Graphdiyne–ZnO Nanohybrids as an Advanced Photocatalytic Material

Aug 28, 2015 - Arulappan Durairaj , Thangavel Sakthivel , Asir Obadiah , Samuel Vasanthkumar. Journal of Materials Science: Materials in Electronics 2...
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Graphdiyne−ZnO Nanohybrids as an Advanced Photocatalytic Material Sakthivel Thangavel,†,‡ Karthikeyan Krishnamoorthy,† Velmurugan Krishnaswamy,§ Nandhakumar Raju,§ Sang Jae Kim,†,∥ and Gunasekaran Venugopal*,‡,⊥ †

Nanomaterials and System Lab, Department of Mechatronics Engineering, Jeju National University, Jeju 690-756, Republic of Korea Department of Nanosciences and Technology and §Department of Chemistry, Karunya University, Karunya Nagar, Coimbatore 641-114, Tamil Nadu, India ∥ School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States ⊥ Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan ‡

S Supporting Information *

ABSTRACT: The utility of carbonaceous materials for hybrid semiconductor photocatalysts has been rapidly increasing in recent years due to the synergetic effect via interfacial charge transfer reactions. In this study, we prepared a novel graphdiyne−ZnO nanohybrid by the hydrothermal method and examined its photocatalytic properties on the degradation of two azo dyes (methylene blue and rhodamine B). Interestingly, the graphdiyne−ZnO nanohybrids showed superior photocatalytic properties than that of the bare ZnO nanoparticles as evidenced by the absorption spectra and total organic carbon analyses. Moreover, the rate constant of graphdiyne−ZnO nanohybrids is nearly 2-fold higher compared to that of the bare ZnO nanoparticles on the photodegradation of both azo dyes. Further, a plausible mechanism for the enhanced photocatalytic properties of the graphdiyne− ZnO nanohybrids has been discussed. This work on the development of graphdiyne-based semiconductor photocatalysis can provide new insights into the design of novel hybrid photocatalysts for potential applications in the environmental remediation sectors.

1. INTRODUCTION Nanostructured semiconducting materials open up new opportunities in the field of photocatalysis and have a huge impact on the energy and environmental sectors.1,2 Several nanostructured metal oxides including TiO2, ZnO, CuO, WO3, MoO3, and binary metal oxides such as BiVO4, BiWO4, SnWO4, and CoWO4 have been examined for photocatalytic applications.3−5 Further, evidential reports are available for the shape and structure dependent photocatalytic properties of nanostructured materials.6,7 Even though various materials are examined for semiconductor photocatalysis during the recent decade, the researchers are focused toward enhancing the efficiency of the photocatalyst via doping, composites, or hybrid materials.8 One of the ways for improving the energy efficiency of the photocatalyst can be achieved by making hybrid nanostructures via integrating the semiconducting material with a carbon material.9 The recent scenario in this field signifies the importance of carbon materials for an advanced photocatalytic applications. Many carbon materials are used for increasing the photocatalytic efficiency including activated carbon, carbon nanomaterials, carbon onions, carbon dots, graphene oxide, graphene, and so on.10−12 Among these, graphene oxide and graphene attracted well due to their sheetlike structures, and the ability to adhere semiconducting © 2015 American Chemical Society

nanomaterial on its basal plane thereby prevents aggregation and inhibits electron−hole pair recombination.13,14 A wide variety of graphene-based hybrid photocatalytic materials such as graphene−TiO2, graphene−ZnO, graphene−WO3, and graphene−CdS in different shapes and structures have been examined well in this decade.15−18 The increasing demand in the development of novel hybrid materials for effective photocatalysis motivated the researchers to utilize the recently emerged 2D carbon allotropes toward advancements in semiconductor photocatalysis. Several allotropes of carbon in two-dimensional forms such as graphyne, graphane, graphdiyne, and halogenated graphenes have recently emerged which are considered as two-dimensional analogue materials to graphene.19 Among these new materials, graphdiyne (GD) nanosheets are considered as novel materials owing to their electronic, optical, and structural properties.20 GD possess both sp and sp2 carbons with two diacetylenic linkages between the adjacent carbon hexagonal structures.21 An earlier study demonstrated that GD possess an electronic conductivity of 2.56 × 10−1 S m−1 (similar to silicon) Received: June 26, 2015 Revised: August 28, 2015 Published: August 28, 2015 22057

