Free-Standing Black Phosphorus Thin Films for Flexible Quasi-Solid

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Energy, Environmental, and Catalysis Applications

Free-standing black phosphorus thin films for flexible quasi-solid-state micro-supercapacitors with high volumetric power and energy density Jie Yang, Zhenghui Pan, Qiang Yu, Qichong Zhang, Xiaoyu Ding, Xinyao Shi, Yongcai Qiu, Kai Zhang, John Wang, and Yuegang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18172 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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ACS Applied Materials & Interfaces

Free-Standing Black Phosphorus Thin Films for Flexible Quasi-SolidState Micro-Supercapacitors with High Volumetric Power and Energy Density Jie Yang,†,‡ Zhenghui Pan,§,* Qiang Yu,† Qichong Zhang,† Xiaoyu Ding,†,‡ Xinyao Shi,† Yongcai Qiu,‖ Kai Zhang, †,‡,♯,* John Wang,§ and Yuegang Zhang†,♣,* †i-Lab,

Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou

215123, China ‡School

of Nano Technology and Nano Bionics, University of Science and Technology of China,

Hefei 230026, China §Department

of Materials Science and Engineering, National University of Singapore, 117574

Singapore, Singapore ‖College ♯CAS

of Environment and Energy, South China University of Technology, Guangzhou

Key Laboratory of Nano-Bio Interface & Key Lab of Nanodevices and Applications,

Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China ♣Department

of Physics, Tsinghua University, Beijing 100084, China

ACS Paragon Plus Environment

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ABSTRACT Micro-supercapacitors (micro-SCs) are significant micro-scale power sources and energy storage components for miniaturized electronic and flexible devices, where electrodes play the key role in determining their electrochemical performance. The efficient intercalation of ions between the stacking layers of 2D layered materials (2DLM) makes them great candidates as thin film electrodes in micro-SCs, where one can achieve much enhanced volumetric capacitance. However, a great challenge is to develop a high-yield production method for high quality 2DLM thin film electrodes. In this work, we have successfully reported a scalable fabrication process for free-standing black phosphorous (BP) thin films, derived from the high quality few-layer BP nanoflakes via a modified electrochemical exfoliation route, for flexible quasi-solid-state microSCs (QMSCs). The as-fabricated QMSCs exhibit an excellent stable electrochemical performance at a high scan rate up to 100 V s-1. More importantly, our QMSC device can not only achieve an outstanding energy density of 3.63 mW h cm-3, a remarkable power density of 10.1 W cm-3 and a superior cycle span (94.3% capacity retention even after 50000 cycles), but also deliver the excellent mechanical flexibility is demonstrated by 91.3% capacity retention after 500 mechanical bending cycles. More interesting, to meet the energy density and power density needs for various practical applications, multiple QMSCs can be successfully integrated in parallel or in series, which is demonstrated by lighting up of the red light emitting diode (LED). The BP-based QMSCs can be patterned on a single substrate with flexible photodetectors based the same BP thin film to form a self-powered optoelectronic system. KEYWORDS: micro-supercapacitors, energy storage, 2D layered materials, black phosphorous, thin film


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1. INTRODUCTION For wearable and portable electronic devices which are becoming increasingly pervasive in the modern society,1-5 there is a rapid growing demand on further miniaturization of thin, flexible and highly efficient micro-scale power sources.6-9 However, most of the commercially available micro-batteries suffer from several limitations, including low energy density, sluggish charge/discharge capability and a short cycle life span.10-12 In addition, it is a big challenge to effective integration of micro-batteries into electronic circuits, which limits the miniaturization of the entire system.13 Indeed, the scale down in power sources components in size makes them inadequately compatible with planar integrated devices on one-single substrate. By comparison, planar micro-supercapacitors (micro-SCs) are emerging as competitive alternatives or even replacement for micro-batteries, as they can offer superior power density, faster charging/discharging rates and much longer cycle life. However, the relatively low energy density of planar micro-supercapacitors hinders their further development.14-20 To this end, as a key component of micro-SC, there is urgent need to develop electrode materials in thin film forms with high volumetric capacitance (Cvol). Recently, it has been documented that an efficient intercalation of ions between the atomically stacking layers of 2D layered materials (2DLM), such as graphene,21-24 transition metal dichalcogenides (TMDs)20,

