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Cation Effects on the Reduction of Colloidal ZnO Nanocrystals Carolyn N. Valdez, Murielle F. Delley, and James M. Mayer J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05144 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018
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Journal of the American Chemical Society
Cation Effects on the Reduction of Colloidal ZnO Nanocrystals Carolyn N. Valdez,§ Murielle F. Delley,§ James M. Mayer* §
These authors contributed equally to this work.
Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, USA
[email protected] Abstract The effects of a variety of monoatomic cations (H+, Li+, Na+, K+, Mg2+, Ca2+) and larger cations (decamethylcobaltocenium and tetrabutylammonium) on the reduction of colloidal ZnO nanocrystals (NCs) are described. Suspensions of ‘TOPO’-capped ZnO NCs in toluene/THF were treated with controlled amounts of one-electron reductants (CoCp*2 or sodium benzophenone anion radical) and cations. Equilibria were quickly established and the extent of NC reduction was quantified via observation of the characteristic near-IR absorbance of conduction band electrons. Addition of excess reductant with or without added cations led to a maximum average number of electrons per ZnO NC, which was dependent on the NC volume and on the nature of the cation. Electrons are transferred to the ZnO NCs in a stoichiometric way, roughly one electron per monovalent cation and roughly two electrons per divalent cation. This shows that cations are charge-balancing the added electrons in ZnO NCs. Overall, our experiments provide insight into the thermodynamics of charge storage and relate the colloidal chemistry of ZnO with bulk oxide semiconductors. They indicate that the apparent band energies of colloidal ZnO are highly dependent on cation/electrolyte composition and concentration, as is known for bulk interfaces, and that electrons and cations are added stoichiometrically to balance charge, similar to the behavior of Li+-batteries.
Introduction The thermodynamics of charge storage in materials is of fundamental and commercial importance for energy technologies as well as other applications.1 A central issue in this area is the energy to add or remove electrons across a solution/solid interface, variously discussed using terms such as the Fermi energy, reduction potential, work function, and/or the band energy of a material.2 These energies are well known to be influenced by the presence of cations at the interface. In aqueous systems, protons are ubiquitous and changing their activity (the pH) is known to change the equilibrium potential of adding electrons to oxide materials.3 Lowering the pH of the contacting solution of a bulk oxide material typically shifts the band energies lower by 59 mV per pH unit. Other cations, like alkali and alkaline metal cations, have also been shown to influence the band energies of semiconducting oxides, such as TiO2 or SnO2.4 Such cation effects have typically been found to be more pronounced in the order Mg2+ ≈ Ca2+ > Li+ > Na+ >
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Cation Effects on the Reduction of Colloidal ZnO Nanocrystals
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tetrabutylammonium (TBA+). This has been related to the cation size-to-charge ratio and the cation’s ability to associate with the oxides via adsorption or intercalation. The influence of cations on charge storage can be discussed in terms of band energy movement, or, on a more fundamental level, as charge balance. These two treatments are different since charge balance requires that electron and cation transfers must be coupled. The one-to-one movement of cations and electrons is central to the behavior of lithium-ion batteries (LIBs)5 and sodium-ion batteries,6 for instance. The addition or removal of each electron to or from the battery anode or cathode is assumed to be stoichiometrically accompanied by addition or removal of a Li+ or Na+.7 For bulk materials, the 1:1 ratio of electrons to cations is necessary to ensure charge-balance. For molecular ions, on the other hand, charges can be balanced by the surrounding electrolyte without explicit concern for counter-ions. In nanoscale systems, such as dye-sensitized solar cells the photo-injection of electrons is not typically considered to be accompanied by a cation.8 Some of the electrons are injected on ultrafast timescales (< 1 ps), which is faster than cation binding or intercalation.9 Still, the much slower changes that occur in these devices during use are often ascribed to proton and/or Li+ intercalation.10 Hence, for these and many other nanoscale oxide materials, it is not clear whether or how electron and cation transfers are coupled, when charge balance occurs during electron transfer processes. Understanding the cation movements and their coupling to electrons is essential to progress in the development and improvement of energy-related processes. Nanoscale materials exhibit electronic bands similar to bulk materials. ZnO is a relatively simple oxide that is typically n-doped.11 Colloidal ZnO nanocrystals (NCs) are perhaps the best characterized oxide NCs in aprotic organic solvents. Capped with dodecylamine or ‘TOPO’ (octylphosphinate-type12) ligands, the size of the NCs, their concentration, surface chemistry, and redox state can be readily controlled and examined by optical, EPR and NMR spectroscopies, ICP-MS and TEM. With all of this in hand, the stoichiometry of electron / cation addition can be probed, similar to what is possible for molecular compounds. Colloidal ZnO is therefore a particularly convenient nanoscale oxide material to study electron transfers. Electrons can be introduced into ZnO NCs via photoreduction, in which an exciton is formed and the transient hole reacts with a reductant such as ethanol to leave an electron in the conduction band (CB).13 Anaerobic solutions of such photoreduced NCs are stable and blue in color.13f Their characteristic optical absorbance features include a band edge bleach at H+, Li+ > Na+ > TBA+, CoCp*2+. We attribute these differences in charging levels to differences in cation valency and to differences in the nature and extent of the association of cations with NCs. To our knowledge, this is the first comparison of this kind of cation effects on the thermodynamic reducibility of colloidal nanocrystals. Our findings are consistent with literature reports describing the influence of cations on the electronic energies of polycrystalline or mesoporous TiO2 or SnO2 thin films, for which the cation effects were found to depend on the size-to-charge ratio of the cation.4