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The Journal of Physical Chemistry C with a semiconducting behavior.22 The recent studies revealed that nitrogen-doped GD can be used for hydrogen purification.23 GD nanosheets showed superior Li-ion storage capacity which demonstrated their applications in energy storage devices.24 The theoretical studies using density functional theory suggested that GD possess a band gap of 0.47 and 1.12 eV (by two different methods), which is an advantage of GD over graphene for optoelectronic and photocatalytic applications.25 Moreover, the preliminary studies showed that GD act as an electron-transport material in the photodegradation of methylene blue.26 This motivated us to develop novel hybrid materials based on GD for advanced photocatalysis. The selection of materials for nanohybrids is an important factor which decides the final properties of the hybrids. In this regard, nanosized ZnO is an interesting material due to its wide band gap (3.2 eV), inexpensive, and environmentally benign with superior physical and chemical properties.27 These intriguing properties of ZnO make them ideal candidates for several applications including solar cells, photocatalytic water splitting, photocatalytic degradation of organic pollutants, and piezo-phototronics.28,29 In addition to this, ZnO possesses high electron mobility almost 2 orders of magnitude higher than that of TiO2 which results in higher photocatalytic efficiency due to the increase in lifetime of photogenerated charge carriers.30 Hence, we aimed at developing advanced photocatalytic materials based on GDZnO by the one-pot hydrothermal method and examined their effects on the photodegradation of two azo dyes, viz. (i) methylene blue and (ii) rhodamine B. Further, the photocatalytic and mineralization efficiency of GD-ZnO nanohybrids on the photodegradation of azo dyes have been examined, and a plausible mechanism for the synergetic photocatalysis has been discussed in detail.

2.3. Preparation of Graphdiyne−Zinc Oxide (GD-ZnO) Nanohybrid Materials. The graphdiyne−zinc oxide (GDZnO) nanohybrids are prepared via one-pot hydrothermal method. Briefly, an aqueous solution containing an appropriate amount of GD is prepared and exfoliated with the aid of ultrasound irradiation for 30 min. Then, aqueous solution containing zinc acetate is added to the above solution followed by the addition of NaOH (in the molar ratio 1:1), and the reaction is preceded for 30 min. After that, the entire solution is transferred into a Teflon vessel followed by the hydrothermal reaction at 180 °C for 24 h. After completion of the reaction, the autoclave is allowed to reach room temperature, and the final products (consisting 0.5% GD in the GD-ZnO nanohybrids) are obtained by repeated centrifugation using water and ethanol. For comparison, pure ZnO was synthesized by same method in the absence of GD. 2.4. Instrumentation. The XRD patterns of ZnO and GDZnO samples were measured using an X-ray diffractometer system (XRD Lab X Shimadzu). The surface morphology and energy dispersive spectra were studied using scanning electron microscope (JEOL JSE-6390) and transmission electron microscope (FEI-Tecnai) measurements. Fourier transformed infrared spectroscopy (FT-IR) spectra were recorded on the Thermo Scientific Nicolet 6700 FTIR spectrometer in the range 400−2000 cm−1. Raman spectroscopy was performed on the samples using a LabRam HR Evolution Raman spectrometer (Horiba Jobin-Yvon, France). The chemical and surface states of the samples are analyzed by using X-ray photoelectron spectroscopy measurements (Model: PHI1600 Quantum ESCA Microprobe System, using the Mg K line of a 300 W Mg X-ray tube as a radiation source at 15 kV voltage). The absorbance spectra were measured using a UV spectrophotometer (JASCO V-60). The photoluminescence spectra of the prepared samples are obtained on a spectrophotometer (FLUOROLOG, HORIBAYVON). The TOC content was determined using a Shimadzu Model TOC-VLPH TOC analyzer. 2.5. Photocatalytic Measurements and Analysis. The photocatalytic properties of the prepared GD-ZnO and bare ZnO materials are evaluated by the photodegradation of two azo dyes (i) methylene blue and (ii) rhodamine B under UV light irradiation. The experiments were performed under ambient atmospheric condition in a Pyrex glass bottle with 100 mL of test dye solution at concentration (1 × 10−3 ML−1) with catalyst (5 mg L−1). Prior to illumination, these suspensions were thoroughly stirred in the dark over 30 min to ensure the establishment of absorption equilibrium of dye on the sample surface (see Figure S1). Subsequently, the suspension was irradiated under a UV light, and the absorption spectra as well as total organic carbon (TOC) removal (%) of the dye were recorded at different time intervals to monitor the photocatalytic process. The photodegradation efficiency of the catalysts during the reaction was determined by measuring the absorbance spectra of the dye samples at regular time intervals. The absorbance value measured before the start of photocatalytic reaction was taken as the initial concentration (C0) and the absorbance at various time (t) intervals during the photocatalytic reaction was represented as Ct. The degradation efficiency of the photocatalytic reaction was calculated using the equation32

