MXene‐enhanced environmentally stable organohydrogel ionic diode toward harvesting ultralow‐frequency mechanical energy and moisture energy

With the accelerating advancement of distributed sensors and portable electronic devices in the era of big data, harvesting energy from the surrounding environment to power electrical devices has become increasingly attractive. However, most mechanical energy harvesters often require high operating frequencies to function properly. Moreover, for practical applications, the survivability of devices in harsh operating environments is a vital issue which must be addressed. Besides, the single‐stimulus responsiveness limits their further applications in complex external environments. Here, a pressure and moisture dual‐responsive ionic diode consisting of two organohydrogels with opposite charges as an energy harvester is proposed. The organohydrogel ionic diode utilizes the migration of cations and anions to form the depletion zone and followed by an enhancement of the built‐in potential along the depletion zone as a result of mechanical stress or humidity, converting ultralow‐frequency mechanical energy or moisture energy into electrical energy. Meanwhile, this mechanism is further confirmed by the finite element analysis. With the increased rectification ratio due to the introduction of MXene, the ionic diode exhibits a relatively large output current (∼10.10 μA cm−2) and power density (∼0.10 μW cm−2) at a mechanical pressure of 0.01 Hz, outperforming most currently available mechanical energy harvesters. More impressively, the incorporation of ethylene glycol provides the hydrogel ionic diode with excellent temperature tolerance and long‐term environmental stability. The organohydrogel ionic diode can also be applied as a moisture‐driven power generator and self‐powered humidity sensor. This study presents promising prospects for the efficient collection of renewable and sustainable energy and the practical application of hydrogel‐based energy harvesters in extreme environments.


INTRODUCTION
2][3][4] Replacing fossil resources by converting the energy from the surrounding environment into electricity in an environmentally friendly way has become one of the most promising strategies. 5,6Among the various renewable energy sources, environmental mechanical energy has attracted worldwide attention due to its ubiquity and ease of access as well as its diverse transduction strategies. 7,8or example, the conversion from mechanical to electrical energy has been realized employing diverse strategies (e.g., piezoelectric, tribological, and electromagnetic converters). 9,10Especially, piezoelectric energy harvesting, as the most promising technology for environmental energy harvesting, has attracted wide interest due to the simple structure, favorable scalability, and high power density. 11,12In addition, humidity is also a ubiquitous source of environmental stimulation.][15] The majority of mechanical energy harvesting devices require high frequencies (>10 Hz) to function efficaciously, 16,17 and the energy output is severely limited at lower operating frequencies. 18Therefore, an innovative energy collector on the basis of "ionic conduction" has been designed to get over this limitation. 19Unlike conventional piezoelectric devices, the ionic energy harvesters utilize the slow diffusion of high concentrations of mobile ions within the material in response to mechanical stimuli, which enables devices to function at low frequencies while yielding large outputs, showing great potential in capturing electrical energy from low-frequency mechanical energy. 202][23] Despite their good electromechanical conversion capability, the environmental stability of hydrogel-based ion devices is usually neglected.Typically, the water-rich structure allows the hydrogel materials to behave like a solid while still acting like a liquid for fast ion transport. 24evertheless, conventional conductive hydrogels utilizing pure water as the dispersion medium generally have two inherent drawbacks that cannot be neglected: The hardening issue induced by freezing at subzero temperatures, which dramatically limits the working temperature range of ionic hydrogels.The other is structural dehydration caused by water evaporation, which greatly shortens the lifetime of the hydrogel at room temperature as well as at high temperature. 25,26To meet practical applications in complex situations, it is essential to develop environmentally adaptive hydrogel-based ionic diodes that possess long-term environmental stability and freezing resistance for ultralow-frequency mechanical energy collection.
][29] However, due to the undesired solubility of the polyelectrolyte in the binary solvent and the limitation of ion mobility by the increased viscosity, the conductivity of the organohydrogels decreases, as well as the rectification rate of organohydrogel-based ionic diodes, which limits the net electrical output of the organohydrogel-based ionic diodes. 30Hence, it is highly desirable to develop a novel hydrogel ionic diode with environmental stability, while improving its rectification performance to enhance the electrical output.To achieve excellent conductivity and enhanced charge collection, integrating conductive fillers into the polymer matrix to improve the rectification performance of ionic diodes has proven to be an effective method. 194][35] MXene nanosheets exhibit substantial potential in the manufacture of conductive hydrogels as a result of their outstanding electrical conductivity, large specific surface area, superior hydrophilicity, and great mechanical properties. 36However, to the best of our knowledge, MXene has not been employed as a conductive filler in ionic diode devices.Moreover, most previously reported ionic diode-based devices are limited to singlestimulus responsiveness, either mechanical energy harvesting or moisture energy harvesting only. 21,23,37Indeed, complex external environments and diverse application demands often call for the development of dual/multiresponsive self-powered devices.Thus, the development of energy harvesting devices with dual/multi-stimulus responsiveness holds great significance.
In view of that, this work presented a novel pressure and humidity dual-responsive environmentally tolerant nanocomposite organohydrogel ionic diode energy harvester for converting mechanical motion at low characteristic frequencies as well as moisture into electrical energy.The ionic diode consisted of a bilayer structure of anionic and cationic organohydrogels embedded with MXene (Ti 3 C 2 T x ), which displayed entropy-induced depletion from moving ions and rectifying characteristics resembling a p-n semiconductor junction.As demonstrated by the finite element method simulations, under external mechanical stimulation, the equilibrium in the ion depletion zone was disrupted and the space-charge zone was expanded, generating an electrical output related to migration of ions rectified from the junction.Owing to the enhanced rectification performance from MXene, the flexible ion mechanical energy harvester exhibited relatively large current output (∼10.10 μA cm −2 ) and peak power output (∼0.10 μW cm −2 ) during each mechanical cycling under an ultralow frequency (0.01 Hz), exceeding most present mechanical energy harvesting devices.More importantly, as a result of the substantial hydrogen bonds between water and ethylene glycol (EG), the prepared organohydrogel ionic diode displayed excellent lowtemperature tolerance (−20 • C), long-term environmental stability, and anti-drying capability (60 • C), which could harvest low-frequency mechanical energy sources stably in extreme environments.In addition, the organohydrogel ionic diode can also be employed as a moisture-driven power generator to generate electricity in humid environments.The prepared organohydrogel ionic diode could be employed to constructing self-powered pressure sensors and humidity sensors.

