Effective photocatalytic dechlorination of 2,4-dichlorophenol by a novel graphene encapsulated ZnO/Co₃O₄ core–shell hybrid under visible light

Abstract

In the present work, a graphene encapsulated ZnO/Co₃O₄ (GE/ZnO/Co₃O₄) core–shell hybrid is fabricated through a facile self-assembly approach, where the mutual electrostatic interaction force drives the ZnO/Co₃O₄ heteronanostructures to be fully wrapped with flexible ultrathin graphene shells. The as-prepared GE/ZnO/Co₃O₄ core–shell hybrid is characterized and exhibits excellent visible light photocatalytic ability toward dechlorination of 2,4-dichlorophenol (2,4-DCP) in the aqueous phase. It is worth noting that 2,4-DCP is almost completely mineralized into CO₂ and H₂O by the GE/ZnO/Co₃O₄ after 5 h of a photocatalytic reaction. This type of higher dechlorination and mineralization efficiency of 2,4-DCP is not generally observed, and is found to be higher than some previous studies. The dechlorination of 2,4-DCP has been achieved under different parametric conditions. The unique architecture of the GE/ZnO/Co₃O₄ core–shell hybrid also provides high stability and recyclability towards degradation of 2,4-DCP. The higher photocatalytic activity of the hybrid can be ascribed to the synergistic effect of ZnO, Co₃O₄ and graphene, and also by an increase in the contact surface between the metal oxide core and the graphene shell, which acts as a continuous path for rapid electron transport to offer a greater number of reactive species.

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Introduction

Semiconductor photocatalytic technology, a well-accepted strategy, has attracted tremendous attention because of its potential for hydrogen generation and pollutant degradation to simultaneously solve energy and environmental crises.¹ In recent years, graphene–inorganic semiconductor composite materials have received widespread concern from researchers in the area of solar energy conversion.^2,3 Graphene, formed of zero band gap two-dimensional (2D) sp²-hybridized carbon atom with a platelike structure possesses very high charge carrier mobility, specific surface area, excellent optical properties, transparency, mechanical flexibility as well as good thermal and chemical stability, which makes it an excellent electron-transport material for photocatalysis even more appropriate than C₆₀, polyaniline and graphite-like carbon.^4–6 In particular, due to the large-size 2-D structure, the graphene sheet could behave as a giant solid support (stabilizer) of nanoparticles through interfacial interactions to avoid particle aggregation, and plenty of p electrons present in the special π-conjugation structure of graphene provide a suitable platform for the growth and stability of inorganic nanoparticles.^7,8 Graphene could enhance the photocatalytic activity of a catalyst by increasing the reaction active sites and facilitating the adsorption of organic contaminants.⁹ Furthermore, a strong heterojunction electric field is formed on the interface between graphene and an inorganic semiconductor material due to the difference in their Fermi levels.¹⁰ Photogenerated electrons are swiftly transferred to the graphene surface and participate in the reduction reactions occurring there, which enhances the separation efficiency of photogenerated electron–hole pairs and also the photocatalytic ability significantly.

Metal oxide nanocomposites composed of two or more dissimilar materials with attractive performance are currently in the spotlight to improve diverse properties of functional materials.¹¹ An interface is created by the partial reaction of two oxides, which shows some novel attractive properties because of the influence of proximity and diffusion phenomena at the nanoscale range.^12,13 Based on this idea, various ZnO-based heterostructures have been designed for various interesting properties.^14–16 ZnO, an n-type semiconductor, is well known for its versatile application,¹⁷ while the p-type Co₃O₄ is recognized for high catalytic activity and lithium storage capacity.^18,19 It is desirable that the synergistic combination of ZnO and Co₃O₄ will display some novel fascinating properties. In recent years, a reasonable number of literature reports are available on ZnO/Co₃O₄ exhibiting their potential application in gas sensing and catalytic behaviour.^17,20 Owing to the rise of graphene in recent times, combinations of graphene with semiconductors such as, TiO₂,²¹ ZnO,²² CdS,²³ SnO₂ ²⁴etc. are becoming popular for various applications. In these materials, the metal oxides are distributed into the graphene surface or between the graphene layers significantly improving the photocatalytic activity. However, it is important to note that the contact area between graphene and the inorganic semiconductor materials is relatively small and the effective area of the heterojunction built on the interface is low, as a result, it cannot display optimum photocatalytic performance.² Also the metal oxide nanoparticles still can strongly aggregate because of non-intimate contact between the graphene layers and metal oxides. One of the most promising strategies to tackle the aggregation problem of metal oxides is either to confine them within individual carbon shells or the usage of graphene coated metal oxide surface. For example, Yang et al. synthesized graphene encapsulated Co₃O₄ for a high- performance Li ion battery.²⁵ Bu and co-workers fabricated a graphene coated ZnO quasi-core–shell composite for high photocatalytic performance.¹⁰ However, the photocatalytic activity of the ZnO/Co₃O₄ hybrid has not been well explored so far. Recently, we have shown higher gas sorption and photocatalytic activity of porous ZnO/Co₃O₄ hybrids obtained from a coordination polymer route.²⁶ However, to the best of our knowledge, the synergistic effect and enhanced photocatalytic activity towards dechlorination of chlorinated phenols for this new type core–shell hybrid of ZnO/Co₃O₄ and graphene have not been explored yet.

