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Self-Reduction Synthesis of New MXene/Ag Composites with Unexpected Electrocatalytic Activity Zhiwei Zhang, Hanning Li, Guodong Zou, Carlos Fernandez, Jie Hu, Baozhong Liu, Qingrui Zhang, and Qiuming Peng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b01698 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 27, 2016

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ACS Sustainable Chemistry & Engineering

SelfSelf-Reduction Synthesis of New MXene/Ag Composites with Unexpected Electrocatalytic Activity Zhiwei Zhang1, Hanning Li1, Guodong Zou1, Carlos Fernandez2, Baozhong Liu3，Qingrui Zhang*,4, Jie Hu4, Qiuming Peng*,1, 1

State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Hebei

Street 438, Qinhuangdao 066004, P.R. China. 2

School of Pharmacy and Life sciences, Rober Gordon University, Aberdeen, AB107GJ, United

Kingdom 3

School of Materials Science and Engineering, Henan Polytechnic University, Century Street 2001,

Jiaozuo 454000, P.R. China 4

Hebei Key Laboratory of Applied Chemistry, School of Environmental and Chemical Engineering,

Yanshan University, Hebei Street 438, Qinhuangdao 066004, P.R. China

*Corresponding emails: [email protected], [email protected]

1

ACS Paragon Plus Environment


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Abstract Differing from graphene, the activated groups on the surface of layered two dimensional titanium carbide (MXene) materials bestow superiority to self-assemble some novel MXene derivatives with intriguing chemical/physical properties. Here we firstly report a series of new MXene-Ag composites by directly mixing AgNO3 and alkalization-intercalated MXene (alk-MXene, Ti3C2(OH/ONa)2) solution containing polyvinylpyrrolidone (PVP) at room temperature, in which an analogous urchin-shaped MXene-Ag0.9Ti0.1 bimetallic nanowire composite exhibits unexpected electro-catalytic activity for oxygen reduction reaction. The addition of PVP solution induces to form five-fold nanotwin Ag seeds, and then they grow into Ag/Ti (Ag0.9Ti0.1) bimetallic nanowires. The unique bimetallic nanowires favor four electron transfer process, and exhibit high current density and good stability by offering numerous oxygen adsorption sites and shortening diffusion path of adsorbed oxygen. The results represent a new step for the electrocatalytic applications of MXene materials, and also motivate enthusiasm in the quest for new MXene derivations.

Keywords: MXene, Nanowires, Oxygen reduction reaction, Bimetal

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1. Introduction The precious metal platinum (Pt) catalysts are suffering from some puddles owing to their high cost, dissolution and aggregation [1]. Thus to cut down the amount of Pt or to develop new catalysts becomes a crucial issue. Silver (Ag) is believed as a potential candidate to substitute Pt in cathode catalysts for fuel cells. Except for its abundance and low cost, Ag has relatively high oxygen reduction reaction (ORR) activity and stability in alkaline electrolyte [2]. Hence some Ag nanoparticles, such as nanocubes, nanowires and nanorods have been synthesized, and show electrocatalytic activity toward ORR [3, 4]. However, their performances have been reported to be low [5]. One of promising strategies for further increasing catalytic properties is to prepare a more conductive supporter coupled with some effective bimetallic catalysts [6]. A family of two-dimensional materials-transition metal carbides and carbonitrides-known as MXene (Ti3C2(OH/F)2), were discovered by the exfoliating “A” laminar component of the MAX matrix phase in HF solutions under mild conditions [7]. The termed MAX phase reflects the chemical composition of the designated compound family, i.e., Mn+1AXn (n=1,2,3), M represents an early transition metal; A assigned to the elements of Group IIIA or IVA and X refers to the C or N component [8, 9]. Especially, a mild Al-removing method by HCl and LiF laid a foundation to potentially apply these 2D layered MXene materials in batteries, photo-catalysis and supercapacitors [10-12]. Note that the surface of MXene after Al-removing is terminated by Ti-OH or Ti-F bonding. These groups not only provide direct ion exchange sites for selectively removing toxic metallic ions [13, 14], but also serve as an effective reductant to certain metallic ions, such as Mn (VII), Cr(VI) and Fe(III) [15]. However, compared with the enthusiasm on the role of MXene as a supporter in supercapacitors or batteries, the reductive role has hardly been concerned. Providing that both 3



