Efficient Photocatalytic Hydrogen Evolution by Iron Platinum Loaded Reduced Graphene Oxide

The production of hydrogen (as a clean energy carrier that could replace fossil fuels) nowadays attracts much attention because of environmental pollution and energy demands1–3. Recently, hydrogen evolution technologies, such as production by steam reforming4, electrolysis5, degradation of organic pollutants in wastewater6, and photoelectrochemical splitting of water7 have been investigated. Among all the methods, photocatalytic hydrogen generation processes8–10 on nanomaterials have gained considerable attention because of their ability to provide a clean and renewable energy source. Photocatalytic splitting of water promises to be a cleaner and greener route towards generation of hydrogen. A key challenge for water splitting is the development of catalysts for the direct and efficient production of hydrogen from protons. Up to now, numerous metal-based photocatalysts have been discovered as catalysts for this reaction11,12, but they are ultimately of low efficiency, high cost, and low abundance13,14. Several strategies have been employed to improve the photocatalytic performance of metallic photocatalysts, for example, textural design15,16, coupling with other metal photocatalysts17,18, etc. In particular, great interest has been devoted to linking carbon nanomaterials19. Conjugated carbon materials, such as fullerenes, graphene, carbon nanotubes, and graphite are excellent candidates for refining the transport of photocarriers during photocatalysis through the formation of electronic interactions with photocatalyst nanoparticles. Among carbon-based materials, graphene has been reported as an efficient co-catalyst for photocatalytic H2 production because of its high specific surface area (theoretical value 2600 m2 g–1), excellent electron mobility (15000 m2 V–1 s–1 at room temperature), thermal conductivity, and high mechanical strength20–22. In recent years, reduced graphene oxide (rGO) has been modified with different nanoparticles, such as ZnO23, TiO2 24 and CdS25. They have been used as photocatalysts under visible-light irradiation. Previously, a few shape-controlled Pt alloy nanocrystals, such as Pt-Fe and Pt-Co26, have been made using diols and other reducing agents. To the best of our knowledge, metals like Pt and Fe play a significant role in the hydrogen evolution process, especially when they are combined with carbon-based composites27–30. Herein, we have attempted to introduce the grapheme-based composite as a novel class of photocatalysts for enhancement of photocatalytic hydrogen evolution. In this work, we have synthesized platinum-iron NPs loaded reduced graphene oxide using Pt(acac)2 and Fe(acac)3 as metal nanoparticle sources. We have also examined the photocatalytic activity of FePt-rGO and FePt nanomaterials for the water splitting reaction to produce hydrogen in the presence of methanol as a sacrificial agent. Efficient Photocatalytic Hydrogen Evolution by Iron Platinum Loaded Reduced Graphene Oxide


Introduction
The production of hydrogen (as a clean energy carrier that could replace fossil fuels) nowadays attracts much attention because of environmental pollution and energy demands [1][2][3] .Recently, hydrogen evolution technologies, such as production by steam reforming 4 , electrolysis 5 , degradation of organic pollutants in wastewater 6 , and photoelectrochemical splitting of water 7 have been investigated.Among all the methods, photocatalytic hydrogen generation processes [8][9][10] on nanomaterials have gained considerable attention because of their ability to provide a clean and renewable energy source.
Photocatalytic splitting of water promises to be a cleaner and greener route towards generation of hydrogen.A key challenge for water splitting is the development of catalysts for the direct and efficient production of hydrogen from protons.Up to now, numerous metal-based photocatalysts have been discovered as catalysts for this reaction 11,12 , but they are ultimately of low efficiency, high cost, and low abundance 13,14 .Several strategies have been employed to improve the photocatalytic performance of metallic photocatalysts, for example, textural design 15,16 , coupling with other metal photocatalysts 17,18 , etc.In particular, great interest has been devoted to linking carbon nanomaterials 19 .Conjugated carbon materials, such as fullerenes, graphene, carbon nanotubes, and graphite are excellent candidates for refining the transport of photocarriers during photocatalysis through the formation of electronic interactions with photocatalyst nanoparticles.Among carbon-based materials, graphene has been reported as an efficient co-catalyst for photocatalytic H 2 production because of its high specific surface area (theoretical value 2600 m 2 g -1 ), excellent electron mobility (15000 m 2 V -1 s -1 at room temperature), thermal conductivity, and high mechanical strength [20][21][22] .In recent years, reduced graphene oxide (rGO) has been modified with different nanoparticles, such as ZnO 23 , TiO 2 24 and CdS 25 .They have been used as photocatalysts under visible-light irradiation.Previously, a few shape-controlled Pt alloy nanocrystals, such as Pt-Fe and Pt-Co 26 , have been made using diols and other reducing agents.To the best of our knowledge, metals like Pt and Fe play a significant role in the hydrogen evolution process, especially when they are combined with carbon-based composites [27][28][29][30] .
Herein, we have attempted to introduce the grapheme-based composite as a novel class of photocatalysts for enhancement of photocatalytic hydrogen evolution.In this work, we have synthesized platinum-iron NPs loaded reduced graphene oxide using Pt(acac) 2 and Fe(acac) 3 as metal nanoparticle sources.We have also examined the photocatalytic activity of FePt-rGO and FePt nanomaterials for the water splitting reaction to produce hydrogen in the presence of methanol as a sacrificial agent.

