Enhanced Daylight-Induced Photocatalytic Activity of Solvent

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Enhanced daylight-induced photocatalytic activity of solvent exfoliated graphene (SEG)/ZnO hybrid nanocomposites towards degradation of Reactive Black 5 Wee-Jun Ong, Seen-Yee Voon, Lling-Lling Tan, Boon Tong Goh, Siek-Ting Yong, and Siang-Piao Chai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5027088 • Publication Date (Web): 10 Oct 2014 Downloaded from http://pubs.acs.org on October 18, 2014

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Industrial & Engineering Chemistry Research

Enhanced daylight-induced photocatalytic activity of solvent exfoliated graphene (SEG)/ZnO hybrid nanocomposites towards degradation of Reactive Black 5 Wee-Jun Ong,† Seen-Yee Voon,† Lling-Lling Tan,† Boon Tong Goh,‡ Siek-Ting Yong,† and SiangPiao Chai*,† †

Multidisciplinary Platform of Advanced Engineering, Chemical Engineering Discipline, School

of Engineering, Monash University, Jalan Lagoon Selatan, Bandar Sunway, 46150 Selangor, Malaysia. ‡

Low Dimensional Materials Research Centre, Department of Physics, Faculty of Science,

University of Malaya, 50603 Kuala Lumpur, Malaysia.

ACS Paragon Plus Environment

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ABSTRACT A series of SEG/ZnO photocatalysts with different ratios of Zn(NO3)2 precursor was synthesized via a facile chemical deposition-calcination approach using solvent exfoliated graphene (SEG) as a precursor of graphene, Zn(NH3)4CO3 as a precursor of ZnO, NaOH as a precipitating agent and poly(vinyl pyrrolidone) as an interface linker. The photocatalytic activity and stability of SEG/ZnO were evaluated by the degradation of Reactive Black 5 (RB5) under visible light. In comparison to pure SEG and ZnO, SEG/ZnO nanocomposites displayed a remarkable RB5 degradation of 97.0% with a rate constant of 0.0199 min-1 under a low-power 15 W energysaving light bulb for 3 h. The optimum ZnO content was 69.0 wt%, which demonstrated a 10fold enhancement after graphene hybridization than that for pure ZnO. This was accredited to effective dye sensitization and suppressed electron-hole recombination via SEG as electron storage. Lastly, a visible-light photocatalytic mechanism for the photoactivity was explored.

KEYWORDS Solvent exfoliated graphene, ZnO, photocatalysis, Reactive Black 5 dye, visible light irradiation


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1. INTRODUCTION The shift into the 21st century overlooks deteriorating conditions in energy and environmental aspects, gradual depletion of fossil fuels and climate change, triggering alarming concerns for mankind.1,2 Little is made known that the release of organic dyes from the textile industries constitutes one of the largest group of organic pollutants in wastewater, with approximately 20% of dyes being purged during the dyeing procedure.3 The discharge of highly coloured pollutants into the environment is a significant cause of eutrophication and perturbations in the aquatic ecosystem.4 Furthermore, these dyes have the potential of undergoing natural reductive anaerobic degradation, which resulted in the production of hazardous and carcinogenic intermediates.5 Due to their complicated nature and most being resistant to bio-degradation, both the physical and conventional biological treatments such as reverse osmosis, flocculation, adsorption using activated carbon and oxidation with chlorine, hydrogen peroxide or ozone6 proved to be less effective for complete degradation; even leading to high regeneration cost due to transfer of contaminants from one phase to another, rather than elimination from the water matrix. Heterogeneous photocatalysis has been considered as a promising and economical destructive technology due to the key advantage: the ability of being conducted at ambient condition leading to complete organic carbon mineralization or dye degradation.7 Although it highlights a new sought after avenue for such an environmental purification, it suffers a cornucopia of obstacles impeding its practical maximization of photocatalytic performance, such as low adsorption capability, rapid recombination of electron-hole pairs, insufficient quantum efficiency, inability for visible light utilization and possible photocorrosion of the catalyst. 8 In the past few years, many photocatalysts such as TiO23,9 and ZnO10,11 have been attempted for dye


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degradation ranging from methylene blue (MB), methyl orange (MO), Rhodamine B (RhB), Reactive Yellow 145 to Reactive Black 5 (RB5). In comparison to the relatively stable TiO2-based materials, the lower cost ZnO

with

higher photoactivity towards dye degradation in the liquid phase holds the disadvantage of photo-instability in aqueous medium from the photocorrosion with light illumination. This markedly reduces its photocatalytic performance for environment purification.12-14 In recent years, numerous techniques have been investigated to enhance its photostability, encompassing formation of surface complex,15 surface organic coating,16 and surface hybridization of ZnO with graphite-like such as monolayer polyaniline,14 fullerenes C6013 and also graphene.17,18 These studies provided the opportunity of applying ZnO in practical applications of removing organic pollutants photocatalytically. Even though the sun provides a plethora of photons, ZnO is active under ultraviolet (UV) light due to its wide band gap of 3.2 eV, which comprises a minimal fraction (~5%) of solar energy that reaches the surface of the Earth as compared with visible light (~45%).15 Accounting for energy conservation and utilization, the engineering of improved visible-light-responsive photocatalysts is both essential and challenging. Since the first study of graphene as the newest member of carbon nanomaterials by condensed-matter physicists Geim and Novoselov back in 2004,19 graphene has promptly established its prominence as an unforeseen and at times unwelcomed newcomer. The professional scepticism with respect to graphene applications that primarily garnered the attention of several researchers is slowly fading under the pressure of recent developments due to its remarkable mechanical, thermal, electrical and optical properties. These include large specific surface area (theoretical value of 2630 m2 g-1), high Young’s modulus (~1100 GPa) and thermal conductivity (~5000 W m-1 K-1), excellent mobility of charge carriers (> 200 000 cm2 V-1 s-1 at


