In situ reduced MXene/AuNPs composite toward enhanced charging/discharging and specific capacitance

In this work, gold nanoparticles (AuNPs) decorated Ti3C2Tx nanosheets (MXene/AuNPs composite) are fabricated through a self-reduction reaction of Ti3C2Tx nanosheets with HAuCl4 aqueous solution. The obtained composite is characterized as AuNPs with the diameter of about 23 nm uniformly dispersing on Ti3C2Tx nanosheets without aggregation. The composite (MXene decorated on 4.8 wt% AuNPs) is further employed to construct supercapacitor for the first time with a higher specific capacitance of 278 F·g−1 at 5 mV·s−1 than that of pure Ti3C2Tx and 95% of cyclic stability after 10,000 cycles. Furthermore, MXene/AuNPs composite symmetric supercapacitor with filter paper as separator and H2SO4 as electrolyte, is assembled. The supercapacitor exhibits a high volumetric energy density of 8.82 Wh·L−1 at a power density of 264.6 W·L−1 and ultrafast-charging/discharging performance. It exhibits as a promising candidate applied in integrated and flexible supercapacitors.


Introduction 
On account of excellent cycle stability, high rate of charge/discharge, and high-power density, supercapacitors † Zixiang Zheng and Wei Wu contributed equally to this work. offer an abundance of active surface area for charge storage through electric double layer capacitance (EDLC) or fast surface redox reactions (pseudocapacitance) [4]. During the last decade, due to relatively better chemical stability, higher specific surface area and active surface sites, excellent hydrophilicity, and higher electrical conductivity, transition metal carbides and/or nitrides (MXene) as a novel family of 2D materials have been applied in different areas including energy storage, water desalination, catalysis, electromagnetic interference shielding, and transparent conductive films [5][6][7]. The general formula of MXenes is M n+1 X n T x (n = 1-3), where M represents a transition metal (such as Sc, Ti, and Zr), X is carbon and/or nitrogen, and T stands for the surface termination (i.e., hydroxyl, oxygen, or fluorine) [5]. The earliest explored and the most widely applied MXene is Ti 3 C 2 T x which shows exceptional performances as a potential electrode material for supercapacitors [8].
However, due to the presence of van der Waals force, almost all of 2D materials, including MXene, have the inevitable problems of aggregation and self-restacking [9]. This problem prevents electrolyte penetration into layers, limits the ion transport, and reduces the active site in supercapacitors. Therefore the restacking has become a huge obstacle to ameliorate the performance of MXene [10]. In order to solve the problem, various interlayer spacers are introduced into 2D materials to prevent the stacking of them, which can accelerate electrolyte penetration and ion diffusion [11,12]. These include carbon nanomaterials such as carbon nanotubes (CNTs) [13] and graphene [14,15], conductive polymers such as polypyrrole (PPy) [16], poly(3,4-ethylenedioxythiophene) (PEDOT) [17], polyaniline (PANI) [18], and metal nanoparticles such as silver nanoparticles (AgNPs) [19] and gold nanoparticles (AuNPs) [11]. In particular, since AuNPs is one of the highest conductivity metals with large specific surface area and high stability at the nanoscale level, it can be available for high performance energy storage devices [20]. To the best of our ability, there are still no reports about Ti 3 C 2 T x nanosheets modified by AuNPs to improve the electrochemical properties.
In this work, AuNPs decorated Ti 3 C 2 T x nanosheets were fabricated through a self-reduction reaction of Ti 3 C 2 T x with HAuCl 4 aqueous solution. By vacuum filtration of the mixture solution, freestanding composite films were fabricated. Then MXene/AuNPs composite was used as electrode material of supercapacitors for the first time. MXene/AuNPs electrodes showed significantly enhanced electrochemical property compared to the pure Ti 3 C 2 T x electrodes. The integrated device can power a red light emitting diode (LED), demonstrating its energy storage capacity.

