Heteroatom nitrogen- and boron-doping as a facile strategy to improve

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Heteroatom nitrogen- and boron-doping as a facile strategy to improve photocatalytic activity of standalone reduced graphene oxide in hydrogen evolution Lutfi Kurnianditia Putri, Boon-Junn Ng, Wee-Jun Ong, Hing Wah Lee, Wei Sea Chang, and Siang-Piao Chai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b12060 • Publication Date (Web): 09 Jan 2017 Downloaded from http://pubs.acs.org on January 10, 2017

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ACS Applied Materials & Interfaces

Heteroatom nitrogen- and boron-doping as a facile strategy to improve photocatalytic activity of standalone reduced graphene oxide in hydrogen evolution Lutfi K. Putri,[a] Boon-Junn Ng,[a] Wee-Jun Ong,[b] Hing Wah Lee,[c] Wei Sea Chang,[d] and Siang-Piao Chai*[a] a

Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon

Selatan, Bandar Sunway, 47500 Selangor, Malaysia b

Institute of Materials Research and Engineering (IMRE), Agency for Science, Technology and

Research (A*STAR), 2 Fusionopolis Way, Innovis, 138634, Singapore c

Nanoelectronics Lab, MIMOS Berhad, Technology Park Malaysia, Kuala Lumpur 57000,

Malaysia d

Mechanical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon

Selatan, Bandar Sunway, 47500 Selangor, Malaysia

KEYWORDS. Photocatalyst; Nitrogen; Boron; Doped; Graphene; Hydrogen

ACS Paragon Plus Environment

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ABSTRACT. Owing to its superior properties and versatility, graphene has been proliferating the energy research scene in the past decade. In this contribution, nitrogen (N-) and boron (B-) doped reduced graphene oxide (rGO) variants were investigated as a sole photocatalyst for the green production of H2 and their properties with respect to photocatalysis were elucidated for the first time. N-, B- rGOs were facilely prepared via the pyrolysis of graphene oxide with urea and boron anhydride as their respective dopant source. The pyrolysis temperature was varied (600oC800oC for N-rGO and 800oC-1000oC for B-rGO) in order to modify dopant loading percentage (%) which was found to be influential to photocatalytic activity. N-rGO600 (8.26 N at%) and BrGO1000 (3.59 B at%), which holds the highest at% from each of their party, exhibited the highest H2 activity. Additionally, the effects of the nature of N and B bonding configuration in H2 photoactivity were also examined. This study demonstrates the importance of dopant atoms in graphene, rendering doping as an effective strategy to bolster photocatalytic activity for standalone graphene derivative photocatalysts.


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1.0

INTRODUCTION

Decarbonizing the energy sector has become a global ambition since the onslaught of global warming, and hydrogen (H2) is predicted to be a potential future, clean energy carrier owing to its high energy density (120 MJ kg-1) and their carbon-free emissions upon combustion. Particularly, the hydrogen production by the resourceful combination of water and light has attracted significant attention as it provides a potential approach for renewable energy production without the dependence on fossil fuels.

In the realization of this vision, over the past few years, graphene, the wonder material made up of carbon atoms densely packed into a two-dimensional (2D) honeycomb lattice, has made great contribution to improving light-driven H2 generation performances.1-4 However, though typically the graphene-related materials such as reduced graphene oxide (rGO) or graphene oxide have been employed extensively as supports for various semiconductors and metal nanoparticles, the graphene moiety in these hybrids is not considered as an active component.5-7 Only recently, there have been reports demonstrating that graphite oxide (GO) or graphene oxide can stand as an independent photocatalyst for H2 generation,8-9 which reveals the possibility to replace expensive metallic semiconductor photocatalysts and encourages the search for highly available, sustainable while remaining efficient metal-free catalysts. Moreover, graphene has the benefit of large surface area, high electrical conductivity for electron transport and short diffusion distances for electrons owing to its few-atom thick structure, suggesting its amenability in the role of photocatalyst.10-11 In addition to that, recently the chemical doping of graphene with heteroatoms (N, B, S, etc.) has been a rising strategy to tailor and bolster the electronic properties of graphene.12-14 In detail, heteroatom doping of graphene can simultaneously introduce a bandgap


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and endow graphene with catalytically active sites, sanctioning its role as an intrinsic photocatalyst on its own. With that, to continue the exploitation of the potential of graphenebased materials as a photocatalyst, we herein report the photocatalytic activity for nitrogendoped reduced graphene oxide (N-rGO) and boron-doped reduced graphene oxide (B-rGO) in H2 generation. Nitrogen and boron are excellent dopant choices as they have compatible atomic size with carbon and thus garnering strong bonds with carbon atoms. Although prior studies exist on sole heteroatom doped graphene as a photocatalyst,15-16 these works concern graphene-like polycyclic aromatic hydrocarbons (PAHs) derived from organic precursors. Contrarily, this work features doped graphene procured from GO as the starting material and to the best of our knowledge, this is the first report on the use of B-rGO and N-rGO for photocatalytic H2 production. For the first time, the photocatalysis-related properties of doped-rGOs were explored and additionally, the doping levels and configuration influence to photoactivity were also systematically investigated in this study.

Up to now, several strategies have been established to synthesize chemically doped graphene, which include chemical vapour deposition (CVD), arc discharge and plasma treatment, but these require rigorous conditions or elaborate instruments, which are often too expensive and troublesome for mass production. Conversely, the thermal annealing of GO in the appropriate presence of dopant precusors is more beneficial owing to the low cost, facile preparation and easy scalability. Not to mention, it also allows flexible controllability over the doping levels and the bonding configuration of the dopant atoms.17 On that account, herein the preparation of NrGO and B-rGO samples was achieved by the pyrolysis of GO in the presence of urea and boric anhydide (B2O3) respectively under inert argon environment in a tubular furnace. N-rGO samples were subjected to various temperatures i.e. 600oC, 700oC and 800oC, while B-rGO