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roles on supporter and reductant can be utilized simultaneously, some new MXene-metal composites might be achieved, which will be used in the electrode of fuel cells. These materials have at least two-fold merits. The layered body structure improves chemical properties of nanosized particles by inhibiting aggregation and increasing electron transfer. In turn, the formation of nanoscale metal particles on the surface of MXene provides a great opportunity to improve the conductivity of MXene, increasing the performance in the applications of fuel cells. Herein, we synthesized a series of MXene/Ag composites by direct reduction of an alkalization-intercalated MXene (alk-MXene, Ti3C2(OH/ONa)2) containing PVP for the first time. Differing from the precursor of as-obtained MXene, the Ti-doping Ag nanowire, instead of nanosized Ag particles [16], will be achieved in the presence of PVP solution. The alk-MXene acts as a supporter coupled with a reductant during reaction process. This new composite composed of layered MXene template and Ag0.9Ti0.1 nanowire composite (MXene/NW-Ag0.9Ti0.1) exhibits unexpected electro-catalytic activity for ORR, of which the onset potential and half-wave potential are higher than those of the commercial 20 wt.% Ag/C and Ag-based catalysts reported previously.

2. Experimental section 2.1 Synthesis of alk-MXene. The Ti3AlC2 powders were made by mixing elemental titanium (Alfa Aesar, Ward Hill, USA, 99 wt.% purity, particle size < 40 µm ), aluminum (Alfa Aesar, Ward Hill, USA, 99 wt.% purity, particle size < 40 µm) and graphite (Alfa Aesar, Ward Hill, USA, 99 wt.% purity, particle size < 48 µm) powders in a 3:1.5:2 molar ratio. The powders were ball-milled for 48 h and cold pressed into cylindrical discs under pressure of 1 GPa. The discs were placed in a tube furnace under flowing argon, Ar, and heated to 1457 oC for 1 h at a heating rate of 20 oC/min. After cooling to 4


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room temperature, the discs were then milled for 2 h to obtain powders (400 mesh). Conventionally, the MXene was prepared by immersing the MAX powder in 40 % HF (Fisher Scientific, Fair Lawn, NJ) at 40 oC for 10 h to exfoliate Al layers. Then the filtered MXene solution was alkaline activation treatment by 6M NaOH solution for 2 h. The solution was centrifuged, dried at 100 oC for 10 h and directly used in the following experiments.

2.2 Synthesis of alk-MXene/metal composites The MXene/NW-Ag0.9Ti0.1 nanowire composites: 100 mg alk-MXene and 100 mg poly (vinylpyrrolidone) (PVP) (Aladdin Reagent, Shanghai, China, Mw=58000) were mixed with 80 ml deionized water, and followed by ultrasound for 20 min and then stirred for 30 min. In a separate solution, 20 ml 30 mM AgNO3 (Aladdin Reagent, Shanghai, China, 99.8 wt%,) was stirred for 30 min to ensure complete dissolution. Then the whole AgNO3 solution was injected slowly into the mixture of alk-MXene, PVP and deionized water. After the injection was completed, the solution was stirred for 5-240 min at room temperature. Finally, the suspension was centrifuged for 30 min at 3500 rpm, rinsed with deionized water and ethyl alcohol, and dried at 40 ℃ for 24 h. The prepared composites with the reaction times of 5 min, 60 min and 240 min were analyzed comparatively. In addition, a reference in the absence of PVP was synthesized by the same procedure.

2.3 Preparation of alk-MXene/Ag composite electrodes The electrochemical characterization was studied by rotating disc electrode (RRDE-3A, ALS Co., Japan). All electrochemical experiments were performed in a classic three-electrode cell at room temperature. The Ag/AgCl electrode serves as the reference electrode, a Pt foil as the counter electrode, and the catalyst supported on a grinded glassy carbon (GC) as the working electrode. The tests were performed in the electrochemical workstation (Bio-logic, VSP). 20 mg catalyst 5



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material was dispersed in 4 ml ultrapure water and 1 ml isopropanol. And then 50 µl Nafion solution（DuPont, 5 wt. %）was added to the solution with sonication for 30 min. Then we took 3 µl of the suspension by the pipette to drip on a grinded glassy carbon (GC) disk electrode of 0.07065 cm2 area (diameter: 3 mm). The solvent was dried in air before used. For comparison, the commercial 20 wt.% Ag/C (Premetek company)was tested under the rotation rate of 1600 rpm.

