3. Property Control
Te is well known as a p-type narrow bandgap (0.35 eV, direct) semiconductor that lacks centrosymmetry; thus, the electrical
[38], optical
[13][38][39], magnetic
[23][32], and other properties
[1][40] of its NTs are greatly controlled by their geometric, structural, physical, and chemical features. Rational optimization of these factors is important to ensure the effective uses of Te NTs in electronic and optoelectronic applications.
3.1. Electrical Properties
Given that Te is a metalloid with relatively large spin–orbit coupling
[41], Te NTs are endowed with the highest electrical conductivity among inorganic elements (2 × 10
2 S·m
−1), p-type narrow bandgap, and high structural rigidity over flexible NBs and NWs, and they generally have highly stable electrical properties depending on their size and heteroatom doping. For another ultralong submicron Te NT, it was found that the trace level doping of Na could thermally scatter their weakly-bonded lattice with ease, thus enabling the resistivity of these Te NTs to decrease upon cooling (5–300 K). By employing an individual of them as the building block, a nanodevice was built through focused-ion-beam deposition to exhibit a quadratic temperature-dependent resistivity, of which the room-temperature resistivity and the ratio of 5 K resistivity/room-temperature resistivity could reach 9.854 μΩ and 0.47, respectively
[38]. Te NTs with an average grain size <10 nm and wall thickness range of 15–30 nm were embedded into a field effect transistor, whose mobility was decreased to ~0.01 cm
2/V·s, as its phonon scattering was dominated by the Te NT lattice that could decrease the thermal conductivity for the increased thermoelectric figure of merit
[23]. Additionally, its field effect mobility was temperature-dependent and obeyed the Conwell–Weisskopf relationship within the temperature of <250 K. For another Te NTs with an easily tunable diameter (40–100 nm), by using the solvothermal method, their surface-to-volume ratio and crystallinity were optimized to fill their surface trap states and crystalline defects with more photo-generated holes. As a consequence, a high photoresponsivity of 1.65 × 10
4 A·W
−1 and photoconductivity gain of 5.0 × 10
6% were observed on the optoelectronic nanodevice based on these Te NTs
[40].
3.2. Optical Properties
As most Te NTs have a single-crystalline structure, their optical properties can be modified through geometric control and chemical treatments. For example, Yu’s research group
[42] reported the single-crystalline trigonal Ne NTs with sloping and hexagonal cross-sections grew along the (001) direction and had the outer diameters/wall thicknesses/lengths within 100–500 nm/50–100 nm/150–200 μm. Further, the 365 nm photoluminescence excited these Ne NTs to present blue–violet emissions (390–550 nm) for the first time, which was highly related to the thickness of the nanostructures and crystallization behavior of the solvothermally reduced Te NTs
[39]. Replacing the distilled water with absolute ethanol, hydrothermally prepared Te NTs which had sloping cross-sections, open ends, and relatively short lengths of 30–50 μm were also well formed. These Te NTs were found to present a concentration-dependent excitation/emission, attributed to their thickness and highly anisotropic crystallization. It was also reported that the further decoration of spherical Te NPs on the shuttle-shaped Te NTs could increase their chirality, considering the inherently helical chain structure with two ends, to induce a brand new strong red emission, beneficial for nano-optical applications
[13]. Except for geometric modifications, the oxygen-related defects formed on the hexagonal column-shaped Te NTs favored the electron radiation transition from the p-antibonding triple of conduction band to the p-bonding triple of valence band in the latter, helping them to obtain a broad photoluminescence peak at ~532 nm
[43].
3.3. Magnetoresistance Properties
Ever since the positive magnetoresistance effect at low temperature was recorded on the Te microtubes by Li et al. in 2003
[32], only a few attempts have been made to explore the magnetoresistance properties of Te NTs. Later, Rheem et al.