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samples, which requires higher calcination temperature for integration were subjected to elevated temperatures of 800oC, 900oC and 1000oC. GO hereby behaves as the backbone for heteroatom incorporation where labile oxygen-containing groups on GO provide active sites for dopant atoms to attack and consequently integrate into the graphene framework. At the same time, this is accompanied by the removal of oxygen-containing functional groups which are concurrently reduced at high temperatures. The evolution of N- or B-rGO structures which are temperaturesensitive was studied to gain an understanding into the relation of N- or B-rGO temperaturedependent properties to photocatalytic activity. 2.0 2.1

EXPERIMENTAL

Chemicals

All chemicals used were of reagent grade. Natural graphite powder (Sigma Aldrich, particle size < 45 µm, > 99.99%), sulfuric acid, H2SO4 (Chemolab supplies, 95–97%), phosphorus pentoxide, P2O5 (Sigma Aldrich, ≥ 98.0%), potassium persulfate, K2S2O8 (Sigma Aldrich ≥ 99.0%), potassium permanganate, KMnO4 (Sigma Aldrich, ≥ 99.0%), hydrogen peroxide, H2O2 (R&M Chemicals, 30%), hydrochloric acid, HCl (Chemolab supplies, 37%), ethanol (Chemolab supplies, 96%), urea (Sigma Aldrich, ≥ 99.0%) and boric anhydride, B2O3 (Sigma Aldrich, ≥ 99.0%), sodium hydroxide, NaOH (Sigma Aldrich, ≥ 98.0%). Throughout the course of the experiment where water is mentioned, deionized (DI) water (> 18.2MΩ cm resistivity) was used.

2.2

Characterizations

The surface morphology and chemical composition of the as-prepared samples were investigated by field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS) and transmitting electron microscopy (TEM). FESEM images and EDS were taken on a


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Hitachi SU8010 and TEM images were obtained using a TECNAI G2 F20 microscope. The TEM samples were prepared by depositing a drop of diluted ethanol suspension containing the samples on lacey-film-coated copper grid. Powder X-ray diffraction (XRD) measurements were collected in the diffraction angle range (2θ) 5-50o on Bruker D8 Discover X-Ray Diffractometer at the scan rate of 0.02o s-1 and using an accelerating voltage and applied current of 40kV and 40mA respectively. Raman spectra were recorded at room temperature on HORIBA LabRam HR Evolution Raman spectrometer using 512nm laser. Ultraviolet-visible (UV-Vis) absorbance spectra were analyzed using a UV-Vis spectrophotometer (Agilent, Cary 100) equipped with an integrated sphere under ambient room conditions. X-ray photoelectron spectroscopy (XPS) measurements were recorded on a scanning X-ray microprobe PHI Quantera II (Ulvac-PHI, INC) using monochromatic Al-Kα (hv = 1486.6 eV) X-ray source that operated at 25.6 W with a beam diameter of 100 µm. The wide scan analysis was performed using a pass energy of 280 eV with 1 eV per step while the narrow scan analysis was performed at a pass energy of 112 eV with 0.1 eV per step for chemical state analysis. Prior to de-convolution, charge correction was performed at C 1s by setting binding energies of C-C and C-H to 284.8 eV. Room-temperature PL spectra were measured on HORIBA LabRAM HR Evolution using 325nm line from a He-Cd laser for PL excitation. Electrodes for electrochemical measurements were prepared by drop casting the samples onto fluorine doped SnO2 (FTO) conducting glass substrates (Sigma Aldrich, ~8Ω/sq) with an active area fixed at 1cm2. The photoelectrochemical (PEC) measurements of the photocatalysts were performed using electrical analyzer (CHI 6005E) in a standard threeelectrode cell with Ag/AgCl and Pt as reference and counter electrode, respectively in 0.5M Na2SO4 as the electrolyte. For transient photocurrent response, the working electrode was illuminated with a 500 W Xe arc lamp as the exciting light source and under an applied bias of


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0.5 V with light intermittently turned on and off. Mott-Schottky plots were measured in the range from -0.8 to 2 V with a potential step of 50 mV at 100 Hz frequency. Lastly, the electrochemical impedance spectroscopy (EIS) was measured in the frequency 10-2 to 106 Hz at an applied potential of 0.2 V vs. Ag/AgCl with an AC perturbation signal of 10 mV. 2.3

Development of nitrogen-doped reduced graphene oxide (N-rGO)

Graphite oxide (GO) was first synthesized following the methods from previously published works.7,

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N-rGO was synthesized from the thermal pyrolysis of solid GO and urea. Briefly,

graphene oxide suspension of 1 mg/mL in DI water (100 mL) was mixed with 300 mg urea (1:3 mass ratio). The mixture was thickened to a paste under heating and continuous stirring. The paste was freeze dried to give a homogenous powder mixture. The mixture was transferred onto a quartz boat and was placed in the centre of the tube furnace where it was subjected to thermal annealing at designated temperature (600oC, 700oC, 800oC) in an argon atmosphere. Prior to heating, the tube was flushed with argon for about 30 min to evacuate the air in the reaction system. Next, the furnace was heated to the scheduled temperature at a rate of 10oC/min and maintained at the temperature for 2 h. Upon completion, argon flow was maintained until the furnace cooled down naturally to room temperature. The produced solid was crushed, washed repetitively with 1M HCl and DI water to remove surface-adsorbed residual species several times, and dried at 60oC overnight to yield N-rGOX, where X denotes 600, 700 or 800 for the designated pyrolysis temperature. 2.4

Development of boron-doped reduced graphene oxide (B-rGO)

The same procedure as in the case of N-rGO was followed to synthesize B-rGO except with boron oxide (B2O3) as the boron source and at annealing temperatures (800oC, 900oC, 1000oC). Finally, the solid product was washed with 2M NaOH under reflux and followed by DI water to


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remove leftover B2O3, and dried at 60oC overnight to yield B-rGOX, where X denotes 800, 900 or 1000 for the designated pyrolysis temperature. 2.5

