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Zinc oxide (ZnO) nanoparticles are under investigation for the synthesis of materials with tunable magnetic and electric properties and for possible medical applications in cancer therapy. In this study, a sample of thiol-capped ZnO nanoparticles was studied by simultaneous thermogravimetry (TG) and differential scanning calorimetry (DSC) using a NETZSCH STA 449 F1 Jupiter® thermal analyzer. The analyzer was coupled to both a NETZSCH QMS 403 Aeolos® mass spectrometer and a Bruker Optics TENSOR™ FTIR spectrometer to perform evolved gas analysis by mass spectrometry (MS) and Fourier transform infrared spectroscopy (FTIR).
The transfer lines, the coupling adapters and the FTIR gas cell were kept at a constant temperature of 200°C. The thiol-capped ZnO nanoparticles sample with a mass of 11.18 mg was pressed on the bottom of a Pt-Rh DSC crucible to form a layer of about 1 mm thickness. The sample was heated from 30-1,200°C with a heating rate of 20 K/min under 60 mL/min nitrogen purge.
Results and Discussion
The TG, DTG (mass change rate), DSC and Gram Schmidt curves are plotted in Figure 1. The TG curve shows five mass loss steps that have corresponding peaks in the DTG curve and corresponding endothermic features in the DSC curve due to desorption and decomposition processes in the sample. The peak temperatures in the Gram Schmidt plot correspond well with the peak temperatures in the DTG curve.
Figure 2 shows the TG and DTG curves, along with the temperature-dependent integrated band areas (traces) for the O-H stretching of H2O, the C-H stretching of hydrocarbons, and the anti-symmetric C=O stretching of CO2. The figure clearly shows that the desorption of H2O and CO2 corresponds with the first four mass loss steps, whereas the hydrocarbons evolve in the mid-temperature range in good correspondence with the second and third mass loss steps in the TG curve.
The MS ion-current curves for H2O (18, 17 and partially 16 amu) and CO2 (44 and partially 16 amu) plotted in Figure 3, along with the TG curve, show more details due to the higher sensitivity of the MS; the results are in agreement with the FT-IR traces that H2O and CO2 evolution corresponds with the first four mass loss steps in the TG curve.
The MS ion-current curves for SO2 (64 and 48 amu) plotted in Figure 4, along with the TG curve, clearly show that small amounts of SO2 evolve at elevated temperatures in correspondence with the fifth mass loss step in the TG curve. Finally, the MS ion-current curves for many different organic fragments plotted in Figure 5 show that these species evolve as two peaks in very good agreement with the FTIR results.
A simultaneous TG/DSC (STA) instrument coupled to MS and FTIR spectrometers is a very powerful combination for sample characterization because it supplies data for the mass change (TG), transformation temperatures and energetics (DSC), and evolved gas analysis (MS, FTIR) in a single measurement. All the data analysis can be carried out with the NETZSCH Proteus® software.
Simultaneous use of MS and FTIR for evolved gas analysis is beneficial because the FTIR can quickly identify functional groups based on their characteristic bands. On the other hand, the MS has higher sensitivity and can detect homonuclear diatomic molecules (e.g., H2, O2, N2) and atomic gases (e.g., He, Ne, Ar, etc.), which are not detectable by FTIR.
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