Thermoelectric technology has potential for converting waste heat into electricity. Although traditional thermoelectric materials exhibit extremely high thermoelectric performances, their scarcity and toxicity limit their applications. Zinc oxide (ZnO) emerges as a promising alternative owing to its high thermal stability and relatively high Seebeck coefficient, while also being earth-abundant and nontoxic. However, its high thermal conductivity (>40 W m−1K−1) remains a challenge. In this study, we use a multi-step strategy to achieve a significantly high dimensionless figure-of-merit (zT) value of approximately 0.486 at 580 K (estimated value) by interfacing graphene quantum dots with 3D nanostructured ZnO. Here, we show the fabrication of graphene quantum dots interfaced 3D ZnO, yielding the highest zT value ever reported for ZnO counterparts; specifically, our experimental results indicate that the fabricated 3D GQD@ZnO exhibited a significantly low thermal conductivity of 0.785 W m−1K−1 (estimated value) and a remarkably high Seebeck coefficient of −556 μV K−1 at 580 K.

Introduction

With the ever-increasing energy demand and consumption of fossil fuels, the researchers are actively focusing on renewable energy technologies1. Among the various renewable energy technologies, the thermoelectric system is gaining increased attention because more than 65% of the globally produced energy from industrial machinery is lost as waste heat. Much of waste heat can be directly converted into valuable electrical power via the Seebeck effect2,3. The efficiency of thermoelectric systems is evaluated using the dimensionless figure-of-merit (zT = σS2T κ−1) depending on intrinsic material parameters—electrical conductivity (σ), Seebeck coefficient (S), thermal conductivity (κ), and absolute temperature (T)4. Over the past decades, various thermoelectric materials including metal tellurides (e.g., Bi2Te35, PbTe6, etc), metal silicides (e.g., Mg4Si77, Mg2Si-Mg2Sn8, etc), and Si-Ge alloys9 with high zT values (>1.5) have been actively reported. Although these materials bring thermoelectric technology closer to commercialization, their toxicity, scarcity, and chemical instability remain open challenges. In this regard, metal oxides are potential alternatives because of their nontoxicity, earth-abundance, and high thermal stability, thereby opening up more possibilities for the commercialization of thermoelectric technology10. Various metal oxides, such as SrTiO3, Ca3Co4O9, and NaxCo2O4, have been developed to achieve competitive zT values with conventional thermoelectric materials, as summarized in Supplementary Table 1. However, the synthesis techniques for metal oxides with high thermoelectric performances are quite challenging due to their complex chemical configurations and crystal structures.

Zinc oxide (ZnO), a well-known n-type thermoelectric metal oxide, has emerged as a promising candidate due to its relatively high S (−100 to −400 μV K−1), wide working temperature (570–1500 K), and facile processability compared to other metal oxides11,12. Despite the increase in the academic and industrial efforts to realize commercial-scale thermoelectric performances using ZnO, relatively high κ (>40 W m−1 K−1) of ZnO remains a critical obstacle and therefore hardly avoids the low zT value (~1.4 × 10−4 at 350 K and ~0.02 at 1000 K)13,14. To address this problem, nanostructuring—such as nano-arrays15, nano-porous materials16, nanocomposite structures17, and grain boundary engineering18,19,20—has been proposed to significantly reduce κ by scattering of phonons at countless interfaces of nanostructures. Notwithstanding advances in this direction, the resulting random-form factors cause inevitable σ degradation or uncontrollable electron scattering. Therefore, it is crucial to design ZnO with uniform nanoscale features to effectively reduce κ while maintaining electrical conduction pathways. In this regard, we previously demonstrated a rational design and fabrication of highly ordered three-dimensional (3D) thin-shell ZnO using an advanced lithographic technique, namely, proximity field nanopatterning (PnP)21,22,23,24,25,26. The design achieved a significant reduction in κ without degrading σ owing to the continuous network, by considering the difference in the phonon and electron scattering lengths27,28,29. Furthermore, the fabricated 3D ZnO has the potential for flexible thermoelectric applications, overcoming the intrinsic brittle nature of ZnO through structural engineering30.

In addition to structural engineering, interface engineering by incorporating nanosized materials into nanostructured supports is regarded as an efficient strategy, because interfacing not only mitigates phonon propagation but also selectively filters out low-energy electrons to enhance S based on the well-established energy filtering effect31. For example, inserting nanocarbon additives (e.g., graphene and carbon nanotube) into nanostructured thermoelectric materials effectively reduces their κ owing to nanoprecipitate phonon scattering and increases S through the filtering of low-energy electrons at additive/host interfaces32,33,34,35. Among nanocarbon materials, graphene quantum dots (GQDs) have an advantage in terms of effective interfacing over graphene or carbon nanotubes because of a sufficient number of active sites to uniformly cover the nanostructured supports. However, a well-matched band alignment with the host metal oxides is essential to form a proper energy barrier at the interface to maximize thermoelectric performance through the interfacing of GQDs. In this sense, lowly-oxidized GQDs fabricated from graphite intercalation compounds (GICs) can be a promising additive for host metal oxides owing to their well-controlled energy states from subdomains that only appear within GQDs under controlled oxidation, compared with defect-rich GQDs synthesized using Hummers’ method36,37,38. Previously, we demonstrated the effectiveness of lowly-oxidized GQDs as an interfacing additive to various metal oxides (e.g., TiO2 and SnO2) in photocatalyst and gas sensor applications39,40,41. The interface of GQD/metal oxides can enhance S by filtering out low-energy electrons while lowering κ by scattering of phonons at a well-controlled interfacial energy barrier. Therefore, a carefully engineered GQD/metal oxide interface and a uniform 3D nanostructure improve its thermoelectric performance.

In this work, we present a multi-step rational strategy for achieving high thermoelectric performance by interfacing lowly-oxidized GQD with 3D nanostructured ZnO (3D GQD@ZnO). The highly periodic 3D thin-shell ZnO considerably reduces κ by effective scattering of long mean free path (MFP) phonons at nanostructured surfaces, which contribute most to the thermal conduction of ZnO at the optimized thickness of 70 nm of the 3D ZnO shell. In addition to the structural factor, grain boundary engineering in the thin-shell ZnO through careful thermal treatment at 250 °C for 2 h induces further phonon scattering and low-energy electrons filtering at grain boundaries, resulting in a slightly improved zT value at an optimum grain size (~30 nm) of thin-shell ZnO. Furthermore, interfacing lowly-oxidized GQDs with the 3D ZnO provides numerous heterogeneous interfaces of GQD/ZnO, forming an interfacial energy barrier of 0.63 eV owing to the energy offset between the lowest unoccupied molecular orbital (LUMO) of GQD and conduction band minima (CBM) of ZnO. This study indicates that the interfacial energy barrier is the key factor for maximizing the zT value by altering electron and phonon transport. Specifically, the low-energy electrons of ZnO, corresponding to energies below the LUMO level of GQD, become trapped at the GQD/ZnO interfacial energy barrier. This electron filtering—allowing only the transport of high-energy electrons above the LUMO level of GQDs—enhances S, resulting in a 74% enhancement of power factor (σS2) when compared with that of the bare 3D ZnO despite a slight decrease in σ owing to a reduced charge carrier concentration. This interfacial energy barrier also leads to the significant reduction in κ from 1.49 to 0.785 W m−1 K−1 through the nanoprecipitate scattering of phonons with relatively short MFP. Consequently, the 3D GQD@ZnO heterostructure exhibits an exceptionally high zT value of 0.486 at 580 K, yielding a record high value among ZnO-based materials.