Low‐Temperature‐Processed Colloidal Quantum Dots as Building Blocks for Thermoelectrics

Colloidal quantum dots (CQDs) are demonstrated to be promising materials to realize high‐performance thermoelectrics owing to their low thermal conductivity. The most studied CQD films, however, are using long ligands that require high processing and operation temperature (>400 °C) to achieve optimum thermoelectric performance. Here the thermoelectric properties of CQD films cross‐linked using short ligands that allow strong inter‐QD coupling are reported. Using the ligands, p‐type thermoelectric solids are demonstrated with a high Seebeck coefficient and power factor of 400 μV K^?1 and 30 µW m^?1 K^?2, respectively, leading to maximum ZT of 0.02 at a lower measurement temperature (<400 K) and lower processing temperature (<300 °C). These ligands further reduce the annealing temperature to 175 °C, significantly increasing the Seebeck coefficient of the CQD films to 580 μV K^?1. This high Seebeck coefficient with a superior ZT near room temperature compared to previously reported high temperature‐annealed CQD films is ascribed to the smaller grain size, which enables the retainment of quantum confinement and significantly increases the hole effective mass in the films. This study provides a pathway to approach quantum confinement for achieving a high Seebeck coefficient yet strong inter‐QD coupling, which offers a step toward low‐temperature‐processed high‐performance thermoelectric generators.     

High Thermoelectric zT in n‐Type Silver Selenide films at Room Temperature

In this work, a zT value as high as 1.2 at room temperature for n‐type Ag₂Se films is reported grown by pulsed hybrid reactive magnetron sputtering (PHRMS). PHRMS is a novel technique developed in the lab that allows to grow film of selenides with different compositions in a few minutes with great quality. The improved zT value reported for room temperature results from the combination of the high power factors, similar to the best values reported for bulk Ag₂Se (2440 ± 192 µW m^?1 K^?2), along with a reduced thermoelectric conductivity as low as 0.64 ± 0.1 W m^?1 K^?1. The maximum power factor for these films is of 4655 ± 407 µW m^?1 K^?2 at 103 °C. This material shows promise to work for room temperature applications. Obtaining high zT or, in other words, high power factor and low thermal conductivity values close to room temperature for thin films is of high importance to develop a new generation of wearable devices based on thermoelectric heat recovery.     

Optimizations of Pulsed Plated p and n‐type Bi2Te3‐Based Ternary Compounds by Annealing in Different Ambient Atmospheres

This work presents a comprehensive study of the fabrication and optimization of electrodeposited p‐ and n‐type thermoelectric films. The films are deposited on Au and stainless steel substrates over a wide range of deposition potentials. The influence of the preparative parameters such as the composition of the electrolyte bath and the deposition potential are investigated. Furthermore, the p‐doped (Bi_xSb_1‐x)₂Te₃ and the n‐doped Bi₂(Te_xSe_1‐x)₃ films are annealed for a period of about 1 h under helium and under tellurium atmosphere at 250 °C for 60h. Annealing in He already leads to significant improvements in the thermoelectric performance. Furthermore, due to the equilibrium conditions during the process, annealing in Te atmosphere leads to a strongly improved film composition, charge carrier density and mobility. The Seebeck coefficients increase to values up to +182 μV K^?1 for p‐doped and–130 μV K^?1 for n‐doped materials at room temperature. The power factors also exhibit improvements with 1320 μW m^?1 K^?2 and 820 μW m^?1 K^?2 for p‐doped and n‐doped films, respectively. Additionally, in‐situ XRD measurements performed during annealing of the films up to 600K under He atmosphere show stepwise improvements of the crystal structure leading to the improvements in thermoelectric parameters. The thermal conductivity is between 1.2 W m^?1 K^?1 and 1.0 W m^?1 K^?1.     