Photocatalytic H2 evolution experiment

The reaction was carried out in an open flow system at atmospheric pressure and ambient temperature. A photocatalyst sample (15 mg) was dispersed in a 0.1M Na2S/Na2SO3 mixtures (120 mL) as the hole scavengers and sonicated prior to being contained in Pyrex vessel. A magnetic stirrer was used to agitate the catalyst-suspended solution at the bottom to prevent sedimentation of the catalyst. The reaction system was evacuated by adequately purging high velocity (50 mL/min) nitrogen (N2) for 30 minutes to eliminate the dissolved air in the reaction solution. The gaseous products were then analyzed at an hourly increment by an integrated gas chromatography (Agilent 7820A, Hayesep Q and mol sieve column, Ar carrier) to avoid air contamination. Lastly, the light was supplied from a side irradiation by a 500 W Xe lamp with visible light cut-off. The entire system was shielded inside a black box during the reaction to prevent interferences from outside light. 3.0 3.1

RESULTS AND DISCUSSION

Materials characterizations

The surface morphology of the as-synthesized samples was analyzed using FESEM and TEM. Both N-rGO600 and B-rGO1000 embodied a planar structure representative of a typical graphene structure as displayed in Figure 1, and the apparent crumpling and folding discerned on the surface of the sheets signify a highly defective structure which was formed upon exfoliation and/or due to the presence of dopant atoms. It is noteworthy that the sheets comprised a lateral dimension mainly in the micron-sizes and no other foreign impurities are visible from the image.


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As shown in the TEM images from Figure 1(c-d), the wrinkled N, B-rGO sheets had low contrast under electron beam, which evidence its low thickness with little overlapping sheets stacked at random, revealing a high exfoliation degree. Overall, the morphology of annealed and doped N and B-rGOs is not significantly changed compared with that of GO (Figure S1) since its laminar morphology, scrollings and foldings were retained. In addition to that, EDS elemental mapping had also verified the successful doping of nitrogen and boron (Figure 1(e-f)). The EDS mapping illustrated the strong presence of carbon (red) and sparse distribution of oxygen (yellow) and nitrogen (green) or boron (blue) in comparison with no additional foreign elements detected (Figure S2), thereby qualitatively authenticating the elemental composition of the samples.


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Figure 1. FESEM images of (a) N-rGO600 (b) B-rGO1000. TEM images of (c) N-rGO600 (d) BrGO1000 and EDS overlay mapping of (e) N-rGO600 and (f) B-rGO1000. Carbon, oxygen, nitrogen and boron are represented in red, yellow, green and blue respectively.

The XRD patterns of all prepared samples were obtained to characterize their crystalline nature and attest their phase purity (Figure 2). Firstly, adequately oxidized GO has its characteristic peak at 2θ = 11o which signifies the expansion of the basal planes spacing owing to the interlamellar incorporation of oxygen functional groups. The interlayer d spacing was obtained


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to be 0.84nm, which is an expansion from its former spacing of 0.34nm for graphite.19 After the annealing process, the peak disappeared and a new peak at ~26o is introduced for N- and BrGOs which is descriptive of the crystalline-like nature of graphite. The broadening of the C (002) peak also indicates the amorphous nature of the synthesized N-rGO and B-rGO. Moreover, it can also be observed that there is a subtle peak shift to lower angle from N-rGOs to B-rGOs samples. This shift can be credited to different composition, stoichiometry and atomic sizes of nitrogen and boron atoms, which give rise to different degree of strain, resulting in the shift. In addition to that, the integrity of the samples can also be ascertained from the XRD peaks. Furthermore, it is to be stressed that graphitic carbon nitride (g-C3N4) can also be possibly converted from the thermal annealing process of urea. Being a photoactive material, g-C3N4,20-21 may potentially temper with the H2 photocatalytic results and thus should be declared free in all N-rGOs samples.22 It is confirmed by XRD that there is no existence of peaks to imply the possible formation of g-C3N4 (2θ = 13o) which is characteristic to its in-plane structural packing of heptazine units.23 Therefore, it is anticipated that nitrogen-containing intermediates (cyanuric acid, ammelide, ammeline and ammonia) from urea pyrolysis were rapidly consumed by reactions with oxygen groups on GO, before any formation of carbon nitride can occur.24-25 This reaction pathway into g-C3N4 is also not likely as urea decomposes and loses its mass completely at temperatures even below 600oC.26 Similarly, residual B2O3 from B-rGOs samples is also deemed unlikely due to the absence of its typical peak at 2θ = 14o.27


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Figure 2. XRD patterns of GO, N-rGOs and B-rGOs samples.

Furthermore, Raman spectroscopy also presented proof on the doping in the graphene lattice. As shown in Figure 3, it is apparent that all samples displayed two prominent bands at ~1300 cm-1 and ~1500 cm-1 which associate to D and G band respectively. The D band expresses the disorder and defects present in the graphite samples. Meanwhile the G band expresses the vibrational modes of sp2 carbon samples and has been established to be sensitive to applied strain and level of doping.28 As the annealing temperature increased, in either case with N-rGO or BrGO, the position of G band is shifted to lower wavenumbers from that observed in GO at 1596.37 cm-1. The G peaks of N-rGO600, N-rGO700 and N-rGO800 were 1586.38cm-1, 1584.71cm-1 and 1583.05 cm-1 respectively. On the other hand, the G peaks of B-rGO800, BrGO900 and B-rGO1000 were 1586.38cm-1, 1579.71cm-1 and 1578.04cm-1 respectively. This downshift of G peaks from GO to N, B- rGOs samples arises from the restoration of conjugated structure and may also be contributed by the electron/hole donating capability of N and B heteroatoms respectively which concurs with previous reports.29-30 Moreover, the ID/IG ratio was


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used to evaluate the degree of disorder in the graphene materials. It was revealed that the ID/IG ratio for all doped graphene samples exceeded that of GO valued at 0.913 (Figure 3 inset). In fact, the ID/IG ratio of N-rGO600, N-rGO700 and N-rGO800 was found to be 1.097, 1.099 and 1.100 respectively. While B-rGO showed a higher range of ID/IG values where B-rGO800, BrGO900 and B-rGO1000 were 1.132, 1.154 and 1.159 respectively, implying more defects and imperfections after doping with boron. This rise in ID/IG ratio can be attributed to several factors. Firstly, the decomposition of oxygen-containing functional groups resulted in the C=C crack and the creation of small nano-crystalline graphene sp2 domains.31 Secondly, it may also be due to the incorporation of foreign heteroatoms that leads to an increased disorder as a result of lattice distortion.30

Figure 3. Raman spectra of GO, N-rGOs and B-rGOs at different pyrolysis temperature and the inset is the corresponding ID/IG ratio.


