Gas-exfoliation assisted fabrication of porous graphene nanosheets

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Gas-exfoliation assisted fabrication of porous graphene nanosheets derived from Plumeria rubra for highly efficient photocatalytic hydrogen evolution Youliang Wang, Ting Song, Piyong Zhang, Tingbo Huang, Ting Wang, Tingting Wang, and Heping Zeng ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01723 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 18, 2018

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ACS Sustainable Chemistry & Engineering

Gas-exfoliation assisted fabrication of porous graphene nanosheets derived from Plumeria rubra for highly efficient photocatalytic hydrogen evolution

Youliang Wang, Ting Song, Piyong Zhang, Tingbo Huang, Ting Wang, Tingting Wang*, Heping Zeng* Key Laboratory of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, 381 Wushan Road, Guangzhou, Guangdong, 510641, P. R. China. *Corresponding author. Tel.: +86-20-87112631; Fax: +86-20-87112631; E-mail: [email protected]，[email protected]

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Abstract An environmentally friendly strategy was performed to prepare porous graphene nanosheets (PGS) through gas-exfoliation assisted KOH activation by using Plumeria rubra as the precursor for the first time. Due to the synergistic reaction of various graphitization temperature and activating agent amounts, the precursor was gradually exfoliated by released gases, graphitizing at high temperatures and forming porous graphene nanosheets during the graphitization. PGS-2-1000 with a hierarchical porous structure possessed high graphitization (ID/IG=0.77, I2D/IG=0.53), ultrahigh specific surface (1581 m2 g-1) and large pore volume (0.916 cm3 g-1). A facile in-situ photoreduction treatment was utilized to form a Cu/PGS photocatalyst composed of non-semiconductor plasmonic Cu NPs and porous graphene nanosheets, exhibiting the hydrogen evolution rate of 4.87 mmol g-1 h-1, which is fourfold as high as that of single Cu NPs. The photostability of Cu/PGS-2-1000 was investigated in five consecutive runs of 30 h accumulative irradiation. The PGS could act as an electron transport bridge to boost the separation of electron-hole pairs and the uniform distribution of Cu NPs, even serve as a photocatalyst for hydrogen generation. Furthermore, a possible mechanism was proposed to illustrate the circumstantial charge transfer channel as well as the improvement of photocatalytic activity. Keywords: Biomass; Plumeria rubra; Graphene nanosheets; Hydrogen evolution


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Introduction Hydrogen evolution from water by solar energy is an effective way to solve environmental pollution and meet the energy needs of the modern and the next generation [1-9]. In recent years, plasmonic metal nanoparticles (Cu, Ag, Au, etc.) exhibited good photocatalytic activities for water splitting [10-15]. It is dominated by the localized surface plasmon resonance (LSPR), defined as the collective motions of the conduction electrons induced by light irradiation [16-18]. Due to its low cost and high photocatalytic activity, Cu NPs as photocatalyst for hydrogen evolution has attracted considerable attention [19,20]. However, the primary challenge is to keep Cu NPs from oxidation and agglomeration. A promising and viable way to overcome these drawbacks is the deposition of metal nanoparticles on some supporters such as carbon materials. As is known to all, graphene has promising superiority in excellent optical transmittance, conductivity and large specific surface which is effectively decreasing electron-hole recombination rate and affording a number of reaction sites [21-27]. Furthermore, existent oxygenate groups could serve as the nucleation center to fix nanoparticles that uniform distributed in the edge of wrinkled graphene nanosheets. In the past years, several methods such as graphite oxide reduction method, chemical vapor deposition (CVD) method and epitaxial growth method were reported for the synthesis of graphene [28-32]. However, these methods were limited by the consumption of time and complicated multiple steps so that it is impossible to massively produce high quality graphene. It is badly in need of a facile, green and reliable method for successfully synthesizing graphene. Biomass with the properties of cheap, non-toxic and readily available has attracted growing attention in the preparation of graphene nanosheets. Therefore, hunting for a valid biomass precursor is a key to exploit facile and reliable method to synthesize graphene nanosheets. So far, some kinds of renewable biomass, such as Artemia cyst shells [33], cellulose [34] and spruce bark [60], have been utilized as precursors to synthetize graphene nanosheets for supercapacitors. To the best of our knowledge, no investigation has