2.4 Electrochemical measurements The modified GC electrode was then brought to take electrochemical measurements immediately. The cyclic voltammetries (CVs) for ORR were performed by scanning the potential from -0.97V to 0.1 V (vs. Ag/AgCl) at a scan rate of 50 mVs-1. All potentials reported in this work were converted from the Ag/AgCl to the reversible hydrogen electrode (RHE) scale using the equation of E (RHE) = E(Ag/AgCl) + 0.97 V in 0.1 M KOH ( Aladdin Reagent, Shanghai, China, 99.8 wt.%). The electrolyte was bubbled with ultra-high purity (99.999%) O2 or Ar for 30 min to make the solution saturated with O2 or Ar. The whole experimental procedures were under an atmosphere of O2 or Ar. The linear scanning voltammograms (LSVs) were also performed in a potential range from 0.1 V to -0.97 V (vs. Ag/AgCl) at a scan rate of 5 mVs-1. The LSVs were measured under the rotation rates of 400, 800, 1200, 1600, 2500 and 3600 rpm, respectively. The ORR kinetics analysis can be conducted using Koutecky–Levich (K–L) equations [17]. I = i + i = ( / ) + i

(1)

= 0.62 ( )/ /

(2)

where Id was the measured current density, id and ik were the kinetic current density and film diffusion limiting current density, respectively, B was the reciprocal of the slope, ω was the angular velocity of the disk, n was the number of electrons in the oxygen reduction, F was the 6


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Faraday constant (96 500 C mol-1), Co was O2 volume concentration (1.14 ×10-6 mol cm-3), n was the kinematic viscosity of the electrolyte (0.01 cm2 s-1), and Do was the diffusion coefficient of O2 in 0.1 M KOH (1.73 ×10-5 cm2 s-1).

2.5 Analysis and characterization The X-ray diffraction (XRD) patterns were obtained with a power diffractometer (Rigaku D/MAX-2005/PC) using Cu Kα radiation (λ=1.5406 Å) at a voltage of 40 kV and a current of 200 mA with a step scan of 0.02° per step and a scanning speed of 2° per min. Si power was added to some samples as an internal standard to calibrate the diffraction angles and the instrumental peak broadening. The elemental contents (Ti, Ag) of the MXene/Ag composites were measured by inductively coupled plasma (ICP) (ICAP 6300 Thermo Scientific, USA). The samples were dissolved in 6M HNO3 solution. The Ar gas was used for protection. A FESEM (FESEM, Hitachi S4800, Japan) was used to obtain images of the prepared samples. TEM characterizations were conducted on a JEOL JEM2010 transmission electron microscope equipped with a Gatan CCD camera working at the accelerating voltage of 200 kV. The elemental compositions were detected by energy dispersive X-ray (EDX) analyzer with AMETEK/EDAX Genesis attachment (EDAX Inc., Mahwah, NJ, USA) mounted on the TEM. Nine random spots have been performed to calculate the elemental compositions. In addition, the area fraction of nanowire was calculation by Image 6 software. The average area fraction which was obtained based on five SEM images corresponds to the nanowire dimension per area. All TEM samples were created by depositing a drop of diluted suspensions in ethanol on a carbon film coated copper grid. XPS analysis of given samples was performed with a spectrometer (ESCALAB-2, Great Britain) equipped with Mg Kα X-ray source (1253.6 eV protons).

