Redox Potentials of Colloidal n-Type ZnO Nanocrystals: Effects of Confinement, Electron Density, and Fermi-Level Pinning by Aldehyde Hydrogenation.
Redox Potentials of Colloidal n-Type ZnO Nanocrystals: Effects of Confinement, Electron Density, and Fermi-Level Pinning by Aldehyde Hydrogenation.
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DOI：
10.1021/jacs.5b06715
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发表时间：
2015-09-02
影响因子：
15
通讯作者：
Gamelin DR
中科院分区：
文献类型：
--
作者：
Carroll GM;Schimpf AM;Tsui EY;Gamelin DR
电子掺杂的胶体半导体纳米晶体提供了宝贵的机会，以探索新的物理和化学性质赋予其过剩的电荷载流子。光掺杂是在自支撑胶体半导体纳米晶体内引入和控制自由载流子密度的有力方法。通过光化学氧化乙醇可以制备出具有离域导带电子（CB）的光还原（n型）ZnO纳米晶。以前的研究已经证明了这种化学反应的光化学电子积累，在某些情况下达到每个ZnO纳米管100个以上的电子，但在迄今为止的每一种情况下，这种化学反应都在一个明确定义的平均电子密度达到最大值，为1020 cm−3。这个最大值的起源从未被确定。在这里，我们使用溶剂化的氧化还原指示剂原位测定还原ZnO的氧化还原电位。正如预期的那样，平均只有一个过量CB电子的各种光掺杂ZnO纳米晶体的费米能级显示出量子限制效应，但比使用Cp*2Co化学还原的相同ZnO纳米晶体的费米能级低>600 meV，反映了它们的电荷补偿阳离子之间的重要差异。在光化学电子积累时，费米能级在约2 × 1019 cm−3以上时与电子体积无关，并在电子体积最大时达到最大值（1.6 ± 0.3）× 1020 cm−3。提出这个最大值是由于乙醛的双电子/双质子氢化引起的费米能级钉扎，这逆转了乙醇的光氧化反应。
Electronically doped colloidal semiconductor nanocrystals offer valuable opportunities to probe the new physical and chemical properties imparted by their excess charge carriers. Photodoping is a powerful approach to introducing and controlling free carrier densities within free-standing colloidal semiconductor nanocrystals. Photoreduced (n-type) colloidal ZnO nanocrystals possessing delocalized conduction-band (CB) electrons can be formed by photochemical oxidation of EtOH. Previous studies of this chemistry have demonstrated photochemical electron accumulation, in some cases reaching as many as >100 electrons per ZnO nanocrystal, but in every case examined to date this chemistry maximizes at a well-defined average electron density of 〈Nmax〉 ≈ (1.4 ± 0.4) × 1020 cm−3. The origins of this maximum have never been identified. Here, we use a solvated redox indicator for in situ determination of reduced ZnO nanocrystal redox potentials. The Fermi levels of various photodoped ZnO nanocrystals possessing on average just one excess CB electron show quantum-confinement effects, as expected, but are >600 meV lower than those of the same ZnO nanocrystals reduced chemically using Cp*2Co, reflecting important differences between their charge-compensating cations. Upon photochemical electron accumulation, the Fermi levels become independent of nanocrystal volume at 〈N〉 above ~2 × 1019 cm−3, and maximize at 〈Nmax〉 ≈ (1.6 ± 0.3) × 1020 cm−3. This maximum is proposed to arise from Fermi-level pinning by the two-electron/two-proton hydrogenation of acetaldehyde, which reverses the EtOH photooxidation reaction.