Hence, in this report, ZnO/Co₃O₄ heteronanostructures are first prepared from a facile thermal treatment of a coordination polymer precursor. The as-prepared ZnO/Co₃O₄ is fabricated with graphene to obtain a graphene encapsulated ZnO/Co₃O₄ (GE/ZnO/Co₃O₄) core–shell type hybrid. The overall synthetic procedure of GE/ZnO/Co₃O₄ involves three steps: firstly, ZnO/Co₃O₄ is modified by surface grafting of aminopropyltriethoxysilane (APTES) to make the surface positively charged²⁵ (Scheme 1). In the second step, the amine-treated ZnO/Co₃O₄ is assembled with negatively charged GO by the mutual electrostatic interactions of the two species. Finally, the resulting aggregates are reduced with hydrazine hydrate to synthesize the GE/ZnO/Co₃O₄ core–shell type hybrid, where the graphene surface encapsulates the ZnO/Co₃O₄ core from the outer shell. The as-prepared GE/ZnO/Co₃O₄ further exhibits excellent performance towards mineralization of 2,4-DCP under visible light. The enhanced photocatalytic activity of the GE/ZnO/Co₃O₄ core–shell hybrid can be ascribed to several factors: (i) synergistic effects of the presence of both metal oxides and graphene improve visible light absorption, (ii) formation of a core–shell type hybrid where graphene shells are very close in contact with the metal oxide core wrapping all around (contact area increases) and acting as a continuous path for very rapid electron transport, (iii) very high surface area of the GE/ZnO/Co₃O₄ hybrid and also the porous ZnO/Co₃O₄ which adsorb organic molecules along with graphene, hence more number of reactant molecules are bound to the catalyst surface, (iv) it suppresses the recombination of electron–holes and hence, prolongs the lifetimes of the carriers resulting in more hydroxyl and superoxide radicals, and (v) the unique core–shell architecture prevents aggregation and photo-corrosion of the ZnO/Co₃O₄ core more effectively and makes it highly stable towards degradation of 2,4-DCP.

Scheme 1 Schematic representation of the formation of the GE/ZnO/Co₃O₄ core–shell hybrid from ZnO/Co₃O₄ prepared through a nano coordination polymer route.

Experimental

Reagents and materials

2,3-Dihydroxybenzaldehyde and zinc acetate dihydrate were obtained from Sigma-Aldrich and SISCO Research Laboratories Pvt Ltd (India), respectively and used as received. Cobalt acetate tetrahydrate was obtained from Loba Chemie Pvt. Ltd (India). 3-Aminopropyltriethoxy silane (APTES) was purchased from Spectrochem Pvt Ltd (India). All other chemicals (solvents) used in this investigation were of analytical grade (99.9%). All the solutions were prepared in Millipore water (Milli-Q system).

Preparation of the ZnO–Co₃O₄ heteronanostructures from nano coordination polymers. The ZnO–Co₃O₄ heteronanostructures were prepared according to our previously reported method²⁶ with slight modifications using 2,3-dihydroxylated salophen ligand (2,3-DHS) and terephthalic acid (1,4-H₂BDC) as linkers with Zn(OAc)₂·2H₂O and Co(OAc)₂·4H₂O in DMF. In a typical method, 2,3-DHS (0.007 mmol) was dissolved in a minimum amount of DMF. Another DMF solution of Zn(OAc)₂·2H₂O (0.021 mmol) and Co(OAc)₂·4H₂O (0.028 mmol) was added successively to the above prepared solution. Finally, 5 mL DMF solution of 1,4-H₂BDC (0.035 mmol) was added to the prepared solution. Within seconds a hexagonal lump shaped NCP was formed. The Zn–Co NCPs obtained were centrifuged and washed with acetonitrile several times and then dried under open air (yield ∼70%) for several hours. The produced Zn–Co NCPs were then placed in a conventional furnace and heated at 550 °C for 75 min; finally hexagonal shaped ZnO/Co₃O₄ nanostructures were generated (Fig. S1 and S2†).

Preparation of GO. Graphene oxide was synthesized from natural graphite by a modified Hummers method.²⁷ In a typical method, 1 g of graphite and 0.5 g of sodium nitrate (NaNO₃) were mixed with 35 mL of concentrated sulphuric acid. After that, the mixture was stirred for 30 min in an ice bath, and 3.6 g of potassium permanganate (KMnO₄) was added under vigorous stirring. The rate of addition was carefully controlled to maintain the reaction temperature lower than 20 °C. The mixture was then continuously stirred at room temperature overnight, and 150 mL of deionized (DI) water was slowly added under vigorous agitation. The obtained diluted suspension was stirred for one day, and 10 mL of 30 wt% H₂O₂ was added to the mixture. Finally, the mixture was washed by rinsing and centrifuging with 5 wt% HCl and DI water several times. Chemical exfoliation of thus-obtained graphite oxide was carried out in an ultrasonic bath for 90 min to obtain GO.

Preparation of the GE/ZnO/Co₃O₄ core shell hybrids. The GE/ZnO/Co₃O₄ core–shell hybrids were obtained via electrostatic interactions between APTES modified oxides and graphene oxide in aqueous solutions. Firstly, ZnO/Co₃O₄ was modified with APTES to introduce a positively charged surface. In a typical process, ZnO/Co₃O₄ (0.5 g) was dispersed into 50 mL of isopropanol via ultrasonication. Then, 0.5 mL APS was added into the above solution and refluxed at 80 °C for 24 h. After washing with ethanol several times, the APTES modified ZnO/Co₃O₄ (APTES/ZnO/Co₃O₄) was obtained. Next, the dried APTES/ZnO/Co₃O₄ was dispersed in 100 mL ultrapure water followed by addition of 150 mL aqueous GO suspension (0.5 mg mL⁻¹) and the overall pH of the solution was measured to be 6.0. At this pH, the modified ZnO/Co₃O₄ was assembled with negatively charged GO by electrostatic interactions. After mildly stirring for 30 min, 10 mL of hydrazine hydrate solution (35 wt%) was added into the above aggregates to reduce the GO, and finally the GE/ZnO/Co₃O₄ was obtained after centrifugation and washing with ultrapure water.

