Aqueous Preparation of Platinum Nanoflowers on Three-Dimensional Graphene for Efficient Methanol Oxidation

A facile aqueous method to construct a platinum nanoflowers (PtNFs)/three-dimensional (3D) graphene electrode for electrochemical catalysis was demonstrated. PtNFs composed of thin Pt nanowires with the length of 6–16 nm and the diameter of 2–3 nm were prepared on 3D graphene foam as a growth template in the aqueous solution without any surfactant. The 3D graphene foam was used for patterning PtNFs and controlling their morphology. The fabricated PtNF/3D graphene electrode was applied for electrocatalytic methanol oxidation. Electrochemical measurements show that the PtNF/3D graphene electrode has higher electrocatalytic activity and better stability than commercial Pt-C modified glassy carbon electrode. It displays promising potential for applications in fuel cells.

In our work, 3D graphene foam was used as a support for direct growth of platinum nanoflowers (PtNFs) under room temperature in aqueous solution without any surfactant.Monomorphic PtNFs composed of thin Pt nanowires were successfully synthesized through the template function of 3D graphene.The 3D graphene-supported PtNFs act as a novel 3D electrode for methanol oxidation reaction (MOR), which holds many advantages including improving the interaction between PtNFs and 3D graphene and enhancing mass transport in the process of electrocatalysis.The fabricated PtNF/3D graphene electrode shows higher electrocatalytic activity and better CO tolerance for methanol oxidation than that of the commercial Pt-C catalyst (Hispec4000) modified glassy carbon electrode (GCE) (Pt-C/GCE).

Results and Discussion
Scanning electron microscopy (SEM) micrographs of 3D graphene foam in Figure 1 show that it has a well-defined macroporous structure with a thin and smooth graphene skeleton.Raman spectra taken from different regions of 3D graphene foam present a G band at ~1560 cm −1 and 2D band at ~2700 cm −1 (Figure 2).The ratio of the intensity of the G band (I G ) and 2D band (I 2D ) is related to the number of layers of graphene [27].The sharp and symmetric 2D band corresponds with a single-layer graphene, and the broad 2D band corresponds with a few-layer graphene in Figure 2, indicating that the prepared 3D graphene foam comprises both single-layer and few-layer graphene [41][42][43][44][45][46].Furthermore, the absence of an obvious disorder-induced D band from the Raman characterization of 3D graphene foam suggests its high quality [45].
In our work, 3D graphene foam was used as a support for direct growth of platinum nanoflowers (PtNFs) under room temperature in aqueous solution without any surfactant.Monomorphic PtNFs composed of thin Pt nanowires were successfully synthesized through the template function of 3D graphene.The 3D graphene-supported PtNFs act as a novel 3D electrode for methanol oxidation reaction (MOR), which holds many advantages including improving the interaction between PtNFs and 3D graphene and enhancing mass transport in the process of electrocatalysis.The fabricated PtNF/3D graphene electrode shows higher electrocatalytic activity and better CO tolerance for methanol oxidation than that of the commercial Pt-C catalyst (Hispec4000) modified glassy carbon electrode (GCE) (Pt-C/GCE).