Preparation and characterization of the organohydrogel ionic diode
The organohydrogel ionic diode with an asymmetric structure was constructed via incorporating a separator between two composite organohydrogels containing polymer polyelectrolytes with opposite charges.Each organohydrogel layer was obtained from mixing water, EG, MXene, agarose, and polyelectrolyte (poly(sodium styrene sulfonate) (PSSNa) or poly(diallyldimethylammonium chloride) (PDACl)).First, highly conductive nanofillers MXene (Ti 3 C 2 T x ) was prepared (the following words "MXene" all mean Ti 3 C 2 T x ).As shown in Figure S1, HCl/LiF was used to selectively etch the Al atomic layer of the MAX phase to produce MXene nanosheets to reduce the risk of concentrated hydrogen fluoride dilution.Figure 1A presents the preparation process of the organohydrogel ionic diode.The gels were doped with water and EG as binary solvents, biocompatible polysaccharide agarose as the gel backbone, and highly conductive MXene as the nanofiller, along with two typical polyelectrolytes, polycationic (PDACl) and polyanionic (PSSNa).The chemical structures of polycationic and polyanionic are pictured in Figure 1A.In the water within the organohydrogel, polyanionic electrolyte dissociated into the fixed polyanionic backbones and mobile counterions (Na + ), similar to the acceptors and the holes in the p-type semiconductor, respectively.Similarly, the fixed polycationic backbones and mobile counterions (Cl − ) produced by the dissolved polycationic electrolyte within organohydrogel resemble the donors and the electrons of the n-type semiconductor, respectively. 20,38The ionic groups of both ionic polymers are fixed on the polymer chains, whereas the counterions can be delocalized.This not only provides substantial amounts of ions inside the organohydrogel but also enables an efficient interaction between the mobile ions and polyelectrolyte chains. 23In addition, the comparatively high level of ion migration along with the rapid response under the stimuli from the outside could be attributed to the tiny sizes of mobile ions.More importantly, the introduced EG forms a large number of hydrogen bonds with the water in the gel, providing the hydrogel with antifreezing and anti-drying performance.Notably, MXene nanosheets are introduced into two organohydrogels for increasing the electrical conductivity, which afford an efficacious way to harvest the electrical output generated as a result of the ions transport.The hydrophilic polytetrafluoroethylene (PTFE) permeable membrane separator, sandwiched between two organohydrogel nanocomposites, allows the passage of water and ions while also avoiding the occurrence of short circuits due to the merging of the organohydrogel layers containing the MXene nanosheets.The electrical output performance is measured by attaching carbon cloth to both ends of the sandwich structure.
Figure 1B presents the operating principle of the organohydrogel ionic diode.Once the ionic diode device is assembled, the mobile Na + and Cl − from the two polyelectrolytes diffuse entropically across the permeable membrane toward the opposite side, respectively.Thus, the presence of an excess of immobilized polyanions and polycations on both sides creates a space-charge zone and establishes an interfacial electric field opposite to the ions diffusion to counteract the additional diffusion of ions, thereby achieving balance.When subjected to a mechanical pressure, the volume of the gel changes, resulting in a corresponding rise in the volume concentrations of Na + and Cl − .As a result, the initial state of balance is broken, leading to an expansion of the space-charge zone. 39Prior to the formation of a new balance, the directed movement of the two mobile ions generates voltages and currents, accomplishing electromechanical conversion and energy harvesting.As exhibited in Figure 1C, the flexible organohydrogel ionic diode energy harvester is capable of operating at ultralow frequencies of 0.01 Hz with relatively large short-circuit current density (∼10.10 μA cm −2 ) and voltage (∼26.50 mV) in a single mechanical cycle.The organohydrogel ionic diode can not only be applied for low-frequency mechanical energy harvesting but also exhibits excellent temperature tolerance and long-term environmental stability, which is expected to be applied in extreme environments.
The following test characterizations were performed to validate the successful fabrication of MXene nanosheets and nanocomposite organohydrogels.First, the MXene (Ti 3 C 2 T x ) nanosheets were manufactured by selectively etching the aluminum atomic layer from the MAX phase (Ti 3 AlC 2 ) and then layering. 40The microscopic morphology of the unetched MAX phase could be observed in Figure S2a, presenting the distinctive layered and stacked aggregates.The accordion-like structure of the etched and unexfoliated Ti 3 C 2 T x could be distinctly seen in Figure S2b.X-ray diffraction (XRD) analysis was employed to identify the crystalline structure transformation of both MAX phase and MXene.In Figure 2A, it can be observed that the (0 0 2) peak of Ti 3 AlC 2 at 9.52 • 2θ angle was clearly shifted to 6.23 • of Ti 3 C 2 T x , which should be attributed to the removal of Al atomic layer from Ti 3 AlC 2 .X-ray photoelectron spectroscopy (XPS) analyses of Ti 3 C 2 T x were provided in Figure 2B and Figure S3, confirming the presence of Ti, C, O, and F elements in the MXene sheet.The oxygen element existed mainly in the polar groups of MXene nanosheets.The groups, such as -O, -F, and -OH, enable MXene nanosheets to display good hydrophilic behavior.Moreover, the atomic force microscope (AFM) image in Figure 2C exhibited that the single MXene nanosheet was relatively thin, with a thickness of only 1.65 nm and a horizontal dimension of about 1 μm, representing the most probably monolayer structure of the MXene nanosheet.In addition, the images from transmission electron microscopy (Figure S4) and scanning electron microscopy (SEM) (Figure S5) could also be observed that MXene nanosheets exhibited irregular 2D nanoflake morphology.It can be noted from Figure S6 that the layered hydrophilic Ti 3 C 2 T x flakes were uniformly dispersed in the aqueous solution, and its colloidal solution exhibited the noticeable Tyndall effect.In addition, the electrical conductivity of the membrane filtered through vacuum with a thickness of 7 μm was measured to be 7.32 × 10 4 S m −1 , presenting an extraordinary conductivity.As demonstrated in Figure S7, the Raman spectra of MXene film obtained by vacuum filtration was highly consistent with the scattering spectra of Ti 3 C 2 T x film.The SEM observation of the Ti 3 C 2 T x film surface revealed a wrinkle-like appearance (Figure 2D), whereas its elemental mapping analysis further demonstrated the homogeneous distribution of the major elements, such as F, O, Ti, and C (Figure 2E).
Additionally, Figure 2F displays optical images of two composite organohydrogels, which present superior mechanical flexibility and are highly expected in stress/strain cycles during the process of mechanical energy harvesting.In addition, the microstructure and elemental distribution of the two composite organohydrogels were also characterized.The SEM images of the two composite organohydrogels after freeze-drying were presented in Figure 2G, where a three-dimensional interconnected porous microstructure in the gel could be observed, providing pathways for the migration of ions.Meanwhile, the elemental mapping analysis revealed the homogeneous distribution of Ti 3 C 2 T x and ionic polymers inside the gels.The intermolecular interactions in the two composite organohydrogels were also illustrated by Fourier transform infrared spectroscopy.For PDACl gels, the peaks at 3354 and 1045 cm −1 in the spectra from pure hydrogels were corresponding to distinctive groups of O-H and the C-O, respectively (Figure 2H).In addition, the introduction of Ti 3 C 2 T x resulted in a shift of the corresponding peaks to 3342 and 1035 cm −1 , suggesting the creation of hydrogen bonds between polymer chains and nanosheets.The addition of EG induced an additional shift of above peaks to 3334 and 1024 cm −1 , indicating the stronger H-bonding interactions between EG and other components within the gel.Similarly, the PSSNa gels displayed the same trend in Figure 2I.

