Multifunctional monoclinic VO₂ nanorod thin films for enhanced energy applications: Photoelectrochemical water splitting and supercapacitor

Reddy, I. Neelakanta,Sreedhar, Adem,Shim, Jaesool,Gwag, Jin Seog Elsevier 2019 Journal of Electroanalytical Chemistry Vol.835 No.-

<P><B>Abstract</B></P> <P>Monoclinic VO<SUB>2</SUB> nanorod thin films were deposited on indium‑tin-oxide-coated glass substrates using radio-frequency reactive magnetron sputtering at a substrate temperature of 300°C and various O<SUB>2</SUB> flow rates. The thin films were characterized via standard analysis techniques. The VO<SUB>2</SUB> thin films exhibited a highly crystalline monoclinic phase with an indirect band gap of ~1.73eV. At optimized O<SUB>2</SUB> flow rate (4sccm), the thin films was observed nanorod structures, exhibited a remarkable photocurrent of ~0.08mAcm<SUP>−2</SUP> during photoelectrochemical water splitting in the visible region. Electrochemical performance tests of the nanorod films revealed a specific capacitance of ~486mFcm<SUP>−2</SUP> at a scan rate of 10mVs<SUP>−1</SUP>. In addition, amperometric <I>I</I>–<I>t</I> curves showed that VO<SUB>2</SUB> thin film electrodes were highly stable during the photo-oxidation process. The nanorod films also exhibited a good specific capacitance of ~120mFcm<SUP>−2</SUP> after 5000cycles at a scan rate of 100mVs<SUP>−1</SUP>. The photocurrents during photoelectrochemical water splitting and the specific capacitance of VO<SUB>2</SUB> thin films deposited at O<SUB>2</SUB> flow rates of 2 and 6sccm were 0.06 and 0.07mAcm<SUP>−2</SUP> and 398 and 37mFcm<SUP>−2</SUP>, respectively. The films deposited under Ar at 8sccm and O<SUB>2</SUB> at 4sccm showed the highest photoelectrochemical water splitting performance and specific capacitance, owing mainly to their nanorod-like morphology.</P> <P><B>Highlights</B></P> <P> <UL> <LI> VO<SUB>2</SUB> nanorod thin films were deposited on glass substrates using magnetron sputtering </LI> <LI> A remarkable photocurrent of ~0.08mAcm<SUP>−2</SUP> during water splitting in the visible region </LI> <LI> The VO<SUB>2</SUB> nanorod films has highest specific capacitance of ~486mFcm<SUP>−2</SUP> </LI> <LI> VO<SUB>2</SUB> nanorod obtained at 4sccm of O<SUB>2</SUB> has the highest water splitting and specific capacitance </LI> </UL> </P>

Structural, optical, and bifunctional applications: Supercapacitor and photoelectrochemical water splitting of Ni-doped ZnO nanostructures

Neelakanta Reddy, I.,Venkata Reddy, Ch.,Sreedhar, Adem,Shim, Jaesool,Cho, Migyung,Yoo, Kisoo,Kim, Dongseob Elsevier 2018 Journal of Electroanalytical Chemistry Vol.828 No.-