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electron densities of 2 × 1011 cm-2) and optical transmittance (~97.7%).20,21 Methods such as physical and chemical routes have been devoted to the synthesis of pristine graphene.22-24 With progress continuing apace, a recent breakthrough was reported by Hersam and co-workers25 with the development of solvent exfoliated graphene (SEG) synthesized through an integrated strategy of reducing defects on graphene and enhancing the interfacial contact between semiconductor nanoparticles and graphene sheets. The synergetic effect of ZnO and graphene is expected to enhance the overall photocatalytic activity as a result of the increase of light absorption intensity, broadening of light absorption range, suppression of charge carrier recombination, improvement of surface active sites as well as enhancement in the chemical stability of photocatalysts. In recent years, strenuous efforts have been devoted to explore graphene/ZnO nanocomposites;17,18,20,26-28 however, there is still a paucity of reports on the fabrication of well-defined graphene/ZnO nanocomposites. The control of the ZnO particle sizes and their dispersion on the graphene was strongly dependent on the methods employed during the synthesis process, which was generally hard to be achieved. Conventionally, for the simple mixing route, the loading of ZnO onto the graphene is limited because of the weak connection between graphene and ZnO. 29 Moreover, other synthesis methods such as chemical vapor deposition and ultrasonic spray pyrolysis require high-end and sophisticated equipment.30 Herein, this article reported the synthesis of SEG/ZnO nanocomposites via a facile and cost-effective chemical deposition-calcination approach without employing any organic solvent. In this study, a hydrophilic linear polymer of poly(vinyl pyrrolidone) (PVP) was employed as an intermediate to incorporate Zn2+ ions with carbon nanomaterials to anchor ZnO on SEG sheets. The SEG/ZnO photocatalysts would be used for the degradation of RB5, which has been


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continuously released from the industries. Research in the field of RB5 degradation has been scarcely reported as compared to other dyes such as MB, MO and RhB. To the extent of our knowledge, the incorporation of ZnO with SEG for RB5 photodegradation has not been reported yet, moreover, for such an application utilizing not only the sought after low-power 15 W energy-saving light bulb, but also under ambient conditions. Quoting Nature editorials,31 “graphene is not a miracle material, just a very promising one and it would take sustained interest to deliver its potential”. This research takes a step further into investigating the optimum graphene loading, harnessing the outstanding properties of such a remarkable composite. It is hoped that the potential of such a composite for environmental remediation is exploited in a more reasonable and rational way, rather than combining the graphene “gold rush”.

2.

EXPERIMENTAL SECTION 2.1. Materials. Natural graphite powder (< 45 µm, ≥ 99.99%) was purchased from Sigma

Aldrich. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, Sigma Aldrich, ≥ 99%), ammonium carbonate ((NH4)2CO3, Sigma Aldrich, ≥ 30% NH3 basis), PVP (Sigma Aldrich, average molecular weight = 40000), N-N-dimethylformamide (DMF, Sigma Aldrich, 99.8%), sodium hydroxide (NaOH, Sigma Aldrich, ≥ 97%), terephthalic acid (Sigma Aldrich, 98%), ethanol (Chemolab supplies, 96%), nitric acid (HNO3, Chemolab supplies, 69%), sulphuric acid (H2SO4, Chemolab supplies, 95–97%) and hydrogen peroxide (H2O2, Chemolab supplies 30 vol%) were of analytical grade. The commercial azo dye RB5 obtained from Sigma Aldrich was used as such. All chemicals were used as received without undergoing extra purification. Deionized water (>18.2 MΩ cm resistivity) was used for all processes.


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2.2. Synthesis of Solvent Exfoliated Graphene (SEG). SEG was synthesized based on our previous study.32 In brief, 1 g of natural graphite powder was added to 100 ml of DMF and sonicated for 3 h. The precipitate was washed with ethanol. Acid pre-treatment was performed with 25 ml of 5 M HNO3 and 75 ml of 5 M H2SO4 (at a volume ratio of 1:3) for 4 h at 100 oC in a non-asbestos heating mantle. The solution was allowed to cool down for 1 h before being repeatedly washed with deionized water until neutral pH was achieved. Lastly, the sample was dried in an oven at 70 oC overnight to obtain SEG powder. 2.3. Fabrication of SEG/ZnO Photocatalysts. The SEG/ZnO nanocomposites have been fabricated via a facile chemical deposition-calcination approach as illustrated in Figure 1. The detailed synthesis procedure was as follows: 25 ml of 0.25 M Zn(NO3)2 aqueous solution, 25 ml of 0.25 M (NH3)4CO3 aqueous solution and 0.5 M NaOH were vigorously mixed to form watersoluble Zn(NH3)4CO3 solution (labelled as solution A). The pH of the solution was altered to pH 8 using 0.5 M NaOH. The as-prepared 0.1 g of SEG was ultrasonicated in deionized water with an equivalent amount of PVP for 1 h to form SEG suspension. This suspension was denoted as solution B. After that, solution A and solution B were magnetically stirred at room temperature for 2 h, followed by vacuum filtration and then repeatedly washed with deionized water. The final product was dried in an oven at 80 oC overnight to acquire dried SEG/ZnO precursor, before further calcination at 400

o

C for 2 h in air to obtain the calcined SEG/ZnO

nanocomposites. For control experiment, the sample was synthesized using a similar procedure, but without the addition of PVP. Also, pure ZnO was developed according to the same procedure without the presence of SEG and was used for comparison purpose. In this study, the concentration of Zn(NO3)2 precursor was varied (25 ml, 12.5 ml, 6.25 ml, 3.125 ml) and the as-


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synthesized SEG/ZnO nanocomposites were labelled as SEG/ZnO-1, SEG/ZnO-2, SEG/ZnO-3 and SEG/ZnO-4, respectively for the decreasing concentration of Zn(NO3)2 precursor.

