<i>In situ</i> reduction and exfoliation of g-C₃N₄ nanosheets with copious active sites <i>via</i> a thermal approach for effective water splitting

Pawar, Rajendra C.,Kang, Suhee,Han, Hyuksu,Choi, Heechae,Lee, Caroline S. The Royal Society of Chemistry 2019 Catalysis Science & Technology Vol.9 No.4

<P>Poor optical absorbance and charge recombination are the major drawbacks of polymeric graphitic carbon nitride (g-C3N4)-based photocatalysts. In this paper, we show for the first time a single-step <I>in situ</I> technique to control the porosity of two-dimensional g-C3N4 sheets and exfoliate them by introducing ascorbic acid (AA) molecules. The AA simultaneously acts as the carbon (C) source and deposits amorphous C onto g-C3N4 sheets. Nanosized pores are also introduced into the g-C3N4 sheets, leading to a large number of active sites. The as-prepared C-doped porous g-C3N4 nanosheets demonstrate a high visible light-photocatalytic H2 production activity of 793 μmol g<SUP>−1</SUP> with the optimum structure, which is almost 25 times higher than the value obtained with bulk g-C3N4 (31 μmol g<SUP>−1</SUP>). This exceptional photocatalytic performance arises from the C-doped conjugated system and porous nanosheets. The enhanced photocatalytic H2 evolution was attributed to the effective separation and transport of charge carriers by the deposition of C onto the nanosheets and an increased number of active sites resulting from the nanopores created inside the g-C3N4 sheets. Moreover, molecular dynamics (MD) simulations confirm that the interaction between AA and melamine molecules at elevated temperatures results in the formation of C-doped porous and exfoliated g-C3N4 structures. Therefore, the present approach is very promising for application to the design of new and efficient photocatalysts for photocatalytic H2 evolution under visible irradiation.</P>

Hybrid photocatalysts using graphitic carbon nitride/cadmium sulfide/reduced graphene oxide (g-C₃N₄/CdS/RGO) for superior photodegradation of organic pollutants under UV and visible light

Pawar, Rajendra C.,Khare, Varsha,Lee, Caroline Sunyong The Royal Society of Chemistry 2014 Dalton Transactions Vol.43 No.33

<P>Graphitic carbon nitride (g-C<SUB>3</SUB>N<SUB>4</SUB>) was hybridized with CdS nanoparticles and reduced graphene oxide (RGO) sheets using a facile chemical method, for the application of catalytic photodegradation of Rhodamine B and Congo red dyes under irradiation with UV and visible light. Fourier-transform infrared (FTIR) spectroscopy and X-ray photoemission spectroscopy (XPS) analyses confirmed the formation of pure g-C<SUB>3</SUB>N<SUB>4</SUB>, as well as g-C<SUB>3</SUB>N<SUB>4</SUB>/CdS, g-C<SUB>3</SUB>N<SUB>4</SUB>/RGO, and g-C<SUB>3</SUB>N<SUB>4</SUB>/CdS/RGO composites. The large surface area of the g-C<SUB>3</SUB>N<SUB>4</SUB>/CdS/RGO composite (70.42 m<SUP>2</SUP> g<SUP>−1</SUP>) resulted in rapid dye adsorption onto the surface of the photocatalyst, leading to effective photodegradation of organic pollutants. The addition of CdS and RGO increased the photocatalytic activity of g-C<SUB>3</SUB>N<SUB>4</SUB> by a factor of approximately twenty compared with that of the commercially available TiO<SUB>2</SUB> catalyst under visible light, and the g-C<SUB>3</SUB>N<SUB>4</SUB>/CdS/RGO composite was found to significantly enhance the catalytic effect compared with pure g-C<SUB>3</SUB>N<SUB>4</SUB> and with the g-C<SUB>3</SUB>N<SUB>4</SUB>/CdS and g-C<SUB>3</SUB>N<SUB>4</SUB>/RGO composites. The superior photocatalytic activity of the g-C<SUB>3</SUB>N<SUB>4</SUB>/CdS/RGO composite is attributed to enhanced separation of the photogenerated electron–hole pairs, as well as increased visible-light absorption. The improved transport of photoelectrons was consistent with the results of transient photocurrent measurements. Therefore, g-C<SUB>3</SUB>N<SUB>4</SUB>/CdS/RGO composites using a facile method are applicable to the development of high-efficiency photocatalytic devices for industrial applications.</P> <P>Graphic Abstract</P><P>Schematic of the electron transport from g-C<SUB>3</SUB>N<SUB>4</SUB> to the CdS nanoparticles and RGO sheets during RhB photodegradation. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4dt01278j'> </P>