<P><B>Abstract</B></P> <P>Over the past few decades, doped ZnO structures have attracted significant attention because of their distinctive properties and a wide range of applications in catalysis and energy-storage devices. However, effective simple synthesis of doped ZnO structures for photoelectrocatalytic and supercapacitor applications still remains challenging. In this study, Ni-doped ZnO structures were synthesized at different Ni concentrations. Analysis of the obtained samples confirmed the formation of Ni-doped ZnO; 1.5 mol% Ni-doped ZnO showed enhanced water splitting activity and supercapacitor properties. The highest photocurrent density of 4.6 mA/cm<SUP>2</SUP> was obtained in a 0.1 M KOH solution at an applied bias photon-to-current efficiency of 4.2%, which is almost twice that obtained with pristine ZnO (2.8%), indicating an enhanced electron-hole separation. Doped ZnO exhibits a photocurrent 1.78 times higher than pristine ZnO under light illumination. Ni-doping induces effective charge separation and transfer, efficiently diminishing the recombination rate and reducing intrinsic defects. Further, the highest specific capacity of ~96 F g<SUP>−1</SUP> was observed for 1.5% Ni-doped ZnO at an applied scan rate of 10 mV s<SUP>−1</SUP>. The optimized sample, 1.5% Ni-doped ZnO, exhibited a high specific capacitance retention and coulombic efficiency of ~98% and ~99.2%, respectively. These results are expected to be very helpful in developing cheap and simple fabrication methods and efficient electrode materials for photoelectrochemical water splitting and supercapacitor applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Photoelectrochemical and supercapacitor properties of Ni-doped ZnO has been systematically investigated. </LI> <LI> The 1.5 mol% Ni-doped ZnO showed the highest photocurrent density of 4.6 mA/cm<SUP>2</SUP>. </LI> <LI> The highest specific capacitance of 1.5 mol% Ni-doped ZnO was obtained of ~ 96Fg<SUP>−1</SUP> at 10mVs<SUP>-1</SUP>. </LI> <LI> The optimal 1.5 % Ni-doped ZnO shows the highest specific capacitance retention and Coulombic efficiency of ~98 % and ~99.2 %, respectively. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

A stable novel nanostructure of ZnFe₂O₄ based nanocomposite for improved photoelectrocatalytic and photocatalytic activities

Neelakanta Reddy, I.,Venkata Reddy, Ch.,Sreedhar, Adem,Shim, Jaesool,Cho, Migyung,Yoo, Kisoo,Kim, Dongseob,Gwag, Jin Seog Elsevier 2018 Journal of electroanalytical chemistry Vol.823 No.-

<P><B>Abstract</B></P> <P>A nonagon ZnO–ZnFe<SUB>2</SUB>O<SUB>4</SUB> nanocomposite was synthesized using high-energy ball milling followed by post-annealing with an initial mixture of high purity magnetite (Fe<SUB>3</SUB>O<SUB>4</SUB>) and zincite (ZnO) powders. The effects of the process parameters on the structural, optical, photocatalytic, and photoelectrocatalytic activities were investigated. X-ray diffraction analysis was used to confirm the presence cubic spinel ZnFe<SUB>2</SUB>O<SUB>4</SUB> and hexagonal ZnO crystal phases. The lowest optical bandgap observed was 1.83 eV, which decreased due to post-annealing. The photocatalytic activity of the 800 °C post-annealed nanocomposite exhibited the highest degradation percentage of 96% over 180 min under visible light, with a degradation rate constant of 1.5 min<SUP>−1</SUP>. Furthermore, the optimized sample showed no substantial decrease in photocatalytic activity after five cycles of the repeated catalysis experiment. In electrochemical studies, when compared to the other post-annealed samples, the 800 °C post-annealed nanocomposite showed the highest water splitting activity with the lowest charge-transfer resistance of 75 Ω, photocurrent density of 4.3 × 10<SUP>−5</SUP> Acm<SUP>−2</SUP>, exchange current density of 4.19 log JA<SUP>−1</SUP> cm<SUP>−2</SUP>, and Tafel slopes of 74.1 mVdec<SUP>−1</SUP>. In dual activities, the performance of the 800 °C post-annealed nanocomposite was noticed to be enhanced. This nanocomposite has potential industrial applications as a cost-effective catalyst.</P> <P><B>Highlights</B></P> <P> <UL> <LI> ZnO-ZnFe<SUB>2</SUB>O<SUB>4</SUB> nonagon was prepared by a combination of ball-milling and calcination method. </LI> <LI> 96% of MO degradation was achieved under visible light illumination. </LI> <LI> ZnO-ZnFe<SUB>2</SUB>O<SUB>4</SUB> composite shows 4.3 × 10<SUP>−5</SUP> Acm<SUP>−2</SUP> photocurrent density. </LI> <LI> Dual performance composites are cost-effective catalysts for hydrogen generation and dye degradations. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Effect of plasmonic Ag nanowires on the photocatalytic activity of Cu doped Fe₂O₃ nanostructures photoanodes for superior photoelectrochemical water splitting applications

