Shen-wei Bai, Hui Mei,*Gang-qiang Zhu, Ming-gang Zhang, Wei-zhao Huang and Lai-fei Cheng
In this paper, the authors develop a Ag core – Zn0.5Cd0.5S semiconductor shell nanoparticle for the photocatalytic production of hydrogen from water (Figure 1). The aim was to develop a nanocomposite which may utilise the intrinsic localised surface plasmon resonance (LSPR) of an Ag metal nanoparticle to enhance the electronic properties of the photolytically active shell.
The key role in which XPS was expected to provide insight into the success of this endeavor was the monitoring of core binding energies in the presence and absence of irradiation. Unfortunately the authors neglect to sufficiently describe the experimental parameters of exactly how this was performed and we are left to assume the light source used was relevant. Since the only experimental detail provided is the use of a Kratos SUPRA we do not know if the light source used was an external monochromatic UV lamp or the internal UPS system (given the presence of a single UPS measurement with the data it is highly likely the internal UPS system was used).
The authors measured the binding energies in the presence and absence of some illumination and determined peak shift following illumination which they ascribe to charge transfer (Figure 2). The magnitude of the shifts, however, are reported to by 0.1/0.2 eV which are experimentally insignificant and the reported values do not appear entirely consistent with the graphical presentation. Furthermore only the peaks of S 2p have been modelled whilst the Zn 2p analysis has clearly taken a peak maxima off-centre. To improve this interpretation, peak fittings and subsequent component maxima should be taken to avoid false shifts due to noise artifacts, whilst changes to energy separations between Ag 3d5/2 and X 2p3/2 (X = S, Cd, Zn) would remove any questions regarding changes to the sample energies via charging effects induced by the addition of exciting energy.
The photocatalytic activities clearly indicate the benefits of these architectures, however the ‘in-situ’ XPS analysis does not quite support the claims of the authors. Furthermore they use it to claim evidence of an LSPR effect in these composites when Ag nanoparticle LSPR absorptions may be readily observed in standard UV-Vis spectra. Whilst there is no clear peak in the UV-Vis, an increased absorption above 400 nm is more evidence for the existence of an LSPR than the XPS, and the UV-Vis spectrum of the bare un-coated Ag nanowires would provide the evidence required to support this. As such it is conspicuous in it’s absence.