Zinc precursor Zn(NH3)4CO3

PVP

Zn2+ (a) Stirring at room temperature

Polymer solution (b) Filtering (c) Calcination at o 400 C for 2 h

SEG

ZnO

Acid pre-treatment

SEG/ZnO

SEG

Figure 1. Schematic of the development of SEG/ZnO hybrid nanocomposites. 2.4. Characterization of Photocatalysts. Thermogravimetric analysis (TGA) was performed under a studied temperature range from room temperature to 1000 oC under a continuous purified air flow (TA Instrument Q50). The surface morphology of the nanocomposites was characterized by Hitachi SU8010 Field Emission Scanning Electron Microscopy (FESEM). Scanning transmission electron microscopy (STEM) image was taken on a FESEM at an accelerating voltage of 30 kV. High resolution transmission electron microscopy (HRTEM) image was acquired using a JEOL JEM-2100F microscope operated at 200 kV. The TEM sample was prepared by depositing a drop of diluted suspensions in ethanol on a lacey-film-coated copper grid. X-ray diffraction (XRD) pattern was recorded with a Bruker D8 Advance X-Ray Diffractometer using Cu Kα (λ = 0.1549 nm) radiation in the diffraction angle range of 5 to 90o under a scan rate of 0.01o s-1 with an accelerating voltage of 40 kV. The crystallite size of ZnO nanoparticles was estimated based on the Debye-Scherrer’s equation:


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(1)

where d is the crystallite size, λ is the wavelength of the irradiation used, k is a constant (0.9 assuming that the particles are spherical), β is the full width at half maximum (FWHM) intensity of the most significant peak of ZnO in radians, and θ is Bragg’s diffraction angle.33 Fourier transform infrared (FTIR) spectra were obtained on Nicolet iS10 Transform Infra-Red Spectrometer over the spectral range of 4000–400 cm-1 using KBr Die Model 129. Raman spectra were recorded using Renishaw inVia Raman Microscope with an excitation of 514.5 nm laser light. The light absorbance properties of samples were evaluated over the spectral region of 200–800 nm on a UV-Vis spectrophotometer (Agilent, Cary 100). The band gap energies were calculated using Equation (2) based on the optical absorption edge. (2) where Eg is the band gap energy, h is Planck’s constant, c is the speed of light and λ is the wavelength of the optical absorption edge. The photoluminescence (PL) emission spectra were analyzed on a fluorescence spectrometer (Perkin Elmer, LS55) at an excitation wavelength of 325 nm with the scanning speed of 600 nm min-1. The widths of emission slit and excitation slit were fixed at 10 nm. All the measurements were performed at room temperature. 2.5. Evaluation of Photocatalytic Activity. The photocatalytic degradation of RB5 solution over the as-prepared SEG/ZnO nanocomposites was performed under visible light irradiation with 15 W energy-saving light bulb (Philips, TORNADO 15 W WW E27 220–240 V 1CT). The average light intensity was measured to be 8.5 mW cm-2 by utilizing a pyranometer (Kipp and Zonen type CMP 6). The light spectrum of the visible light was recorded using an Avantes fiber


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optic spectrometer (AvaSpec-128) equipped with a cosine collector (Figure S1, Supporting Information), with the distance apart between the light source and the reactor being 5 cm. For effective degradation of RB5, 30 mg of the catalyst (SEG, ZnO, SEG/ZnO) was dispersed in a RB5 suspension (20 mg/l, 50 ml) under ambient conditions and constant stirring. Before turning on the light source, the suspensions were magnetically stirred for 30 min in the dark to attain the adsorption-desorption equilibrium. 3 ml of aliquots was sampled every 15 min and centrifuged (13500 rpm, 15 min) to completely remove the photocatalyst. The solution was analyzed using a UV-Vis spectrophotometer (Agilent, Cary 100) at the maximal absorption wavelength of RB5, and its characteristic absorption peak was chosen to be 597 nm. The durability and stability test was also conducted by following the similar approach as discussed earlier. Three consecutive cycles, in which the duration of each cycle was 3 h, was performed on the as-developed SEG/ZnO photocatalysts. After the end of each cycle, the product was centrifuged (13500 rpm, 15 min) and thoroughly washed with deionized water. It was then added to the new RB5 solution. At least two experimental runs were performed and the results were averaged to determine the reproducibility of the photocatalytic results. For all cases, the experimental error was within ±5%. 2.6. Trapping Experiments for Radicals and Holes. The influence of various reactive species such as holes (h+), hydroxyl (•OH) and superoxide (•O2-) radicals on the RB5 degradation was examined to understand the photocatalytic mechanism in the SEG/ZnO system. Various scavenging reagents such as triethanolamine (TEOA), isopropanol (IPA), and benzoquinone (BQ) were used in this study. The concentrations of each scavenging reagent in the reaction system were all 10 mM. The analysis procedure was exactly identical to the photodegradation process.


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2.7. Analysis of Hydroxyl Radicals (•OH). Using terephthalic acid as a probe molecule, the formation of •OH at the illuminated photocatalyst/water interface was identified by the fluorescence

method.

Briefly,

terephthalic

acid

reacts

with

•OH

to

produce

2-

hydroxyterephthalic acid, which is known as a fluorescent product.5,34,35 Its fluorescence signal was measured at 425 nm. The fluorescence intensity of 2-hydroxyterephthalic acid is proportional to the as-produced •OH radicals. The experimental process was identical to the photoactivity measurement except that the 5 × 10-3 M terephthalic acid solution in addition to the 10 × 10-3 M NaOH was used to substitute the RB5 solution. The fluorescence spectra of the 2hydroxyterephthalic

acid

were

examined

on

a

Perkin

Elmer

LS55

fluorescence

spectrophotometer with an excitation wavelength of 315 nm Xenon lamp. For every interval of 15 min upon light irradiation, the solution was centrifuged (13500 rpm, 15 min). The fluorescence intensity at a wavelength of 425 nm was then determined.

