Noble metal nanoclusters for electrocatalysis, catalysis, enzyme-mimicking, and photoelectrochemistry analysis

Abstract

Noble metal NCs (such as Pt, AuNCs.) possess unique physical and chemical properties, making them competitive candidates in a wide application field, such as electrocatalysis, photo(electro)chemical, energy conversion, nanomedicine. This thesis focuses on the synthesis of noble metal NCs materials and their applications in catalysis, electrocatalysis, enzyme mimics, photoelectrochemical analysis and others. The main contents and conclusions of the research are as follows: 1. We synthesized ligand-free PtNCs by using the template of ZIF-8, denoted as PtNCs@ZIF-8. From the EM images, the homogeneous distribution of PtNCs was observed with an average core diameter of dc = 1.86 ± 0.20 nm. Meanwhile, the shifts of the high-resolution XPS spectra of N 1s and Zn 2p, and the FTIR spectra were confirmed the interaction of PtNCs and ZIF-8 template. We can predict the PtNCs formation mechanisms, in which the unsaturated Zn and free N- extremities on the external surface of ZIF-8 or at defects can hold the Pt precursor (PtCl4-) where NC nucleation initiates during the in situ reductions. In the meantime, when optimizing the parameter during the synthesis, the concentration of the PtCl4- influences the distribution of PtNCs in ZIF-8. To evaluate the catalytic potential of this catalyst, the reduction reaction upon the catalytic 4-NP was applied, the mass activity parameter and activation energy of which was be 1.29 × 102 s-1 g-1, and Ea = 15.1 kJ mol-1, respectively, indicating the high catalytic activity compared to those of previous reports. Furthermore, HER tests show that Pt-2@ZIF-8 have a much small Eη of 1.4 mV and larger SA, MA parameters compared to other Pt-based catalysts, suggesting the better HER activity of our catalysts. We speculate that the high catalytic performance maybe due to the smaller size, ligand-free of PtNCs. In addition, ORR catalytic performance of Pt@ZIF-2 was also carried out, showing a similar Tafel slope in comparison with the commercial 20 wt.% Pt/C catalyst. Moreover, these PtNCs can be extracted to both aqueous and non-aqueous solvents. This study and finding provides a rapid synthesis method to prepare efficient catalyst and offer avenues to obtain homogenous PtNCs for further applications. 2. Based on the previous research, we prepared a PtAg alloy with the previously modified method. Two different protocols of (I) co-reduction (II) Pt-guided methods were applied to obtain larger PtAg NPs (dc = 9.10 ± 2.31 nm) and smaller PtAgNCs (dc = 1.78 ± 0.12 nm) in ZIF-8, respectively, confirmed by EM images. Further characterization of PXRD further indicated that the non-visible, additional features in PtAgNCs case were probably due to the incorporation of PtAgNCs with Pr-imidazole groups of ZIF-8, unlike the PtAgNPs case formed outside of the ZIF-8 with intact peaks compared with those of pristine ZIF-8. This is consistent with the TEM and FTIR results. Furthermore, enzyme mimicking properties of both PtAgNPs and PtAgNCs catalysts were investigated by the oxidation of OPD in the presence of H2O2, decomposition of H2O2, and the oxidation of TMB in the presence of dissolved oxygen, representing POD- , CAT- and OD-like activities, respectively. These findings show that the PtAgNCs possess more efficient enzyme-mimicking properties than those of PtAgNPs, pure Pt@ZIF-8 and other nanozyme catalyst reported previously, further indicating the enhanced catalytic performance may originate from the synergistic effect from the addition of Ag element. Further mechanism studies showed that the nature of the enzyme mimics behaviors of PtAgNCs may be due to their ability to accelerate the electron-transfer process between the substrates and H2O2 or dissolved O2. 3. We demonstrated a plasmon-enhanced PEC sensing platform of AuNCs based on AuNRs as an enhancer to sensor H2O2. The enhanced PEC platform (ePEC) was built in a layer by layer method, and its PEC performance was tested for revealing the enhancement phenomenon at the different potentials in PBS solution. We observed a larger photocurrent enhancement after the enhancer (AuNRs@SiO2) introduction and verified that the enhancement mechanism results from energetic changer transfer from the AuNRs to the nearby AuNCs by measuring the response of photocurrents to light power and wavelength dependent. More importantly, compared to traditional PEC method, the reported ePEC platform presents enhanced sensing ability toward H2O2, suggesting a strategy that using the plasmon of the plasmonic metals to enhance the sensing ability of the PEC sensors. To offer more proof for the sensing possibility of ePEC sensor, we are going to measure other redox species such as catechol, ferrocene or Ru-hexamine complexes. which are also important for biosensing and biochemical applications. In summary, we conducted a systematic study related to the synthesis of noble metal NCs (Pt, PtAg, and Au) and the applications in the areas of catalyst, electrocatalysis, enzyme mimics as well as photoelectrochemistry. The small size PtNCs in ZIF-8 not only exhibit improved catalytic properties but also reduce the cost of the Pt-based catalyst. The alloy PtAgNCs show advanced enzyme mimics performance. The plasmon enhanced PEC sensing properties were investigated on AuNCs-based sensor incorporated with AuNRs@SiO2, showing the potential of sensing H2O2. In future research work, while exploring the maximal photocurrent with different silica thickness of AuNRs@SiO2, it is necessary to figure out the plasmon enhanced mechanism. In the meanwhile, thetheoretical experiment was also carried out for further mechanism investigation. Once it is clear, the various use of the plasmon-enhanced AuNCs sensor can go further.