2. EXPERIMENTAL SECTION 2.1. Materials. The precursor materials used for preparation of GD such as trimethylsilylacetylene, n-butyllithium, hexabromobenzene, tetrakis(triphenylphosphine)palladium(0), zinc chloride, tetrahydrofuran, dimethylformamide, diethyl ether, copper foil, trichlorobutane, dry ice, and toluene are purchased from Merck Chemicals Ltd., India. The materials used for ZnO preparation such as zinc acetate, ammonia, and sodium hydroxide are obtained from Sigma-Aldrich, India. Methylene blue (MB), rhodamine B (RhB), and phenol were obtained from Merck Chemicals Ltd., India. Doubly deionized water is used throughout the experiments. 2.2. Preparation of Graphdiyne Nanosheets. The graphdiyne nanosheets were prepared according to the method reported in the literature.31 The GD nanosheets were grown on the surface of copper foil through a cross-coupling reaction using hexaethynylbenzene as the starting material. Breifly, the hexaethynylbenzene monomer was synthesized in good yield (62%) by addition of TBAF to THF solution of hexakis[(trimethylsilyl)ethynyl]benzene for 10 min at 8 °C. The GD was successfully grown on the surface of copper foil in the presence of pyridine by a cross-coupling reaction of the monomer of hexaethynylbenzene for 72 h at 60 °C under a nitrogen atmosphere. The as-grown GD on copper foil was immersed in a solution containing trichlorobutane and sonicated at 60 °C for few hours for obtaining the GD powders. The detailed experimental procedure is given in the Supporting Information (section S1).

Degradation Efficiency (DE %) = [1 − (Ct /C0)] × 100 (1) 22058

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The Journal of Physical Chemistry C The mineralization of dye during the photocatalytic reaction was determined by measuring the TOC removal in the dye samples at regular time intervals. The TOC measured before the start of photocatalytic reaction was taken as the initial TOC concentration (TOC0), and the TOC concentration at various time (t) intervals during the photocatalytic reaction was represented as TOCt. The mineralization efficiency of the photocatalytic reaction was calculated using the equation33

