Solution-Plasma-Assisted Bimetallic Oxide Alloy Nanoparticles of Pt

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Energy, Environmental, and Catalysis Applications

Solution Plasma-assisted Bimetallic Oxide Alloy Nanoparticles of Pt and Pd Embedded within Two-dimensional Ti3C2Tx Nanosheets as Highly Active Electrocatalysts for Overall Water-splitting Bingbing Cui, Bin Hu, Jiameng Liu, Minghua Wang, Yingpan Song, Kuan Tian, Zhihong Zhang, and Linghao He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06568 • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 27, 2018

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ACS Applied Materials & Interfaces

Solution Plasma-assisted Bimetallic Oxide Alloy Nanoparticles of Pt and Pd Embedded within Two-dimensional Ti3C2Tx Nanosheets as Highly Active Electrocatalysts for Overall Water-splitting Bingbing Cui a‡, Bin Hu a‡, Jiameng Liua, Minghua Wanga, b, Yingpan Songa, Kuan Tiana, b, Zhihong Zhang a, b*, Linghao Hea, b*

a

Henan Provincial Key Laboratory of Surface and Interface Science, Zhengzhou

University of Light Industry, No. 136, Science Avenue, Zhengzhou 450001, China.

b

Henan Collaborative Innovation Center of Environmental Pollution Control and

Ecological Restoration, School of Materials and Chemical Engineering, Zhengzhou University of Light Industry, No. 136, Science Avenue, Zhengzhou 450001, China. KEYWORDS: bimetallic oxide alloy nanoparticles of Pt and Pd, solution plasma, 2D Ti3C2Tx nanosheets, hydrogen evolution reaction, oxygen evolution reaction, water splitting

Corresponding authors: Zhihong Zhang, Linghao He *

E-mail addresses: [email protected] or [email protected]

‡ These authors contributed equally 1

ACS Paragon Plus Environment

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ABSTRACT Exploiting high-efficiency and low-cost bifunctional electrocatalysts for hydrogen evolution (HER) and oxygen evolution reactions (OER) has been actively encouraged because of their potential applications in the field of clean energy. In this paper, we reported a novel electrocatalyst based on an exfoliated two-dimensional (2D) MXene (Ti3C2Tx) loaded with bimetallic oxide alloy nanoparticles (NPs) of Pt and Pd (represented by PtOaPdObNPs@Ti3C2Tx), which was synthesized via solution plasma (SP) modification. The prepared materials were then utilized as highly efficient bifunctional electrocatalysts toward HER and OER in alkaline solution. At a high plasma input power (200 W), bimetallic oxide alloy nanoparticles of Pt and Pd or nanoclusters with different metallic valence states deposited onto the Ti3C2Tx nanosheets. Due to the synergism of the noble metal NPs and the Ti3C2Tx nanosheets, the electrocatalytic results revealed that the as-prepared PtOaPdObNPs@Ti3C2Tx nanosheets under the plasma input power of 200 W for 3 min catalyst only required a low overpotential to attain 10 mA cm−2 for HER (57 mV) in 0.5 M H2SO4 solution and OER (1.63 V) in 0.1 M KOH sollution. Moreover, water electrolysis using this catalyst achieved a water splitting current density of 10 mA cm-2 at a low cell voltage of 1.53 V in 1.0 M KOH solution. These results suggested that the hybridization of the ultra-extremely low usage of PtOa/PdOb NPs (1.07 µg cm-2) and Ti3C2Tx nanosheets by SP will expand the applications of other clean energy reactions to achieve sustainable energy.

2


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■ INTRODUCTION The increasing demand for energy and fossil fuel consumption indicates the urgent need to fabricate innovative strategies with high energy utilization searching for environmentally friendly, renewable, and clean energy.1 Among the many potential techniques for developing energy sources, hydrogen is considered one of the most main candidates to replace fossil fuels because it is clean, abundant, and renewable.2,3 The methods for producing hydrogen include water electrolysis,4,5 water photoelectrocatalysis,6 and photocatalytical water splitting.7 For electrocatalysis reactions, hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have gained significance in promising clean energy technologies, such as metal-air batteries, fuel cells, and water splitting devices.8 However, the sluggish kinetics of HER and OER have severely limited the development of clean energy technologies. Therefore, active electrocatalysts that can strengthen catalytic efficiency must be developed by minimizing the overpotential of HER.9,10 To enhance the electrochemical performance of energy conversion devices, specifically fuel cells and metal-air batteries, low-cost non-noble metal bifunctional electrocatalysts with high electrocatalytic performance for both HER and OER must be developed.11,12 At present, precious metal-based electrocatalysts, such as platinum (Pt), ruthenium (Ru), rhodium (Rh), iridium (Ir), and palladium (Pd), are the best known HER and OER electrocatalysts.13 Pt-based electrocatalysts are considered the best candidates because of its attractive physical and chemical performances and highly efficient catalytic activity.14 However, the wide application of Pt-based electrocatalysts is hindered by 3