Integration of ZnO with g-C3N4 structures in coreeshell approach via sintering process for rapid detoxification of water under visible irradiation

Rajendra C. Pawar,Yeonho Son,Jongryul Kim,Sung-Hoon Ahn,Caroline Sunyong Lee 한국물리학회 2016 Current Applied Physics Vol.16 No.1

We integrated ZnO with g-C3N4 nanostructures via successful coreeshell formation using single step simple sintering process for photocatalysis application. Analytical tools show that the g-C3N4 shell with an average thickness of 3 nm has been coated onto surface of ZnO nanoparticles. Additionally, few ZnO particles are sporadically distributed on g-C3N4 sheets forming their nanocomposites. Further, these core eshell nanocomposites are used as photocatalysts for the degradation of Rhodamine B (RhB) under visible irradiation. As a result of photocatalysis measurements, it was found that 0.75 wt% zinc source with g-C3N4 exhibited almost two times high kinetic rate constant (k ¼ 6.87 103 min1) as compared with that of pure g-C3N4 (k ¼ 3.38 103 min1) and ZnO (k ¼ 1.03 103 min1) nanostructures. This enhanced photocatalytic performance of coreeshell nanocomposites was aroused from effective interfacial charge separation and transportation, resulted into rapid degradation of RhB molecules. Moreover, increased optical absorption in visible region of spectrum revealed the large number of generation of charge carriers. Fabricated ZnO/g-C3N4 coreeshell nanocomposites are also stable upto four cycles, indicating its good stability. Therefore, present coreeshell nanocomposites can be applied to water purification devices under visible irradiation.

Minimization of Recombination Losses in 3D Nanostructured TiO₂ Coated with Few Layered g-C₃N₄ for Extended Photo-response

Kang, Suhee,Pawar, Rajendra C.,Park, Tae Joon,Kim, Jin Geum,Ahn, Sung-Hoon,Lee, Caroline Sunyong The Korean Ceramic Society 2016 한국세라믹학회지 Vol.53 No.4

We have successfully fabricated 3D (3-dimensional) nanostructures of $TiO_2$ coated with a $g-C_3N_4$ layer via hydrothermal and sintering methods to enhance photoelectrochemical (PEC) performance. Due to the coupling of $TiO_2$ and $g-C_3N_4$, the nanostructures exhibited good performance as the higher conduction band of $g-C_3N_4$, which can be combined with $TiO_2$. To fabricate 3D nanostructures of $g-C_3N_4/TiO_2$, $TiO_2$ was first grown as a double layer structure on FTO (Fluorine-doped tin oxide) substrate at $150^{\circ}C$ for 3 h. After this, the $g-C_3N_4$ layer was coated on the $TiO_2$ film at $520^{\circ}C$ for 4 h. As-prepared samples were varied according to loading of melamine powder, with values of loading of 0.25 g, 0.5 g, 0.75 g, and 1 g. From SEM and TEM analysis, it was possible to clearly observe the 3D sample morphologies. From the PEC measurement, 0.5 g of $g-C_3N_4/TiO_2$ film was found to exhibit the highest current density of $0.12mA/cm^2$, along with a long-term stability of 5 h. Compared to the pristine $TiO_2$, and to the 0.25 g, 0.75 g, and 1 g $g-C_3N_4/TiO_2$ films, the 0.5 g of $g-C_3N_4/TiO_2$ sample was coated with a thin $g-C_3N_4$ layer that caused separation of the electrons and the holes; this led to a decreasing recombination. This unique structure can be used in photoelectrochemical applications.

Solvent-polarity-induced hematite (α-Fe₂O₃) nanostructures for lithium-ion battery and photoelectrochemical applications

Pawar, Rajendra C.,Um, Ji Hyun,Kang, Suhee,Yoon, Won-Sub,Choe, Heeman,Lee, Caroline S. Elsevier 2017 ELECTROCHIMICA ACTA Vol.245 No.-