The surface morphology and composition of the prepared GD-ZnO nanohybrids were investigated using SEM, TEM, and EDAX analyses. The SEM micrograph of GD-ZnO nanohybrid is represented in Figure 2a which revealed the spherical ZnO nanoparticles anchored on the GD nanosheets uniformly, thus preventing the aggregation of GD as well as ZnO nanoparticles in the nanohybrids. Further, the TEM micrograph shown in Figure 2b suggested that ZnO nanoparticles are tightly bounded on the surface of the 2D GD nanosheets. This uniform anchoring of ZnO nanoparticles on the GD nanosheets is due to the ultrasound assisted absorption of Zn ions on the GD surface as well the hydrothermal reaction which results in the formation of ZnO nanoparticle densely packed on the GD nanosheets. The chemical composition of the GD-ZnO nanohybrid is measured by the EDAX analysis (as shown in Figure 2c) which clearly showed the presence of carbon (C), zinc (Zn), and oxygen (O) elements in the prepared GD-ZnO nanohybrids. The Zn and O elements originated from the ZnO nanoparticles whereas the C was obtained from the GD nanosheets. The chemical bonding nature between the GD and ZnO in the prepared GD-ZnO nanohybrids are examined by the FT-IR spectroscopy. Figure 3 shows the FT-IR spectra of bare ZnO and GD-ZnO nanohybrids. The FT-IR spectrum of bare ZnO nanoparticles showed the presence of a band at 440 cm−1; this is due to the vibration of Zn−O.37 The FT-IR spectrum of GDZnO nanohybrids showed the presence of bands at 440, 1450, and 1586 cm−1. The band observed at 440 cm−1 corresponds to the Zn−O vibrational modes in GD-ZnO. The bands observed 1450 and 1586 cm−1 are due to the vibration of the aromatic ring in the GD nanosheets which are in close agreement with the earlier study.38 Further, the prepared GD-ZnO nanohybrids were characterized using laser Raman spectroscopy, which is one of the prominent tools for understanding the crystalline nature, and defect levels present in nanostructures including carbon materials (such as graphene, GO, and CNT) and hybrid materials.39 The Raman spectrum of GD-ZnO is provided in Figure 4, which revealed the presence of bands originated due to ZnO and GD in the nanohybrids. It showed the presence of E2 (high) mode at 438 cm−1 which is the characteristic of wurtzite ZnO phase40 as well as the G and D bands of GD. The G band was observed at 1569 cm−1 corresponds to the firstorder scattering of the E2g mode in carbon materials and the D band (observed at 1362 cm−1) can be assigned to the presence of defects in GD, as similar to that of graphene, GO, and rGO.31,41 Moreover, the calculated ID/IG ratio of GD is about 0.54, which is lower compared to the GO (prepared via the modified Hummers method) and reduced graphene oxide (prepared via the hydrothermal method) as in our earlier report.42 This further evidence that the prepared GD is of high crystallinity with low defects.31,41 The chemical and surface states of elements present in the GD-ZnO nanohybrids are examined by X-ray photoelectron spectroscopy. The XPS survey spectra of the GD-ZnO nanohybrids are presented in Figure 5a which indicated the presence of C, Zn, and O groups at binding energies 285, 1030, and 532 eV, respectively. The C 1s spectrum of GD-ZnO nanohybrids is shown in Figure 5b can be deconvoluted into C−C (sp), C−C (sp2), CO, and C−O at binding energies 284.5, 285.2, 286.2, and 288.3 eV, respectively. The observed result is in consistent with the previous report on the XPS of GD nanosheets.43 The Zn 2p deconvoluted spectrum (Figure 5c) shows the presence of Zn 2p3/2 and Zn 2p1/2 at the binding

Mineralization Efficiency (ME %) = [1 − (TOCt /TOC0)] × 100

(2)

3. RESULTS AND DISCUSSION This study employed a hydrothermal route for the preparation of GD-ZnO nanohybrids using GD (synthesized by crosscoupling reaction), zinc acetate, and sodium hydroxide as the starting precursors. At first, the prepared GD is dispersed in water using ultrasonication for exfoliation process followed by the addition of zinc acetate. During ultrasound irradiation, the exfoliation of GD into few layers has been achieved, and simultaneously the addition of zinc acetate leads to the anchoring of zinc ions on the surface of the GD nanosheets. Further, sodium hydroxide is added to the reaction medium followed by the hydrothermal reaction at 180 °C for 24 h, which leads to the formation of GD-ZnO nanohybrids. The Xray diffraction pattern of the bare ZnO and GD-ZnO nanohybrids are shown in Figure 1. The bare ZnO shows the

Figure 1. X-ray diffraction pattern of ZnO and GD-ZnO nanohybrids.