Ultralow-frequency mechanical energy harvesting performance
Efficient diffusion of the cations and anions in contrary directions is the crucial factor in the functioning of our innovative mechanical energy harvester based on organohydrogel ionic diode.This necessitates both high ion concentrations and high migration rates for efficient operation.Therefore, the high electrical conductivity and good rectification capability are essential requirements for hydrogel ionic diodes.The rectification principle of the ionic diode is explained below: The interfacial electric field established resulting from the diffusion of moving ions endows the organohydrogel diode with current rectification behavior, similar to that of a semiconductor p-n junction. 38,41During forward bias application, the applied bias voltage opposes the direction of the built-in electric field created at the interface between the upper and lower gel layers, resulting in a reduction of the built-in potential.Under forward voltage, the counterions are attracted toward the gel/gel interface, causing a contraction of the ionic bilayer and facilitating current flow across the gel/gel junction.During reverse bias application, the bias voltage aligns with the built-in electric field established at the gel/gel interface, resulting in an enhancement of the built-in potential.And under the reverse voltage, the counterions are drawn away from the gel interface, resulting in a wider ionic bilayer and impeding the flow of current.Thus, the current flow can only pass in one direction and is blocked in the other, exhibiting the typical characteristics of a diode.
First, only the polyelectrolytes were introduced into the agarose hydrogel without EG and MXene to investigate the effects of the thickness of the hydrogel and the concentration of the polyelectrolytes on the rectification performance.After determining the optimal thickness of the hydrogel and the optimal concentration of the polyelectrolytes, EG and MXene were introduced into the hydrogel for further investigation.It was found that the optimal thickness of the hydrogel film was 300 μm and the optimal concentration of the polyelectrolytes was PSS 7.0 wt%, PDACl 5.0 wt% (Figure S8).The extreme temperature tolerance of hydrogel ionic diodes is facilitated by the introduction of EG solvent, a low-molecular vicinal alcohol that acts as a nontoxic inhibitor of water freezing and drying. 28However, it was found that there is a trade-off regarding the amount of EG added in our experiment.As presented in Figure 3A, we investigated the rectification Here, the rectification ratio of the hydrogel diode is determined as the ratio of the current in the forward biased and reverse biased states at the bias voltage of 5 V, with a sweep speed of 100 mV s −1 .It could be found the rectification ratio of the hydrogel ionic diode decreased with the increase of EG content (Figure S9).This is due to the undesired solubility of the ionic polymer in the EG solvent, which reduces the ionic conductivity of the EG/water binary solvent system. 30Additionally, the increased viscosity can inhibit the migration rate of ions, which also reduces the rectification performance.In order to ensure that the hydrogels possessed low-temperature freezing resistance, the ionic conductivity of hydrogels containing different amounts of EG was tested at −20 • C to explore the optimal amount of EG.As shown in Figure 4A, the differential scanning calorimetry (DSC) test results revealed that the hydrogel containing 30% EG content had already exhibited an exothermic peak at −20 • C, although its freezing point was −21.04 • C, which indicated that a small amount of ice crystals had already been formed at this time.As a result, the movement of a small portion of the ions inside the hydrogel would be hindered at this time, as reflected in the decrease in ionic conductivity shown in Figure S10.When the content of EG in the hydrogel was 40%, the freezing point was much lower than −20 • C. In this case, there was no ice crystal formation within the hydrogel, and the movement of all ions was not hindered.As a result, the ionic conductivity is higher compared to the hydrogel with 30% EG content.In addition, due to the undesired solubility of the ionic polymer in the EG solvent, the ionic conductivity rather decreased when continuing to increase the EG content to 50%; therefore, the hydrogel sample with 40% EG content exhibited the maximum conductivity at −20 • C. Hence, the optimal content of EG in the hydrogel was 40% to balance rectification ability and temperature tolerance.However, the rectification ratio of the hydrogel ionic diode was relatively low at this time.In order to improve the rectification ratio, the highly conductive MXene was introduced into the organohydrogel.As displayed in Figure 3B, as the MXene content gradually increased to 0.3 wt%, the forward current density gradually increased as well as the rectification ratio (Figure S11).In addition, the rectification ratio increased from 3.48 to 7.31.The increase of the rectification ratio can be attributed to the following reasons.Ion-electron interconversion is usually achieved by capacitive behavior or redox reactions on the electrodes.We used the inert electrodes and no redox reaction occurred in this work.Ion migration produces electron transport through the charge adsorption.In this paper, the current collection capability was enhanced by introducing highly conductive MXene as a current collector into the gels.On the one hand, the highly conductive MXene improved the conductivity of organohydrogels.(Figure S12 demonstrates that the single-layer organohydrogel containing 0.3 wt% MXene exhibited noticeably improved conductivity compared to the organohydrogel without MXene.)On the other hand, MXene could penetrate deeply into the gel matrix, which increased the accessible surface area of the electrode and the gel electrolyte and minimized the distance between the adsorbed ions and the electrode to improve the bilayer capacitance.Consequently, the combination of outstanding charge conductivity and high capacitance resulted in a dramatic improvement of ion-electron conversion in the gel electrolyte, achieving a substantial increase of the rectification ratio.The degradation of rectifying performance and current caused by an excessive amount of MXene may be explained by the uneven distribution of MXene.It is because the aggregation of MXene could cause the conductivity of the ionic diode to be more dominated by electrons than ions.In addition, we also verified the effect of MXene on the rectification performance of pure hydrogels.Figure S13 exhibits that the optimal amount of MXene improved the rectification ratio of the pure hydrogel from 5.13 to 10.11.However, rectification performance was not available in the symmetrical PDACl/MXene/EG//PDACl/MXene/EG and PSSNa/MXene/EG//PSSNa/MXene/EG devices with the same charge (Figure S14), confirming that the asymmetric interface between two oppositely charged gel polyelectrolytes was essential in forming a rectifying junction.Besides, the parameters of the PTFE membrane used and its critical importance in ion rectification were also investigated (Figure S15).The large pore size, hydrophilicity, and material structure of the PTFE membrane greatly influenced the permeability of water molecules and ions, which was critical for the ion rectification performance.
To quantify the output performance of the organohydrogel ionic diode, the electrical characterization tests were performed.It was worth noting that an initial potential of about 45.00 mV was observed in Figure 3C, which was attributed to the built-in potential of the junction.When the ion diode was connected in the forward direction to the digital multimeter, a voltage of approximately 26.50 mV was generated under the ultralow-frequency mechanical pressure of around 0.01 Hz.][44] Furthermore, the average duration of current exceeding one-half of the maximum value was approximately 15.1 s, which was much greater than that of traditional piezoelectric power devices and other mechanical energy harvesters based on different principles, [45][46][47] manifesting the superior capability of generating abundant charges and energy utilizing pressure stimulation.The significant difference in the time duration stemmed from the involvement of the entirety of organohydrogels in generating electrical charges, whereas the production of charges in the majority of other energy harvester materials occurred mainly at their surface.On the basis of the peak area for the current curve, the organohydrogel diode yielded a charge of ∼0.19 mC cm −2 in an individual pressure cycling, which was several magnitudes larger than that of other piezoelectric and triboelectric devices. 45,48The previous results demonstrate the converting of mechanical energy into electrical energy.It was found that the reverse voltage signals were generated when the organohydrogel ionic diode was connected to the digital multimeter in the reverse direction, which was equal in magnitude to that of the forward connection (Figure S16a, b).To further confirm the observed variation in electrical response was indeed caused by the asymmetric double-layered organohydrogels within ionic diode, the electrical output of organohydrogel complexes containing only PSSNa or PDACl was measured in response to pressure cycling.As shown in Figure S17, no electrical signal generation was detected under the mechanical stimuli, indicating that the electrical output actually came from the redistribution of ions in the gel ionic diode and was unrelated to the ionic polymer gel itself.
To further understand the operation principle of the prepared energy harvester, we have validated it employing the finite element method.Details of the simulation process and the various parameters (Table S1) are provided in the Supporting Information.The connection pattern of the double-layered ionic gels with each other is analogous to that of a p-n semiconductor junction.As the PSSNa and PDACl organohydrogel materials are assembled together, creating a junction (Figure 3E), two mobile counterions (Na + and Cl − ) near the interfaces of the upper and lower gel layers transport across the permeable membrane in opposite directions as a result of the entropy drive.As the cross-linked networks of the gel do not allow long-distance movement of the fixed polyelectrolyte chains, the excessive charges of the immobilized polycations and polycation near the junction interface create a space-charge zone when the mobile ions diffuse.The formation of a heterojunction between polyanion and polycnion results in the so-called ionic double layer, resembling the depletion layer in the p-n semiconductor junction. 41,49At the same time, a built-in electric field opposite to ion diffusion is induced to establish and a built-in potential (φ b ) is created.As the diffusion movement proceeds, the space-charges gradually increase and the space-charge area gradually expands.Meanwhile, the built-in potential gradually increases and the drifting motion of the ions progressively intensifies.When the diffusion and drift of ions finally reach dynamic equilibrium, the concentration curves of quantification derived from the finite element method simulations verify that the mobile counterions in the region near the junction interface have been depleted (Figure 3F), creating a stable zone of space-charge as a result (Figure 3G and S18a).In addition, the finite element method provides further validation for the potential barrier formation (Figure 3H and S18b).When inputting a mechanical stress, additional ion diffusion occurs due to changes in the volume of the gels.The volume concentrations of the ions inside the gels increase as the stress increase (Figure 3I,J), which can break the initial state of balance and create an expanded space-charge zone (Figure 3K and S18c).In the newly developed state of balance, the potential difference between the built-in potential (φ s ) under mechanical pressure in the depletion zone and the initial potential (φ b ) is referred to as V g .In addition, the V g implies that the resulting output voltage with the mechanical input.As depicted in Figure 3L and Figure S18d, the generated φ s resulting from the mechanical input is also demonstrated via the finite element method.Therefore, the ion motion inside the organohydrogel ionic diode, caused by mechanical stress, can result in current generation via induction of the external circuitry (Figure 3I).As the mechanical force is removed (Figure 3E), the built-in potential gradually recovers to the original level as the drift movement of ions gradually returns to the initial state of balance, resulting in a progressive decreasing of the current flowing via the electric circuit. 19,50oreover, the electrical outputs of the prepared energy harvesting devices could be integrated and amplified by connecting multiple ion devices in series and parallel.As displayed in Figure 3M and Figure S19, the multilayered device with five organohydrogel diodes exhibited an electrical output of 50.4 μA cm −2 and 131.9 mV, indicating a linear correlation between the electrical output value and the amount of devices.Figure 3N presents the output voltage and output current values obtained from the fabricated energy harvester at various applied stresses.It can be found that the electrical output values increased from 0.82 mV and 0.23 μA cm −2 to 26.50 mV and 10.10 μA cm −2 , over a relatively wide pressure range.In addition, the organohydrogel diode was no longer responsive to mechanical forces over 7.30 kPa, owing to the fact that the loaded ionic polymer had reached saturation at this pressure.The sensitivity of the ionic diode was 0.013 V kPa −1 over a range of 0.06-1 kPa.Furthermore, the voltage signals of organohydrogel diode under different pressures displayed excellent stability and reversibility (Figure S20).The prepared ionic diode exhibited high sensitivity and low detection limits, suggesting its great potential as a high-performance flexible self-powered sensor.Figure 3O and Figure S21 present that the electrical outputs of the fabricated energy harvester increased as the mechanistic frequency decreased, which was mainly related to the diffusive features of the free ions.Besides, the cycling stability of the organohydrogel ionic diode was also tested.The output current and voltage of the gel diode displayed superior stability over 5000 cycles at 0.01 Hz, as exhibited in Figure 3P and Figure S22.The slight decrease of the signals at the late stage of cycling may be caused by the loss of trace water in gels during the prolonged cycling.The variation of electrical output and power density with the external load resistance from 50 Ω to 1 MΩ is depicted in Figure S23.The achieved maximum power density was 0.1 μW cm −2 when the optimum resistance was 10 kΩ, which was higher than previously reported ionic diodes and piezoelectrochemical devices. 19,51Furthermore, we performed current output measurements on the mechanical energy harvesting of the device using different inert electrodes (graphite paper, carbon cloth, and platinum sheet electrode), and the results showed that the overall difference in the output values was not significant (Figure S24).

