Surface nitrided MXene sheets with outstanding electroconductivity and oxidation stability.

Two-dimensional Ti3C2Tx MXenes are promising candidates for a wide range of film- or fiber-based devices owing to their solution processability, high electrical conductivity, and versatile surface chemistry. The surface terminal groups (Tx) of MXenes can be removed to increase their inherent electrical performance and ensure chemical stability. Therefore, understanding the chemical evolution during the removal of the terminal groups is crucial for guiding the production, processing, and application of MXenes. Herein, we investigate the effect of chemical modification on the electron-transfer behavior during the removal of the terminal groups by annealing Ti3C2Tx MXene single sheets under argon (Ar-MXene) and ammonia gas (NH3-MXene) conditions. Annealing in ammonia gas results in surface nitridation of MXenes and preserves the electron-abundant Ti3C2 structure, whereas annealing MXene single sheets in Ar gas results in the oxidation of the titanium layers. The surface-nitrided MXene film exhibits an electrical conductivity two times higher than that of the Ar-MXene film. The oxidation stability is quantified by calculating the oxidation rate constants for severe reactions with H2O2. The surface-nitrided MXene is 13 times more stable than Ar-MXene. The investigation of MXene single sheets provides fundamental insights that are valuable for designing electrically conductive and chemically stable MXenes.

Flexible and Safe Additives-Based Zinc-Binder-Free-Hierarchical MnO2 -Solid Alkaline Polymer Battery for Potential Wearable Applications.

The next-generation flexible wearable electronics are among the most rapidly growing industries due to their extended use in everyday applications resulting in an increased demand for cheaper, safer, and flexible energy storage devices. This study aims to investigate and enhance the overall performance of a Zn-MnO2 alkaline battery and make it suitable for safe and flexible wearable applications. To achieve high cyclability and performance of the cathode, issues of low active-material availability for redox reactions and inactive-phase formations are overcome by fabricating a binder-free hierarchical (increased surface area) additives (enabled reversible compound formation) based MnO2 cathode. Furthermore, zinc/stainless steel composite anode (to reduce anode shape changes) and calcium hydroxide coated polymer electrolyte (to stop zincate ion transfer) are used to improve cyclability. By assembling the above mentioned layers, excellent rate capabilities, high-capacity utilization (487 mAh g-1 ), long cycling stabilities (1000 cycles with 70% retention), and high energy density (400 Wh kg-1 ) are achieved. Moreover, bending, hammering, puncturing, and lighting up an light emitting diode are conducted (under flat, bent, and cut) to demonstrate the cells' safety, flexibility, and robustness. The successful findings in this study can chart new pathways to the development of safe, flexible, and cost-effective next-generation energy storage sources for wearables.

Large-scale wet-spinning of highly electroconductive MXene fibers.

Ti3C2Tx MXene is an emerging class of two-dimensional nanomaterials with exceptional electroconductivity and electrochemical properties, and is promising in the manufacturing of multifunctional macroscopic materials and nanomaterials. Herein, we develop a straightforward, continuously controlled, additive/binder-free method to fabricate pure MXene fibers via a large-scale wet-spinning assembly. Our MXene sheets (with an average lateral size of 5.11 µm2) are highly concentrated in water and do not form aggregates or undergo phase separation. Introducing ammonium ions during the coagulation process successfully assembles MXene sheets into flexible, meter-long fibers with very high electrical conductivity (7,713 S cm-1). The fabricated MXene fibers are comprehensively integrated by using them in electrical wires to switch on a light-emitting diode light and transmit electrical signals to earphones to demonstrate their application in electrical devices. Our wet-spinning strategy provides an approach for continuous mass production of MXene fibers for high-performance, next-generation, and wearable electronic devices.

Room-Temperature, Highly Durable Ti3C2Tx MXene/Graphene Hybrid Fibers for NH3 Gas Sensing.

