE = 87.02 mV and a variation of the electric power of Δp = 114.14 mW cm−2. The use of dual fuels could improve the performance of experimental fuel cells.

Optical properties and novelty preparation PVA/PVP doping with Cu as surface plasmonic ions

This paper presents a low-cost microwave-assisted chemical reduction method to synthesize copper nanoparticles using sodium borohydride (NaBH4) and ascorbic acid as reducing and stabilizing agents. Furthermore, the preparation of copper-doped PVA and copper-doped PVA/PVP composite films was operated using a solution casting technique. Low-cost copper nanoparticles are considered to have a broad and weak surface plasmon resonance (SPR). With the help of scanning electron microscopy (SEM) and UV–vis near-infrared spectroscopy (UV–VIS–NIR), the characterization of copper nanoparticles was analyzed. The peak of the surface plasmon resonance shifts from 240 to 300 nm. Optical characterizations of copper doped PVA and PVA/PVP composite films were established. The transmittance spectra of 1.851 wt% Cu-doped PVA and 0.590 wt% Cu-doped PVA/PVP showed light blocking in the wavelength ranges of 190–282 nm (PVA) and 289 nm (PVA/PVP). According to the absorption spectra, the surface plasmon resonance (SPR) is induced by Cu NPs at a wavelength of 260 nm. The indirect energy gap, Eg, was determined and found to decrease with an increase in the doping of Cu. Nonlinear optical properties such as nonlinear refractive index, linear optical susceptibility, and third-order nonlinear optical susceptibility were obtained. These findings will aid in producing a plasmonic novel material for optical applications.

Synthesis and antibacterial activity of silver-copper nano-composites formed by microwave assisted chemical reduction

Abstract Bimetallic copper silver systems are promising antibacterial agents with wider activity range and enhanced activity than single metallic systems. In this study we present the synthesis of copper silver bimetallic composites utilizing a fast, microwave assisted chemical reduction method. The method requires low reaction times and yields nano-composites with a morphology best described as nano silver decorated copper particles. Water leaching studies show that copper follows a fast exponential leaching kinetics compared to silver. The release of silver remains slow and is sustained over a much longer period compared to copper. Consequently, the antibacterial effects of the composites will be dictated by the fast leaching copper and modulated by the composition of silver in the composite. The antibacterial effects of the composite on pseudomonas aeruginosa bacterium confirm this effect.

Synthesis and antifungal activity of soluble starch and sodium alginate capped copper nanoparticles

In this work copper nanoparticles (Cu NPs) were prepared by using biopolymers soluble starch and sodium alginate (NaAlg) with its abundance of hydroxyl groups and its biocompatibility as the capping agents. Soluble starch capped Cu NPs (CuS NPs) and sodium alginate capped Cu NPs (CuA NPs) were first prepared by microwave assisted chemical reduction method using hydrazine, copper sulfate, soluble starch and sodium alginate as the reduction, oxidation and capping agents, respectively. The synthesized nanoparticles were characterized by x-ray diffraction (XRD), scanning electron microscopy (SEM), dynamic light scattering analysis (DLS), thermogravimetric analysis (TGA). The antifungal activities of the Cu NPs against Candida albicans and Candida krusei were determined depending on the stabilizing agents. The results showed that CuS NPs have better antifungal activity and stability under the open environment atmosphere than CuA NPs. For this reason, CuS NPs are thought to have the potential to be used in areas such as biotechnology and food packaging.

Antibacterial Property of Composites of Reduced Graphene Oxide with Nano-Silver and Zinc Oxide Nanoparticles Synthesized Using a Microwave-Assisted Approach.