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XPS was used to probe the elemental composition for individual samples. The survey spectra affirmed the presence of C, O and relevant heteroatom being N or B and devoid of contamination from other impurities as presented in Figure 4. Precisely, the atomic percentages of constituent atoms for all samples are summarized and tabulated in Table 1. The complete XPS survey and high-resolution scans can be found in the Supporting Information (Figures S3-S4). All annealed samples displayed a significant reduction in oxygen level since majority of oxygen are removed by pyrolysis even below 400oC due to the release of carbon dioxide and water.25 In regards to NrGOs, the N at% trend decreased as the annealing temperature is increased, which is coincident to precedent studies.32 In addition to that, N-rGOs structures are complex with a diversity of N functionalities (Figure 4(a)), including: (1) Pyridinic N (~398 eV) which occurs in 6 members ring and donates one p-electron to aromatic π system, (2) Pyrrolic N (~399 eV) which occurs in 5 members ring and contributes two p-electrons to the π system, (3) Graphitic N (~401 eV) which corresponds to N atoms link with 3 carbon atoms in the graphene basal plane, and (4) N-O (~402 eV) for oxidized N.33-34 Initially, oxygen functional groups on the GO interact with nitrogencontaining intermediates from the thermal decomposition of urea, establishing the early incorporation of N into the graphene lattice. These N atoms mainly exist at edge and defect sites in the form of pyridinic N or pyrrolic N.35 At increasing pyrolysis temperatures, these are favorably transformed to graphitic form as they are more thermally stable.36 This evolution of N functionalities can be observed from XPS investigation by plotting the N at% against pyrolysis temperature as shown in Figure 5(a). The decrease in percentage of pyridinic N and pyrrolic N accompanied with the increase in graphitic N infer the transformation of pyrrolic N and pyridinic N to graphitic N from 600oC to 800oC.


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Figure 4. XPS survey spectra of (a) N-rGO600 and (b) B-rGO1000 and the inset is the high resolution N 1s and B 1s respectively.

Contrarily in B-rGOs, the B at% gradually increased with increasing pyrolysis temperatures which has also been observed in earlier reports.37 The high resolution B1s peak can be deconvoluted into 3 peaks corresponding to 3 boron-containing bonding configurations (Figure 4(b)): (1) B-C (~190 eV) bond which makes up graphitic BC3 substitution in the graphene planar sheet, (2) B-O bonds in BC2O, and (3) B-O bonds in BCO2.38-39 From Figure 5(b), boron transformation was such that graphitic BC3 proportion decreased and oxidized B-O proportion increased with increasing pyrolysis temperatures. Furthermore, the XPS results showed an


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absence of extra peak located at ~193.7 eV, which verified the absence of any adhering B2O3 from the reaction.40

Figure 5. N and B functionalities percentage of (a) N-rGO and (b) B-rGO prepared at different annealing temperature.

The synthesis approach and the schematic structure of the N-rGOs and B-rGOs samples are demonstrated in Figure 6. Their parent GO comprised dominantly of epoxy and hydroxyl groups which reside at the basal planes and carboxylic acid groups that dangle at the edges.41 These oxygen functionalities were confirmed by the FTIR spectrum of GO presented in Figure S5. In this scenario, the labile oxygen groups are vital in the incorporation of dopant atoms as they serve as initial integration points. During heating, the volatile gases from dopant sources (i.e. urea or B2O3) react with dangling oxygen groups to form initial anchorage on the graphene sheet. Alternatively, the removal process of temperature-labile oxygen species through heating also creates chemically active edges for opportune integration points of dopants into the graphene lattice. As the temperature increases, bond reconstruction occurs for a more thermally stable


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configuration, allowing the adjustability of dopant levels and configurations on the final dopedrGO product.

Figure 6. Synthesis approach and schematic structure of (a) N-rGO and (b) B-rGO. The green, blue and red spheres represent nitrogen, boron and oxygen atoms respectively.

All in all, the preceding characterization data firmly supplemented the formation of N-doped graphene and B-doped graphene by the pyrolysis of GO. The pre-existing oxygen groups were extensively reduced, whereas dopant atoms were successfully introduced to the graphene lattice and their laminar morphologies were retained in the process.

3.2

Evaluation of photocatalytic H2 evolution and mechanism behind enhancement

The photocatalytic H2 evolution properties of all these metal-free N-rGO and B-rGO sheets under UV irradiation and in the presence of 0.1M Na2S/Na2SO3 mixture as sacrificial hole scavengers were investigated. This study will probe the photocatalytic activity of N-rGO and B-


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rGO in relation to not only the dopant level, but also the type of dopant configuration which was controlled by varying the pyrolysis temperature. To the best of our knowledge, this is the first study of its kind. First and foremost, it is vital to avouch the significance of the role of N and B dopants in boosting the H2 evolution reaction and thus, GO and rGO were included as reference photocatalysts. The rGO sample was prepared at calcination temperature of 800oC since this is the temperature that both N-rGOs and B-rGOs samples shared. As can be seen from Figure 7, regardless of dopant nature, all doped rGO samples, whether is N-rGO or B-rGO, exhibited at least 2-fold increase in photoactivity when compared to GO. GO has an O content totaling to a massive 35.65 at%. Despite oxygen groups being helpful for water dissolution, this slump in activity may be accountable to excessive oxygen groups on GO that impose heavy sp3 structural defects in the lattice thus deteriorating the carrier transport.42-43 Furthermore, electronegative oxygen has a tendency to trap carriers which are then vulnerable as recombination centers.44-45 Hence, this implies that post-pyrolysis samples of GO with lesser density oxygen groups are more meritable for photocatalysis. Though surprisingly, rGO, obtained by the pyrolysis of GO at 800oC but devoid of any dopant atoms, displayed a more suppressed activity than that of GO despite having diminished oxygen groups. In this case, from these control experiments, it is therefore conclusive that dopant atoms, whether is N or B, can be pinpointed as the root cause in the enhancement of photoactivity.