The surface charges of APTES/ZnO/Co₃O₄ and GO were examined at different pH solutions by zeta potential measurements. As shown in Fig. S3,† the surface of GO was negatively charged (zeta potential = −23–53 mV) over the investigated pH range (2–11) due to ionization of the carboxylic acid and phenolic hydroxyl groups located on the GO.²¹ The surface charge of the APTES/ZnO/Co₃O₄ switched from positive (zeta potential = +50 mV) to negative (zeta potential = −45 mV) with an increase of the pH value from 2 to 11. It indicates that the mutual assembly can only be triggered when APTES/ZnO/Co₃O₄ and GO were oppositely charged, hence suggesting that the electrostatic interaction was the driving force.

Visible light photocatalytic experiments

The photocatalytic activity of the as-synthesized GE/ZnO/Co₃O₄ core–shell hybrid was measured in 30 mL of reaction mixture of 2,4-DCP (20 mg L⁻¹) in an aqueous medium. 1 g L⁻¹ of the hybrid catalyst was taken in a Pyrex glass vessel and stirred in the dark for 30 min to maintain adsorption equilibrium between the pollutant and the surface of the catalyst under room air-equilibrated conditions. Then, the final mixture was irradiated with 300 W Phillips visible lamps containing aqueous NaNO₂ solution (10% w/w) as a UV-cut off filter. After a certain regular interval of time, exactly 5 mL of an aliquot was taken from the reaction mixture and centrifuged to separate the catalyst. The obtained clear solution was preserved for spectral analysis.

GC-MS study

The photocatalytic degradations of 2,4-DCP were repeated several times under optimized conditions. After 2 h and 5 h of the photoreaction, the reaction mixtures (after centrifuged) were collected separately. Then, the obtained mixtures were extracted firstly with chloroform, dichloromethane and ethyl acetate, and finally with diethyl ether. All the organic solvents were collected and removed under reduced pressure using a rotary evaporator. Finally, the products obtained were dried under vacuum and dissolved in HPLC grade methanol for GC-MS analysis.

Results and discussion

Characterization of the synthesized GE/ZnO/Co₃O₄ core–shell hybrid

To determine the phase composition and surface functional in GE/ZnO/Co₃O₄ core/shell heterostructures FT-IR studies were carried out. Fig. 1 shows FTIR spectra of graphite, GO, bare ZnO–Co₃O₄, and GE/ZnO/Co₃O₄. It is noted that no significant peak is found for the graphite, while the presence of different types of oxygen functionalities in graphene oxide is confirmed by O–H stretching vibrations at 3400 cm⁻¹, O–H deformation vibrations at 1415 cm⁻¹, C=O stretching vibrations at 1720 cm⁻¹, carboxyl C–OH stretching vibrations at 1216 cm⁻¹ and alkoxy C–O–C stretching at 1055 cm⁻¹. Besides, the aromatic C=C skeletal vibration of the sp² domains (1565 cm⁻¹) is also present in the GO.

Fig. 1 FT-IR spectra of graphite, GO, ZnO/Co₃O₄ and the GE/ZnO/Co₃O₄ hybrid.

Compared with the IR spectrum of GO, it is observed that the oxygen-containing functional peaks decrease to a large extent and even disappeared for the GE/ZnO/Co₃O₄ hybrid, further indicating that the GO is reduced to graphene after the solvothermal reaction. In addition, the skeletal vibration of the graphene sheets²⁸ emerged at 1545 cm⁻¹ for the GE/ZnO/Co₃O₄ hybrid. In the IR spectra of the GE/ZnO/Co₃O₄ hybrid, absorption bands at 482 cm⁻¹, 572 and 677 cm⁻¹ are attributed to the vibration of Zn–O, Co³⁺–O and Co²⁺–O bonds,²⁵ respectively, indicating the existence of ZnO and Co₃O₄ in the hybrid. Also, three new absorption bands at 1005 cm⁻¹, 1085 cm⁻¹ and 1185 cm⁻¹ originating from Si–O–Zn, (Si–O)_n and Si–O–Co bond vibration, respectively²⁵ are due to grafting of APTES moieties successfully onto the surface of ZnO–Co₃O₄.