Graphene oxide (GO) composites with various metal nanoparticles (NPs) are attracting increasing interest owing to their broad scope in biomedical applications. Here, microwave-assisted chemical reduction was used to deposit nano-silver and zinc oxide NPs (Ag and ZnO NPs) on the surface of reduced GO (rGO) at the following weight percentages: 5.34% Ag/rGO, 7.49% Ag/rGO, 6.85% ZnO/rGO, 16.45% ZnO/rGO, 3.47/34.91% Ag/ZnO/rGO, and 7.08/15.28% Ag/ZnO/rGO. These materials were tested for antibacterial activity, and 3.47/34.91% Ag/ZnO/rGO and 7.08/15.28% Ag/ZnO/rGO exhibited better antibacterial activity than the other tested materials against the gram-negative bacterium Escherichia coli K12. At 1000 ppm, both these Ag/ZnO/rGO composites had better killing properties against both E. coli K12 and the gram-positive bacterium Staphylococcus aureus SA113 than Ag/rGO and ZnO/rGO did. RedoxSensor flow cytometry showed that 3.47/34.91% Ag/ZnO/rGO and 7.08/15.28% Ag/ZnO/rGO decreased reductase activity and affected membrane integrity in the bacteria. At 100 ppm, these two composites affected membrane integrity more in E. coli, while 7.08/15.28% Ag/ZnO/rGO considerably decreased reductase activity in S. aureus. Thus, the 3.47/34.91% and 7.08%/15.28% Ag/ZnO/rGO nanocomposites can be applied not only as antibacterial agents but also in a variety of biomedical materials such as sensors, photothermal therapy, drug delivery, and catalysis, in the future.

A Z-scheme ternary Bi2MoO6/Au/TiO2 photocatalytic films with high-efficient catalytic performance

TiO2 nanofiber film (TiO2 NFF) with visible light scattering ability was successfully synthesized by an ethylene glycol-assisted hydrothermal method, Au nanoparticles (Au NPs) were then deposited on the surface of TiO2 film using microwave-assisted chemical reduction process, and at last, Bi2MoO6/Au/TiO2 ternary photocatalytic film was successfully synthesized via a simple alcohol-thermal method using Au/TiO2 nanofilms as templates. The photocatalytic activities of as-fabricated samples were investigated by the oxidation of organic pollutants, like methylene blue (MB) and phenol, and the reduction of aqueous Cr(VI) under visible light. It shows that the Bi2MoO6/Au/TiO2 composite displays a much higher visible-light-responsive photocatalytic performance compared to pure TiO2, Au/TiO2 NFF, and Bi2MoO6/TiO2 complex. The improved visible light photoactivity could be ascribed to the excellent light scattering structure of TiO2 NFF, the surface plasmon resonance effect of Au NPs, and the highly effective separation of photogenerated electrons and holes through the Z-scheme Bi2MoO6/Au/TiO2 system, in which Au NPs plays a key role in transmitting charges in the photocatalytic process.

Preparation of Noble-metal Nanoparticles by Microwave-assisted Chemical Reduction and Evaluation as Catalysts for Nitrile Hydrogenation under Ambient Conditions

Preparation of Noble-metal Nanoparticles by Microwave-assisted Chemical Reduction and Evaluation as Catalysts for Nitrile Hydrogenation under Ambient Conditions

Advances in Microwave-Assisted Production of Reduced Graphene Oxide.

Efficient reduction of graphene oxide to obtain high-quality graphene nanosheets is desirable for energy storage, catalysis, electronics and environmental remediation. In this brief review, we mainly focus on the microwave-assisted production of reduced graphene oxide in three categories: (1) microwave-assisted chemical reduction of graphene oxide; (2) microwave-assisted thermal reduction of graphene oxide; (3) microwave-assisted simultaneous thermal exfoliation & thermal reduction of graphite oxide. We also summarize common techniques for characterizing reduction efficiency and quality of as-obtained rGO.

Rapid Microwave-Assisted Synthesis of Platinum Nanoparticles Immobilized in Electrospun Carbon Nanofibers for Electrochemical Catalysis

A one-pot microwave-assisted synthesis method was developed to produce carbon nanofiber (CNF) supported platinum (Pt) catalyst nanoparticles as potential electrode materials for polymer electrolyte membrane fuel cells (PEMFC). CNF-supported Pt (Pt/CNF) hybrid structures were prepared through electrospinning of poly(acrylonitrile-co-N-vinylpyrrolidone) (P(AN-co-nVP)) copolymer containing PtCl2 as a precursor and subsequent microwave-assisted chemical reduction in hydrazine hydrate solution and carbonization. First, the effect of carbonization temperatures (600 to 1000 °C) on the growth of Pt nanoparticles and their electrochemical activity was studied. Uniform particle distribution with an average particle size of 4.25 nm and the highest electrochemical surface area (ECSA) (47.4 m2 g–1) were observed for the sample carbonized at 800 °C. Then microwave-assisted chemical reduction of PtCl2 in the nanofibers prior to carbonization was investigated, which resulted in better control of particle size and distribution and higher electrochemical activity. After carbonization at 800 °C of those treated with microwave-assisted chemical reduction with irradiation time of 15 and 30 s, CNF-supported Pt nanoparticles with a particle size of 1.75 and 2.85 nm and catalytic activity of 71.2 m2 g–1 and 74.6 m2 g–1, respectively, were obtained. Finally, the addition of highly conductive SWCNTs into Pt/CNF hybrids was investigated in terms of its effect on Pt particle size and distribution and electrochemical activity of hybrid materials. SWCNT-containing Pt/CNF, obtained by chemical reduction with 15 s microwave irradiation followed by carbonization at 800 °C, had nanoparticles with an average particle size of 0.86 nm and showed high electrochemical active surface area of 90.0 m2 g–1.

