Unveiling hole trapping and surface dynamics of NiO nanoparticles

Abstract

Mesoporous NiO is used as p-type material in photoelectrochemical energy conversion devices. The presence of two kinds of hole traps can affect device performance. Here, after band-gap excitation, the relaxation of the hole into two different traps was observed and characterized.

Insights into the mechanism and aging of a noble-metal free H2-evolving dye-sensitized photocathode

“…We note that such transient current spikes are not observed on control NiO ǀ RBG-174 ǀ C 10 P electrodes suggesting that these peaks arise from the possibility of accumulating electrons on the cobalt centers. Although a detailed study of the dynamics of such a light-driven process is beyond the scope of this study, the observation of distinct behaviors depending on the applied potential is likely in line with the existence of two populations of holes 71 with distinct dynamics 72 at the surface of NiO nanoparticles. In terms of photocurrent densities, the performance of co-grafted NiO ǀ RBG-174 ǀ Co C11P electrodes is close to those of reported covalent constructs based on the same catalytic centers 22 , 25 and compares well to other NiO-based molecular photocathodes.…”

Understanding the Role of Surface States on Mesoporous NiO Films

“… 45 In previous findings, in dye-free NiO, the filled intra-bandgap states ((NiO IBS-R ) − ) recombine with holes on a 100-ns time scale, indicating that the lifetime of trapped electron is much long-lived compared to the lifetime (e.g., t 1/2 = 6 ps) of PB6 – on NiO. 23 I 3 – species seem to have reasonable time to diffuse near NiO IBS-R – in order to complete electron transfer. In any case, reduced dye molecules will first transfer electrons to NiO IBS-R , and then (NiO IBS-R ) − will transfer the electron to the associated I 3 – or solvated I 3 – .…”

“…The DOS value is consistent with other reported values of mesoporous NiO films. 23 , 25 However, the DOS decreased to ca. 3.68 × 10 19 cm –3 in same potential range (0.02 to −0.9 V) in NiO-A films.…”

“…The density of states (DOS) in the pristine mesoporous NiO films prepared by the well-known sol–gel method has been characterized. 23 In this study, the DOS as a function of potential was derived from the cathodic scanning curve of CV (see SI for the detailed calculations). 25 , 31 The DOS obtained is similar to that reported in a previous study, where the DOS of surface states was extracted from the impedance spectra in an identically prepared NiO films.…”

NiO Pseudocapacitance and Optical Properties: Does The Shape Win?

“…The values of band gaps and nanoparticle diameters obtained for the different NiO preparations are reported in Table 3. The nature of the trapping was recently investigated by D'Amario et al [43], who generated electron-hole pairs in the band gap of NiO and followed the recombination dynamics. They associated the process with the generation of "Ni 3+ " and "Ni 4+ " ions, of which the lifetime may be affected by surrounding structures.…”