Information regarding the phase composition, purity, and crystallinity of the as-synthesized samples was procured by powder X-ray diffraction (PXRD) analysis, and the results are shown in Fig. 2. The XRD peak of natural graphite powder is found at 26.71° exhibiting an interlayer distance of 3.2 Å. In comparison to the natural graphite, a wide peak at 2θ = 10.27° is observed corresponding to the (001) diffraction of GO, which indicates that the graphite is fully oxidized into GO with an interlayer distance of 8.60 Å. GO has a larger interlayer distance than graphite since H₂O molecules and various oxide groups intercalate between its layers.²⁹ From the XRD pattern of the GE/ZnO/Co₃O₄ core–shell hybrid, it is observed that the intensive peaks at 2θ values of 31.7, 34.4, 36.2, 47.5, 56.5, 62.8, and 68.0 can be respectively indexed to (100), (002), (101), (102), (110), (103) and (112) diffractions of the hexagonal wurtzite phase of the ZnO, which is indexed by an asterisk mark in the figure. Again, the measured peaks observed at 2θ = 31.13, 36.76, 38.5, 44.8, 55.5, 59.2, 65.26 and 77.4 correspond to (111), (220), (311), (222), (400), (422), (511), (440) and (533) planes of the cubic phase of Co₃O₄ crystals, respectively (JCPDS, 42-1467).³⁰ No peaks of impurities are observed in the XRD pattern indicating the formation of single-phase ZnO/Co₃O₄ heteronanostructures. Compared with pure ZnO/Co₃O₄, an additional characteristic peak around 2θ = 22.27° is found in the XRD pattern, which corresponds to (002) diffraction of the graphene in the GE/ZnO/Co₃O₄ core–shell hybrid demonstrating that GO is fully reduced to graphene by hydrazine upon thermal treatment. The interlayer distance of graphene is found to be 3.5 Å, which is reduced from the interlayer distance of 8.60 Å of GO indicating successful removal of oxygenated functional groups.

Fig. 2 Powder X-ray diffraction pattern of the graphite, synthesized GO and the GE/ZnO/Co₃O₄ hybrid.

X-ray photoelectron spectra (XPS), usually are used to analyze the element component and valence of the materials, and also give information about the interfacial interactions between the components. Fig. 3 shows the XPS spectra of the GE/ZnO/Co₃O₄ core–shell hybrid. From the full survey spectrum, the peaks belonging to C, Zn, O, Co, Si and N elements can be observed (Fig. 3a). For detailed information, the high-resolution XPS spectra of the C, Zn, Co and O elements are also provided. It is clearly seen (Fig. 3b) that the XPS spectrum of C 1s can be deconvoluted into three peaks centred at 282.7, 284.6 and 286.5 eV. The peak at 284.6 eV is attributed to the sp² carbon atom, while the peak positioned at 286.5 eV is assigned to the C–O groups. The peak located at 282.7 eV is closely associated with the C (sp³)–Si bond of the APTES.³¹ The C species belonging to the sp² carbon atom have a high percentage than the other carbon species, and the intensities of all the related oxygen peaks are sharply decreased in the GE/ZnO/Co₃O₄ sample compared to GO indicating that the GE/ZnO/Co₃O₄ contains far less oxygen and delocalized p conjugation is restored in the GE/ZnO/Co₃O₄, hence confirming its high quality. The XPS survey also showed a pronounced peak for N 1s of the C–N bond at 399.5 eV,³² which is due to the presence of amine functionalization of APS. Also the Si–O–C bond of APS appears at 101.2 eV in the XPS survey of the GE/ZnO/Co₃O₄ hybrid.³² Besides the C 1s peak, as shown in Fig. 3d, the peaks of Zn 2p_3/2 and Zn 2p_1/2 are located at 1021.4 and 1044.4 eV in the GE/ZnO/Co₃O₄ hybrid. The high resolution XPS fitting of Co 2p shows (Fig. 3c) prominent peaks at around 779.3 eV and 794.4 eV, which are attributed to Co³⁺ 2p_3/2 and Co³⁺ 2p_1/2 configurations of the Co₃O₄, respectively, and the fitting peaks at 780.7, 789.1 and 796.4 eV are due to Co²⁺ of the Co₃O₄ ³³ in the GE/ZnO/Co₃O₄ core–shell hybrid.

Fig. 3 (a) XPS full survey of the synthesized GE/ZnO/Co₃O₄ core–shell hybrid, (b) high resolution XPS measurement of the C 1s, (c) Co 2p and (d) Zn 2p of the GE/ZnO/Co₃O₄ core–shell hybrid.

Convincing proof of the core/shell formation in GE/ZnO/Co₃O₄ heterostructures is revealed by TEM and HRTEM studies (Fig. 4). In Fig. 4a, the TEM image shows the presence of bare GO sheets with a uniform distribution. However, after hybrid formation of GO with ZnO/Co₃O₄ and followed by reduction of GO, the TEM images show (Fig. 4b–d) that in most cases thin graphene shells are wrapped around the surface of ZnO/Co₃O₄ particles. The diameters of the ZnO and Co₃O₄ in the GE/ZnO/Co₃O₄ heterostructure are found to be around 40 and 20 nm, respectively. Due to the presence of flexible and ultrathin graphene, the GE/ZnO/Co₃O₄ exhibit crinkled and rough textures. The TEM images clearly show that the ZnO/Co₃O₄ particles are firmly attached or interconnected to each other by thin graphene shells via electrostatic interactions (also see, Fig. S4†). As shown in Fig. 5a and b, the HRTEM image of the GE/ZnO/Co₃O₄ core–shell hybrid clearly exhibits the interface of the ZnO or Co₃O₄ core and the thin graphene shell.

Fig. 4 (a) TEM image of the prepared GO, and (b–d) TEM images of the GE/ZnO/Co₃O₄ core–shell hybrid.

Fig. 5 HR-TEM images of the synthesized GE/ZnO/Co₃O₄ hybrid showing graphene shell with (a) Co₃O₄ core and (b) ZnO core and lattice fringes; (c) EDX analysis of the GE/ZnO/Co₃O₄ hybrid and (d) SAED pattern of the GE/ZnO/Co₃O₄.