CdSe modified TiO2 nanotube arrays with Ag nanoparticles as electron transfer channel and plasmonic photosensitizer for enhanced photoelectrochemical water splitting

Ag nanoparticles (NPs) and a CdSe nanocrystal were successively deposited on the surface of a TiO2 nanotube array film (CdSe/Ag/TiO2 NTs) via microwave assisted chemical reduction and in situ fabrication methods. The nanotubular structure of TiO2 preserves well after loading Ag and CdSe, as the ternary composite not only exhibits an excellent visible light harvesting ability but also has the smallest charge transfer resistance according to electrochemical impedance measurement. The most positive flat band potential of CdSe/Ag/TiO2 NTs facilitates charge carrier transfer at the semiconductor-electrolyte interface. Notably, the introduction of Ag nanoparticles forms a unique carrier-transfer-channel in the CdSe/Ag/TiO2 NTs system, in which the Ag nanoparticles can take advantage of its high carriers transfer rate as a linker. In addition, Ag serving as a surface plasmon resonance source further improves photoelectric catalytic activities of the sample. It is noteworthy that CdSe/Ag/TiO2 NTs exhibits the desired photoelectrochemical performance with the highest H2 production rate of 413.99 µmol cm−2 after 2.5 h irradiation.

Microwave-assisted synthesis of palladium nanoparticles supported on copper oxide in aqueous medium as an efficient catalyst for Suzuki cross-coupling reaction

We report here a reliable green method for the synthesis of palladium nanoparticles supported on copper oxide as a highly active and efficient catalyst for Suzuki cross-coupling reaction. The experimental synthetic approach is based on microwave-assisted chemical reduction of an aqueous mixture of palladium and copper salt simultaneously using hydrazine hydrate as reducing agent. The catalyst was fully characterized using various techniques showing well-dispersed palladium nanoparticles. The catalytic activity and recyclability of the prepared catalyst were experimentally explored in the ligand-free Suzuki cross-coupling reaction with a diverse series of functionalized substrates. The synthesized Pd/CuO catalyst shows many advantages beside its high catalytic efficiency such as the recyclability of up to five times with negligible loss of catalytic activity, short reaction times, use of environmentally benign solvent systems, and mild reaction conditions.

Design of Copper and Titanium Dioxide Nanoparticles Doped with Reduced Graphene Oxide for Hydrogen Evolution by Water Splitting

TiO2-graphene (P25-GR, PG) nanocomposite was fabricated from P25 titania and graphite oxide by hydrothermal method, and then Cu nanoparticles (Cu NPs) were assembled in P25-GR composite (Cu- P25-GR, CPG) under microwave-assisted chemical reduction. The prepared samples were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), UV–Vis absorption (UV–Vis) and Raman spectroscopies. Cu NPs were well dispersed on the surface of PG and are in metallic state. The ternary Cu-P25-GR (CPG) nanocomposites show an extended light absorption range and more efficient charge separation properties compared to binary P25-GR (PG) composite. Methylene blue photodegradation experiment proved that surface plasmon resonance (SPR) phenomenon had an effect on photoreaction efficiency. The corresponding hydrogen evolution rate for CPG prepared using 0.002 M Cu(NO3)2 solution was 10 times higher than with pure P25, and 2.3 times higher than with PG in the same test conditions. The improved photocatalytic performance can be attributed to the presence of GR in the prepared composite and to the SPR effect, leading to the longer lifetime of photogenerated electronhole pairs and faster interfacial charge transfer rate. We expect that our work would be useful for the further exploration of GR-based nanocomposites.