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and MXenes,26 can make them great candidates as the thin film

electrode materials in micro-SCs, where enhanced volumetric capacitances can be achieved. Such as, Gao et al. have developed that graphene thin films based micro-SCs which exhibit a volumetric capacitance of 3.1 F cm-3, where the thin film structure promotes the access of electronics/ions to the electrode surface.20 Inspired by this process, a high volumetric capacitance of 7.45 F cm-3 was achieved with micro-SCs made of MoS2 thin films.21 On the one hand, these


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micro-SCs have demonstrated much improved capacitance. On the other hand, there is plenty space for further advance in the overall performance of these micro-SCs, as some of these 2DLM thin films are not desirable in terms of delivering Cvol, which is proportional to the packing density (ρ) (Cvol = Cwt × ρ, Cwt is the gravimetric capacitance) of the material. Among the 2DLM thin films, MXenes represents an attractive choice for micro-SCs of high volumetric capacitance of 200 F cm-3 due to their ultra-high packing density (4.93 g cm-3), when properly controlled. However, there is complexity in manufacturing of MXenes, as well as high cost.26 Black phosphorous (BP), a newly emerging 2DLM, has shown several advantages as an electrode materials for SCs, because of their relatively good packing density of 2.69 g cm-3 (1.1 g cm-3 of graphene and 1.35 g cm-3 of MoS2), excellent electrical conductivity (300 S m-1) and fast ion/electrons diffusion properties.27, 28, 29 Furthermore, the unique puckered lamellae structure of BP crystal presents a large plane spacing of 5.3 Å between adjacent puckered layers (3.3 Å of graphite and 6.15 Å of MoS2), which can be beneficial for both high volumetric energy and power densities.27, 30-35 Liquid-exfoliated BP nanoflakes have been exploited for quasi-solid-state SCs (QSSCs), which deliver an excellent volumetric capacitance of 13.75 F cm-3 and exceptional cyclability (15.5% capacitance decay after 10000 cycles).27 However, when BP films are fabricated by drop-casting process, the dispersed nanoflakes can be largely damaged by the prolonged sonication in the liquid-exfoliated process. Therefore, the volumetric capacitance of QSSCs decreases rapidly with increasing charge/discharge rates. Herein, we develop a highly efficient and scalable fabrication process for free-standing BP thin films with outstanding mechanical flexibility, derived from the few-layer BP nanoflakes through a modified electrochemical exfoliation process, for mechanically flexible quasi-solidstate micro-SCs (QMSCs). Noted that the neutral aqueous solution (0.5 M Na2SO4) is employed


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instead of acid that has been commonly used as the electrolyte for electrochemical exfoliations (H2SO4, Tables S1), which is to benefit the formation of the high-quality and large-quantity of exfoliated BP nanoflakes.30, 35 The as-exfoliated BP nanoflakes show a relatively low degree of oxidation, and an intrinsic structure with lateral size as large as 90 μm. Significantly, the QMSCs derived exhibit a highly stable electrochemical performances including an excellent scan rate up to 100 V s-1, a high volumetric capacitance of 26.67 F cm-3 at 0.5 A cm-3, a volumetric energy density of 3.63 mW h cm-3 while maintaining a maximum power density of 10.1 W cm-3. They also show impressive long cycle stability with 94.3% capacity retention after 50000 cycles as well as above 91.3% capacity retention after 500 bending cycles. The mechanically flexible QMSCs are also demonstrated being able to withstand harsh bending and allow for multiple integrations, by lighting up a red light emitting diode (LED). The BP-based QMSCs can be patterned on a single substrate as flexible photodetectors to function as a self-powered optoelectronic system.


