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Photocurrent generation in carbon nanotube/cubic-phase HfO 2 nanoparticle hybrid nanocomposites

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ABSTRACT

A hybrid material consisting of nonfunctionalized multiwall carbon nanotubes (MWCNTs) and cubic-phase HfO2 nanoparticles (NPs) with an average diameter of 2.6 nm has been synthesized. Free standing HfO2 NPs present unusual optical properties and a strong photoluminescence emission in the visible region, originating from surface defects. Transmission electron microscopy studies show that these NPs decorate the MWCNTs on topological defect sites. The electronic structure of the C K-edge in the nanocomposites was probed by electron energy loss spectroscopy, highlighting the key role of the MWCNT growth defects in anchoring HfO2 NPs. A combined optical emission and absorption spectroscopy approach illustrated that, in contrast to HfO2 NPs, the metallic MWCNTs do not emit light but instead expose their discrete electronic structure in the absorption spectra. The hybrid material manifests characteristic absorption features with a gradual merger of the MWCNT π-plasmon resonance band with the intrinsic defect band and fundamental edge of HfO2. The photoluminescence of the nanocomposites indicates features attributed to combined effects of charge desaturation of HfO2 surface states and charge transfer to the MWCNTs with an overall reduction of radiative recombination. Finally, photocurrent generation under UV–vis illumination suggests that a HfO2 NP/MWCNT hybrid system can be used as a flexible nanodevice for light harvesting applications.

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(a) HAADF-STEM image of the MWCNT:HfO2, (b) higher magnification image of (a), (c) HAADF-HRSTEM images of the HfO2 NPs on the MWCNTs, (d) NPs attached to the side walls of a MWCNT, (e) NPs attached at buckled edges of the MWCNT, (f) wall of the nanotube showing defects and irregularities to which nanoparticles are attached, (g) HRTEM images of randomly oriented agglomerates, (h) <110>-oriented, and (i) <001>-oriented nanoparticles indicated by arrows and all attached to curved regions of the CNT.
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Figure 1: (a) HAADF-STEM image of the MWCNT:HfO2, (b) higher magnification image of (a), (c) HAADF-HRSTEM images of the HfO2 NPs on the MWCNTs, (d) NPs attached to the side walls of a MWCNT, (e) NPs attached at buckled edges of the MWCNT, (f) wall of the nanotube showing defects and irregularities to which nanoparticles are attached, (g) HRTEM images of randomly oriented agglomerates, (h) <110>-oriented, and (i) <001>-oriented nanoparticles indicated by arrows and all attached to curved regions of the CNT.

Mentions: In Fig. 1, an overview of the hybrid material shows successful attachment of the nanoparticles to the CNT. The cubic HfO2 nanoparticles exhibit an average diameter of 2.6 nm [20]. In the overview of Fig. 1, the nanoparticles seem to decorate only specific sites either singularly or in agglomerates. A higher magnification HAADF-STEM image in Fig. 1 further reinforces the selective decoration of the HfO2 nanoparticles as several regions of the CNT are devoid of nanoparticles. An explanation to this selective anchoring is provided below and also in the EELS section. In Fig. 1, a HAADF-HRSTEM image exhibits small agglomerates of these nanoparticles on the MWCNT, where the diameter of the MWCNT has reduced due to buckling. The nanoparticles remain crystalline as displayed by the atomic column resolution, even after 2 h of sonication. In Fig. 1, a defective nanotube presenting successive kinks is shown with the HfO2 nanoparticles preferentially attached around the kinked area. A HRSTEM image in one such buckled/kinked regions of the MWCNT is illustrated in Fig. 1, where the nanoparticles are attached to two different kinked areas. In effect, in the kinked region, there is a breakdown in the curvature and CNTs with individual walls exhibit different curvatures. Moreover, when the MWCNT is bent, the changes in the curvature induce a modified atomic arrangement and local break down of symmetry. This implies a local change of the electronic structure [36] along with an increase in π mismatch. This in turn accentuates the reactivity of these MWCNTs and converts these defects into receptors for functional groups on the surface of the nanoparticles. Buckling and kinking of the MWCNT arises due to rearrangement of C atoms around the curved area giving rise to vacancies and dangling bonds. These defective areas are well known for their high reactivity to foreign atoms [37].


