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The carbonization of various types of metal–organic frameworks (MOFs) was carried out under N2 gas flow and high temperature. The formation of carbonized MOFs (CMOFs) was monitored by Raman spectroscopy. In addition to the well-known D and G bands in Raman spectra, the salient G′ band feature was observed only in Mn-, Fe-, Co-, and Ni-containing CMOFs. On the other hand, CMOFs containing other metals (Al, Cr, V, Cu, and Zr) do not show the G′ band. Furthermore, the G′ band was also observed when we mixed the nitrate salts of Mn(II), Fe(III), and Co(II) with Al-containing MOFs using the same treatment conditions as in the formation of CMOFs. The G′ band is known to be related to the stacking order of graphitic layers. The presence of the Raman G′ band in CMOFs can be ascribed to the catalytic activity of Mn, Fe, Co, and Ni. The trend of the G′ band to G band intensity ratio resembles the “volcano curve” in the description of the behavior of catalytic activities of transition metals. The G′ bands in Mn-, Fe...

Under inert gas flow and high temperature, carbonization of aluminum-based metal–organic frameworks (MOFs) was carried out. The formation rate of carbonized MOFs (CMOFs) was monitored by the variation of the Raman D band to G band intensity ratio with heat treatment duration. Powder x-ray diffraction (PXRD) and scanning electron microscope (SEM) techniques were used to confirm the formation of CMOFs. The activation energy was extracted from the temperature-dependent rate constants using the Arrhenius equation and correlated with the structural properties of precursor MOFs such as pore size and the number of carbon atoms per ligand. A reaction mechanism is proposed and discussed for the formation of CMOFs based on Raman observation. Copyright © 2016 John Wiley & Sons, Ltd.

4-mercaptobenzoic acid (4-MBA) adsorbed on Ag colloids has been investigated by surface-enhanced Raman scattering (SERS) spectroscopy. From the intensity variation of the symmetric stretching COO− vibration band corresponding to different pH values, it shows that 4-MBA molecules are bonded to the Ag surface only through the S atoms at acidic conditions, but through the S atoms and the COO− groups at neutral or alkaline conditions. The 4-MBA bonding to Ag shows SERS activity with multilayer adsorption (Freundlich isotherm model). Raman spectra calculated by density functional theory (DFT) with emphasis on long-range charge-transfer effect were compared with experimental observation to interpret the adsorption behavior of 4-MBA on the Ag surface. From both DFT simulations and experimental observation, we have verified that the contribution of electromagnetic and chemical enhancements in the SERS spectra of 4-MBA adsorbed on Ag are equally important.

Surface-enhanced Raman scattering (SERS) spectra of azo dyes (methyl orange and p-methyl red) adsorbed on ZnO nanoparticles were observed. Hydrothermally synthesized ZnO nanoparticles were characterized by powder X-ray diffraction and X-ray photoelectron spectroscopy. The ZnO nanoparticle size, monitored with X-ray diffraction, was tuned by calcination to optimize SERS intensities. The observed SERS effect of azo dyes adsorbed on ZnO can be ascribed to charge-transfer resonance effect. Time-dependent density functional theory was used to calculate the optical spectra and interpret the chemical enhancement observed in the experiment. The SERS enhancement factors for methyl red on ZnO were boosted by nearly four times and twice with O2 plasma and H2 plasma, respectively. However, plasma treatment showed no effect on the enhancement factors of methyl orange on ZnO. We conclude that plasma-induced defect formation and band gap shift in ZnO and the coupling of energy levels between ZnO and azo dye molecules are responsible for the observed enhancement of SERS intensities. Copyright © 2014 John Wiley & Sons, Ltd.

We report the direct observation of surface-enhanced Raman scattering (SERS) effect using metal–organic frameworks (MOFs) as substrates. Without the aid of any metal colloids or enhancing agents, the SERS signals of methyl orange (MO) adsorbed in MOFs were observed and even remained active if the organic linkers in MOFs were completely removed by high temperature and O2 plasma treatments. It implies that the SERS active site is at the metal oxide clusters. The ultraviolet-visible spectra of MO, MOFs, and MO–MOF complexes show that absorption peaks are far from laser excitation line. Thus, conventional resonance enhancement effect should be ruled out, and charge-transfer mechanism is the most likely scenario responsible for the observed SERS effect. Density functional theory (DFT) was used to interpret the chemical enhancement mechanism and the adsorption orientation-dependent SERS spectra in our observation. The preferred adsorption orientations calculated by DFT method are consistent with the observed SERS results. Copyright © 2013 John Wiley & Sons, Ltd.

Multiwalled carbon nanotubes (MWCNTs) were treated with a low-pressure water plasma, and the formation of defects on the MWCNT surface was monitored via the changes in the Raman D band to G band intensity ratio that occurred with different plasma treatment times and different temperatures. A kinetic model with two competing processes (defect formation and defect scavenging) was adopted to interpret the observed nonlinear time-dependent intensity ratio trends. The fitted activation energy (Ea) for the defect formation process was found to be higher than that of the defect scavenging process. This was ascribed to the fact that the OH radicals were more effective in reacting with carbon defects and impurities than with pure CNTs. The Raman-determined Ea’s were also found to be excitation energy-dependent, with maximum values for red light. Such a unique dependence is characteristic of highly π-conjugated carbon systems.