3. RESULTS AND DISCUSSION 3.1. Characterizations of the Photocatalysts. Although SEG has lower defect density than graphite oxide (GO) sheets, SEG resulted in a weak interfacial contact due to insufficient hydrophilic functional groups.36 The lower defect density of SEG relative to GO could be evidenced by the Raman spectroscopy which will be discussed in detail in the later section. Thus, acid pre-treatment was carried out to introduce more oxygen-containing groups on the SEG sheets. At the same time, to further functionalize the SEG surface, PVP was added besides functioning as an interface linker.29 It could be seen that solution of precursor for SEG/ZnO without PVP showed precipitation after 24 h (Figure S2B(b), Supporting Information), whereas


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precursors of SEG/ZnO in the presence of PVP demonstrated well dispersion without obvious aggregation (Figure S2B(a), Supporting Information), similar to the as-prepared form in Figure S2A(a) (Supporting Information). The introduction of ZnO precursors markedly modified the charge density and caused disruption in stability of SEG colloid,29 leading to agglomerations which eventually form deposition at the base of the sampling bottle. Studies have revealed that hydrophobic interaction between PVP chains and sp2 conjugated carbon caused effective solubilization of graphene in water.37 Moreover, PVP, which comprises of nitrogen (N) and oxygen (O) atoms, decreases the surface free energy through its attachment on the ZnO surface, leading to slow growth speed of the facets and subsequently, formation of nano-sized particles.38 Therefore, in this study, the slight modification of SEG through the incorporation of PVP resulted in additional hydrophilic functional groups on the surface of SEG. This led to a stable and well-dispersed SEG coupled with size confinement of ZnO particles within the nanometer range. TGA is employed to determine the ZnO content in the SEG/ZnO nanocomposites. TGA curves of ZnO, SEG and SEG/ZnO samples were depicted in Figure 2. Pure ZnO annealed at 400 o

C experienced a complete weight loss of 2.2%, accredited to the loss of water adsorbed on the

ZnO nanoparticles surfaces (Figure 2a). The SEG sample displayed a dramatic weight loss initialed at 580 oC, corresponding to decomposition of carbon and also oxygen-containing groups.39 It can be seen that SEG exhibited a complete decomposed temperature at 820 oC (Figure 2f). As for the SEG/ZnO hybrid nanocomposites, the weight loss could be divided into two main stages: 100–250 oC and 670–880 oC (Figure 2b–e). In the first stage ranging from 100– 250 oC, the weight loss was attributed to the water loss from the photocatalyst surface. On the other hand, the second stage from 670–880 oC was due to the carbon decomposition in the


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graphene-based materials. However, it is believed that the existence of PVP caused the delay of SEG weight loss resulting from the thermal shielding effect in all the SEG/ZnO nanocomposites.29 Hence, the contents of ZnO in SEG/ZnO-1, SEG/ZnO-2, SEG/ZnO-3 and SEG/ZnO-4 nanocomposites were approximately calculated to be 81.7 wt%, 69.0 wt%, 52.4 wt% and 33.9 wt% respectively.

(a)

97.8

(b)

81.7

(c)

69.0

(d)

52.4

(e) 33.9

(f)

0.0

Figure 2. TGA curves of (a) ZnO, (b) SEG/ZnO-1, (c) SEG/ZnO-2, (d) SEG/ZnO-3, (e) SEG/ZnO-4 and (f) SEG. FESEM was performed to observe the morphological structure of SEG and SEG/ZnO nanocomposites as a result of variation in the concentration of Zn(NO3)2 precursor used. The ATSEG demonstrated a two-dimensional (2D) folded paper-like and transparent structure, consisting of 3 to 6 layers of graphene sheets after ultrasonication for 3 h and acid pre-treatment for 4 h (Figure 3a–b). In Figure 3c for SEG/ZnO-1, ZnO was observed to be fairly distributed and anchored onto graphene sheets, implying good combination between SEG sheets and ZnO.


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Figure 3d exemplified an enlarged view of Figure 3c and it could be observed that ZnO displayed a nanorod structure with an average length and diameter of 100 nm and 15 nm, respectively. However, a decrease in ZnO content to 69.0 wt% for SEG/ZnO-2 displayed a change in ZnO morphology. It was observed that SEG sheets were decorated densely by ZnO nanoparticles (Figure 3e–f). Similarly, SEG/ZnO-3 and SEG/ZnO-4 displayed nearly similar morphological features as SEG/ZnO-2 (Figure S3, Supporting Information). The main discrepancy among SEG/ZnO-2, SEG/ZnO-3 and SEG/ZnO-4 was that the particle size of ZnO on the SEG surface reduced with a decreasing ZnO concentration from SEG/ZnO-2 to SEG/ZnO-4. Size distribution of ZnO nanoparticles was within the range of 10– 30 nm for SEG/ZnO-2; a further reduction to 25 nm and finally, 20 nm was obtained for SEG/ZnO-3 and SEG/ZnO-4, respectively. This could be explained as follows: at low concentrations of Zn(NO3)2 precursor, the high nucleation rate competing with the lower growth rate resulted in a large number of nucleation before a suitable environmental condition for nuclei growth was being developed.40 Thus, the FESEM results demonstrated that SEG served as a substrate for the deposition of ZnO nanocrystals, in which variation in the concentration of Zn(NO3)2 precursor resulted in a change in the morphology and nanocrystallite size range.