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Journal of the American Chemical Society Cation Effects on the Reduction of Colloidal ZnO Nanocrystals
Valdez, Delley, and Mayer
Number of transferred electrons per cation. The number of electrons injected into ZnO NCs per cation added to the solution was evidenced in the regime before the maximum number of electrons per NC () was reached. For the number of transferred electrons per cation we examined the "slope" of vs. plots. We draw some of our conclusions from direct comparisons of experiments with the same NC batch. This can be more indicative of trends observed due to batch to batch variations inherent to the nanocrystalline system studied. Qualitatively, the data and trends found are reproducible across different NC batches. Figures 7A and 7B are plots of electrons per NC vs. added cations per NC for the same reductant CoCp*2, where the data for each cation has been fitted with a sigmoidal curve as discussed above (see Figures S14 and S15 for experimental details, fit coefficients and data beyond the maximal shown here). In Figure 7A data using H+, Na+ or Li+ are compared where all other conditions were kept the same. Initially these monovalent cations behaved nearly identically and led to the transfer of similar amounts of electrons per added cation (ca. 0.4 e– per cation). An example of more typical slope values for monovalent cations, i.e. close to 1, is shown in Figure 7B, where H+ and Na+ initially behaved the same and transferred both 0.9 e– per cation (0.9(2) or 0.9(4) e– per H+ or Na+, respectively). Overall, there is some scatter to the data, but taking into account data from a number of NC batches, we find that (1) H+, Na+, and Li+ cations initially transferred a similar number of electrons to ZnO NCs and (2) that the steepest slope between ne in the NCs and cation added was roughly 1. These data support our prior conclusion of e–/H+ slopes of ca. 1.21b The data show only small differences between the different cations of the same charge. While excess H+ gives a higher maximum charge density than Na+ with the same reductant, the behavior at lower amounts of cations is almost the same for the two cations. Similarly, the behaviors with added Mg2+ or Ca2+ are quite similar (see below). The finding of equal slopes in vs. for various monovalent cations follows that the differences in (M+) are not due to differences in slope, but are likely due to more cation binding. The divalent cations Mg2+ and Ca2+ had steeper slopes in the plot of observed vs. added. Figure 7B shows these plots for different batches of NCs of comparable average size. Figure S13 presents a direct comparison of these dications with Na+ using the same batch of NCs. For the divalent cations, the ∆/∆ slopes were 1.4(2) for Mg2+ and 1.3(3) for Ca2+. These values are ~1.5 times as steep as found for monovalent cations. Thus ca. 1.5 times as many electrons were transferred to ZnO per added cation for divalent cations compared to monovalent cations. Figure 5 above shows a related experiment with twice the slope for Mg2+ and Ca2+ (1.2(1) and 1.06(8) for ∆/∆) than for Na+ (0.52(10) for ∆/∆). While other factors also are significant, these data are consistent with the primary function of the cation being charge balance for the added electron, that M2+ should be twice as effective as M+ and stabilize two electrons per cation.
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Journal of the American Chemical Society Cation Effects on the Reduction of Colloidal ZnO Nanocrystals
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Figure 7. Plots of vs. for ZnO NCs reductions with excess CoCp*2 and various cations: A) Monovalent cations behave the same initially under similar conditions. (B) The divalent cations Mg2+ and Ca2+ transfer ca. 1.5 as many electrons per added cation to ZnO than the monovalent cations H+ and Na+. The black dotted lines of slope ∆/∆ = 1.4 or 0.9 are added to guide the eye. Overall, the vs. slopes indicate that the ZnO NCs behave less like outersphere electron transfer agents and more like batteries. In a battery model the addition of an electron requires a charge-balancing cation, whether it is H+ or Li+ or Na+, in line with what we previously observed for proton and electron addition to ZnO.21b This model also suggests that the addition of two electrons per cation can be balanced by divalent cations, consistent with our observations. The use of divalent cations, such as Mg2+, as working ions in order to stabilize higher electron densities is in fact a recent trend in battery research to double the energy carried per volume.34 Without a sufficient charge-balancing cation (as is the case with CoCp*2+ and TBA+), the thermodynamic cost for adding an electron to the NC is significantly higher. It would be valuable to know whether the monatomic cations intercalate into the lattice, but such studies are challenging due to the very low cation loading of
