Achieving Highly Electrocatalytic Performance by Constructing Holey

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Achieving Highly Electrocatalytic Performance by Constructing Holey Reduced Graphene Oxide Hollow Nanospheres Sandwiched by Interior and Exterior Pt Nanoparticles Xiaoyu Qiu, Xiaoxiao Yan, Ke Cen, Dongmei Sun, Lin Xu, and Yawen Tang ACS Appl. Energy Mater., Just Accepted Manuscript • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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ACS Applied Energy Materials

Achieving Highly Electrocatalytic Performance by Constructing Holey Reduced Graphene Oxide Hollow Nanospheres Sandwiched by Interior and Exterior Pt Nanoparticles Xiaoyu Qiu†, Xiaoxiao Yan†, Ke Cen, Dongmei Sun, Lin Xu*, Yawen Tang*

Jiangsu Key Laboratory of New Power Batteries, Jiangsu Collaborative Innovation Center of Biomedical Functional Materials, School of Chemistry and Materials Science, Nanjing Normal University, Nanjing 210023, PR China

KEYWORDS: 3D r-GO nanospheres, Pt nanoparticles, ORR, MOR, fuel cells.

ABSTRACT: Two dimensional (2D) graphene nanosheets are considered as an attractive support to load metal nanoparticles for applications in fuel cells due to their extraordinary physicochemical properties arising from the 2D nanostructure. However, fabricating graphene/metal nanoparticle nanohybrids with superior electrochemical performance remains a great challenge to date. In this manuscript, we, for the first time, demonstrate a novel and ingenious approach to fabricate holey reduced graphene oxide hollow nanospheres sandwiched by interior and exterior Pt nanoparticles (denoted as Pt@holey r-GO@Pt hollow nanospheres), using uniform

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SiO2 nanospheres as the templates. The Pt@holey r-GO@Pt hollow nanospheres represent a new type of metal/graphene heteroarchitecture due to their rich porosity, abundant active sites, facilitated reaction kinetics and outstanding structural stability. Thanks to these distinguished merits, the as-prepared Pt@holey r-GO@Pt hollow nanospheres exhibit greatly enhanced electrocatalytic performances toward the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) as compared with the intact counterparts and commercial Pt/C catalyst, showing great potential in fuel cell devices. The smart strategy outlined here should be readily applicable to rational design of other graphene/metal nanoparticle nanohybrids for energy storage and conversion applications in the future.

Due to the unique physicochemical properties, noble metal-based nanoparticles have been considered as a class of versatilely functional materials and hold great potentials in various fields, including biomedicine, catalysis, electronics and sensors, etc.1-7 When used as electrocatalysts, it has extensively documented that hybridization of noble metal nanoparticles with carbonaceous supports could further improve their activity and stability by enhancing the overall conductivity and alleviating the migration and aggregation of nanoparticles.8-13 Among various carbonaceous supports, graphene, an atomically thin two-dimensional (2D) carbon nanosheet, represents an attractive platform for the construction of noble metal/graphene nanohybrids owing to its large theoretical surface area (~ 2600 m2 g-1), excellent electrical conductivity, superior mechanical strength, and high electrochemical stability.14-17 In spite of a lot


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of appealing properties, 2D graphene nanosheets still suffer from some severe problems when utilized as a catalyst support. Specifically, the hydrophobic surface of graphene nanosheets renders a poor dispersibility in water and goes against the permeation and adsorption of reactants and electrolytes. Furthermore, the inert surface of graphene is not favorable to the deposition and immobilization of nanocrystals.18-20 For this reason, graphene oxide (GO) and reduced graphene oxide (r-GO) are regarded as a class of ideal supports due to the hydrophilic feature, rich surface functional groups and ease of surface modification.21-23 Unfortunately, the strong π−π interactions and van der Waals force between graphene-based nanosheets can result in serious aggregation and irreversible restacking (Figure 1a), which definitely reduces the accessible surface area, impedes mass transport and buries active catalytic sites, and thus leads to an unsatisfied catalytic activity. 24-29