been reported about taking Plumeria rubra as precursor to fabricate porous graphene nanosheets for photocatalytic hydrogen evolution. In this work, a universal approach for the facile synthesis of porous graphene nanosheets (PGS) was designed through gas-exfoliation assisted KOH activation from Plumeria rubra. During the synthesis process, the carbon precursor was gradually exfoliated by the continuously released gases (H2, CO2, CO) as the reaction carried out between carbon precursor and KOH. Various KOH amounts were thoroughly discussed to tune highly porous structure that increases the specific surface and provides more reaction sites. Furthermore, the different graphitization temperatures were systematically investigated to study their impact on the graphitization. An efficient in-situ method for the generation of plasmonic Cu NPs modified with porous graphene nanosheets under vacuum conditions was utilized to assess the photocatalytic performance of Cu/PGS. The hydrogen evolution rate of Cu/PGS was higher than that of Cu NPs alone with lactic acid as a sacrificial reagent under Xe lamp irradiation. Several recycles were monitored for studying the persistent photostability of Cu/PGS-2-1000. Furthermore, a possible photocatalytic and charge transfer mechanism was discussed in detail.

Experimental Preparation of PGS The Plumeria rubra was cleaned with distilled water by several times and dried in 80 °C overnight, and then the brown solid was crushed and ground to obtain the Plumeria rubra powder. The as obtained powder was put into the tube furnace and heated to 500 °C for 2 h at a heating rate of 1 °C min-1 under nitrogen atmosphere and subsequently raised the calcination temperature to 850 °C for 1 h at a heating rate of 3 °C min-1. After cooling down to room temperature, the carbonized materials were impregnated with KOH in an aqueous solution (the mass ratios of KOH to carbonized materials was 1 : 1, 2 : 1, 3 : 1 and 4 : 1). The mixed suspension was sonicated for 30 min and stirred continuously for 60 min, and dried at 80 °C in conventional oven overnight. The as obtained material was activated in the tube furnace for 1 h at 500 °C


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under a nitrogen atmosphere, subsequently calcined at pre-set temperature (800, 900, 1000 and 1100 °C) for 0.5 h at a heating rate of 3 °C min-1. Finally, the collected samples were washed repeatedly with diluted HCl solution (0.1 mol L-1) and deionized water to remove residual inorganic impurities, until a pH value of 7 was reached, followed by drying at 80 °C. The final product was obtained and denoted as PGS-X-Y where X referred to the KOH/C weight ratio in g/g and Y represented the graphitization temperature in °C. Preparation and photocatalytic hydrogen evolution of Cu/PGS The photocatalytic experiments were carried out in a 300 mL Pyrex reaction cell which was connected to an evacuation system and closed gas circulation. Cu/PGS nanocomposite was synthesized by an in-situ photoreduction method [20]. 10 mg of PGS and different volume of copper acetate solution (2 mmol L-1) were added into a reaction cell, which contains 60 mL deionized water and 10 mL lactic acid. The aqueous solution was completely degassed about 1 h and then irradiated by a 300 W Xe lamp (PLS-SXE300CUV, Perfect light. Co. Ltd, Beijing) without filter. After irradiation, Cu NPs were generated on the surface of PGS and PGS was reduced at the same time. The theoretical Cu NPs content in Cu/PGS is 1.3, 6, 11.3, 16, 20.4 and 24.2 wt%, respectively. Hydrogen evolution was determined by online gas chromatography (Tian mei, GC-7900, 5 Å molecular sieve column, nitrogen as a carrier gas) with a thermal conductivity detector (TCD). Characterization Raman spectra were obtained by Laser Confocal Raman Microscopy system (LabRAM Aramis, H.J.Y, France) with excitation wavelength of 532 nm. X-ray diffraction (XRD) patterns were performed on a D8 X-ray diffractometer (German, Bruker AXS) with Cu-K radiation. Fourier transform infrared (FTIR) spectra were recorded on FTIR spectrometer (Nicolet 670). The specific surface of samples was measured in a 3H-2000PS1 static volume method instrument. Scanning electron microscopy (SEM) with a Zeiss Merlin (Germany, Zeiss Co.) emission scanning electron microscope was performed to investigate the morphologies and structure.