high cost, inherently sluggish kinetics, and declining activity.15 Therefore, highly efficient and low-cost Pt-based bifunctional electrocatalysts must be established for HER and OER. Synthesizing Pt-based catalysts is one of the feasible ways to overcome these disadvantages. For instance, PtPd alloy can be embedded in nitrogen-rich graphene nanopores and utilized as highly efficient bifunctional electrocatalysts for HER and OER.13 The Au/Pd alloy bimetallic NPs on microwave-irradiated carbon ceramic electrodes displayed a higher electrocatalytic activity for HER.16 In general, the available catalysts are efficient for HER in acidic solution or OER in basic solution. Nonetheless, selecting what electrode reactions to pair in an integrated electrolyzer for real applications remains a challenge because the stability and activity of these catalysts are mismatched in wide pH ranges. In particular, alkaline water electrolysis has been recently applied as a mature technology for hydrogen production that can produce high-purity hydrogen from a moderate energy input. Thus, efficient bifunctional electrocatalysts with high activity for both OER and HER in alkaline solution must be developed to improve the overall alkaline water electrolysis. Nevertheless, in view of the usage of the stabilizer or stabilizing agents in the conventional preparation of precious metal NPs,17 a number of drawbacks were observed, such as long aging and reduction times, the involvement of organic solvents in certain processes, the necessity to remove the stabilizers at the final stage, the preparation of complex molecular precursors,18 and the use of chemical reducing agents.19 Thus, novel approaches must be explored to overcome these disadvantages 4


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during the synthesis of precious metals. Solution plasma (SP) is a new electrical discharge process that can generate an atmospheric non-equilibrium plasma at room temperature in a liquid environment, such as an aqueous solution or an organic compound.20 SP is a convenient procedure for synthesizing nanoparticles (NPs), including Au NPs,21 PdCu,22 Mn3O4 catalysts,23 FeOx-kaolinite nanocomposites,24 and Pd/PdO nanoparticles.25 It demonstrated that Pd/PdO and Pt/GNs/TiO2 (GNs, graphene nanosheets) catalysts exhibited a promoting catalytic activity toward methanol oxidation.26 However, the catalytic mechanism of catalysts prepared by SP and their related applications in catalytic fields, particularly the development of electrocatalysts for water splitting, have yet to be fully exploited. In addition, to avoid the aggregation of NPs and decrease catalytic activity, novel carbon supports must be developed to promote the performance of electrocatalysts.27 The performance of performance of Pt-based catalysts can be markedly improved by using carbon materials, including carbon nanotubes,28 carbon nanofibers,29 carbon nanocoils,30 ordered mesoporous carbons31 and graphene,32 which have a sequential structure and high electrical conductivity. In recent years, the family of 2D materials have been enriched by a new but potentially huge group of early transition metal carbides/carbonitrides known as MXenes (e.g., Ti3C2, Ti2C, Nb2C, V2C, Ti3CN, and Mo2C, etc.).33 Given their high electronic conductivity, hydrophilic nature, and good stability, MXenes have been successfully used in various electrochemical applications, including supercapacitors,34 Li and multivalent ion batteries,35 and electrocatalysts for OER.36 A recent theoretical study posited the potential of 2D MXenes as effective 5



HER catalysts.37 However, limited reports have explored the use of MXenes as electrocatalysts for the HER. For example, Mo2CTx was found to be a promising system for HER, suggesting that the basal plane was active for HER and thereby eliminating the need for a deliberate and complex material design to maximize the density of the exposed edge sites.38 Moreover, as a class of materials with ultralow work function and electronegative surfaces, MXenes may potentially alter the electrophilicity of the active centers of catalysts and consequently change the catalytic properties in multicomponent catalyst systems. Hierarchical MoS2/Ti3C2-MXene@C nanohybrids with excellent structural stability, electrical properties, and strong interfacial coupling have been fabricated by assembling carbon-coated few-layered MoS2 nanoplates on carbon-stabilized Ti3C2 MXene.39 These nanohybrids exhibited exceptional performance for HER, achieving a positive onset potential, a low overpotential, and long-term stability in acidic solution. The hybrid material of 2D cobalt 1,4-benzenedicarboxylate with Ti3C2Tx nanosheets via an interdiffusion reaction-assisted process was applied in OER at a current density of 10 mA cm−2 and a potential of 1.64 V vs. a reversible hydrogen electrode and a Tafel slope of 48.2 mV dec−1 in 0.1 M KOH.36 A novel photocatalyst consisting of 2D Ti2C and graphitic carbon nitride (g-C3N4) was synthesized to enhance water splitting activities due to the efficient synergistic interaction between Ti2C and g-C3N4.40 Nevertheless, these investigations suggested that although the Ti3C2Tx loaded electrocatalytically active components displayed a considerably better electrocatalytic performance than that of the pristine Ti3C2Tx nanosheets, the tedious preparation procedure of Ti3C2Tx-based 6