<P><B>Abstract</B></P> <P>A range of morphology-controlled hematite superstructures, including two-dimensional sheet assembly, microspheres with a vein-like network, hollow urchin with tapered nanorods, and flowers with porous branch-like structures, were obtained from different solvents via the solvothermal approach. The results showed that the variation of the solvent from protic to aprotic facilitated morphological modifications, with the solvent playing a key role in determining the overall architecture of the oxide particles under different polar conditions. A porous microsphere grown from water solvent delivered a reversible capacity of 880mAhg<SUP>−1</SUP> over 60 cycles at 200mAg<SUP>−1</SUP> with good rate stability. Here, the bicontinuous nanoporous structure, with well-developed mesoporosity in the microsphere, reduced the severity of volume changes during the cycling process and stabilized passivation layer growth. The hematite nanostructure grown from water/dimethylformamide solvent exhibited the highest photocurrent density of 190μA/cm<SUP>−2</SUP> vs. Ag/AgCl, which was attributed to the rapid transport of charge carriers in the porous nanostructure. Thus, solvent-mediated synthesis can be used to prepare a range of hematite porous nanostructures for use as an anode in lithium-ion batteries and as a photoanode in photoelectrochemical cells, as well as for water splitting, gas sensors, and catalytic applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Solvent polarity induced α-Fe<SUB>2</SUB>O<SUB>3</SUB> nanostructures via solvothermal method. </LI> <LI> Polar aprotic/protic solvent controls growth kinetics. </LI> <LI> Excellent lithium ion storage and photoelectrochemical properties. </LI> <LI> Mesoporus structure improves the actives sites and performances. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>Cycle performance of the porous microsphere grown from H<SUB>2</SUB>O solvent at 200mAg<SUP>−1</SUP> compared to similar hematite structures from the literature with veins network (inset TEM image).</P> <P>[DISPLAY OMISSION]</P>

Defect-controlled growth of ZnO nanostructures using its different zinc precursors and their application for effective photodegradation

Rajendra C. Pawar,김형섭,이선영 한국물리학회 2014 Current Applied Physics Vol.14 No.4

Various zinc precursors, such as zinc acetate, zinc nitrate, zinc sulfate, and zinc chloride, have been used to control the formation of zinc oxide (ZnO) nanostructures onto aluminum substrate by chemical means. FESEM images of the ZnO nanostructures showed the formation of different morphologies, such as flakes, nanowalls, nanopetals, and nanodisks, when the nanostructures were synthesized using zinc acetate, zinc nitrate, zinc sulfate, and zinc chloride precursors, respectively. The TEM image of disk-like ZnO nanostructures formed using zinc chloride as a precursor revealed hexagonally shaped particles with an average diameter of 0.5 mm. Room-temperature photoluminescence (PL) spectra revealed a large quantity of surface oxygen defects in ZnO nanodisks grown from zinc chloride compared with those using other precursors. Furthermore, the ZnO nanostructures were evaluated for photocatalytic activity under ultraviolet (UV) light illumination. Nanostructures having a disk-like shape exhibited the highest photocatalytic performance (k ¼ 0.027 min1) for all the ZnO nanostructures studied. Improved photocatalytic activity of ZnO nanodisks was attributed to their large specific surface area (4.83 m2 g1), surface oxygen defects, and super-hydrophilic nature of their surface, which is particularly suitable for dye adsorption.

Study of multi-faceted CoS2 introduced graphene aerogel hybrids via chemical approach for an effective electrocatalytic water splitting

Pawar Rajendra C.,강수희,Khan Haritham,한혁수,이선영 한국물리학회 2021 Current Applied Physics Vol.32 No.-

The present article reports the synthesis of hybrid structure along with non-precious cobalt-disulfide. A simple hydrothermal method was used to fabricate multi-faceted CoS2 introduced graphene aerogels. Studies on electrocatalytic activity showed that the presence of CoS2 facets along with graphene aerogel played a prominent role in the enhancement of proton reduction to hydrogen gas. The CoS2/graphene aerogel hybrid sample exhibits extremely low overpotential (160 mV vs. RHE), and high current density for HER in acidic solution. The activity enhancement can be attributed to increasing the active electrochemical surface area of graphene aerogel and faceted particles inside the 3D matrix of graphene. Furthermore, the CoS2/graphene hybrid retained its high activity even after 1000 cycles of cyclic voltammetry scans, signifying longer stability under acidic condition. The results suggest that CoS2/graphene aerogel hybrids show their potential application to hydrogen evolution reaction.

