Electronic structure and dielectric properties of ZrO₂-CeO₂ mixed oxides

Varshney, Mayora,Sharma, Aditya,Chae, Keun Hwa,Kumar, Shalendra,Won, Sung Ok Elsevier 2018 The Journal of physics and chemistry of solids Vol.119 No.-

<P><B>Abstract</B></P> <P>ZrO<SUB>2</SUB>–CeO<SUB>2</SUB> mixed ceramics were studied for their local electronic structure and dielectric properties. X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray absorption fine structure (XAFS) and frequency dependence dielectric constant measurements were performed on the samples. Rietveld refinements and quantitative phase analysis of ZrO<SUB>2</SUB>-CeO<SUB>2</SUB> ceramics showed an increased concentration of tetragonal ZrO<SUB>2</SUB> and cubic Zr<SUB>0.5</SUB>Ce<SUB>0.5</SUB>O<SUB>2</SUB> phases at the expense of parent monoclinic ZrO<SUB>2</SUB>. The morphology of the ZrO<SUB>2</SUB>-CeO<SUB>2</SUB> ceramics remained invariant for all of the fractional percentages of CeO<SUB>2</SUB> loading. O K-edge and Ce M<SUB>5,4</SUB>-edge NEXAFS showed the oxygen vacancies and Ce<SUP>3+</SUP>-Ce<SUP>4+</SUP> redox pair formation in ZrO<SUB>2</SUB>CeO<SUB>2</SUB> ceramics, which is further corroborated by Ce L<SUB>3</SUB>-edge XAFS. The higher CeO<SUB>2</SUB> molar ratios containing ZrO<SUB>2</SUB>–CeO<SUB>2</SUB> ceramics showed higher ε′, ε″ and σ<SUB>ac</SUB> and lower tan δ.</P> <P><B>Highlights</B></P> <P> <UL> <LI> ZrO<SUB>2</SUB>-CeO<SUB>2</SUB> mixed ceramics were prepared using a solid-state reaction technique. </LI> <LI> XRD, FE-SEM, XAFS and frequency-dependent dielectric-constant measurements were performed. </LI> <LI> O K-edge, Ce M<SUB>5,4</SUB>-edge and Ce L<SUB>3</SUB>-edge XANES have confirmed Ce<SUP>3+</SUP> and Ce<SUP>4+</SUP> redox pairs. </LI> <LI> ZrO<SUB>2</SUB>-CeO<SUB>2</SUB> ceramics have shown higher <I>ε′, ε″</I> and <I>σ</I> <SUB> <I>ac</I> </SUB> and lower <I>tan δ</I> as a function of frequency. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

NEXAFS and EXAFS study of Zr and Hf doped CeO2 Nanoparticles

Aditya Sharma,Mayora Varshney,Tae-Kyun Ha,Hyun-Joon Shin 한국진공학회 2014 한국진공학회 학술발표회초록집 Vol.2014 No.8

XANES, EXAFS and photoluminescence investigations on the amorphous Eu:HfO₂

Sharma, Aditya,Varshney, Mayora,Shin, Hyun-Joon,Chae, KeunHwa,Won, Sung Ok Elsevier 2017 Spectrochimica acta. Part A, Molecular and biomole Vol.173 No.-