3. Results and Discussion 7



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3.1 Material characteristics After Al-removing from MAX (Ti3AlC2) in a 40 %HF solution, a standard alkalization-intercalated method (6M NaOH, 2h) has been performed to obtain a layered alk-MXene (Ti3C2(OH/ONa)2) sample [13]. The typical field emission electron microscopy (FESEM) image (Fig. Fig. 1a) 1a reveals that the layer-shaped structure of MXene (Ti3C2(OH)0.8F1.2 [18]) still remains well. The c lattice of MXene is ~9.75Å in terms of the (0002) peak at ~90 (Fig. Fig. 1b), 1b and it corresponds to the mixing of OH and F groups, which is consistent with the previous results [8, 19]. However, a new peak at ~6.70 is observed after alkalization-intercalated treatment. The full width at half maximum (FWHM) also increases, indicating that not only does the c lattice increase (~15.14 Å), but also they become less uniform [20]. It believes that two reactions occurred during alkalization intercalated treatment [7,13]. Specifically, F groups are substituted by OH groups in a strong base solution. Concurrently, some active OH sites change into ONa groups owing to ion exchange, resulting in the increment of c lattice. A new urchin-like structure has been prepared by mixing AgNO3 and alk-MXene (Ti3C2(OH/ONa)2) solution containing PVP addition (Fig. Fig. 1c). 1c The layered morphology maintains integral, and some long wires (termed A) and dot-like particles (termed B) are detected on the surface of the alk-MXene. Four diffraction peaks (Fig. Fig. 1b) are indexed to the (111), (200), (220), and (311) planes of face-centered-cubic Ag single crystals. The calculated d spacing constant based on the XRD pattern is 4.01 Å, close to the reported date [5, 21]. The transmission electron microscopy (TEM) images (Fig. Fig. 1d1d-e and Fig. 2a) reveal that the surface of the nanowires is rough. The length is more than 5 m and the width is ~42±5 nm. The silver nanowires are mainly composed of Ag and Ti elements, and the mole ratio of Ag/Ti is about 9:1. The corresponding

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selected-area electron diffraction (SAED) pattern is taken from the edge of an individual Ag0.9Ti0.1 nanowire. A typical twin diffraction pattern is observed. In addition, the diameter of Ag particles is 20~50 nm (Fig. 1f and Fig. 2c). 2c The typical five-fold twin morphology is also observed. However, differing from the element compositions of the Ag0.9Ti0.1 nanowire, the nanotwin (NT) Ag particles merely contain Ag element. The area fractions of NW-Ag0.9Ti0.1 and NT-Ag particles measured by SEM are 89% and 11%, respectively. Thus, this new structure which mainly consisted of MXene and nanowire Ag0.9Ti0.1 is denoted as MXene/NW-Ag0.9Ti0.1. To clarify the influencing factors (PVP addition and incubating time) on the formation of MXene-Ag composites, phase compositions and morphology as a function of incubating time were investigated. The results were summarized in Table 1. 1 For the reference without PVP solvent, a large number of dot-like particles with the diameter of ~10 nm form immediately (Fig. Fig. 3). With increasing the incubating time, the number of particles decreases but the diameter of the particles increases continuously. The SAED image confirms that the particle corresponds to a nanosized Ag (N-Ag) single crystal. This structure is denoted as MXene/N-Ag, wherein both five-fold twin structure and Ti-doping phenomena are hardly observed (Fig. Fig. 3e3e-f). Nevertheless, when AgNO3 solution was added into alk-MXene solution containing PVP, a large number of dot-like nanotwin Ag (NT-Ag) particles formed firstly (~5 min, Fig. 3g), 3g and the structure is defined as MXene/NT-Ag. As the incubation time increased, some dot-like particles transformed into long bar-shaped precipitates. The width continuously increased with increasing the incubation time, while the fraction of Ag particles reduced correspondingly (Fig. Fig. 4a). The NT-Ag particles ripened into NT-Ag/Ti wires (~60 min, Fig. 3h, MXene/NW-Ag0.9Ti0.1). When the incubation time was over 240 min, the nanowire varied into snow-shaped dendrite structure. 9



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Moreover, the element ratio of snow-shaped dendrite structure (SS-Ag/Ti) was the same as that of NW-Ag0.9Ti0.1 in terms of elemental analysis. Therefore, this new morphology is signified as MXene/SS-Ag0.9Ti0.1 (Fig. 3i). Thus the whole reaction process at room temperature can be shown as follows: %&'() , +,-

alk − MXene ./////////0 MXene/N − Ag %&'() ,6 +,-

+,-

(3)

: +,-

alk − MXene + PVP .////////0 MXene/NT − Ag .///0 MXene/NW − Ag.9 Ti. .////0 MXene/SS − Ag .9Ti.