Results and Discussion
Scanning electron microscopy (SEM) micrographs of 3D graphene foam in Figure 1 show that it has a well-defined macroporous structure with a thin and smooth graphene skeleton.Raman spectra taken from different regions of 3D graphene foam present a G band at ~1560 cm −1 and 2D band at ~2700 cm −1 (Figure 2).The ratio of the intensity of the G band (IG) and 2D band (I2D) is related to the number of layers of graphene [27].The sharp and symmetric 2D band corresponds with a singlelayer graphene, and the broad 2D band corresponds with a few-layer graphene in Figure 2, indicating that the prepared 3D graphene foam comprises both single-layer and few-layer graphene [41][42][43][44][45][46].Furthermore, the absence of an obvious disorder-induced D band from the Raman characterization of 3D graphene foam suggests its high quality [45].3D graphene foam was directly used as a growing template for synthesis of PtNFs.The experimental details can be seen in the Materials and Methods section.The morphology and structure of PtNFs growing on 3D graphene can be modulated by the addition of a Pt precursor.These were studied by SEM and transmission electron microscopy (TEM) characterizations.As shown in Figures 3A and 4A-C, 3D PtNFs uniformly cover the surface of 3D graphene by a considerable amount.When the addition of H2PtCl6•6H2O increases from 2.2 to 4.4 mg, the distribution and number of PtNFs per unit surface area of 3D graphene increases.The TEM images (Figure 3B,C) reveal that many Pt nanowires with the length of 6-16 nm are located together to form 3D flower-like nanostructures.High-resolution TEM (HRTEM) characterizations (Figures 3D and 4D) demonstrate that each PtNF consists of many Pt nanowires with the diameter of 2-3 nm, and the amount of Pt nanowires increases with the increase of Pt precursor.The magnified HRTEM image in Figure 4D shows that the nanowire grown on 3D graphene is one single crystal with the lattice spacing of 0.225 nm, which is attributed to the (111) crystal plane of Pt [47,48].The energy-dispersive X-ray spectroscopy (EDS) characterization (Figure 5A) further confirms the formation of PtNFs on 3D graphene.3D graphene foam was directly used as a growing template for synthesis of PtNFs.The experimental details can be seen in the Materials and Methods section.The morphology and structure of PtNFs growing on 3D graphene can be modulated by the addition of a Pt precursor.These were studied by SEM and transmission electron microscopy (TEM) characterizations.As shown in Figures 3A  and 4A-C, 3D PtNFs uniformly cover the surface of 3D graphene by a considerable amount.When the addition of H 2 PtCl 6 •6H 2 O increases from 2.2 to 4.4 mg, the distribution and number of PtNFs per unit surface area of 3D graphene increases.The TEM images (Figure 3B,C) reveal that many Pt nanowires with the length of 6-16 nm are located together to form 3D flower-like nanostructures.High-resolution TEM (HRTEM) characterizations (Figures 3D and 4D) demonstrate that each PtNF consists of many Pt nanowires with the diameter of 2-3 nm, and the amount of Pt nanowires increases with the increase of Pt precursor.The magnified HRTEM image in Figure 4D shows that the nanowire grown on 3D graphene is one single crystal with the lattice spacing of 0.225 nm, which is attributed to the (111) crystal plane of Pt [47,48].The energy-dispersive X-ray spectroscopy (EDS) characterization (Figure 5A) further confirms the formation of PtNFs on 3D graphene.3D graphene foam was directly used as a growing template for synthesis of PtNFs.The experimental details can be seen in the Materials and Methods section.The morphology and structure of PtNFs growing on 3D graphene can be modulated by the addition of a Pt precursor.These were studied by SEM and transmission electron microscopy (TEM) characterizations.As shown in Figures 3A and 4A-C, 3D PtNFs uniformly cover the surface of 3D graphene by a considerable amount.When the addition of H2PtCl6•6H2O increases from 2.2 to 4.4 mg, the distribution and number of PtNFs per unit surface area of 3D graphene increases.The TEM images (Figure 3B,C) reveal that many Pt nanowires with the length of 6-16 nm are located together to form 3D flower-like nanostructures.High-resolution TEM (HRTEM) characterizations (Figures 3D and 4D) demonstrate that each PtNF consists of many Pt nanowires with the diameter of 2-3 nm, and the amount of Pt nanowires increases with the increase of Pt precursor.The magnified HRTEM image in Figure 4D shows that the nanowire grown on 3D graphene is one single crystal with the lattice spacing of 0.225 nm, which is attributed to the (111) crystal plane of Pt [47,48].The energy-dispersive X-ray spectroscopy (EDS) characterization (Figure 5A) further confirms the formation of PtNFs on 3D graphene.The fabricated PtNF/3D graphene electrode was used for electrocatalytic methanol oxidation against Pt-C/GCE.The TEM and XRD characterizations of used commercial Pt-C catalyst are presented in Figure S1.From the cyclic voltammograms (CVs) of Pt-C/GCE and PtNF/3D graphene electrodes in N2-saturated 0.5 M H2SO4 solution (Figure 6A), the PtNF/3D graphene electrode shows stronger electrochemical peaks ascribed to hydrogen adsorption/desorption than that of Pt-C/GCE, indicating its larger electrochemically active surface area.The electrochemically active surface areas of Pt-C/GCE and PtNF/3D graphene were evaluated from their CVs to be 29.2 cm 2 g -1 and 67.5 cm 2 g -   The fabricated PtNF/3D graphene electrode was used for electrocatalytic