To

address

these

issues,

rational

design

of

three-dimensional

(3D)

graphene-based materials can not only inherit excellent intrinsic properties of 2D graphene, but also generate some new intriguing properties, including prevented nanosheet restacking, enhanced mechanical robustness, and fast mass/electron transport kinetics, which are expected to make great contributions to the enhancement of electrocatalytic performance.30-34 To date, various synthetic strategies, such as self-assembly approach, hard template-engaged synthesis, and chemical vapor deposition (CVD) method, have been developed for the fabrication of 3D graphene-based nanoarchitectures for diverse applications.35-39 Recently, we have developed a general approach to fabricate 3D hollow GO nanospheres decorated with



noble metal nanoparticles by employing SiO2 nanospheres as templates.24, 31 Because of the minimized corrugation and excellent structural integrity, the resultant 3D hybrid hollow nanospheres exhibit greatly enhanced performances with respect to the 2D graphene sheets-based counterparts for electrocatalysis. However, in such formed architecture, the interior surface of the GO hollow nanospheres cannot be effectively utilized due to the intact wall and closed sphere. As such, the available exposed surface would be noticeably reduced and mass diffusion/transport would be greatly restricted, as illustrated in Figure 1b. We envision that holey GO hollow nanospheres with high porosity may represent an effective solution to overcome these limitations.40-41 Through fabricating holey GO hollow nanospheres, as depicted in Figure 1c, the interior surfaces of the holey nanospheres can be also utilized for the deposition of nanoparticles, thus significantly increasing the number of catalytically active sites. In addition, holey spherical nanostructures enable to offer shortened mass diffusion distance and facile electrolyte penetration, thus facilitating to expedite the reaction kinetics.

Herein, for the first time, we demonstrate a novel and ingenious approach to fabricate holey reduced GO hollow nanospheres sandwiched by interior and exterior Pt nanoparticles (denoted as Pt@holey r-GO@Pt hollow nanospheres), using uniform SiO2 nanospheres as the sacrificial templates. Thanks to the unique structural advantages, these Pt@holey r-GO@Pt hollow nanospheres exhibit greatly enhanced electrocatalytic performances toward the oxygen reduction reaction (ORR) and methanol oxidation reaction (MOR) as compared with the intact counterpart and


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commercial Pt/C catalyst, making them a promising electrocatalyst in fuel cell devices.

The overall synthesis process for the Pt/holey r-GO/Pt hollow nanospheres is schematically illustrated in Figure S1. Specifically, it involves (i) in-situ growth of sparse Pt nanoparticles onto the positively charged SiO2 nanospheres to form Pt nanoparticle-decorated SiO2 nanospheres (denoted as SiO2@Pt nanospheres), (ii) conformal coating of the resultant SiO2@Pt nanospheres with graphene oxide, leading to the formation of SiO2@Pt@GO nanospheres, (iii) digging holes on GO layer of the SiO2@Pt@GO nanospheres to generate SiO2@Pt@holey reduced GO (r-GO) nanospheres through a novel chemical etching strategy, which will be discussed in detail later, (iv) further deposition of a layer of ultrafine Pt nanoparticles on the surface of SiO2@Pt@holey r-GO nanospheres, forming SiO2@Pt@holey r-GO@Pt nanospheres and finally (v) removal of interior SiO2 nanospheres from the SiO2@Pt@holey r-GO@Pt nanospheres by NaOH solution, producing holey r-GO hollow nanospheres sandwiched by internal and external Pt nanoparticles, namely, Pt@holey r-GO@Pt hollow nanospheres.

The morphological evolution from initial SiO2 nanospheres to final Pt@holey r-GO@Pt hollow nanospheres is examined by transmission electron microscopy (TEM) images of the products collected at different reaction stages, as presented in Figure 2. Uniform SiO2 nanospheres with an average diameter of 160 nm (Figure 2a) were initially prepared through a facile stöber method,42 and served as sacrificial