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catalysts limits its applications in clean energy or solar batteries. In view of its efficiency, stability, and cost, PtPd alloy embedded in exfoliated Ti3C2Tx nanosheets should be explored as co-catalysts. In this work, a novel 2D exfoliated Ti3C2Tx-based nanocomposite loaded with PtPd alloy NPs was prepared via coprecipitation strategy, in which the PtPd oxide alloy NPs were synthesized by SP method using H2PtCl6·xH2O and PdCl2 as the precursors (Scheme 1). In this way, the PtPd oxide NPs were seamlessly coated onto the surfaces of the Ti3C2Tx nanosheets (PtOa(a=0, 1, 2)PdOb(b=0,1)@Ti3C2Tx). In view of the combined advantages of two components, namely, the highly efficient electrocatalytic activity of Pt and Pd NPs as well as the excellent electrochemical performance, chemical,41 and structural stability of the exfoliated Ti3C2Tx nanosheets,42 the

as-obtained

hybrid

materials

exhibited

extremely

high

electrocatalytic activity toward HER in both acidic (H2SO4) and alkaline solution (KOH), as well as an excellent activity toward OER with a Tafel slope of 48.2 mV dec−1 in 0.1 M KOH at a current density of 10 mA cm−2 and a low potential of 1.63 V. As such, water electrolysis using this catalyst achieved a high water splitting current density of 10 mA cm-2 at a low cell voltage of 1.53 V. This study demonstrated that the hybridization of these components is promising for the development of Ti3C2Tx-based composites for clean energy applications. ■ EXPERIMENTAL SECTION Chemicals and Materials. H2PtCl6·xH2O and PdCl2 were purchased from Shanghai sss Reagent Co., Ltd, China. Ti3AlC2 MXene was purchased from Shanghai Puhan 7



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Chemical Reagent Co., Ltd, China. Hydrofluoric acid, dimethyl sulfoxide and ethylene glycol were obtained from Tianjin Fuyu Chemical Reagent Co., Ltd. Commercial Pt black was purchased from Alfa Aesar, England. Milli-Q water (>18.2 Ω cm) was used throughout this study. Synthesis PtOaPdObNPs@Ti3C2Tx nanosheets. The preparation of exfoliated Ti3C2Tx nanosheets was supplied in the S1 section (See Supporting information). The SP modification was performed in a HQ-2 PECVD system manufactured by the Institute of Microelectronics of the Chinese Academy of Sciences, China. The radio frequency generator was operated at 13.56 MHz. Ti3C2Tx (2 mg) was dispersed in 1800 µL ethylene glycol in a glass vial (4.0 mL) and sonicated for 15 min. Then, 100 µL of H2PtCl6·xH2O (0.01 g mL-1) and 100 µL of PdCl2 (0.01 g mL-1) were added into the mixture and sonicated for 15 min again. The glass vial was put into the plasma chamber, following by the irradiation under plasma input power of 200 W for various periods (1, 3, and 5 min) at a continuous wave with the pressure kept constant at 0.1 Pa. As a result, the series of PtOaPdObNPs@Ti3C2Tx nanosheets dispersions were prepared and stored in 4 °C for further electrochemical measurements. According to the different valence states of Pt or Pd in the metal oxides, the Pt NPs and Pd NPs were represented as PtOa and PdO, respectively. It will be discussed in the 3.1 Section. For comparison, the PtOaNPs@Ti3C2Tx nanosheets were synthesized following the exact same procedure as that of PtOaPdObNPs@Ti3C2Tx without PdCl2, whereas the

PdObNPs@Ti3C2Tx

nanosheets

were

prepared

8


without

H2PtCl6·xH2O.

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Additionally, PtOa and PdO nanoparticles were also prepared by the same way in the absence of Ti3C2Tx nanosheets using H2PtCl6·xH2O and PdCl2 as the precusor, seperatedly. As for the preparation of PtOaPdOb NPs, H2PtCl6·xH2O and PdCl2 were used concurrently without Ti3C2Tx nanosheets. PtOa NPs, PdO NPs, PtOaPdOb NPs, PtOaNPs@Ti3C2Tx, and PdObNPs@Ti3C2Tx nanosheets were obtained under the input power of 200 W for 3 min. Additionally, for the contrast experiments, the series of PtOaNPs@Ti3C2Tx, PdObNPs@Ti3C2Tx, and PtOaPdObNPs@Ti3C2Tx nanosheets were also prepared by the hydrothermal (HT) method, which are denoted by HT-PtOaNPs@Ti3C2Tx, HT-PdObNPs@Ti3C2Tx, and HT-PtOaPdObNPs@Ti3C2Tx nanosheets, accordingly, as supplied in the S2 section (See Supporting Information). Characterizations. Powder X-ray diﬀraction (XRD) measurements were recorded on a Rigaku D/Max-2500 X-ray diﬀractometer using Cu Kα as a radiation. Powder samples were prepared by crushing single crystals. The corresponding intensity data were collected in the step-scan mode with a scan rate of 5° min-1 and a step size of 0.02°. Raman spectra were obtained on a Renishaw inVia Raman spectrometer with a solid-state laser (excitation at 532 nm) at room temperature in the range of 100-3000 cm-1. X-ray photoelectron spectroscopy (XPS) analysis was conducted using a Thermo Fisher ESCALAB 250Xi spectrometer equipped with an Al anode (Al-Kα 1486.6 eV). Surface morphologies of all samples were characterized by Field emission scanning electron microscopy (FE-SEM) (JSM-6490LV scanning electron microscope) and high-resolution transmission electron microscopy (TEM, 9



