GraphdiyneâZnO Nanohybrids as an Advanced Photocatalytic

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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 J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b06138 • Publication Date (Web): 28 Aug 2015 Downloaded from http://pubs.acs.org on August 29, 2015

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The Journal of Physical Chemistry

Graphdiyne-ZnO Nanohybrids as an Advanced Photocatalytic Material Sakthivel Thangavel,1,2# Karthikeyan Krishnamoorthy,1# Velmurugan Krishnaswamy,3 Nandhakumar Raju,3 Sang Jae Kim,1,4 Gunasekaran Venugopal2,5*

1

Nanomaterials and System Lab, Department of Mechatronics Engineering, Jeju National University, Jeju 690-756, Republic of Korea

2

Department of Nanosciences and Technology, Karunya University, Karunya Nagar, Coimbatore 641-114, Tamil Nadu, India

3

Department of Chemistry, Karunya University, Karunya Nagar, Coimbatore 641-114, Tamil Nadu, India. 4

School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332-0245, United States. 5

Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

ACS Paragon Plus Environment


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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 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 graphdiyneZnO nanohybrids is nearly two 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 on the design of novel hybrid photocatalysts for potential applications in the environmental remediation sectors. KEYWORDS: Graphdiyne, ZnO, Nanohybrids, Photocatalysis, Luminescence quenching.


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1. INTRODUCTION Nanostructured semiconducting materials opens-up new opportunities in the field of photocatalysis and have 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 Eventhough, various materials are examined for semiconductor photocatalysis during the recent decade; the researchers are focused towards 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 sheet-like structures, ability to adhere semiconducting 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 shape 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 towards advancements in semiconductor photocatalysis.



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Several allotropes of carbon in two dimensional forms such as graphyne, graphane, graphdiyne, and halogenated graphenes, are 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 its 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 Sm-1 (similar to silicon) 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, environmentally benign with superior physical and chemical properties.27 These intriguing properties of ZnO make them an ideal candidate for several applications including solar cells, photocatalytic water splitting, photocatalytic degradation of organic pollutants and piezo-phototronics.28,29 In addition to this, ZnO possess high electron mobility almost two orders of magnitude higher than that of TiO2 which results in higher photocatalytic efficiency due to the increase in life time of photogenerated charge carriers.30 Hence, we aimed at developing advanced photocatalytic


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materials based on GD-ZnO by 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. EXPERIMENTAL 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 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 ºC under 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. detailed experimental procedure is given in supporting document (Section. S1).


The


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2.3 Preparation of graphdiyne-zinc oxide (GD-ZnO) nanohybrid materials. The graphdiyne-zinc oxide (GD-ZnO) nanohybrids are prepared via one-pot hydrothermal method. Briefly, an aqueous solution containing 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 pattern of ZnO and GD-ZnO 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 300WMg X-ray tube as a radiation source at 15 kV voltages). 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.


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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 Fig. 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 (Cο) 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 given equation32: Degradation efficiency (DE %) = [1- (Ct/ Co)] × 100 ……………. (1) 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 (TOCο) 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 given equation33:



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Mineralization efficiency (ME %) = [1- (TOCt/ TOCo)] × 100 ……………. (2) 3. RESULTS AND DISCUSSION This study employed a hydrothermal route for the preparation of GD-ZnO nanohybrids using GD (synthesized by cross-coupling 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 ºC for 24 h which leads to the formation of GD-ZnO nanohybrids. The X-ray diffraction pattern of the bare ZnO and GD-ZnO nanohybrids are shown in Fig. 1. The bare ZnO shows the 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-05-0664).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 Fig. 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 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


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nanohybrid is represented in Fig. 2 (a) which revealed the spherical ZnO nanoparticles anchored on the GD nanosheets uniformly, thus prevent the aggregation of GD as well as ZnO nanoparticles in the nanohybrids. Further, the TEM micrograph shown in Fig. 2(b) 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 Fig. 2(c)) 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 respectively. 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 GD-ZnO nanohybrids showed the presence of bands at 440, 1450 and 1586 cm-1 respectively. The band observed at 440 cm-1 corresponds to the Zn-O vibrational modes in GDZnO. 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 GDZnO nanohybrids were characterized using laser Raman spectroscopy, which is one of the prominent tools for understanding the crystalline nature, and defects levels present in nanostructures including carbon materials (such as graphene, GO, CNT) and hybrid materials.39 The Raman spectrum of GD-ZnO is provided in Fig. 4 which revealed the presence of bands



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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 first-order 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 modified Hummers method) and reduced graphene oxide (prepared via hydrothermal method) as in our earlier report.42 This further evidence that the prepared GD is of high crystallinity with low defects31,41. The chemical and surface states of elements present in the GD-ZnO nanohybrids are examined by the X-ray photoelectron spectroscopy. The XPS survey spectrum of the GD-ZnO nanohybrids are presented in Fig. 5(a) which indicated the presence of C, Zn and O groups at binding energies 285, 1030, and 532 eV respectively. The C1s spectrum of GD-ZnO nanohybrids is shown in Fig. 5(b) can be deconvoluted into C-C (sp), C-C (sp2), C=O, 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 spectra (Fig. 5(c)) shows the presence of Zn 2p3/2 and Zn 2p1/2 at the binding 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 (Fig. 5(d)) 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