<P><B>Abstract</B></P> <P>We report detailed investigations on the local electronic/atomic structure and photoluminescence properties of chemically synthesized Eu:HfO<SUB>2</SUB> powders. X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), extended X-ray absorption fine structure (EXAFS) and photoluminescence (PL) measurements were performed to analyze the crystal structure, local atomic/electronic structure and luminescence properties of the samples. No crystalline phases were detected with Cu <I>K</I>α (λ=1.5418Å) based XRD; however, local monoclinic structure was confirmed by the Hf <I>L</I>-edge XANES and EXAFS. O <I>K</I>-edge XANES spectral features could be deconvoluted with doublets and triplets in <I>e</I> <SUB> <I>g</I> </SUB> and <I>t</I> <SUB> <I>2g</I> </SUB> orbitals, respectively, which ascribed to the local monoclinic structure for all of the samples. Eu <I>M</I> <SUB>5,4</SUB>-edge XANES confirmed the pre-dominancy of Eu<SUP>3+</SUP> ions in the HfO<SUB>2</SUB> samples with a fractional amount of Eu<SUP>2+</SUP> ions. PL spectra revealed the electric dipole allowed (<SUP>5</SUP> <I>D</I> <SUB>0</SUB>-<SUP>7</SUP> <I>F</I> <SUB>0,2,4</SUB>) emission properties of Eu:HfO<SUB>2</SUB> samples. The orange-red emission is ascribed to the Eu interstitial/surface segregation induced defects.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Eu:HfO<SUB>2</SUB> samples were synthesized using co-precipitation method. </LI> <LI> XRD, XANES, EXAFS and photoluminescence measurements were performed. </LI> <LI> Eu<SUP>3+</SUP> ions were dominantly present in Eu doped samples with marginal Eu<SUP>2+</SUP> ions. </LI> <LI> Hf <I>L</I> <SUB>1</SUB>-edge XANES/EXAFS and O <I>K</I>-edge XANES confirmed local monoclinic structure in amorphous samples. </LI> <LI> High emission characteristics are achieved by Eu insertion in the weakly emitting HfO<SUB>2</SUB>. </LI> </UL> </P> <P><B>Graphical Abstract</B></P> <P>[DISPLAY OMISSION]</P>

Electronic structure study of Ce_1−<i>x</i>A_<i>x</i>O₂ (A = Zr & Hf) nanoparticles: NEXAFS and EXAFS investigations

Sharma, Aditya,Varshney, Mayora,Shin, Hyun-Joon,Park, Yong Jun,Kim, Min-Gyu,Ha, Tae-Kyun,Chae, Keun Hwa,Gautam, Sanjeev The Royal Society of Chemistry 2014 Physical Chemistry Chemical Physics Vol.16 No.37

<P>Single phase nanoparticles (NPs) of CeO<SUB>2</SUB>, Ce<SUB>0.5</SUB>Zr<SUB>0.5</SUB>O<SUB>2</SUB>, Ce<SUB>0.5</SUB>Hf<SUB>0.5</SUB>O<SUB>2</SUB> and Ce<SUB>0.5</SUB>Hf<SUB>0.25</SUB>Zr<SUB>0.25</SUB>O<SUB>2</SUB> were successfully synthesized by co-precipitation method at constant pH and temperature. The X-ray diffraction results revealed that the additive atoms did not segregate to form secondary phases but led to grain size variation in the NPs. The 10 Dq values in the near edge X-ray absorption fine structure (NEXAFS) spectra at the O K-edge did not vary in the same way as the average grain size was changed for the doped CeO<SUB>2</SUB> NPs. The deconvolution of Ce M<SUB>5</SUB>-edge and detailed analysis of O K pre-edge peak have shown the higher Ce<SUP>+3</SUP>/(Ce<SUP>+3</SUP> + Ce<SUP>+4</SUP>) ratio in the Zr- and Hf-doped samples. The local atomic structure around the Ce, Zr and Hf atoms was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy at Ce K-edge, Zr K-edge and Hf L<SUB>3</SUB>-edge, respectively, and the EXAFS data were fitted with the theoretical calculations. The 4f occupancy, Ce<SUP>+3</SUP>/(Ce<SUP>+3</SUP> + Ce<SUP>+4</SUP>) ratio of Ce ions, coordination number of Ce and Ce–Ce/Ce–O bond distances were sensitive to the additive atoms but not explicitly changed according to the grain size variation in the NPs.</P> <P>Graphic Abstract</P><P>4f-occupation, Ce<SUP>+3</SUP>/Ce<SUP>+4</SUP> ratio and grain-size effects in Zr- and Hf-substituted CeO<SUB>2</SUB> nanoparticles are investigated by NEAXFS and EXAFS. <IMG SRC='http://pubs.rsc.org/services/images/RSCpubs.ePlatform.Service.FreeContent.ImageService.svc/ImageService/image/GA?id=c4cp02409e'> </P>

X-ray absorption spectroscopy study and photocatalyst application of CuO and Cu0.9Ti0.1O nanoparticles