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Figure 7. Photocatalytic rate of H2 evolution in 0.1M Na2S/Na2SO3 solution mixture for all studied samples.

Furthermore, from Figure 7, it is discernable that the pyrolysis temperature of doped rGO samples has a profound influence on the photocatalytic activity. This dependence of photocatalytic activity on the pyrolysis temperature is correlated with the evolution of the dopedrGO structure during pyrolysis. This information is then utilized to gain useful insights into the temperature-property relation of doped-rGO and its accountability towards photocatalysis. In the case of N-rGOs, it is apparently visible that the photoactivity of N-rGOs decreases with increasing pyrolysis temperature. This increase in temperature is in alignment with the decrease in the total N at%. From Table 1, N at% of N-rGO600, N-rGO700 and N-rGO800 showed a decreasing trend from 8.26 at% to 7.60 at% and 6.51 at%, respectively, each corresponding to an H2 evolution rate of 64.6, 61.8 and 55.4 µmolg-1h-1. In company to this, the increase in the C at% signifies the elimination of oxygen and the restoration of graphitic lattice at higher temperatures.46 Additionally, the influence of specific N bonding configuration with respect to H2 photoactivity was also thoroughly investigated.


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Table 1. Nomenclature of N-rGO and B-rGO samples used in the study and their respective atomic constituent and photocatalytic activity data. Sample GO rGO N-rGO600 N-rGO700 N-rGO800 B-rGO800 B-rGO900 B-rGO1000

N at%

B at%

C at%

O at%

8.26 7.60 6.51 -

2.74 2.93 3.59

62.05 93.05 88.83 89.05 91.31 87.34 85.90 85.68

35.65 6.95 2.91 3.35 2.18 9.92 11.18 10.73

H2 production (µmolg-1h-1) 24.11 12.53 64.62 61.84 55.42 52.86 63.95 66.71

Through XPS analysis, it has been established that as the pyrolysis temperature increased, pyridinic fraction remained relatively constant, pyrrolic fraction reduced while graphitic and N-O fractions increased (Figure 5(a)). In this context, pyrrolic configuration which is at its maximum at% at 600oC is the most favourable N bond for H2 generation while graphitic basal N which is at its minimum at% at 600oC configuration does not promote H2 generation.

As for B-rGOs, their photoactivity increases with increasing pyrolysis temperature. Contrary to N-rGOs, increasing pyrolysis temperature for B-rGO caused the increase in B at% content. The B at% for B-rGO800, B-rGO900 and B-rGO1000 raised from 2.74at% to 2.93at% and 3.59 at%, respectively, each corresponding to an H2 evolution rate of 52.9, 64.0 and 66.7 µmolg-1h-1 respectively. In addition to that, the C at% of B-rGOs samples conversely reduced with increasing pyrolysis temperatures, different from that of N-rGO. This indicated that for both NrGOs and B-rGOs, photoactivity is governed dominantly by the at% of dopant atoms (N or B) irrespective of the amount of O or C at%. In the case of B-rGOs, as the pyrolysis temperature


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increased, the fraction of graphitic boron (BC3) decreased and the oxidized boron fractions (BC2O and BCO2) increased (Figure 5(b)). This is plausible since boron incorporates to the graphene lattice from reactions with B2O3 vapors which is generated at high temperatures. Therefore, adhering oxygen groups from B2O3 lead to the increased in oxygen-containing boron configurations.47 Similar to N-rGOs, these results also allude that graphitic basal boron (BC3) is not favourable as active sites for H2 generation.

In light of the fact that oxygen functional groups are susceptible to photoreduction, this thus questions its chemical stability for its direct use in a prolonged temporal setting in photocatalysis. It is therefore crucial to investigate the reusability and stability of the photocatalysts at play. Taking the best sample of B-rGO1000 as the representative photocatalyst, the regeneration and reusability of B-rGO1000 were investigated for four photoreaction cycles. For each cycle, B-rGO1000 was collected after washing with water three times and re-dispersed in fresh solutions containing hole scavengers. As displayed in Figure 8, no apparent deactivation of photocatalyst was observed after four consecutive runs, which signifies the high stability of the B-rGO1000 sample. This implied that the chemical constitution, which is accountable for photo-responsivity and the active sites, did not degrade under extended exposure to light and from prolonged H2 evolution activity.


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Figure 8. Stability tests of B-rGO1000 sample: (a) Hourly evolution of H2 production. (b) Total evolution of H2 in the span of 6 hours for four consecutive cycles.

Aside from that, it is noteworthy to mention that all doped-rGO samples are active solely in the presence of UV light and do not display any visible-light active behavior. This agrees with the results from UV-Vis analysis which sheds light in the optical properties of the doped rGOsamples. As shown in Figure 9, all samples exhibited an onset absorption edge of ca. ~220nm. Noticeably, this absorption edge is only marginally shifted with the change of pyrolysis temperature or dopant percentage as can be seen from the magnified inset. Extrapolation of the band edge to the x-intercept (Eg = 1240/λ) yielded a band gap value of approximately ~5eV.


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Figure 9. UV-Vis spectra of (a) N-rGOs and (b) B-rGOs samples. Insets of (a) and (b) contain a high magnification of the range between 200 and 300nm.

Although this study proves the applicability of doped-rGO as a sole photocatalyst, the fundamental mechanistic of the doped-rGO in behaving as a standalone photocatalyst is still elusive at the present. This is made even more complex due to the inhomogeneity of the rGOs structures, wherein the precise distribution of oxygen groups is an ambiguous subject. Essentially, the origins of photosensitivity can be assigned by a combination of factors. First, it has been noted that the rich cocktail of foreign functionalities comprising oxygen, boron or nitrogen could act in concert to break graphene’s former lattice symmetry, opening a gap between π and π* bands, transforming the zero gap semi-metallic graphene to semiconducting and creating extrinsic states.48 Secondly, Eda et al. has also postulated that isolated sp2 nanodomains are capable for exhibiting quantum confinement-induced semiconducting behavior.49 The local energy gaps of π- π* transition then vary depending on the size, shape and


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fraction of these sp2 domains. The smaller this sp2 domain, the higher the outcome of the energy gap. The calculated energy gap between HOMO and LUMO for a cluster of 37 rings is ~2eV, and this energy gap progressively increases to ~7eV for a single benzene ring.50 For the case of doped-rGOs, the latter seemed to be the governing factor. The formations of small sp2 nanodomains are what render them UV-active. This is since small nanocrystalline sp2 graphene domains were formed in the rGOs samples from the random removal of oxygen-bonding and the formation of C=C cracks and voids due to the the harsh thermal reduction process. This phenomenon is illustrated in Figure 10. Based on this model, we can attribute the UV photoactivity of the rGOs samples to the intrinsic states associated with these isolated sp2 nanodomains procured from the reduction process.

