研究背景
ZnO是一种热稳定的N型半导体材料。ZnO二维纳米片由于其独特的性能，如直接带隙和室温下的强激子结合能，已经获得了大量的关注。这些材料被广泛用于压电学储能、光电探测器、发光二极管、太阳能电池、气体传感器和光催化。ZnO纳米片的化学性质和性能在很大程度上取决于纳米结构，而纳米结构可以通过调节合成策略进行调节和控制。
研究成果
本综述对设计和开发二维ZnO纳米材料进行了全面的研究，然后强调了其潜在的应用。首先，讨论了各种合成策略和ZnO二维纳米片的属性，然后重点讨论了方法学和反应机制。然后，进一步讨论了它们对电池、超级电容器、电子/光电子、光催化、传感和压电平台的审议。最后，基于其目前的发展，对挑战和未来的机会进行了介绍。相关报道以“2D Zinc Oxide – Synthesis, Methodologies, Reaction Mechanism, and Applications”为题发表在Small期刊上。
图文导读
Figure 1. Schematic illustration of synthesis strategies and applications of ultrathin 2D ZnO.
Figure 2. a–c) TEM image of ZHDS nanomembranes. a) TEM image at low magnification, inset: electron diffraction pattern of a single grain. b) Crosssectional TEM image of ZHDS nanomembrane, c) Hillock pyramid-like the hexagonal surface of a single grain. d) Schematic illustration of evaluation of ZHDS nanomembrane at the water–air interface. e–i) Rectangular ZHDS sheets e) optical image, f,g) AFM scan topographical images of flat-surfaced rectangular sheet and spiral growth rectangular sheet, respectively, with line profiles, h) TEM image of single rectangular ZHDS sheet with electron diffraction pattern (inset), i) TEM image of the exfoliated sheet, inset: HRTEM image and corresponding electron diffraction pattern.
Figure 3. a) Schematic representation of the formation of ZnO nanosheet at the water–air interface, b) AFM topography scan (scale bar 5 µm), c) SEM image of individual ZnO nanosheet (scale bar 5 µm), d) HRTEM image shows overgrowth of nanosheet (scale bar 2 nm), e) schematic representation of time-dependent evolution of ZnO nanosheet. . f) SEM image of ZnO nanosheet at different surface pressure, g) AFM image of ZnO nanosheet with height profile, h) HRTEM image shows wurtzite lattice structure with Fast Fourier Transform FFT pattern (lower inset) and electron diffraction pattern, (upper inset). i) Large area SEM image of ZnO nanosheet (inset magnified image), j) 3D nanosheet with height profile (inset image), k) HRTEM image of wurtzite ZnO with inset SAED (right top) and FFT (right bottom) pattern of ZnO nanosheet which confirms polycrystalline nature of nanosheet. Reproduced with permission
Figure 4. a,b) SEM image of an as-synthesized flat surface, b) Hillock hexagonal nanoplates, c) AFM topography scan of dislocation core, d) HRTEM image of ZHS with inset SAED and FFT pattern, respectively, e) schematic representation of dislocation-driven growth of different nanomaterials. f,g) SEM images, h) AFM images, and i) HRTEM images inset: SAED pattern of ultrathin Ni-doped ZnO nanosheet with SOS.
Figure 5. a) TEM image, b) AFM image, c) HRTEM image with SAED pattern (inset) of ZnO nanosheet. d) Low magnification, e) higher magnification SEM image of as-synthesized bilayer hexagonal ZnO disks, f) HRTEM image and SAED pattern (inset) of ZnO disks. g) SEM image of ZnO nanosheet (scale bar 200 nm), h) HRTEM image of ZnO nanosheet (scale bar 5 nm) and inset: HRTEM images of the crystal lattice structure of the nanosheets (scale bar 1 nm). TEM and HRTEM image of i) Zn5(OH)8Cl2H2O, j) ZnO-300, k) ZnO-500, l) ZnO-700.
Figure 6. a) SEM image with EDS elemental mapping (inset), b) AFM image, inset: height profile image of ZnO nanosheet. c) Low magnification TEM, d,f) HRTEM image of monolayer ZnO nanosheet with crystal lattice (inset). e) TEM image with corresponding SAED pattern (inset). g) SEM image, h) AFM image with corresponding line profile (inset image) of a monolayer of gZnO nanosheet, i) nanobeam electron diffraction pattern of 3-layer gZnO j) HRSTEM image of a ZnO and rGO composite with intensity profile diagram. SEM image of k) as-synthesized and l) annealed at 800 °C of ZnO nanosheet respectively. m) SEM, n,o) TEM image of synthesized porous ZnO nanosheet on GO template.