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(a)

(b)

Graphene stacking layers

(c)

(d) SEG

ZnO

(e)

(f)

ZnO

SEG

Figure 3. (a) STEM image of SEG. (b) HRTEM image of the thin edge of SEG, revealing 3–6 layers of graphene. FESEM images of (c–d) SEG/ZnO-1 and (e–f) SEG/ZnO-2. FTIR spectra of PVP, SEG and SEG/ZnO-2 were illustrated in Figure 4A. For pure PVP (Figure 4A(a)), the characteristic peaks at 2930 and 2850 cm-1 were ascribed to the –


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CH3 asymmetric and symmetric stretching vibrations, respectively with the band at 1430 cm-1 being allocated to the pyrrolidone-ring-related bonds.41 The peak at 1285 cm-1 was due to C–N bending mode42 whereas the broad absorption bands at 3425 and 1650 cm-1 were identified to the hydroxyl groups of adsorbed H2O molecules. Meanwhile, for the SEG (Figure 4A(b)), the peak at 1630 cm-1 represented the skeletal vibration of unoxidized graphitic domains while the presence of O–H deformations of the C–OH groups was visible at 1390 cm-1.43 The broad absorption peak at 1085 cm-1 was recognized as the characteristic stretching vibration of C–O situated on the SEG sheets.44 This showed the successful synthesis of SEG with additional hydrophilic functional groups due to the acid pre-treatment process. In the spectrum of SEG/ZnO-2 (Figure 4A(c)), several new characteristic bands of PVP were observed. This includes the stretching vibrations of alkyl C–H at 2921 and 2830 cm-1, and C–N bonds at 1235 cm-1. The shifts of PVP peaks were attributed to the synergistic effect between the N atoms and C–O of PVP and ZnO crystallographic plane decorated on the SEG.37 Moreover, several changes were observed when comparing SEG/ZnO-2 with pure SEG. The intensities of absorption bands of C–O (1085 cm-1) and O–H band attributed to the deformation of C–OH (1390 cm-1) were dramatically reduced. Nevertheless, a broad absorption band at 3425 cm-1 was observed, which was due to the residual O–H groups of SEG. This indicated that SEG had been partially reduced during the calcination process, which contained residual oxygen-containing functional groups. Thus, ZnO could well interact with these functional groups in the nanocomposites during the synthesis process. More importantly, the absorption band at 460 cm-1 belonging to Zn–O vibration was clearly identified, implying successful preparation of ZnO from the Zn(NH3)4CO3 precursor after the calcination process.


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The crystal phase of SEG/ZnO nanocomposites was characterized by XRD as presented in Figure 4B. The XRD pattern of the as-fabricated SEG/ZnO-2 disclosed sharp peaks and indicated that the sample was crystalline. As seen, the ZnO nanoparticles manifested peaks at 2θ of 31.7o, 34.5o, 36.2o, 47.5o, 56.6o, 62.8o, 66.3o, 68.0o, 69.1o, 72.6o and 77.0o which could be referred to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004) and (202) crystal planes of the hexagonal phase wurtzite ZnO structure (space group P63mc), respectively.10 All these peaks observed for ZnO nanoparticles were in agreement with those taken from the JCPDS file no. 36-1451 (a = 3.2498 Å, b = 3.2498 Å, c = 5.2066 Å). This demonstrated that the presence of SEG did not result in alterations in preferential orientations of ZnO or development of new crystal orientations. The additional peaks at 2θ of 26.5o and 44.5o accounted for the graphitic characteristic peak, which directly corresponded to the (002) and (100) planes, respectively.45 By employing Equation (1), the average crystallite size of the ZnO formed in the nanostructure was 13.6 nm based on a β value of 0.7o for the (101) diffraction peak. Raman spectroscopy is a non-destructive and effective method to characterize the graphitic materials. Figure 4C exemplifies the Raman spectra of graphite, SEG and SEG/ZnO-2. From Figure 4C(a), two characteristic peaks were observed in the Raman spectrum of graphite. The D (disordered) peak and G (graphitic) peak were centered at 1340 and 1575 cm-1, respectively. The D band is due to the distortion of the symmetrical hexagonal graphitic lattice resulting from internal structural defects and edge defects. On the other hand, the G band is ascribed to the symmetric sp2 hybridized graphene domains. After the acid pre-treatment of graphite, the Raman spectrum of SEG (Figure 4C(b)) exhibited an increased intensity in D band relative to that of the graphite. In general, the intensity ratio of the D and G band (ID/IG) is an


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important tool to measure the degree of disorder and average size of the sp2 domains in carbonbased materials.46 The ID/IG of SEG was 0.31, which was greater than 0.17 as compared to graphite. This observation was attributed to the introduction of oxygen-containing functional groups, leading to a considerable decrease in the size of in-plane sp2 domains. Hence, acid pretreatment had successfully formed defects on the graphene sheets, which served as a platform for the deposition of ZnO through charge interaction. More importantly, the as-fabricated SEG yielded a lower ID/IG value (0.31) than GO sheets (0.94) using modified Hummers’ method (Figure S4, Supporting Information), as reported previously in our work.33 This was in concord with the fact that SEG in DMF had a lower defect density as compared to the widely employed modified Hummers’ method, which enabled SEG with longer electronic mean path and enhanced electrical mobility.36 Furthermore, the shift in G band for SEG from 1575 to 1580 cm-1 as compared to graphite could be accredited to the presence of functional groups on the carbon network of SEG. Compared with SEG, SEG/ZnO-2 nanocomposite revealed a blue shift in D band from 1340 to 1350 cm-1 and a red shift in G band from 1580 to 1570 cm-1. The recovery of the hexagonal carbon network and the chemical interaction between SEG and ZnO were identified to be plausible explanations of these shifts respectively.47 According to the theory, hexagonal wurtzite ZnO structure has the following optic modes: 1A1 + 2B1 + 1E1 + 2E2.48 The Raman active is corresponded to A1, E1 and E2 while B1 is forbidden. The polar modes of A1 and E1 can be split into longitudinal optical (A1 LO and E1 LO) and transverse optical (A1 TO and E1 TO) phonons. Meanwhile, the non-polar E2 modes can be split into high- and low- frequency phonons (E2 high and E2 low). As seen in Figure 4C(c), the Raman peaks at 325 and 430 cm-1 were attributed to the second-order vibration mode E2 low and E2 high, correspondingly while the other peak at 580 cm-1 corresponded to the longitudinal