Reddy, I. Neelakanta,Reddy, Ch. Venkata,Sreedhar, Adem,Cho, Migyung,Kim, Dongseob,Shim, Jaesool Elsevier 2019 Journal of electroanalytical chemistry Vol.842 No.-

<P><B>Abstract</B></P> <P>The Present study focuses on the synthesis and analysis of Cu doped hematite (α-Fe<SUB>2</SUB>O<SUB>3</SUB>) nanostructures for effectively enhancing the optical properties as well as their implementation as photoelectrodes for energy-harvesting applications. In addition to this, the influence of noble metal plasmonic layer of Ag nanowires as a bottom layer for undoped and doped Fe<SUB>2</SUB>O<SUB>3</SUB> photoanodes has been investigated. Herein, we studied the influence of dopant on morphology, structural, and optical properties of Fe<SUB>2</SUB>O<SUB>3</SUB>. X-ray diffraction technique and X-ray photoelectron spectroscopy analysis were confirmed Cu ion substitution into host Fe<SUB>2</SUB>O<SUB>3</SUB> nanostructures. The optical band gap decreases from ~ 1.95eV to ~ 1.38eV with increasing of Cu dopant concentration. Impedance analysis reveals that the Cu dopant works as an electron donor and improves the Fe<SUB>2</SUB>O<SUB>3</SUB> charge carrier density. The photoelectrochemical water splitting studies reveals that the photoanodes without plasmonic layer was shown improved photocurrents compare to the undoped sample, thus improving the absorption of the incident light. Significantly, the optimized 0.2mol% Cu-doped α-Fe<SUB>2</SUB>O<SUB>3</SUB> photoelectrodes without Ag layer reached the maximum photocurrent density of ~0.31mA/cm<SUP>2</SUP>, ~ 28-fold that of pure Fe<SUB>2</SUB>O<SUB>3</SUB> (0.011mA/cm<SUP>2</SUP>). Further, the same photoanode with plasmonic Ag nanowires showed a significantly improved photocurrent density of 1.48mA/cm<SUP>2</SUP>, which is ~ 135-fold that of pure Fe<SUB>2</SUB>O<SUB>3</SUB> and ~ 5-folds that of 0.2mol% Cu doped α-Fe<SUB>2</SUB>O<SUB>3</SUB> photoelectrodes without plasmonic nanowire layer. The superior photocurrent is ascribed to the enhanced electron donor density and reduced charge recombination rate, as an outcome of optimized Cu doping and Ag nanowires.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Cu-doped Fe<SUB>2</SUB>O<SUB>3</SUB> and Ag plasmonic nanostructures were synthesized by hydrothermal and polyol methods. </LI> <LI> Optimal 0.2mol% Cu doped Fe<SUB>2</SUB>O<SUB>3</SUB> photoanode showed the highest photocurrent density of 1.48mA/cm<SUP>2</SUP>. </LI> <LI> Photoanode with Ag plasmonic increases the photocurrents of 135-fold that of pure Fe<SUB>2</SUB>O<SUB>3</SUB>. </LI> <LI> Excellent photoelectrochemical water splitting activity was achieved. </LI> </UL> </P>

Structural, optical, and XPS studies of doped yttria for superior water splitting under visible light illumination

Reddy, I. Neelakanta,Reddy, Ch. Venkata,Cho, Migyung,Kim, Dongseob,Shim, Jaesool Elsevier 2019 Journal of electroanalytical chemistry Vol.848 No.-