Figure 7. a) Schematic representation of ZnO-MSN synthesis, b,c) low magnification and (lower inset) enlarged image of the selected area, (upper inset) SAED pattern. d) HRTEM image of ZnO-MSN, e) atomic resolution TEM image and f–h) magnified atomic-resolution TEM images.
Figure 8. TEM image of a) ZnO (scale bar 1 µm) with SAED pattern (inset image), b) AFM image of ZnO thin layers. c) Schematic representation of formation mechanism of transition metal oxide, d) SEM, e) TEM, f) SAED pattern, g) enlarged TEM and h) HRTEM image of ZnO nanosheet.
Figure 9. SEM image of a) pure ZnO, b) ZnO nanosheet, and c) low magnification ZnO/GO nanocomposite. d) SEM imaging of ZnO grown at 800 °C in a 0.5 mbar pure O2 environment with a 45° slanted view, e) bright-field TEM picture of a single ZnO nanosheet with corresponding SAED pattern, revealing the inclination angle of the growth path with the ZnO hexagonal c axis, f) a brightfield image of a nanosheet tip reveals a kink beneath the Au catalyst droplet, g) SEM images of a sample grown at 800 °C in a 0.5 mbar mixed ambient of 5% O2 to 95% Ar, with a 45° tilted view and cross-sectional (inset) image, h) a bright-field TEM image of a single nanowire with its accompanying SAED pattern, i) HRTEM micrograph of a nanowire and its Fourier transform, grown in a 0.5 mbar mixed ambient of 5% O2 to 95% Ar, j) HRTEM image and its Fourier transform of a nanosheet grown in a 0.5 mbar pure O2.
Figure 10. Electrochemical performance of ZnO a–e) nanosheet, a) cyclic voltammetry at 0.1 mV s−1, galvanostatic charge–discharge curve at b) 0.2 A g−1 for different cycles, and d) different current densities, c) cyclic stability at 0.2 A g−1, e) rate performance at different current densities. Galvanostatic charge–discharge curve at 0.05 A g−1, f) porous ZnO nanosheets and g) commercial ZnO powders, h) cyclic stability of porous ZnO nanosheets and commercial ZnO powders at the current density of 0.5 A g−1, i) Rate performance of ZnO nanosheet. j) CV and k) GCD curve of ZnO nanomembrane, l) Ragone plot of energy density versus power density.
Figure 11. a) Optical microscope image of ZHDS nanomemberane-fabricated FETs on a PET substrate. Transfer characteristics of b) as-synthesized and annealed at 95 °C for 48 h ZHDS. c) ZnO nanosheet. Output characteristics of c) as-synthesized and d) annealed ZHDS sample at different VD = 0–15 V and VG = 0–20 V. e) Optical microscope image of ZnO nanosheet on Si wafer coated with 50 nm Al2O3 (f) plot of ID versus VG at VD = 5V and VG = 7 to −7 V. g) Plot of ID versus VD at VG = 2 to −7 V with a 1V step. h) I–V Characteristics of 2D nanosheets. Magnified ZnO nanosheet (inset). i) The plot of photoresponse study of 2D nanosheets. j–l) Comparison of the relation between photovoltage and time of ZnO photodetectors thin films for UV, white, and green illuminations. j) NPs photodetector, k) NSs photodetector, and l) NFs photodetector
Figure 12. a) Represent response of ZnO nanosheet sensor with different facets to ethanol vapor of various concentrations. The low concentrated response–recovery curve was magnified as inset b) selectivity of the sensors to different interfering gases. c) Long-term stability study. d,e) Response curve at d) different operating temperatures, e) different concentrations of ethanol, and f) different gases at 100 ppm concentration. Comparative response study between 3 sensors g) at different ethanol concentrations. h) Different interfering gases. i–k) Response and recovery versus i) ethanol concentration, inset is the gas response of the sensors to 0.01–1 ppm. j) At different interfering gases at 400 °C. k) Stability curve of 200 ppm ethanol concentration at 400 °C.
Figure 13. a) Gas responses of Pd-ZnO-NSs and ZnO-NSs sensors to acetone at concentrations measuring from 10 to 500 ppm. The inset image represented response versus concentration curves. b) Acetone selectivity over other interfering gases. c) Long-term stability study. The sensing performance of pure ZnO, ZnO nanosheets, and ZnO/GO nanocomposites sensors. d) Selectivity and e) sensing characteristics to 100 ppm acetone of 240 °C. The sensing activity of porous ZnO nanosheets. f) Sensing characteristics and g) stability to 2000 ppm of ethylene and h) response to various gases.
