Effect of HNO₃ functionalization on large scale graphene for enhanced tri-iodide reduction in dye-sensitized solar cells

Das, Santanu,Sudhagar, P.,Ito, Eisuke,Lee, Dong-yoon,Nagarajan, S.,Lee, Sang Yun,Kang, Yong Soo,Choi, Wonbong The Royal Society of Chemistry 2012 Journal of materials chemistry Vol.22 No.38

<P>Improving the electro-catalytic activity of graphene has recently been the subject of intense research for high efficiency flexible energy storage and conversion devices. We report the synthesis of a large scale graphene film by a CVD method and its electro-catalytic activity by functionalization with HNO<SUB>3</SUB> for a high efficiency electrochemical electrode in DSSCs. We found that HNO<SUB>3</SUB> functionalization on graphene enhances the tri-iodide reduction rate by three times in a dye sensitized solar cell compared to that of pristine graphene. The X-ray photoelectron spectroscopy (XPS) and ultra-violet photoemission spectroscopy (UPS) studies confirm the covalently attached C–OH, C(O)OH and NO<SUP>3−</SUP> moieties to carbon atoms through sp<SUP>2</SUP>–sp<SUP>3</SUP> hybridization, and this results in the Fermi level shift towards p-type doping. We believe that the covalently attached functional groups cause the enrichment of the electro-catalytically active sites along with facilitating the charge transfer kinetics from graphene counter electrodes to redox couples. The enhanced catalytic effect of functionalized graphene offers insights into new types of electrode development opportunities in graphene based energy storage and conversion devices.</P> <P>Graphic Abstract</P><P>Nitric acid doped p-type graphene shows higher electro-catalytic activity towards tri-iodide reduction in dye sensitized solar cells. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2jm32481d'> </P>

Effect of aluminum doping on the structural and luminescent properties of ZnO nanoparticles synthesized by wet chemical method

Kumar, R. Saravana,Sathyamoorthy, R.,Sudhagar, P.,Matheswaran, P.,Hrudhya, C.P.,Kang, Yong Soo Elsevier 2011 Physica E, Low-dimensional systems & nanostructure Vol.43 No.6

<P><B>Abstract</B></P><P>The critical role that dopants play in semiconductor devices has stimulated research on the properties and the potential applications of semiconductor nanocrystals. Hence the investigation of the role of dopant concentration on the properties of semiconductor nanoparticles is very important from the viewpoints of basic physics as well as applications. In this context, in the present work Al-doped ZnO (AZO) nanoparticles were synthesized by simple wet chemical route. The structure and morphology of the nanoparticles analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM) revealed hexagonal wurtzite structure with flower-like clusters consisting of multi-nanorods. Energy Dispersive Spectrum (EDS) confirms the substitution of Al into ZnO lattice. Defect analysis and excitonic effect of the nanoparticles were investigated by photoluminescence (PL) and UV–Vis absorption measurements, respectively. Optical absorption showed band gap broadening due to quantum confinement effect. PL measurements exhibited both near band edge (NBE) and deep level (DL) emissions. The effect of doping concentration on the growth, crystallization and defect distribution of AZO nanoparticles was studied.</P> <P><B>Research highlights</B></P><P>► Al-doped ZnO nanoparticles were synthesized by facile wet chemical approach. ► Influence of Al doping on the physical properties of ZnO nanoparticles was investigated. ► Crystallinity of the ZnO nanoparticles deteriorated with Al incorporation. ► Broadening of the band gap due to quantum confinement effect was observed upon Al doping. ► Al doping causes annihilation of defect emission and enhancement of UV emission in ZnO.</P>

Interfacial engineering of quantum dot-sensitized TiO₂ fibrous electrodes for futuristic photoanodes in photovoltaic applications

Sudhagar, P.,Gonzá,lez-Pedro, Victoria,Mora-Seró,, Ivá,n,Fabregat-Santiago, Francisco,Bisquert, Juan,Kang, Yong Soo The Royal Society of Chemistry 2012 Journal of materials chemistry Vol.22 No.28