JEOL JEM-2100) with a field emission gun of 200 kV. The chemical element analysis was measured by Energy dispersive X-ray spectrometry (EDS) coupled with SEM. Raman spectra of the compounds were collected by LabRam HR Evolution, whereas grating 532 nm wavelength laser was used as an excitation source under low laser power (7 mW) to avoid local heating. Electrochemical measurements. The obtained suspension (10 µL) was loaded onto the glass carbon electrode (GCE) with 3 mm in diameter, and dried naturally at room temperature. The final loading for all catalysts and commercial Pt/C electrocatalyst on the GCEs is about 0.141 µg cm-2. HER tests were performed in a conventional three-electrode system at an electrochemical station (CHI660E, Shanghai Chenhua Instruments Co., China), using Ag/AgCl (3.5 M KCl solution) electrode as the reference electrode, Pt mesh as the counter electrode, and GCE as the working electrode. Linear sweep voltammetry (LSV) was measured in 0.5 M H2SO4 at a scan rate of 5 mV s-1 deaerated with argon at room temperature for 30 min. The Ag/AgCl (3.5 M KCl) reference electrode was calibrated with respect to a reversible hydrogen electrode (RHE). Alternating current (AC) impedance measurements were carried out in the same configuration when the working electrode was biased at the overpotential of 100 mV from 105 to 10-1 Hz with an AC voltage of 5 mV. OER measurements were conducted in a three-electrode system on an electrochemical station (Autolab 302N) coupled with a rotating disk electrode (RDE). A Pt plate was used as the counter electrode, a Ag/AgCl electrode as the reference electrode, and the catalyst-modified GC RDE as the working electrode with a rotating 10


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speed of 1600 rpm. LSV was measured in 0.1 M KOH at the scan rate of 5 mV s-1. All measured potentials vs Ag/AgCl were converted to RHE scale based on the Nernst equation below: ERHE =EAg/AgCl + 0.0591×pH + EθAg/AgCl, where ERHE is the applied potential vs RHE; EAg/AgCl is the applied potential vs Ag/AgCl reference electrode, pH is the pH value of the electrolyte, and EθAg/AgCl is the standard potential of the Ag/AgCl reference electrode. In 0.5 M H2SO4 solution, ERHE= EAg/AgCl + 0.22 V, while ERHE = EAg/AgCl +0.915 V in 0.1 M KOH. The HER and OER kinetics of the as-synthesized samples were obtained using corresponding Tafel plots by fitting the linear regions to the Tafel equation (η = b log j + α, η for overpotential, b for Tafel slope, j for current density, and α for Tafel constant), and the Tafel slopes and exchange current densities were calculated for all samples, RuO2, and Pt/C (20%) catalysts.43 The potential at a current density of 1 mA cm−2 is defined as the onset potential. The electrochemical double-layer capacitance (Cdl) is collected between the potential range with no faradic processes at varied scan rates, which is used to calculate the electrochemical active area (ECSA) of the catalysts.44

■ RESULTS AND DISCUSSION Basic characterizations of the exfoliated Ti3C2Tx nanosheets. The XRD patterns, Raman spectra, XPS spectra, and SEM images were obtained to explore the chemical and physical performances of the exfoliated Ti3C2Tx nanosheets (Figure S1). As illustrated in Figure S1a (curve i), the XRD patterns of Ti3AlC2 present four peaks at 25.3°, 37.0°, 38.1°, and 38.9°, which originate from anatase phase of TiO2 (JCPDS 11



card no. 00−021−1272). As shown by the curve ii, the fully exfoliated Ti3C2Tx nanosheets demonstrate one sharp peak at 25.3°, which corresponds to the (101) plane of anatase TiO2 and is produced by the local heat generated during the HF treatment of the MAX phase.42 Figure S1b shows the corresponding Raman spectra of the samples. The chemical structure of the exfoliated Ti3C2Tx nanosheets is transformed into a graphene-like nanostructure (Figure S1b, curve i). The characteristic D and G bands of carbon are detected at 1371 and 1551 cm−1 with a relatively low ratio of intensity (ID/IG = 0.46), suggesting that substantial amounts of disordered carbon are formed in the nanostructure.45 This type of disordered and defective carbon can enhance catalytic activity.46 In addition, the peak at 2680 cm-1 is attributed to the characteristic 2D band of graphene, further confirming the preceding hypotheses. The Raman spectrum of the exfoliated Ti3C2Tx (curve ii) reveala that these peaks are very strong. The surface morphologies of the stacked Ti3C2Tx and the exfoliated Ti3C2Tx nanosheets are shown in Figures S1c and S1d, respectively. As shown, the exfoliated Ti3C2Tx nanosheets exhibit a flat surface with a lateral size of hundreds of nanometers (Figure S1d), whereas the pristine multilayered Ti3C2Tx nanosheets are overlaid together (Figure S1c). Moreover, the surface feature of the resultant exfoliated Ti3C2Tx nanosheets are similar to that of the reduced graphene oxide, which is favorable to the deposition of the Pt or Pd NPs.47 The XPS survey scan spectrum of the exfoliated Ti3C2Tx nanosheets is displayed in Figure S2, in which strong signals of C 1s (284.6 eV) as well as weak signals of Ti 2p (461.4 eV) and O 1s (532. 6 eV) are observed in the exfoliated Ti3C2Tx nanosheets. 12