Experimentally, this phenomenon can be reflected and verified by measurements from the Raman data. Previous studies have announced that the intensity ratio of the D and G band (ID/IG) band was inversely proportional to the in-plane crystallite sizes, La.51-53 Considering that La is the average interdefect distance, this is reciprocal to the average size of sp2 clusters located within defects in the graphene structure such as sp3 C matrix and/or vacancies.40 The average size of the sp2 domains, designated as La, was calculated by means of using Knight‘s empirical formula, denoted in equation (1):54-55

=

. () ⁄

From this, the sizes of the sp2 domains of GO, N-rGO600 and B-rGO1000 were measured to be 4.77, 3.96 and 3.75 respectively. This tallies with past observations reporting the size of sp2


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domains in GO to be in the range 3-8nm.45, 56-59 After reduction and doping, the size of these sp2 domains decreases, signifying that concurrent annealing and doping induce the nucleation of the sp2 domains within sp3 matrix, which thus increases the density of small sp2 nanodomains thereby decreasing the average size of the sp2 domains (Figure 10). In principle, this smaller sp2 domain size led to larger band gaps, therefore rendering the doped-rGOs samples to be only UV active.

Figure 10. Structural model for the thermal reduction of GO (red: sp3 bonding region due to oxygen functionalities; white: sp2 bonding region; V: voids) and crystallite sizes of GO, N-rGO600 and BrGO1000.

However, it is to be stressed that doped graphene derived from organic precursors which was investigated in earlier reports are able to exhibit visible-light photoactivity.15-16 They are to be distinguished from these doped-rGOs as they did not undergo any reduction process and therefore do not generate any sp2 nanodomains. Unlike doped-rGOs, bottom-up produced doped graphene-like PAHs has large area of sp2 and therefore their photoresponsivity can be mostly associated with the extrinsic states originating from from dopant functional groups on the graphene backcbone. As a result, smaller bandgap can be achieved and visible-light activity can be seen.


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Additionally, transient photocurrent tests have also asserted the photoresponsivity of the doped rGOs samples, and the results are summarized in Figure 11. As formerly explained, the origin of this photoactivity is due to the additon of various functional groups from dopants and the fraction of sp2 domains.60 For all samples, it is clear that fast photocurrent responses were observed. The photoresponsive phenomenon was also entirely reversible which is shown by the immediate dissipation and recovery of current when the light was intermittenly turned off and on repeatedly. For N-rGOs, the photocurrent intensity was in a descending order of N-rGO600 > N-rGO700 > N-rGO800, which also corresponds to a decreasing amount of N content. Similary, B-rGOs photocurrent displayed a decreasing order of B-rGO1000 > B-rGO900 > B-rGO800, which is also coincident with the decrease in B content. These results from photocurrent tests are in good agreement with photocatalytic H2 production results.


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Figure 11. Photocurrent transient tests for (a) N-rGOs and (b) B-rGOs samples excited under chopped irradiation.

Unlike crystallite semiconductors, where conduction band is typically comprised the cation d and sp orbitals and valence band is composed of the anion p orbitals, chemically modified graphene such as the present case for heteroatom-doping is not as straightforward.61 For a carbon material containing a mixture of sp2 and sp3 bonding like GO/rGOs, its photo-responsivity can be ascribed to the combination of localized, electronic band gaps of sp2 domains within the sp3 matrix,49, 62-64 and the extrinsic states derived from its various functional groups.65-66 However, due to the uncertainty of the distribution of these elements and other nanoscale inhomogeneities found in GO/rGO structure, it is challenging to define a clear LUMO-HOMO illustration like in


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the event of typical crystallite semiconductors. Mott-Schottky (M-S) analysis is a well-adopted tool to determine the built-in potential of many semiconductor materials on the basis of the space charge region by changing the applied DC bias.67-68 Hence in this study, to unveil the band structure of the doped graphene photocatalysts and consequently uncover the reaction mechanism, M-S analysis was performed. The M-S relationship is represented by the following equation (2): 2 1 = − − (2) Where C is the space charge capacitance, q is the electronic charge (1.602 × 10-19 C), ε and εo are the dielectric constant of material and permittivity in vacuum (8.85 × 10-14 F cm-2), respectively, N is the charge carrier density, E and Efb are the applied potential and flat-band potential, respectively, k is Boltzmann constant, N is the charge carrier density and lastly T is temperature in Kelvin. The value of flat band potential, Efb, can be retrieved by the extrapolation of the linear portion of the curve to the x-axis. From Figure 12(a), it was revealed that the integration of GO with nitrogen or boron dopant triggered a negative potential shift in the position of the band edges for both cases. As evidenced from the M-S plot, the Efb values of GO, N-rGO600 and B-rGO1000 were measured to be -0.34, -0.37 and -0.42 V vs. Ag/AgCl respectively. The electrode potentials were converted to the NHE scale using the equation (3):69 (

!."#$)

= (

!. %&⁄%&'()

+ *%&'( (3)

Where E0Ag/AgCl equates to 0.197 for a saturated solution. Thereupon, the Efb values of GO, NrGO600 and B-rGO1000 were determined to be -0.143, -0.173 and -0.223 V (vs. NHE),


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respectively. It is commonly accepted that the conduction band minimum (CBM) is positioned about 0.3 V more negative than Efb,70 the CBM values of GO, N-rGO600 and B-rGO1000 were then calculated to be -0.44, -0.47 and -0.52 V (vs. NHE). This shift towards negative potential upon doping is propitious for hydrogen evolution as higher over-potential connotes to greater driving force for electrons to be consumed for H+ reduction reactions into hydrogen. This is in well agreement to photocatalytic H2 results where B-rGO1000>N-rGO600>GO.