Temperature tolerance and anti-drying performance
From the perspective of practical applications, the excellent environmental stability, that is, temperature tolerance and anti-drying performance, is essential for the stable outputs of hydrogel-based ionic diodes under harsh conditions.The PDACl organohydrogels were used as the representative to investigate the stability in extreme and open environments to better assess their weather resistance.First, the anti-freezing ability of gels was characterized using the DSC. Figure 4A presents a distinct exothermic peak was appeared at −6.12 • C for the EG-free gel, suggesting a substantial transformation of the abundant free water within the gel into freezing crystal. 29,52In addition, the deviation of the freezing point from the conventional 0 • C was ascribed to the interaction between the hydrogel polymer chains and the water molecules via hydrogen bonding.As the content of EG increased, the representative peaks shifted to lower temperatures and became smaller and narrower.The freeze temperature of gel with the addition of 40 vol% EG was reduced to −50.78 • C.These results demonstrated that the incorporation of EG can be effective in reducing the freezing point of hydrogel.The outstanding anti-freezing performance stems from the hindrance of freezing crystal appearance by the hydrogen bonds (H-bonds) created between the hydroxyl groups from EG and H 2 O. 53 Electrochemical impedance spectroscopy (EIS) was employed to quantitatively measure the effect of EG on the conductivity of organohydrogels (Figure S25).It was found that the conductivity of organohydrogels decreased with the increasing content of EG, which was attributed to the undesired solubility of the ionic polymer in the EG solvent.In addition, the temperaturedependent conductivity of the organohydrogel was also investigated by the EIS.As shown in Figure 4B and Figure S26, the organohydrogel containing 40 vol% EG presented a positive correlation between electrical conductivity and temperature.The organohydrogel achieved superior conductivity at low temperatures by inhibiting the growth of ice crystals.In order to visualize the conductivity of the organohydrogels, hydrogel samples with or without 40 vol% EG stored at −20 and 60 • C for 24 h were integrated into a closed circuit connected to a green LED to further verify the temperature tolerance of the organohydrogel.Figure 4C displays that the LED in the circuit connected with the organohydrogel was illuminated at both low and high temperatures, whereas the hydrogel was out of function.As depicted in Figure 4D, the organohydrogel was flexible and in the natural sagging state both at −20 and 60 • C, whereas the hydrogel became rigid and inflexible.All these results demonstrated that the organohydrogel possessed outstanding temperature tolerance.
The long-term stability of the organohydrogel in the open environment was also investigated.The weight changes of the organohydrogel and hydrogel were recorded over 15 days at 23.9 • C and 59% relative humidity (RH), respectively.After the initial one day of placement, the appearance and size of the organohydrogel remained virtually unchanged, whereas the shape of the hydrogel underwent noticeable changes (Figure 4E).After 15 days, the changes of organohydrogel were still minimal, showing a minor volumetric contraction compared to its original state.In contrast, the hydrogel underwent long-term water loss and shrank significantly in size.Generally speaking, the thinner the hydrogel, the more likely it is to lose water.As presented in Figure S27, the thin organohydrogel with a thickness of 300 μm still displayed flexibility after 3 days of storage.However, the hydrogel with the same thickness became rigid due to water loss.The weight retention of hydrogels containing different concentrations of EG was also quantitatively investigated.As depicted in Figure 4F, the pure hydrogel evaporated most of the water within 15 days and maintained a weight retention of 30.16%.In contrast, the organohydrogel containing 40 vol% EG could retain 75.25% of its initial weight.In addition, the weight retention exhibited an upward trend with the increasing amount of EG.The SEM in Figure S28 reveals that a few pores emerged in the hydrogel after 15 days of storage resulting from the considerable water evaporation, whereas no discernible pores were noticed in the organohydrogel after the same storage period.Furthermore, the water loss of hydrogel and organohydrogel containing 40 vol% EG was also studied at 60 • C, respectively (Figure S29).The hydrogel exhibited significant weight loss within 180 min, whereas little change was observed for the organohydrogel.The above results demonstrated the organohydrogel was less vulnerable to evaporation, which is as a result of the existence of multiple H-bonds formed between the introduced EG and H 2 O. 54 Moreover, the effect of EG on enhancing hydrophilicity was examined and the water contact angle reduced from 40.10 • to 19.90 • as the content of EG increased, indicating the contribution of EG to the long-term moisturizing performance (Figure S30). Figure 4G illustrates schematically the characteristics of the organohydrogel ionic diode.The organohydrogel ionic diode could be applied for low-frequency mechanical energy harvesting in the extreme environments due to its excellent temperature resistance and long-term environmental stability.We examined the change in electrical output of the organohydrogel ionic diode after storage in an open environment for different time.As shown in Figure 4H, the output voltage values exhibited only a slight decrease with increasing storage time, resulting from the inevitable evaporation of a small amount of water in the organohydrogel.Meanwhile, the electrical output performance of the organohydrogel ionic diode at different temperatures was also tested (Figure 4I).No apparent electrical output loss was noticed as the temperature increased from −20 to 60 • C, indicating the superior stability of the organohydrogel ionic diode over a wide operating temperature range.
Moreover, we utilized density functional theory calculations to calculate the interaction energy of hydrogen bonds between the water molecules and the gel matrix to demonstrate the environmental stability of organohydrogel.As presented in Figure 4J and Table S2, compared with the weak interaction of H 2 O-H 2 O (−0.6745 Ha), the binding energy of H 2 O-EG was greater (−1.6117Ha), suggesting the strong interaction between water molecules and EG.The other components of the gel also exhibited high binding energies with the water molecules, for instance, −12.9332 and −5.3471 for H 2 O-Agar and H 2 O-MXene, respectively, which was consistent with the results of the DSC tests.Consequently, the superior environmental stability could be ascribed to the synergistic effects.The strong interactions between the water molecules and EG or other components can disturb the hydrogen bonding of H 2 O-H 2 O, thus affecting the vapor pressure of the water and reducing the freezing point.Noticeably, in terms of binding energies, it seemed that the other components of the gel played a more important role in environmental stability than EG, but the substantial presence of EG offered more hydrogen bonding interacting with water.The organohydrogel possesses excellent environmental stability (temperature tolerance and resistance to dryness), which facilitates the expansion of applications in harsh environments such as cold northern regions and dry tropical areas.