S. E. Moradi *
Young Researchers and Elite Club, Islamic Azad University-Sari Branch, Sari, Iran In this work, graphene oxide (GO) was prepared by the Hummers method from natural graphite, and modified with iron and platinum nanoparticles by the solvothermal method.The structural order and textural properties of the grapheme-based materials were studied by BET, TEM, XRD, TG-DTA, and XPS techniques.UV−Vis diffuse reflectance spectra indicate the band gap for FePt and FePt-rGO composites to be 3.2 and 2.8 eV, respectively.FePt-rGO showed a hydrogen generation rate higher than that of the FePt nanoparticles.A detailed study of Pt effect on the photocatalytic H 2 production rates showed that Pt NPs could act as an effective co-catalyst, enhancing photocatalytic activity of FePt-rGO.The FePt-rGO gave a H 2 production rate of 125 µmol g -1 h -1 .This is ascribed to the presence of Pt NPs (acting as electron sinks) and graphene oxide (as an electron collector and transporter) in FePt-rGO composites.

Synthesis of GO adsorbent
GO was synthesized from expandable graphite using a modified Hummers' method 31 .An amount of 1 g of graphite powder was added to 23 mL of concentrated H 2 SO 4 in an ice bath.KMnO 4 (3 g) was then added slowly with stirring and cooling to keep the temperature of the reaction mixture below 293 K.The temperature of the reaction mixture was increased and maintained at 308 K for 30 minutes.When 46 mL of deionized water was added slowly to this mixture, the temperature was increased to 371 K.After 15 minutes, 140 mL of deionized water was added followed by 10 mL of 30 % H 2 O 2 solution.The solid product was separated by centrifugation.It was washed repeatedly with 5 % HCl solution until the sulfate ions had been removed, and then washed with distilled water repeatedly until free of chloride ions.The product was then filtered and washed 3-4 times with acetone to make it moisture-free, and the residue dried in an oven at 338 K overnight.The GO was suspended in water and exfoliated by ultrasonication for 3 hours.

Synthesis of FePt-rGO composites
FePt-rGO was obtained by the method as described by Chen et al. 32 Prior to the synthesis of FePt-rGO, the as-prepared GO was dispersed in deionized water by ultrasonication (KQ-50B supersonic cleaner, Kun Shan Ultrasonic Instruments Co., Ltd, China; ultrasonic frequency: 40 kHz; ultrasonic power: 80W) for 3 hours.FePt-rGO composites were synthesized by the solvothermal method using ethylene glycol (EG)-water as the solvent.In a typical synthesis, Pt(acac) 2 (0.25 mmol, 0.0985 g) was dissolved in EG (15 mL) under magnetic stirring with a short heating (90-100 °C, 5 min).Fe(acac) 3 (0.25 mmol, 0.0883 g) was dissolved in another 15 mL of EG under magnetic stirring with a short heating period (below 100 °C), which was subsequently added dropwise into the EG solution containing Pt(acac) 2 .Then, 10 mL of GO (5 mg mL -1 ) aqueous dispersion was added dropwise into the EG solution.After 30 minutes of stirring, the mix-ture was transferred to, and sealed in, a 50-mL Teflon-lined stainless steel autoclave, and heated to 160 °C for 24 h, and then cooled to room temperature.The precipitate was collected and washed alternately with ethanol and deionized water by centrifugation (10,000 rpm, 5 min), and then dried at 60 °C in vacuum.

