Thermoelectric materials are capable of converting thermal energy into electrical energy. This happens when two dissimilar metals or semiconductors connected together experience a change in temperature, where electrons flow from the hot part to the cold end of the material. Thomas Johann Seebeck observed this occurrence, leading to the term ‘Seebeck effect’. As they can create power from heat, thermoelectric materials are of use in a range of applications where waste-heat is a byproduct, such as powerplants and factories. Further to this, with modern society’s ever-growing need for more power to keep our gadgets energized, thermoelectric materials might also play a pivotal role here.

To begin with, the researchers prepared and cleaned their cloth, an electrically conductive carbon fabric (see top left image), before repeatedly dipping it into a solution of Zn(CH3CO2)2·2H2O and NaOH dissolved in C2H6O. This dip process seeded the carbon fabric with a uniform coating of ZnO, which was monitored through electron-probe analyses (see top right image). Creating conventional ZnO thermoelectric materials can be complex, involving high temperatures (several hundreds of degrees Celsius for many hours), which is not ideal for the kinds of comfortable fabric people might wear. Instead, Suresh Prasanna and colleagues decided on faster, microwave-assisted solvothermal reactions, where the reagents were only exposed to temperatures of 150 °C (at most) for no more than 30 minutes. Submersing the ZnO-seeded fabric in the same dip-solution, the researchers fine-tuned both the microwave power and time to grow the seeds into nanorods with average heights of 1,553 nm and 152 nm in diameter. The optimal reaction parameters were found to be 100 W for 15 minutes, where the best thermoelectric properties were obtained due to the crystallinity of the nanorods, the greatest density of nanorods grown on the fabric, and the vertical alignment of the rods, leading to a better packing efficiency. The thermoelectric and Hall measurements of this sample yielded a Seebeck coefficient of ~11.5 μV K–1 with an electrical conductivity of ~54 S cm–1, respectively. With lesser microwave power, or with the same power but over shorter times, the thermoelectric and electrical conductivity measurements were noticeably lower. Also, when the material was subjected to 200 W for 10 minutes, the Seebeck coefficient dropped considerably to ~8 μV K–1, but the electrical conductivity increased to ~58 S cm–1. Further analysis of these 200 W samples showed that the nanorods had coalesced to form a mixture of ZnO nanorods and nanosheets, where the formation of the sheets is believed to be the reason for the increased electrical conductivity. The experiments clearly highlight the importance of understanding and controlling the reaction parameters during the growth phase of thermoelectric materials.