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templates to regulate the overall morphology during the entire synthesis. After surface modification with 3 layers of polyelectrolytes (PDDA/PSS/PDDA), the positively charged SiO2 nanospheres were decorated with sparse Pt nanoparticles (Figure 2b). It is noteworthy that the coverage of Pt nanoparticles on the SiO2@Pt nanospheres at this stage is deliberately controlled at a relatively low level to expose enough positively charged surfaces of the SiO2 nanospheres for the subsequent GO coating. Meanwhile, the sparse Pt layer can effectively avoid excessive agglomeration during subsequent thermal treatment. Since SiO2@Pt nanospheres and GO nanosheets are oppositely charged, the negatively charged GO nanosheets with high flexibility could spontaneously encapsulate the SiO2@Pt nanospheres through strong electrostatic attractions, forming SiO2@Pt@GO nanospheres (Figure 2c). Subsequently, creating holes on GO layer of the SiO2@Pt@GO nanospheres was accomplished by surface adsorption of Fe(III) ions, in-situ pyrolysis and acid leaching processes, resulting in the formation of SiO2@Pt@holey r-GO nanospheres (Figure 2d). As revealed by the TEM image, the pre-deposited Pt nanoparticles inevitably become larger after heat treatment and holes on the SiO2@Pt@holey r-GO nanospheres can be clearly observed. For clarity, 2D pristine GO nanosheets (Figure S2a) were employed as a model to illustrate the detailed digging processes. First, Fe ions can absorb strongly on the surface of GO nanosheets due to the electrostatic attraction forces. While heating at 800 oC under an inert atmosphere, the adsorbed Fe ions would be pyrolysed into Fe2O3 nanoparticles and GO would be partially reduced to r-GO at the same time, producing

Fe2O3

nanoparticle-decorated

r-GO

nanosheets


(Figure

S2b-c).

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Subsequently, Fe2O3 nanoparticles could be selectively etched away by H2SO4 solution, generating holey r-GO nanosheets with numerous pores (Figure S2d-f). After the hole-digging process, the exterior surface of the resultant SiO2@Pt@holey r-GO nanospheres were further covered by a layer of Pt nanoparticles through in-situ growth, producing SiO2@Pt@holey r-GO@Pt nanospheres (Figure 2e). Combining with inner sparse Pt layer, the full use of graphene can be realized by construction of mass transfer holey channels. Eventually, the innermost SiO2 nanospheres were removed from the SiO2@Pt@holey r-GO@Pt nanospheres by NaOH solution, resulting in Pt@holey r-GO@Pt hollow nanospheres (Figure 2f). It is noteworthy that the spherical skeleton and structural integrity of the prepared Pt@holey r-GO@Pt hollow nanospheres can be perfectly maintained without any deformation or collapse after the removal of SiO2 template, demonstrating their excellent mechanical robustness.

Representative scanning electron microscopy (SEM) image (Figure 3a) of the as-fabricated Pt@holey r-GO@Pt hollow nanospheres exhibits a spherical morphology with rough surface and uniform size. High-magnification SEM image (Figure 3b) suggests that the shells are composed of numerous closely interconnected Pt nanoparticles. A broken nanosphere (marked with an arrow in Figure 3b) clearly reveals the hollow feature of the obtained nanospheres. The TEM image (Figure 3c) further confirms that these Pt@holey r-GO@Pt nanospheres are completely hollow. Moreover, no isolated Pt nanoparticles can be observed away from the nanospheres, indicating the complete in-situ growth of Pt nanoparticles on the surface of Pt@holey



r-GO nanospheres. From the TEM image (Figure 3d) of an individual hollow nanosphere, it can be clearly observed that the shell thickness of the hollow nanosphere is as thin as only a few nanometers. The corresponding selected-area electron diffraction (SAED) pattern (inset of Figure 3d) suggests the polycrystalline nature of the hollow nanosphere and the diffraction rings can be readily assigned to Pt with face-centered-cubic (fcc) phase. The magnified TEM image (Figure 3e) further verifies that the shell consists of interlinked fine nanoparticles with plenty of interparticle mesopores distributed throughout the entire shell. The presence of pores could significantly provide more active sites and facilitate mass diffusion and transport, which are beneficial for catalytic applications. From the high-resolution TEM (HRTEM) image (Figure 3f), one can clearly see that the size of Pt nanoparticles is estimated to be approximately 4 nm. Additionally, notable grain boundaries are also clearly distinguishable between the varied oriented grains. The d-spacing for adjacent lattice fringes are measured to be 0.224 nm, corresponding to that of the (111) planes of fcc Pt.