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optical phonon mode with E1 symmetry (E1 LO) of oxygen vacancy of ZnO crystals.49,50 In addition, a spectral peak that revealed the longitudinal optical phonon mode with A1 symmetry (A1 LO) was observed at 1120 cm-1. These results further confirmed the presence of ZnO nanoparticles on SEG sheets, which are in concordant with the XRD result presented earlier. The light absorption properties of the pure ZnO and SEG/ZnO nanocomposites were examined by UV-Vis diffuse reflectance spectroscopy (Figure 4D). A large absorption band below 400 nm could be seen in the spectrum of pure ZnO with an absorption edge at 380 nm in the UV region. Also, the incorporation of SEG resulted in a substantial effect on the optical properties for all the studied samples. It is noted that there was an improved absorbance and broad background in the visible light region (400 to 800 nm) with increasing SEG content. This was in accordance with the change in color from white to greyish-black (inset in Figure 4D). A substantial red shift to a higher wavelength (ca. 420 nm) could be observed in the absorption edge of the SEG/ZnO samples that that of pure ZnO. Hence, the remarkable improvement in the light absorption and the extended absorption edge can be accredited to the synergistic interaction between SEG and ZnO, which is analogous to the carbon nanotube- and other graphene-based composite materials.1,33 Overall, the improved light absorption intensity for SEG/ZnO nanocomposites implied that a remarkable photoactivity could be demonstrated. This hypothesis was justified by evaluating the photodegradation of RB5 under ambient conditions.


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(A)

(C) (c)

(B)

(D)

580

325 200

430

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1120

G 1570

(e)

(d)

D 1350

D 1340

(b) D 1340

G 1580

(b)

(c)

G 1575

(a)

(b)

(c)

(d)

(a)

(e)

(a)

Figure 4. (A) FTIR spectra of (a) PVP, (b) SEG and (c) SEG/ZnO-2 nanocomposites. (B) XRD pattern of SEG/ZnO-2 nanocomposite (G and Z denote graphitic and ZnO peaks, respectively). (C) Raman spectra of (a) graphite, (b) SEG and (c) SEG/ZnO-2 nanocomposites. (D) UV-Vis spectra of pure ZnO and SEG/ZnO nanocomposites (inset is the colors of photocatalysts). 3.2. Photocatalytic Performance and Mechanisms of Photocatalytic Enhancement. The photocatalytic degradation of RB5 on the as-developed SEG/ZnO nanocomposites was conducted under visible light irradiation (Figure 5). The photoactivities of pure ZnO and SEG were performed under the same conditions for comparison. The control experiment indicated that RB5 was not degraded under visible light illumination in the absence of photocatalysts, denoting that RB5 is a stable compound and that the mechanism of photolysis could be neglected. Further comparative experiment was also performed to evaluate the degradation activity in the dark along with the photocatalysts (ZnO and SEG/ZnO). It could be observed that adsorption-


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desorption equilibrium of RB5 was achieved within 30 min in the dark as evidenced in Figure S5 (Supporting Information). Apparently, the adsorptivity of RB5 molecules on the SEG/ZnO was greater relative to that on pure ZnO due to the incorporation of SEG with ZnO. The addition of SEG enhanced adsorptivity of RB5 via π-π conjugation between aromatic regions of SEG and RB5 molecules.51 The improved adsorptivity on the photocatalyst surface plays an important role for photodegradation of RB5 under visible light irradiation. Therefore, the overall degradation of RB5 was resulted from both the adsorption and photocatalytic reaction with negligible photolysis.

(a)

(b)

Figure 5. (a) Photodegradation of RB5 over pure SEG, ZnO and SEG/ZnO hybrid nanocomposites under visible light illumination. (b) Percentage of RB5 degradation efficiencies after 180 min irradiation with visible light. Notably, decreasing Zn(NO3)2 precursor solution during the synthesis process (from SEG/ZnO-1 to SEG/ZnO-4) indirectly implied increasing amount of SEG present in the SEG/ZnO nanocomposites. With respect to the SEG/ZnO nanocomposites, the photocatalytic activities increased gradually with increasing SEG content and then decreased progressively after reaching the optimum SEG content. The amount of SEG present in the nanocomposites had been discussed earlier under the TGA section. As illustrated in Figure 5a, the efficiency of RB5


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photodegradation followed the order SEG/ZnO-2 > SEG/ZnO-1 > SEG/ZnO-3 > SEG/ZnO-4 > ZnO > SEG. After 3 h of reaction, the degradation efficiencies of RB5 were approximately 0.38%, 29.2%, 76.2%, 97.0%, 58.7% and 42.0% for pure SEG, pure ZnO, SEG/ZnO-1, SEG/ZnO-2, SEG/ZnO-3 and SEG/ZnO-4, respectively (Figure 5b). Generally, we attributed the significantly improved photoactivity of SEG/ZnO to the band gap narrowing of the hybrid materials (Figure 4D) in comparison to the pure ZnO. It was found that the SEG/ZnO-2 sample improved by ca. 21% compared with SEG/ZnO1, indicating that the addition of an appropriate amount of SEG could markedly enhance the photocatalytic activity. The excellent features of SEG greatly improved the efficient separation of electron-hole pairs due to the fact that graphene has a high charge carrier mobility of more than 200 000 cm2 V-1 s-1 at room temperature and the 2D π-conjugation structure to effectively hinder the charge recombination.32,52,53 As can be seen in Figure 4D, SEG is a unique lightharvesting material evidenced by the enhanced light absorption of the photocatalyst with increasing SEG content. However, with the excess addition of SEG, the photoactivity deteriorated, implying that a synergistic effect between ZnO and SEG is a pre-requisite in order to enhance the activity. Therefore, it can be concluded that excessive SEG in the nanocomposite is detrimental for the photoactivity, which is by and large the main constraint encountered for all the graphene-based composites.52 The decrease in photocatalytic activity for SEG/ZnO-3 and SEG/ZnO-4 nanocomposites could be due to the increase in the light scattering and opacity.54 Excess SEG content shielded the active sites on the photocatalyst surface and decreased light intensity through the depth of the reaction medium. Surprisingly, pure ZnO exhibited photocatalytic activity towards degradation of RB5 (with degradation efficiency of 29.2%) due to the dye sensitization effect and also the presence