Figure 12. (a) Mott-Schottky plots and (b) EIS plots of GO, N-rGO600 and B-rGO1000 samples.

EIS was also performed to elucidate the charge separation and migration characteristic of the doped rGOs samples and the results are shown in Figure 12(b). The arc radius of the Nyquist circle reflects the effectiveness of charge transfer occurring at the contact interface between the working electrode and electrolyte solution. In general, the smaller the radius on the Nyquist plot, the lower the charge-transfer resistance. It was found that the arc radii of samples after doping with nitrogen and boron are smaller than its parent GO, implying that the addition of nitrogen and boron dopants and thermal reduction led to a lower electronic and charge diffusion resistance. The charge transfer impedance of the samples is in the descending sequence of GO >


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N-rGO600 > B-rGO1000. The smallest arc radius of B-rGO1000 indicates that the lowest electron-hole recombination and better transfer rate of electron and holes occurred for the sample, which is consistent with the photocatalytic results.

Additionally, steady-state PL is also considered as a powerful analysis device to reflect the nature of radiative recombination of charge kinetics and the optical properties for a photocatalyst. From Figure S6, the rugged PL spectra are indicative of many emission peaks, similar to previously reported results.40, 71-72 This is due to the plethora of oxygen groups and a mixture of sp2 and sp3 fraction which induce many local energy gaps within the structure. It is apparent that upon simultaneous reduction and doping with nitrogen and boron, the PL emission intensity decreases as compared with that of the parent GO. This quenched PL emission intensity testified that N, B-doped rGOs exhibit improved separation of charge carriers, which markedly facilitated electron shuttling for H+ reduction reactions to H2 under light illumination.

Moreover, in conjunction to granting graphene with energy gap, these dopants also bestowed graphene with the essential active sites to propel hydrogen evolution reactions. Graphene, as we know, it is chemically inert since its unpaired electrons are strongly bound and passivated in its delocalized π system, which impede its adsorptivity and reactivity.73 However in this setting, graphene acts as an individual photocatalyst, different from its typical role as an electron relay medium. Therefore, in addition to being photoresponsive, it should necessarily garner active sites to foster H2 reactions. According to a theoretical study performed by Yang et al., adding foreign heteroatoms with a different electronegativity than carbon will break the electroneutrality in graphene and create unbalanced charged areas which serve as active sites.74 A study by Zhang revealed that heteroatoms with unpaired electrons cause a localized distribution of molecular


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orbitals which makes it susceptible to chemical reactivity.75 Heteroatom doping can also lead to the asymmetry of spin density which results in polarization allowing easy adsorption of molecules.76 Added to that, active sites also exist in the form of structural defects which can arise from the strain in the lattice imparted from the atomic size differences with the dopants.77 Besides functionalities, defects and graphene edges can also partially account for improved catalytic activity. As testimony, from the Raman spectra (Figure 3), B-rGO samples claimed higher ID/IG ratios than N-rGO, and in concordance, the H2 evolution of B-rGOs surpassed that of N-rGOs. This markedly signifies that the generation of defects and dopants could indeed increase catalytic activity. Therefore, doping is deemed as an effective strategy to simultaneously grant graphene with the energy gap and active sites required to perform as an independent photocatalyst.

Lastly, on the basis of M-S results and UV-Vis results, a possible reaction mechanism and pathway was proposed schematically in Scheme 1. Upon UV irradiation, generation of excited electron-hole pairs to high-energy states occured. This results in the promotion of electron (e-) to the conductive band and formation of a positive hole (h+) in the valence band. The CBM values incrementally increased from -0.44 to -0.47 and to -0.52 V (vs. NHE) for GO, N-rGO600 and BrGO1000 respectively, signifying improved potential driving force upon doping with nitrogen and boron. As these values far exceed the potential requirement for hydrogen reduction reaction at H+/H2 = 0 V (vs. NHE), this implies that the directional transfer of electrons from these conduction bands for H2 evolution is feasible. At the same time, active sites on the doped graphene sheets in the form of unbalanced charge areas, localized orbitals, asymmetrical spins and structural defects intimately adsorbed H+ on its surface which were then able to catalyze and consume electrons for the generation of H2.74-77 Meanwhile at the valence band side, sacrificial


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reagent acted as hole scavengers, whereby S2-/SO32- irreversibly oxidized by holes to form S2O32and SO42-, enriching electrons density in the conduction band, which in turn enhanced the H2 evolution reaction.

Scheme 1. Schematic of proposed reaction mechanism for photocatalytic hydrogen evolution over doped-rGO photocatalysts under light irradiation.

4.0

CONCLUSIONS

In the present study, it has been demonstrated that nitrogen and boron doping were facilely accomplished by the thermal solid-state reaction of GO with urea and boric anhydride respectively. The nitrogen and boron content was tuned by simply varying the pyrolysis temperature. All doped-rGO samples were used as a standalone photocatalyst and exhibited an enhanced photocatalytic H2 activity than the parent GO which were conditional to the dopant content. N-rGO600 (8.26 N at%) and B-rGO1000 (3.59 B at%), which bore the highest dopant content from each party, displayed the highest activity than the rest by hoisting the photocatalytic performance by 2.6- and 2.7-fold than its predecessor GO, and 5.2- and 5.3-fold than rGO


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reduced at 800oC respectively. The incorporation of dopant atoms led to the reformation of bonds and properties, which was favorable and critical to photocatalytic H2 production. We also believe that this study provides new insights for understanding the optical properties of dopedrGOs. The improved performance implied that they are ideally suited as a photocatalyst and it is anticipated that in the future, more of these doped-rGO materials could be employed for better elucidation, not only constrained to H2 production, but other applications such as CO2 reduction and environmental remediation process.