<P>Herein we report generic surface treatment approaches to improve the electronic interfaces of quantum dot-sensitized TiO<SUB>2</SUB> fiber electrodes, thereby promoting their photoanode performance. Highly dense, continuous and nanostructured TiO<SUB>2</SUB> fibrous membranes, without the inclusion of a scattering layer, unlike conventional TiO<SUB>2</SUB> particulate electrodes, showed feasible photoconversion performance under the proposed interfacial engineering modification. The proposed interfacial treatment concerns fibrous membranes both before and after calcination. The chemical vapor pre-treatment on an as-deposited fibrous membrane using tetrahydrofuran (THF) reinforces the physical contact between the fibrous membrane and the transparent conducting substrate and reduces significantly the recombination rate. In the case of post-treatment by F-ion on a fibrous surface, together with the interfacial engineering approach, the ZnS surface passivation layer markedly improves the photoanode performance of the TiO<SUB>2</SUB> fibrous membrane nearly to a factor of 3.2% with a remarkable open-circuit voltage <I>V</I><SUB>oc</SUB> = 0.69 V and <I>J</I><SUB>sc</SUB> = 13 mA cm<SUP>−2</SUP> under 1 sun illumination (100 mW cm<SUP>−2</SUP>). This report provides an excellent platform for studying and understanding the interfacial contacts and mechanisms related to the charge transfer at CdS/CdSe QD-sensitized TiO<SUB>2</SUB> fibrous assemblies. Such implications of this interfacial treatment strategy can be successfully extended to a wide range of photoanode candidates in energy conversion systems and confirm the effectiveness of some alternative nanostructured electrodes for the development of semiconductor-sensitized solar cells.</P> <P>Graphic Abstract</P><P>The generic surface treatment approaches were demonstrated to improve the electronic interfaces of quantum dot-sensitized TiO<SUB>2</SUB> fiber electrodes, thereby improving their photoanode performance in solar cells. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c2jm31599h'> </P>

Enhanced photocatalytic performance at a Au/N–TiO₂ hollow nanowire array by a combination of light scattering and reduced recombination

Sudhagar, P.,Devadoss, Anitha,Song, Taeseup,Lakshmipathiraj, P.,Han, Hyungkyu,Lysak, Volodymyr V.,Terashima, C.,Nakata, Kazuya,Fujishima, A.,Paik, Ungyu,Kang, Yong Soo The Royal Society of Chemistry 2014 Physical chemistry chemical physics Vol.16 No.33

<P>We demonstrate one-step gold nanoparticle (AuNP) coating and the surface nitridation of TiO<SUB>2</SUB> nanowires (TiO<SUB>2</SUB>-NWs) to amplify visible-light photon reflection. The surface nitridation of TiO<SUB>2</SUB>-NW arrays maximizes the anchoring of AuNPs, and the subsequent reduction of the band gap energy from 3.26 eV to 2.69 eV affords visible-light activity. The finite-difference time-domain (FDTD) simulation method clearly exhibits the enhancement in the strengths of localized electric fields between AuNPs and the nanowires, which significantly improves the photocatalytic (PC) performance. Both nitridation and AuNP decoration of TiO<SUB>2</SUB>-NWs result in beneficial effects of high (e<SUP>−</SUP>/h<SUP>+</SUP>) pair separation through healing of the oxygen vacancies. The combined effect of harvesting visible-light photons and reducing recombination in Au/N-doped TiO<SUB>2</SUB>-NWs promotes the photocatalytic activity towards degradation of methyl orange to an unprecedented level, ∼4 fold (1.1 × 10<SUP>−2</SUP> min) more than does TiO<SUB>2</SUB>-NWs (2.9 × 10<SUP>−3</SUP> min<SUP>−1</SUP>). The proposed AuNP decoration of nitridated TiO<SUB>2</SUB>-NW surfaces can be applied to a wide range of n-type metal oxides for photoanodes in photocatalytic applications.</P> <P>Graphic Abstract</P><P>Maximizing the Au nanoparticle decoration on TiO<SUB>2</SUB> nanowire through nitrogen doping for simultaneous enhancement in visible light scattering and electron–hole charge separation. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4cp02009j'> </P>

Synergistic Catalytic Effect of a Composite (CoS/PEDOT:PSS) Counter Electrode on Triiodide Reduction in Dye-Sensitized Solar Cells

Sudhagar, P.,Nagarajan, S.,Lee, Yong-Gun,Song, Donghoon,Son, Taewook,Cho, Woohyung,Heo, Miyoung,Lee, Kyoungjun,Won, Jongok,Kang, Yong Soo American Chemical Society 2011 ACS APPLIED MATERIALS & INTERFACES Vol.3 No.6

<P>Inorganic/organic nanocomposite counter electrodes comprised of sheetlike CoS nanoparticles dispersed in polystyrenesulfonate-doped poly(3,4-ethylenedioxythiophene (CoS/PEDOT:PSS) offer a synergistic effect on catalytic performance toward the reduction of triiodide for dye-sensitized solar cells (DSSCs), yielding 5.4% power conversion efficiency, which is comparable to that of the conventional platinum counter electrode (6.1%). The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry measurements revealed that the composite counter electrodes exhibited better catalytic activity, fostering rate of triiodide reduction, than that of pristine PEDOT: PSS electrode. The simple preparation of composite (CoS/PEDOT:PSS) electrode at low temperature with improved electrocatalytic properties are feasible to apply in flexible substrates, which is at most urgency for developing novel counter electrodes for lightweight flexible solar cells.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2011/aamick.2011.3.issue-6/am2003735/production/images/medium/am-2011-003735_0008.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/am2003735'>ACS Electronic Supporting Info</A></P>