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2. RESULT AND DISSCUSSION

Figure 1. (a) Schematic diagram of the electrochemical exfoliation of bulk BP into few-layers BP nanoflakes. The bulk BP crystal is exfoliated in a neutral aqueous solution (0.5 M Na2SO4) by applying a DC voltage. (b) Photographs of the bulk BP crystal and the electrochemically exfoliated BP nanoflakes dispersed in DMF. (c) Schematic illustration of the proposed


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mechanism of electrochemical exfoliation. (d) Schematic diagram of the as-proposed mechanism of the modified electrochemical exfoliation. The desired free-standing BP films and electrochemical and photoelectric devices are made from the few-layers BP nanoflakes derived from a modified electrochemical exfoliation process, as has been described above. The experimental setup for the modified electrochemical exfoliation of bulk BP is shown in Figure 1(a). Typically, a bulk BP crystal (Figure 1b), a Pt wire, and a 0.5 M Na2SO4 aqueous solution were used as the working electrode, counter electrode, and electrolyte, respectively. Note that the neutral aqueous solution is employed instead of acid that has been commonly used as the electrolyte for electrochemical exfoliations (H2SO4, Tables S1). This is to benefit the formation of the high-quality and large-quantity of exfoliated BP nanoflakes. Moreover, the aqueous salt solution containing sulfate anions (SO42-) is known to give rise high exfoliation efficiency compared to other anions such as NO3- and ClO4-. The excellent exfoliation efficiency of sulfate salts is due to the reduction potential from SO42- to SO2 is as low as +0.20 V (+0.96 V for NO3- and+1.42 for ClO4-) (Scheme S1). The BP crystal begins to expand when a positive bias of +10 V was applied on BP electrode for 10 min, and then many BP nanoflakes dissociate and spread into the electrolyte as the time increased (Figure S1). A summary of the exfoliation voltages and electrolytes optimized in this paper is given in Table S1, together with the corresponding key results. Afterwards, the as-exfoliated BP nanoflakes were collected by centrifuging and then re-dispersed in dimethylformamide (DMF), which was seen stable for several weeks without obvious agglomeration (Figure 1c). Significantly, the exfoliation procedure can be readily scaled up depending on the size of the BP electrode used.


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The electrochemical exfoliation of bulk BP crystals into nanoflakes is explained as follows, as depicted in Figure 1(d). Firstly, applying a positive bias in a reduce of water at the working electrode produces hydroxyl (OH-) and oxygen radicals (O-) that assemble around the bulk BP crystal. The SO42-, OH- and O- radicals insert themselves between the BP layers, and then loosen the van der Waals interactions between the interlayers. Secondly, oxidation of the OH- and Oradicals and SO42- anions lead to the release of O2 and SO2, as evidenced by the vigorous gas evolution during the electrochemical process, which causes expansion of the interlayer distance of BP. Finally, BP nanoflakes are separated from the bulk BP crystal by the gushing gas and are then suspended in the electrolyte.


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Figure 2. XPS spectra of (a) bulk BP crystal and (b) as-exfoliated BP nanoflakes. (c) Raman spectrum of the as-exfoliated few-layer BP nanoflakes. (d) AFM image of an as-exfoliated BP nanoflake placed on a silicon wafer. (e) A typical low-magnification TEM image of the electrochemically exfoliated BP nanoflakes. (f) HRTEM image of a few-layer BP nanoflake (Inset: The corresponding electron diffraction pattern).