<P><B>Abstract</B></P> <P>Recently, there has been much research on developing highly efficient semiconductor photoelectrodes for photoelectrochemical (PEC) water splitting applications. In this study, we synthesized and investigated Cu-doped Y<SUB>2</SUB>O<SUB>3</SUB> photoelectrodes for this application using the hydrothermal method. The effects of Cu (0.0, 0.1, 0.3, and 0.5 mol) concentration on the crystal structure, morphology, and optical properties and their impact as photoelectrodes for energy-harvesting applications have been systematically explored. XPS analysis confirmed the incorporation of Cu ions into the host Y<SUB>2</SUB>O<SUB>3</SUB> photoelectrodes. The optical band gap of Y<SUB>2</SUB>O<SUB>3</SUB> photoelectrodes considerably decreases from ~4.35 eV to ~2.67 eV as a function of the Cu concentration, and the lowest bandgap was obtained for 0.3 mol% Cu-doped Y<SUB>2</SUB>O<SUB>3</SUB> electrodes. Electrochemical impedance spectroscopy analysis confirmed that the Cu dopant enhanced the charge carrier density of Y<SUB>2</SUB>O<SUB>3</SUB> and that it acts as an electron donor. PEC studies of Y<SUB>2</SUB>O<SUB>3</SUB> revealed that the photoanodes with 0.3 mol% of Cu dopant exhibited the highest photocurrent density of ~7.5 × 10<SUP>−5</SUP> A/cm<SUP>2</SUP>, which is ~15-fold that of pure Y<SUB>2</SUB>O<SUB>3</SUB> (5.0 × 10<SUP>−6</SUP> A/cm<SUP>2</SUP>). The superior photocurrent density is attributed to the improved electron donor density, the significantly reduced particle size, and the electron–hole recombination rate, which improved because of the Cu doping.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Doped Yttria photoelectrodes synthesis via hydrothermal method. </LI> <LI> Superior reduction of bandgap was achieved through Cu doping. </LI> <LI> Enhanced photocurrent density was attained at 0.3 mol% Cu doped Yttria. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Mn-doped ZrO₂ nanoparticles prepared by a template-free method for electrochemical energy storage and abatement of dye degradation

Reddy, Chandragiri Venkata,Reddy, I. Neelakanta,Akkinepally, Bhargav,Harish, V.V.N.,Reddy, Kakarla Raghava,Jaesool, Shim Elsevier 2019 CERAMICS INTERNATIONAL Vol.45 No.12

<P><B>Abstract</B></P> <P>A template-free technique was used to prepare pure and Mn-doped ZrO<SUB>2</SUB> photocatalyst samples in this study. The effect of doping on the structural, optical, photocatalytic, and supercapacitor properties were investigated. X-ray diffraction analysis revealed that the prepared samples had a tetragonal crystal structure. The optical band gap was narrowed due to the incorporation of Mn ions. The photocatalytic activity of methyl orange organic dye degradation with the Mn-doped ZrO<SUB>2</SUB> sample was examined under visible-light irradiation. The doped sample showed 83% of dye degradation after 100 min of irradiation. Cyclic voltammetry was used to study the supercapacitor properties, and the doped samples exhibited capacitance seven times greater than that of the pure sample.</P>

Systematic studies of Bi₂O₃ hierarchical nanostructural and plasmonic effect on photoelectrochemical activity under visible light irradiation

Reddy, I. Neelakanta,Reddy, Ch. Venkata,Sreedhar, Adem,Cho, Migyung,Kim, Dongseob,Shim, Jaesool Elsevier 2019 CERAMICS INTERNATIONAL Vol.45 No.14

<P><B>Abstract</B></P> <P>The effect of Ag plasmonic nanowire layers on the hierarchical nanostructure of Bi<SUB>2</SUB>O<SUB>3</SUB> photoelectrodes for water splitting under visible light irradiation was studied for the first time. In addition, the impact of various Bi<SUB>2</SUB>O<SUB>3</SUB> nanostructures on light harvesting and generation of relative photocurrent have been investigated. The cubic structure of Bi<SUB>2</SUB>O<SUB>3</SUB> was confirmed using X-ray diffraction analysis. Optical bandgaps of 2.14 and 2.30 eV have been achieved for the Bi<SUB>2</SUB>O<SUB>3</SUB> nanoparticles and nanoflowers photoelectrodes (BP and BF), respectively. The photocurrent density (J) of the BP featuring Ag plasmonic layer photoelectrode (Ag/BP) was 6.47 mA cm<SUP>−2</SUP>, and was higher than that of the BF featuring Ag plasmonic layer photoelectrode (Ag/BF), which was 4.33 mA cm<SUP>−2</SUP>. These values were approximately 647 and 2165 times higher than those of BP and BF, respectively. However, the J value of BP was 2.13 mA cm<SUP>−2</SUP> higher than that of BF. The superior J values of Ag/BP and Ag/BF were attributed to the increased light absorption and reduced electron-hole recombination rate at the time scale beyond a few 10<SUP>−12</SUP> s, owing to the Ag nanowires. In addition, the plasmonic field was able to reduce the charge recombination rate of the nanostructured electrodes in reactor cells.</P>