Figure 4a presents the X-ray diffraction (XRD) pattern of the synthesized Pt@holey r-GO@Pt hollow nanospheres. The broad peak positioned at 23.3o indicated by an asterisk can be ascribed to the (002) peak of graphitic carbon, and the rest diffraction peaks can be indexed to fcc-phased Pt. According to the Scherrer formula43, the average crystallite size is estimated to be 3.8 nm, in good accordance with the above TEM observation. Thermogravimetry analysis (TGA) is carried out under air atmosphere to quantify the exact Pt content in the final Pt@holey r-GO@Pt


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hollow nanospheres. As shown in Figure 4b, the final residue with 53.9 wt% of the original weight is the oxidized Pt in the form of PtO2. Therefore, the initial weight percentage of Pt in the as-prepared Pt@holey r-GO@Pt hollow nanospheres is calculated to be 46.4 wt%. The graphitization degree of the carbon in the Pt@holey r-GO@Pt hollow nanospheres is investigated by Raman spectrum. As illustrated in Figure 4c, the D band is associated with the defects and disordered carbon and the G band can be attributable to sp2-hybridized carbon.44-45 Evidently, the peak intensity ratio of D band to G band (ID/IG) for the Pt@holey r-GO@Pt hollow nanospheres is noticeably lower than that of the pristine GO nanosheets (0.89 vs. 1.02), suggesting a well-crystallized graphitic carbon in the Pt@holey r-GO@Pt hollow nanospheres. Such a high graphitization degree would be undoubtedly favorable for improving the overall electronic conductivity of the final product. This result is strongly corroborated by the X-ray photoelectron spectroscopy (XPS) results. As displayed in Figure 4d, the peak intensity of C-O peak in the Pt@holey r-GO@Pt hollow nanospheres is drastically lower than that in the pristine GO nanosheets, indicating a deep reduction of GO during the hole-digging process. As revealed by Figure 4e, Pt exists predominantly as zero-valent state in the resultant product. The N2 sorption isotherms (Figure 4f) of the Pt@holey r-GO@Pt hollow nanospheres can be categorized as type-IV isotherms with an obvious hysteresis loop, suggestive the existence of mesopores.46-47 Additionally, the steep nitrogen uptake at a high relative pressure (p/p0 > 0.9) manifests the presence of larger micropores corresponding to the hollow

interior

of

the

Pt@holey

r-GO@Pt

hollow


nanospheres.48

The


Brunauer-Emmett-Teller (BET) surface area of the Pt@holey r-GO@Pt hollow nanospheres is measured to be 186.7 m2 g-1.

The formed Pt@holey r-GO@Pt hollow nanospheres may offer combined features of highly holey architecture and abundant catalytically active sites, making it an appealing candidate for high-efficiency electrocatalysis. Therefore, the electrocatalytic properties of the Pt@holey r-GO@Pt hollow nanospheres for the ORR were initially appraised to demonstrate their architectural advantages in electrocatalysis. In order to highlight the importance of hole-digging, intact GO hollow nanospheres sandwiched by internal and exterior Pt nanoparticles (denoted as Pt@r-GO@Pt hollow nanospheres, Figure S3) were prepared through a similar protocol expect without hole-digging procedure and their electrocatalytic properties were evaluated under the identical test conditions. Additionally, commercial Pt/C (20 wt%) catalyst was also employed as a benchmark. Figure S4a presents the cyclic voltammograms (CVs) of Pt@holey r-GO@Pt hollow nanospheres, Pt@r-GO@Pt hollow nanospheres and commercial Pt/C catalyst obtained in N2-saturated 0.5 M H2SO4 solution at a sweep rate of 50 mV s-1. As displayed in Figure S4b, the electrochemically active surface areas (ECSAs) of the three catalysts were estimated to be 54.69, 48.41 and 53.05 m2 g-1, respectively.