Aditya Sharma,Mayora Varshney,하태균,채근화,신현준 한국물리학회 2015 Current Applied Physics Vol.15 No.10

We report detailed investigations on the electronic structure and photocatalyst application of CuO and Cu0.9Ti0.1O nanoparticles (NPs). The NPs were prepared by co-precipitation method and subsequent annealing. Crystal structure and morphology of the NPs were investigated by synchrotron X-ray diffraction and high resolution transmission electron microscope, respectively. The local atomic structure around the Cu atoms was investigated by the extended X-ray absorption fine structure (EXAFS) at the Cu K-edge. Electronic structure determination was done using near edge X-ray absorption fine structure (NEXAFS) at the O K-edge, Cu L-edge, Cu K-edge and Ti L-edge. From the structural and electronic structure investigations, it is inferred that the Ti substitutes the Cu in CuO lattice without forming any secondary phases and the valence state of Cu is not affected by the Ti substitution; however the Cu - O bond length is found to be shorten in the Ti doped sample. As prepared NPs exhibit excellent photocatalyst application toward the degradation of methyl orange (MO) and potassium dichromate (PD) pollutant dyes under the visible light irradiation. The mechanism of the photodegradation of MO and PD pollutants, by the smaller sized CuO and larger sized Cu0.9Ti0.1O NPs, is briefly discussed.

Copper insertion induced augmentation in the photocatalyst and electronic structure properties of TiO₂ and ZnO nanostructures

Aditya Sharma,Mayora Varshney,Byeong-Hyeon Lee,HyeMi Jo,Keun-Hwa Chae,Sung Ok Won 한국진공학회 2017 한국진공학회 학술발표회초록집 Vol.2017 No.2

Mechanistic investigations on emission characteristics from g-C3N4, g-C3N4@Pt and g-C3N4@Ag nanostructures using X-ray absorption spectroscopy

Aditya Sharma,Mayora Varshney,채근화,원성옥 한국물리학회 2018 Current Applied Physics Vol.18 No.11

An improved method for the preparation of g-C3N4 is described. Currently, heating (> 400 C°) of urea is the common method used for preparing the g-C3N4. We have found that sonication of melamine in HNO3 solution, followed by washing with anhydrous ethanol, not only reduce the crystallite size of g-C3N4 but also facilitate intriguing electronic structure and photoluminescence (PL) properties. Moreover, loading of metal (Pt and Ag) nanoparticles, by applying the borohydride reduction method, has resulted in multicolor-emission from g-C3N4. With the help of PL spectra and local electronic structure study, at C K-edge, N K-edge, Pt L-edge and Ag K-edge by X-ray absorption spectroscopy (XAS), a precise mechanism of tunable luminescence is established. The PL mechanism ascribes the amendments in the transitions, via defect and/or metal states assimilation, between the π* states of tris-triazine ring of g-C3N4 and lone pair states of nitride. It is evidenced that interaction between the C/N 2p and metal 4d/5d orbitals of Ag/Pt has manifested a net detraction in the δ*→LP transitions and enhancement in the π*→LP and π*→π transitions, leading to broad PL spectra from g-C3N4 organic semiconductor compound.

Structural, electronic structure and antibacterial properties of graphene-oxide nano-sheets

Sharma, Aditya,Varshney, Mayora,Nanda, Sitansu Sekhar,Shin, Hyun Joon,Kim, Namdong,Yi, Dong Kee,Chae, Keun-Hwa,Ok Won, Sung Elsevier 2018 Chemical physics letters Vol.698 No.-