methanol oxidation against Pt-C/GCE.The TEM and XRD characterizations of used commercial Pt-C catalyst are presented in Figure S1.From the cyclic voltammograms (CVs) of Pt-C/GCE and PtNF/3D graphene electrodes in N2-saturated 0.5 M H2SO4 solution (Figure 6A), the PtNF/3D graphene electrode shows stronger electrochemical peaks ascribed to hydrogen adsorption/desorption than that of Pt-C/GCE, indicating its larger electrochemically active surface area.The electrochemically active surface areas of Pt-C/GCE and PtNF/3D graphene were evaluated from their CVs to be 29.2 cm 2 g -1 and 67.5 cm 2 g -    The fabricated PtNF/3D graphene electrode was used for electrocatalytic methanol oxidation against Pt-C/GCE.The TEM and XRD characterizations of used commercial Pt-C catalyst are presented in Figure S1.From the cyclic voltammograms (CVs) of Pt-C/GCE and PtNF/3D graphene electrodes in N2-saturated 0.5 M H2SO4 solution (Figure 6A), the PtNF/3D graphene electrode shows stronger electrochemical peaks ascribed to hydrogen adsorption/desorption than that of Pt-C/GCE, indicating its larger electrochemically active surface area.The electrochemically active surface areas of Pt-C/GCE and PtNF/3D graphene were evaluated from their CVs to be 29.2 cm 2 g -1 and 67.5 cm 2 g - The fabricated PtNF/3D graphene electrode was used for electrocatalytic methanol oxidation against Pt-C/GCE.The TEM and XRD characterizations of used commercial Pt-C catalyst are presented in Figure S1.From the cyclic voltammograms (CVs) of Pt-C/GCE and PtNF/3D graphene electrodes in N 2 -saturated 0.5 M H 2 SO 4 solution (Figure 6A), the PtNF/3D graphene electrode shows stronger electrochemical peaks ascribed to hydrogen adsorption/desorption than that of Pt-C/GCE, indicating its larger electrochemically active surface area.The electrochemically active surface areas of Pt-C/GCE and PtNF/3D graphene were evaluated from their CVs to be 29.2 cm 2 g −1 and 67.5 cm 2 g −1 , respectively.The MOR of Pt-C/GCE and PtNF/3D graphene electrode in N 2 -saturated 0.5 M H 2 SO 4 solution is shown in Figure 6B.The PtNF/3D graphene electrode has higher electrocatalytic activity for MOR than Pt-C/GCE.The mass activity of PtNF/3D graphene electrode for MOR was 306.3 mA mg −1 Pt at 0.7 V, which was 2.69 times higher than that of Pt-C/GCE (113.8 mA mg −1 Pt) (Figure 6C).The ratio of the forward oxidation peak intensity (I f ) to the reverse peak intensity (I b ) (I f /I b ), is generally used to evaluate the CO and other carbonaceous species tolerance of catalysts.The value of I f /I b of PtNF/3D graphene electrode (I f /I b = 1.3) is higher than that of Pt-C/GCE (I f /I b = 0.72) (Figure 6C), indicating that PtNF/3D graphene electrode can electrochemically oxidize methanol more effectively and generate less poisoning species than Pt-C/GCE during the forward scan.The durability of electrocatalysts is very significant for practical applications of catalysts.Chronoamperometric measurements of the PtNF/3D graphene electrode and Pt-C/GCE at 0.7 V for MOR show that the decay of peak current density of Pt-C/GCE for MOR is faster than that of the PtNF/3D graphene electrode (Figure 6D), suggesting that the PtNF/3D graphene electrode is more stable for MOR in acidic electrolytes.The high electrocatalytic activity and good stability of PtNF/3D graphene electrode are attributed to the high specific surface area of 3D graphene foam and exposed crystal plane (111) of thin Pt wires in a PtNF/3D graphene electrode.Specific surface area of 3D graphene foam (670 m 2 g −1 ) [51] is larger than that of carbon black of commercial Pt-C (250 m 2 g −1 ) [52].Furthermore, the crystal plane (111) was proved to have a low tendency to be poisoned by CO intermediates [53].
Catalysts 2018, 8, x FOR PEER REVIEW 5 of 9 1 , respectively.The MOR of Pt-C/GCE and PtNF/3D graphene electrode in N2-saturated 0.5 M H2SO4 solution is shown in Figure 6B.The PtNF/3D graphene electrode has higher electrocatalytic activity for MOR than Pt-C/GCE.The mass activity of PtNF/3D graphene electrode for MOR was 306.3 mA mg −1 Pt at 0.7 V, which was 2.69 times higher than that of Pt-C/GCE (113.8 mA mg −1 Pt) (Figure 6C).
The ratio of the forward oxidation peak intensity (If) to the reverse peak intensity (Ib) (If/Ib), is generally used to evaluate the CO and other carbonaceous species tolerance of catalysts.The value of If/Ib of PtNF/3D graphene electrode (If/Ib = 1.3) is higher than that of Pt-C/GCE (If/Ib = 0.72) (Figure 6C), indicating that PtNF/3D graphene electrode can electrochemically oxidize methanol more effectively and generate less poisoning species than Pt-C/GCE during the forward scan.The durability of electrocatalysts is very significant for practical applications of catalysts.Chronoamperometric measurements of the PtNF/3D graphene electrode and Pt-C/GCE at 0.7 V for MOR show that the decay of peak current density of Pt-C/GCE for MOR is faster than that of the PtNF/3D graphene electrode (Figure 6D), suggesting that the PtNF/3D graphene electrode is more stable for MOR in acidic electrolytes.The high electrocatalytic activity and good stability of PtNF/3D graphene electrode are attributed to the high specific surface area of 3D graphene foam and exposed crystal plane (111) of thin Pt wires in a PtNF/3D graphene electrode.Specific surface area of 3D graphene foam (670 m 2 g −1 ) [51] is larger than that of carbon black of commercial Pt-C (250 m 2 g −1 ) [52].Furthermore, the crystal plane (111) was proved to have a low tendency to be poisoned by CO intermediates [53].