The ORR measurements were performed in O2-saturated 0.5 M H2SO4 solution by using a glassy carbon rotating ring electrode (RDE) at a sweep rate of 5 mV s-1 and a rotating rate of 1600 rpm. As evidenced from the ORR polarization curves (Figure


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5a), the Pt@holey r-GO@Pt hollow nanospheres exhibit the highest ORR activity among the studied three catalysts with the most positive onset potential and half-wave potential. As summarized in Figure 5b, the onset potential of Pt@holey r-GO@Pt hollow nanospheres is determined to be 0.942 V, which is higher than that of Pt@r-GO@Pt hollow nanospheres (0.939 V) or commercial Pt/C catalyst (0.928 V), suggesting that the ORR occurs more favorably on Pt@holey r-GO@Pt hollow nanospheres. Similarly, the half-wave potential of Pt@holey r-GO@Pt hollow nanospheres positively shifts more than 20 mV as compared with the reference samples, verifying the enhanced ORR activity with a relatively small overpotential. From the corresponding Tafel plots (Figure 5c), one can clearly observe that the Pt@holey r-GO@Pt hollow nanospheres deliver the highest specific activity at the same applied potential among the three catalysts. For instance, as depicted in Figure 5d, the Pt@holey r-GO@Pt hollow nanospheres exhibit a specific activity of 2.12 mA cm-2 at 0.80 V, which is significantly higher than those of Pt@r-GO@Pt hollow nanospheres (1.63 mA cm-2) and commercial Pt/C catalyst (1.66 mA cm-2). Likewise, the current density of Pt@holey r-GO@Pt hollow nanospheres at 0.85 V reaches a value of 1.10 mA cm-2, almost 1.64 times larger than that of commercial Pt/C catalyst. The long-term stability of the prepared Pt@holey r-GO@Pt hollow nanospheres is further investigated by accelerated durability tests (ADTs). As illustrated in Figure 5e, the Pt@holey r-GO@Pt hollow nanospheres show a small negative shift of 14 mV in half-wave potential after 3000-cycle test. By contrast, the degradations of Pt@r-GO@Pt hollow nanospheres and commercial Pt/C catalyst are more severe with



remarkable decreases of their half-wave potentials (42 mV for Pt@r-GO@Pt hollow nanospheres and 68 mV for commercial Pt/C catalyst) under the identical test conditions, as shown in Figure 5f and Figure S5. Moreover, the Pt@holey r-GO@Pt hollow nanospheres do not show an evident loss in the ECSAs, as shown in Figure S 6a. Additionally, TEM image (Figure S6b) manifests that the spherical morphology of the Pt@holey r-GO@Pt hollow nanospheres could be well preserved after the ADTs, confirming their structural robustness.

Impressively, the Pt@holey r-GO@Pt hollow nanospheres also exhibit efficient performance toward the methanol oxidation reaction (MOR) with high activity and superior durability. Figure 6a shows the Pt mass-normalized CV curves of the three catalysts. Apparently, the Pt@holey r-GO@Pt hollow nanospheres exhibit a much more negative onset oxidation potential with much higher activity as compared with the Pt@r-GO@Pt hollow nanospheres and commercial Pt/C. As shown in Figure 6b, the Pt mass-normalized current density of Pt@holey r-GO@Pt hollow nanospheres affords a value of 575.2 A g-1, which is 1.3-fold and 1.7-fold larger than those of the Pt@r-GO@Pt hollow nanospheres (440.4 A g-1) and commercial Pt/C (335.1 A g-1), respectively. Additionally, the specific kinetic activity (Figure 6b) of Pt@holey r-GO@Pt hollow nanospheres (10.51 A m-2) is approximately 1.67-times larger than that of the commercial Pt/C catalyst (6.31 A m-2), indicating a higher intrinsic activity of the Pt@holey r-GO@Pt hollow nanospheres. As disclosed by the CO-stripping CV plots (Figure 6c), both the onset potential and the peak potential of CO oxidation on Pt@holey r-GO@Pt hollow nanospheres negatively shift as compared with those on


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Pt@r-GO@Pt hollow nanospheres and commercial Pt/C, revealing a better anti-poisoning

ability

of

the

Pt@holey

r-GO@Pt

hollow

nanospheres.