Applications
Next, we investigated the applications of organohydrogel ionic diodes as self-powered sensors in pressure imaging and tactile sensing.As exhibited in Figure 5A, the 5 × 5 pixel ionic diode array was tested by encapsulating it with poly(dimethylsiloxane) (PDMS) and the pressure imaging application was demonstrated by placing colored polystyrene foam letters "Huazhong University of Science and Technology (HUST)" on the ionic diode array.The ionic diode self-powered sensor array displayed the same signal image as the original object by applying a slight pressure (∼0.5 kPa) to the letters.Furthermore, the prepared devices were attached to the fingertips, allowing for the detection and recognition of finger gripping motions (Figure 5B).The 250 and 500 mL beakers were gripped by volunteers (both containing the identical volume of solution), and the electrical signals associated with each finger were measured.The results suggested notable variations in the time-domain signal values across different fingers (Figure S31).And gripping the beaker of 250 mL displayed in a fainter signal response, possibly because of the smaller size of the beaker and the decreased strength required for gripping.Hence, the magnitude of the signal could serve as an indicator for estimating the size of a beaker.Figure 5C presents that the pattern analogous to the letter "T" was plotted on the ionic diode array by a pen tip.In addition, an image similar to the original letter graphic was visualized in the corresponding electrical signal plot.The result demonstrated that the ionic diode self-powered sensor array can be suitable for applications in the analysis of the spatial movement of objects.Moreover, Figure 5D presents the variations of output voltage sensed at every pixel as the central pixel was subjected to normal touch (∼1 kPa), which suggested that organohydrogel ionic diodes held great potential for tactile energy harvesting and self-powered sensing.