2 Preparation of MXene/AuNPs composite
In view of the preparation of Ti 3 C 2 T x , minimally intensive layer delamination (MILD) method was used [21]. Briefly, 12 M HCl of 40 mL solution and LiF of 2 g were mixed with stirring of 30 min. After the powder was completely dissolved, Ti 3 AlC 2 powders of 2 g were mixed slowly with the LiF-HCl solution. The as-prepared solution was stirred with 500 rpm for 24 h at 35 ℃. The suspension was completely transferred to a centrifuge tube. Then the etched powder was washed using deionized water to ensure the pH value of the solution higher than 6. Each wash process included 2500 rpm for 3 min centrifugation and 2-4 min hand shaking. The as-prepared multilayer Ti 3 C 2 T x powder was dispersed in 100 mL deionized water by sonicating for 1 h and centrifuging at 3500 rpm to obtain the Ti 3 C 2 T x colloidal solution. Finally, Ti 3 C 2 T x film was obtained by vacuum filtered on a cellulose membrane filter.

4 Electrochemical measurements
CHI 660E electrochemical workstation was used in all electrochemical measurements. In a three-electrode system, electrochemical properties of Ti 3 C 2 T x and MXene/AuNPs electrode were studied by cyclic voltammetry (CV), galvanostatic charging/discharging (GCD), and electrochemical impedance spectroscopy (EIS) in 1 M H 2 SO 4 electrolyte. Ag/AgCl was served as reference electrodes, Pt sheet was used as the counter electrode, and Ti 3 C 2 T x and MXene/AuNPs were served as the working electrode. A symmetric supercapacitor was sandwiched in a coin cell with MXene/AuNPs composite film (Φ16 mm in diameter) as electrode, filter paper (Φ20 mm in diameter) as separator, and 1 M H 2 SO 4 as electrolyte.

1 Characterization and preparation mechanism
Fabrication of MXene/AuNPs composite film is shown in Fig. 1. By selective etching of Al layer of Ti 3 AlC 2 powders, multilayer Ti 3 C 2 T x powders are obtained. Then, multilayer Ti 3 C 2 T x powders are sonicated into Ti 3 C 2 T x nanosheets. There are functional groups on the surface of Ti 3 C 2 T x nanosheets, such as -F, -O, and -OH. These functional groups make Ti 3 C 2 T x nanosheets have a dispensability in aqueous solutions. Finally, adding the HAuCl 4 solution into Ti 3 C 2 T x nanosheet dispersion, the composite material will be formed due to the reduction reaction between AuCl 4 and the functional groups on the surface of Ti 3 C 2 T x nanosheets. Additionally, this film obtained by vacuum filtration can be easily wrapped around a glass bar and retain its integrity, as showed in Fig. 1, indicating good flexibility.
From Fig. 2, the high diffraction peak at 6.9° for Ti 3 C 2 T x corresponds to the (002) facet, suggesting the existing of the Ti 3 C 2 T x phase. For the MXene/AuNPs composite film, the (002) peak positions of Ti 3 C 2 T x do not vary, implying the presence of MXene and no Au embedded into the lattice of MXene [5]. The four diffraction peaks, as shown in Fig. 2, at 38.1°, 44.4°, 64.5°, and 77.5°, correspond to (111), (200), (220), and (311) planes of AuNPs, respectively. As HAuCl 4 content increasing, the four diffraction peaks become stronger and sharper, implying that the content of AuNPs decorated on Ti 3 C 2 T x is increasing.  In view of the morphology of the pure Ti 3 C 2 T x film ( Fig. S1 in the Electronic Supplementary Material (ESM)), the surface is crumpled (Fig. S1(a) in the ESM). The Ti 3 C 2 T x nanosheets have analogous wrinkles and high transparency, which are similar to graphene features ( Fig. S1(b) in the ESM). The high resolution TEM (HRTEM) image shows that the fringe spacing of 0.253 nm corresponds to the (002) lattice plane of Ti 3 C 2 T x (Fig. S1(c) in the ESM). The selected area electron diffraction (SAED) pattern ( Fig. S1(d) in the ESM) shows characteristic diffraction rings corresponding to the (002) planes of Ti 3 C 2 T x , clearly revealing the polycrystal of Ti 3 C 2 T x . As shown in Figs. 3(a) and 3(b), MXene/AuNPs composite film exhibits similar morphologies of pure Ti 3 C 2 T x film, but the surface is relatively rougher. Figures 3(b) and 3(d) reveal a uniform distribution of AuNPs on Ti 3 C 2 T x nanosheets with the average particle size about 23 nm (Fig. 3(e)). The film exhibits crumpled morphology stacked by layers ( Fig. 3(c)). And the intercalation of AuNPs has little effect to the thickness of MXene films. As shown in Fig. S2 in the ESM, the thickness of MXene films with different amounts of AuNPs at the same areal weight loading is almost 6.6 μm. As shown in Fig. 3(e), the HRTEM image shows that the fringe spacings of 0.235 and 0.203 nm correspond to the (111) and (200) lattice planes of Au, respectively. The SAED pattern ( Fig. 3(f)) shows characteristic diffraction rings corresponding to the (200) and (220) planes of Au. Figure S3 and Table S1 in the ESM show the EDS of MXene/AuNPs composite, indicating that AuNPs are decorated on the surface of Ti 3 C 2 T x nanosheets after HAuCl 4 treatment in the absence of reducing agents and under the ambient condition. As shown in Fig. S4 in the ESM, the elemental mapping images indicate the even distribution of AuNPs.