Chronoamperometry (CA) measurements (Figure 6d) suggest that the i-t curve of Pt@holey r-GO@Pt hollow nanospheres shows a slower current attenuation relative to the other two references, affirming that the Pt@holey r-GO@Pt hollow nanospheres exhibit a better tolerance to CO species generated during the MOR. Furthermore, the MOR current density on Pt@holey r-GO@Pt hollow nanospheres is significantly larger than those on the Pt@r-GO@Pt hollow nanospheres and commercial Pt/C, further proving the extraordinary stability of the Pt@holey r-GO@Pt hollow nanospheres.

The outstanding electrocatalytic activity of the Pt@holey r-GO@Pt hollow nanospheres toward both ORR and MOR could be reasonably ascribed to the unique structural superiorities. (1) Due to the presence of numerous pore on the walls of the Pt@holey r-GO@Pt hollow nanospheres, such holey hollow architectures engender large exposed both exterior and interior surfaces, which could provide high molecular permeability and plenty of accessible active sites for catalytic reactions. (2) The broad void space inside the holey hollow architectures could facilitate mass transfer and diffusion for electrocatalysis reactions, which could improve the reaction efficiency. (3) The high electric conductivity of the holey r-GO hollow nanospheres and the intimate contact of Pt nanoparticles with the r-GO support not only ensure a rapid electron transfer during the reactions, but also help to stabilize Pt nanoparticles.



Besides, the surface-modified PDDA also contribute to the better electrocatalytic performance due to the “metallization” effect of PDDA on Pt nanoparticles. 49-51

In conclusion, for the first time, we have presented an elaborate strategy to construct holey reduced GO hollow nanospheres sandwiched by interior and exterior Pt nanoparticles using uniform SiO2 nanospheres as the sacrificial templates. Through tactfully digging holes on GO hollow nanospheres, not only can both of the exterior and interior surfaces of GO surfaces be fully utilized for Pt nanoparticle deposition, but the molecular permeability and mass diffusion can also be remarkably facilitated. Due to the unique highly open and porous structure, the synthesized Pt@holey r-GO@Pt hollow nanospheres serve as a high-efficiency electrocatalyst toward the ORR and MOR with greatly improved activity and durability as compared with Pt@r-GO@Pt hollow nanospheres and commercial Pt/C catalyst, making them a promising electrocatalyst in fuel cell devices. We believe that the smart strategy presented here may open an avenue to rational synthesis of other metal@ holey r-GO@metal hollow nanospheres with multiple compositions for various applications in the future.

Experimental Section Reagents and Chemicals Poly (diallyl dimethyl ammonium chloride) (PDDA, Mw < 500 000 Da), and poly (sodium 4-styrenesulfonate) (PSS, Mw < 700 000 Da) were supplied by Alfa Aesar


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Co. Ltd. Tetraethyl orthosilicate (TEOS), potassium tetrachloroplatinate(II) (K2PtCl4), iron (III) chloride hexahydrate (FeCl3•6H2O, 99.99%), trisodium citrate dehydrate, and sodium borohydride (NaBH4) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Graphene oxide (GO) was provided by Nanjing XFNANO Materials Tech Co., Ltd (Nanjing, China). Commercial Pt/C was purchased from Johnson Matthey Corporation. All chemicals were used as received without further purification. Synthesis of SiO2@Pt nanospheres In a typical synthesis, uniform SiO2 nanospheres with an average diameter of 150 nm were initially synthesized via a modified stöber method and used as hard templates to regulate the overall morphology of the final product. Prior to Pt deposition, the as-obtained SiO2 nanospheres were further surface modified with positive charges through a layer-by-layer self-assembly approach according to our previous protocols. Subsequently, 30 mg of the positively charged SiO2 nanospheres, 10 mg of K2PtCl4, 60 mg of trisodium citrate dehydrate were added into 40 mL of water and sonicated for 30 min. Then 10 mg of NaBH4 powder was rapidly added into the above suspension to in-situ reduce K2PtCl4 on the surface of the SiO2 nanospheres, forming Pt nanoparticle-decorated SiO2 nanospheres (denoted as SiO2@Pt nanospheres). Synthesis of SiO2@Pt@ holey r-GO nanospheres 0.5 g SiO2@Pt nanospheres and 0.25 g GO nanosheets were disperse into 20 mL of distilled water by stirring for 5 h. Due to the electrostatic attraction, the negatively