The HRTEM images clearly exhibit lattice fringes with interplanar distances of 0.286 nm and 0.28 nm, which corresponds to the (220) plane of the cubic Co₃O₄ and (100) plane of the hexagonal wurtzite ZnO³⁴ and thin graphene shells are wrapped around the metal oxide core. The SAED pattern (Fig. 5d) indicates the polycrystalline nature of the GE/ZnO/Co₃O₄ hybrid. Energy Dispersive X-ray spectroscopy (EDS) displays intense signals of C, Zn, Co and O (Fig. 5c) on positioning the electron probe through the core–shell of the GE/ZnO/Co₃O₄. The copper and silicon signals come from the Cu grid used for TEM and APTES moiety, respectively. Thus, the TEM and HRTEM analyses confirm the core/shell morphology of GE/ZnO/Co₃O₄ hybrids.

TGA analysis and BET surface area of the synthesized GE/ZnO/Co₃O₄ hybrid

The quality of the as-prepared GE/ZnO/Co₃O₄ hybrid is assessed by thermogravimetric analysis (TGA). The samples are heated from room temperature to 600 °C at 5 °C min⁻¹ under a N₂ atmosphere using Al₂O₃ crucible and the results are shown in Fig. 6a. The bare ZnO/Co₃O₄ sample shows high thermal stability up to 600 °C without any loss of weight. In contrast, the GE/ZnO/Co₃O₄ hybrid exhibits relatively lower thermal stability than bare ZnO/Co₃O₄ due to the presence of the graphene shell.

Fig. 6 (a) TGA curve of the synthesized (i) bare ZnO/Co₃O₄ and (ii) GE/ZnO/Co₃O₄ hybrid; (b) BET-N₂ gas sorption isotherm curves of the synthesized GE/ZnO/Co₃O₄ and bare ZnO/Co₃O₄ (inset).

The total weight loss for the GE/ZnO/Co₃O₄ is only 9% at 600 °C indicating incorporation of a very low weight fraction of the graphene shell into the core–shell hybrid. Hence, the weight fraction of ZnO/Co₃O₄ in the core–shell hybrid is as high as 91%, which is much higher than those of reported core–shell metal oxide/carbon composites (40–75%).^35–37 A minor weight loss occurred between 100 and 225 °C indicating the release of CO, CO₂ and steam from the most labile functional groups during pyrolysis.³⁸ The minor mass-loss is attributed to the absence of most oxygen functional groups of the GO after thermal reduction.

Moreover, the porosity and surface area of the synthesized GE/ZnO/Co₃O₄ core–shell hybrid are characterized by Brunauer–Emmett–Teller (BET) nitrogen adsorption–desorption isotherm measurements. As shown in Fig. 6b, the BET isotherms of the prepared hybrid follow type III behaviour, which is a weak van der Waals force of attraction between the adsorbate (N₂ gas) and adsorbents (hybrid). Thus, the isotherms are reversible in nature as soon as the pressure is released. All the curves explain the multilayer sorption properties of the materials.³⁹ It is noted that the BET specific surface area (250.2 m² g⁻¹) and pore volume (0.435 cc g⁻¹) of the GE/ZnO/Co₃O₄ core–shell hybrid are significantly higher than those of the bare ZnO/Co₃O₄ (100.4 m² g⁻¹ and 0.223 cc g⁻¹, respectively).

The high specific surface area and porosity of the GE/ZnO/Co₃O₄ could be attributed to the presence of the wrapped graphene shell as well as the formation of secondary pores between the porous ZnO/Co₃O₄ and graphene. Hence, the high surface area and porosity of the GE/ZnO/Co₃O₄ hybrid could promote higher photocatalytic activity by adsorbing more number of organic reactant molecules on the catalyst surface.

Visible light photocatalytic degradation of 2,4-DCP

Chlorophenol compounds are a major class of environmental pollutants and potential human carcinogens.⁴⁰ This class of compounds can easily be accumulated in biological tissues, which poses a great threat to agriculture, food chain and human health.⁴¹ 2,4-DCP, a highly toxic and an important reagent in the manufacture of pesticides and herbicides⁴² has been identified as a primary pollutant by the United States Environmental Protection Agency (USEPA). Therefore, it is highly desirable to develop efficient methods or catalysts for the removal of 2,4-DCP from water bodies transforming this class of compounds into a non-toxic product or CO₂ and H₂O. Consequently, the removal of 2,4-DCP is relevant in this current report. Recently, visible light assisted photocatalytic degradation of various organic pollutants and synthesis of some important organic molecules have been reported by our group.^43–47

The photocatalytic activities of the GE/ZnO/Co₃O₄ core–shell hybrids are evaluated by taking 30 mL of reaction mixture of 2,4-DCP (20 mg L⁻¹) in aqueous medium and catalytic amount (1 g L⁻¹) of the hybrids, and before irradiation, it is kept in the dark under magnetic stirring for 30 min to maintain adsorption/desorption equilibrium between the pollutant and the surface of the catalyst. The adsorption of 2,4-DCP on the catalysts could be attributed to the adsorption of 2,4-DCP on the RGO shell and on the core of the porous metal oxide nanostructure. The former seemed to be much more favourable because of its giant p-conjugational plane, which strongly interacts with DCP molecules via a p–p stacking with a face-to-face orientation. The photodegradation of 2,4-DCP is monitored using both HPLC and UV-Visible absorption spectrophotometer by measuring the concentration of 2,4-DCP at different time intervals.