Energy-dispersive X-ray (EDX) was carried out on a SEM with an EDX spectrometer. Transmission electron microscopy (TEM) images were taken on a JEOL JEM-2100 electron microscope. The atom force microscopy (AFM) images were obtained on Multimode 8. X-ray photoelectron spectroscopy (XPS) data were collected on a Kratos Axis-Ultra DLD (equipped with AES) instrument with monochromatized Al-K as line source (150 W). The UV–vis diffuse reflection absorption spectra were obtained by a UV–vis spectrometer (Japan, Hitachi, U3010, referenced by BaSO4) equipped with an integrating sphere accessory in the diffuse reflectance mode. The analysis of photoluminescence spectra (PL) was performed on Hitachi F-4500 fluorescence spectrophotometer. Electrochemical experiments were carried out in CHI660C Instruments electrochemical workstation and a conventional three-electrode system. Electrochemical impedance spectroscopy (EIS) spectra were recorded at the frequency range of 0.1 Hz to 100 kHz. The light source was a 300 W Xenon lamp for the photocurrent measurement and electrochemical impedance spectroscopy. Electron probe X-ray microanalyser (EPMA) was performed in EPMA-1600/EDAX.

Results and discussion Formation and characterization of PGS In the overall synthetic route (Fig. 1) of PGS, Plumeria rubra was chosen as carbon precursor, KOH as the activating agent to fulfill the synchronous carbonization and graphitization. Firstly, K2CO3 formed due to the presence of KOH and carbon precursor at 400–600 °C (eqn (1)) [35,37-39]. Superfluous KOH was totally consumed at 600 °C. Until above 700 °C, K2CO3 started to decompose into K2O and CO2. With the increasing of temperature, the produced CO2 and K solid could take reaction with intermediate product over 700 °C further (eqn (3)–(5)). During the graphitization, a large amount of gas was released, overcoming the van der Waals attraction and intercalating into the carbon lattice. These gases would cause a continuous and homogenous corrosion, leading to the formation of a hierarchical amorphous structure and abundant micro/mesoporous at the meantime [33]. Additionally, elevated temperature caused intermediate product to vary volume


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shrinkage irreversibly. This volume shrinkage not only increased the degree of graphitization and specific surface, but also effectively influenced the microscopic morphology. 6KOH+2C=2K+3H2↑+2K2CO3

(1)

K2CO3=K2O+CO2↑

(2)

CO2+C=2CO↑

(3)

K2CO3+2C=2K+3CO↑

(4)

K2O+C=2K+CO↑

(5)

Fig. 1. Schematic illustration of the synthesis process of PGS, using Plumeria rubra as the precursor and KOH as activating agent.

Raman spectroscopy was employed to examine the graphitization and crystallinity degree. Raman spectra showed three characteristic bands of graphene (Fig. 2a), including the D band at ∼1350 cm-1 caused by a breathing mode of out-plane vibration of disordered sp3-hybridized carbon atoms, the G band at ∼1580 cm-1 originated from the in-plane vibration of sp2-bonded carbon atoms in aromatic macromolecule, and the 2D band at ∼2700 cm-1 related to the second order of zone-boundary phonons and sensitive to stacking of graphene nanosheets [41,42]. The intensity ratio of D band to G band (ID/IG) was generally employed to determine the degree of ordering or defect density of graphene. With the increasing amount of



