Roughly, 50% of primary energy worldwide is rejected as waste heat over a wide range of temperatures. Waste heat above 573 K has the highest Carnot potential ( > 50 % ) to be converted to electricity due to higher Carnot efficiency. Thermoelectric (TE) materials have gained significant attention as potential candidates for efficient thermal energy conversion devices. Silicon nanowires (SiNWs) are promising materials for TE devices due to their unique electrical and thermal properties. In this study, we report the successful fabrication of high-quality double-sided SiNW arrays using advanced techniques. We engineered the double-sided structure to increase the surface area and the number of TE junctions, enhancing TE energy conversion efficiency. We also employed non-agglomeration wire tip engineering to ensure uniformity of the SiNWs and designed effective Ohmic contacts to improve overall TE efficiency. Additionally, we post-doped the double-sided SiNW arrays to achieve high electrical conductivity. Our results showed a significant improvement in the TE performance of the SiNW array devices, with a maximum figure-of-merit (ZT) value of 0.24 at 700 K, fabricated from the single SiNW with ZT of 0.71 at 700 K in our previous work [Yang et al., Nat. Commun. 12(1), 3926(2021)].

We report a scalably synthesized WO3/BiVO4 core/shell nanowire photoanode in which BiVO4 is the primary light-absorber and WO3 acts as an electron conductor. These core/shell nanowires achieve the highest product of light absorption and charge separation efficiencies among BiVO4-based photoanodes to date and, even without an added catalyst, produce a photocurrent of 3.1 mA/cm(2) under simulated sunlight and an incident photon-to-current conversion efficiency of ∼ 60% at 300-450 nm, both at a potential of 1.23 V versus RHE.

The basal plane of molybdenum disulfide (MoS2) was recently activated for hydrogen evolution reaction (HER) by creating sulfur (S) vacancies (MoS2-x). However, the HER activity of those S-vacancies depends on the concentration of S-vacancies, imposing a dilemma for either improving activity per site or increasing overall active site density. Herein, we use density functional theory (DFT) calculations and experiments to show that the HER activities of MoS2-x are greatly enhanced by adding cobalt (Co) clusters on the basal plane. Our DFT results show that the highest HER activity is achieved when the Co clusters are anchored on the S-vacancies with the interface of Co-Mo as the preferred active site. Our experiments confirm that the addition of Co enhances the activity per unit active site and increases the electrochemical active surface area. These results demonstrate the basal plane activity of MoS2-x can be enhanced by decorating S-vacancies with transition-metal clusters.

We demonstrate the growth, assembly, and characterization of ultrahigh quality polaritonic systems based on α-MoO3 microplates and nanoribbons. These micro- and nanostructures are bottom-up-synthesized single crystals with minimal impurities. By optimizing the growth conditions, we also realize morphology control of the α-MoO3 structures. We observe highly confined polariton modes in the individual structures by using scattering-type scanning near-field optical microscopy. These highly confined polariton modes are of fundamental and technological interest.

Copper ferrite (CuFe2O4) possesses an indirect bandgap in the range of 1.54-1.95 eV. It is used as an attractive p-type photocathode in photoelectrochemical (PEC) water splitting, and theoretically it can yield a maximum photocurrent density of ∼27 mA/cm2 and a maximum solar-to-hydrogen conversion efficiency of ∼33%. To date, only a few reports have been published on CuFe2O4 photocathodes with very low-photocurrent densities, with a maximum value of 0.4 mA/cm2 at 0.4 V vs RHE. Herein, we prepared a CuFe2O4 photocathode on FTO glass with the sol-gel method followed by either high-temperature flame annealing or furnace annealing. We found that the flame-annealed CuFe2O4 photocathode generated a photocurrent density of 1.82 mA/cm2 at 0.4 V vs RHE that is approximately 3.5 times higher than the furnace-annealed CuFe2O4 (0.52 mA/cm2). This photocurrent density is also higher than those of all the reported CuFe2O4 photocathodes, and any Cu containing ternary oxide (Cu-M-O, M: Fe, Bi, V, and Nb) photocathode (0.1-1.3 mA/cm2 at 0.4 V vs RHE). An improved PEC performance of the flame-annealed CuFe2O4 photocathode is elicited owing to the beneficial effects of flame annealing on the physical, optical, and electrical properties of CuFe2O4. Flame annealing enhances the light absorption property of the CuFe2O4 photocathode by slightly reducing the bandgap, and by forming a thicker film with increased porosity. Flame annealing also reduces the oxygen vacancy concentration in CuFe2O4, thus facilitating charge transport and interfacial charge transfer processes. Moreover, flame annealing requires only 16 min, which is much shorter than the time required for furnace annealing (∼9 h). These results demonstrate that flame annealing is a rapid and effective means for fabricating metal oxide photoelectrodes with an enhanced PEC water splitting performance.

van der Waals nanomaterials supporting phonon polariton quasiparticles possess extraordinary light confinement capabilities, making them ideal systems for molecular sensing, thermal emission, and subwavelength imaging applications, but they require defect-free crystallinity and nanostructured form factors to fully showcase these capabilities. We introduce bottom-up-synthesized α-MoO₃ structures as nanoscale phonon polaritonic systems that feature tailorable morphologies and crystal qualities consistent with bulk single crystals. α-MoO₃ nanoribbons serve as low-loss hyperbolic Fabry-Pérot nanoresonators, and we experimentally map hyperbolic resonances over four Reststrahlen bands spanning the far- and mid-infrared spectral range, including resonance modes beyond the 10th order. The measured quality factors are the highest from phonon polaritonic van der Waals structures to date. We anticipate that bottom-up-synthesized polaritonic van der Waals nanostructures will serve as an enabling high-performance and low-loss platform for infrared optical and optoelectronic applications.

Thermoelectrics operating at high temperature can cost-effectively convert waste heat and compete with other zero-carbon technologies. Among different high-temperature thermoelectrics materials, silicon nanowires possess the combined attributes of cost effectiveness and mature manufacturing infrastructures. Despite significant breakthroughs in silicon nanowires based thermoelectrics for waste heat conversion, the figure of merit (ZT) or operating temperature has remained low. Here, we report the synthesis of large-area, wafer-scale arrays of porous silicon nanowires with ultra-thin Si crystallite size of ~4 nm. Concurrent measurements of thermal conductivity (κ), electrical conductivity (σ), and Seebeck coefficient (S) on the same nanowire show a ZT of 0.71 at 700 K, which is more than ~18 times higher than bulk Si. This ZT value is more than two times higher than any nanostructured Si-based thermoelectrics reported in the literature at 700 K. Experimental data and theoretical modeling demonstrate that this work has the potential to achieve a ZT of ~1 at 1000 K.