The full XPS spectra of Ti 3 C 2 T x and MXene/AuNPs are shown in Fig. 4(a). The presence of MXene together with -F, -Cl, -OH, and -O groups can be proved by observing the common peaks of Ti 2p, C 1s, F 1s, Cl 2p, and O 1s from 0 to 800 eV. The C 1s spectra of Ti 3 C 2 T x and MXene/AuNPs both have four characteristic peaks locating at 288.9, 286.4, 284.8, and 282.1 eV, as shown in Figs. 4(b) and 4(f), which correspond to the groups of O-C=O, C-OH, C-C, and C-Ti-O, respectively [22,23]. As shown in Figs. 4(c) and 4(g), the Ti 2p spectra of Ti 3 C 2 T x and MXene/ AuNPs are both indexed with three characteristic peaks locating at 454.5, 455.9, and 457.4, corresponding to tetravalent Ti-C, Ti-X (TiC x , x < 1), and Ti x O y , but there is a characteristic peak locating at 458.8 eV in the Ti 2p spectra of MXene/AuNPs corresponding to TiO 2 , indicating the slight oxidation of Ti 3 C 2 T x after mixing with HAuCl 4 [24][25][26].   From above results, the AuNPs are assembled and decorated on the surfaces of Ti 3 C 2 T x nanosheets evenly. It should be pointed out that no additional reducing agent was added in this work. Cheng et al. [27] have demonstrated that MXenes with -OH terminations can reduce noble metal ions into zero-valent metals. Since the surfaces of Ti 3 C 2 T x nanosheets are covered by -F, -O, and -OH, AuCl 4 can be directly reduced by -OH and formed AuNPs [28]. AuCl 3e Au 4Cl which is consistent with reduction in O content calculated above. The AuNPs and Ti 3 C 2 T x nanosheets are combined into a structure of conducting network, which depicts the pronounced contributions of AuNPs to the enhancement of conductivity. This structure contributes to the electrolyte penetration, the ion transport, and the active site increase in electrochemical reaction. Figure 5(a) shows the CV curves of Ti 3 C 2 T x and MXene/AuNPs electrodes at 5 mV·s -1 in 1 M H 2 SO 4 solution. Due to the contribution of pseudocapacitance derived from the varied valence of transition metal Ti atoms, CV curves of Ti 3 C 2 T x and MXene/AuNPs film exhibit deformed rectangle shape. According to the CV curves of Ti 3 C 2 T x , MXene/AuNPs-1, MXene/AuNPs-2, and MXene/AuNPs-3 electrodes, the specific capacitance is calculated as 228, 234, 278, and 250 F·g -1 , respectively. MXene/AuNP-2 composite electrode shows the largest CV area, which is about 1.2 times larger than that of Ti 3 C 2 T x electrode, indicating it possesses better electrochemical capacitive performance by wedging AuNPs. Figure 5(b) presents CV curves of MXene/AuNPs-2 varying from 5 to 100 mV·s -1 . The shape deformation of CV curves becomes heavier with the scan rate increasing. This phenomenon is possibly caused by slow ion response at high rates. GCD measurement was performed to investigate electrochemical performance of MXene/AuNPs-2 at current density ranging from 1 to 10 A·g -1 as shown in Fig. 5(c). GCD curves of MXene/AuNPs are almost symmetric and linear triangles, indicating that MXene/AuNPs has reversible charge/discharge process and good capacitance behavior. The inset of Fig. 5  capacitance decreases with the excessive addition of HAuCl 4 solution. The Nyquist plots shown in Fig. 5(e) are composed of two regions with a semicircle at the high-frequency part and an inclined line at the low-frequency part. The intercept of the x-axis represents the equivalent series resistance, which is related to the resistance of the electrode [29,30]. The equivalent series resistance of the MXene/AuNPs-2 (1.4 Ω) shows the lowest resistance value compared to those of the Ti 3 C 2 T