Characterization and hydrogen evolution
The morphology and surface structure of GO and FePt-rGO were examined by X-ray diffraction (XRD, Philips Xpert MPD, Co Kα irradiation, λ = 1.78897A°), and the X-ray photoemission spectroscopy (XPS) analysis was acquired by using a Scienta ESCA 200 analyzer (Gammadata, Sweden) equipped with a monochromatized Al Ka X-ray source.Transmission electron microscope (TEM) analysis was conducted with a JEM 2100 transmission electron microscope (JEOL, Japan) at 200 kV.The composition and thermal properties of GO and FePt-rGO were determined by TGA with a PL Thermal Sciences; model PL-STA using a heating rate of 10 K min -1 from room temperature to 1073 K.The measurements were conducted using approximately 3-mg samples, and then weight retention/temperature curves were recorded.
Volumetric nitrogen sorption studies were taken at 77 K using a Micromeritics ASAP 2020 system.Before performing the measurements, the samples were degassed below 1.33 Pa at 90 °C for 1 h and heated (10 °C min -1 ) to 350 °C for 10 h.The specific surface area (SBET) was calculated by the BET method in the relative pressure range of 0.04-0.20.Total pore volume (V t ) was calculated at relative pressure p/p 0 = 0.98.The microporous volume (V mi ) was determined by applying Dubinin-Radushkevich (DR) analyses on the corresponding isotherms in the relative pressure range 10 −4 -10 −2 .The volume of pores smaller than 1 nm (V < 1 nm) was determined by the cumulative pore volume in the relative pressure range 10 −6 -10 −4 using the Horvath-Kawazoe (HK) method.The meso-and micropore sizes of samples were analyzed by the Barrett-Joyner-Halenda (BJH) and HK methods, respectively.A Shimadzu spectrophotometer (Model 2501 PC) was used to record the UV-Vis diffuse reflectance spectra of the samples with the region of 200 to 800 nm.
The photocatalytic hydrogen evolution tests were carried out at room temperature under atmospheric pressure in a closed quartz reactor system.The light intensity was measured to be 80 mW cm -2 by an optical power meter (1 L, 1400 A, International Light) from a 400 W high-pressure Hg lamp with a water filter to remove the infrared part of the spectrum.Typically, the photocatalysts (5 mg) were suspended in an aqueous methanol solution (80 mL of distilled water, 20 mL of methanol) by means of a magnetic stirrer within the reactor.Prior to the experiment, the mixture was dispersed by ultrasound treatment for 15 minutes, followed by purging N 2 gas for 30 minutes.The amount of evolved H 2 was determined by a GC5890F gas chromatograph (thermal conductivity detector, molecular sieve 5A, 99.999 % N 2 carrier).