Chemical oxidation of multiwalled carbon nanotubes (MWCNTs) using H 2 SO 4 /HNO 3 solution has been monitored by micro-Raman spectroscopy and X-ray absorption spectroscopy. The diameter distribution variation in MWCNTs due to chemical oxidation has been measured by scanning electron microscopy and transmission electron microscopy. The Raman dispersion behaviors of the intensity ratio and the band positions of the D, G, and G′ bands were found to be correlated with the MWCNT diameter distribution. It was also found that, during the nanotube unzipping process, defect formation complicates the observation of the diameter-dependent Raman dispersion effect. The curvature effect plays an important role in the intensity ratio trend. On the other hand, defect formation dominates the band position trend.

We have adopted different fitting procedures to study the Raman dispersion behavior of the G' band in plasma-treated multi-walled carbon nanotubes (MWCNTs). Our analysis shows that the trend of the G' band dispersion is related to the presence of defects in the MWCNTs. The intensity ratio dispersions involving the G' 21 , G' 32 , G' 41 , and G' 42 components of the G band behave in a similar way to the well known D to G band ratio (D/G), which is a good criterion for estimating the defect content in MWCNTs. The G' 21 , G' 32 , G' 41 , and G' 42 peaks are shown to belong to the lower-frequency components of the G' band by using two-, three-, and four-peak fitting procedures. Conversely, the higher-frequency components G' 22 , G' 33 , G' 43 , and G' 44 behave like the Gband and are related to the non-defect content. Consequently, the intensity ratios G' 21 /G' 22 ; G' 32 /G' 33 , and G' 41+42 /G' 43+44 show the same dispersion behavior as that of D/G. The difference of band position between the high and low frequency G' band components is found to be sensitive to plasma treatment.

Plasma treatment of multi-walled carbon nanotubes (MWCNTs) using an atmospheric pressure hydroxyl radical (OH) source has been monitored with micro-Raman spectroscopy. We use dispersion behavior of the intensity ratio, band position, and linewidth of the D, G, D′, and G′ bands to probe the defect formation and oxidation process in MWCNTs. A simple kinetic model is adopted to interpret the observed dispersion trends in plasma-treated MWCNTs. X-ray photoelectron spectroscopic analysis, scanning electron microscopy inspection, and Raman dispersion characterization of MWCNT surfaces suggest that the dominant effect of OH plasma on MWCNTs is reduced π-conjugated states due to creating structural defects and attachment of oxygen-containing functional groups.

Multiwalled carbon nanotubes (MWCNTs) were treated with an atmospheric pressure plasma source using an argon/water mixture. Optical emission diagnostics has shown that hydroxyl radicals (OH) were the major reactive species in the plasma. The structural changes in MWCNTs were monitored by micro-Raman spectroscopy. The observed variation of the D and G band intensity ratio and position dispersion with plasma treatment time was ascribed to the change in structural disorder on MWCNT surfaces. Scanning electron microscopic study showed that some defects can be induced in MWCNTs during plasma treatment. Results of thermogravimetric analysis indicated that atmospheric pressure OH plasma is as effective as traditional wet methods for purifying MWCNTs.

Multi-walled carbon nanotubes (MWCNTs) were treated with a radio-frequency discharge. We found that MWCNTs showed opposite trends in electrical conductivity when treated with oxygen and hydrogen plasmas. MWCNTs showed enhanced electrical conductivity when placed at cathode with oxygen plasma treatment, whereas MWCNTs treated at positive column did not show such a trend. In contrast, the conductivity of MWCNTs dropped sharply with hydrogen plasma treatment. The measured conductivity trends of MWCNTs are correlated with observed Raman spectral shift. The possible mechanisms of the change in electrical conductivity in plasma-treated MWCNTs are discussed.

We report the fluorescence and surface-enhanced Raman scattering (SERS) analysis of methyl red (MR) isomers (o-MR, m-MR, p-MR) on silver (Ag) colloids at different pH conditions. The changes in simultaneous fluorescence quenching around 400 nm accompanied by enhancement around 540 nm with pH are closely related to energy transfer efficiency and the interaction between the carboxylic group and the Ag surface. The change of SERS signals with pH values can be ascribed to different adsorption orientations of MR on the Ag surface, from near flat (o-MR) to near perpendicular (p-MR).

Carbonaceous dust particles have been synthesized from benzene using an rf glow discharge. Scanning electron microscope inspection revealed that the plasma-synthesized dust particles can be classified into two types. Shell-structured dust particles showed a wide size distribution from 3 to40 μm. The other type, with different degrees of aggregation, appeared to be dense and spherical with a very distinctive yellow color and size distribution from 100 nm to 2 μm. Analyses using micro-Raman and Fourier transform infrared microscopy indicated that the main components of the dust particles are polyphenyls and hydrogenated amorphous carbon (HAC). The luminescence background in Raman spectra and the infrared C–H stretching vibrational features observed around 3.4 μm for the dust particles are attributed to HAC. The formation mechanisms and spectroscopic characterization of carbonaceous dust particles are discussed.