Graphene-based fibers (GFs) have aroused enormous interest in portable, wearable electronics because of their excellent mechanical flexibility, electrical conductivity, and weavability, which make them advantageous for wearable electronic devices. Herein, we report the development of metal binder-free Ti3C2Tx MXene/graphene hybrid fibers by a scalable wet-spinning process. These hybrid fibers exhibit excellent mechanical and electrical properties for applications in flexible wearable gas sensors. The synergistic effects of electronic properties and gas-adsorption capabilities of MXene/graphene allow the created fibers to show high NH3 gas sensitivity at room temperature. The hybrid fibers exhibited significantly improved NH3 sensing response (ΔR/R0 = 6.77%) compared with individual MXene and graphene. The hybrid fibers also showed excellent mechanical flexibility with a minimal fluctuation of resistance of ±0.2% and low noise resistance even after bending over 2000 cycles, enabling gas sensing during deformation. Furthermore, flexible MXene/graphene hybrid fibers were woven into a lab coat, demonstrating their high potential for wearable devices. We envisage that these exciting features of 2D hybrid materials will provide a novel pathway for designing next-generation portable wearable gas sensors.

Tailored nanoplateau and nanochannel structures using solution-processed rutile TiO2 thin films for complementary and bipolar switching characteristics.

We synthesized two different nanostructures of rutile TiO2 (r-TiO2) thin films on a fluorine-doped tin oxide (FTO) substrate at the lowest temperature reported until now and fabricated resistive random access memory (RRAM) devices with these r-TiO2 thin films having the stacking sequence of Ag/r-TiO2/FTO. Complementary resistive switching (CRS) and bipolar resistive switching (BRS) were observed in different thicknesses of r-TiO2 based devices. Benefiting from the in situ growth of the solution processed thin films and modulating the reaction growth rates, we successfully attained two different morphologies of r-TiO2 with a nanoplateau at a controlled deposition rate and pre-defined nanochannels at a higher deposition rate. The RRAM devices with nano-plateaus of r-TiO2 showed excellent CRS as well as unprecedented simultaneous observations of BRS. These CRS and BRS characteristics were reversible and reproducible. On the other hand, the tailored pre-defined nanochannels in r-TiO2 led to forming-free BRS with a pulse endurance higher than 107 without any degradation in the high and low resistance states. We propose a plausible switching mechanism of these unprecedented events using various physical and electrical characterization studies of low-temperature processed r-TiO2 RRAM devices. This work suggests the importance of solution-processed thin film engineering for RRAM switching with reliable and reproducible characteristics.

Porous Graphene-Carbon Nanotube Scaffolds for Fiber Supercapacitors.

Fiber nanomaterials can become fundamental devices that can be woven into smart textiles, for example, miniaturized fiber-based supercapacitors (FSCs). They can be utilized for portable, wearable electronics and energy storage devices, which are highly prospective areas of research in the future. Herein, we developed porous carbon nanotube-graphene hybrid fibers (CNT-GFs) for all-solid-state symmetric FSCs, which were assembled through wet-spinning followed by a hydrothermal activation process using environmentally benign chemicals (i.e., H2O2 and NH4OH in deionized water). The barriers that limited effective ion accessibility in GFs were overcome by the intercalation of CNTs in the GFs which enhanced their electrical conductivity and mechanical properties as well. The all-solid-state symmetric FSCs of a precisely controlled activated hybrid fiber (a-CNT-GF) electrode exhibited an enhanced volumetric capacitance of 60.75 F cm-3 compared with those of a pristine CNT-GF electrode (19.80 F cm-3). They also showed a volumetric energy density (4.83 mWh cm-3) roughly 3 times higher than that of untreated CNT-GFs (1.50 mWh cm-3). The excellent mechanical flexibility and structural stability of a miniaturized a-CNT-GF are highlighted by the demonstration of negligible differences in capacitance upon bending and twisting. The mechanism of developing porous, large-scale, low-cost electrodes using an environmentally benign activation method presented in this work provides a promising route for designing a new generation of wearable, portable miniaturized energy storage devices.