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of defects (e.g. oxygen vacancies) in the ZnO, which is consistent with several reported studies.55-57 The presence of defects in the ZnO could be evidenced using Raman and PL analyses (Figure S8, Supporting Information). Meanwhile, when SEG itself was used as a photocatalyst acting as a control sample, there was no appreciable photocatalytic degradation of RB5 under visible light, highlighting that bare SEG was undoubtedly not active due to the absence of holes and active •OH radicals as evidenced in the latter section on PL analysis. The time-dependent absorption spectra changes of RB5 by SEG/ZnO-2 could be vividly presented in Figure 6a. The peak intensity at 597 nm, which was the main UV-Vis absorption of RB5 molecule, reduced progressively with increasing duration under visible light illumination. The peak virtually disappeared after 3 h of reaction, manifesting complete mineralization of RB5. Evidently, the RB5 solution gradually changed its color from blue to colorless after the end of reaction (inset of Figure 6a). The dye degradation could be described as a pseudo first order kinetic reaction based on the Langmuir-Hinshelwood model: ln(C0/C) = kt, where C0 is the equilibrium concentration of RB5, C is the concentration at time t and k is the apparent first order reaction rate constant,58,59 which has been illustrated and summarized in Figure 6b and Table S1 (Supporting Information). The calculated k value for SEG/ZnO-2 was 0.0199 min-1, which exhibited a remarkable 10-fold enhancement after graphene hybridization as compared to that for pure ZnO. The order of k value was found to be 0.0199 min-1 (SEG/ZnO-2) > 0.0080 min-1 (SEG/ZnO-1) > 0.0053 min-1 (SEG/ZnO-3) > 0.0031 min-1 (SEG/ZnO-4) > 0.0020 min-1 (ZnO) > 3.0 × 10-5 min-1 (SEG), which is concordant with the results in Figure 5a. The results clearly demonstrated that the optimum SEG content was 31.0 wt% (or 69.0 wt% of ZnO) present in the SEG/ZnO-2 nanocomposites as compared to the other samples with different percentages of SEG and without SEG. Hence, these experimental findings exemplified the crucial role of optimum


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SEG or ZnO content, and interfacial contact between SEG and ZnO in improving the photocatalytic performance.

(a)

Initial

0 min

15 min

30 min

45 min

60 min

75 min

90 min

105 min

120 min

135 min

150 min

165 min

180 min

(b)

Figure 6. (a) Time-dependent UV-Vis absorption spectral pattern of RB5 solution using SEG/ZnO-2 photocatalysts. The inset shows the sequence in color change of RB5 solution. (b) The pseudo first order kinetic reaction of RB5 degradation using various photocatalysts under visible light illumination.


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To examine the radical species on the RB5 degradation over SEG/ZnO-2 and further understand the underlying reaction mechanism, the effect of reactive species such as h+, •OH and •O2- radicals in the photodegradation was thoroughly explored. Various scavengers such as isopropanol (IPA, •OH scavenger), triethanolamine (TEOA, h+ scavenger) and benzoquinone (BQ, •O2- scavenger) were employed in this study. As depicted in Figure 7a, the photoactivity was markedly suppressed as a result of quenching effect. When BQ scavenger was added, the photocatalytic activity of SEG/ZnO-2 was reduced most significantly to 20.7% compared with no scavenger. The addition of IPA and TEOA scavengers also decreased the photodegradation performance to a smaller suppression extent in comparison to the case of addition of BQ. Therefore, it is evident that •OH, holes and •O2- radicals all play an important role on the photodegradation of RB5. To get more insight into the charge carrier separation efficiencies and oxidation ability of SEG/ZnO, pure ZnO and SEG, the catalytic reaction was further explored by employing terephthalic acid as a probe molecule to detect the production of •OH radicals via fluorescence method. Referring to Figure 7b, a continuing increase of the fluorescence intensity at 425 nm was observed with time for the SEG/ZnO-2. However, the fluorescence intensity did not increase without the presence of visible light or the SEG/ZnO photocatalysts, signifying that it was excited by 2-hydroxyterephthalic acid due to the reactions between •OH radicals and terephthalic acid. Figure 7c depicts a comparison of different samples on the fluorescence intensity upon visible light irradiation for 3 h. As mentioned earlier, the fluorescence intensity is directly related to the amount of •OH radicals. For 3 h of reaction, the amount of •OH radicals in the SEG/ZnO-2 was the maximum evidenced by the highest fluorescence intensity among all the samples


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studied. This inferred that SEG/ZnO-2 displayed the highest separation efficiency of charge carriers for enhanced photocatalytic activity, which agreed well with the photocatalytic results as shown in Figure 5a. Furthermore, no fluorescence intensity was detected for the bare SEG due to the absence of holes and •OH radicals, manifesting that SEG itself was not a photocatalyst. Overall, the •OH radical analysis deduced that the •OH radicals were considered to be one of the active species during photocatalytic reactions.


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(a)

(b)

(c)

Figure 7. (a) Effect of different scavengers on the RB5 photodegradation in the presence of SEG/ZnO-2 under visible light for 180 min. (b) •OH trapping fluorescence spectra of SEG/ZnO2 in the terephthalic acid solution under visible light irradiation. (c) •OH trapping fluorescence spectra of various samples in terephthalic acid solution irradiated for 180 min.