Realizing High Thermoelectric Performance in n‐Type Highly Distorted Sb‐Doped SnSe Microplates via Tuning High Electron Concentration and Inducing Intensive Crystal Defects

In this study, a record high figure of merit (ZT) of ≈1.1 at 773 K is reported in n‐type highly distorted Sb‐doped SnSe microplates via a facile solvothermal method. The pellets sintered from the Sb‐doped SnSe microplates show a high power factor of ≈2.4 µW cm^?1 K^?2 and an ultralow thermal conductivity of ≈0.17 W m^?1 K^?1 at 773 K, leading a record high ZT. Such a high power factor is attributed to a high electron concentration of 3.94 × 10¹⁹ cm^?3 via Sb‐enabled electron doping, and the ultralow thermal conductivity derives from the enhanced phonon scattering at intensive crystal defects, including severe lattice distortions, dislocations, and lattice bent, observed by detailed structural characterizations. This study fills in the gaps of fundamental doping mechanisms of Sb in SnSe system, and provides a new perspective to achieve high thermoelectric performance in n‐type polycrystalline SnSe.     

High Thermoelectric Performance in p‐type Polycrystalline Cd‐doped SnSe Achieved by a Combination of Cation Vacancies and Localized Lattice Engineering

Herein, a high figure of merit (ZT) of ≈1.7 at 823 K is reported in p‐type polycrystalline Cd‐doped SnSe by combining cation vacancies and localized‐lattice engineering. It is observed that the introduction of Cd atoms in SnSe lattice induce Sn vacancies, which act as p‐type dopants. A combination of facile solvothermal synthesis and fast spark plasma sintering technique boosts the Sn vacancy to a high level of ≈2.9%, which results in an optimum hole concentration of ≈2.6 × 10¹⁹ cm^?3 and an improved power factor of ≈6.9 µW cm^?1 K^?2. Simultaneously, a low thermal conductivity of ≈0.33 W m^?1 K^?1 is achieved by effective phonon scattering at localized crystal imperfections, as observed by detailed structural characterizations. Density functional theory calculations reveal that the role of Cd atoms in the SnSe lattice is to reduce the formation energy of Sn vacancies, which in turn lower the Fermi level down into the valence bands, generating holes. This work explores the fundamental Cd‐doping mechanisms at the nanoscale in a SnSe matrix and demonstrates vacancy and localized‐lattice engineering as an effective approach to boosting thermoelectric performance. The work provides an avenue in achieving high‐performance thermoelectric properties of materials.     

Flexible Thermoelectric Generators Composed of n‐and p‐Type Silicon Nanowires Fabricated by Top‐Down Method

This study demonstrates the fabrication and characterization of a flexible thermoelectric (TE) power generator composed of silicon nanowires (SiNWs) fabricated by top‐down method and discusses its strain‐dependence analysis. The Seebeck coefficients of the p‐ and n‐type SiNWs used to form a pn‐module are 156.4 and ?146.1 µV K^?1, respectively. The maximum power factors of the p‐ and n‐type SiNWs are obtained as 8.79 and 8.87 mW (m K²)^?1, respectively, under a convex bending of 1.11%, respectively; these are the largest values among the power factors hitherto reported for SiNWs. The dimensionless figure of merit (ZT ) values of the SiNWs at room temperature are 6.8 × 10^?2 and 6.7 × 10^?2 for the convex bent p‐ and n‐type SiNWs, respectively, with a strain of 1.11%. The thermoelectric properties of the pn‐module and its component SiNWs are characterized under strain conditions ranging from ?1.11% to 1.11%. The maximum Seebeck coefficient and power factor of the pn‐module are obtained as 448 µV K^?1 and 14.2 mW (m K²)^?1, respectively, under convex bending of 1.11%. Moreover, the mechanical stability of the TE characteristics of the pn‐module is demonstrated through a continuous bending test of 3000 cycles under convex bending of 0.66%.     

Optimization of Electrodeposited p‐Doped Sb2Te3 Thermoelectric Films by Millisecond Potentiostatic Pulses

A systematic optimization of p‐type Sb₂Te₃ thermoelectric films made by potentiostatic electrodeposition on Au and stainless steel substrates is presented. The influence of the preparative parameters of deposition voltage, concentration, and the deposition method are investigated in a nitric acid solution. As a postdeposition step, the influence of annealing the films is investigated. The use of a potential‐controlled millisecond‐pulsed deposition method could improve both the morphology and the composition of the films. The samples are characterized in terms of composition, crystallinity, Seebeck coefficient, and electrical resistivity. Pulsed‐deposited films exhibit Seebeck coefficients of up to 160 μV K^?1 and an electrical conductivity of 280 S cm^?1 at room temperature, resulting in power factors of about 700 μW m^?1 K^?2. After annealing, power factors of maximum 852 μW m^?1 K^?2 are achieved. Although the annealing of DC‐deposited films significantly increased the power factor, they do not reach the values of the pulsed‐deposited films in the preannealing state. Structural analysis is performed with X‐ray diffraction and shows the crystalline structure of Sb₂Te₃ films. The performance is tuned by annealing of deposited films up to 300 °C under He atmosphere while performing in‐situ X‐ray diffraction and resistivity measurements. The chemical analysis of the films is performed by inductively coupled plasma optical emission spectroscopy (ICP‐OES) as well as scanning electron microscope energy dispersive X‐ray analysis (SEM‐EDX).     