[23] demonstrated that the unique magnetoresistance properties behavior could be observed on the Te NTs fabricated by galvanic displacement, which presented a magnetoresistance ratio up to 37% (260 K); however, the related mechanism was unclear. The latest analogues of Te NTs including the layered transition metal dichalcogenides NiTe
2, PdTe
2 and PtTe
2, irrespective of which did or did not host Weyl or Type-II Dirac fermions, had high intrinsic carrier mobility, but its high purity was the prerequisite to observe maximal magnetoresistance effects
[44]. In this case, the strong interconnection between carrier mobility and magnetoresistance contributed to the temperature dependence for an individual sample or the difference between the samples via forcing carriers on Landau orbits by the applied transverse field.
In addition to the above, Te NTs also have many characteristics, such as sensitive gas sensing
[1][45], outstanding mechanical properties
[6], and stability
[7][42]. Note, doped heteroatoms, especially metals, could endow Te NTs with additional excellent properties, such as high thermopower, small thermal conductivity
[16][46][47], antibacterial ability
[48], roll-to-roll processability
[49], and catalytic ability
[46][50], etc.
4. Applications
Benefiting from the above characteristics, Te NTs are proven to be versatile and applicable in sensing and decontamination [1][2][45][51], energy storage [52][53], thermoelectrics [16][47][54][55][56], and templating for catalysts [46][50]. Particularly, Te NTs possess certain benefits for these applications that include a high specific surface area, tailorable charge transfer/transport, and the ability to heterostructure with other nanomaterials [18][49][50].
4.1. Sensing and Decontamination
Recently, various sensors based on semiconductors have been extensively studied due to their small size, relative simplicity, and low cost. As a p-type semiconductor, elemental Te can respond to the ppb (nM) level of many gassy compounds (ions) at room temperature, and thus Te NTs are expected to offer much a lower limit of detection (LOD) because of the high specific surface area resulted from their hollow tubular structure. In principle, Te NTs can serve as the resistors in gas sensors to present resistivity change in response to the chemical environment change upon absorbing specific atoms in gases. To the best of researchers' knowledge, Te NTs were reported to be sensitive to both oxidative (NO, NO2, and CO) and reductive (NH3 and H2S) gases, for which the further doping of Au, Pt, and Ag could obtain even higher anti-interference. On exposure to oxidizing NO (reducing NH3 or H2S), the resistance of a vacuum-deposited Te NT-built gas sensor [1] decreased (increased), which was ascribed to the surficial-adsorbed O-induced TeO2 formation from Te and electrons trapping that could increase hole density, and conductivity was facilitated (passivated) by NO (NH3 and H2S). Affected by the similar redox reaction, the hexagonal column-shaped Te NTs [45] enabled the thus-constructed gas sensor to detect CO (30 ppm) and NO2 (3 ppm) within 11 min and 6.5 min, respectively, at room temperature. Based on another microwave-assisted polyol reflux-synthesized cylindrical Te NTs [2], a chemiresistive sensor exhibited an ultralow LOD of NO2 (≥0.5 ppb within ≥3 min), even in the real environment, as well as good selectivity toward CH4, NH3, CO, and H2 due to the much higher electron affinity of NO2 when adsorbed on this sensor. Except for gases, Te NTs are also viable for sensing ions through the redox reaction. For example, Wei et al. [51] developed a trigonal Te NTs-incorporated agarose gel membrane that had a high surface area and strong hybridization between the p (Te) and d (Hg) electronic states at the valence band edge (Hf (HgTe) = 244 kJ·mol−1) to act as an optical sensor for Hg2+. Even for Hg2+ spiked in real environment water, a low LOD down to 10 nM and a high removal above 97% could be selectively and quickly obtained through the following displacement reaction.
2Hg2+ + Te + 3H2O → 2Hg +TeO32-+ 6H+
In addition to gases and ions, Te NTs could also be employed to fabricate working electrodes for photodetection. The epitaxial growth of Se on Te NTs formed the Te@Se roll-to-roll NTs [49], in which the increased optical absorption, enhanced built-in electric field, and suppressed carrier transport contributed to the sharp increases in the photocurrent density (7.79 μA·cm−2) and photoresponsivity (98.8 μA·W−1) and excellent tolerance to aqueous solutions (HCl, NaCl, and KOH) of the photodetector. Zhang et al. [18] recently reported the Bi quantum-dots-decorated Te NTs (Te@Bi) van der Waals heterojunctions that exhibited a high photocurrent density (16.87 μA·cm−2) and photoresponsivity (142.97 μA·W−1), which was proven to result from the interfacial plasma effects and the van der Waals between Te NTs and Bi quantum dots.