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As we know, BP materials are rather unstable in an open air environment, easily generating oxide layers on the surface. Thus, a similar supposition is whether the oxidation would be generated in the BP nanoflakes during electrochemical exfoliation. In order to verify this hypothesis, X-ray photoelectron spectroscopy (XPS) was carried out on both the as-exfoliated BP nanoflakes and the bulk BP. The XPS survey spectrum of the as-exfoliated BP nanoflakes shows no obvious changes compared to that of bulk BP, with featured P peaks detected in both samples (Figure S2). High-resolution P 2p XPS spectra enable a precise investigation of the oxidation state of P (Figure 2). As presented in Figure 2(a), the bulk BP shows a well-defined P 2p peak, which can be divided into P 2p3/2 (128.3 eV) and P 2p1/2(129.1 eV).30-32 While, the asexfoliated BP nanoflakes show a less intense P 2p signal, which could be ascribed to the oxide contribution (Figure 2b). Furthermore, the P ox peak occurred at about 132.5 eV is clearly more intense compared to that of the bulk BP, suggesting that the as-exfoliated BP nanoflakes are more likely to oxidation.33-34 However, the enhanced degree of oxidation in BP nanoflakes would not influence the fast diffusion of ions and efficient intercalation among the puckered interlayers. The increased degree of oxidation can inversely not only ameliorate the wettability of BP nanoflakes resulting in realizing of the free-standing BP films, but also improve the compatibility of BP with the electrolyte, and leading to an enhanced electrochemical performance.35 Raman spectra are used to provide further information on the microstructure of the asexfoliated BP nanoflakes. As shown in Figure 2(c), three characteristic bands located at about 361 cm-1, 439 cm-1 and 468 cm-1 can be detected, corresponding to the characteristic A1g, B2g, and A2g vibration modes of BP, respectively.30 This well proves that the as-exfoliated BP nanoflakes can retain the orthorhombic structure of BP after electrochemical exfoliation.30, 36 It is noted that the intensity of the characteristic bands decreases for the exfoliated BP nanoflakes


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compared to the BP bulk, indicating a reduced thickness (Figure S3a).34 In addition, the blueshift of the A2g band for the as-exfoliated sample also verify the reduced number of layers (Figure S3b).34 Atomic force microscopy (AFM) result further checks that most of the BP nanoflakes show a thickness of about 8 nm, a value of about three to four-layer thicknesses (Figure 2d).37 Interestingly, the AFM image confirms that the lateral size of the as-exfoliated thin BP nanoflake can be up to 50 μm, demonstrating the superiority of electrochemical exfoliation in producing large-sized flakes compared to other route, such as ultrasonication. The high quality of the few-layer BP nanoflakes was further evaluated by using transmission electron microscopy (TEM). As revealed in Figure 2(e), the as-exfoliated BP nanoflakes are not only very clean without visible impurities, but also look transparent under the electron beam irradiation, indicating the very thin nature of the BP nanoflakes. High-resolution TEM (HRTEM) image obviously presents the inter-planar spacing of 0.42, and 0.32 nm with the BP nanoflakes, corresponding to (001), and (021) planes of the orthorhombic BP (Figure 2f), respectively, which is proved by the X-ray diffraction results (Figure S4).37, 38


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Figure 3. Electrochemical performance of the flexible QMSC-electrochemical exfoliated (QMSC-EE) based on the free-standing BP thin films. (a, b) CV profiles of as-fabricated flexible QMSC-EE under distinct scan rates. (c) Discharging current densities function of scan rates for the QMSC-EE device. (d) GCD profiles of the flexible QMSC-EE at distinct current densities. (e) Capacitances versus distinct current densities for the QMSC-EE. (f) Energy densities and power densities of our QMSC-EE device with free-standing BP thin film compared with some reported SCs devices. As shown in Figure S5, to demonstrate the application potential of the electrochemical exfoliated BP nanoflakes in electrochemical energy-storage devices, we have fabricated the flexible QMSC devices based on the free-standing BP thin film, which was prepared via vacuum filtrating of the as-prepared BP nanoflake dispersion (the details can be observed in Figure S6, S7, S8 and the experiment section). For comparison, the BP nanoflakes by liquid exfoliation were also used to prepare the thin film electrodes for the assembly of QMSC.39 The devices fabricated with the electrochemical exfoliated BP nanoflakes and liquid exfoliated BP nanoflakes are denoted as QMSC-EE (QMSC-electrochemical exfoliated) and AMSC-LE (QMSC-liquid