Engineering the Thermoelectric Transport in Half‐Heusler Materials through a Bottom‐Up Nanostructure Synthesis

Half‐Heusler (HH) alloys are among the best promising thermoelectric (TE) materials applicable for the middle‐to‐high temperature power generation. Despite of the large thermoelectric power factor and decent figure‐of‐merit ZT (≈1), their broad applications and enhancement on TE performance are limited by the high intrinsic lattice thermal conductivity (κ_L) due to insufficiencies of phonon scattering mechanisms, and the fewer powerful strategies associated with the microstructural engineering for HH materials. This study reports a bottom‐up nanostructure synthesis approach for these HH materials based on the displacement reaction between metal chlorides/bromides and magnesium (or lithium), followed by vacuum‐assisted spark plasma sintering process. The samples are featured with dense dislocation arrays at the grain boundaries, leading to a minimum κ_L of ≈1 W m^?1 K^?1 at 900 K and one of the highest ZT (≈1) and predicted η (≈11%) for n‐type Hf_0.25Zr_0.75NiSn_0.97Sb_0.03. Further manipulation on the dislocation defects at the grain boundaries of p‐type Nb_0.8Ti_0.2FeSb leads to enhanced maximum power factor of 47 × 10^?4 W m^?1 K^?2 and the predicted η of ≈7.5%. Moreover, vanadium substitution in FeNb_0.56V_0.24Ti_0.2Sb significantly promotes the η to ≈11%. This strategy can be extended to a broad range of advanced alloys and compounds for improved properties.     

Large Voltage Generation of Flexible Thermoelectric Nanocrystal Thin Films by Finger Contact

This paper demonstrates that thermal energy radiated from a human finger can be converted efficiently into electricity by a nanocrystal (NC) thin film that substantially suppresses thermal conduction, but still allows electric conduction. The converting efficiencies of the chalcogenide NC thin films with dimensions 40 µm × 20 µm × 20 nm, prepared on flexible substrates by a solution process, are maximized by adjusting the NC size. A Seebeck coefficient of S = 1829 µV K^?1, and a dimensionless thermoelectric figure‐of‐merit, ZT = 0.68 are achieved at ambient temperature for p‐ and n‐type NC thin films, respectively. A thermoelectric array consisting of p‐ and n‐type NC thin films generates a voltage of 645 mV for a temperature gradient of 10 K. Furthermore, the donut‐shaped pn array can generate a voltage of 170 mV from the heat supplied by an individual's finger.     

Wearable All‐Fabric‐Based Triboelectric Generator for Water Energy Harvesting

Realizing energy harvesting from water flow using triboelectric generators (TEGs) based on our daily wearable fabric or textile has practical significance. Challenges remain on methods to fabricate conformable TEGs that can be easily incorporated into waterproof textile, or directly harvest energy from water using hydrophobic textile. Herein, a wearable all‐fabric‐based TEG for water energy harvesting, with additional self‐cleaning and antifouling properties is reported for the first time. Hydrophobic cellulose oleoyl ester nanoparticles (HCOENPs) are prepared from microcrystalline cellulose, as a low‐cost and nontoxic coating material to achieve superhydrophobic coating on fabrics, including cotton, silk, flax, polyethylene terephthalate (PET), polyamide (nylon), and polyurethane. The resultant PET fabric‐based water‐TEG can generate an instantaneous output power density of 0.14 W m^?2 at a load resistance of 100 MΩ. An all‐fabric‐based dual‐mode TEG is further realized to harvest both the electrostatic energy and mechanical energy of water, achieving the maximum instantaneous output power density of 0.30 W m^?2. The HCOENPs‐coated fabric provides excellent breathability, washability, and environmentally friendly fabric‐based TEGs, making it a promising wearable self‐powered system.     