x (2.5 Ω), MXene/AuNPs-1 (1.5 Ω), and MXene/AuNPs-3 (1.4 Ω) electrodes. The semicircle represents the charge transfer resistance (R ct ) at the electrode−electrolyte interface, which is 0.008, 0.007, 0.006, and 0.008 Ω for Ti 3 C 2 T x , MXene/AuNPs-1, MXene/AuNPs-2, and MXene/AuNPs-3, respectively. The Warburg impedance stemming from the slope of the curve reflects the ion diffusion of the electrode [31], which is 8.7, 8.5, 11.1, and 8.3 S 1/2 for Ti 3 C 2 T x , MXene/AuNPs-1, MXene/AuNPs-2, and MXene/ AuNPs-3, respectively. Among them, the MXene/ AuNPs-2 electrode has the smallest R ct and the highest Warburg impedance, indicating excellent ion diffusion capabilities. The mechanism of MXene/AuNPs-2 with the best electrochemical performance can be mainly attributed to the following points: i) Compared with MXene/AuNPs-1 (AuNPs of ~18 nm in diameter and 0.43 at%), MXene/AuNPs-2 (AuNPs of ~23 nm in diameter and 0.95 at%) with larger spacing of MXene nanosheets possesses higher conductivity for electronic, better diffusion for electrolyte and ions, and larger specific surface area for active sites ( Fig. S5 and Table S1 in the ESM); ii) compared with MXene/AuNPs-3, as the increasing of content of Au (1.74 at%), AuNPs will aggregate and grow up (~50 nm), which making the spacing of MXene nanosheets too large. This is harmful to the conductivity of MXene film (Fig. S5(d) in the ESM) [32]. Therefore, compared with MXene/AuNPs-1 and MXene/AuNPs-3, MXene/AuNPs-2 has the best electrochemical performance. The cyclic stability of MXene/AuNPs-2 electrodes is shown in Fig. 5(f). Notably, at 50 mV·s -1 , it exhibits excellent stability and the specific capacitance of MXene/AuNPs-2 still remains 95.0% after 10,000 cycles. The high stability and good electrochemical performance of MXene/AuNPs composite electrode could be mainly derived from three aspects: (1) The intrinsic properties of MXene and gold. MXene possesses both excellent EDLC and pseudo-capacitance. Gold has excellent stability and conductivity. (2) As shown in Fig. 6(a), the AuNPs as the spacer prevents selfstacking between Ti 3 C 2 T x nanosheets and lead to more space between the tightly stacked nanosheets, which significantly enlarge specific surface area of the composite material [33]. The BET results of Ti 3 C 2 T x and MXene/AuNPs, as shown Fig. 6(b), reveal that the specific surface area of MXene/AuNPs is about 1.8 times larger than that of Ti 3 C 2 T x . The larger specific surface area provides more active sites, improving the electrochemical performance. (3) The AuNPs as the spacer prevents self-stacking of Ti 3 C 2 T x nanosheets, which is beneficial for electrolyte penetration and the transport of ions [11]. As shown in Fig. 5(e), the smaller solution resistance (R s ) of MXene/AuNPs implies MXene/ AuNPs have faster ion response and lower inherent resistance. (4) The conductivity of MXene/AuNPs composite is significantly improved due to AuNPs' excellent conductivity and the structure of conducting network between the AuNPs and Ti 3 C 2 T x nanosheets [33]. As shown in Fig. S6 in the ESM, the conductivity of MXene/AuNPs is 1.67 times higher than that of Ti 3 C 2 T x . Compared with the performances of the various MXene-based supercapacitors in Table 1, the specific capacitance of MXene/AuNPs with an electrolyte of 1 M H 2 SO 4 in this work is calculated to be 278 F·g -1 (5 mV·s -1 ). This value is 1.2 times higher than that of MXene (i.e., 238 F·g -1 ) and is better than that of Ti 3 C 2 T x -Cl [34], Ti 3 C 2 T x clay [35], and Ti 3 C 2 T x -N 2 H 4 [36].