presence of sharp well-defined peaks at 2θ = 31.47°, 34.09°, 36.01°, 47.29°, 56.39°, 62.68°, and 67.63°, which are corresponding to planes (100), (002), (101), (102), (110), (103), and (201), respectively. The observed diffraction pattern and interplanar spacing of the ZnO nanoparticles are in good agreement with the standard diffraction pattern of ZnO corresponding to hexagonal wurtzite structure (JCPDS-050664).34 There is no peak corresponding to any impurities or hydroxides, suggesting the hydrothermal process results in high purity ZnO nanoparticles. The XRD pattern of GD-ZnO (shown in Figure 1) indicates the presence of diffraction peaks corresponding only to ZnO nanoparticles. This is due to the fact that ZnO nanoparticles are uniformly anchored on either side of the GD nanosheets. The similar type of observations has been already noticeable in graphene-ZnO hybrid materials and other graphene decorated metal oxides.35,36 22059

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Figure 2. (a) Scanning electron micrograph, (b) Transmission electron micrograph, and (c) EDAX analysis of GD-ZnO nanohybrids.

Figure 4. Laser Raman spectrum of GD-ZnO nanohybrid prepared by the hydrothermal method.

Figure 3. Fourier transform-infrared spectra of ZnO and GD-ZnO nanohybrids.

spectra at 664 nm due to the hetero-polyaromatic linkage with a shoulder was located at 292 nm, due to benzene ring.47 RhB is the basic dye with absorbance maximum at 554 nm. Moreover, both azo dyes (RhB and MB) used in this study are resistant to biodegradation and direct photolysis.48 The photocatalytic degradation of these dyes catalyzed by GD-ZnO nanohybrids are examined with the UV−vis spectroscopic analysis. Figure 6a,b shows the time dependent absorbance spectra of MB and RhB which revealed a linear photodegradation of these dyes catalyzed by GD-ZnO nanohybrids. The control experiments such as the photodegradation reaction under light irradiation (in the absence of catalyst) as well as in dark conditions (in the presence of catalysts) are also performed in order to understand the catalytic performance of the prepared GD-ZnO nanohybrids. There is no significant changes occurred in the UV−vis

energies 1022 and 1045 eV, respectively, corresponding to the states of Zn in ZnO.44 This result indicated that the oxidation state of Zn is +2 in the GD-ZnO nanohybrid. Further, O 1s spectra (Figure 5d) is observed at the binding energy 532 eV which is originated from the oxygen content in the metal oxide (ZnO) surface and is in agreement with the previous reports.45 The photocatalytic activity of the prepared GD-ZnO nanohybrids (in comparison with bare ZnO nanoparticles) are evaluated by the photodegradation of two azo dyes. viz. (i) methylene blue (MB) and (ii) rhodamine B (RhB) under UVlight irradiation. Both MB and RhB dyes are the most commonly used coloring agents in textile industry and severe environmental contaminants owing to their toxicity as well as carcinogenic effects.46 MB exhibits the strong absorbance 22060

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Figure 5. X-ray photoelectron spectra of GD-ZnO nanohybrid: (a) Survey spectrum, (b) C 1s spectrum, (c) Zn 2p spectrum, and (d) O 1s spectrum.

dyes. Similarly, the degradation efficiency of the GD-ZnO and bare ZnO is about 68 and 42% for the photodecomposition of MB, whereas in the case of RhB it is about 55 and 35%, respectively. This increase clearly indicated that GD-ZnO nanohybrid possess superior photocatalytic activity than that of the bare ZnO for the photodegradation of both azo dyes MB and RhB. Further, the TOC removal efficiency of the photocatalyst is evaluated to understand the effective rate of organic carbon removal from the dyes. Figure 7 shows the timedependent concentration of total organic carbon (TOC) in the dye solutions during the photocatalytic process. From the TOC detection, we can clearly see that the mineralization efficiency is higher for GD-ZnO nanohybrids compared to bare ZnO for both azo dyes MB and RhB, respectively. This is due to the rapid charge-transfer property of the GD nanosheets in the GD-ZnO hybrids. In the view of understanding the photocatalytic performance of novel materials, it is highly recommended to access their activity with standards such as phenol (a photostable organic pollutant).50,51 Therefore, the effectiveness of GD-ZnO hybrids for the photodegradation of phenol (concentration of 40 mg/L) was also examined in this study. Figure 8a shows the photodegradation of phenol catalyzed by GD-ZnO nanohybrid monitored in a time dependent manner. Figure 8b demonstrates that the GDZnO nanohybrids possess superior photocatalytic property compared to bare ZnO nanoparticles in the photodegradation of phenol. This study further supports that the GD-ZnO