Freely Shapable and 3D Porous Carbon Nanotube Foam Using Rapid Solvent Evaporation Method for Flexible Thermoelectric Power Generators

A rapid solvent evaporation method based on the triple point of a processing solvent is presented to prepare carbon nanotube (CNT) foam with a porous structure for thermoelectric (TE) power generators. The rapid solvent evaporation process allows the preparation of CNT foam with various sizes and shapes. The obtained highly porous CNT foam with porosity exceeding 90% exhibits a low thermal conductivity of 0.17 W m^?1 K^?1 with increased phonon scattering, which is 100 times lower than that of a CNT film with a densely packed network. The aforementioned structural and thermal properties of the CNT foam are advantageous to develop a sufficient temperature gradient between the hot and cold parts to enhance TE output characteristics. To improve the electrical conductivity and Seebeck coefficient further, p‐ and n‐molecular dopants are easily introduced into the CNT foam, and the optimized condition is investigated based on the TE properties. Finally, optimized p‐ and n‐doped CNT foams are used to fabricate a vertical and flexible TE power generator with a combination of series and parallel mixed circuits. The maximum output power and output power per weight of the TE generator reach 1.5 µW and 82 µW g^?1, respectively, at a temperature difference of 13.9 K.     

Conductive,Solution‐Processed Dioxythiophene Copolymers for Thermoelectric and Transparent Electrode Applications

Conjugated polymers with high electrical conductivities are attractive for applications in capacitors, biosensors, organic thermoelectrics, and transparent electrodes. Here, a series of solution processable dioxythiophene copolymers based on 3,4‐propylenedioxythiophene (ProDOT) and 3,4‐ethylenedioxythiophene (EDOT) is investigated as thermoelectric and transparent electrode materials. Through structural manipulation of the polymer repeat unit, the conductivity of the polymers upon oxidative solution doping is tuned from 1 × 10^?3 to 3 S cm^?1, with a polymer consisting of a solubilizing alkylated ProDOT unit and an electron‐rich biEDOT unit (referred to as PE₂) showing the highest electrical conductivity. Optimization of the film casting method and screening of dopants result in AgPF₆‐doped PE₂ achieving a high electrical conductivity of over 250 S cm^?1 and a thermoelectric power factor of 7 μW m^?1 K^?2. Oxidized spray cast films of PE₂ are also assessed as a transparent electrode material for use with another electrochromic polymer. This bilayer shows reversible electrochemical switching from a colored charge‐neutral state to a highly transmissive color‐neutral, oxidized state. These results demonstrate that dioxythiophene‐based copolymers are a promising class of materials, with ProDOT–biEDOT serving as a soluble analog to the well‐studied PEDOT as a p‐type thermoelectric and electrode material.     

High‐Performance Thermoelectric Paper Based on Double Carrier‐Filtering Processes at Nanowire Heterojunctions

As commercial interest in flexible power‐conversion devices increases, the demand for high‐performance alternatives to brittle inorganic thermoelectric (TE) materials is growing. As an alternative, we propose a rationally designed graphene/polymer/inorganic nanocrystal free‐standing paper with high TE performance, high flexibility, and mechanical/chemical durability. The ternary hybrid system of the graphene/polymer/inorganic nanocrystal includes two hetero­junctions that induce double‐carrier filtering, which significantly increases the electrical conductivity without a major decrease in the thermopower. The ternary hybrid shows a power factor of 143 μW m^?1 K^?1 at 300 K, which is one to two orders of magnitude higher than those of single‐ or binary‐component materials. In addition, with five hybrid papers and polyethyleneimine (PEI)‐doped single‐walled carbon nanotubes (SWCNTs) as the p‐type and n‐type TE units, respectively, a maximum power density of 650 nW cm^?2 at a temperature difference of 50 K can be obtained. The strategy proposed here can improve the performance of flexible TE materials by introducing more heterojunctions and optimizing carrier transfer at those junctions, and shows great potential for the preparation of flexible or wearable power‐conversion devices.     