Preparation of PtNF/3D Graphene Electrode
3D graphene foams were prepared by chemical vapor deposition using ethanol as the precursor and nickel foam as a template according to the previously reported method [41,42].The synthesized 3D graphene foams were used for growth of PtNFs through chemical reduction of chloroplatinic acid hexahydrate with formic acid.In a typical procedure, 0.40 mL formic acid and 0.44 mL chloroplatinic acid hexahydrate (10 mg mL −1 ) were added into a 10-mL flask and sonicated for 30 min.3D graphene foam was immersed into the mixed solution, and then the reaction proceeded in an airtight flask at room temperature for up to 4 d without stirring.The 3D graphene foam loaded with PtNFs was pulled out and washed with deionized water many times, and then was dried at 60 C in a vacuum drying oven.Since the reaction time of 4 d is enough to finish the reaction, the amount of PtNFs on 3D graphene in our work could be modulated by the addition of chloroplatinic acid hexahydrate.

Electrochemical Measurements
Pt loading on PtNF/3D graphene was estimated by inductive coupled plasma-optical emission spectroscopy (ICP-OES).Pt loading per unit geometric surface area of PtNF/3D graphene electrode and Pt-C/GCE (0.158 mg/cm 2 ) was kept the same for electrochemical tests.Electrochemical experiments were implemented by using Ag/AgCl (3 M KCl) as a reference electrode, Pt wire as a counter electrode, and PtNF/3D graphene or Pt-C/GCE as a work electrode.The work electrode was firstly activated by CVs from −0.2 to 0.8 V in N 2 -saturated 0.5 M H 2 SO 4 solution until CV becomes steady.MOR was measured by CVs from 0 to 1.0 V in 0.5 M H 2 SO 4 solution containing 1.0 M CH 3 OH.The scan rate for CVs was 50 mV s −1 .

Conclusions
In summary, a PtNF/3D graphene electrode was constructed by a simple aqueous method with 3D graphene foam as the growth template of PtNFs.PtNFs were easily and controllably formed on the 3D graphene foam under room temperature without any assistance of surfactants.The fabricated PtNF/3D graphene electrode shows high electrocatalytic activity and good stability for MOR.This easy method is meaningful for designing and preparing highly efficient 3D inorganic nanostructures/graphene electrodes in the applications of fuel cells.

Figure 2 .
Figure 2. Raman spectra recorded at different regions of 3D graphene foam.

Figure 2 .
Figure 2. Raman spectra recorded at different regions of 3D graphene foam.

Figure 2 .
Figure 2. Raman spectra recorded at different regions of 3D graphene foam.

Figure 6 .
Figure 6.(A) Cyclic voltammogram (CV) measurements of Pt-C/glassy carbon electrode (GCE) and PtNF/3D graphene electrode in N 2 -saturated 0.5 M H 2 SO 4 solution.(B) CV measurements of Pt-C/GCE and PtNF/3D graphene electrode in N 2 -saturated 0.5 M H 2 SO 4 solution containing 1.0 M CH 3 OH.(C) Mass activity at 0.7 V and I f /I b of Pt-C/GCE and PtNF/3D graphene for methanol oxidation reaction (MOR).(D) Amperometric curves of Pt-C/GCE and PtNF/3D graphene at 0.7 V in N 2 -saturated 0.5 M H 2 SO 4 solution containing 1.0 M CH 3 OH.Scan rate in CV measurements of Figure 6A,B is 50 mV s −1 .Current density in Figure 6D was normalized with the geometric surface area of electrodes.
: Figure S1.(A) TEM and (B) XRD characterizations of the used commercial Pt-C catalyst.Author Contributions: Y.Z., L.X. and Z.L. proposed the concept and supervised the research work.Z.S., Q.S. and M.L. designed and performed the experiments.All authors analyzed the data and wrote the paper.