Modulating the interaction between gold and TiO₂ nanowires for enhanced solar driven photoelectrocatalytic hydrogen generation

Sudhagar, P.,Song, Taeseup,Devadoss, Anitha,Lee, Jung Woo,Haro, Marta,Terashima, Chiaki,Lysak, Volodymyr V.,Bisquert, Juan,Fujishima, Akira,Gimenez, Sixto,Paik, Ungyu The Royal Society of Chemistry 2015 Physical chemistry chemical physics Vol.17 No.29

<P>The interaction strength of Au nanoparticles with pristine and nitrogen doped TiO<SUB>2</SUB> nanowire surfaces was analysed using density functional theory and their significance in enhancing the solar driven photoelectrocatalytic properties was elucidated. In this article, we prepared 4-dimethylaminopyridine capped Au nanoparticle decorated TiO<SUB>2</SUB> nanowire systems. The density functional theory calculations show {101} facets of TiO<SUB>2</SUB> as the preferred phase for dimethylaminopyridine–Au nanoparticles anchoring with a binding energy of −8.282 kcal mol<SUP>−1</SUP>. Besides, the interaction strength of Au nanoparticles was enhanced nearly four-fold (−35.559 kcal mol<SUP>−1</SUP>) at {101} facets <I>via</I> nitrogen doping, which indeed amplified the Au nanoparticle density on nitrided TiO<SUB>2</SUB>. The Au coated nitrogen doped TiO<SUB>2</SUB> (N–TiO<SUB>2</SUB>–Au) hybrid electrodes show higher absorbance owing to the light scattering effect of Au nanoparticles. In addition, N–TiO<SUB>2</SUB>–Au hybrid electrodes block the charge leakage from the electrode to the electrolyte and thus reduce the charge recombination at the electrode/electrolyte interface. Despite the beneficial band narrowing effect of nitrogen in TiO<SUB>2</SUB> on the electrochemical and visible light activity in N–TiO<SUB>2</SUB>–Au hybrid electrodes, it results in low photocurrent generation at higher Au NP loading (3.4 × 10<SUP>−7</SUP> M) due to light blocking the N–TiO<SUB>2</SUB> surface. Strikingly, even with a ten-fold lower Au NP loading (0.34 × 10<SUP>−7</SUP> M), the synergistic effects of nitrogen doping and Au NPs on the N–TiO<SUB>2</SUB>–Au hybrid system yield high photocurrent compared to TiO<SUB>2</SUB> and TiO<SUB>2</SUB>–Au electrodes. As a result, the N–TiO<SUB>2</SUB>–Au electrode produces nearly 270 μmol h<SUP>−1</SUP> cm<SUP>−2</SUP> hydrogen, which is nearly two-fold higher than the pristine TiO<SUB>2</SUB> counterpart. The implications of these findings for the design of efficient hybrid photoelectrocatalytic electrodes are discussed.</P> <P>Graphic Abstract</P><P>The interaction strength of Au nanoparticles with pristine and nitrogen doped TiO<SUB>2</SUB> nanowire surfaces was analysed using density functional theory and their significance in enhancing the solar driven photoelectrocatalytic properties was elucidated. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c5cp01175b'> </P>

Three dimensional-TiO₂ nanotube array photoanode architectures assembled on a thin hollow nanofibrous backbone and their performance in quantum dot-sensitized solar cells

Han, Hyungkyu,Sudhagar, P.,Song, Taeseup,Jeon, Yeryung,Mora-Seró,, Ivá,n,Fabregat-Santiago, Francisco,Bisquert, Juan,Kang, Yong Soo,Paik, Ungyu The Royal Society of Chemistry 2013 Chemical communications Vol.49 No.27

<P>Facile synthesis of TiO<SUB>2</SUB> nanotube branched (length ∼0.5 μm) thin hollow-nanofibers is reported. The hierarchical three dimensional photoanodes (H-TiO<SUB>2</SUB>-NFs) (only ∼1 μm thick) demonstrate their excellent candidature as photoanodes in QD-sensitized solar cells, exhibiting ∼3-fold higher energy conversion efficiency (<I>η</I> = 2.8%, <I>J</I><SUB>sc</SUB> = 8.8 mA cm<SUP>−2</SUP>) than that of the directly grown nanotube arrays on a transparent conducting oxide (TCO) substrate (<I>η</I> = 0.9%, <I>J</I><SUB>sc</SUB> = 2.5 mA cm<SUP>−2</SUP>).</P> <P>Graphic Abstract</P><P>Facile synthesis of TiO<SUB>2</SUB> nanotube branched (length ∼0.5 μm) thin hollow-nanofibers is reported. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c3cc40439k'> </P>