Moisture energy harvesting and self-powered sensing performance
The prepared organohydrogel ionic diode can be employed not only for the mechanical energy harvesting but also for the moisture energy harvesting.Both PDACl and PSSNa organohydrogels were properly dried to improve the responsiveness of the organohydrogel ionic diode in humidity.Additionally, in order to dissociate more ions in the presence of moisture, the concentrations of PDACl and PSS in the organohydrogels were increased to 8% and 10%, respectively.The electrode we used was also not changed, and the SEM images of the carbon cloth electrode revealed the porous structure (Figure S32), which facilitated the entrance of moisture.Owing to the strong hydrophilicity of the polymer network and the hygroscopic EG medium in the gel (Figure S30), the organohydrogels are highly sensitive to the moisture.When exposed to the moisture, the hygroscopically charged organohydrogels will rapidly absorb water from the environment and the polymer chains will gradually hydrate and dissociate the counterions (e.g., positive Na + from PSSNa and negative Cl − from PDACl), and the spontaneous diffusion of free dual ions of opposite polarity will form the needed ion concentration difference for power generation (Figure 6A and Figure S33).The mechanism for the generation of electricity from moisture is similar to that for the power generation under pressure stimulation.Variations in humidity induce the hydration/dehydration process in the organohydrogel ionic diode followed by the changes of ionic gradients, which ultimately are converted into the output of electrical energy.As displayed in Figure 6B, it could be noticed that a gradual increase in the voltage of the organohydrogel ionic diode when exposed to the moisture, which was stable within approximately 15 min.In addition, the maximum output voltage of the ionic diode increased with the gradual increase in humidity, which was ascribed to the enhanced dissociation and diffusion of the counterions at high humidity levels. 55,56The humidity sensitivity of the self-powered organohydrogel ionic diode was ∼0.93 mV/RH% at low humidities (<90%), increasing to ∼1.35 mV/RH% as the humidity exceeded 90% (Figure 6C).
As presented in Figure 6D, the output voltage could revert to its original state once the ionic diode was kept in an extremely dry environment with a RH of 20% for ∼18 min.In addition, the output electrical signals that followed the variation of humidity could be repeated consecutively.Moreover, the cycling stability of the device in terms of moisture energy harvesting was explored.As shown in Figure S34, after 1000 cycles of water adsorption and desorption, the output voltage of the ionic diode can still maintain over 96% of its initial value, indicating that the ionic diode exhibited excellent stability and cycling performance in humidity energy harvesting.A linear correlation of the output voltage with the seriesconnected ionic diode was shown in Figure 6E, which suggested the superior scaling capability of the organohydrogel ionic diode.Benefiting from the outstanding and stable moisture-electric performance, the ionic diode can be employed as self-powered humidity sensors for the corresponding application scenarios.When integrated with a normal mask, the ionic diode could be applied for real-time monitoring of human breathing based on the large amount of moisture released during the breath of body (Figure 6F and Movie S1).The variations in output voltage can reveal features of human breath such as breathing intensity and frequency, which can be employed as a method of diagnosing human health conditions.More interestingly, there are some distinctions between mouth breathing and nose breathing.As illustrated in Figure 6G, the output voltage values are greater for mouth breathing, mainly due to the greater humidity in the mouth compared with the nasal cavity.Furthermore, owing to the existence of sweat glands in the body, the surface of the natural skin can become moist, which will alter the humidity level of the surrounding environment. 57As displayed in Figure 6H, S35, and Movie S2, as a finger moved gradually toward the surface of the self-powered device, the output voltage of the ionic diode exhibited a corresponding progressive increase.Therefore, the organohydrogel ionic diode possessed the capability of perceiving the distance from it to the skin.The ionic diode sensor array could sense the 3D signal distribution of the humidity surrounding the finger.As the surface of the 3 × 3 self-powered sensor array was ∼2 mm away from the finger, the obtained corresponding humidity response distribution is presented in Figure 6I, with a maximum output voltage value of 8 mV.
In general, the pressure and humidity sensing capabilities cannot be cooperated with each other.However, the unique application that only be achieved by our dual-responsive self-powering sensor was demonstrated.Human skin integrates several kinds of sensors under the visible appearance, which helps us to sense almost everything in the daily life.When some organisms such as a human hand get closer to our skins, we can perceive changes in the surrounding environment, such as humidity.Furthermore, when it finally touches the skin, the microreceptors in the dermis with the function of mechanical stimuli sensing allow humans to feel the external pressure such as touching.These series of sensing are easy and common to humans.However, to any inanimate object, for example, robots, perceiving a hand approaching and touching itself is a challenging problem.Our dual-responsive ionic diode self-powering sensor has the potential to solve the above problem.Given the excellent dual-responsive sensing features of the ionic diode, we can employ it as an artificial skin for robotic hands to enable interactions with surroundings in both tactile and touchless modes.For example, the ionic diode was fixed to a robot finger to recognize the interaction between a human hand and a robot hand.The voltage variations in time for the artificial skin upon approaching a human finger, touching it, and then withdrawing from the finger were monitored by a peripheral circuit in real time.In addition, the robot manipulation system was connected to peripheral circuit.As the robot finger gradually approached and touched the human finger, two different response signals could be detected.As displayed in Figure S36a, as the robot finger moved gradually toward the human finger, the output voltage of the ionic diode exhibited a corresponding progressive increase.It was worth noting that the output signal value increased instantly when the robot finger touched the surface of the human finger, which was attributed to the further diffusion of ions when the ionic diode device was subjected to a slight pressure.Therefore, the robot could analyze the current interaction mode from real-time signals based on the response time and signal values recognized by the control system.The signal mode changes from touchless (slowly increased signal) to tactile (rapidly increased signal) working mode, intrinsically rendering discrimination between these two interactions unambiguously.
In addition, as the robot finger moved gradually toward and touched the wetted human finger, the output voltage value of the approach process increased significantly due to the increase in humidity (Figure S36b).Thus, the ion diode can help the robot to distinguish the degree of wetness of the contacting object.Besides, as the robot finger fixed with the ionic diode gradually approached and touched the finger of another robot, it could be found that no voltage signal was generated during the approach process due to the absence of humidity on the surface of the robot finger (Figure S36c).Moreover, the voltage generated by contact was greater than that generated by contact with the human finger, due to the fact that the robot finger is harder compared to the human skin and the ionic diode is subjected to more pressure when touched.The robot can recognize the present interaction modes from real-time signal analysis, without the need of providing the historical behaviors of signal changes.This distinct feature simplifies signal processing and thus speeds up systematic response time in future applications.Thus, our ionic diode self-powered sensors can help robots to simulate human socialization scenarios and distinguish between contact with a human or a robot, enabling human skinlike sensing functions.All in all, the organohydrogel ionic diode exhibited superior moisture-electricity conversion and humidity sensing performance, creating more possibilities for moisture-based self-powered flexible wearable electronics.