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As illustrated in Figures S2b and S2c, the core-level Ti 2p and C 1s XPS spectra of the exfoliated Ti3C2Tx nanosheets were deconvulated and analyzed. The results indicate that C 1s (93.3%) is present in the exfoliated Ti3AlC2, whereas only Ti 2p (0.88%) remains after the etching by HF and exfoliation at 600 W for 4 h. This finding suggest that the two main peaks at 463.65 and 458.04 eV are fitted in the Ti 2p core-level XPS spectrum and are attributed to Ti 2p1/2 and Ti 2p3/2, respectively. For the C 1s core-level XPS spectrum, the main peak at 283.4 eV is assigned to the C-C group, whereas the weak peaks at 284.3, 285.5, and 287.5 eV are ascribed to C-O, C=O, and COOH, respectively. A very weak peak at 289.5 eV is observed and ascribed to the conjugated π−π* structure of the exfoliated Ti3C2Tx nanosheets, further confirming the formation of a graphene-like structure after the high-powered exfoliation. This chemical structure transforms from Ti3C2Tx to graphene-like nanosheets, thereby improving both the electrochemical activity and the catalytic ability of the nanomaterials.48 Therefore, the presence of oxygen-related groups indicate that the Ti3C2Tx nanosheets could be caused by oxygen pollution, as revealed by XPS,49 or by being oxidized during the exfoliation procedure.50 Chemical structure and components of all samples. The XRD pattern and Raman spectra of the PtOaPdObNPs@Ti3C2Tx nanosheets are shown in Figure S3. No diffraction peak of PtOa NPs and PdOb NPs is observed. The XRD patterns and Raman spectra of the three samples are the same as those of the Ti3C2Tx nanosheets. The probable reason is that very few metal particles in the Ti3C2Tx nanosheets are in the resultant samples, leading to only a slight change in the structure of the Ti3C2Tx 13



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nanosheets. To further explore the chemical structures and components of the prepared samples, XPS characterizations were performed and analyzed using XPSPEAK4.1 software. The XPS survey scan spectra of the PtOa NPs, PdO NPs, PtOaPdOb NPs are presented in Figure S4a, and those of PtOaNPs@Ti3C2Tx, PdObNPs@Ti3C2Tx, and a series of PtOaPdObNPs@Ti3C2Tx nanosheets are shown in Figure S4b. As illustrated in Figure S4a, only a weak Pd 3d (338.9 eV) peak is observed in PdO NPs, whereas a weak Pt 4f (70.7 eV) signal is present in the PtOa NPs. Pd 3d and Pt 4f signals coexisted in the bimetallic PtOaPdOb NPs. The weak Pd 3d (338.9 eV) signal is found in the PdObNPs@Ti3C2Tx nanosheets, whereas Pt 4f (70.7 eV) is present in the PtOaNPs@Ti3C2Tx nanosheets. The coexistence of Pt 4f and Pd 3d was also observed in the series of PtOaPdObNPs@Ti3C2Tx nanosheets, indicating that the Pd and Pt NPs are concurrently hybridized with the Ti3C2Tx matrix. To understand the effect of the plasma deposition time on the chemical structure and components, the atomic% values of each element contained in all samples are displayed in Table 1. Apart from C 1s, O 1s, and Ti 2p, Pt 4f (1.61%) and Pd 3d (0.43%) are separately observed in the PtOaNPs@Ti3C2Tx

and

PdObNPs@Ti3C2Tx

nanosheets.

For

the

series

of

PtOaPdObNPs@Ti3C2Tx nanosheets prepared at 200 W, the atomic contents of Pt 4f and Pd 3d increase from 1.02% to 1.57% and from 0.38% to 0.72%, respectively as the plasma deposition time is prolonged from 1 min to 5 min. To determine the chemical surface structure of the as-obtained samples, including

