REVIEW www.advancedscience.com Energy-Saving Pathways for Thermoelectric Nanomaterial Synthesis: Hydrothermal/Solvothermal, Microwave-Assisted, Solution-Based, and Powder Processing Nagaraj Nandihalli,* Duncan H. Gregory, and Takao Mori* The pillars of Green Chemistry necessitate the development of new chemical methodologies and processes that can benefit chemical synthesis in terms of energy efficiency, conservation of resources, product selectivity, operational simplicity and, crucially, health, safety, and environmental impact. Implementation of green principles whenever possible can spur the growth of benign scientific technologies by considering environmental, economical, and societal sustainability in parallel. These principles seem especially important in the context of the manufacture of materials for sustainable energy and environmental applications. In this review, the production of energy conversion materials is taken as an exemplar, by examining the recent growth in the energy-efficient synthesis of thermoelectric nanomaterials for use in devices for thermal energy harvesting. Specifically, “soft chemistry” techniques such as solution-based, solvothermal, microwave-assisted, and mechanochemical (ball-milling) methods as viable and sustainable alternatives to processes performed at high temperature and/or pressure are focused. How some of these new approaches are also considered to thermoelectric materials fabrication can influence the properties and performance of the nanomaterials so-produced and the prospects of developing such techniques further. N. Nandihalli, T. Mori National Institute for Materials Science (NIMS) International Center for Materials Nanoarchitectonics (WPI-MANA) Namiki 1-1, Tsukuba 305-0044, Japan E-mail: nandihalli.nagaraj@gmail.com; MORI.Takao@nims.go.jp D. H. Gregory WestCHEM School of Chemistry University of Glasgow Glasgow G82 5EZ, UK The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/advs.202106052 © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. DOI: 10.1002/advs.202106052 1. Introduction Thermoelectric (TE) materials are energy harnessing solid-state semiconductors that transform thermal energy into electricity or create a temperature difference from an ap- plied voltage. [1] The potential applications of TE materials are numerous. [2] Among many exciting and prominent emerging applications are wearable TE devices that can convert body heat into electrical en- ergy. Such devices could drive low power consumption implants such as deep-brain stimulators, artificial cochlea, health pa- rameter sensors, and various similar com- ponents within the internet-of-things (IoT) to monitor patient health remotely. [3] Even in contemporary society, many state-of-the- art devices require little input power so that sensors such as electronic tracking tags, which are able to operate at 0.1 mW, are ideal candidates to be driven by TE devices. [4] Other potential applications of TEs lie at completely the opposite scale; TE generator systems that can harvest indus- trial and automotive waste heat to produce electrical energy are likely to be major contributors to sustainable power generation and could play a major role in the reduction of carbon emissions over the years to come. [5] TE devices have a number of salient features that characterize their attractive- ness as a superior energy conversion technology: high reliability, easy miniaturization, geometrically accommodative, an absence of moving parts, noise-free, low maintenance, extended lifetimes, high-precision temperature control, and an ability to function in extreme environments (which can be ideal for both power gener- ation and remote sensing). 1.1. Governing Parameters in Thermoelectrics and the Role of Nanostructuring The parameter that enumerates the performance of a thermo- electric (TE) material is its figure-of-merit (zT) and is given by zT = S 2 T  (1) Adv. Sci. 2022, 2106052 © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH 2106052 (1 of 61)