activating agent, the ID/IG values became larger since superfluous activating agent corroded large scale graphene to pieces which led to massive defect and structural collapse irreversibly. The ID/IG values of PGS-2-800, PGS-2-900, PGS-2-1000 and PGS-2-1100 were 0.91, 0.99, 0.771 and 1.104, respectively, indicating appropriate temperature could facilitate ordering structure and good crystallinity of the graphene nanosheets. Along with increasing of temperature to 1000 °C, a pronounced increase of the 2D band intensity was clearly observable. The intensity ratio between 2D and G band (I2D/IG) of PGS-2-1000 was calculated as 0.53, demonstrating higher graphitization degree and few layers (≤ 7 layers) feature of PGS-2-1000 [43]. It’s noteworthy that the synergistic reaction of KOH as activating agent and high temperature was in favor of graphitization. The crystallinity degree of all samples was analyzed by X-ray diffraction (XRD) patterns. As shown in Fig. 2b, there was little difference between PGS-1-800, PGS-2-800, PGS-3-800 and PGS-4-800, indicating the amount of potassium hydroxide has slight effect on graphitization of PGS. PGS-2-800 and PGS-2-900 only showed a broad and weak diffraction peak, indicating that it was an amorphous structure with low graphitization degree. But PGS-2-1000 and PGS-2-1100 appeared to display two sharp diffraction peaks locating at 26° and 43.1°, which are indexed as the (002) and (101) planes of graphene, respectively. This finding was consistent with Raman's results that high temperatures contribute to the graphitization. It was noted that the broad (002) diffraction peaks of the sample gradually left shift with increasing graphitization temperature, resulting from the different degree of corrosion by KOH that led to a breakdown of aligned structural and a random distribution of aromatic carbon sheets [58]. FTIR spectra (Fig. 2c) showed that the peak at 3431 cm-1 was correlated to the –OH vibration stretching and the peak at 1624 cm-1 was related to the C=C vibration stretching. The peaks of epoxy C-O-H (1382 cm-1) and alkoxy C-O-C (1066 cm-1) groups situating at the edges of graphene also could be found. From FTIR spectra, we could see that KOH and high temperature have mild effect on functional group of


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graphene.

Fig. 2. (a) Raman spectra, (b) XRD patterns, (c) FTIR spectra and (d) nitrogen adsorption/desorption isotherms of PGS.

Scanning electron microscope (SEM) was utilized to analyze the morphologies and microstructures of the as-prepared samples. SEM in Fig. S1 showed the effect of KOH in different weight on graphene nanosheets. It could be observed that synergistic reaction of carbonization and graphitization processes could well maintain original Plumeria rubra structure and form substantial micropores, following by enlarging to mesopores. The average pore sizes gradually enlarged with the increase of activating agent at the same graphitization temperature. Porous structure was essential for the ion diffusion/transport. The abundant mesopores offered more active sites and macropores minimized the ion diffusion distance [47]. Additionally, the crystalline structure of samples has changed significantly with the increase of the graphitization temperature. When the graphitization temperature was 800 °C, sample exhibited an



amorphous structure. After elevating temperature to 900 °C, samples began graphitization. When the temperature was further increased to 1000 °C, it was started to large-area emerging continuous lamellar structure (Fig. 3a-c). The TEM images as shown in Fig. 3d-e demonstrated a randomly wrinkled and folded morphology of graphene nanosheets, meanwhile showing an amorphous structure with irregular domains corresponding to defects and functional groups. In addition, high-resolution TEM image (HRTEM) (Fig. 3f) revealed the multilayers (typically 5 layers) characteristic of the as prepared PGS-2-1000, with an average adjacent interlayer distance of 0.34 nm [43]. This result was accorded with the observations of Raman.

Fig. 3. (a–c) Scanning electron microscope (SEM) image of PGS-2-1000 with different magnification, (d-e) TEM images, and (f) high–resolution TEM images of PGS-2-1000.

Atomic force microscopy (AFM) was conducted to study the top-view images and cross-sectional thickness of graphene nanosheets. As shown in Fig. 4, the randomly selected nanosheets displayed an approximate thickness of 5 nm. Due to the existence of oxygen containing functional epoxy and hydroxyl groups, meandering wrinkles introduce an additional ∼0.44 nm, increasing the height of monolayer graphene from ∼0.34 nm to ∼0.8 nm [62,63]. Therefore, PGS-2-1000 exhibited 5-7 layers characteristic, which was consistent well with the above HRTEM and Raman


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results.