ASSOCIATED CONTENT Supporting Information. FESEM images of GO, rGO. EDS spectra of N-rGO600 and BrGO1000. Complete XPS data for all samples. FTIR spectrum of GO. PL spectra of GO, NrGO600 and B-rGO1000. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Present Addresses Chemical Engineering Discipline, School of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 47500 Selangor, Malaysia Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENTS This work was funded by the Ministry of Science, Technology and Innovation (MOSTI) Malaysia under the e-Science Fund (Ref. no. 03-02-10-SF0244). ABBREVIATIONS GO, graphite oxide; rGO, reduced graphite oxide. REFERENCES 1. Zhang, N.; Yang, M.-Q.; Liu, S.; Sun, Y.; Xu, Y.-J. Waltzing with the Versatile Platform of Graphene to Synthesize Composite Photocatalysts. Chem. Rev. 2015, 115 (18), 10307-10377. 2. Han, C.; Zhang, N.; Xu, Y.-J. Structural Diversity of Graphene Materials and Their Multifarious Roles in Heterogeneous Photocatalysis. Nano Today 2016, 11 (3), 351-372. 3. Zhang, N.; Xu, Y.-J. The Endeavour to Advance Graphene-Semiconductor CompositeBased Photocatalysis. CrystEngComm 2016, 18 (1), 24-37. 4. Yang, M.-Q.; Zhang, N.; Pagliaro, M.; Xu, Y.-J. Artificial Photosynthesis Over Graphene-Semiconductor Composites. Are We Getting Better? Chem. Soc. Rev. 2014, 43 (24), 8240-8254. 5. Zhang, Y.; Tang, Z.-R.; Fu, X.; Xu, Y.-J. TiO2−Graphene Nanocomposites for Gas-Phase Photocatalytic Degradation of Volatile Aromatic Pollutant: Is TiO2−Graphene Truly Different from Other TiO2−Carbon Composite Materials? ACS Nano 2010, 4 (12), 7303-7314. 6. Zhang, N.; Zhang, Y.; Xu, Y.-J. Recent Progress on Graphene-Based Photocatalysts: Current Status and Future Perspectives. Nanoscale 2012, 4 (19), 5792-5813. 7. Ong, W.-J.; Putri, L. K.; Tan, Y.-C.; Tan, L.-L.; Li, N.; Ng, Y. H.; Wen, X.; Chai, S.-P. Unravelling Charge Carrier Dynamics in Interfacing Protonated g-C3N4 with Carbon Nanodots as Co-Catalysts Toward Enhanced Photocatalytic CO2 Reduction: A Combined Experimental and First-Principles DFT Study. Nano Res. 2016, DOI: 10.1007/s12274-016-1391-4. 8. Yeh, T. F.; Syu, J. M.; Cheng, C.; Chang, T. H.; Teng, H. Graphite Oxide as a Photocatalyst for Hydrogen Production from Water. Adv. Funct. Mat. 2010, 20 (14), 2255-2262. 9. Hsu, H.-C.; Shown, I.; Wei, H.-Y.; Chang, Y.-C.; Du, H.-Y.; Lin, Y.-G.; Tseng, C.-A.; Wang, C.-H.; Chen, L.-C.; Lin, Y.-C. Graphene Oxide as a Promising Photocatalyst for CO2 to Methanol Conversion. Nanoscale 2013, 5 (1), 262-268. 10. Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M.; Geim, A. K. Fine Structure Constant Defines Visual Transparency of Graphene. Science 2008, 320 (5881), 1308-1308. 11. Yang, M.-Q.; Xu, Y.-J. Photocatalytic Conversion of CO2 Over Graphene-Based Composites: Current Status and Future Perspective. Nanoscale Horiz. 2016, 1 (3), 185-200. 12. Chang, D. W.; Baek, J.-B. Nitrogen-Doped Graphene for Photocatalytic Hydrogen Generation. Chem. – Asian J. 2016, 11 (8), 1125-1137. 13. Putri, L. K.; Ong, W.-J.; Chang, W. S.; Chai, S.-P. Heteroatom Doped Graphene in Photocatalysis: A Review. Appl. Surf. Sci. 2015, 358, 2-14.


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Table of Contents Graphic and Synopsis:

Nitrogen and boron doped reduced graphene oxide as functional standalone photocatalyst for reduction of water to hydrogen.


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Figure 1. FESEM images of (a) N-rGO600 (b) B-rGO1000. TEM images of (c) N-rGO600 (d) BrGO1000 and EDS overlay mapping of (e) N-rGO600 and (f) B-rGO1000. Carbon, oxygen, nitrogen and boron are represented in red, yellow, green and blue respectively.


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Figure 2. XRD patterns of GO, N-rGOs and B-rGOs samples.



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Figure 3. Raman spectra of GO, N-rGOs and B-rGOs at different pyrolysis temperature and the inset is the corresponding ID/IG ratio.


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Figure 4. XPS survey spectra of (a) N-rGO600 and (b) B-rGO1000 and the inset is the high resolution N 1s and B 1s respectively.



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Figure 5. N and B functionalities percentage of (a) N-rGO and (b) B-rGO prepared at different annealing temperature.


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Figure 6. Synthesis approach and schematic structure of (a) N-rGO and (b) B-rGO. The green, blue and red spheres represent nitrogen, boron and oxygen atoms respectively.



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Figure 7. Photocatalytic rate of H2 evolution in 0.1M Na2S/Na2SO3 solution mixture for all studied samples.


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Figure 8. Stability tests of B-rGO1000 sample: (a) Hourly evolution of H2 production. (b) Total evolution of H2 in the span of 6 hours for four consecutive cycles.



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Figure 9. UV-Vis spectra of (a) N-rGOs and (b) B-rGOs samples. Insets of (a) and (b) contain a high magnification of the range between 200 and 300nm.


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Figure 10. Structural model for the thermal reduction of GO (red: sp3 bonding region due to oxygen functionalities; white: sp2 bonding region; V: voids) and crystallite sizes of GO, N-rGO600 and BrGO1000.



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Figure 11. Photocurrent transient tests for (a) N-rGOs and (b) B-rGOs samples excited under chopped irradiation.


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Figure 12. (a) Mott-Schottky plots and (b) EIS plots of GO, N-rGO600 and B-rGO1000 samples.



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Scheme 1. Schematic of proposed reaction mechanism for photocatalytic hydrogen evolution over doped-rGO photocatalysts under light irradiation.


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Heteroatom nitrogen- and boron-doping as a facile strategy to improve

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