Effect of ball milling on optical properties and visible photocatalytic activity of Fe doped ZnO nanoparticles

Reddy, I. Neelakanta,Reddy, Ch. Venkata,Sreedhar, M.,Shim, Jaesool,Cho, Migyung,Kim, Dongseob Elsevier 2019 Materials science and engineering B. Advanced Func Vol.240 No.-

<P><B>Abstract</B></P> <P>The Fe doped ZnO nanoparticles were prepared using high-energy ball milling technique. The X-ray diffraction was utilized to study the effect of ball milling time on the crystalline structure. Field emission scanning electron microscopy was used to analyze the morphology of Fe doped ZnO. The intensity of the peaks decreases with the ball milling time and also no evidence was found the appearance of Fe<SUB>3</SUB>O<SUB>4</SUB> peaks at above 30 h, indicates a decrement in crystallinity owing to the incorporation of Fe into ZnO sites and it confirmed through peak shift in phase analysis. The electron structure was studied by X-ray photoelectron spectroscopy. It was observed that the both Zn and Fe are in 2+ oxidation state. The effect of milling time on optical properties and photocatalytic activity of Fe doped ZnO was determined using UV–visible spectrophotometry. Photocatalytic efficiency of the prepared samples was estimated by degradation of methylene orange dye in aqueous solution under sunlight irradiation. Maximum degradation of ∼98.7% for MO was achieved using 40 h ball milled Fe doped ZnO nanoparticles.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Fe doped ZnO nanoparticle was successfully grown by ball milling. </LI> <LI> The lowest bandgap of 2.55 eV was achieved for 40 h milled Fe doped ZnO sample. </LI> <LI> Fe doped ZnO nanoparticles exhibited high the visible light absorption properties. </LI> <LI> Maximum degradation of ∼98.7% for MO was observed under solar light irradiation. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Enhanced visible-light photocatalytic performance of Fe3O4 nanopyramids for water splitting and dye degradation

Reddy, I. Neelakanta,Sreedhar, Adem,Reddy, Ch. Venkata,Shim, Jaesool,Cho, Migyung,Kim, Dongseob,Gwag, Jin Seog,Yoo, Kisoo Springer-Verlag 2018 Journal of solid state electrochemistry Vol.22 No.11

Template-free synthesis of tetragonal Co-doped ZrO₂ nanoparticles for applications in electrochemical energy storage and water treatment

Reddy, Ch Venkata,Reddy, I. Neelakanta,Reddy, Kakarla Raghava,Jaesool, Shim,Yoo, Kisoo Elsevier 2019 ELECTROCHIMICA ACTA Vol.317 No.-

<P><B>Abstract</B></P> <P>Template-free synthesis of pure and Co-doped ZrO<SUB>2</SUB> tetragonal structured nanoparticles was achieved by the hydrothermal method and their photocatalytic, photoelectrocatalytic, and electrochemical energy storage supercapacitive properties were investigated for the first time. XRD revealed tetragonal crystal structure, while BET surface analysis suggested that Co-doped ZrO<SUB>2</SUB> has higher surface area (186.4 m<SUP>2</SUP>g<SUP>−1</SUP>) than that of the undoped sample (99.5 m<SUP>2</SUP>g<SUP>−1</SUP>). The band gap value was reduced from 4.95 to 2.12 eV due to incorporation of cobalt. Under visible-light illumination, photocatalytic and photoelectrocatalytic activities for methyl orange (MO) dye degradation were evaluated. The working electrodes have been prepared with 85 wt% active material (pure and doped ZrO<SUB>2</SUB>), 10 wt% active carbon black, and 5 wt% polyvinylidene difluoride (PVDF) in order to perform the supercapacitive properties. The Co-doped ZrO<SUB>2</SUB> material exhibited 93.7% and 27.4% MO dye degradation within 100 min with the highest specific capacitance value of 2151.2 Fg<SUP>-1</SUP>.</P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>