The UV-Visible absorption spectra of the 2,4-DCP degradation are shown in Fig. 7a. 2,4-DCP exhibits its maximum absorption at 285 nm, hence a decrease in the intensity of the peak at 285 nm is used to measure the concentration of 2,4-DCP. After 30 min, the peak at 285 nm is blue-shifted to 275 nm due to dechlorination of 2,4-DCP to o-chlorophenol (o-CP), which is further decomposed to phenol identified by the peak at 270 nm in UV spectra after 60 min.⁴⁸ Finally, the phenol is mineralized into CO₂ and water. This information has been also supported by a total ion chromatogram of the reaction products of 2,4-DCP carried out by HPLC (Fig. 7b). 2,4-DCP was decomposed into o-CP, p-CP and phenol after 120 min of photocatalytic reaction. The concentration of 2,4-DCP is sufficiently decreased, and the peak intensity demonstrates that the generation of o-CP is more than that of p-CP. However, after carrying out the reaction up to 5 h, the products o-CP, p-CP and 2,4-DCP vanish and a very small amount of phenol is observed indicating maximum mineralization of the above products. The products o-CP, p-CP and phenol are identified by the retention time of the standard samples under identical conditions and same concentrations (Fig. S5†).

Fig. 7 (a) UV-Visible absorption spectra of the 2,4-DCP degradation in the presence of the GE/ZnO/Co₃O₄ hybrid ([2,4-DCP] = 20 mg L⁻¹, [catalyst] = 1.0 g L⁻¹, pH = 5.0), and (b) HPLC analysis of total ion chromatogram for the degradation of 2,4-DCP under different intervals of time: 0 h, 2 h and 5 h.

To measure the mineralization efficiency of the GE/ZnO/Co₃O₄ catalyst, TOC is analysed after different time intervals. The results show that up to 2 h, the TOC removal efficiency is only 35% and this can be attributed to the higher rate of conversion of 2,4-DCP to the o-CP, p-CP and phenol during the first 2 h. And, the TOC removal finally reached 93% after 5 h of reaction (Fig. S6†). However, complete mineralization (100%) of 2,4-DCP has not been achieved by the GE/ZnO/Co₃O₄ catalyst, as a very small percentage of phenol is left as the final product indicated by HPLC. However, the high TOC removal value indicates its excellent efficiency toward degradation of 2,4-DCP converting to CO₂ and H₂O, and the value is found to be higher than other reported literature values.^49–51

It can be seen from Fig. 8 that 2,4-DCP is stable under visible light irradiation without the presence of any photocatalyst. It is slightly degraded (∼10%) in the presence of GO. However, in the presence of the catalyst GE/ZnO/Co₃O₄, the degradation of 2,4-DCP is remarkably enhanced (91% degraded only after 150 min) with rate constants of about 3.38 × 10⁻² min⁻¹ (Fig. S7†) under visible irradiation. The visible light photo-degradation rate of 2,4-DCP in the presence of the GE/ZnO/Co₃O₄ catalyst is faster than other reported literature rates.^50,52 However, the photocatalytic activity is significantly decreased (2 times lower) with a rate constant of about 1.7 × 10⁻² min⁻¹ when bare ZnO/Co₃O₄ is applied as a catalyst. Interestingly, when the photodegradation of 2,4-DCP in the presence of the GE/ZnO/Co₃O₄ hybrids is carried out under dark conditions, no significant degradation is observed (∼12%). This result infers that visible light is an essential component for degradation of 2,4-DCP.

Fig. 8 Kinetic plots (C_t/C₀vs. Time) for visible light (λ ≥ 420 nm) photocatalytic degradation of 2,4-DCP in the presence of GE/ZnO/Co₃O₄ hybrid with control studies.

Optimization of reaction conditions

Effect of catalyst dose. The photodegradation of 2,4-DCP is carried out at various catalyst dosages from 0.2 to 2.0 g L⁻¹ to optimize the reaction conditions. Fig. 9a shows that the photodegradation efficiency is increased with increase in catalyst loading up to 1.25 g L⁻¹ and then reaches a plateau. A further increase in catalyst dosage did not increase the degradation efficiency. This enhancement at low catalyst loading can be attributed to the increased number of available active sites for 2,4-DCP adsorption and light absorption, which leads to higher number of hydroxyl and/or superoxide radicals.⁵³ Beyond the optimum point (1.25 g L⁻¹), an adverse effect of higher catalyst dosage on the degradation rate is observed. In fact, particle agglomeration at higher concentrations limits light penetration as well as 2,4-DCP adsorption on the available surface of the catalyst. Furthermore, with high catalyst dosages, increased turbidity of the suspension and also light scattering by the particles prevent efficient light-harvesting,⁵⁴ which result in a decrease in illuminated active sites on the catalyst surface.⁵⁵

Fig. 9 Effect of (a) catalyst loading and (b) pH of solution for the dechlorination of 2,4-DCP. Experimental conditions: [GE/ZnO/Co₃O₄] = 0.25–1.75 g L⁻¹, pH = 3–9, [2,4-DCP] = 20 mg L⁻¹.

Effect of pH of the solution. In heterogeneous photocatalysis, surface properties of semiconductors play a significant role as the reaction takes place on the surface of the photocatalyst. Therefore, acid–base conditions of the metal oxide surfaces can have a large impact on the adsorption–desorption and photocatalytic degradation performance.⁵⁴ The influence of pH on the photodegradation of 2,4-DCP is examined between pH 3.0 and 9.0, as shown in Fig. 9b. The rate of 2,4-DCP degradation is very fast between pH 3 and 6, but with a further increase in pH the degradation rate declines slightly at pH 7, and significantly beyond pH 8.0 to 9.0. Hence, the degradation of 2,4-DCP is favoured under slightly acidic conditions. This can be explained based on the point of zero charge pH (pH_pzc) of the semiconductors. The pH_pzc for Co₃O₄ and ZnO are found to be 7.3 and 9.0, respectively from zeta potential analysis.^56,57 Therefore, both the Co₃O₄ and ZnO surfaces are positively charged under acidic conditions (pH < 7.3), whereas they are negatively charged under alkaline conditions (pH > 9.0). Hence, under slightly acidic to neutral conditions, more 2,4-DCP molecules are adsorbed on the positively charged surface of the catalyst, thereby enhancing the degradation rate. In contrast, under alkaline conditions, less surface interaction is possible between the negatively charged surfaces and negatively charged phenolic molecules, impeding the adsorption on the catalyst surface, hence degradation rate drastically decreases. However, it is important to note that at pH 8, the Co₃O₄ surface is negatively charged and ZnO is positively charged, and these two effects oppose each other, hence the photodegradation rate of 2,4-DCP remains at an intermediate stage.