3 Electrochemical performance of MXene/AuNPs symmetric supercapacitor
To evaluate the performance of MXene/AuNPs-2 as supercapacitor, the CV, GCD, and EIS were studied. In the working potential window of 0.0-0.6 V with the scan rates ranging from 5 to 100 mV·s -1 , as shown in Fig. 7(a), CV curves are almost rectangular, demonstrating its ideal characteristic of electrical double-layer capacitor and excellent rate capability. At current density ranging from 1 to 10 A·g -1 , the GCD curves demonstrate a good capacitive performance due to its good linear potential-time profiles as well as nearly symmetrical with their discharging counterpart ( Fig. 7(b)). There is no obvious semicircular in the high-frequency Nyquist diagram (Fig. 7(c)), indicating the low R ct and the good electrical conductivity of the composite. The cyclic stability of MXene/AuNPs-2 electrodes is shown in Fig. 7(d). The specific capacitance of symmetric supercapacitor remains 95.0% after 10,000 cycles at 50 mV·s -1 , which exhibits excellent stability. As shown in Fig. S3 in the ESM with the increase of the scanning rate, the gravimetric capacitance obviously decreases, which is due to slow ion transport rate. By calculation, the gravimetric capacitance is 213.8, 196.1, 186.4, 160.9, and 141.1 F·g -1 from 100 to 5 mV·s -1 , which can be also found in Fig. S7 in the ESM. Figure 7(e) shows the comparation of the volumetric energy density and power density of MXene/AuNPs with other supercapacitors reported in the literature [44][45][46][47]. Calculated from the data in Fig. 7(a), the volumetric power density and energy density at 5, 10, 20, 50, and 100 mV·s -1 are calculated to be 264. 6 [46], and Ti 3 C 2 T x paper (18.5 Wh·L -1 , 240 W·L -1 ) [47], the energy density of MXene/AuNPs in this work is comparable to these MXene-based symmetric or asymmetric supercapacitor devices while the power density is higher than those. (Table S2 in the ESM). As shown in Fig. S8 in the ESM, the time constant is calculated as 5 s. Leakage current of the sueprcapacitor has been recorded at 0.6 V. Leakage current diminishes significantly to 0.049 mA within few seconds and then it further reduces to its low value of 0.023 mA. We performed self-discharge test of the sueprcapacitor for 2 h. Figure S9 in the ESM displays the good capability of 0.32 V after 2 h self-discharge test. In addition, a red LED (1 W, 2.6-2.8 V, 350 mA) is successfully powered by three prepared symmetric supercapacitors in series, as shown in Fig. 7(f), convincingly demonstrating the energy storage capacity of MXene/AuNPs symmetric supercapacitor.

Conclusions
In summary, MXene/AuNPs composite is fabricated with AuNPs evenly distributed on the surface of Ti 3 C 2 T x nanosheets. The obtained AuNPs are directly reduced from AuCl 4 by the groups of -OH on the surface of Ti 3 C 2 T x nanosheets. The AuNPs and Ti 3 C 2 T x nanosheets are combined into a structure of conducting network, which contributes to rapid electron transfer in electrochemical reactions. Composite electrodes (MXene decorated on 4.8 wt% AuNPs) show enhanced charge storage ability with a capacitance of 278 F·g -1 at 5 mV·s -1 . The cyclic stability reaches 95.0% after 10,000 cycles. Furthermore, a MXene/AuNPs symmetric supercapacitor with filter paper as separator and H 2 SO 4 as electrolyte, exhibits a high volumetric energy density of 8.82 Wh·L -1 at a power density of 264.6 W·L -1 . The integrated device can power a red LED demonstrating its energy storage capacity.