Study of Ethanol Electrooxidation Reaction at Room Temperature on Nanometric Pt-Ru, Pt-Sn and Pt-Ru-Sn in Direct Alcohol Fuel Cells

Ethanol electrooxidation in acid medium was investigated on Pt-Ru-Sn/C, Pt-Ru/C and Pt-Sn/C. The electrocatalysts were synthesized by microwave assisted chemical reduction reaction. The samples were characterized by transmission electron microscopy (TEM), X-ray diffraction analysis (XRD) and electrochemical analysis for the electrooxidation of ethanol. The ternary electrocatalyst was evaluated in an experimental Direct Ethanol Fuel Cell (DEFC). The method of synthesis used in this work allowed the formation of nanostructured electrocatalysts. The results obtained by electrochemical studies showed that the ternary system Pt-Ru-Sn/C exhibited the highest activity with respect to the binary systems Pt-Ru/C and Pt-Sn/C for carrying out the ethanol electrooxidation reaction. 0.4 mg∙cm-2 of electrocatalytic load of Pt-Ru-Sn/C was placed in the anode of an experimental fuel cell operating at room temperature. It was possible to obtain a power density of 0.14, 0.12 and 0.11 mW∙cm-2 after 20, 40 and 60 minutes respectively. The experiments were carried out at a controlled temperature of 297 K and they showed the feasibility to produce electricity at room temperature by using this ternary electrocatalyst in Direct Ethanol Fuel Cells.

Graphene based catalyst supports for high temperature PEM fuel cell application

In this study, the effect of graphene nanoplatelet (GNP) and graphene oxide (GO) based carbon supports on polybenzimidazole (PBI) based high temperature proton exchange membrane fuel cells (HT-PEMFCs) performances were investigated. Pt/GNP and Pt/GO catalysts were synthesized by microwave assisted chemical reduction support. X-ray diffraction (XRD), Thermogravimetric analysis (TGA), Brauner, Emmet and Teller (BET) analysis and high resolution transmission electron microscopy (HRTEM) were used to investigate the microstructure and morphology of the as-prepared catalysts. The electrochemical surface area (ESA) was studied by cyclic voltammetry (CV). The results showed deposition of smaller Pt nanoparticles with uniform distribution and higher ECSA for Pt/GNP compared to Pt/GO. The Pt/GNP and Pt/GO catalysts were tested in 25 cm2 active area single HT-PEMFC with H2/air at 160 °C without humidification. Performance evaluation in HT-PEMFC shows current densities of 0.28, 0.17 and 0.22 A/cm2 for the Pt/GNP, Pt/C and Pt/GO catalysts based MEAs at 160 °C, respectively. The maximum power density was obtained for MEA prepared by Pt/GNP catalyst with H2/Air dry reactant gases as 0.34, 0.40 and 0.46 W/cm2 at 160 °C, 175 °C and 190 °C, respectively. Graphene based catalyst supports exhibits an enhanced HT-PEMFC performance in both low and high current density regions. The results indicate the graphene catalyst support could be utilized as the catalyst support for HT-PEMFC application.

The fabrication and photoelectrocatalytic study of composite ZnSe/Au/TiO2 nanotube films

In this paper, anatase TiO2 nanotube (NT) film photoelectrodes are successfully fabricated by a simple and effective hydrothermal method. Subsequently, an aqueous-phase processing technique is adopted to construct highly dispersed ZnSe quantum dots (QDs) on Au/TiO2 NT films prepared by microwave-assisted chemical reduction, which formed composite ZnSe/Au/TiO2 NT film systems (ZATs) with excellent performance in photoelectrocatalytic (PEC) applications. The morphology and performance of as-obtained ZATs were investigated based on various characterizations. The investigation revealed that as-obtained ZATs not only greatly extend spatial separation of charges and restrain the recombination rate of photogenerated electron–hole pairs, but also improve the efficiency to use visible light and display a wide and strong absorption in the visible light region ranging from 400 nm to 800 nm. Moreover, we observe a larger fluorescence quenching of ZATs compared with that of pure TiO2 NT films and binary composites. Experimental results indicate that the photocurrent densities of pure TiO2, 0.8 Au/TiO2, 60 min ZnSe/TiO2, and ZATs are 0.020 mA cm−2, 0.032 mA cm−2, 0.037 mA cm−2 and 0.070 mA cm−2, respectively, which is approximately 2–3.5 times higher than that of pure TiO2 NT films and binary compound photoelectrodes. Additionally, experimental results suggest that the as-prepared ZATs photoelectrode has exhibited considerable stability and significantly increased PEC activity for the degradation of methylene blue (MB) in distilled water under 100 mW cm−2 xenon lamp irradiation. The degradation efficiency on MB of 45 min ZnSe/0.8 Au/TiO2 NT films approaches 91%; however, the counterpart of TiO2 NT films is less than 10%. Eventually, the mechanism for the improvement of the PEC performance of ZATs is discussed to point out that ZATs display prominent charges transport performance, and a stepwise band alignment structure is built up in its photoelectrode, which indicates that charge separation and migration across the interfaces should favor vectorial electron transfer according to the scheme of ZnSe → TiO2 → Au → Ti substrates.