Results and discussion
Characterization of the GO and FePt-rGO samples Fig. 1 shows the nitrogen adsorption-desorption isotherms of GO and FePt-rGO, which were used to investigate the surface area and porous structure.The surface area of GO and FePt-rGO calculated by the Brunauer-Emmett-Teller (BET) theory, which explain the physical adsorption of gas molecules on a solid surface 33 , are 741.2 and 1018.7 m 2 g -1 , respectively.
As shown in Fig. 2a, the broad and relatively weak diffraction peak at 2θ = 10.5°(d = 0.87 nm), which corresponds to the typical diffraction peak of graphene oxide adsorbent, is attributed to the (002) plane 34 .The 2θ values for FePt-rGO at 40.77°, 46.92°, 69.30°, and 83.10° can be indexed to diffraction planes of (111), ( 200), (220), and (311), respectively.The mentioned planes can be attributed to chemically disordered fcc metal nanoparticle 12 .No peaks from the iron oxides were observed in the XRD data, confirming that the iron nanoparticles were not oxidized.The peak at ca. 24.32° (2θ) is related to carbon peak of reduced graphene oxide 35 .
A representative TEM image of the obtained FePt-rGO is shown in Fig. 3a.The TEM image of FePt-rGO also shows that platinum-iron nanoparticles are relatively well-dispersed on the reduced graphene oxide sheet.The mean size of FePt-rGO calculated from the TEM image is around 16.0 nm.The histogram of particle size for FePt-rGO show that the size distribution is relatively narrow (Fig. 3b).
Fig. 4 displays Pt 4f and Fe 2p X-ray photoelectron spectroscopy (XPS) spectra of FePt-rGO composites.The two characteristic peaks at 71.3 eV (Pt 4f 7/2 ) and 74.5 eV (Pt 4f 5/2 ) may be observed in Moreover, no signal for the C in the XPS is provided, which would be useful to confirm the reduction of GO.
The TGA was also performed on GO and the FePt-rGO samples to determine the structure and thermal stability of the GO and the FePt-rGO (Fig. 5a and Fig. 5b).GO shows two significant weight losses close to 100 °C and 500 °C with 9 % and 30 % weight loss, respectively.They are related to the evaporation of the water molecules in the material and thermal decomposition of oxygen carrying functionalities and oxidation of carbon.The FePt-rGO shows a 2 % loss below 100 °C which should be due to the removal of adsorbed water, a 3 % loss at ca. 200 °C should be assigned to the decomposition of the residual oxygen containing groups, and a 13 % loss from 440 to 590 °C should be associated with the pyrolysis of the carbon skeleton of reduced graphene oxide 36 .These results suggest that FePt-rGO is thermally stable at higher temperatures.
To investigate the optical properties, UV-Vis absorption spectra of FePt and FePt-rGO are shown in Fig. 6.It can be observed that FePt and FePt-rGO show sharp absorption edges at around 470 nm.The hydrogen generating capability of the composite photocatalysts was investigated in methanol aqueous solution.The photocatalytic results of the reduction of water to produce H 2 over FePt and FePt-rGO catalysts are shown in Fig. 7.The rate of hydrogen evolution continuously increased at the initial reaction duration, and after several hours, it reached a constant value of 125 µmol g -1 h -1 (Fig. 7).The total amount of H 2 evolved from FePt-rGO after 6 h of irradiation was 890 µmol g -1 , which was higher than that of FePt (460 µmol g -1 ).The higher photocatalytic activity might be attributable to good light absorption of FePt-rGO compared to FePt NPs.It is reasonable to imagine the formation of a Schottky-like barrier between the closely contacted iron and platinum NPs surface species, which would facilitate electron-hole separation, similar to the action of traditional conductive platinum co-catalysts 37 .
The highest rates of H 2 generation with the above amounts of nanomaterials are shown in Fig. 7.We can see a drastic increase in the amount of evolved H 2 gas when FePt is used with reduced graphene oxide.This is due to the synergy effect of rGO to FePt.In the case of noble metal, such as Pt, an electron from photoexcited iron oxide is transferred to Pt, and then it reduces H + ion to produce H 2 gas.Platinum nanoparticles loaded on the rGO surface are known to act as electron sinks.This strongly enhances the photocatalytic activity of iron oxide through the formation of a Schottky barrier (retarding the electron/hole recombination) at the FePt-rGO surface.This phenomenon appears to promote an efficient separation of holes and electrons charges carriers photo-generated under near-UV light.As a result, the interfacial charge transfer and the efficiency of the photocatalytic reaction are enhanced [38][39][40] .Platinum deposits can serve as a temporary electron chamber.Fig. 8 shows the hydrogen evolution by recycled FePt-rGO.After each cycle, the amount of hydrogen evolution from FePt-rGO remained constant.Fig. 8 shows that the photocatalytic evolution rate of hydrogen remained above 95 % of the initial rate.After 30 h of irradiation, the system produced 850 µmol g -1 of H 2 without noticeable catalyst deactivation.

Conclusion
Hydrogen evolution from water containing methanol was performed on FePt-rGO by medium-pressure Hg lamp.XPS analysis confirmed that Fe and Pt species were coordinated to reduced graphene oxides.Diffuse reflectance spectra revealed an increase in absorption with Fe and Pt NPs implying that the doping modified the electronic properties of reduced graphene oxide.The significant promotion effect of hydrogen evolution using FePt-loaded reduced graphene oxide photocatalysts was observed.The Pt and Fe NPs enhance the photocatalytic hydrogen evolution activity of FePt-rGO.The hydrogen evolution of FePt-rGO was 125 µmol g -1 h -1 with a maximum of 890 µmol g -1 , which exceeded 1.9 times compared with FePt NPs.

F i g . 1 -
Fig. 4a, confirming the formation of metallic Pt.The peaks at 711.1 eV (Fe2p 3/2 ) and 725.6 eV (Fe 2p 1/2 ) presented in Fig. 4b, show the existence of Fe-O or Fe-OOH group in FePt-rGO composites.Moreover, no signal for the C in the XPS is provided, which would be useful to confirm the reduction of GO.The TGA was also performed on GO and the FePt-rGO samples to determine the structure and thermal stability of the GO and the FePt-rGO (Fig.5aand Fig.5b).GO shows two significant weight losses close to 100 °C and 500 °C with 9 % Fig. 6b displays the plot of the transformed Kubelka-Munk function versus energy of light, by which the estimated band gaps are 3.2 and 2.8 eV, corresponding to FePt and FePt-rGO, respectively.The narrow band gap of FePt-rGO nanoparticles was attributed to the interaction between rGO and FePt.

F i g . 4 -
Photocatalytic hydrogen evolution activity and mechanism