Stability of the GE/ZnO/Co₃O₄ hybrid in multiple runs

To check the photostability of the GE/ZnO/Co₃O₄ core–shell hybrid, photodegradation of 2,4-DCP is repeated up to seven cycles under the same conditions, and a set of five cycles is provided in the discussion. The repeatability cycles are performed after washing with ethanol and water several times to remove any unreacted adsorbed DCP molecules from the surface of the core–shell hybrid catalyst to bring the material to its original condition. The GE/ZnO/Co₃O₄ catalyst maintained high photocatalytic activity and does not show any noticeable decline after five successive cyclic operations to degrade 2,4-DCP (Fig. 10). It is worth noting that the GE/ZnO/Co₃O₄ catalyst possesses much higher photostability and activity than other graphene based ternary composites where this type of core–shell hybrid has not been applied.^58–60 It is quite interesting to see that the bare ZnO/Co₃O₄ in the absence of the graphene shell significantly loses its catalytic efficiency after each successive cycle. This is due to photoleaching and photocorrosion, which are normally observed for the metal oxide heteronanostructures.

Fig. 10 Reusability of the GE/ZnO/Co₃O₄ and ZnO/Co₃O₄ hybrid for the 2,4-DCP degradation.

Nevertheless, introduction of a thin graphene shell protects significantly the ZnO/Co₃O₄ core hybrid from photocorrosion or photodissolution and ion leaching, and therefore, provides the catalyst excellent long-term stability and good catalytic performance, and hence offers great promise for environmental purification of aromatic compounds. The catalyst samples before and after photodegradation of 2,4-DCP are also investigated by PXRD and TEM studies. The PXRD patterns (Fig. S8a†) indicate that there is no remarkable alteration or shift of the peak in the crystal structure of the catalyst after 5^th cycle of operation. However, there is only 1% loss of photoactivity after the 5^th cycle, which may be due to some loss of samples during washing. The TEM image of the hybrid after the 5^th cycle shows (Fig. S8b†) the ZnO/Co₃O₄ core still wrapped by graphene shells and hence no noticeable change in morphology is observed for the hybrids.

Scavenger study and photocatalytic mechanism of the GE/ZnO/Co₃O₄ core–shell hybrid

To gain a more in-depth knowledge about the photocatalytic mechanism, benzoquinone⁶¹ (BQ), dimethylsulfoxide⁶² (DMSO) and ammonium oxalate⁶³ have been used as scavenging species in order to investigate the generation and roles of O₂˙⁻, OH˙ and (h⁺), respectively. The scavengers (10⁻³ M) are introduced prior to the addition of the catalyst to the solution of 2,4-DCP. Fig. 11 shows that the degradation of 2,4-DCP is inhibited in the presence of these scavengers in the following order BQ (3.6 × 10⁻³ min⁻¹) > DMSO (1.7 × 10⁻² min⁻¹) > formate (2.4 × 10⁻² min⁻¹).

Fig. 11 (i) Kinetics plots (C_t/C₀vs. time) of the photodegradation of 2,4-DCP under different scavengers; (ii) the kinetic constants for the photocatalytic degradation of 2,4-DCP under visible light irradiation in the presence of (a) GO, (b) GE/ZnO/Co₃O₄, (c) GE/ZnO/Co₃O₄ + DMSO, (d) bare ZnO/Co₃O₄, (e) GE/ZnO/Co₃O₄ + formate and (f) GE/ZnO/Co₃O₄ + BQ ([catalyst] = 1.0 g L⁻¹, pH = 5.0, [2,4-DCP] = 20 mg L⁻¹).

The GE/ZnO/Co₃O₄ exhibits only 15% and 30% degradation efficiency of 2,4-DCP in 150 min under visible light after introducing BQ and DMSO into the system, respectively. However, in the presence of ammonium oxalate, the degradation of 2,4-DCP is not decreased significantly indicating that holes are not main active species formed in the reaction. Thus, it is postulated that O₂˙⁻ followed by OH˙ are the primarily active intermediates substantially contributing to the degradation of 2,4-DCP, while the role of h⁺ is less significant. The catalytic process of the GE/ZnO/Co₃O₄ is further confirmed by the detection of OH˙ by a photoluminescence (PL) technique using terephthalic acid (TA) as a probe molecule.⁶³Fig. 12 shows the changes of PL spectra of TA solution with the catalyst under visible light irradiation with irradiation time.

Fig. 12 PL spectra changed with visible light irradiation time on the GE/ZnO/Co₃O₄ hybrid in a 5 × 10⁻⁴ mol L⁻¹ basic solution of TA (excitation at 325 nm).

