Modulation of stoichiometry, morphology and composition of transition metal oxide nanostructures through hot wire chemical vapor deposition

Chakrapani, Vidhya; Brier, Matthew; Puntambekar, Ajinkya; DiGiovanni, Thomas

doi:10.1557/jmr.2015.366

Cited by 23 publications

(16 citation statements)

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“…Nanowires with a range of vanadium-to-oxygen ratio were prepared with high purity without the introduction of extraneous reducing agents by simply tuning the filament power and partial pressure of oxygen and water vapor in the chamber, as was shown in our recent work for other oxides. 32,33 Figure 1C shows the scanning electron micrograph (SEM) of as-synthesized oxide grown on fluorinated tin oxide (FTO) substrates. From the image, the as-synthesized deposits are seen to be vertically-aligned, single crystalline nanowires with diameters in the range of 60-80 nm and length extending to 1-2 microns.…”

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“…As is common in most TMOs, the color of the sample provides a visual measure of the extent of deviation from stoichiometry. 32 In vanadium oxide, fully stoichiometric phase (V 2 O 5 ) is orange in optical coloration, while increasing oxygen deficiency shifts the sample color sequentially from orange to yellow-brown to dark green to grayish-black seen in the highly reduced, V 2 O 5-x , phase. Six samples, referred to as S1 to S6 in the text, were prepared with increasing oxygen deficiency, consisting of stoichiometric V 2 O 5 (orange, Sample S1) to highly reduced V 2 O 5-x phase (yellow to black, Samples S2 to S6).…”

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confidence: 99%

“…32 The blue-shift seen in V 2 O 5 may be a consequence of the low dispersion of conduction band states, which leads to band filling even with relatively small concentration of e− injection. In-situ Raman and PL measurements were performed with HORIBA Scientific LabRAM HR Evolution spectrometer.…”

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Defect-induced Burstein-Moss shift in reduced V2O5 nanostructures

et al. 2016

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The main effects of oxygen vacancy defects on the electronic and optical properties of V 2 O 5 nanowires were studied through in-situ Raman, photoluminescence, absorption, and photoemission spectroscopy. Both thermal reduction and electrochemical reduction via lithium insertion leads to the creation of oxygen vacancy defects in the crystal that gives rise to new electronic mid-gap defect states at energy 0.75 eV below the conduction band edge. The defect formation results in delocalization and injection of excess electrons into the conduction band, as opposed to localized electron injection as previously suggested. Contrary to what is seen in most oxides, the presence of vacancy defects leads to band filling and an increase in the optical band gap of V 2 O 5 from 1.95 eV to 2.45 eV, which is attributed to the Burstein-Moss effect. Other observed changes in the optical properties are correlated to the changes in the electronic structure of the oxide as result of defect formation. Further, in-situ Raman measurements during the electrochemical reduction at room temperature show that the oxygen atom that is most readily reduced is the three-fold coordinated oxygen (O3). Reduction of V 2 O 5 through the formation of oxygen vacancy, V O , defects have been studied both by theoretical and experimental studies. 10,11,12,13,14,15 In this work, we address some of the key unanswered questions regarding the correlation between optical properties and electronic structure of The rise of absorbance in the NIR spectral range in reduced oxide is a result of optical excitation of V O defect related mid-gap states. In addition, using in-situ Raman measurements during reduction process, we show that the oxygen atom that most readily participates during electrochemical reduction at room-temperature and gets reduced is the three-fold coordinated oxygen (O3).Vacancies in vanadium oxide have been probed using a variety of spectroscopic techniques, such as scanning tunneling microscopy, 9,26,27 electron energy loss spectroscopy, 28,29 electron paramagnetic resonance studies, 16,12,23 X-ray absorption measurements, 30 and X-ray photoemission spectroscopy (XPS). 15,17 However, most of these techniques either require ultrahigh vacuum (UHV) or controlled environment for operation that makes it difficult to adapt it to 6 in-situ catalytic studies. Here, we use near-infrared photoluminescence (NIR-PL) and Raman spectroscopy that allows for study of vacancy defects and their interaction with redox species under in-operando electrochemical conditions at room temperature, as shown in our recent study. 31 The spectral range of various types of electronic transitions in V 2 O 5 is shown in Fig.1A.Representative PL spectra of stoichiometric and non-stoichiometric V 2 O 5 are shown in Fig.1B.The optical gap of nominally undoped V 2 O 5 is in the range of 1.9-2.5 eV and hence emission spectrum lies in the ultraviolet-to-visible part of the spectral range. In most TMOs, deviation from stoichiometry is a result of the presence of high density ...