Subtle Roles of Sb and S in Regulating the Thermoelectric Properties of N‐Type PbTe to High Performance

A high ZT (thermoelectric figure of merit) of ≈1.4 at 900 K for n‐type PbTe is reported, through modifying its electrical and thermal properties by incorporating Sb and S, respectively. Sb is confirmed to be an amphoteric dopant in PbTe, filling Te vacancies at low doping levels (<1%), exceeding which it enters into Pb sites. It is found that Sb‐doped PbTe exhibits much higher carrier mobility than similar Bi‐doped materials, and accordingly, delivers higher power factors and superior ZT . The enhanced electronic transport is attributed to the elimination of Te vacancies, which appear to strongly scatter n‐type charge carriers. Building on this result, the ZT of Pb_0.9875Sb_0.0125Te is further enhanced by alloying S into the Te sublattice. The introduction of S opens the bandgap of PbTe, which suppresses bipolar conduction while simultaneously increasing the electron concentration and electrical conductivity. Furthermore, it introduces point defects and induces second phase nanostructuring, which lowers the lattice thermal conductivity to ≈0.5 W m^?1 K^?1 at 900 K, making this material a robust candidate for high‐temperature (500–900 K) thermoelectric applications. It is anticipated that the insights provided here will be an important addition to the growing arsenal of strategies for optimizing the performance of thermoelectric materials.     

Synergistically Optimizing Electrical and Thermal Transport Properties of BiCuSeO via a Dual‐Doping Approach

The layered oxyselenide BiCuSeO system is known as one of the high‐performance thermoelectric materials with intrinsically low thermal conductivity. By employing atomic, nano‐ to mesoscale structural optimizations, low thermal conductivity coupled with enhanced electrical transport properties can be readily achieved. Upon partial substitution of Bi³⁺ by Ca²⁺ and Pb²⁺, the thermal conductivity can be reduced to as low as 0.5 W m^?1 K^?1 at 873 K through dual‐atomic point‐defect scattering, while a high power factor of ≈1 × 10^?3 W cm^?1 K^?2 is realized over a broad temperature range from 300 to 873 K. The synergistically optimized power factor and intrinsically low thermal conductivity result in a high ZT value of ≈1.5 at 873 K for Bi_0.88Ca_0.06Pb_0.06CuSeO, a promising candidate for high‐temperature thermoelectric applications. It is envisioned that the all‐scale structural optimization is critical for optimizing the thermoelectricity of quaternary compounds.     

Thermally Chargeable Solid‐State Supercapacitor

Ubiquitous low‐grade thermal energy, which is typically wasted without use, can be extremely valuable for continuously powering electronic devices such as sensors and wearable electronics. A popular choice for waste heat recovery has been thermoelectric energy conversion, but small output voltage without energy‐storing capability necessitates additional components such as a voltage booster and a capacitor. Here, a novel method of simultaneously generating a large voltage from a temperature gradient and storing electrical energy without losing the benefit of solid‐state no‐moving part devices like conventional thermoelectrics is reported. Thermally driven ion diffusion is used to greatly increase the output voltage (8 mV K^?1) with polystyrene sulfonic acid (PSSH) film. Polyaniline‐coated electrodes containing graphene and carbon nanotube sandwich the PSSH film where thermally induced voltage‐enabled electrochemical reactions, resulting in a charging behavior without an external power supply. With a small temperature difference (5 K) possibly created over wearable energy harvesting devices, the thermally chargeable supercapacitor produce 38 mV with a large areal capacitance (1200 F m^?2). It is anticipated that the attempt with thermally driven ion diffusion behaviors initiates a new research direction in thermal energy harvesting.     

Significantly Enhanced Thermoelectric Properties of PEDOT:PSS Films through Sequential Post‐Treatments with Common Acids and Bases

Thermoelectric (TE) materials are important for the sustainable development because they enable the direct harvesting of low‐quality heat into electricity. Among them, conducting polymers have attracted great attention arising from their advantages, such as flexibility, nontoxicity, easy availability, and intrinsically low thermal conductivity. In this work, a novel and facile method is reported to significantly enhance the TE property of poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) films through sequential post‐treatments with common acids and bases. Compared with the as‐prepared PEDOT:PSS, both the Seebeck coefficients and electrical conductivities can be remarkably enhanced after the treatments. The oxidation level, which significantly impacts the TE property of the PEDOT:PSS films, can also be well tuned by controlling the experimental conditions during the base treatment. The optimal PEDOT:PSS films can have a Seebeck coefficient of 39.2 µV K^?1 and a conductivity of 2170 S cm^?1 at room temperature, and the corresponding power factor is 334 µW (m^?1 K^?2). The enhancement in the TE properties is attributed to the synergetic effect of high charge mobility by the acid treatment and the optimal oxidation level tuned by the base treatment.     