(4)

In addition, two separated (002) peaks (Fig. Fig. 1b and Fig. 4b) imply that there are two different activated sites (ONa and OH) on the surface of the alk-MXene. At an early stage, the ONa peak at ~6.7o decreased rapidly (Fig. Fig. 4c). Thereafter the intensity of OH peak at ~9o became weak with increasing Ag concentration. Finally both peaks disappeared when Ag concentration was over 300 m/L. It suggests that both ONa and OH sites possess reduction activity, which is consistent with the presence of Ag particles in the primitive MXene. Moreover, the ONa sites have higher reaction activity than OH sites, which agrees well with facile ion exchange of Na during the alkalization-intercalated procedure in terms of the first-principle calculations [13,14].

3.2 Self-reduction mechanism The reason for this unexpected reduction role is interpreted by XPS. The distinguished C1s, Ti2p and O1s peaks onto the primitive alk-MXene demonstrates the intrinsic C/Ti and O/Ti compositions, and the emerged Ag 3d peak (~368 eV) within the MXene/NW-Ag0.9Ti0.1 sample implies the successful Ag loading (Fig. Fig. 5a). As for the primitive alk-MXene (Fig. Fig. 5b), the strong Ti 2p3/2 and Ti 2p1/2 peaks are located at ~458.8 eV and~464.5 eV, respectively, which are ascribed to the stabilized TI(IV)-O compositions [22, 23]. Interestingly, dissimilar to conventional 10


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titanium-based materials, a broad peak is observed on the alk-MXene ranging from ~452 eV to ~457 eV, which might be related to distinctive Ti species, corresponding to Ti(II) 2p3/2 (455.6 eV) and Ti(III) 2p3/2 (454.7 eV) [17]. Such low valence Ti species result in the formation of metallic Fig. 5d) are located at ~367.8 and ~373.8 eV, Ag. The representative Ag 3d5/2 and Ag 3d3/2 peaks (Fig. respectively [24, 25]. The slitting of the 3d doublet of Ag is 6.0 eV, demonstrating the formation of Ag(0) within the MXene/NW-Ag0.9Ti0.1. Additionally, nearly all Ag(0) peaks instead of other Ag+ and Ag2+ species fitted using the software XPS peak-fit after a Shirley background subtraction also confirm the strong reductive activity of the low valence Ti(II) and Ti(III) species. This mechanism is the similar to that of the Ag reduction in the presence of as-obtained MXene precursor [16]. Fig. 5e-f) on the primitive alk-MXene are Additionally, the representative spectrums of O1s (Fig. divided into three separated O species, corresponding to Ti-O (~530.1 eV) and Ti-OH (~531.4 eV) and Oactivated (~532.5 eV) with area fractions of 74.4%, 15.8% and 9.8% respectively. It is noteworthy that the unique Oactivated represents the activated chemisorbed oxygen, which corresponds to oxygen vacancies for catalysis enhancement [26, 27]. The Ag(0)-Ti hybridization can stimulate an increment of Oactivated (28.8%) and a loss of Ti-OH species (6.6%), resulting in the efficient catalysis activity of the MXene/NW-Ag0.9Ti0.1 samples [28].

3.3 Electrochemical catalytic properties The cyclic voltammetry (CV) curves for four electrodes (Fig. Fig. 6 and Fig. 7) show that no apparent redox peaks appeared under Ar condition, suggesting that these composites are stable in the tested potential range. Comparatively, under O2 condition, a main characteristic reduction peak at 0.5-0.6 V(RHE) is detected, which corresponds to ORR [29, 30]. The values of the peak potentials 11