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exfoliated), respectively. The cyclic voltammetry (CV) curves of the QMSCs at various scan rates were collected to evaluate their electrochemical performance, and are presented in Figure 3(a-b) and Figure S9. The nearly rectangular shapes of these CV curves demonstrate the rather ideal electrical double-layer capacitive (EDLC) behavior at scan rates from 0.2-5.0 V s-1 (Figure 3a). Significantly, the CV rectangular shape is well maintained at scan rates ranging from 10-100 V s-1 (Figure 3b), which indicates the exceptional reversibility and ultrafast charging/discharging capability of our QMSC-EE. A high linear dependence in the capacitive region can be obtained up to 50 V s-1, meaning an ultrahigh power capability for the QMSC-EE (Figure 3c)3. More importantly, our QMSC-EE can still work well even under a high charging/discharging at 100 V s-1, which is comparable to those of previously reported 2D LM based QMSCs, such as graphene-based QMSC (100 V s-1)40 and MXene-based QMSC (100 V s-1).27 The electrochemical performance of the our as-fabricated QMSC-EE was further assessed under different current densities ranging from 0.5 to 20 A cm-3 with the corresponding galvanostatic charge/discharge (GCD) curves almost linear and symmetric (Figure 3d), again suggesting the excellent capacitive behavior and good reaction reversibility of the QMSC-EE. Notably, the GCD profile just displays a tiny IR drop of 47 mV even at 20 A cm-3, implying that the device is of relativity low series resistance (RS) (Figure S10). As shown in Figure 3(e) and Figure S11, the QMSC-EE device exhibits a higher volumetric capacitance of 26.67 F cm-3 (an area capacitance of 40 mF cm-2) at 0.5 A cm-3 than that of QMSC-LE (16.4 F cm-3, 24.6 mF cm2).

This value is also high than most of those reported ones till now (Table S2). A comparison of

the volumetric power/energy densities of the QMSC-EE (based on the whole device volume including two micro-electrodes and gel electrolyte) with those of the previously reported QMSCs and some commercially available energy storage devices is presented in Figure 3(f) and Table


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S3. The highest volumetric energy density of 3.63 mW h cm-3 can be achieved at a power density of 0.247 W cm-3 which can remain at 1.53 mW h cm-3 even at a maximum power density of 10.1 W cm-3. Significantly, such stack volumetric energy density value is more than three-times higher than those of commercially available SCs (2.75 V/44 mF, 5.5 V/100 mF, < 1 mW h cm-3) and even comparable to the thin-film lithium ion battery (4 V/500 μA h, 1-5 mW h cm-3). The volumetric energy density is also superior to those in other reports13,19,21,24,41-46, for example, including QMSC-BP (2.47 mW h cm-3 at 0.009 W cm-3),27 QMSC-PEDOT (0.66 mW h cm-3 at 0.004 W cm-3),42 QMSC-graphene (1.29 mW h cm-3 at 0.003 W cm-3)24, QMSC-MVN@NC NWs (0.97 mW h cm-3 at 0.019 W cm-3)43-44 and QMSC-MnO2 (0.41 mW h cm-3 at 0.001 W cm3).19

The maximum gravimetric (areal) energy density of 2.59 mWh kg-1 (5.44 W h cm-2) at a

gravimetric power density of 0.17 W kg-1 (0.37 mW cm-2) is also achieved (Figure S12).


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Figure 4. (a) The cycling performance of the QMSC device as a function of cycle number. (b) CV profiles of the QMSC device at distinct bending angles (0.2 V s-1). (c) Capacitance retention of distinct bending cycles. CV profiles for a single QMSC-EE device and two QMSC devices integrated in series (d) and in parallel (e) at a scan rate of 0.2 V s-1. (f) Relationship between number of QMSC devices integrated and total device capacitance. (g-i) Photographs of two powered QMSC devices connected in series lighting on a red LED. For practical application, good capacitance retention of the QMSC device is also an important factor. As displayed in Figure 4(a) and Figure S13-15, the QMSC also exhibits superior cycle life and maintain a high 94.3% retention after 50000 charge/discharge cycles at a current density of 0.2 A cm-3. The capability to tolerate harsh bending is another important factor for the real application of QMSCs in flexible electronics.43 To this end, a series of mechanical flexibility


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experiments were performed on our QMSC devices. As shown in