4.2. Energy Storage
With respect to energy conversion and storage, Te seems one of the most promising electrode candidates due to its high electrical conductivity and theoretical volumetric capacity (2621 mA·h·cm−3). Additionally, compared with the same main group elements (such as sulfur and selenium), Te exhibits relatively smaller volume expansion and faster charge–discharge kinetics, showing particular potential as an attractive electrode material for Li-ion batteries. Therefore, it is expected that the high specific surface area, single-crystallinity, flexibility, and film-forming ability will increase the electrochemical active sites in Te NTs to present faster ion diffusion and better charge storage. In the flexible all-solid-state Li-ion batteries constructed with a novel Te NTs-grown carbon fiber cloth cathode [52], the embedding of the hexagonal phase Te NTs with high single crystalline quality and solvent-free feature increased the electrochemical–structural stability, electrical conductivity, and flexibility. Thus, the high gravimetric capacity of 316 mA·h·g−1 and volumetric capacity of 1979 mA·h·cm−3 (@100 mA·g−1) after 500 cycles were obtained. After being uniformly deposited on a nanofibrillated cellulose film as an anode for Li-ion batteries, Te NTs [53] favored the electrode–electrolyte contact, ion diffusion, structural integrity maintenance, and consequently reached a high volumetric capacity (1512 mA·h·cm−3 @200 mA·g−1), high capacity retention (104% over 300 cycles), and excellent rate performance (833 mA·h·cm−3 @100 mA·g−1).
4.3. Thermoelectrics
Thermoelectrics are a kind of electronics that can enable heat to be converted into electrical current, which is considered one of the solutions to the severe environmental issues arising from the increasing overuse of energy. As is well known, the crucial issue in thermoelectrics’ study is to increase
Z·
T (=
α2T/
ρk), where
Z,
T,
α,
ρ, and
k represent the figure of merit (K
−1), the absolute temperature (K), the Seebeck coefficient (V·K
−1), electrical conductivity (S·m
−1), and thermal conductivity (W·m·K
−1), respectively
[54]. In other words, simultaneously achieving a high power factor (
α2/
ρ) and low thermal conductivity is the prerequisite for high-performance thermoelectrics
[16]. Jung et al.
[55] proposed the Sb-doped Te-rich NTs with hollow hexagonal cross-sections that could act as the phase-change memory switch for an electrical pulse. Benefiting from their small cross-sectional area, geometrical effect on the heat transport, high electrical conductivity, and low thermal conductivity, a much lower writing current was needed for the induction of amorphization voltages pulses. As they are still evolving, Te NTs will be frequently utilized as the secondary components for fabricating thermoelectrics in the 15 years ahead. It was reported by Kyung et al.
[47] that by growing Bi
2Te
3 on Te NTs through polyol refluxing, many single-crystalline Bi
2Te
3 NPs were bonded on the nanotubular structure that reduced the lattice thermal conductivity of the thus-generated powdery nanocomposite due to the increased phonon scattering. Furthermore, the use of Te NTs as the substrate to grow Bi
2Te
3 realized the formation of a core/shell heterostructured Te/Bi
2Te
3 composite, of which the rough serrated interfaces and hollow structures synergistically enhanced the phonon scattering inside it to present as a low thermal conductivity of 0.43 W·m·K
−1 within 300–400 K
[56].