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(EP) are summarized in Table 2, in which the most positive initial EP value of ~0.564 V(RHE) for the MXene/NW-Ag0.9Ti0.1 catalyst indicates it possesses the best ORR activity. As the cycle times increase, all Ep values shift toward negative ones correspondingly. Attractively, the least reduced amount (0.016 V vs. RHE) for the MXene/NW-Ag0.9Ti0.1 catalyst after 1000 cycles reveals it has the best structure stability and reversibility. The ORR activity of the MXene/NW-Ag0.9Ti0.1 catalyst is further studied by the linear sweep voltammetry (LSV) (Fig. Fig. 6), wherein the current density increases with increasing rotation speeds. For comparison, the polarization curves of other three samples and the reference of 20 wt.% Ag/C are also investigated at a rotation rate of 1600 rpm (Fig. Fig. 6). It shows that the Table 1) exhibits the onset potential (EORR) and the half-wave MXene/NW-Ag0.9Ti0.1 catalyst (Table potential (E1/2) at 1600 rpm which are 0.921 V (RHE) and 0.782 V (RHE), respectively. Compared with other three samples, both EORR and E1/2 shift towards positive direction. Prominently, the EORR and E1/2 of the MXene/NW-Ag0.9Ti0.1 catalyst are more positive than those of the reference of Ag/C catalyst (0.881V/0.85V and 0.571V/0.56 [31]), indicating that the MXene/NW-Ag0.9Ti0.1 sample exhibits high activity toward the ORR. This value is even better than those of pure silver nanowires reported recently (Table Table 1) [24，31-37]. The electron transfer numbers (n) toward the ORR were analyzed by the Koutecky–Levich equation (Fig. Fig. 6c and Fig. 7) [6]. The MXene/NW-Ag0.9Ti0.1 sample reduces O2 to OH- in alkaline media principally through a 4-electron transfer process (n=3.95) in the entire calculated potential range. However, 2-electron reduction plays a main role for both MXene/N-Ag and MXene/NT-Ag samples. The durability of the MXene/NW-Ag0.9Ti0.1 catalyst toward the ORR has been tested with a US Department of Energy (DOE) protocol for accelerated durability test in Ar-saturated 0.1 M

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KOH solution. The LSV curves before and after 2000 cycles with the rotating speed of 2500 rpm (Fig. Fig. 6d) show that the EORR and E1/2 are negatively shifted to ~ 5 mV and ~15 mV, respectively. Basically, the Ti-doping Ag nanowire significantly improves the ORR activity and cycle stability due to its unique morphology and electron structure. Firstly, akin to Ag/C catalyst, the conductive body structure of 2D-alk-MXene consists of Ti and C, which serves as not only the supports for nanoparticles, but also some effectively conductive channels for electrons [25]. The layered material is more convenient for the electron transfer from Ti or C to Ag. Moreover, these positively charged surfaces on the adjacent Ti or C atoms establish favorable sites for the side-on O2 adsorption and facilitate the reduction of oxygen [6], just as Au/Ag [38] and Pd/Ag [39] bimetallic catalysts. Secondly, the Ti-doping increases the concentration of defects or vacancies, which provides more adsorption sites and increases the concentration of adsorbed oxygen (Fig. 5f), resulting in the increment of limiting diffusion current density. Thirdly, the nanowire is mostly composed of 90%Ag and 10% Ti, so highly dispersed Ti-doping Ag nanowires can provide a synergistic effect acting as a bifunctional catalyst to catalyze different oxygen reduction reaction steps, and especially the presence of Ti accelerates the decomposition of H2O2 [40]. Accordingly, the HO2intermediate generated on the Ag surfaces can easily diffuse to the neighboring Ti surfaces or the Ag-Ti interfaces, undergoing prompt disproportionation into OH- and O2 for further oxygen reduction in this hybrid catalyst [6, 24].

4. Conclusions In summary, the results offer a one-step method to fabricate some novel MXene-metal nanocomposites with unexpected chemical properties by means of the direct reduction role of 2D 13



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MXene materials. Taking Ag as an example, we prepared a new urchin-like MXene-Ag0.9Ti0.1 nanowire composite by directly reduction of AgNO3 in alk-MXene solution containing PVP. This unique structure show exceptional electrochemical activity in the ORR by enhancing the conductivity and increasing abundant active sites, which is better than commercial Ag/C catalyst and pure Ag naowires. Based on its low cost, high stability and simple route, it demonstrates that this alk-MXene-Ag0.9Ti0.1 nanowire composite would be a promising candidate for non-Pt cathode catalysts in alkaline fuel. We foresee an enormous potential impact of this methodology in exploiting new and unexpected physical and chemical properties of different MXene-bimetal nanocomposites.