Successful demonstration of an efficient I⁻/(SeCN)₂ redox mediator for dye-sensitized solar cells

Song, Donghoon,Kang, Moon-Sung,Lee, Yong-Gun,Cho, Woohyung,Lee, Jung Hyun,Son, Taewook,Lee, Kyoung Jun,Nagarajan, S.,Sudhagar, P.,Yum, Jun-Ho,Kang, Yong Soo The Royal Society of Chemistry 2012 Physical chemistry chemical physics Vol.14 No.2

<p>A new I<SUP>−</SUP>/(SeCN)<SUB>2</SUB> redox mediator has favorable properties for dye-sensitized solar cells (DSCs) such as less visible light absorption, higher ionic conductivity, and downward shift of redox potential than I<SUP>−</SUP>/I<SUB>3</SUB><SUP>−</SUP>. It was then applied for DSCs towards increasing energy conversion efficiency, giving a new potential for improving performance.</p> <P>Graphic Abstract</P><P>A novel I<SUP>−</SUP>/(SeCN)<SUB>2</SUB> redox mediator, for the first time, showed favourable electrochemical properties for DSCs, resulting in an increased energy conversion efficiency. <img src='http://pubs.rsc.org/ej/CP/2011/c1cp23457a/c1cp23457a-ga.gif'> </P>

Effective Passivationof Nanostructured TiO₂ Interfaces with PEG-Based OligomericCoadsorbents To Improve the Performance of Dye-Sensitized Solar Cells

Lee, Yong-Gun,Park, Suil,Cho, Woohyung,Son, Taewook,Sudhagar, P.,Jung, June Hyuk,Wooh, Sanghyuk,Char, Kookheon,Kang, Yong Soo AmericanChemical Society 2012 The Journal of Physical Chemistry Part C Vol.116 No.11

<P>A novel poly(ethylene glycol) (PEG) based oligomericcoadsorbent was employed to passivate TiO<SUB>2</SUB> photoanodesresulting in the large increase in both open-circuit voltage (<I>V</I><SUB>oc</SUB>) and short-circuit current density (<I>J</I><SUB>sc</SUB>) primarily because of the reduced electronrecombination by the effective coverage of vacant sites as well asthe negative band-edge shift of TiO<SUB>2</SUB>. The effective suppressionof electron recombination was evidenced by electrochemical impedancespectroscopy (EIS) and by stepped light-induced transient measurementsof photocurrent and voltage (SLIM-PCV). The work function measurementsalso showed that the existence of coadsorbents on TiO<SUB>2</SUB> interfacesis capable of shifting the band-edge of TiO<SUB>2</SUB> photoanodesupwardly resulting in the increase in photovoltage. In addition, thecoadsorbent was proven to be effective even in the presence of commonadditives such as LiI, 4-<I>tert</I>-butylpyridine, andguanidinium thiocyanate. The effect of Li<SUP>+</SUP> cation trappingby ethylene oxide units of the coadsorbent was particularly notableto significantly increase <I>V</I><SUB>oc</SUB> at a smallexpense of <I>J</I><SUB>sc</SUB>. Consequently, the introductionof novel PEG-based oligomeric coadsorbents for TiO<SUB>2</SUB> photoanodesis quite effective in the improvement of photovoltaic performancebecause of the simultaneous increase in both <I>V</I><SUB>oc</SUB> and <I>J</I><SUB>sc</SUB>.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2012/jpccck.2012.116.issue-11/jp210360n/production/images/medium/jp-2011-10360n_0001.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp210360n'>ACS Electronic Supporting Info</A></P>

Synergistic Metal–Metal Oxide Nanoparticles Supported Electrocatalytic Graphene for Improved Photoelectrochemical Glucose Oxidation

Devadoss, Anitha,Sudhagar, P.,Das, Santanu,Lee, Sang Yun,Terashima, C.,Nakata, K.,Fujishima, A.,Choi, Wonbong,Kang, Yong Soo,Paik, Ungyu American Chemical Society 2014 ACS APPLIED MATERIALS & INTERFACES Vol.6 No.7

<P>We report the fabrication of graphene–WO<SUB>3</SUB>–Au hybrid membranes and evaluate their photocatalytic activity towards glucose oxidase mediated enzymatic glucose oxidation. The dual-functionality of gold nanoparticles in the reinforcement of visible light activity of graphene–WO<SUB>3</SUB> membranes and improving the catalytic activity of immobilized enzymes for unique photoelectrochemical sensing application is demonstrated. This work provides new insights into the fabrication of light-sensitive hybrid materials and facilitates their application in future.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/aamick/2014/aamick.2014.6.issue-7/am4058925/production/images/medium/am-2013-058925_0011.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/am4058925'>ACS Electronic Supporting Info</A></P>