Plenary Lectures

Triboelectric nanogenerator (TENG) was invented by my group in 2012, which is based on coupling of triboelectrification and electrostatic induction effects for converting mechanical energy into electric power. TENG is playing a vitally important role in the distributed energy and self-powered systems, with applications in internet of things, environmental/infrastructural monitoring, medical science, environmental science, and security. There are now over 12,000 scientists distributed in 83 countries and regions around the globe who have published papers on TENG. This presentation will first focus on the advances in fundamental science made due to the discovery of TENG. Then we will focus on the potential industrial impacts that have been made by TENG. We will show how this new field will benefit to the sustainable development of humankinds.

References:

1. L. Wang and A.C. Wang “On the origin of contact electrification“ (Review), Materials Today, 30 (2019) 34-51; https://doi.org/10.1016/j.mattod.2019.05.016
2. Shiquan Lin^#, Xiangyu Chen^#, and Zhong Lin Wang* “Contact-electrification at liquid-solid interface” (Review), Chemical Review， 122 (2022) 5209-5232; https://doi.org/10.1021/acs.chemrev.1c00176
3. L. Wang “From conctact electrication to triboelectric nanogenerators“ (Review), Report on Progress in Physics, 84 (2021) 096502; https://doi.org/10.1088/1361-6633/ac0a50
4. Jiajia Shao, Morten Willatzen^*, and Zhong Lin Wang^* “Theoretical modelling of triboelectric nanogenerators (TENGs)” （Review）, Applied Physics, 128 (2020) 111101;  https://doi.org/10.1063/5.0020961
5. Zhong Lin Wang “ On the expanded Maxwell’s equations for moving charged media system – general theory, mathematical solutions and applications in TENG”， Materials Today， 52 (2022) 348-363; https://doi.org/10.1016/j.mattod.2021.10.027
6. L. Wang “ Maxwell’s equations for a mechano-driven, shape-deformable, charged media system, slowly moving at an arbitrary velocity field 𝒗(𝒓,𝑡)”, J. Phys, Communication，6 (2022) 085013; https://doi.org/10.1088/2399-6528/ac871e;
7. L. Wang “ The expanded Maxwell’s equations for a mechano-driven media system that moves with acceleration”, Intern. J. of Modern Physics (2022) 2350159; https://doi.org/10.1142/S021797922350159X

Understanding the local chemical ordering propensity in random solid solutions, and tailoring its strength, can guide the design and discovery of complex, paradigm-shifting multi-component alloys. A simple thermodynamic framework was employed, based solely on binary enthalpies of mixing, to select optimal alloying elements to control the nature and extent of chemical ordering in high entropy alloys (HEAs). Subsequently, high resolution electron microscopy, atom probe tomography, and computational modeling have been coupled to demonstrate how controlled additions of Al and Ti and subsequent annealing drive chemical ordering (from short-range ordered domains to long-range ordered precipitates) in nearly random equiatomic FCC CoFeNi solid solution, boosting its tensile yield strength by a factor of four while also substantially improving ductility, which breaks the so-called strength-ductility paradox. The generality of this approach was demonstrated via addition of Al for introducing chemical ordering and enhancing mechanical properties in another nearly random BCC refractory NbTaTi HEA.

Keywords: High entropy alloys, Thermodynamics, Chemical ordering

Fig. 1. Schematic figure representing the thermodynamic framework employed for introducting local chemical ordering within the random solid solution of FCC HEAs, the coupled computational-expertimental evidence for such ordering, and its impact on mechanical properties.

Photocatalytic water splitting has been receiving growing interest as a means of producing renewable hydrogen, because systems based on particulate photocatalysts may be spread over large areas through potentially inexpensive processes. A solar hydrogen production system based on 100-m² arrayed photocatalytic water splitting panels and an oxyhydrogen gas-separation module was built recently, and its performance and system characteristics including safety issues were reported.¹⁾ Nevertheless, it is essential to radically improve the solar-to-hydrogen energy conversion efficiency (STH) of particulate photocatalysts and develop suitable reaction systems.²⁾ In my talk, recent progress in photocatalytic materials and reaction systems will be presented.