CONCLUSION
In summary, we have designed a pressure and moisture dual-responsive flexible environmentally tolerant energy harvesting device based on organohydrogel ionic diode that can efficiently convert ultralow-frequency mechanical energy and moisture energy into electricity.The ionic diode consisted of two organohydrogels with opposite charges, which exhibited entropy-induced depletion from moving ions and rectifying characteristics resembling a pn semiconductor junction.The applied external mechanical or moisture stimulus caused a rise in the built-in potential of the depletion zone, which converted mechanical energy or moisture energy into electrical energy.In addition, the mechanism was also supported by the finite element method simulations.MXene was introduced into the organohydrogel matrix to enhance the overall conductivity and charge collection of the ionic diode, resulting in superior electromechanical conversion performance.The ionic diode exhibited relatively large short-circuit current density (∼10.10 μA cm −2 ) and peak power density (∼0.10 μW cm −2 ) during each mechanical cycling under an ultralow frequency (0.01 Hz), exceeding most present mechanical energy harvesting devices.More importantly, the organohydrogel energy harvester displayed excellent temperature tolerance (−20-60 • C) and long-term environmental stability, which facilitated the expansion of applications in harsh environments.Furthermore, the ionic diode can also be employed as a moisture-driven power generator for spontaneous moist-electric generation.We have faith that this work will provide a promising direction toward the effective harvesting of renewable energy and the exploration of self-powered portable sensors.
All the above reagents were analytically pure and did not require any additional purification.

Synthesis of MXene (Ti 3 C 2 T x ) nanosheet solution
First, lithium fluoride (1.50 g) and hydrochloric acid (20.0 mL, 9.0 M) were mixed with stirring at low speed to react to generate the hydrofluoric acid.Next, Ti 3 AlC 2 (1.00 g) was slowly added to the abovementioned solution in several portions, and then, stirred for 24.0 h at 35 • C. Following complete etching, the mixed solution was repeatedly centrifuged and washed before reaching the pH value over 6.Lastly, the colloidal solution of few-layer or monolayer nanosheets was yielded by sonication for 2.0 h at a low temperature and Ar conditions and centrifugation for 1.0 h at 3500 rpm.

Fabrication of the nanocomposite organohydrogels
PDACl of 5 wt% was dissolved in deionized water with stirring over 1 h.Different amounts of MXene solution were subsequently added and sonicated for 20 min.Next, different volume percentages of EG were mixed into the abovementioned solution with stirring for 20 min.Afterward, agarose (2 wt%) was introduced to above solution and resulting mix was heated to 100 • C for 10 min.To avoid undesired oxidization of MXene at hot temperatures, above heating process was performed under a nitrogen atmosphere.Finally, the mixture was poured into the special mold and cooled at room temperature to form the gel.The PSS nanocomposite organohydrogel was synthesized in the same way as described above, except that 7 wt% of PSS was added to the mixed solution.In order to dissociate more ions in the presence of moisture, the concentrations of PDACl and PSS in the organohydrogels used for the moisture test were increased to 8 and 10 wt%, respectively.Both PDACl and PSSNa organohydrogels were dried over a period of 3.5 h under 65 • C for improving the responsiveness of the organohydrogel ionic diode in humidity.

Fabrication of the device
The device was assembled from two nanocomposite organohydrogels with a diameter of 1 cm, separated by a PTFE permeable membrane (1 μm pore diameter, Sterlitech Co.).In addition, the organohydrogels were inserted into the holes of PDMS spacers (diameter of 1 cm, thickness of ∼300 μm) and PDMS molds were manufactured utilizing the Sylgard 184 from Dow Corning.The signals of electricity were collected using the carbon cloth electrodes and connected to the digital multimeter via the copper wire.