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Figure 4(b), there are

insignificant changes in the CV curves under a scan rate of 0.2 V s-1 with different bending angles from 0° to 180o, indicating the outstanding electrochemical performances retention of our device. Furthermore, 91.3% capacity retention after bending at 90° for even more than 500 cycles (Figure 4c), again implying the great mechanical property of our device. Due to the needing for the high energy or power density in practical applications, the capability of scalable integration of our QMSC devices both in series and in parallels are systematically evaluated. As shown in Figure 4(d) and 4(e), the working window and output currents are almost doubled under the same discharge current when two QMSC devices were integrated in series or in parallels, respectively. The shapes of the voltage profiles are also completely maintained, demonstrating the outstanding stability of the as-connected devices (Figure S16). Moreover, multiple QMSC devices are connected in parallel by integrating a number of devices (up to 10) (Figure 4f and Figure S17). The overall performance of the thus-assembled QMSC devices improved linearly with the number of QMSC devices, implying the idea scalability. In addition, a practical application of our QMSC by powering LED was demonstrated. As can be seen, two full charged tandem QMSC devices can successfully light a red LED under both the normal and bending states after charging (under an angle of 180o, Figure 4g-4i).


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Figure 5. Performances of the free-standing BP thin films based flexible photodetector. (a) Schematic illustration of the fabrication procedure of the flexible photodetector based on the BP thin films derived from electrochemically exfoliated BP nanoflakes. The inset is the photograph of flexible BP photodetectors on PET substrates. (b) Schematic of our flexible QMSC-EE powered a flexible photodetector based on the same free-standing BP thin film on a single substrate. (c, d) The I-V curves and the photoresponse of the flexible photodetector powered by a fully charged QMSC-EE device. To further demonstrate the high quality and application potential of the electrochemically exfoliated BP nanoflakes for optoelectronic devices, a flexible photodetector made of the freestanding BP thin film was fabricated on a flexible PET substrate.42 The detail fabrication process was illustrated in Figure 5(a), where the BP thin film was first filtered on a PTFE substrate


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followed by a simple dry transfer procedure on a flexible PET substrate. The Cr/Au (10 nm/90 nm) electrodes were subsequently deposited on the BP film by using the interdigital electrodes as the shadow mask. The superior mechanical flexibility of our as-fabricated photodetector device was confirmed in the inset of Figure 5(a). More impressively, as another potential integration demonstration, the flexible photodetector were driven by a fully charged QMSC device

on

a

single substrate (Figure 5b). Figure 5(c) shows the typical I-V curves of the flexible photodetector under the dark and under laser illumination conditions. The idea linearity of the IV curves demonstrates the high quality of BP thin film and good contacts between metal electrodes and channel material. A clearly current increase is also noted with the light illuminating on the device. Figure 5(d) displays the time-dependent photocurrent of the device under illuminating. It is obviously that the device shows almost the same “on” and “off” current with cycles. No evident photo degradation is observed from the device, even under the largest light intensity ( ≈ 29.7 nW) that we can provide. Furthermore, the photodetector device also displays a high stability under 532 nm (Figure S18).

3. CONCLUSIONS In conclusion, we demonstrate that the novel modified electrochemical exfoliation of bulk BP is a hopeful process for the fabrication of few-layer BP nanoflakes in both high quality and scalable yield. For the first time, the free-standing BP thin films, derived from vacuum filtrating of the BP nanoflakes dispersion, are successfully used in making of the flexible all-solid-state micro-SCs. The as-fabricated QMSCs exhibit an excellent stable electrochemical performance at a high scan rate up to 100 V s-1. More importantly, Our QMSC device can not only achieve an outstanding energy density of 3.63 mW h cm-3, a remarkable power density of 10.1 W cm-3 and a superior


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cycle span with 94.3% retention even after 50000 cycles, but also deliver extraordinary flexibility such as above 91.3% capacity retention after 500 bending cycles. Furthermore, we have proved that the multiple QMSCs devices can be successfully connected in parallel or in series for meeting the energy density and power density needs in various practical applications, for example a red LED indicator and also being capable of integrating with a patterned BP thin film based flexible photodetector on a single substrate. These encouraging results make the BP thin film great promise for a wide range of penitential applications including flexible microelectromechanical storage systems and flexible optoelectronic devices.