PtOa

NPs,

PdO

NPs,

PtOaPdOb

14


NPs,

PdObNPs@Ti3C2Tx,

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PtOaNPs@Ti3C2Tx, and the series of PtOaPdObNPs@Ti3C2Tx nanosheets, the high-resolution XPS spectra of each element in all samples were analyzed. The Ti 2p, Pd 3d, and C 1s core-level XPS spectra of the PdObNPs@Ti3C2Tx nanosheets are displayed in Figure S5. The Pd 3d core-level XPS spectrum reveals two peaks at 337.5 and 342.8 eV, which are attributed to Pd 3d5/2 and Pd 3d3/2 of PdO NPs, respectively.51 The reactive species formed in situ via SP, such as hydrogen and hydroxyl radicals, can be used for nanomaterial processing. In addition, metal ions or complexes in aqueous solution can be reduced to form metal NPs when SP is generated in the solution.52 Organic solutes contribute to the reduction by reducing the corresponding metal ions. Under SP irradiation, the oxygen element contained in the solvent, i.e., ethylene glycol, can be decomposed into different kinds of radicals (O·, O-·, or O2-·), positive ions (O+ and O2+), negative ions (O- and O2-), and electrons in the reaction channels.53 Most electrons would be retarded before reaching the substrate surface (the surface of the Ti3C2Tx nanosheets). Part of the negative ions (Oand O2-) can combine with Pd2+ to produce the precipitate of PdO and be deposited on the surface of the Ti3C2Tx nanosheets.54 The presence of PdO plays a significant role in enhancing the electrocatalytic activity of the Ti3C2Tx nanosheets. For the Ti 2p and C 1s high-resolution XPS spectra of PdObNPs@Ti3C2Tx, similar results were observed to those of the pristine Ti3C2Tx nanosheets. These findings suggest that the PdO NPs can be combined with the Ti3C2Tx nanosheets. Furthermore, the Pt 4f core-level XPS spectrum of the PtOaNPs@Ti3C2Tx nanosheets was fitted, as shown in Figure S6. The core-level XPS spectrum of Pt 4f 15



exhibits three doublets at binding energies of 70.4–73.9, 71.25–75.15, and 72.11–77.54 eV, which corresponds to Pt0, Pt(II), and Pt(IV), respectively.55 The Pt(II) and Pt(IV) species may be present in the form of PtO and PtO2, respectively. According to the peak area, the percentages of Pt(0), Pt(II), and Pt(IV) are approximately 23.3%, 49.3%, and 27.4%, respectively,56 indicating that PtO is the main species on the surface of PtOaNPs@Ti3C2Tx. As reported previously by Mizukoshi et al.,57 Pt(IV) ions can be reduced to Pt(0) via the formation of Pt(II) ions during plasma irradiation. Consequently, the Pt(IV) species in PtCl62- can be partially reduced to Pt(II) and Pt(0), of which the Pt(II) species is present in PtO form. The reserved Pt(IV) reacted with the active O species, producing PtO2. Therefore, all precipitates are deposited onto the Ti3C2Tx surface to form the PtOaNPs@Ti3C2Tx nanocomposite. For the high-resolution XPS spectra of Ti 2p and C 1s, the same results are found as those for the Ti3C2Tx nanosheets. For comparison, the Pt 4f and Pd 3d core-level XPS spectra of the PdO, PtOa, and PtOaPdOb NPs prepared at a plasma input power of 200 W for 3 min were also measured and analyzed (Figure S7). In the Pt 4f core-level XPS spectrum of the PtOa NPs, four peaks are observed (Figure S7a), whereas the characteristic peak of Pt(IV) is not found in contrast to the PtOaNPs@Ti3C2Tx nanocomposite, indicating that the PtCl62- ions are completely reduced to Pt(II) and Pt(0) under glue discharge irradiation. In Figure S7b, the two main peaks at 336 (Pd 3d5/2) and 341.2 eV (Pd 3d3/2), which are as same as those of the PdObNPs@Ti3C2Tx nanosheets, are deconvulated. Given that the valence states of Pt and Pd play an important role in the catalytic ability of 16


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catalysts, the Pt 4f and Pd 3d core-level XPS spectra of the series of PtOaPdObNPs@Ti3C2Tx prepared at a plasma input power of 200 W for different periods, i.e., 1, 3, and 5 min, were analyzed, and the data are displayed in Figure 1. The same position of each component for Pt 4f is observed in the different samples, including 70.4–73.9, 71.25–75.15, and 72.11–77.54 eV, which correspond to Pt(0), Pt(II), and Pt(IV), respectively, but with different peak intensities for each component. The relative contents of the different species were calculated based on the ratio of each peak area to the sum area,58 and the data are summarized in Table S1. The results indicate the percentage of Pt(0) (33% in the three kinds of Pt-related species, i.e., Pt(0), Pt(II), and Pt(IV)) in the PtOaPdObNPs@Ti3C2Tx sample obtained at 3 min is the highest among the three samples, but it has the lowest content of Pt(IV) (25.6%). By contrast, the percentage of Pt(IV) of PtOaPdObNPs@Ti3C2Tx (5 min) is the highest (64.2%), whereas that of Pt(0) is the lowest (9.7%). The Pd 3d core-level XPS spectra of

the

three

samples

were

simulated

and

summarized.