Low-Temperature Solution-Processed Thiophene-Sulfur-Doped Planar ZnO Nanorods as Electron-Transporting Layers for Enhanced Performance of Organic Solar Cells.

1-D ZnO represents a fascinating class of nanostructures that are significant to optoelectronics. In this work, we investigated the use of an eco-friendly, metal free in situ doping through a pure thiophene-sulfur (S) on low temperature processed (<95 °C) and annealed (<170 °C), planar 1-D ZnO nanorods (ZnRs) spin-coated as a hole-blocking and electron transporting layer (ETL) for inverted organic solar cells (iOSCs). The TEM, HRTEM, XPS, FT-IR, EDS and Raman studies clearly reveal that the thiophene-S (Thi-S) atom is incorporated on planar ZnRs. The investigations in electrical properties suggest the enhancement in conductivity after Thi-S doping on 1-D ZnRs. The iOSCs of poly(3-hexylthiophene-2,5-diyl) and phenyl-C61-butyric acid methyl ester (P3HT: PC60BM) photoactive layer containing thiophene-S doped planar ZnRs (Thi-S-PZnRs) as ETL exhibits power conversion efficiency (PCE) of 3.68% under simulated AM 1.5 G, 100 mW cm-2 illumination. The ∼47% enhancement in PCE compared with pristine planar ZnRs (PCE = 2.38%) ETL is attributed to a combination of desirable energy level alignment, morphological modification, increased conductivity and doping effect. The universality of Thi-S-PZnRs ETL is demonstrated by the highest PCE of 8.15% in contrast to 6.50% exhibited by the iOSCs of ZnRs ETL for the photoactive layer comprising of poly[4,8-bis(5-(2-ethylhexyl)thiophene-2-yl)benzo[1,2-b;4,5-b]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)]: phenyl-C71-butyric acid methyl ester (PTB7-Th: PCB71M). This enhancement in PCE is observed to be driven mainly through improved photovoltaic parameters like fill factor (ff) as well as photocurrent density (Jsc), which are assigned to increased conductivity, exciton dissociation, and effective charge extraction, while; better ohmic contact, reduced charge recombination, and low leakage current density resulted in increased Voc.

Low-Temperature Solution-Processed SnO2 Nanoparticles as a Cathode Buffer Layer for Inverted Organic Solar Cells.

SnO2 recently has attracted particular attention as a powerful buffer layer for organic optoelectronic devices due to its outstanding properties such as high electron mobility, suitable band alignment, and high optical transparency. Here, we report on facile low-temperature solution-processed SnO2 nanoparticles (NPs) in applications for a cathode buffer layer (CBL) of inverted organic solar cells (iOSCs). The conduction band energy of SnO2 NPs estimated by ultraviolet photoelectron spectroscopy was 4.01 eV, a salient feature that is necessary for an appropriate CBL. Using SnO2 NPs as CBL derived from a 0.1 M precursor concentration, P3HT:PC60BM-based iOSCs showed the best power conversion efficiency (PCE) of 2.9%. The iOSC devices using SnO2 NPs as CBL revealed excellent long-term device stabilities, and the PCE was retained at ∼95% of its initial value after 10 weeks in ambient air. These solution-processed SnO2 NPs are considered to be suitable for the low-cost, high throughput roll-to-roll process on a flexible substrate for optoelectronic devices.

Work-Function and Surface Energy Tunable Cyanoacrylic Acid Small-Molecule Derivative Interlayer on Planar ZnO Nanorods for Improved Organic Photovoltaic Performance.

