Al-doped ZnO (AZO) thin films have been prepared by mist chemical vapor deposition and magnetron sputtering. The band gap shift as a function of carrier concentration in n-type zinc oxide (ZnO) was systematically studied considering the available theoretical models. The shift in energy gap, evaluated from optical absorption spectra, did not depend on sample preparations; it was mainly related to the carrier concentrations and so intrinsic to AZO. The optical gap increased with the electron concentration approximately as ne2∕3 for ne≤4.2×1019 cm−3, which could be fully interpreted by a modified Burstein–Moss (BM) shift with the nonparabolicity of the conduction band. A sudden decrease in energy gap occurred at 5.4−8.4×1019 cm−3, consistent with the Mott criterion for a semiconductor-metal transition. Above the critical values, the band gap increased again at a different rate, which was presumably due to the competing BM band-filling and band gap renormalization effects, the former inducing a band gap widening and the latter an offsetting narrowing. The band gap narrowing (ΔEBGN) derived from the band gap renormalization effect did not show a good ne1∕3 dependence predicated by a weakly interacting electron-gas model, but it was in excellent agreement with a perturbation theory considering different many-body effects. Based on this theory a simple expression, ΔEBGN=Ane1∕3+Bne1∕4+Cne1∕2, was deduced for n-type ZnO, as well as p-type ZnO, with detailed values of A, B, and C coefficients. An empirical relation once proposed for heavily doped Si could also be used to describe well this gap narrowing in AZO.

͑Zn,Al͒O thin films have been prepared by a dc reactive magnetron sputtering system with the Al contents in a wide range of 0 -50 at. %. The structural, optical, and electrical properties of ͑Zn,Al͒O films were detailedly and systematically studied. The amount of Al in the film was nearly the same as, but often lower than, that in the sputtering target. The growth rate of films monotonically decreased as the Al content increased. In a low Al content region ͑Ͻ10 at. % ͒, Al-doped ZnO ͑AZO͒ thin films could be obtained at 400°C in an Ar-O 2 ambient with good properties. The optimal results of n-type AZO films were obtained at an Al content of 4 at. %, with low resistivity ϳ10 −4 ⍀ cm, high transmittance ϳ90% in the visible region, and acceptable crystal quality with a high c-axis orientation. The band gap could be widened to 3.52 eV at 4 at. % Al due to the Burstein-Moss shift ͓E. Burstein, Phys. Rev. 93, 632 ͑1954͔͒ modulated by many-body effects. An appropriate Al-doping concentration served effectively to release the residual, compressive stress in film, which may be the reason for the improvement in film stability and the increment in grain size as well. In a medium Al content region ͑10-30 at. % ͒, however, the film quality was degraded, which was presumably due to the formation of clusters or precipitates in the grains and boundaries. Besides the ͑002͒ plane, other diffraction peaks such as ͑100͒ and ͑101͒ planes of ZnO were observed, but the ͑Zn,Al͒O films still exhibited a single-phase wurtzite ZnO structure. An intragrain cluster scattering mechanism was proposed to interpret the reduction of carrier mobility in films with the Al contents in the 7 -20 at. % region. The solubility limit of Al in ZnO film was identified to be in the 20-30 at. % range, much higher than the thermodynamic solubility limit of 2 -3 at. % in ZnO. In a high Al content region ͑ജ30 at. % ͒, there were distinct observations for ͑Zn,Al͒O films. As the Al content was 30 at. %, the film appeared in a two-phase nature with ZnO hexagonal and Al 2 O 3 rhombohedral structures. At the 50 at. % Al content, the matrix of the ͑Zn,Al͒O film was Al 2 O 3 , and no evident trace of wurtzite ZnO was observed. The electrical and optical properties for both cases were also very different from those at the Al contents below 30 at. %.

Emerging single‐atom catalysts (SACs) hold great promise for CO2 electroreduction (CO2ER), but the design of highly active and cost‐efficient SACs is still challenging. Herein, a gas diffusion strategy, along with one‐step thermal activation, for fabricating N‐doped porous carbon polyhedrons with trace isolated Fe atoms (Fe1NC) is developed. The optimized Fe1NC/S1‐1000 with atomic Fe‐N3 sites supported by N‐doped graphitic carbons exhibits superior CO2ER performance with the CO Faradaic efficiency up to 96% at −0.5 V, turnover frequency of 2225 h−1, and outstanding stability, outperforming almost all previously reported SACs based on N‐doped carbon supported nonprecious metals. The observed excellent CO2ER performance is attributed to the greatly enhanced accessibility and intrinsic activity of active centers due to the increased electrochemical surface area through size modulation and the redistribution of doped N species by thermal activation. Experimental observations and theoretical calculations reveal that the Fe‐N3 sites possess balanced adsorption energies of *COOH and *CO intermediates, facilitating CO formation. A universal gas diffusion strategy is used to exclusively yield a series of dimension‐controlled carbon‐supported SACs with single Fe atoms while a rechargeable Zn–CO2 battery with Fe1NC/S1‐1000 as cathode is developed to deliver a maximal power density of 0.6 mW cm−2.

Electrocatalysts playak ey role in accelerating the sluggish electrochemical CO 2 reduction (ECR) involving multi-electron and proton transfer.W en ow develop ap roton capture strategy by accelerating the water dissociation reaction catalyzedb yt ransition-metal nanoparticles (NPs) adjacent to atomically dispersed and nitrogen-coordinated single nickel (NiÀN x )a ctive sites to accelerate proton transfer to the latter for boosting the intermediate protonation step,a nd thus the whole ECR process.A berration-corrected scanning transmission electron microscopy, X-ray absorption spectroscopy, and calculations reveal that the Ni NPs accelerate the adsorbed H (H ad )g eneration and transfer to the adjacent Ni À N x sites for boosting the intermediate protonation and the overall ECR processes.T his proton capture strategy is universal to design and prepare for various high-performance catalysts for diverse electrochemical reactions even beyond ECR.