In

contrast

to

PdObNPs@Ti3C2Tx, two doublets at binding energies of 335–340.5 and 336.7–341.7 eV are fitted and assigned to the Pd(0) and Pd(II) species, respectively. The presence of Pd(0) suggests that the Pd(II) species are partly reduced to its metal state, whereas the remaining part transfer to PdO because Cl signal is not found. The Pd(II) state is found only in the PdObNPs@Ti3C2Tx nanosheets, suggesting that the presence of the Pt-related NPs promote the reduction of Pd(II) to Pd(0). Similarly, the contents of Pd(0) and Pd(II) in the Pd-related NPs were calculated, and the data are summarized in Table S1. The percentage of Pd(0) (71.8%) is the highest among three samples, 17



Page 18 of 63

suggesting that the reduction degree of Pd(II) depends on the plasma irritation duration and is consistent with the trend of Pt(0). For comparison, the Pt 4f and Pd 3d core-level XPS spectra of the PtOaPdOb NPs were also fitted, and the data are summarized in Figures S7c and S7d. Similar results are observed to those of the sole PtOa and PdO NPs, but each species has a different density. Surface morphology characterizations. SEM and TEM characterizations were performed

to

understand

the

surface

performances

of

PtOaNPs@Ti3C2Tx,

PdObNPs@Ti3C2Tx, and the series of PtOaPdObNPs@Ti3C2Tx nanosheets. Compared with the exfoliated Ti3C2Tx, the surface morphologies of PtOaNPs@Ti3C2Tx and PdObNPs@Ti3C2Tx roughens because of the coverage of PtOa or the deposition of PdOb NPs, as shown in Figures S8 and S9, respectively. The TEM images of the PdOb@Ti3C2Tx nanosheets (Figures S9a and S9b) reveal that PdO NPs with the size of 2–3 nm are uniformly distributed on the Ti3C2Tx nanosheets surface. Continuous lattice fringes are observed in the HR-TEM image (the inset in (b) and (d) shows the FFT pattern of the HR-TEM image), suggesting that the PdO NPs successfully attach onto the Ti3C2Tx support. The interplanar spacing of the particle are 0.199 and 0.213 nm, which are assigned to the (102) and (110) planes of the PdO NPs [JCPDS no. 01-085-0713]. For the PtOa@Ti3C2Tx nanosheets, uniformly distributed PtOa NPs with the size of 2–3 nm are observed on the surface of the Ti3C2Tx support, and the lattice spacing of the Pt-related NPs is distributed over a wide range (Figures S9c and S9d). Among them, 0.226 and 0.196 nm correspond to Pt (111) and (200) planes [JCPDS no. 03-065-2868], respectively, whereas 0.217 and 0.171 nm are ascribed to 18


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the PtO (110) [JCPDS no. 00-043-1100] and PtO2 (121) planes [JCPDS no. 00-021-0613], respectively. These results are consistent with the XPS results, further confirming the formation of different metal valence states of Pt, PtO, and PtO2 of PtOaNPS@Ti3C2Tx and the metal valence states of Pd and PdO of the PdObNPs@Ti3C2Tx nanosheets. The SEM images of the series of PtOaPdObNPs@Ti3C2Tx nanosheets (Figure 2) reveal no substantial change among the different samples prepared under different plasma conditions. To investigate the distribution of the main elements (Pt and Pd), EDS spectrum and elemental mapping characterization was conducted on the PtOaPdObNPs@Ti3C2Tx nanosheets (3 min). Results show that Pt and Pd are uniformly dispersed in the composite (Figure S10), which is desirable for high catalytic efficiency. Owning to the low usage of Pt- and Pd-related catalysts, their color of each element containing in the PtOaPdObNPs@Ti3C2Tx nanosheets is very weak, consistent with the results of the XPS characterization. The TEM images of the three samples (Figure 3) show that the diameter of the PtOaPdOb NPs are increased with prolonged plasma treatment time. The inset size distribution histograms indicate that the average size of the alloy particles are 2.2, 2.5, and 6 nm for the PtOaPdObNPs@Ti3C2Tx samples obtained at 1, 3, and 5 min, respectively. These results suggest that increased plasma treatment time favors crystal growth. The PtOaPdObNPs@Ti3C2Tx samples obtained at 3 min are more evenly dispersed than those treated for 1 and 5 min. The alloy particles are composed of numerous single crystals, as evidenced by the HR-TEM image, of which the corresponding FFT 19



Page 20 of 63

patterns are shown in Figure 3c and the inset of Figure 3d. Moreover, the lattice spacing of (0.195 and/or 0.225 nm), which correspond to the Pt or Pd (200) and (111) planes, respectively, are observed in the HR-TEM images of the three samples, indicating the formation of Pd(0) in the presence of the Pt-related NPs. These findings agreed with the results of the XPS measurements. The coexistence of the Pt-related and Pd-related NPs in the three samples implies that plasma treatment can promote the synthesis of the PtPd alloy nanocomposites. Electrocatalysis performances of PtOaPdObNPs@Ti3C2Tx for water splitting. HER and OER are the two half reactions of electrocatalytic water splitting and are the key electrocatalytic reactions for obtaining clean and sustainable energy. Herein, the HER

and

OER

performances

of

the

as-obtained

electrocatalyst,

i.e.,

PtOaPdOdNPs@Ti3C2Tx nanosheets, were investigated simultaneously. As discussed, the plasma conditions used in preparing the PtOaPdOdNPs@Ti3C2Tx nanosheets determine the chemical components and the surface nanostructure, which can afford the various electrochemical activities for HER and OER. HER performances of all samples. To obtain further insight into the high HER performance of the PtOaPdObNPs@Ti3C2Tx nanosheets, the interaction between the inherent features and the catalytic performance must be understood. For comparison, the electrocatalytic activities of the PtOa NPs, PdO NPs, PtOaPdOb NPs, PtOaNPs@Ti3C2Tx, and PdObNPs@Ti3C2Tx prepared at a plasma input power of 200 W for 3 min were investigated. Figure 4a shows the HER performances of Pt/C (20%), the exploited pristine Ti3C2Tx, PtOa NPs, PdO NPs, PtOaPdObNPs, 20