Fig. 4. AFM images with section analysis of PGS-2-1000

Fig. 2d showed the N2 adsorption/desorption isotherm which was obtained to characterize the specific surface and pore size distribution. It could be seen that the isotherms of PGS-X-800 exhibited type-І isotherm with a sharp adsorption at low relative pressures and the adsorption remained almost constant at higher relative pressures. Furthermore, hysteresis loop of isotherm was not obvious, indicating the substantial micropores were produced by the corrosion of potassium hydroxide on carbon precursor. Noticeably, it was observed that the adsorption capacity increased with the rise of the amount of KOH. When the ratio of potassium hydroxide to carbon material reached 4 : 1, the specific surface decreased to 278.6 m2 g-1. This is because the reaction between the carbon precursor and KOH was accelerated, which led to the enlargement of corrosion and irreversible collapse of carbon structure [40]. However, with further increasing the graphitization temperature, the isotherms of PGS-2-1000 and PGS-2-1100 changed to a typical IV type isotherm with oblique N2 adsorption curve at low relative pressures, and a clear hysteresis loop was observed in the relative pressure P/P0 region of 0.4 to 0.9. As shown in Fig. S6, PGS contained a large



number of micropores and a small number of mesopores. Mesoporous pore size is approximately 4 nm. As the temperature increased, the number of mesopores increased, implying high temperature promoted the reaction of KOH and carbon precursor. Furthermore, with graphitization temperature increasing from 800, 900 to 1000 °C, the specific surface area dramatically increased from 541.9, 718.1 to 1581 m2 g-1. Interestingly, it was observed that the adsorption capacity decreased to 1085 m2 g-1 after the temperature was raised to 1100 °C, resulting from excessively corroded large scale graphene to pieces. The appropriate graphitization temperature and amount of KOH could boost the formation of pores and widen the pores at the same time. Electron probe X-ray microanalyser (EPMA) was carried out to investigate the elemental content of PGS-2-1000 (Fig. S7). The result showed that PGS-2-1000 only consisted of C (98.06 at%) and O (1.94 at%) without impurities. X-ray photoelectron spectroscopy (XPS) was performed to measure the surface chemical composition and analyze the functional groups of PGS-2-1000. As shown in Fig. 5a, the XPS result also disclosed that PGS-2-1000 only consisted of carbon and oxygen. After fitting by Gaussian–Lorentzian method (Fig. 5b), the C 1s spectrum could be deconvoluted into four peaks, including sp2-bonded graphite (C=C) at 284.6 eV, sp3-bonded graphite (C-C) at 285.8 eV, C–O (286.7 eV) and a satellite peak of π–π* (291.5 eV) in the higher-binding energy region, which indicates the formation of abundant conjugated systems [48,49]. The C-O bond was belonging to the hydroxyl and epoxy functional groups at the edge of PGS-2-1000.


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Fig. 5. (a) XPS survey spectra and (b) high-resolution C 1s spectrum of the PGS-2-1000

Characterization of Cu/PGS After in-situ photoreduced Cu NPs formed on PGS under vacuum, samples were collected and examined by SEM. As shown in Fig. S9, individual Cu NPs exhibited apparent agglomeration and not uniform in size, which went against the transportation of electrons. However, Fig. 6a showed that Cu NPs were uniformly distributed in wrinkled PGS-2-1000. Due to the presence of oxygenate groups on PGS which served as the nucleation center to fixed nanoparticles, PGS contributes towards a good dispersion and helps to prevent the aggregation of Cu NPs. Further magnification of the image (Fig. 6b) revealed that Cu NPs were successfully loaded on the nanosheets surface, implying intimate contact with the graphene and forming heterostructures [45]. The EDX analysis in Fig. 6c exhibited that the composite consists of Cu, C and O elements. The corresponding elemental mappings (Fig. 6d-g) were consistent with the EDX spectrum. Fig. 6h showed a pronounced nanosheets structure of PGS-2-1000 and dark spheres were belonging to Cu NPs. The HRTEM image (Fig. 6j) revealed that the interplaner spacing was 0.21 nm in response to the (111) plane of metallic Cu, implying that Cu NPs effectively formed via the photoreduction approach.



Fig. 6. Morphology of Cu/PGS-2-1000: (a-b) SEM image with different magnification, (c) EDX spectrum, (d-g) elemental mapping patterns, (h-j) TEM image with different magnification.