charged GO nanosheets with high flexibility could spontaneously encapsulate the SiO2@Pt nanospheres, leading to the formation of SiO2@Pt@GO nanospheres. Subsequently, 20 mL of 0.5 M FeCl3 solution was added into the resultant suspension. After stirring for 1 h, the sample was collected through centrifugation. After drying, the obtained sample was transferred to tube furnace and heated to 800 oC under N2 atmosphere for 3 h. After pyrolysis, the nanospheres were subjected to 40 mL of 1 M H2SO4 solution at 60 oC to remove the generated Fe2O3 nanoparticles, forming SiO2@Pt@ holey r-GO nanospheres. Synthesis of Pt@holey r-GO@Pt hollow nanospheres 30 mg of the SiO2@Pt@ holey r-GO nanospheres were dispersed into 60 mL of 0.5 M NaCl solution by sonication, followed by introducing 0.75 g of PDDA to get the surface of SiO2@Pt@ holey r-GO nanospheres positively charged. Subsequently, 30 mg of K2PtCl4 and 60 mg trisodium citrate dehydrate were added into the above suspension and the resultant mixture was further sonicated for 30 min. Then 10 mg of NaBH4 powder was added into the above well-dispersed suspension to obtain SiO2@Pt@ holey r-GO@Pt nanospheres. After centrifugation and drying, the SiO2 templates were finally etched by 15 mL of 2 M NaOH solution, forming Pt@holey r-GO@Pt hollow nanospheres. Characterization TEM and HRTEM images were taken on a JEOL JEM-2010 at an accelerating voltage of 200 kV. SEM images were captured on a Hitachi S-4800 scanning electron microscope, operating at 5 kV. XRD pattern were acquired on a Model D/max-rC


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X-ray diffractometer by using Cu Kα radiation source (λ = 1.5406 Å), operating at 40 kV and 100 mA. N2 adsorption/desorption isotherms were measured on Thermo Fisher Scientific Surfer Gas Adsorption Porosimeter. XPS was carried out on a thermo VG scientific ESCALAB 250 spectrometer with an Al Kα radiator, and the binding energy was calibrated by means of the C 1s peak energy of 284.6 eV. The compositional analysis of the catalysts was determined using inductively coupled plasma atomic emission spectrum (ICP-AES) techniques. Electrochemical measurements The electrochemical measurements were performed in a standard three-electrode system on an electrochemical workstation (CHI 660 C electrochemical analyzer, CH instruments, Shanghai, Chenhua Co.) at 25 oC. The three-electrode system was composed of a reference electrode of saturated calomel electrode (SCE), a catalyst modified glassy carbon electrode (GCE, 3 mm in diameter, 0.071 cm2) as the working electrode, and a platinum wire as the auxiliary electrode. For preparing working electrode, 10 mg of the electro-catalyst was dispersed in 5 mL of water by sonication to form a homogeneous ink. 6 µL of the electrocatalyst ink was dropped onto the surface of a glassy carbon electrode, and 3 µL nafion solution (5 wt. %) was added to fix the electrocatalysts on the GCE surface. Cyclic voltammetry (CV) measurements were performed in N2-saturated 0.5 M H2SO4 solution. The ORR measurements were conducted in 0.5 M H2SO4 solution under a flow of O2 using the rotating disk electrode (RDE) at a rotation rate of 1600 rpm with a sweeping rate of 5 mV s-1.



The electrochemical measurements for MOR were performed in N2-saturated 0.5 M H2SO4 + 0.5 M CH3OH solution. For CO-stripping tests, CO was pre-adsorbed on the surface of the catalyst by bubbling CO into a 0.5 M H2SO4 solution while holding the working electrode at 0 V vs. SCE for 15 min. The remnant CO was driven away by flowing N2 for 20 min before CO-stripping tests. For ORR and MOR activities, the specific activity and mass activity of the catalysts were normalized to ECSA and Pt mass, respectively. All potentials in this work were converted with respect to the reversible hydrogen electrode (RHE).