4.4. Templating for Catalysts
In recent years, there has been a trend toward nanoarchitectured catalysts enabled by the sacrificial templating of Te NTs, which can act as a physical scaffold and reactant to induce the resultant to form in the same geometry as Te NTs. For inorganic materials that are unlikely to assemble into tubular nanostructures by using existing technologies, the use of Te NTs as the sacrificial template turns out to be a reliable route to confer them with hollow interiors and nanotubular geometry, as well as a highly tailorable shape and size. Making use of uniform dispersion, geometric stability, and temperature-dependent reactivity, Te-NTs were employed as a self-sacrificial template to generate various shish-kebab-like CdS-Te@(Pt, Pd) multiheterostructures
[50]. The increased and stable photocatalytic production of H
2 of them was attributed to the intimate interaction between CdS-Te and other components, along with more active sites, improved separation of photogenerated carriers, and enhanced light adsorption. Te NTs are also desirable templates of electrocatalysts because of their well-controlled size and morphology to ensure as ideal a geometry as possible, thus leading to the optimized electrocatalytic activity. Lou et al. had devised a facile route to produce porous Pt NTs from the bamboo-like Te NTs template, and the unique tubiform structure and nanoporous framework helped the former to exhibit excellent electrocatalytic activities toward methanol oxidation, alternatively available to direct methanol fuel cells
[46].
5. Current Challenges and Outlook
Te NTs are single crystalline 1D hollow semiconductor nanomaterials that could change the landscape in many fields of science and technology. Although the development is rapid, it is nevertheless in graduated stages and some issues remain unsolved. Firstly, it is a prerequisite to have an unambiguous and generally acceptable definition of those shapes of Te NTs, including cylindrical, prismatic, bamboo-like, shuttle-like, or otherwise. Only by making the distinction clearer can the literature and the applications of Te NTs be made easier to understand, repeat, and optimize. Secondly, regardless of the synthetic strategies, relatively broader distributions of length, diameter, wall thickness, and aspect ratio are yielded for most Te NTs, correspondingly covering a very wide range of electrical, optical, magnetoresistance, and other properties. Therefore, a promising application for Te NTs based on one certain synthesis may also be feasible for that synthesized by another one. In other words, it is highly required to find out these fabrication method counterparts so that the engineering of Te NTs to present well-defined characteristics is more experimentally accessible to researchers in the community, as well as more effective applications that can be achieved more easily.
For sensing and decontamination, energy storage, thermoelectrics, and templating for catalysts, the disadvantage of Te NTs is the lack of economic scale-up fabrication. Unlike carbon NTs, currently, the gram-scale production of Te NTs can hardly satisfy the demands of continuously developing fundamental research and practical applications. Therefore, more efforts need to be paid to the development of the fabrication methods, especially the relatively low-cost and efficient routes, including polyol reflux, the electrochemical method, and thermal evaporation. On the other hand, although it is difficult to produce as many Te NTs as their 1D nanomaterials, the uniqueness of the former can still guarantee their application in specific fields. In addition, currently, the reportage of Te NTs in biomedicine is rare, although it may only require a small amount, far below the clinical poisoning threshold, of Te NTs as the additives.
The electrical, optical, magnetoresistance, and other properties of Te NTs are highly related to their size, shape, and chemical defects. Experimentally isolating their influence is challenging, as the reports about precisely controlling the geometry of Te NTs are rare, and so deeper theoretical studies are needed to reveal more insights. Currently, knowledge about the interplay between these factors is still limited, which strictly hinders the comprehensive improvement of Te NTs in various applications, let alone the finding of new properties in them for new applications. In addition, the growth mechanisms of Te NTs have been frequently reported; however, the exclusivity/compatibility among some of them is rarely studied. Moreover, the detailed investigation and understanding of growing heterostructured NTs of Te or the alloying Te NTs with other elements/compounds are highly anticipated.
The current scope of Te NTs applications has already covered a wide range of topics, and it is still expanding as coronavirus disease 2019 continues to spread worldwide. Just like other 1D Te nanomaterials, Te NTs are expected to engineer and improve the properties of more and more nanomaterials. Because of their good uniformity and tailorable geometry, Te NTs may serve as a new class of sacrificial templates for many applications. Overall, the study of Te NTs is very young, but still has great potential if current challenges can be tackled and barriers can be removed. Thus, the desirable merits of Te NTs will be understood in a deeper way and more efficiently utilized.