<P><B>Abstract</B></P> <P>Correlation between the structural/electronic structure properties and bio-activity of graphene-based materials need to be thoroughly evaluated before their commercial implementation in the health and environment precincts. To better investigate the local hybridization of sp<SUP>2</SUP>/sp<SUP>3</SUP> orbitals of the functional groups of graphene-oxide (GO) and their execution in the antimicrobial mechanism, we exemplify the antibacterial activity of GO sheets towards the Escherichia coli bacteria (<I>E. coli</I>) by applying the field-emission scanning electron microscopy (FESEM), near edge X-ray absorption fine structure (NEXAFS) and scanning transmission X-ray microscope (STXM) techniques. C K-edge and O K-edge NEXAFS spectra have revealed lesser sp<SUP>2</SUP> carbon atoms in the aromatic ring and attachment of functional oxygen groups at GO sheets. Entrapment of <I>E. coli</I> bacteria by GO sheets is evidenced by FESEM investigations and has also been corroborated by nano-scale imaging of bacteria using the STXM. Spectroscopy evidence of functional oxygen moieties with GO sheets and physiochemical entrapment of <I>E. coli</I> bacteria have assisted us to elaborate the mechanism of cellular oxidative stress-induced disruption of bacterial membrane.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Large-scale synthesis of graphene-oxide nano-sheets by sonication based synthesis. </LI> <LI> Structural and morphology investigations by XRD, FTIR, Raman and HR-TEM. </LI> <LI> Amalgamation of active oxygen species is investigated by C & O K-edge NEXAFS. </LI> <LI> Antibacterial properties are studied by FE-SEM and STXM. </LI> <LI> Mechanistic understanding of cellular oxidative stress-induced disruption of bacteria. </LI> </UL> </P> <P><B>Graphical abstract</B></P> <P>[DISPLAY OMISSION]</P>

Investigation on the local electronic/atomic structure properties using XANES/EXAFS and photocatalyst application of Zr1xCuxO2 (0 x 0.2)

Aditya Sharma,MAYORA VARSHNEY,Keun Hwa Chae,Hyun-Joon Shin,원성옥 한국물리학회 2016 Current Applied Physics Vol.16 No.10

We investigated local electronic/atomic structure properties by using X-ray absorption spectroscopy and demonstrated the photocatalyst application of Zr1xCuxO2 (0 x 0.2) samples. X ray diffraction (XRD) results revealed the amorphous nature of the samples; however, transmission electron microscopy (TEM) depicted the formation of <2 nm sized particles in all of the samples. Local electronic structure was investigated by X-ray absorption near edge structure (XANES) at O K-edge, Zr K-edge and Cu K-edge. The O K-edge XANES suggested strong hybridization of s-p-d orbitals upon Cu doping. The Cu K-edge XANES confirmed þ1 valence state of Cu (Cuþ1) in all of the Cu doped ZrO2samples. Extended X-ray absorption fine structure (EXAFS) analysis at the Zr K-edge nullifies the formation of trivial phases and confirms the local monoclinic structure in all of the samples. Photocatalyst properties of the samples were investigated by monitoring the net degradation in the methyl orange dye aqueous solution under the UV light irradiation. The Cu doped ZrO2 samples exhibited superior photo-catalyst properties over the undoped ZrO2.

X-ray absorption spectroscopy investigations on electronic structure and luminescence properties of Eu:SnO2-SnO nanocomposites

Aditya Sharma,MAYORA VARSHNEY,Hyun-Joon Shin,Keun Hwa Chae,원성옥 한국물리학회 2016 Current Applied Physics Vol.16 No.10

Tin oxide nanostructures are nontoxic in nature and biocompatible. However, they exhibit weak emission characteristics that limit their applications in cellular and biology fields. Therefore, it is important to enhance luminescence properties in this oxide and establish a relationship between the local electronic structure and emission characteristics. In this report, we demonstrate significant advancement in the photoluminescence (PL) properties in the Eu incorporated SnO2-SnO nanocomposites. Eu:SnO2-SnO nanocomposites were synthesized by chemical precipitation method. X-ray diffraction (XRD), X-ray absorption near edge structure (XANES) and PL measurements were performed to investigate the structural, electronic structure and emission properties of the samples. Eu3þ ions, as confirmed by the Eu M5,4-edge XANES studies, lead to defect formation and local electronic structural perturbation in the nanocomposites. The O K-edge and Sn M5,4-edge XANES studies confirm a net decrease in the hybridization of O 2p and Sn 5s/5p states and formation of Sn defects, upon Eu doping. Significant enhancement in the Eu3þ related interband transitions (5D0-7FJ; J ¼ 0,1,2,3,4) is achieved upon increasing the Eu concentrations, indicating synthesis of emission characteristic rich SnO2-SnO nanocomposites.