Figure 1. Different configurations of GO supported Pt nanoparticles. (a) 2D GO nanosheet supported Pt nanoparticles, (b) 3D GO hollow nanosphere supported Pt nanoparticles, and (c) 3D holey GO hollow nanosphere sandwiched by both internal and external Pt nanoparticles.


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Figure 2. Representative TEM images of the products collected at different reaction stages. (a) SiO2 nanosphere template, (b) SiO2@Pt nanospheres, (c) SiO2@Pt@GO nanospheres, (d) SiO2@Pt@holey r-GO nanospheres, (e) SiO2@Pt@holey r-GO@Pt nanospheres, and (f) Pt@holey r-GO@Pt hollow nanospheres.



Figure 3. Morphological characterizations of the resultant Pt@holey r-GO@Pt hollow nanospheres. (a)-(b) SEM images, (c)-(d) TEM images and SAED pattern (Inset of panel d), (e) Magnified TEM image, (f) HRTEM image.


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Figure 4. Compositional analyses of the as-prepared Pt@holey r-GO@Pt hollow nanospheres. (a) XRD pattern, (b) TGA curve, (c) Raman spectra, (d) C1s region in XPS spectrum, (e) Pt 4f region in XPS spectrum, and (f) N2 sorption isotherms.

Figure 5. Comparison of ORR performances of Pt@holey r-GO@Pt hollow nanospheres, Pt@r-GO@Pt hollow nanospheres and commercial Pt/C catalyst. (a) ORR polarization curves recorded in an O2-saturated 0.5 M H2SO4 solution with a



sweep rate of 5 mV s-1 and a rotation rate of 1600 rpm, (b) onset potentials and half-wave potentials of different catalysts, (c) Tafel plots, (d) comparison of mass activity at 0.80 and 0.85 V, (e) ORR polarization curves of Pt@holey r-GO@Pt hollow nanospheres before and after the durability test, and (f) half-wave potential degradations of different catalysts.

Figure 6. Comparison of MOR performances of Pt@holey r-GO@Pt hollow nanospheres, Pt@r-GO@Pt hollow nanospheres and commercial Pt/C catalyst. (a) Pt mass-normalized CV curves recorded in an N2-saturated 0.5 M H2SO4 +0.5 M CH3OH solution with a sweep rate of 50 mV s-1. (b) histogram of mass and specific activities, (c) CO-stripping curves, and (d) i-t curves recorded at 0.8 V.

ASSOCIATED CONTENT


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The characterization of samples (including TEM images, SEM and XRD patterns) and electrochemical test results are provided in Supporting Information.

AUTHOR INFORMATION * E-mail: [email protected], [email protected] (Lin Xu) [email protected] (Yawen Tang) † Xiaoyu Qiu and Xiaoxiao Yan contributed equally.

Acknowledgements This work was financially supported by National Natural Science Foundation of China (21503111 and 21576139), Natural Science Foundation of Jiangsu Province (BK20171473) and Natural Science Foundation of Jiangsu Higher Education Institutions of China (16KJB150020). The authors are also grateful for the supports from National and Local Joint Engineering Research Center of Biomedical Functional Materials and a project sponsored by the Priority Academic Program Development of Jiangsu Higher Education Institutions. References 1. Zhang, J.; Winget, S. A.; Wu, Y.; Su, D.; Sun, X.; Xie, Z. X.; Qin, D., Ag@Au Concave Cuboctahedra: A Unique Probe for Monitoring Au-Catalyzed Reduction and Oxidation Reactions by Surface-Enhanced Raman Spectroscopy. ACS Nano 2016, 10, 2607-2616.



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Table of Contents Graphic Title: Achieving Highly Electrocatalytic Performance by Constructing Holey Reduced Graphene Oxide Hollow Nanospheres Sandwiched by Interior and Exterior Pt Nanoparticles

We present an elaborate strategy to construct holey reduced GO hollow nanospheres sandwiched by interior and exterior Pt nanoparticles using uniform SiO2 nanospheres as templates. These hollow spherical nanohybrids demonstrate high performances toward the ORR and MOR with greatly improved activity and durability.


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Achieving Highly Electrocatalytic Performance by Constructing Holey

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