Highly efficient and stable Au/Bi2MoO6/Bi2WO6 heterostructure with enhanced photocatalytic activity for NO gas removal under visible light irradiation

The ternary composite photocatalyst Au/Bi2MoO6/Bi2WO6 has been successfully synthesized by a facile hydrothermal process and microwave-assisted chemical reduction method for the first time. A series of characterization results reveal that Au nanoparticles are uniformly dispersed on the surface of Bi2MoO6/Bi2WO6 microspheres. The unique photocatalytic performance of the novel composite material is evaluated by the photocatalytic removal of NO gas at ppb levels under visible light irradiation. The photocatalytic removal rate of NO gas in the presence of Au/Bi2MoO6/Bi2WO6 sample is the highest, which directly attributes to the effective separation of photoinduced charge carriers through the formation of the heterojunction and the Schottky junction at the interface between Au nanoparticles and Bi2MoO6 or Bi2WO6. Additionally, it is indicated that the h+ and play indispensable role in photocatalytic process by free radicals trapping experiments and the oxidation products of NO gas on the surface of the catalysts are nitrate by drawing the standard curve of total N content in nitrate. Meanwhile, a possible mechanism for the photocatalytic activity enhancement of the Au/Bi2MoO6/Bi2WO6 composites is proposed. These results demonstrate that the Au/Bi2MoO6/Bi2WO6 photocatalyst is a promising candidate for indoor air purification under solar light irradiation.

Microwave-Assisted Synthesis of Pd Nanoparticles and Catalysis Application for Suzuki Coupling Reactions

A facile and efficient method was developed for the synthesis of highly active palladium nanoparticles (PdNPs) by microwave-assisted chemical reduction. The PdNPs with different morphology were prepared with or without KBr (capping agent) using ethylene glycol (EG) and citric acid (CA) as reducing agent in the presence of polyvinylpyrrolidone (PVP), respectively. The as-prepared PdNPs were characterized by (UV-Visible Spectrophotometer) UV-Vis, transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. The results showed that the PdNPs reduced by EG without KBr (PdNPs-EG) has smaller particle size than other PdNPs. The remarkable catalytic activity of the PdNPs catalysts are obtained using a low amount of PdNPs (0.1 mmol‰) using K2CO3 as base and EtOH/H2O as solvent toward Suzuki coupling reactions of aryl bromides and phenylboronic acid for 15 min.

Fabrication of Polyaniline/Self-Doped TiO2 Nanotubes Hybrids as Supercapacitor Electrode by Microwave-Assisted Chemical Reduction and Electrochemical Deposition

We report a combination method of microwave-assisted chemical reduction with NaBH4 and electrochemical deposition to fabricate high-performance polyaniline/bulk abundant Ti3+ self-doped TiO2 nanotubes (PANI/MR-TiO2NTs) for supercapacitor. At the current density of 0.6 A g−1, PANI/MR-TiO2 NTs nano-composite exhibits high specific capacitance of 1084 F g−1, which is the best value based on TiO2 nanotubes (TiO2 NTs) or its composites with PANI (PANI/TiO2 NTs) using as supercapacitor electrodes as we know. With the current density increased by 30 times, the specific capacitance of PANI/MR-TiO2 NTs still remains at 616 F g−1 denoting good rate capability. PANI/MR-TiO2 NTs also shows a good long-life cycle performance with a low reduction of 13.3% of the initial capacitance after 2000 cycles at a high current density of 18.0 A g−1. The EIS measurement reveals that compared with PANI/TiO2 NTs, PANI/MR-TiO2 NTs exhibits lower solution resistance and charge-transport but higher pseudocapacitance (CPE1), indicating a superior electron transport ability and charge-transfer rate. The data indicate great promise for the PANI/MR-TiO2 NTs used as a supercapacitor electrode.