Description
When a suspension is irradiated with a laser pulse, the electromagnetic wave is locally absorbed by the suspended agglomerates, resulting in a temperature rise. Depending on their temperature, possible phase formations/transitions take place in a short time of the pulse duration. Immediately, the particles are cooled to ambient temperature by thermal equilibrium with the solvent molecules, which are transparent to the laser beam. This process is alternatively repeated during pulsed laser irradiation of suspensions (PLIS), initially known as pulsed laser melting in liquid (PLML). In 2007, scientists reported for the first time that unlike the laser ablation process used to produce smaller nanoparticles, PLML uses lower laser fluences and forms larger particles than the original constituents by melting. In our latest publications [M.S.Shakeri, et al. Adv. Funct. Mater. 2023, 33, 2304359], we proposed that during melting of nanoparticles physical phase transitions and chemical oxidation/reduction processes will occur. We used linear combination fitting of Extended X-ray Absorption Fine Structure (EXAFS) spectra (Figure 1) not only to determine the chemical composition of samples, but also to recognize which one of physical/chemical transitions control the formation of various phases. EXAFS spectra were used to interpret both the mechanism and exact composition of samples.
Fig. 1. The exponential/logarithmic changes of chemical composition derived from LCF of EXAFS spectra
Hopefully, our investigation led to proposing a scientifically acceptable mechanism for particle formation and as a next logical step we tried to dig more into the analyzing the oxidation states by altering the synthesizing parameters using X-ray absorption spectroscopy (XAS) method. We successfully conducted XAS spectra for powders resulted from pulsed laser irradiation of CuO/Fe3O4/organic solvent colloids. X-ray Absorption Near Edge Spectra (XANES) showed the meaningful change in oxidation state of both copper and iron K-edges. We also tried to use Feff code in order to analyse how the structural changes interpret the observed variations in XANES spectra.
This work is mostly supported by the Polish National Science Centre Program No. 2018/31/B/ST8/03043. It is also supported partially by the Polish National Science Centre Program No. 2022/06/X/ST3/01743. The computational works were done in cooperation with Prometheus Cluster, Cyfronet, AGH University of Science and Technology, Krakow using Grant No. PLG/2022/015573. The X-ray absorption spectroscopy measurements were performed at the SOLARIS synchrotron center in Krakow, Poland, under experiment number 221926 and Elettra synchrotron facility, under the proposal number 20232086.