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According to all the above results, analysis and opinion of the published works, a postulated visible-light photocatalytic mechanism of SEG/ZnO-2 was illustrated in Figure 8, which encompassed two mechanisms. The first mechanism was due to the excitation of the semiconductor. The electrons in the VB of ZnO were excited to the CB upon visible light illumination, leaving holes in the VB. Next, the photoexcited electrons were transferred to SEG, which has a lower band-edge position (-0.08 eV vs. NHE),53 to achieve charge equilibrium and stabilization between these two components, forming the resulting Fermi level (EF*). Also, the transfer of electrons to SEG subsequently reduced the trapping of electrons in the lattice of ZnO for effective charge separation to hinder the charge carrier recombination. Moving on, the RB5 molecules were transferred from the solution to the photocatalyst surface and adsorbed via π-π conjugation between aromatic regions of SEG and RB5 with offset face-to-face orientation.51 The second mechanism involved the RB5 dye excitation. In this mechanism, dye served as a sensitizer of visible light and injected electrons to ZnO and SEG. The enriched photogenerated electrons on the SEG reacted with molecular O2 in solution to produce reactive •O2- followed by the generation of •OH radicals (Equations (A2)–(A3)). Correspondingly, the holes in the VB of ZnO underwent charge transfer with hydroxide species or adsorbed water to form •OH radicals (Equations (A4)–(A5)). The adsorbed RB5 was finally oxidized by the holes, •OH radicals and •O2- radicals to produce mineralized products. The possible photocatalytic decomposition reactions were shown in Equations (A1)–(A6).


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Energy (eV) e

-

e

-

O2

RB5* –0.31 3.20 eV

CB

e

–0.11

•O2-

•OH

-

e

-

EF

*

RB5 Degraded products

2.80 eV

RB5 2.89

2.69

+

VB

H2O •OH

h

ZnO

RB5 Degraded products

SEG

Figure 8. Schematic drawing of the photocatalytic reaction process for the RB5 degradation and charge transfer and separation of the SEG/ZnO-2 hybrid nanocomposites under visible light irradiation. 3.3. Stability Evaluation. From the viewpoint of practical applications, the recycling of the catalysts is an important task. The SEG/ZnO-2 was selected to examine the reusability of the composite by reusing it towards the photocatalytic degradation of RB5 for three times under similar conditions. Based on Figure 9, the as-prepared SEG/ZnO-2 nanocomposites demonstrated a good catalytic stability after three consecutive cycles, maintaining about 85% of reactivity. The slight decrease in the activity was originated from the inevitable photocatalyst loss during recycling process. Through analyses of the results of XRD patterns, FTIR spectra and FESEM image of fresh and used SEG/ZnO-2 (Figure S6, Supporting Information), it was found that the phase, chemical structure as well as morphology of the SEG/ZnO-2 remained intact without obvious change after three successive photocatalytic runs, revealing the high stability of the photocatalyst. Therefore, the SEG/ZnO nanocomposites synthesized by this facile method were very promising in the practical application of visible-light photodegradation of pollutants.


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(a)

(b)

Figure 9. (a) Three recycling runs in the degradation of RB5 over SEG/ZnO-2 nanocomposites under visible light. (b) Recycling efficiency of SEG/ZnO-2 in the photocatalytic degradation of RB5.


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4. CONCLUSIONS In short, SEG/ZnO heterostructured photocatalyst has been successfully synthesized by a facile chemical deposition-calcination approach without the introduction of organic solvent. The SEG/ZnO hybrid nanocomposite exemplified significantly enhanced photocatalytic performance of RB5 under a low-power 15 W energy-saving light bulb as compared to the pure ZnO. The optimum ZnO content in the SEG/ZnO was found to be 69.0 wt% which demonstrated a high RB5 photodegradation of 97.0% after 3 h of visible light irradiation with a rate constant of 0.0199 min-1. This was associated with the enhanced photo-responsive ability, improved adsorptivity of dyes and high charge separation efficiency owing to synergistic interactions between SEG and ZnO. In addition, the as-prepared SEG/ZnO photocatalyst had relatively high stability after three consecutive photocatalytic runs under visible light. Our results highlight the importance of the charge transfer between light harvesting semiconductor and highly conductive graphene for the improvement in the catalytic applications. More importantly, the present study not only offers a stable and efficient photocatalyst, but also sheds new inroads for engineering cost-effective nanocomposites which could be extended to the synthesis of various SEGmodified hybridized materials and other binary compounds. Overall, research on the unique properties of graphene is just gearing up and it is hoped that the current research would stimulate incessant interest in the design of visible-light-active graphene-based photocatalysts for environmental decontamination and renewable clean energy supplies in the light of combating the ever-increasing environmental concerns and fossil fuel depletion crisis.


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ASSOCIATED CONTENT Supporting Information. Light spectrum of the 15 W energy-saving daylight lamp, FESEM images of the as-prepared SEG/ZnO photocatalysts, Raman spectra, adsorption ability of the pure SEG, ZnO and SEG/ZnO hybrid nanocomposites, defects in ZnO and SEG/ZnO, reaction mechanisms for the photocatalytic enhancement of RB5 degradation by SEG/ZnO photocatalysts, and electronegativity calculation method for the band structure of SEG/ZnO and pure ZnO photocatalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author * Tel.: +603-551 46234. Fax: +603-551 46207. E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work was funded by Ministry of Science, Technology and Innovation (MOSTI) Malaysia under the e-Science Fund (Ref. no.: 03-02-10-SF0244).


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Page 42 of 42

For Table of Contents Only Zinc precursor Zn(NH3)4CO3 2+

Zn

PVP

(a) Stirring at room temperature

Polymer solution (b) Filtering (c) Calcination at o 400 C for 2 h

SEG

ZnO

SEG/ZnO

Acid pre-treatment

SEG

Photocatalytic Degradation of RB5 Dye Initial

0 min

15 min

30 min

45 min

60 min

75 min

90 min

105 min

120 min

135 min

150 min

165 min

180 min


42

Enhanced Daylight-Induced Photocatalytic Activity of Solvent

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