Unveiling hole trapping and surface dynamics of NiO nanoparticles

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.


ns TAS traces by excitation at 266 nm
To confirm that the BG excitation occurs really in the BG an excitation with more powerful photon was made and the ns TAS are compared in Figure S2.

S2
Heating effect on the absorption spectrum In Figure S3 we report the ∆-Abs of NiO after BG excitation and the difference spectrum betweenthe absorption spectrum recorded at RT and the one at 180 • . Figure S3: Comparison between the ∆-Abs at 200 ns after BG ex. and the difference between the NiO absorption spectrum at RT and the one at 180 • .

Further fs-TAS data
In this section we present the further fs data that were recorded.
fs-TAS with Ni 3+ free NiO Figure S4 shows the DAS of a BG excitation of a NiO sample that was heated at 200 • C. This treatment removes the Ni 3+ impurities from the NiO nanoparticle surface. The features that were assigned to Ni 4+ and Ni 3+ are not present in the DAS any more. Only a broad band in the red is present that is assigned to the direct electron-hole recombination from conduction band to valence band.

fs-TAS raw spectra and kinetic traces
In Figure S5 the raw spectra recorded in the fs-TAS experiment are reported. The delays ranged from 500 fs to 1.9 ns and are indicated in the plot. In Figure S6 the kinetic traces of specific wavelengths are shown.

NiO Band Gap excitation in presence of electron acceptor
The reactivity of the electron in the CB of NiO was tested performing a band gap excitation to a film in contact with an electron scavenger. A saturated solution of methyl viologen was used with this scope. The solution was kept in contact with the film by mean of a microscope cover glass.
In Figure S9 the TA of the wet film recorded 500 ns after excitation is compared with the reference bare NiO TA.

S7
On the kinetic mechanism of electron-hole recombination In the following the monochromatic traces of the main ∆-Absare reported. Figure S10: Monochromatic traces recorded after gap excitation of NiO. Trace 330 nm with respective multi exponential fitting.
In the following plot the comparison of the 430 nm band and the 650 nm one is presented.
The traces show a similar kinetics. The 430 nm is now fit with a second order kinetics.
As discussed in the main article, the 430 nm trace shows the kinetics of the electron-hole trap annihilation: The second order kinetics should be supported by the evidence of initial concentration rate dependence. In other words the reaction should be faster at greater initial concentration of reactants that in this case means higher excitation light intensities. A study was done to confirm this and the results for the 430 nm trace is reported in figure   S12. It seems that the kinetics is faster at higher intensities, thus the kinetics of the trap annihilation is believed to be of the second order.
In Figure S13 the inverse of the ∆-Absis plotted versus time and fit with a straight line. The fitting does not follow completely the data set. This could be due to several factor. First of all the amplitude of these decays are very small considering the nature of the sample (film). Secondly the local temperature of the film is changing of several decades of degree Celsius which can speed up the kinetics in the initial part of the decay.
Thirdly the presence of Ni 2+ was not consider affecting the kinetics while it could have an effect, considering the set of reactions for example: Hole relaxation to a Ni 3+ (h + ) trap: Hole transport through the Ni 2+ or Ni 3+ net: Ni And finally the hole-electron trap annihilation: This mechanism take in consideration the Ni 2+ states that were not created from an electron trapping, i.e. that were already existing in the surface of NiO. These states can work as a hole carrier, as Ni 3+ (h + ) , transporting the hole trap to to the electron trap Ni 2+ where they can finally recombine. This mechanism would complicate quite much the kinetics. In this case the Ni 4+ spectrum vanishes in the first reaction that is a second order reaction with two different starting concentration. From our data it seems that this is not the case since the linear plot in Figure S13 and the light intensity dependence shown in Figure S12. Further experiment should be made to verify this hypothesis. S10 Δ Absorption   Figure S13: Second order fit of the 430 nm trace.