Characterization and measurement
The SEM images and TEM images were obtained by highresolution SEM (FEI Nova NanoSEM 450, 10 kV) and TEM (Titan G2 60-300), respectively.XRD (PANalytical B.V. Empyrean) and AFM (AIST-NT) were employed to identify the structure.FT-IR (VERTEX 70), XPS (AXIS-ULTRA DLD-600 W), and Raman Spectroscope (Horiba Jobin Yvon Aramis, 532 nm laser) were performed to analyze the components.Impedance spectra of organohydrogels were measured using the electrochemical workstation (Chen Hua CHI660E).Contact angles of organohydrogels were tested by a contact angle measuring instrument (JC2000D).The anti-freezing capability of organohydrogel was tested utilizing the differential scanning calorimeter (Diamond DSC).XRD data, AFM data, DSC data, and SEM data were obtained in analysis and testing center of HUST.The rectification behavior of the ionic diode was analyzed by a Keithley 2450 sourcemeter.The electrical outputs of the prepared energy harvesting device were measured by Keithley DMM7510.The RH values were regulated using a saturated salt solution in a sealed container, for example, RH ∼99% (Na 2 HPO 4 ), RH ∼90% (KNO 3 ), RH ∼80% (NH 4 SO 4 ), RH ∼70% (KI), and RH ∼60% (NH 4 NO 3 ), and calibrated with a digital RH meter (AR847+, Shanghai Asahi Chang Industrial Equipment Co., Ltd.).

F I G U R E 1
Organohydrogel ionic diode design: (A) schematic illustration of the preparation process of organohydrogel ionic diode; (B) the operation mechanism of organohydrogel ionic diode; and (C) mechanical energy harvesting measurement of organohydrogel ionic diode for voltage and current density signals.

F I G U R E 3
Electric output performance and working principle of the organohydrogel ionic diode device.(A) I-V curves of the organohydrogel ionic diodes with varying ethylene glycol (EG) contents.(B) I-V curves of the organohydrogel ionic diodes with different MXene contents.(C and D) Mechanical energy harvesting measurement with a forward connection for voltage signals (C); and current density signals (D). (E) Schematic illustration of the principle of the organohydrogel ionic diode in the initial or mechanically released state.(F) Quantitative concentration profiles of Na + and Cl − in the organohydrogel ionic diode.(G) Space-charge density, and (H) potential profile within the organohydrogel ionic diode simulated in the initial or released state.(I) Schematic illustration of the principle of the organohydrogel ionic diode in the stressed state.(J) Quantitative concentration profiles of Na + and Cl − in the organohydrogel ionic diode in the stressed state.(K) Space-charge density, and (L) potential distribution within the organohydrogel ionic diode calculated in the stressed state.(M) Output voltages and output currents related to the parallel number of device units.(N) Output voltage and current generated at different applied pressures.(O) Output voltages and currents at different input mechanical frequencies.(P) Stability performance of the ionic diode device with 5000 cycles of output current at 0.01 Hz. performance of the organohydrogel ionic diode containing different contents of EG at room temperature (25 • C).

F I G U R E 4
Temperature tolerance and anti-drying performance of the organohydrogel ionic diode.(A) Differential scanning calorimetry spectra of gels containing different ethylene glycol (EG) contents with a cooling rate of 5 • C/min.(B) Conductivity variation of the poly(diallyldimethylammonium chloride) (PDACl) MXene organohydrogel from −20 to 60 • C. (C) Circuits involving PDACl MXene organohydrogel and PDACl MXene hydrogel in series with a green LED indicator after being frozen at −20 • C or heating at 60 • C. (D) Photographs of the temperature-tolerant behavior of PDACl MXene organohydrogel and PDACl MXene hydrogel after being frozen at −20 • C or heating at 60 • C. (E) Morphological change comparison of PDACl MXene organohydrogel and PDACl MXene hydrogel after storing for 15 days at 23.9 • C and 59% relative humidity (RH).(F) Weight variation of PDACl MXene gels with different EG contents during 15 days of storage at 23.9 • C and 59% RH.W 0 is the initial weight.W t is the weight at different storage time.(G) Illustration of the characteristics of organohydrogel ionic diode.(H) Output voltage signals of the organohydrogel ionic diode after storage in an open environment for various time.(I) Output current signals of the organohydrogel ionic diode at different temperatures.(J) Density functional theory (DFT) modeling of water (W) with W, EG, agar, and MXene.

F I G U R E 5
Applications of organohydrogel ionic diodes as self-powered sensors.(A) Pressure imaging display detection of Styrofoam letters via ionic diode self-powered sensor array.(B) Photographs of hand-holding beakers with various sizes and the corresponding distribution of electrical signals on the five fingers (the weight of the solution in the beaker is the same).(C) Drawing the letter "T" by a pen tip on the ionic diode self-powered sensor array and the corresponding pressure imaging.(D) Mapping of the voltage signals generated by pressing the central pixel of the ionic diode self-powered sensor array.

F I G U R E 6
Moisture energy harvesting and self-powered sensing performance of the organohydrogel ionic diode device.(A) Schematic representation of the organohydrogel ionic diode for the moisture energy harvesting.(B) Output voltage versus time of organohydrogel ionic diode under various relative humidity (RH) conditions.(C) The humidity sensitivity of the self-powered organohydrogel ionic diode.(D) The output voltage of the organohydrogel ionic diode with alternating humidity levels from 99% to 20%.(E) Plot of output voltage with different series numbers of organohydrogel ionic diode units.(F) Output voltage signals of organohydrogel ionic diode fixed to the mask at different breathing frequencies.(G) The humidity response and recovery curves of nose breathing and mouth breathing for volunteers.(H) Variation curve of output voltage in relation to the distance between the organohydrogel ionic diode and a finger.(I) Detection of a finger humid distribution on the organohydrogel ionic diode sensor arrays.
The authors acknowledge the funding support from the National Natural Science Foundation of China (NSFC: 51872106 [Nishuang Liu], NSFC: 11874025 [Yihua Gao]), and the Natural Science Foundation of Hubei Province, NSFHB: 2016CFB432 (Nishuang Liu).The material characterization tests were supported by Technology Analytical & Testing Center of Huazhong University of Science and Technology.The authors thank engineer Jun Su in the Center of Optoelectronic Micro & Nano Fabrication and Characterizing Facility, Wuhan National Laboratory for Optoelectronics of Huazhong University of Science and Technology for the support in SEM and TEM tests.C O N F L I C T O F I N T E R E S T S TAT E M E N T The authors declare no conflicts of interest.O R C I D Nishuang Liu https://orcid.org/0000-0002-2507-7229R E F E R E N C E S