Material characterization
SEM was used to analyze the morphology of particles and membranes in this study. SEM-EDS was performed to study the chemical composition within the Ag-ZIF-62 particles and membranes. Samples were prepared by mounting them onto an aluminium SEM sample stub using carbon tape. Particulate samples were dispersed onto a silicon wafer before being secured onto the SEM sample stub. The samples and the stub were dried in the oven overnight at 70 oC under a vacuum to remove any residual solvents. The samples were thinly coated with platinum for morphological analysis (FE-SEM and conventional SEM) (ca. 15 nm thickness) or carbon for Energy Dispersive Spectroscopy analysis (SEM-EDS) (ca. 30 nm thickness) using Quorum Q150T high-resolution sputter coater. Scanning Electron Microscopy was performed either using JEOL JSM-7001F or JEOL JSM-7100F for Field Emission Scanning Electron Microscopy (FE-SEM) and Hitachi SU3500-A for conventional SEM imaging. All SEM analysis for morphological analysis was performed under secondary electron imaging mode at 5kV and a working distance of 10mm. Point Energy Dispersive Spectroscopy (SEM-EDS) were performed using JEOL JSM-7100F and JEOL JSM-7001F. Mapping Energy Dispersive Spectroscopy (SEM-EDS) was performed using Hitachi SU3500-A. All SEM-EDS analyses were performed at 20 kV and a working distance of 10 mm.
FT-IR and synchrotron THz-IR analysis was performed to understand the underlying bonds within the particles and membranes. Fourier-Transform Infrared (FT-IR) spectra were obtained using Nicolet 6700 by Thermo Scientific equipped with a diamond attenuated total reflection (ATR) objective. All spectra were collected between 600 cm-1to 4000 cm-1. THz/Far-IR absorption spectra were collected at the THz/Far-IR beamline at the Australian Synchrotron with a Bruker IFS 125/HR Fourier Transform (FT) spectrometer. The bolometer was cooled with liquid helium to reduce the background noise. A 6 µm thick Multilayer Mylar beam splitter was applied. The measurement was performed with an attenuated total reflection (ATR) stage. Powder samples were mounted on the surface of the diamond crystal window. In situ spectra were gathered with the ATR heating stage under a flowing argon environment (ca. 20 mL min-1). For data processing, the Extended ATR correction algorithms in the OPUS 8.0 software, together with NumPy module v1.15 together with Python v3.9, were applied for spectral data correction and peak fitting.
ICP-OES were performed to study the Ag to Zn ratios within the Ag-ZIF-62 particles. The Ag/Zn molar ratio of Ag-doped ZIF-62 was analysed with a Thermofisher iCap Pro inductively coupled plasma optical emission spectrometer (ICP-OES) instrument. The samples were digested with 35 wt% nitric acids before the ICP-OES test.
X-ray diffraction (XRD) was performed to analyse the crystalline and glassy structure of the ZIF-62 and Ag-doped-ZIF-62. Room temperature X-ray diffraction measurements were carried out under ambient conditions with Rigaku Miniflex 600 Benchtop X-Ray Diffractometer and a Cu Kα (λ = 1.5406 Å) radiation source. Samples in particle or small solid form are ground using a pestle and mortar before being dispersed onto a zero-background holder. Membrane or film samples were cut into squares with a size of ca. 2 cm x 2 cm before being secured onto a zero-background holder using a blue tack on the edge of the holder. The secured samples were then mounted onto the sample stage for X-Ray diffraction measurements. The 2θ range was 5o to 50o, with a step size of 0.03o and a 1.5o min-1 scan speed.
In situ thermal XRD was also performed to understand the structural transformation between solid, liquid and cooling phases. In situ thermal x-ray diffraction was performed using Anton Paar BTS 500 benchtop heating stage to study the thermal dynamics of different particles and thin film MOFs. The particles were dispersed onto a zero-background holder before being secured onto the benchtop heating stage for analysis. The heating rate was fixed at 20 oC min-1. All in situ thermal XRD analyses were performed under inert conditions via nitrogen gas flow. Samples within the benchtop heating stage were purged for 30 minutes before the temperature was increased for analysis. The collected XRD data were transformed into xy data format using PowDLL software (N. Kourkoumelis, ICDD Annual Spring Meetings (ed. Lisa O’Neill), Powder Diffraction, 28 (2013) 137-48) before being analysed using Excel. Literature comparison CIF data were obtained from the Cambridge Crystallography Data Centre (CCDC). The CIF data were processed using Mercury software obtained from CCDC, and the XRD data were extracted in xy data type format.
Thermogravimetric analysis was performed to understand the thermal decomposition behaviour of particles and membranes. Thermogravimetric analysis (TGA) was carried out using a METTLER TOLEDO TGA/DSC 1 STARe System. The sample was heated to 900 °C at a rate of 10 °C min-1 under flowing air or nitrogen as required (20 mL min-1).
Differential Scanning Calorimetry was used to study the melting temperature (Tm) and glassy transition temperature (Tg) of the samples. Differential scanning calorimetry (DSC) analysis was conducted using a separate Mettler Toledo DSC 1 STARe system. To determine the melting temperature (Tm), the samples were heated from 40 °C to 450 °C at 20 °C min−1 under a nitrogen atmosphere. For determining the glass transition temperature (Tg), each sample was heated above the inherent Tg and then cooled back to room temperature at a ramping rate of 10 °C to eliminate the effect of thermal history on glass transition temperature (Tg) determination. Then the Tg was determined from the following up-scans.
XPS was performed to study the different elemental states on the surface of the ZIF-62 and Ag-ZIF-62 crystals and melted variations. X-ray photoelectron spectroscopy (XPS, AXIS Supra+, KRATOS Analytical) was performed using a mono Al X-ray gun with an emission current of 10.00 mA and pass energy 160 eV for survey scan and 20 eV for high-resolution spectra, respectively. The C–C peak position was set to 284.8 eV and used as an internal standard.
1H liquid state NMR was performed to study the molar ratio of benzimidazole to imidazole in the as synthesised ZIF-62 particles. 5.0 mg of sample was digested in a mixture of DMSO-d6 (0.5 mL) and DCl/D2O (35%; 0.1 mL). The 1H NMR measurement was performed with a Bruker Avance 500 high-resolution NMR spectrometer interfaced to an 11.7 Tesla 51 mm bore magnet system.13C solid-state NMR was performed to study the molecular interaction surrounding carbon atoms within both the crystalline and melted variations of ZIF-62 and Ag-ZIF-62. Solid state NMR for 13C studies was performed using Bruker 300 MHz Avance III solid-state (SS) NMR spectrometer with 7.41 Tesla (300 MHz) wide bore superconducting magnet interfaced with a two-channel Bruker Avance III spectrometer.
Adsorption isotherms were measured using Micromeritrics TriStar II 3020. Samples were degassed for 24h under a high vacuum before adsorption analysis. CO2 isotherms at 273K were performed from high vacuum up to 130 kPa through one adsorption-desorption cycle.