Acknowledgements We greatly acknowledge the financial support from NSFC (51422105, 51578476), NSF of Hebei Province (B2016203056), The Science Foundation for the Excellent Youth Scholars from Universities of Hebei Province (E2015203404, GCC2014058) and Chinese. Heibei Province Youth Top-notch Talent Program.

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Energy Environ. Sci. 2014; 2014 7: 1719-1724. [2] Blizanac B. B.; Ross P. N.; Markovic N.M.; Oxygen reduction on silver low-index single-crystal surfaces in alkaline solution: rotating ring disk Ag(hkl) Studies, J. Phys. Chem. B, 2006, 110 (10), pp 4735–4741. [3] Fazil A.; Chetty R.. Synthesis and evaluation of carbon nanotubes supported silver catalyst for alkaline fuel cell, Electroanalysis. 2014; 2014 26: 2380-2387. 14


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Table 1. The summarized fundamental data of the different MXene-Ag products. Samples

M-nAg

Reaction conditions

Phase composition

Ag-containing phase fractions* Ag

Ag/Ti

MXene, nano-sized Ag particles

100%

-

MXene, nanotwin Ag particles

100%

-

21%

79%

8%

92%

20 ml 30 mM AgNO3, 100 mg alk-MXene dispersed in 80 ml DI Water, RT, 5-60 min

M-ntAg

20 ml 30 mM AgNO3, 100 mg alk-MXene dispersed in 80 ml DI Water,10 mg PVP, RT, ~5 min

M-nwAg/Ti

20 ml 30 mM AgNO3, 10 mg

MXene,

alk-MXene dispersed in 2 ml DI

Ag0.9Ti0.1 nanowire, nanotwin Ag

Water,10 mg PVP, RT, ~60 min M-ssAg/Ti

20 ml 30M AgNO3, 10 mg

MXene,

alk-MXene dispersed in 2 ml DI

snow-shaped Ag0.9Ti0.1, nanotwin Ag

Water,10 mg PVP, RT, over 240 min

* The Ag concentration was confirmed by ICP. The average phase fraction was calculated based on the phase area fraction in per area. The average values were obtained on five random SEM graphs.

20


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Table 2. The important parameters determined from experimental results (1600 rpm) and some related data on Ag-based catalysts reported previously. Samples

EORR, V

E1/2, V

J mAcm-2

n

Ep (V) 1

100

Refs.

500

1000

MXene/N-Ag

0.880

0.571

3.31

2.06

0.514

0.506

0.501

0.495

MXene/NT-Ag

0.901

0.631

3.34

2.19

0.526

0.523

0.512

0.503

MXene/NW-Ag0.9Ti0.1

0.921

0.782

3.64

3.95

0.565

0.558

0.553

0.549

0.881

0.554

2.78

3.15

0.508

0.504

0.504

0.499

0.57

3.29

3.71

0.78

3.51

3.85

[31]

0.56

3.29

MXene/SS-Ag0.9Ti0.1 20 wt.%Ag/C Supportless Ag nanowire 20 wt.%Ag/C

0.88 0.92

3.70

[30]

Ag nanorods

0.85

0.57

3.80

[24]

Ag-GNR

0.618

3.51

[32]

Ag/B-MWCNTs

0.69

3.80

[33]

Ag–MnO2/graphene

0.67

3.70

[34]

Ag/GNP

0.72

4

[35]

Ag/TiO2

0.69

4

[36]

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Fig. 1. (a) A representative FESEM image of the layered alk-MXene sample. (b) XRD patterns of different samples. The inset corresponds to a high magnification of low angle range. (c) A typical FESEM image of the MXene/NW-Ag0.9Ti0.1. A and B represent nanowire and nano-sized particle, respectively. (d) The typical TEM image of the MXene/NW-Ag0.9Ti0.1. (e) The high magnification TEM image of nanowire A. The width is ~42±5 nm (e-1). The SAED fringe (e-2) confirms the nanotwined Ag structure. (f) The high magnification TEM image of nanosized particle B with a five-fold twin, the inset corresponds to the SAED of particle B.