4. EXPERIMENTAL SECTION 4.1 Electrochemical exfoliation of BP. The as-obtained bulk BP (1 cm in length) and a Pt wire were used as the working electrode and counter electrode, respectively. Electrochemical exfoliation was performed in a 0.5 M Na2SO4 solution by applying a positive DC bias of + 10 V for 0-1 h to exfoliate the BP crystal into nanoflakes. The current remained stable during the exfoliation procedure. The as-exfoliated nanoflakes were then collected by centrifuging, washed repeatedly with DI water. After drying, the as-obtained powders were dispersed in DMF. 4.2 Preparation of free-standing BP thin films and microelectrode. To make a freestanding BP thin film, the as-dispersed DMF solution was first vacuum-filtered via a porous membrane. After being dried in 70 oC for 24 h, the free-standing BP thin film is obtained by dissolving the membrane in acetone. The BP thin films microelectrode was made by a laser machining process (the detailed can be seen in Table S2). 4.3 Preparation of flexible QMSC made of BP thin films. Preparation of PVA/H3PO4 electrolyte was performed by our previous works3. Two microelectrodes were transferred onto a


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flexible PDMS substrate while applying mild pressure for firm contact. The PVA/H3PO4 gel electrolyte was then carefully drop casted onto the coplanar microelectrodes, and subsequently retained at 70 oC for 24 h to evaporate extra water in the as-prepared PVA/H3PO4 electrolyte. 4.4 Materials characterizations. Raman measurements were carried out in a Horiba HR Evolution. AFM was used to analyze the morphology of each sample, and a Thermo Scientific ESCALAB 250Xi X-ray photoelectron spectroscopy (XPS) was used to characterize the elemental and chemical nature. Transmission electron microscopy (TEM) studies were conducted with a JEOL JEM-2100F (field-emission) scanning transmission electron microscope operated at 200 kV, equipped with an Oxford INCA X-sight EDS Si (Li) detector. 4.5 Electrochemical measurements. Electrochemical performances of the as-prepared samples were evaluated using a CHI 760E electrochemical work station (CHI Instruments Inc.). The volumetric capacitance at different current densities was calculated by Cv = I (ΔV/Δt) (F cm3),

where I is the current density (A cm-3) and ΔV/Δt is the slope after drop at the beginning of

each discharge. The energy density was calculated using the equation of E = 1/2 CvV2 and the power density was calculated by P = E/t, where Cv is the volumetric capacitance, V is the working voltage and t is the discharge time.

ASSOCIATED CONTENT Supporting Information Available. Cross-section SEM image of a representative free-standing BP thin film. Summary of parameters optimization in as-exfoliation experiments in Table S1. XPS survey spectra of bulk BP crystal and as-exfoliated BP nanoflakes. Raman spectra of bulk BP crystal and as-exfoliated BP nanoflakes. XRD pattern of as-exfoliated BP nanoflakes. GCD curves of two QMSC-EE devices connected in series and in parallel. Performance of a flexible


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photodetector based on the free-standing BP thin films. Time-dependent photocurrent of the photodetector device with incident light of 532 nm. These materials are available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (Y.Z) *E-mail: [email protected] (K.Z) *E-mail: [email protected] (Z.P) Notes The authors declare no competing interests.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant No.11574349), the Natural Science Foundation of Jiangsu province (Grant Nos. BK20150365, BK20170424), the Key Research Program of Frontier Sciences of Chinese Academy of Sciences (QYZDB-SSW-SLH031), and the Hundred Talent Program of Chinese Academy of Sciences. Y. G. Zhang acknowledges the National Natural Science Foundation of China (No. 21433013 and No. U1832218). The technical support from the Vacuum Interconnected Nanotech Workstation (Nano-X) of Suzhou Institute of Nano-tech and Nano-bionics (SINANO), Chinese Academy of Sciences is also acknowledged.

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Free-Standing Black Phosphorus Thin Films for Flexible Quasi-Solid

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