XRD was performed to investigate the formation of Cu/PGS (Fig. S2). Weak diffraction peaks appearing at 43.3° and 50.3° could be attributed to the (111) and (220) planes of face centered cubic copper (JCPDS No. 04-0836), respectively, indicating that Cu NPs were successfully formed by the photoreduction, as demonstrated by the HRTEM [45]. Cu/PGS were collected and examined by FTIR. Almost all of these absorbance peaks of the oxygenate groups in the spectrum of Cu/PGS-2-1000 strongly decreased, implying PGS-2-1000 was successfully reduced by the photoreduction process [61] (Fig. S3). Raman spectroscopy was employed to investigate the defect degree of Cu/PGS-2-1000 (Fig. S10). D band dramatically decreased and the ID/IG values of Cu/PGS-2-1000 decreased to 0.07, indicating the oxygenate groups and the degree of defect on PGS-2-1000 was reduced through photoreduction. Fitting results for PGS-2-1000 and Cu/PGS-2-1000, including peak position and chemical state content, are compared in Table S2. Note that the contents ratio of oxygen-containing groups (C-O) decreased from 0.155 for PGS-2-1000 to 0.14 for Cu/PGS-2-1000. The contents ratio of sp3 C-C decreased from 0.177 for PGS-2-1000 to 0.122 for Cu/PGS-2-1000. Above all, PGS-2-1000 was indeed reduced by photoreduction. EPMA was carried out to investigate the Cu content on the PGS (Fig. S8). Cu/PGS nanocomposite was synthesized by an in-situ photoreduction method, and then was collected by centrifugation and washed by deionized water and ethanol. This process inevitably lost part of the Cu NPs, so the Cu content (15.77 wt%) was lower than that of the theoretical value (16 wt%). The general XPS survey of Cu/PGS-2-1000 demonstrated that the sample consisted of copper, carbon and oxygen without impurity. The high resolution XPS spectrum of C 1s could be deconvoluted into four peaks (Fig. 7a). The peaks at 284.6, 285.8, 286.7 and 291.5 eV were assigned to the sp2-bonded carbon, sp3-bonded carbon, C–O configurations and π–π*,


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respectively. Compared with the C 1s spectrum of PGS-2-1000, the amount of oxygenate groups in Cu/PGS-2-1000 greatly diminished after photoreduction, implying that numbers of oxygenate groups were reduced successfully, as shown in FTIR. As shown in Fig. S4a, the Cu 2p spectrum showed the binding energy of Cu 2p1/2 and Cu 2p3/2, locating at 932.9 and 952.4 eV, respectively [45]. The absence of satellite peak located at 943 eV was demonstrating that Cu2+ species were completely reduced [19,20]. Cu LMM XAES provided an effective means to distinguish Cu0 and Cu2O [50]. The peak at 918.2 eV was attributed to Cu0 (Fig. S4b). Comprehensive test results of HRTEM, XRD, FTIR and XPS suggested that Cu NPs and PGS were successfully reduced by photoreduction. Photocatalytic hydrogen evolution Hydrogen evolution was carried out for the samples in aqueous solution with lactic acid as sacrificial reagent under Xe light irradiation. As shown in Fig. 7b, along with augmenting the volume of copper acetate, the hydrogen evolution rate of Cu/PGS increased to the highest value at 16 wt% of Cu NPs. When the copper acetate content was further increased, the hydrogen evolution rate decreased because excess Cu NPs agglomerated and the size of Cu NPs augmented which was unfavorable to the separation of electrons and holes. Furthermore, hydrogen evolution of PGS-2-Y with 16 wt% of Cu NPs was performed to investigate the influence of the degree of graphitization on hydrogen production. As shown in Fig. 7c, PGS-2-1000 does not show photocatalytic activity, implying that PGS-2-1000 could not produce photoelectron under Xe light irradiation. Furthermore, a relatively low hydrogen evolution rate (1.29 mmol g-1 h-1) was measured for Cu NPs, and other samples combined with PGS showed higher hydrogen evolution than that of Cu NPs alone. Hydrogen evolution rate increased to the maximum value (4.87 mmol g-1 h-1) for Cu/PGS-2-1000. These results were ascribed to the swift transportation of photoelectrons from Cu NPs to the edge of PGS-2-1000, which suppressed the recombination of the photoinduced electron-hole pairs and its great graphitization degree was beneficial to diminish electron-hole recombination centers in the