Photocurrent generation in carbon nanotube/cubic-phase HfO 2 nanoparticle hybrid nanocomposites
(a) HAADF-STEM image of the MWCNT:HfO2, (b) higher magnification image of (a), (c) HAADF-HRSTEM images of the HfO2 NPs on the MWCNTs, (d) NPs attached to the side walls of a MWCNT, (e) NPs attached at buckled edges of the MWCNT, (f) wall of the nanotube showing defects and irregularities to which nanoparticles are attached, (g) HRTEM images of randomly oriented agglomerates, (h) <110>-oriented, and (i) <001>-oriented nanoparticles indicated by arrows and all attached to curved regions of the CNT.
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Related In: Results  -  Collection

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Figure 1: (a) HAADF-STEM image of the MWCNT:HfO2, (b) higher magnification image of (a), (c) HAADF-HRSTEM images of the HfO2 NPs on the MWCNTs, (d) NPs attached to the side walls of a MWCNT, (e) NPs attached at buckled edges of the MWCNT, (f) wall of the nanotube showing defects and irregularities to which nanoparticles are attached, (g) HRTEM images of randomly oriented agglomerates, (h) <110>-oriented, and (i) <001>-oriented nanoparticles indicated by arrows and all attached to curved regions of the CNT.
Mentions: In Fig. 1, an overview of the hybrid material shows successful attachment of the nanoparticles to the CNT. The cubic HfO2 nanoparticles exhibit an average diameter of 2.6 nm [20]. In the overview of Fig. 1, the nanoparticles seem to decorate only specific sites either singularly or in agglomerates. A higher magnification HAADF-STEM image in Fig. 1 further reinforces the selective decoration of the HfO2 nanoparticles as several regions of the CNT are devoid of nanoparticles. An explanation to this selective anchoring is provided below and also in the EELS section. In Fig. 1, a HAADF-HRSTEM image exhibits small agglomerates of these nanoparticles on the MWCNT, where the diameter of the MWCNT has reduced due to buckling. The nanoparticles remain crystalline as displayed by the atomic column resolution, even after 2 h of sonication. In Fig. 1, a defective nanotube presenting successive kinks is shown with the HfO2 nanoparticles preferentially attached around the kinked area. A HRSTEM image in one such buckled/kinked regions of the MWCNT is illustrated in Fig. 1, where the nanoparticles are attached to two different kinked areas. In effect, in the kinked region, there is a breakdown in the curvature and CNTs with individual walls exhibit different curvatures. Moreover, when the MWCNT is bent, the changes in the curvature induce a modified atomic arrangement and local break down of symmetry. This implies a local change of the electronic structure [36] along with an increase in π mismatch. This in turn accentuates the reactivity of these MWCNTs and converts these defects into receptors for functional groups on the surface of the nanoparticles. Buckling and kinking of the MWCNT arises due to rearrangement of C atoms around the curved area giving rise to vacancies and dangling bonds. These defective areas are well known for their high reactivity to foreign atoms [37].

View Article: PubMed Central - HTML - PubMed

ABSTRACT

A hybrid material consisting of nonfunctionalized multiwall carbon nanotubes (MWCNTs) and cubic-phase HfO2 nanoparticles (NPs) with an average diameter of 2.6 nm has been synthesized. Free standing HfO2 NPs present unusual optical properties and a strong photoluminescence emission in the visible region, originating from surface defects. Transmission electron microscopy studies show that these NPs decorate the MWCNTs on topological defect sites. The electronic structure of the C K-edge in the nanocomposites was probed by electron energy loss spectroscopy, highlighting the key role of the MWCNT growth defects in anchoring HfO2 NPs. A combined optical emission and absorption spectroscopy approach illustrated that, in contrast to HfO2 NPs, the metallic MWCNTs do not emit light but instead expose their discrete electronic structure in the absorption spectra. The hybrid material manifests characteristic absorption features with a gradual merger of the MWCNT &pi;-plasmon resonance band with the intrinsic defect band and fundamental edge of HfO2. The photoluminescence of the nanocomposites indicates features attributed to combined effects of charge desaturation of HfO2 surface states and charge transfer to the MWCNTs with an overall reduction of radiative recombination. Finally, photocurrent generation under UV&ndash;vis illumination suggests that a HfO2 NP/MWCNT hybrid system can be used as a flexible nanodevice for light harvesting applications.

No MeSH data available.


Related in: MedlinePlus