The issue of work-function and surface energy is fundamental to "decode" the critical inorganic/organic interface in hybrid organic photovoltaics, which influences important photovoltaic events like exciton dissociation, charge transfer, photocurrent (Jsc), open-circuit voltage (Voc), etc. We demonstrate that by incorporating an interlayer of cyanoacrylic acid small molecular layer (SML) on solution-processed, spin-coated, planar ZnO nanorods (P-ZnO NRs), higher photovoltaic (PV) performances were achieved in both inverted organic photovoltaic (iOPV) and hybrid organic photovoltaic (HOPV) devices, where ZnO acts as an "electron-transporting layer" and as an "electron acceptor", respectively. For the tuned range of surface energy from 52.5 to 33 mN/m, the power conversion efficiency (PCE) in bulk heterojunction (BHJ) iOPVs based on poly(3-hexylthiophene) (P3HT) and phenyl-C60-butyric acid methyl ester (PC60BM) increases from 3.16% to 3.68%, and that based on poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5b']dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene)-2-carboxylate-2-6-diyl)] (PTB7:Th):[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) photoactive BHJ increases from 6.55% to 8.0%, respectively. The improved PV performance in iOPV devices is majorly attributed to enhanced photocurrents achieved as a result of reduced surface energy and greater electron affinity from the covalent attachment of the strong electron-withdrawing cyano moiety, while that in HOPV devices, where PCE increases from 0.21% to 0.79% for SML-modified devices, is ascribed to a large increase in Voc benefitted due to reduced work function effected from the presence of strong dipole moment in SML that points away from P-ZnO NRs.

Co-functionalized organic/inorganic hybrid ZnO nanorods as electron transporting layers for inverted organic solar cells.

In an unprecedented attempt, we present an interesting approach of coupling solution processed ZnO planar nanorods (NRs) by an organic small molecule (SM) with a strong electron withdrawing cyano moiety and the carboxylic group as binding sites by a facile co-functionalization approach. Direct functionalization by SMs (SM-ZnO NRs) leads to higher aggregation owing to the weaker solubility of SMs in solutions of ZnO NRs dispersed in chlorobenzene (CB). A prior addition of organic 2-(2-methoxyethoxy)acetic acid (MEA) over ZnO NRs not only inhibits aggregation of SMs over ZnO NRs, but also provides enough sites for the SM to strongly couple with the ZnO NRs to yield transparent SM-MEA-ZnO NRs hybrids that exhibited excellent capability as electron transporting layers (ETLs) in inverted organic solar cells (iOSCs) of P3HT:PC60BM bulk-heterojunction (BHJ) photoactive layers. A strongly coupled SM-MEA-ZnO NR hybrid reduces the series resistance by enhancing the interfacial area and tunes the energy level alignment at the interface between the (indium-doped tin oxide, ITO) cathode and BHJ photoactive layers. A significant enhancement in power conversion efficiency (PCE) was achieved for iOSCs comprising ETLs of SM-MEA-ZnO NRs (3.64%) advancing from 0.9% for pristine ZnO NRs, while the iOSCs of aggregated SM-ZnO NRs ETL exhibited a much lower PCE of 2.6%, thus demonstrating the potential of the co-functionalization approach. The superiority of the co-functionalized SM-MEA-ZnO NRs ETL is also evident from the highest PCE of 7.38% obtained for the iOSCs comprising BHJ of PTB7-Th:PC60BM compared with extremely poor 0.05% for non-functionalized ZnO NRs.

Improved Photoelectrochemical Cell Performance of Tin Oxide with Functionalized Multiwalled Carbon Nanotubes-Cadmium Selenide Sensitizer.