The author’s group has studied various particulate semiconductor materials as photocatalysts for water splitting.³⁾ The apparent quantum yield (AQY) of overall water splitting using SrTiO₃ has been improved to more than 90% at 365 nm, equivalent to an internal quantum efficiency of almost unity, by refining the preparation conditions of the photocatalyst and the loading conditions of cocatalysts.⁴⁾ This observation means that particulate photocatalysts can drive the endergonic overall water splitting reaction with almost no recombination loss as in photon-to-chemical conversion processes during the photosynthesis. For practical solar hydrogen production, however, it is essential to develop photocatalysts that are active under visible light. Ta₃N₅,⁵⁾ Y₂Ti₂O₅S₂,⁶⁾ BaTaO₂N,⁷⁾ and SrTaO₂N⁸⁾ were recently shown to be active in photocatalytic overall water splitting via one-step excitation under visible light.

It is also possible to combine hydrogen evolution photocatalysts (HEPs) and oxygen evolution photocatalysts (OEPs) to split water into hydrogen and oxygen via two-step excitation. Such a process is recognized as Z-scheme widely. Particulate photocatalyst sheets consisting of La- and Rh-codoped SrTiO₃ as the HEP and Mo-doped BiVO₄ as the HEP immobilized onto Au and C layers split water into hydrogen and oxygen with STH values exceeding 1.0%.^9,10) Some (oxy)chalcogenides and (oxy)nitrides with long absorption edge wavelengths are also applicable to Z-schematic photocatalyst sheets and hold the promise of realizing greater STH values.

1. Nishiyama et al. Nature 2021, 598, 304.
2. Hisatomi et al. Next Energy 2023, 1, 100006.
3. Chen et al. Nat. Rev. Mater. 2017, 1, 17050.
4. Takata et al. Nature 2020, 581, 411.
5. Wang et al. Nat. Catal. 2018, 1, 756.
6. Wang et al. Nat. Mater. 2019, 18, 827.
7. Li et al. ACS Catal. 2022, 12, 10179.
8. Chen et al. J. Am. Chem. Soc. 2023, 145, 3839.
9. Wang et al. Nat. Mater. 2016, 15, 611.
10. Wang et al. J. Am. Chem. Soc. 2017, 139, 1675.

High-entropy alloys (HEAs) comprise a novel class of scientifically and technologically interesting materials. Among these, the equatomic CrMnFeCoNi alloy with the face-centered cubic (FCC) structure and some of its derivative equiatomic quaternary and ternary alloys are noteworthy because its ductility and strength increase with decreasing temperature while maintaining outstanding fracture toughness at cryogenic temperatures. However, the origin of these excellent mechanical properties has yet to be understood clearly, because mostly of the luck of studies using their single crystals. In the present study, the plastic deformation behavior of single crystals of the FCC equatomic CrMnFeCoNi high-entropy alloy and its derivative quaternary (CrFeCoNi) and ternary (CrCoNi) medium-entropy alloys has been investigated in a temperature range of 10-1273 K. Deformation occurs via slip of the {111}<110> system exclusively in the whole temperature range for all alloys investigated. The CRSS values increase with decreasing temperature, especially below room temperature, so that the concept of ‘stress equivalence’ is obeyed for all alloys investigated. This is a clear indication that the strength of these alloys should be described by a mechanism based on solid-solution hardening. The CRSS values extrapolated to 0 K for polycrystals of equiatomic quinary, quaternary and ternary alloys are reported to be well scaled with the mean-square atomic displacement from the regular FCC lattice points (calculated based on density-functional theory). This seems also the case for the CRSS values at 10 K for single crystals of the present three alloys, although some modifications are definitely needed. Deformation twinning occurs on the conjugate system in the form of the Lüders type deformation in the later stage of deformation at low temperatures in all of the three alloys. The correlation between the twinning stress and the stacking-fault energy in these alloys will be discussed. The occurrence of short-range ordering was confirmed in CrCoNi through the measuring of electrical resistivity, while conventional x-ray and electron diffraction failed to detect short-range ordering because of the very small difference in atomic scattering factor for the constituent elements in a wide range of scattering angle. The extent of short-range oredering in CrCoNi was proved to be the largest after annealing at 673 K. However, no significant variation of the CRSS values, twinning stress and the stacking-fault energy was noted upon the formation of short-range order by heat-treatment.