Molecular Level Insight into Enhanced n‐Type Transport in Solution‐Printed Hybrid Thermoelectrics

Perylene diimide (PDI) derivatives hold great promise as stable, solution‐printable n‐type organic thermoelectric materials, but as of yet lack sufficient electrical conductivity to warrant further development. Hybrid PDI‐inorganic nanomaterials have the potential to leverage these physical advantages while simultaneously achieving higher thermoelectric performance. However, lack of molecular level insight precludes design of high performing PDI‐based hybrid thermoelectrics. Herein, the first explicit crystal structure of these materials is reported, providing previously inaccessible insight into the relationship between their structure and thermoelectric properties. Allowing this molecular level insight to drive novel methodologies, simple solution‐based techniques to prepare PDI hybrid thermoelectric inks with up to 20‐fold enhancement in thermoelectric power factor over the pristine molecule (up to 17.5 µW mK^?2) is presented. This improved transport is associated with reorganization of organic molecules on the surface of inorganic nanostructures. Additionally, outstanding mechanical flexibility is demonstrated by fabricating solution‐printed thermoelectric modules with innovative folded geometries. This work provides the first direct evidence that packing/organization of organic molecules on inorganic nanosurfaces is the key to effective thermoelectric transport in nanohybrid systems.     

Nanostructured p‐n Junctions for Kinetic‐to‐Electrical Energy Conversion

Piezoelectric ZnO nanorods grown on a flexible substrate are combined with the p‐type semiconducting polymer PEDOT:PSS to produce a p‐n junction device that successfully demonstrates kinetic‐to‐electrical energy conversion. Both the voltage and current output of the devices are measured to be in the range of 10 mV and 10 μA cm^?2. Combining these figures for the best device gives a maximum possible power density of 0.4 mW cm^?3. Systematic testing of the devices is performed showing that the voltage output increases linearly with applied stress, and is reduced significantly by illumination with super‐band gap light. This provides strong evidence that the voltage output results from piezoelectric effects in the ZnO. The behavior of the devices is explained by considering the time‐dependent changes in band structure resulting from the straining of a piezoelectric material within a p‐n junction. It is shown that the rate of screening of the depolarisation field determines the power output of a piezoelectric energy harvesting device. This model is consistent with the behavior of a number of previous devices utilising the piezoelectric effect in ZnO.     

Synergistical Enhancement of Thermoelectric Properties in n‐Type Bi2O2Se by Carrier Engineering and Hierarchical Microstructure

Oxygen‐containing compounds are promising thermoelectric (TE) materials for their chemical and thermal stability. As compared with the high‐performance p‐type counterparts (e.g., ZT ≈1.5 for BiCuSeO), the enhancement of the TE performance of n‐type oxygen‐containing materials remains challenging due to their mediocre electrical conductivity and high thermal conductivity. Here, n‐type layered Bi₂O₂Se is reported as a potential TE material, of which the thermal conductivity and electrical transport properties can be effectively tuned via carrier engineering and hierarchical microstructure. By selective modification of insulating [Bi₂O₂]²⁺ layers with Ta dopant, carrier concentration can be increased by four orders of magnitude (from 10¹⁵ to 10¹⁹ cm^?3) while relatively high carrier mobility can be maintained, thus greatly enhancing the power factors (≈451.5 µW K^?2 m^?1). Meanwhile, the hierarchical microstructure can be induced by Ta doping, and the phonon scattering can be strengthened by atomic point defects, nanodots of 5–10 nm and grains of sub‐micrometer level, which progressively suppresses the lattice thermal conductivity. Accordingly, the ZT value of Bi_1.90Ta_0.10O₂Se reaches 0.36 at 773 K, a ≈350% improvement in comparison with that of the pristine Bi₂O₂Se. The average ZT value of 0.30 from 500 to 823 K is outstanding among n‐type oxygen‐containing TE materials. This work provides a desirable way for enhancing the ZT values in oxygen‐containing compounds.