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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.

No MeSH data available.


(a) HAADF-STEM image of the area of interest where EELS was performed, (b) C K-edge core loss EELS spectra on the area of interest indicated in Fig. 2.
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Figure 4: (a) HAADF-STEM image of the area of interest where EELS was performed, (b) C K-edge core loss EELS spectra on the area of interest indicated in Fig. 2.

Mentions: Most nanoparticles are attached to sites with defects and changes in CNT curvature, creating π-orbital mismatches that increase the reactivity and allow nanoparticles with organic moieties to attach. However, there are regions on the CNT that have almost no curvature, as illustrated in Fig. 1. Since we have not functionalized the CNT, there are no functional groups created on the walls that facilitate NP anchorage. In such a case, it is relevant to gain insight into the nature of the graphitic layers in such areas of the CNT by probing their electronic structure with regards to sp hybridization. Energy loss near edge structure (ELNES) of the C K-edge was therefore used to probe the local electronic structure of the CNT at the nanoparticle interface in the noncurved regions of the CNT anchoring nanoparticles. In fact, several EELS spectra were taken from the hybrid nanocomposite at different places near the nanoparticles attached to the CNT. The EELS spectra were acquired from regions located far away from the carbon film of the TEM grid in order to exclude or minimize its influence on the C K-edge measurement. These areas of interest were at the holes of the holey carbon grid. Differences in the electronic structure along various points of the carbon nanotubes were probed in order to understand the affinity of these nanoparticles to only certain regions of the CNT. Accordingly, spectrum 1 in Fig. 4 was acquired from point 1 in Fig. 4, which is devoid of nanoparticles, and indicates typical peaks for sp2/sp3 hybridization corresponding to multiwalled CNTs. The π* peak at 285 eV is related to the 1s to unoccupied antibonding π* transitions, and the exciton peak σ* at 292 eV arises due to transitions to the antibonding σ*. As we approach the region of the CNT decorated by nanoparticles (point 2, Fig. 4), we observe that the σ* peak in Fig. 4 is smeared out along with a decrease in the intensity of the π* 285 eV edge. This smear is manifested as a broad hump starting above 288 eV and extending up to 305 eV, corresponding to the C 1s → σ* transition for disordered carbon–carbon bonds. On a similar note, other noticeable features are usually observed via X-ray absorption spectroscopy (XAS) in the region between π* and σ* transitions. These resonances are ascribed to the oxygen containing functional groups, i.e., a peak at 288.2 eV related to carbonyl (C=O) and another peak at 289.7 eV related to carboxylic (–COOH) [54]. In our case, we have not functionalized the nanotubes, therefore, the slight increase in the π*/σ* in ELNES C K-edge, arises due to the damage of the CNT walls [55], resulting in a loss of features in the σ* peak [54,56–58]. In the region of the CNT decorated by nanoparticles represented by point 3 in Fig. 4, we observe that the σ* peak in spectrum 3 of Fig. 4 is even more smeared out with an increase in the intensity of the 285 eV edge and an even more featureless broad hump starting above 288 eV and extending up to 305 eV due to the C 1s → σ* transition, typical for extremely disordered carbon-carbon bonds. In fact, the smearing of this edge is characteristic of amorphous carbon, similar to the C-K edge of the amorphous carbon support of the TEM grid used as a reference, shown in Fig. 4.


Photocurrent generation in carbon nanotube/cubic-phase HfO 2 nanoparticle hybrid nanocomposites
(a) HAADF-STEM image of the area of interest where EELS was performed, (b) C K-edge core loss EELS spectra on the area of interest indicated in Fig. 2.
© Copyright Policy - Beilstein
Related In: Results  -  Collection

License 1 - License 2
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Figure 4: (a) HAADF-STEM image of the area of interest where EELS was performed, (b) C K-edge core loss EELS spectra on the area of interest indicated in Fig. 2.
Mentions: Most nanoparticles are attached to sites with defects and changes in CNT curvature, creating π-orbital mismatches that increase the reactivity and allow nanoparticles with organic moieties to attach. However, there are regions on the CNT that have almost no curvature, as illustrated in Fig. 1. Since we have not functionalized the CNT, there are no functional groups created on the walls that facilitate NP anchorage. In such a case, it is relevant to gain insight into the nature of the graphitic layers in such areas of the CNT by probing their electronic structure with regards to sp hybridization. Energy loss near edge structure (ELNES) of the C K-edge was therefore used to probe the local electronic structure of the CNT at the nanoparticle interface in the noncurved regions of the CNT anchoring nanoparticles. In fact, several EELS spectra were taken from the hybrid nanocomposite at different places near the nanoparticles attached to the CNT. The EELS spectra were acquired from regions located far away from the carbon film of the TEM grid in order to exclude or minimize its influence on the C K-edge measurement. These areas of interest were at the holes of the holey carbon grid. Differences in the electronic structure along various points of the carbon nanotubes were probed in order to understand the affinity of these nanoparticles to only certain regions of the CNT. Accordingly, spectrum 1 in Fig. 4 was acquired from point 1 in Fig. 4, which is devoid of nanoparticles, and indicates typical peaks for sp2/sp3 hybridization corresponding to multiwalled CNTs. The π* peak at 285 eV is related to the 1s to unoccupied antibonding π* transitions, and the exciton peak σ* at 292 eV arises due to transitions to the antibonding σ*. As we approach the region of the CNT decorated by nanoparticles (point 2, Fig. 4), we observe that the σ* peak in Fig. 4 is smeared out along with a decrease in the intensity of the π* 285 eV edge. This smear is manifested as a broad hump starting above 288 eV and extending up to 305 eV, corresponding to the C 1s → σ* transition for disordered carbon–carbon bonds. On a similar note, other noticeable features are usually observed via X-ray absorption spectroscopy (XAS) in the region between π* and σ* transitions. These resonances are ascribed to the oxygen containing functional groups, i.e., a peak at 288.2 eV related to carbonyl (C=O) and another peak at 289.7 eV related to carboxylic (–COOH) [54]. In our case, we have not functionalized the nanotubes, therefore, the slight increase in the π*/σ* in ELNES C K-edge, arises due to the damage of the CNT walls [55], resulting in a loss of features in the σ* peak [54,56–58]. In the region of the CNT decorated by nanoparticles represented by point 3 in Fig. 4, we observe that the σ* peak in spectrum 3 of Fig. 4 is even more smeared out with an increase in the intensity of the 285 eV edge and an even more featureless broad hump starting above 288 eV and extending up to 305 eV due to the C 1s → σ* transition, typical for extremely disordered carbon-carbon bonds. In fact, the smearing of this edge is characteristic of amorphous carbon, similar to the C-K edge of the amorphous carbon support of the TEM grid used as a reference, shown in Fig. 4.

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 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.

No MeSH data available.