Figure 14. a,b) The response curve of a) 50 ppm NO2 at different temperatures, b) different concentrations of NO2, c) response–recovery curve at 50 ppm NO2 concentration. d–f) Sensitivity plot of ZnO nanowall-based sensor: d) at the different annealing temperatures; e) at different NO2 concentrations; and f) stability study at 50 ppm NO2. g–i) Response curve: g) at different temperatures; h) at different concentrations; and i) selectivity over other interfering gases. Response of j) pristine and Au-functionalized ZnO nanosheets at 1–5 ppm NO2 gas, and k) selectivity.
Figure 15. a) Schematic diagram of fabricated 2D ZnO-based piezotronic device. b,d) Charge transport characteristic curve at series of applied b) stress and d) pressure. c) Band diagram of ZnO nanosheet while negative piezoelectric polarization charges at interface. e) The plot of change of SBH versus applied pressures. f) Schematic representation of hole distribution in nanosheet.
Figure 16. a,b) Effect of the width of channel gate in the device a) holes distribution at different strain values. b) The plot of current and resistivity at different pressure. c) The plot of SBH as a function of thickness. d) Hole concentration distribution under strain-free conditions. e) Band diagram. f,g) Thickness dependence effect of the width of channel gate and f) characteristic curve of electrical transport properties. g) The plot of normalized current versus strain at different thicknesses. h) Schematic diagram of fabricated 2D ZnO-based piezotronic device. i) The characteristic curve of electrical transport properties at different applied pressure. j) The plot of photoresponsivity as a function of applied pressures.
总结与展望
在过去的几十年里，超薄二维纳米材料的研究得到了惊人的发展。在这篇综述中，作者重点讨论了合成策略，包括 ILE、CVD、二维模板合成、溶剂热合成、自组装等。二维ZnO纳米结构的厚度、尺寸、表面特性和结晶度都取决于合成方法，这对不同的应用是有利的。从合成的角度来看，湿化学合成方法存在一些挑战：1)很难合成单层纳米结构；2) 纳米结构的横向尺寸小于1 μm。此外，基于表面活性剂的方法展示了一种有前途的、独立的、大面积的二维ZnO纳米结构的合成方法，这对具有复杂部件的设备制造是有利的。此外，提高纳米材料的生产速度以与商业来源竞争也是学术研究人员的一个主要挑战。
从应用的角度来看，ZnO纳米结构作为一种电极材料，由于其高理论容量、高导电性和大表面积，显示出在储能装置中的潜在应用。此外，由于氧缺陷、纳米结构的形态和取向，它还显示出光催化活性。
此外，基于 ZnO的纳米结构被用于传感应用。N型ZnO半导体中的供体缺陷水平占主导地位。存在于表面的缺陷增强了ZnO表面的吸附位点，从而加强了氧化还原反应效率，导致对ZnO基纳米材料的传感性能产生影响。提高灵敏度、选择性和稳定性以及减少ZnO基传感器的反应/恢复时间是传感应用中的重大挑战。此外，湿度条件也会影响ZnO基传感器的传感性能。在实验室规模的传感中，研究是在受控环境下进行的。然而，在工业层面上，湿度不断变化。因此，当务之急是研究在各种湿度条件下的灵敏度。
此外，ZnO材料在电极上直接沉积在电极上减少了形貌的破坏，提供了较高的表面积和有效的载流子传输通道。因此，提高了ZnO基传感器的性能。此外，二维纳米结构因其独特的带隙、大激子带隙能量（60 meV）和高击穿电压而广泛应用于光电器件。然而，仍有一些未解决的问题。例如，响应性、灵敏度和响应速度、稳定性、耐久性、环保和经济有效的加工。除此之外，在小领域大规模生产和设计复杂电路是未来研究的重大挑战。
上述二维ZnO纳米结构的合成策略可能有助于研究vdW异质结构的实时应用，因为目前vdW异质结构是通过剥离方法制备的，由于横向尺寸/面积小而有局限性。通过优化反应时间、表面压力、前驱体浓度、使用适当的两亲分子和表面还原化学来合成无皱二维纳米片可能对该领域的应用研究产生重大影响。
文献链接
2D Zinc Oxide – Synthesis, Methodologies, Reaction Mechanism, and Applications
https://doi.org/10.1002/smll.202206063.
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