Abstract

This review is concerned with the leading methods of bottom-up material preparation for thermal-to-electrical energy interconversion. The advantages, capabilities, and challenges from a material synthesis perspective are surveyed and the methods are discussed with respect to their potential for improvement (or possibly deterioration) of application-relevant transport properties. Solution chemistry-based synthesis approaches are re-assessed from the perspective of thermoelectric applications based on reported procedures for nanowire, quantum dot, mesoporous, hydro/solvothermal, and microwave-assisted syntheses as these techniques can effectively be exploited for industrial mass production. In terms of energy conversion efficiency, the benefit of self-assembly can occur from three paths: suppressing thermal conductivity, increasing thermopower, and boosting electrical conductivity. An ideal thermoelectric material gains from all three improvements simultaneously. Most bottom-up materials have been shown to exhibit very low values of thermal conductivity compared to their top-down (solid-state) counterparts, although the main challenge lies in improving their poor electrical properties. Recent developments in the field discussed in this review reveal that the traditional view of bottom-up thermoelectrics as inferior materials suffering from poor performance is not appropriate. Thermopower enhancement due to size and energy filtering effects, electrical conductivity enhancement, and thermal conductivity reduction mechanisms inherent in bottom-up nanoscale self-assembly syntheses are indicative of the impact that these techniques will play in future thermoelectric applications.

The intercalation-induced phase transition of MoS2 from the semiconducting 2H to the semimetallic 1T' phase has been studied in detail for nearly a decade; however, the effects of a heterointerface between MoS2 and other two-dimensional (2D) crystals on the phase transition have largely been overlooked. Here, ab initio calculations show that intercalating Li at a MoS2hexagonal boron nitride (hBN) interface stabilizes the 1T phase over the 2H phase of MoS2 by ~ 100 mJ m -2 , suggesting that encapsulating MoS2 with hBN may lower the electrochemical energy needed for the intercalation-induced phase transition. However, in situ Raman spectroscopy of hBN-MoS2-hBN heterostructures during electrochemical intercalation of Li + shows that the phase transition occurs at the same applied voltage for the heterostructure as for bare MoS2. We hypothesize that the predicted thermodynamic stabilization of the 1T'-MoS2-hBN interface is counteracted by an energy barrier to the phase transition imposed by the steric hindrance of the heterointerface. The phase transition occurs at lower applied voltages upon heating the heterostructure, which supports our hypothesis. Our study highlights that interfacial effects of 2D heterostructures can go beyond modulating electrical properties and can modify electrochemical and phase transition behaviors. Main TextVan der Waals heterostructures comprised of different two-dimensional (2D) materials 1,2 can exhibit novel electronic and optical properties [3][4][5] , in which the effect of heterointerfaces is important. Heterointerfaces may also play a central role in determining electrochemical properties and phase transitions of 2D materials. For MoS2, intercalation of alkali metal ions such as Li + into

Mesoporous α-MoO 3−x combined with poly-(diallyldimethylammonium chloride)−functionalized reduced graphene oxide (PDDA−rGO) is introduced as an inexpensive and efficient oxygen reduction reaction (ORR) catalyst. The mesoporous catalysts are wrapped by conductive rGO sheets via an electrostatic interaction induced by a PDDA polyelectrolyte. The thermal interaction of PDDA with MoO 3 efficiently reduces the metal oxide to MoO 3−x at 400− 600 °C, creating a surface oxygen vacancy. Through a combination of density functional theory and experiments, the role of the surface oxygen vacancy sites in the ORR activity of MoO 3−x is identified. For the first time, all the energy barriers against ORR are calculated at each step for MoO 3 with no oxygen vacancies and MoO 3−x with surface oxygen vacancies. It is shown that the presence of an Mo v 4 O-+ •• oxygen vacancy site on the surface significantly reduces the energy barriers against ORR in the reaction pathways. An overpotential of 0.86 V (vs a reversible hydrogen electrode) with excellent electrochemical stability was obtained with the newly designed catalyst, with only a 9% decrease in the activity after ∼17 h. These results offer a new paradigm in the defect engineering of metal oxides with a potential for the synthesis of stable and active noble metal-free ORR electrocatalysts.

Surface functionalization with organic electron donors (OEDs) is an effective doping strategy for 2D materials, which can achieve doping levels beyond those possible with conventional electric field gating. While the effectiveness of surface functionalization has been demonstrated in many 2D systems, the doping efficiencies of OEDs have largely been unmeasured, which is in stark contrast to their precision syntheses and tailored redox potentials. Here, using monolayer MoS2 as a model system and an organic reductant based on 4,4′‐bipyridine (DMAP‐OED) as a strong organic dopant, it is established that the doping efficiency of DMAP‐OED to MoS2 is in the range of 0.63 to 1.26 electrons per molecule. The highest doping levels to date are also achieved in monolayer MoS2 by surface functionalization and demonstrate that DMAP‐OED is a stronger dopant than benzyl viologen, which is the previous best OED dopant. The measured range of the doping efficiency is in good agreement with the values predicted from first‐principles calculations. This work provides a basis for the rational design of OEDs for high‐level doping of 2D materials.