Controlling Carrier Densities in Photochemically Reduced Colloidal ZnO Nanocrystals: Size Dependence and Role of the Hole Quencher

Abstract

Photodoped colloidal ZnO nanocrystals are model systems for understanding the generation and physical or chemical properties of excess delocalized charge carriers in semiconductor nanocrystals. Typically, ZnO photodoping is achieved photochemically using ethanol (EtOH) as a sacrificial reductant. Curiously, different studies have reported over an order of magnitude spread in the maximum number of conduction-band electrons that can be accumulated by photochemical oxidation of EtOH. Here, we demonstrate that this apparent discrepancy results from a strong size dependence of the average maximum number of excess electrons per nanocrystal, <nmax>. We demonstrate that <nmax> increases in proportion to nanocrystal volume, such that the maximum carrier density remains constant for all nanocrystal sizes. <nmax> is found to be largely insensitive to precise experimental conditions such as solvent, ligands, protons or other cations, photolysis conditions, and nanocrystal or EtOH concentrations. These results reconcile the broad range of literature results obtained with EtOH as the hole quencher. Furthermore, we demonstrate that <nmax> depends on the identity of the hole quencher, and is thus not an intrinsic property of the multiply reduced ZnO nanocrystals themselves. Using a series of substituted borohydride hole quenchers, we show that it is possible to increase the nanocrystal carrier densities over 4-fold relative to previous photodoping reports. When excess lithium and potassium triethylborohydrides are used in the photodoping, formation of Zn(0) is observed. The relationship between metallic Zn(0) formation and ZnO surface electron traps is discussed.

Redox Chemistries and Plasmon Energies of Photodoped In2O3and Sn-Doped In2O3(ITO) Nanocrystals

“…43 Absorption spectra of all samples are provided as Supporting Information ( Figure S1). In 2 O 3 and ITO nanocrystals were photodoped in the same manner as detailed 32,33,38 for ZnO nanocrystals (see Experimental Methods). Specifically, exposure to UV illumination under rigorously anaerobic conditions and in the presence of a sacrificial reductant (EtOH) leads to the accumulation of conduction-band electrons (Figure 1, top).…”

“…To correct for volume effects, 38 the resulting data are presented as average electron densities, ⟨N⟩. plots the electron densities, ⟨N as-prepared ⟩, ⟨N photo ⟩, and ⟨N tot ⟩ as a function of Sn concentration.…”

“…This value is slightly higher than that found for ZnO nanocrystals photodoped under the same conditions (1.4 ± 0.4 × 10 20 cm −3 ). 38 These titrations determine carrier densities without relying on a model-dependent analysis of LSPR energies, which presents an opportunity for critical comparison with use of the extended Drude model to analyze carrier concentrations in ITO nanocrystals based on LSPR spectra. 43,51,52 Spectra of the asprepared ITO and all the photodoped nanocrystals were fit using a parametrized dielectric function that included a frequency-dependent damping function, as typically found for ITO.…”

Chemical Control of Plasmons in Metal Chalcogenide and Metal Oxide Nanostructures

“…In another demonstration, they used the "nanocrystal-in-glass" approach to achieve unprecedented dynamic optical switching behavior. The ITO nanocrystals embedded in a niobium oxide glass matrix allows independent manipulation of the visible and nearinfrared transmittance through the fi lm through electrochemical biasing (Figure 3 C), as reported by Llordés et al [ 14 ] Photodoping may also be employed to tune the LSPR, as seen in the work of Schimpf et al in ZnO [ 47 ] and ITO nanocrystals, [ 48 ] which allows independent control of the LSPR energies from the initial carrier density introduced through aliovalent doping (Figure 3 D).…”

Facile preparation of yttrium and aluminum co-doped ZnO via a sol–gel route for photocatalytic hydrogen production

“…[3][4][5][6][7] Besides, ZnO has become a promising photocatalyst, owing to its lower cost, exible preparation method, non-toxicity, high photosensitivity, chemical and thermal stability. 8-12 Thus, more and more scientists are attracted to the intensive research on the photocatalytic H 2 production with ZnO recently.…”