composite [52]. Operational lifetime is an important feature for photocatalyst. Five continuous runs of cumulative 30 h under the identical conditions were monitored to investigate the cycling stability of Cu/PGS-2-1000. Interestingly, gradual increase in activity was observed upon repeated use (Fig. 7d), owing to the fact that PGS-2-1000 was reduced, turning into photocatalyst to promote water splitting and facilitating conductivity [47,61]. These results demonstrated the remarkable stability of the catalyst.

Fig. 7. (a) High-resolution C 1s spectrum of Cu/PGS-2-1000, (b) Hydrogen evolution on PGS with different volume of copper acetate solution under solar light irradiation, (c) photocatalytic hydrogen evolution of Cu NPs and PGS-2-Y photocatalytic hydrogen evolution of Cu NPs and(d) repeatability of measurements of photocatalytic hydrogen evolution for Cu/PGS-2-1000.

Taking advantage of UV-vis absorption spectra to evaluate light-absorbance properties of samples is an important way to confirm the photocatalytic activity (Fig. 8a). PGS modified by Cu NPs has uniform absorption band in the whole absorption range from ultraviolet through visible to infrared region, which is in accordance with the literature [53]. A pronounced enhancement in light absorption intensity was


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achieved when PGS-2-1000 formed composite with Cu NPs. This result was relying on PGS-2-1000 with high graphitization serving as photosensitizer to expand the range of light absorption, and improving the light absorption ability as well [47]. The efficiency of the separation of electron-hole pairs is a key factor for hydrogen evolution rate. Therefore, transient photocurrent response was performed to investigate the excellent properties of charge transfer. As shown in Fig. 8b, PGS-2-1000 did not show photocurrent response during repeated on/off illumination cycles, indicating that PGS-2-1000 could not produce electrons under illumination. However, the photocurrent values of the other samples exhibited obvious prompt responses upon each illumination and then presented a sharp decrease to a steady-state value with the interruption of irradiation. The photocurrent intensity of Cu/PGS-2-1000 was distinctly higher than that of other samples. Moreover it was about fourfold as high as that of Cu NPs alone. This result confirmed that PGS-2-1000, as an ideal electron sink and electron transport channel, led to charges transport from the surface of Cu NPs to PGS-2-1000. The generation and separation efficiency of the photogenerated charge were also disclosed by photoluminescence (PL) under 400 nm light excitation (Fig. 8c). No response of PGS-2-1000 in the PL spectrum proved that no photogenerated electrons were in PGS-2-1000. The introduction of PGS could distinctly reduce the PL intensity in comparison of Cu NPs alone. Furthermore, Cu/PGS-2-1000 exhibited the weakest fluorescence intensity, implying that the charge transfer rate of Cu/PGS-2-1000 was much higher than that of others. In addition, high graphitization was attributed to preventing the recombination of electron-hole pairs [54]. The results of hydrogen evolution, photocurrent response and PL spectrum demonstrated that the photoelectron was generated in Cu NPs and transferred to PGS. Electrochemical Mott–Schottky measurements were performed to further investigate the difference of carrier density between all samples. Fig. 8d showed that the Mott–Schottky plot of samples has positive slope. The gradient of Cu/PGS-2-1000 was lowest, implying that carrier density of Cu/PGS-2-1000 was larger than that of other samples. This should be due to the electron swift



transportation from Cu NPs to PGS-2-1000, reducing the recombination of hot electrons and hot holes. This result was consistent with the results of hydrogen evolution and photocurrent experiment. Electrochemical impedance spectroscopy (Fig. S5) revealed that the diameter of the semicircular extrapolated in the Nyquist diagram of Cu/PGS-2-1000 exhibited a significant reduction compared with the other samples. It was shown that Cu/PGSs-2-1000 exhibited fast separation of photogenerated electron-hole pairs and effective interfacial charge transfer, accompanied by a sharp increase in photocatalytic activity. Generally speaking, PGS-2-1000 has a significant effect on promoting available photoinduced charge separation.