Direct coating of a g-C₃N₄ layer onto one-dimensional TiO₂ nanocluster/nanorod films for photoactive applications

Kang, Suhee,Jang, Joonyoung,Pawar, Rajendra C.,Ahn, Sunghoon,Lee, Caroline Sunyong The Royal Society of Chemistry 2018 Dalton Transactions Vol.47 No.21

<P>We report the coating of metal-free graphitic carbon nitride (g-C3N4) onto titanium dioxide (TiO2) nanorods <I>via</I> a thermal evaporation method. Prior to g-C3N4 coating, TiO2 nanoclusters were grown on TiO2 nanorods to enhance the surface area by dipping in a TiCl3 solution for 12, 24 and 36 h. The prepared films were analyzed to assess the improvement in absorbance and reduction in recombination losses. Nanoclustered TiO2 grown for 24 h and then coated with a g-C3N4 film (<I>i.e.</I>, TC_24h_CN) had the highest photocurrent of 235 and 290 μA, respectively, when measured by transient photocurrent and linear sweep voltammetry techniques. The enhanced performance resulted from a reduced recombination of electron-hole pairs. The TC_24h_CN film displayed an excellent photoresponse over 15 h of exposure to visible light and hence could potentially be used in water purification device technology.</P>

Size-controlled BiOCl–RGO composites having enhanced photodegradative properties

Kang, Suhee,Pawar, Rajendra C.,Pyo, Youngjun,Khare, Varsha,Lee, Caroline Sunyong Taylor Francis 2016 Journal of experimental nanoscience Vol.11 No.4

<P>Visible light-active bismuth oxychloride-reduced graphene oxide (BiOCl-RGO) composite photocatalysts were synthesised using a hydrothermal method at low temperature, and at a low cost. This approach reduced the recombination of electron-hole pairs and thereby provided more efficient photocatalysts. The size of BiOCl structure was controlled by polyvinylpyrrolidone (PVP) addition. Furthermore, formation of nanosized BiOCl sheets and BiOCl-RGO composites were confirmed by X-ray diffraction, X-ray photoelectron spectroscopy, field-emission scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Fabricated BiOCl-RGO composite with PVP exhibited better photocatalytic activity than pure BiOCl grown with and without PVP towards degradation of Rhodamine B (RhB). It was found that the composite photocatalyst degrades RhB completely within 310 min as compared with several hours for pure BiOCl. The improved photocatalytic performance of BiOCl-RGO composite was attributed to its high specific surface area (22.074 m(2) g(-1) and existence of polar surfaces, compared with 9.831 m(2) g(-1) for pure BiOCl). The analyses indicated that RGO helped to reduce recombination losses and improve electron transport. It also showed that presence of polar surfaces improved photocatalytic activity of BiOCl. Hence, BiOCl-RGO composite is a promising catalyst for the degradation of organic pollutants under visible light and could be used in applications such as water purification devices.</P>

Electrospun one-dimensional graphitic carbon nitride-coated carbon hybrid nanofibers (GCN/CNFs) for photoelectrochemical applications

장준영,강수희,Rajendra C. Pawar,이선영 한국물리학회 2018 Current Applied Physics Vol.18 No.9

Coupling of graphitic carbon nitride (GCN) with electrospun carbon nanofibers (CNFs) enhanced the photoelectrochemical (PEC) performance of a pristine GCN photoanode. Polyacrylonitrile (PAN) was electrospun to form fibers that were then carbonized to form one-dimensional (1D) CNFs, which were then used to fabricate the GCN structure. The optimum GCN/CNFs hybrid structure was obtained by controlling the amount of GCN precursors (urea/thiourea). The surface morphology of the hybrid structure revealed the coating of GCN on the CNFs. Additionally, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and X-ray diffraction confirmed the phases of the GCN/CNFs hybrids. PEC results showed a higher photocurrent of 3 μA for the hybrid compared with that of 1 μA for the pristine GCN. The high photocurrent for the hybrid structures indicated the formation of heterojunctions that resulted from a lower recombination rate of charge carriers. Moreover, UTh0.075 (0.075 g of urea and 0.075 g of thiourea) hybrid sample showed the highest performance of hydrogen generation with its numerical value of 437 μmol/g, compared to those of UTh0.1(0.1 g of urea and 0.1 g of thiourea) and UTh0.05 (0.05 g of urea and 0.05 g of thiourea) composite samples. This higher hydrogen production could be explained again with successful formation of heterojunctions between GCN and CNFs. Overall, we report a new approach for obtaining 1D hybrid structures, having better PEC performance than that of pristine GCN. These hybrids could potentially be used in energy-related devices.