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PtOaNPs@Ti3C2Tx, PdObNPs@Ti3C2Tx, and the series of PtOaPdObNPs@Ti3C2Tx synthesized at a plasma input power of 200 W for 1, 3, and 5 min. Table S2 summarizes the onset potential (η0) and overpotential at a current density of 10 mA cm-2 (η10) of all samples. As illustrated in Figure 4a, the PtOa NPs demonstrate good catalytic performances with an extremely small η0 of −3.8 mV) and a η0 of (28 mV), further confirming that the PtOa NPs produced by SP can enhance electrocatalytic activity. For the PdO NPs fabricated by SP, a slightly high η0 of 18.8 mV and a η10 of 51.1 mV are required, demonstrating its poorer HER performance than that of the PtOa NPs.59 For the exfoliated Ti3C2Tx nanosheets, large η0 values of 47.5 and 100.3 mV are required, indicating that the electrocatalytic activity of the exfoliated Ti3C2Tx nanosheets outperformed that of the Ti3C2Tx reported by Seh et al., which exhibited only a slight activity for HER, thereby requiring an overpotential to reach a current density of 10 mA cm-2 (609 mV).38 Among the polarization curves of the different samples, the PtOaPdObNPs@Ti3C2Tx (3 min) nanosheets exhibit the best HER performance with η0 of −17.6 and 26.5 mV, validating the synergistic effect among the different components. Figure 4b shows the calculated Tafel slopes of all samples. The exfoliated Ti3C2Tx nanosheets have a low Tafel slope of 55 mV dec-1, suggesting the fast kinetics of the electrocatalytic behavior toward HER. Compared with other carbon-based supports, such as reduced grapheme60,61 and carbon nanotubes,62 the exfoliated Ti3C2Tx displays better catalytic performance. Therefore, the exfoliated Ti3C2Tx is selected as the support of noble metal catalysts to reduce their usage and enhance the 21



electrocatalytic

activity.

Furthermore,

PtOa

NPs,

Page 22 of 63

PtOaNPs@Ti3C2Tx,

PdOdNPs@Ti3C2Tx, and PtOaPdObNPs@Ti3C2Tx exhibit very close Tafel slopes, resulting in their comparable catalytic kinetics toward HER. By contrast, PdO NPs have a relatively high Tafel slope of 58 mV dec-1, which is mainly due to the poorer electrocatalytic activity of the Pd-related catalysts for HER than that of the Pt-related catalysts.59 Among all samples, the PtOaPdObNPs@Ti3C2Tx nanosheets (3 min) show the lowest Tafel slope of 39 mV·dec−1 (Figure 4b), which is comparable to that of the Pt/C electrocatalyst (37 mV dec−1) and lower than those of the PtOa NPs (49 mV dec−1), PtOaPdOb NPs (47 mV dec−1), PtOaNPs@Ti3C2Tx (49 mV dec−1), PdObNPs@Ti3C2Tx nanosheets (48 mV dec−1), and other PtOaPdObNPs@Ti3C2Tx nanosheets (53 and 51 mV dec−1 for 1 and 5 min, respectively). Three principal steps based on classical theory were proposed for HER: Volmer reaction with a Tafel slope of ∼120 mV dec−1, Heyrovsky reaction with a Tafel slope of ∼40 mV dec−1, and Tafel reaction

with

a

Tafel

slope

of

∼30

mV

dec−1.63,64

Therefore,

the

PtOaPdObNPs@Ti3C2Tx nanosheets (200 W, 3 min) obey the Heyrovsky-Tafel reaction mechanism (H + H → H2↑), further confirming the facile kinetics and high catalytic activity for HER. For the other two PtOaPdObNPs@Ti3C2Tx catalysts (200 W, 1 and 5 min), their Tafel slopes are less than 60 mV dec−1, implying that their HER routes follow the Volmer–Heyrovsky reaction mechanism65. To further assess the superiority of the electrocatalytic activity caused by PtOaPdObNPs@Ti3C2Tx, nanocomposites with similar chemical structures, including HT-PtOaNPs@Ti3C2Tx, HT-PdObNPs@Ti3C2Tx, and HT-PtOaPdObNPs@Ti3C2Tx, 22


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synthesized by hydrothermal method were utilized as electrocatalysts for HER and OER. As shown in their polarization curves (Figure 5a), even the onset potentials of these three samples are very small and required a relatively high η10, which varies from 46.2 to 65.5 mV. The calculated Tafel slopes of these samples are displayed in Figure

5b.

The

rank

PtOaPdObNPs@Ti3C2Tx (3

of

the

min)

Tafel

Solution-Plasma-Assisted Bimetallic Oxide Alloy Nanoparticles of Pt

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