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Defect-induced Burstein-Moss shift in reduced V2O5 nanostructures

et al. 2016

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“…Two-dimensional VO 2 platelets were synthesized using a two-step process involving reductive phase transformation of V 2 O 5 platelet films. Single crystalline V 2 O 5 platelets were synthesized on fluorinated tin oxide (FTO)-coated glass slides by hot filament chemical vapor deposition, as reported previously 26 , 70 . Briefly, the technique consists of resistively heating a vanadium wire in the presence of oxygen at a pressure of 800 mTorr at a substrate temperature between 450 and 500 °C.…”

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confidence: 99%

Effects of charge fluctuation and charge regulation on the phase transitions in stoichiometric VO2

Joshi

Smieszek

Chakrapani

2020

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Detailed electrical and photoemission studies were carried out to probe the chemical nature of the insulating ground state of VO2, whose properties have been an issue for accurate prediction by common theoretical probes. The effects of a systematic modulation of oxygen over-stoichiometry of VO2 from 1.86 to 2.44 on the band structure and insulator–metal transitions are presented for the first time. Results offer a different perspective on the temperature- and doping-induced IMT process. They suggest that charge fluctuation in the metallic phase of intrinsic VO2 results in the formation of e− and h+ pairs that lead to delocalized polaronic V3+ and V5+ cation states. The metal-to-insulator transition is linked to the cooperative effects of changes in the V–O bond length, localization of V3+ electrons at V5+ sites, which results in the formation of V4+–V4+ dimers, and removal of $$\pi^{*}$$ π ∗ screening electrons. It is shown that the nature of phase transitions is linked to the lattice V3+/V5+ concentrations of stoichiometric VO2 and that electronic transitions are regulated by the interplay between charge fluctuation, charge redistribution, and structural transition.

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“…This confirms that the holes charge carriers majorly dominate the conductance of the heterostructures system. The dominance of holes in the heterostructure is expected as both NiO [24,46] and NiWO 4 [47,48] are p-type semiconductors.…”

Section: Electrical and Gas Sensing Characteristicsmentioning

confidence: 99%

Novel Christmas Branched Like NiO/NiWO₄/WO₃ (p–p–n) Nanowire Heterostructures for Chemical Sensing

Kaur

Zappa

Maraloiu

et al. 2021

Adv Funct Materials

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Establishing a platform comprising different nanostructured oxides is an emerging idea to develop highly sensitive and selective sensing devices. Herein, novel 3D-heterostructures (p-p-n) consisting of 1D nanowires of NiO and WO 3 along with their intermediate reactive product, i.e., NiWO 4 seed, are produced by a two-steps vapor phase growth method. In-depth morphological and structural investigations describing the growth mechanism of these heterostructures are presented. Finally, the p-p-n heterostructures are integrated into conductometric sensing devices and their performances are investigated toward different gases. It is observed that by modulating the charge-carrier transport with temperature, the heterostructure sensors exhibit selective behavior toward different gas analytes. Indeed, at 300 °C, the heterostructure sensors show relatively selective behavior toward NO 2 , while at 400 °C, high selectivity toward VOCs is observed. The improvement in sensing performances is mainly based on charge carrier transport through the two interfaces (one at WO 3 /NiWO 4 (n-p) and the other at NiWO 4 /NiO (p-p)) and the modulation of charge carriers in the electron depletion layer of WO 3 and hole accumulation layer of NiO and NiWO 4 . The remarkable performance of these complex heterostructures with low ppb-level detection limits makes them excellent candidates for chemical/ gas sensing applications in e-noses.

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Modulation of stoichiometry, morphology and composition of transition metal oxide nanostructures through hot wire chemical vapor deposition

Cited by 23 publications

References 44 publications

Defect-induced Burstein-Moss shift in reduced V2O5 nanostructures

Defect-induced Burstein-Moss shift in reduced V2O5 nanostructures

Effects of charge fluctuation and charge regulation on the phase transitions in stoichiometric VO2

Novel Christmas Branched Like NiO/NiWO₄/WO₃ (p–p–n) Nanowire Heterostructures for Chemical Sensing

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Modulation of stoichiometry, morphology and composition of transition metal oxide nanostructures through hot wire chemical vapor deposition

Cited by 23 publications

References 44 publications

Defect-induced Burstein-Moss shift in reduced V2O5 nanostructures

Defect-induced Burstein-Moss shift in reduced V2O5 nanostructures

Effects of charge fluctuation and charge regulation on the phase transitions in stoichiometric VO2

Novel Christmas Branched Like NiO/NiWO4/WO3 (p–p–n) Nanowire Heterostructures for Chemical Sensing

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Novel Christmas Branched Like NiO/NiWO₄/WO₃ (p–p–n) Nanowire Heterostructures for Chemical Sensing