Here we report functionalized multiwalled carbon nanotubes (f-MWCNTs)-CdSe nanocrystals (NCs) as photosensitizer in photoelectrochemical cells, where f-MWCNTs were uniformly coated with CdSe NCs onto SnO2 upright standing nanosheets by using a simple electrodeposition method. The resultant blended photoanodes demonstrate extraordinary electrochemical properties including higher Stern-Volmer constant, higher absorbance, and positive quenching, etc., caused by more accessibility of CdSe NCs compared with pristine SnO2-CdSe photoanode. Atomic and weight percent changes of carbon with f-MWCNTs blending concentrations were confirmed from the energy dispersive X-ray analysis. The morphology images show a uniform coverage of CdSe NCs over f-MWCNTs forming a core-shell type structure as a blend. Compared to pristine CdSe, photoanode with f-MWCNTs demonstrated a 257% increase in overall power conversion efficiency. Obtained results were corroborated by the electrochemical impedance analysis. Higher scattering, more accessibility, and hierarchical structure of SnO2-f-MWCNTs-blend-CdSe NCs photoanode is responsible for higher (a) electron mobility (6.89 × 10(-4) to 10.89 × 10(-4) cm(2) V(-1) S(1-)), (b) diffusion length (27 × 10(-6)),

Interfacial Engineering Importance of Bilayered ZnO Cathode Buffer on the Photovoltaic Performance of Inverted Organic Solar Cells.

The role of cathode buffer layer (CBL) is crucial in determining the power conversion efficiency (PCE) of inverted organic solar cells (IOSCs). The hallmarks of a promising CBL include high transparency, ideal energy levels, and tendency to offer good interfacial contact with the organic bulk-heterojunction (BHJ) layers. Zinc oxide (ZnO), with its ability to form numerous morphologies in juxtaposition to its excellent electron affinity, solution processability, and good transparency is an ideal CBL material for IOSCs. Technically, when CBL is sandwiched between the BHJ active layer and the indium-tin-oxide (ITO) cathode, it performs two functions, namely, electron collection from the photoactive layer that is effectively carried out by morphologies like nanoparticles or nanoridges obtained by ZnO sol-gel (ZnO SG) method through an accumulation of individual nanoparticles and, second, transport of collected electrons toward the cathode, which is more effectively manifested by one-dimensional (1D) nanostructures like ZnO nanorods (ZnO NRs). This work presents the use of bilayered ZnO CBL in IOSCs of poly(3-hexylthiophene) (P3HT)/[6, 6]-phenyl-C60-butyric acid methyl ester (PCBM) to overcome the limitations offered by a conventionally used single layer CBL. We found that the PCE of IOSCs with an appropriate bilayer CBL comprising of ZnO NRs/ZnO SG is ∼18.21% higher than those containing ZnO SG/ZnO NRs. We believe that, in bilayer ZnO NRs/ZnO SG, ZnO SG collects electrons effectively from photoactive layer while ZnO NRs transport them further to ITO resulting significant increase in the photocurrent to achieve highest PCE of 3.70%. The enhancement in performance was obtained through improved interfacial engineering, enhanced electrical properties, and reduced surface/bulk defects in bilayer ZnO NRs/ZnO SG. This study demonstrates that the novel bilayer ZnO CBL approach of electron collection/transport would overcome crucial interfacial recombination issues and contribute in enhancing PCE of IOSCs.

Development of highly transparent seedless ZnO nanorods engineered for inverted polymer solar cells.

This work reports on inverted polymer solar cells (IPSCs) based on highly transparent (>95%), hydrophobic, seedless ZnO nanorods (NRs) as cathode buffers with extremely enhanced electrical characteristics. The transparent NR suspension with stability for more than a year is achieved by adding a small amount of 2-(2-methoxyethoxy) acetic acid (MEA). The ability of the stable nanorod suspension to easily spin-coat is certainly an advance to the fabrication of films over large areas and to replace the conventional seeding method to grow one-dimensional nanostructures for use in optoelectronic devices. We observe a strong correlation between the photovoltaic performance and the transparency of ZnO NRs. IPSCs using poly-3-hexylthiophene (P3HT) and [6,6]-phenyl C60 butyric acid methyl ester (PCBM) mixtures in the active layer and transparent (MEA-capped) ZnO NRs as cathode buffers exhibit a power conversion efficiency of 3.24% under simulated AM 1.5G, 100 mW cm(-2) illumination.