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blue (MB) and (ii) rhodamine B (RhB) under UV-light 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 spectra at 664 nm due to the heteropolyaromatic 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 6(a & 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 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 spectra of the dyes for experiments under dark (in 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/Cο) is found to be directly proportional to the normalized change in maximum absorbance values (λmax) of the dyes before and after light (A/Aο) irradiation at desired time periods. Moreover, the decolorization of azo dyes in presence of photocatalyst under light irradiation could be assigned to the pseudo-first order reaction kinetics provided by LangmuirHinshelwood model49 as given below: ln (C/Cο) = -kt………………………… (3)



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where “Cο” and “C” are the initial and final concentration of the dye, “t” is the irradiation time and “k” is the apparent rate constant. Figure 6 (c&d) shows the plot of ln (C/Cο) against the irradiated time “t” for the MB and RhB catalyzed by ZnO, GD-ZnO and in absence of catalyst. The rate constant “k” of GD-ZnO and bare ZnO for the photodecomposition of MB is about 0.00426, and 0.00181 min-1 whereas in the case for RhB, the rate constant of GD-ZnO and bare ZnO are about 0.00298 and 0.00166 min-1 respectively. The rate constant “k” obtained for the GD-ZnO is nearly two fold increase when compared to that of bare ZnO in the photocatalytic degradation of both 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 time-dependent 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 8 (a) shows the photodegradation of phenol catalyzed by GD-ZnO nanohybrid monitored in a time dependent manner. Figure 8 (b) demonstrated that the GD-ZnO nanohybrids possess


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superior photocatalytic property compared to bare ZnO nanoparticles in the photodegradation of phenol. This study further supports that the GD-ZnO 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 Fig. S2). Three weight ratios such as 0.25, 0.5 and 0.75 wt% of GD in the GD-ZnO has been examined for photocatalytic assessments to study the effect of GD loading on the photocatalytic performance. Fig. S2 revealed 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 resulted 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 Fig. 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 Fig. S4), which evidenced that there is 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



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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 Fig. 9. The PL spectra of bare ZnO shows the broad emission peak at 560 nm which is due to the emission from the defect related band such as oxygen vacancies, zinc vacancies as well as donor-acceptor pairs.55,56 The observed emission from ZnO is quenched after hybridization with GD as seen in the PL spectrum of GD-ZnO 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 GD-ZnO 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 (Fig. 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


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electron on GD can react with dissolved oxygen to form superoxide radicals (O2) whereas the holes react with water to form hydroxyl radicals (OH) which decomposes the azo dyes. The recent theoretical and experimental studies on GD-TiO2 hybrid systems supported our claim.26,38 Thus, GD-ZnO nanohybrids act as an advanced photocatalytic material. 4. CONCLUSION In conclusion, a novel graphdiyne-ZnO nanohybrids has been prepared via hydrothermal method and the intermolecular interactions between the ZnO and GD nanosheets has been studied using XRD, SEM, TEM, FT-IR, laser Raman, and XPS analysis, respectively. The GDZnO 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. Based on the experimental findings, we believe that GD nanosheets can create new insights in the developing advanced semiconductor photocatalysis for environmental remediation. ASSOCIATED CONTENT SUPPORTING INFORMATION The 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 are provided in the supporting information. This information is available free of charge via the Internet at “http://pubs.acs.org”. AUTHOR INFORMATION Corresponding author



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*Email: [email protected] (or) [email protected] #

These authors contributed equally.

Notes The authors declare no competing financial interests. ACKNOWLEGMENTS Author S.T. is so thankful to the Indo-Korean Research Internship (IKRI) program between DST (India) and MEST-NRF (South Korea). This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2013R1A1A2064471). REFERENCES: (1) Xu, J.; Luo, L.; Xiao, G.; Zhang, Z.;

Lin, H.; Wang, X.; Long, X. Layered

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FIGURE CAPTIONS Figure 1. X-ray diffraction pattern of ZnO and GD-ZnO nanohybrids. Figure 2. (a) Scanning electron micrograph, (b) transmission electron micrograph, and (c) EDAX analysis of GD-ZnO nanohybrids. Figure 3. Fourier transformed-infra red spectra of ZnO and GD-ZnO nanohybrids. Figure 4. Laser Raman spectrum of ZnO-GD nanohybrid prepared by hydrothermal method. Figure 5. X-ray photoelectron spectra of ZnO-GD nanohybrid ((a) survey spectrum, (b) C 1s spectrum, (c) Zn 2p spectrum, and (d) O 1s spectrum. 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. 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. Figure 9. Photoluminescence spectra of ZnO and GD- ZnO nanohybrids. Figure 10. Schematic representation of GD- ZnO nanohybrid photocatalysis.


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Figure 1.



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Figure 2.


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Figure 3.



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Figure 4.


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Figure 5.



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Figure 6.


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Figure 7.



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Figure 8.


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Figure 9.



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Figure 10.


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GraphdiyneâZnO Nanohybrids as an Advanced Photocatalytic

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