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Fig. 2. (a) The high magnification TEM image of nanowire A in the MXene/NW-Ag0.9Ti0.1 sample. (b) The element composition of nanowire A. The mole ratio of Ag/Ti is about 9:1. (c) The high magnification TEM image of nano-sized particle B in the MXene/NW-Ag0.9Ti0.1 sample. (b) The element composition of nano-sized particle B. No Ti element was detected.

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keV

Fig. 3. The morphology variation without PVP as a function of time, 5 min (a), 30 min (b), 60 (c) min. (d) The typical TEM image of the MXene/N-Ag sample (60 min). (e) The high magnification of nanosized Ag particle. The inset is the SEAD of N-Ag particle along [112] zone. (f) The elemental composition of nanoparticles. (g) The typical FESEM image of the MXene/NT-Ag (5 min). The inset corresponds to TEM image of NT-Ag particle. (h) 60 min. Both particle B and nanowire A appeared. (i) The typical FESEM image of the MXene/SS-Ag0.9Ti0.1 (240 min). The inset corresponds to elemental composition of snow-shaped Ag/Ti. The mole ratio of Ag/Ti is about 9:1.

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Fig. 4 (a) The variation of width and area fraction of nanowires Ag0.9Ti0.1 dependence on the incubation time. The area fraction of nanowire was calculation by Image 6 software. The average area fraction which was obtained based on five images corresponds to the nanowire dimension per area. (b) The variation of phase compositions dependence on the concentration of Ag ions. (c) The corresponding high magnification of yellow range in (b).

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Fig. 5. (a) High-resolution XPS spectra of the alk-MXene and MXene/NW-Ag0.9Ti0.1. (b) Ti 2p spectra of the alk-MXene. (c) Ti 2p spectra of the MXene/NW-Ag0.9Ti0.1. (d) Ag 3d spectra of the MXene/NW-Ag0.9Ti0.1. (e) O 1s of the Alk-MXene. (f) O 1s of the MXene/NW-Ag0.9Ti0.1.

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Fig. 6. ORR properties of the MXene/NW-Ag0.9Ti0.1 sample in Ar/O2-saturated 0.1 M KOH solution. (a) The long-term CVs of the MXene/NW-Ag0.9Ti0.1 electrode, Sweep rate: 50 mVs-1. (b) LSV curves of the MXene/NW-Ag0.9Ti0.1 electrode at various rotation rates. Sweep rate: 5 mVs-1. (c) The Koutecky–Levich plots at different electrode potentials. (d) Comparison of RDE polarization curves of oxygen reduction for the MXene/NW-Ag0.9Ti0.1 catalyst before and after 2000 cycles. Rotation rate: 2500 rpm. Sweep rate: 50 mVs-1. (e) ORR polarization curves in O2-saturated 0.1 M KOH. Rotation rate: 1600 rpm. Sweep rate: 5 mVs-1.

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Fig. 7. 7 The long-term CVs of the different MXene/Ag electrodes in O2-saturated 0.1 M KOH solution. Sweep rate: 50 mVs-1. (a) MXene/N-Ag. (c) MXene/NT-Ag. (e) MXene/SS-Ag0.9Ti0.1. Their accompanying graphs provide the Koutecky-Levich plots at different electrode potentials (b, d and f). Sweep rate: 5 mVs-1.

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TOC

SelfSelf-Reduction Synthesis of New MXene/Ag NanowireNanowire-Shaped Composites with Unexpected Electrocatalytic Activity Zhiwei Zhang, Hanning Li, Guodong Zou, Carlos Fernandez, Baozhong Liu，Qingrui Zhang*, Jie Hu, Qiuming Peng*

The critical issue of fuel cells, as one of sustainable energy, focuses on high efficient catalysts. This new alk-MXene-Ag0.9Ti0.1 nanowire composite would be a promising candidate for non-Pt cathode catalysts, paving a potential avenue to solve energy crisis and green house.

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Ag Composites with

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