spectra of the dyes for experiments under dark (in the presence of catalysts) and under light irradiation (in the absence of catalyst), suggesting the observed photodegradation of dyes are due to the synergetic photocatalytic effect of the GD-ZnO nanohybrids and not due to the self-oxidation of dye. The normalized change in concentration of the dyes before and after light irradiation (C/C0) is found to be directly proportional to the normalized change in maximum absorbance values (λmax) of the dyes before and after light (A/A0) irradiation at desired time periods. Moreover, the decolorization of azo dyes in the presence of photocatalyst under light irradiation could be assigned to the pseudo-first-order reaction kinetics provided by the Langmuir−Hinshelwood model49 as given below:

ln(C /C0) = −kt

(3)

where C0 and C are the initial and final concentration of the dye, t is the irradiation time, and k is the apparent rate constant. Figure 6c,d shows the plot of ln(C/C0) against the irradiated time t for the MB and RhB catalyzed by ZnO, GD-ZnO, and in the absence of catalyst. The rate constants k of GD-ZnO and bare ZnO for the photodecomposition of MB are about 0.004 26 and 0.001 81 min−1, whereas in the case for RhB, the rate constants of GD-ZnO and bare ZnO are about 0.002 98 and 0.001 66 min−1, respectively. The rate constant k obtained for the GD-ZnO is nearly 2-fold increase when compared to that of bare ZnO in the photocatalytic degradation of both 22061

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Figure 6. Photocatalytic performance of GD-ZnO nanohybrids and ZnO nanoparticles over the degradation of (a) methylene blue and (b) rhodamine B measured by UV−vis spectroscopy. Rate kinetic of (c) methylene blue and (d) rhodamine B photodegradation by GD-ZnO nanohybrids and ZnO nanoparticles.

Figure 7. Total organic carbon removal of (a) methylene blue and (b) rhodamine B photodegradation using GD-ZnO nanohybrids and ZnO nanoparticles.

nanohybrids can act as an advanced photocatalytic material for the photodegradation of standard pollutant phenol as well. Further, the effect of GD concentration on the nanohybrids was also examined (as shown in Figure S2). Three weight ratios such as 0.25, 0.5, and 0.75 wt % of GD in the GD-ZnO have been examined for photocatalytic assessments to study the effect of GD loading on the photocatalytic performance. Figure S2 reveals that the sample containing 0.5 wt % GD showed superior photocatalytic efficiency, and an increase in GD concentration (about 0.75 wt %) results in a decrease in photocatalytic efficiency. The increase in GD concentration

may result in engulfing the promotion of synergetic charge transfer, hence resulting in inferior catalytic efficiency. In order to study the advantages of GD as an additive material for effective photocatalysis over other carbon materials such as GO or reduced graphene oxide (rGO), we examined the photocatalytic efficiency of GO-ZnO and rGO-ZnO and compared with GD-ZnO. As observed from Figure S3, the GD-ZnO nanohybrids possess superior photocatalytic activity compared to that of GO-ZnO and rGO-ZnO. Moreover, the stability of the photocatalyst has been evaluated over four consecutive cycles (as shown in Figure S4), which evidenced that there is 22062

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Figure 8. (a) Photocatalytic performance of GD-ZnO nanohybrids over the degradation of phenol measured by UV−vis spectroscopy. (b) Rate kinetics of phenol degradation by GD-ZnO nanohybrids and ZnO nanoparticles.