Fig. 8. (a) UV-visible diffuse reflectance spectra, (b) On-off current-time curves under light (on) and in dark (off), (c) photo-luminescence (PL) spectroscopy and (d) Nyquist plots of samples.

Based on aforementioned discussion, the possible way of how Cu/PGS generates and transfers electron-hole pairs was illustrated in Fig. 9. When the frequency of incident photons matches the frequency of surface electrons oscillating against the restoring force of positive metal nuclei, Cu NPs were induced the resonant collective


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oscillation of electrons under the excitation of Xe light, and then declined into hot electron-hole pairs through Landau damping afterwards [55]. The hot electrons rapidly separated from hot holes on the time scales of femtoseconds. Subsequently, the hot electrons shifted to the metal surface before recombining with hot holes in short order. A large amount of electrons accumulated on the surface of the Cu NPs transferred to the edge of PGS rapidly, reducing the recombination of electrons and holes [24,56,57]. Subsequently the electrons reduced the hydroxyl functional groups on PGS and the protons into H2, whereas the holes were consumed by lactic acid [47]. The pronounced improvement in the hydrogen evolution of Cu/PGS ascribed to the outstanding conductivity and porous nanosheets which provided a lot of active sites. Not only could PGS act as an electron transport channel to boost charges separation, but also it served as a photosensitizer for hydrogen evolution to extend absorption of light. Furthermore, as the photoreduction goes on, PGS with higher degree of oxidation and disorder was adjusted to a more suitable band gap through tuning the degree of oxidation. PGS maybe became a photoactive material for hydrogen generation [24,47].

Fig. 9. Schematic of photocatalytic hydrogen evolution over the Cu/PGS.



Conclusion In summary, high quality porous graphene nanosheets were synthesized by a facile, green and reliable method from Plumeria rubra. The as-prepared PGS possessed inherent properties of graphene and the special structure configuration, which is ascribed to the synergistic reaction of KOH and calcination temperature and exfoliation of graphene nanosheets by releasing abundant gases during reaction. The ratio of potassium hydroxide to carbon material has a significant influence on porosity structure, which plays a critical role in the ion diffusion/transport. The graphitization and lamellar thickness were controlled by calcination temperature, which acted on the degree of graphitization and wrinkle and high specific surface. The unique structure endowed the material with enhanced photocatalysis activity when using an in-situ photoreduction route to form Cu/PGS as a photocatalyst to split water. The result revealed that PGS contributed to the valid separation of electron-hole pairs and led to electron transfer from Cu NPs to the edge of PGS. Furthermore, Cu/PGS-2-1000 exhibited a remarkable photostability and a possible mechanism was established to circumstantially illustrate the charge transfer channel as well as the improvement of photocatalytic activity. This synthetic strategy opens up an effective way to produce graphene from renewable biomass wastes for energy relative applications.

Supporting Information SEM images, adsorption parameters and pore size distribution of PGS, XRD patterns, FTIR spectra, Raman spectra and Nyquist plots of Cu/PGS, high-resolution Cu 2p spectrum and Cu LMM spectrum of Cu/PGS-2-1000, comparison of C 1s peaks fitting results of PGS-2-1000 and Cu/PGS-2-1000, EPMA of PGS-2-1000 and Cu/PGS-2-1000, SEM images of Cu NPs.

Acknowledgements We thank the National Natural Science Foundation of China (No. 21571064, 21371060), and the PhD Start-up Fund of Natural Science Foundation of Guangdong Province and the Fundamental Research Funds for the Central Universities for


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financial support.



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Abstract Graphic

Graphene produced from Plumeria rubra was in-situ loaded with copper nanoparticles for highly efficient photocatalytic hydrogen evolution.


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Abstract Graphic

Graphene produced from Plumeria rubra was in-situ loaded with copper nanoparticles for highly efficient photocatalytic hydrogen evolution.

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Gas-exfoliation assisted fabrication of porous graphene nanosheets

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