Performance enhancement in inverted solar cells by interfacial modification of ZnO nanoparticle buffer layer.

Polymer solar cells (PSCs) have attracted increasing attention in recent years. The rapid progress and mounting interest suggest the feasibility of PSC commercialization. However, critical issues such as stability and the weak nature of their interfaces posses quite a challenge. In the context of improving stability, PSCs with inverted geometry consising of inorganic oxide layer acting as an n-buffer offer quite the panacea. Zinc oxide (ZnO) is one of the most preferred semiconducting wide band gap oxides as an efficient cathode layer that effectively extracts and transports photoelectrons from the acceptor to the conducting indium-doped tin oxide (ITO) due to its high conductivity and transparency. However, the existence of a back charge transfer from metal oxides to electron-donating conjugated polymer and poor contact with the bulk heterojunction (BHJ) active layer results in serious interfacial recombination and leads to relatively low photovoltaic performance. One approach to improving the performance and charge selectivity of these types of inverted devices consists of modifying the interface between the inorganic metal oxide (e.g., ZnO) and organic active layer using a sub-monolayer of interfacial materials (e.g., functional dyes). In this work, we demonstrate that the photovoltaic parameters of inverted solar cells comprising a thin overlayer of functional dyes over ZnO nanoparticle as an n-buffer layer are highly influenced by the anchoring groups they possess. While an inverted PSC containing an n-buffer of only ZnO exhibited an overall power conversion efficiency (PCE) of 2.87%, the devices with an interlayer of dyes containing functional cyano-carboxylic, cyano-cyano, and carboxylic groups exhibited PCE of 3.52%, 3.39%, and 3.21%, respectively, due to increased forward charge collection resulting from enhanced electronic coupling between the ZnO and BHJ active layers.

Effect of solvent annealing on heterojunction polymer solar cells based on P3HT/PCBM.

Heterojunction (HJ) structure has the advantages of high charge collection and transport efficiency, but it also has the disadvantage of low charge collection, due to limited p-n interfaces, compared with bulkheterojunction (BHJ) structure. In order to overcome this disadvantage of HJ, we fabricated HJ solar cells with large p-n interfaces, based on poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butylric acid methyl ester (PC61 BM), treated by a solvent annealing (SA) process under atmospheric condition, which is a simple and low-cost process. The SA process induced a P3HT-PC61 BM interdiffusion layer between the P3HT and PC61 BM layers, by a diffusion of each component, and resulted in an increase of p-n interface areas. These results were confirmed by X-ray photoelectron spectroscopy (XPS), and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). The performance of a HJ device fabricated with a SA process achieved a 30% higher power conversion efficiency (PCE) value (approximately 3.3%), than that without a SA process. Interestingly, the HJ SA device showed higher long-term stability, than that without a SA process.

Low temperature chemically synthesized rutile TiO2 photoanodes with high electron lifetime for organic dye-sensitized solar cells.

Electron lifetime in mesoporous nanostructured rutile TiO2 photoanodes, synthesized via a simple, cost-effective, low temperature (50-55 °C) wet chemical process, annealed at 350 °C for 1 h and not employing any sprayed TiO2 compact layer, was successfully tailored with 0.2 mM TiCl4 surface treatment that resulted in light to electric power conversion efficiency up to 4.4%.

Polythiophene infiltrated TiO2 nanotubes as high-performance supercapacitor electrodes.

We report exceptional electrochemical specific capacitance (640 F g(-1)) for polythiophene (PTh) infiltrated into TiO(2) nanotubes (TNTs) by a controlled electropolymerization route. The resulting PTh-TNTs also exhibit excellent electrochemical behavior with long-term stability. This reproducible and superior performance of PTh-TNT electrodes makes them attractive candidates for energy storage.