A gradual increase in PL intensity at 445 nm is observed with time for GE/ZnO/Co₃O₄ hybrids, which suggests that the fluorescence arises from the chemical reactions between TA and OH˙ produced during catalytic reactions.^64,65 It is obvious that PL intensity is proportional to the amount of produced hydroxyl radicals. Based on the results, a probable photocatalytic mechanism for the GE/ZnO/Co₃O₄ hybrids is proposed and illustrated in Fig. 13. Under visible-light irradiation, electrons (e⁻) are excited from the valence band (VB) of Co₃O₄ to its conduction band (CB), leaving the holes in the VB, thereby forming the electron–hole pairs. The photogenerated electrons in the CB can rapidly transfer to the graphene shell due to the intimate interfacial contact between Co₃O₄ and graphene.

Fig. 13 Schematic diagram of the photocatalytic mechanism of the GE/ZnO/Co₃O₄ core–shell hybrid for the 2,4-DCP degradation under visible light.

As graphene has excellent electron conductivity, the photogenerated electrons rapidly transfer to the graphene shell of the ZnO resulting in significantly enhanced lifetime of photogenerated electron–hole charge carriers. Concurrently, molecular oxygen in the reaction system is activated by reacting with the photogenerated electrons forming superoxide radicals (O₂˙⁻), and the holes of the valence band (VB) of Co₃O₄ react with water to give the hydroxyl radical (OH˙) as evidenced by the control experiments in Fig. 11a. Large amounts of O₂˙⁻ and OH˙ are generated in the system due to the presence of the graphene shell, which act as effective electron transporters and a continuous electron transfer path is established through the graphene shell on the surface of the metal oxide core. 2,4-DCP adsorbed on the surface of GE/ZnO/Co₃O₄ hybrids, firstly undergoes dechlorination to form o-chlorophenol, p-chlorophenol and finally phenol, which subsequently reacts with the active species (OH˙) and converts to para-benzoquinone, which in turn mineralizes into CO₂ and H₂O via some intermediates (Fig. 14). This has also been confirmed by the GC-MS spectrum (Fig. 15) of the degraded products after 2 h of the photocatalytic reaction, which exhibits the peaks of (a) unreacted 2,4-DCP (m/z = 162.15) and its fragmented products, like (b) chlorophenols (m/z = 128.21), (c) phenol (m/z = 94.04), (d) p-benzoquinone (m/z = 108.2), (e) acetic acid (m/z = 60.02) and (f) 1,3-butadiene (m/z = 53.23). However, after 5 h of the photoreaction, no peaks related to 2,4-DCP or chlorophenols are observed (Fig. S9†) in the GC-MS spectrum indicating maximum mineralization of the 2,4-DCP molecules and also their conversion into fragmented products (of ultra-trace level concentrations) like phenol (m/z = 94.04), p-benzoquinone (m/z = 108.2), malonic acid (m/z = 104.01), butenedioic acid (m/z = 116.01), acetic acid (m/z = 60.02) and 1,3-butadiene (m/z = 53.23).

Fig. 14 Proposed photodegradation or dechlorination pathways of the 2,4-DCP in the presence of the GE/ZnO/Co₃O₄ catalyst under visible light.

Fig. 15 GC-MS spectrum of the photo-degraded products of 2,4-DCP after 2 h of the photocatalytic reaction.

Conclusions

In summary, a novel GE/ZnO/Co₃O₄ core–shell hybrid is successfully fabricated through a facile self-assembly approach driven by mutual electrostatic interaction of graphene and a ZnO/Co₃O₄ heteronanostructure prepared from thermal decomposition of a nano coordination polymer. The graphene shells enhance the visible light photocatalytic performance of the ZnO/Co₃O₄ core significantly toward degradation and mineralization of aqueous 2,4-DCP. According to different parametric studies, 91% degradation of 2,4-DCP (20 mg L⁻¹) is achieved in only 150 min of visible light irradiation in the presence of 1.0 g L⁻¹ of the GE/ZnO/Co₃O₄ catalyst at pH 5.0. The photodegradation of 2,4-DCP follows pseudo-first order kinetics with significantly higher rate constants, 3.3 × 10⁻² min⁻¹. It is worth noting that the dechlorination efficiency of the GE/ZnO/Co₃O₄ hybrid is significantly higher than some previous reports. Moreover, the GE/ZnO/Co₃O₄ core–shell hybrid possesses high stability in comparison with normal graphene based hybrids, and has been successfully recycled up to five cycles without any significant loss of photocatalytic activity. The improvement in photocatalytic activity and stability of the GE/ZnO/Co₃O₄ core–shell hybrid can be ascribed to an increase in the contact surface between the metal oxide core and the graphene shell more effectively than the traditional graphene–metal oxide composites. We presume that the association of graphene contact serves as a continuous path for rapid shuttle of electrons producing more reactive species (superoxide and hydroxyl). On the other hand, it also helps preventing the aggregation and photocorrosion of the ZnO/Co₃O₄ core in a more effective way. Overwhelmingly, the higher rate of mineralization (93% in 5 h) offered by the GE/ZnO/Co₃O₄ hybrid would make it a potential candidate for visible light photocatalytic degradation or dechlorination of chlorinated phenols, wastewater treatment as well as in different energy conversion systems.

Acknowledgements

The author, MR, thanks the CSIR for a research fellowship. Further, the authors extend their acknowledgments to CRF-IITKGP for TEM, HRTEM, and EDX analyses. The authors also acknowledge the SERB, New Delhi for financial support (Project code: SDP & Ref. No. SR/FT/CS-146/2011) and DST-FIST for XPS facility (Department of Physics, IIT Kharagpur).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5pp00305a

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