no significant change in the catalytic efficiency. Altogether, these findings suggested that GD-ZnO nanohybrids can act as a superior photocatalytic material for the degradation of dyes. The experimental analysis on the photocatalytic activity of the GD-ZnO nanohybrids studied by the absorbance spectra and mineralization efficiency demonstrated that the prepared hybrid material possess superior catalytic efficiency than that of the bare ZnO. This enhanced photocatalytic efficiency observed in the GD-ZnO nanohybrids can be explained as follows: In general, the mechanism of semiconductor photocatalysis is due to the formation of electron−hole pairs on the semiconductor surface upon absorption of light energy higher than that of the semiconductor band gap energy.52 The formation of reactive oxygen species such as superoxide and hydroxyl radicals due to the reaction of free charge carriers with water and oxygen will decomposes the organic dye molecule into CO2 and H2O.53 Hence, minimizing the rate of electron−hole pair recombination will produce in enhanced catalytic efficiency.54 The inclusion of GD in the prepared GD-ZnO nanohybrids decreases the absorption edge of ZnO, thereby utilizing more photon energy which results in enhanced photocatalytic reaction rate. In order to examine this, we measured the photoluminescence spectra of bare ZnO and GD-ZnO nanohybrids and are shown in Figure 9. The PL spectra of

bare ZnO show the broad emission peak at 560 nm, which is due to the emission from the defect related band such as oxygen vacancies, zinc vacancies, and donor−acceptor pairs.55,56 The observed emission from ZnO is quenched after hybridization with GD as seen in the PL spectrum of GDZnO nanohybrids. The mechanism of PL quenching can be attributed to the electron acceptor nature of the GD nanosheets as similar to that of graphene.57 It might be also due to the interaction between the ZnO surfaces with the diacetylene groups in GD nanosheets which needed further investigation in detail. Moreover, this observed luminescence quenching in GDZnO nanohybrids indicated the prevention of electron−hole recombination due to the effect of hybridization. The similar type of observations has been observed in the previous studies on graphene−ZnO photocatalytic systems.58 The advantage of GD over graphene is due to the diacetylene linkages between the hexagonal carbon rings may create an impurity band edge in the GD-ZnO nanohybrid which provide more effective transfer of electron from valence band to conduction band (Figure 10). The conductive nature of GD makes them a viable candidate for effective transfer of photogenerated electrons from ZnO to GD which suppresses recombination of free charge carriers.41 The trapped electron on GD can react with dissolved oxygen to form superoxide radicals (O2•), whereas the holes react with

Figure 9. Photoluminescence spectra of ZnO and GD-ZnO nanohybrids.

Figure 10. Schematic representation of GD-ZnO nanohybrid photocatalysis. 22063

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The Journal of Physical Chemistry C water to form hydroxyl radicals (OH•) which decompose the azo dyes. The recent theoretical and experimental studies on GD-TiO2 hybrid systems supported our claim.26,38 Thus, GDZnO nanohybrids act as an advanced photocatalytic material.

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4. CONCLUSION In conclusion, a novel graphdiyne−ZnO nanohybrid has been prepared via the hydrothermal method, and the intermolecular interactions between the ZnO and GD nanosheets have been studied using XRD, SEM, TEM, FT-IR, laser Raman, and XPS analysis. The GD-ZnO nanohybrids showed enhanced photocatalytic properties that bare ZnO in the photocatalytic degradation of two azo dyes MB and RhB. A plausible mechanism on the role of GD for the enhancement in photocatalytic properties of ZnO was discussed in detail using photoluminescence analysis. On the basis of the experimental findings, we believe that GD nanosheets can create new insights into developing advanced semiconductor photocatalysis for environmental remediation.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b06138. Detailed procedure for the GD preparation and experimental results such as adsorption analysis, effect of GD weight ratio on the photocatalytic property of the nanohybrids, comparative photocatalytic efficiency of GD-ZnO with GO-ZnO and rGO-ZnO, and photocatalytic cyclic tests (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected] (G.V.). Author Contributions

S.T. and K.K. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.T. is so thankful to the Indo-Korean Research Internship (IKRI) program between DST (India) and MEST-NRF (South Korea). This research was partly supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013R1A1A2064471) and (2013R1A2A2A01068926). The authors G.V. and S.T. gratefully acknowledge the continuous encouragement from the Vice Chancellor Prof. Dr. Sundar Manoharan, of Karunya University, India. The corresponding author (G.V.) is an International Research Fellow of the Japan Society for the Promotion of Science.

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