ZnO Heterojunctions with Enhanced Photocatalytic Activity

Article Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Novel Green Biomimetic Approach for Preparation of Highly Stable Au-ZnO Heterojunctions with Enhanced Photocatalytic Activity Manoj Kumar Choudhary,*,†,‡,§ Jyoti Kataria,‡ and Shweta Sharma*,§ †

Nanomaterial Research Laboratory, Department of Chemistry, Guru Nanak National College, Doraha, Ludhiana 141421, Punjab, India ‡ P. G. Department of Chemistry, Panjab University Research Centre, GGDSD College, Sector 32-C, Chandigarh 160030, India § Institute of Forensic Science and Criminology, Panjab University, Chandigarh 160014, India S Supporting Information *

ABSTRACT: A simple, ecofriendly, and biomimetic approach using cumin seeds extract (CSE) was developed for the formation of Au-ZnO Schottky contact without employing any chemical capping agents or stabilizers. The various unique phytoconstituents available in cumin seeds extract synergistically convert Au3+ ions into Au0 on the surface of ZnO, as each phytoconstituent is unique in context to its molecular structure and properties. The as-prepared biogenic Au-ZnO hybrid composites were examined using various spectroscopic and microscopic techniques. The TEM investigation and XRD patterns clearly depict the well dispersed AuNPs with the size range 10−15 nm and face centered cubic lattice on wurtzite ZnO nanostructures. The optical study of the nanocomposites showed two absorption bands: one intense band around 390 nm, which corresponds to ZnO, and a second broad band approximately around 540 nm, which corresponds to Au. The photocatalytic efficacy of Au-ZnO nanocatalysts was investigated by observing the mineralization of an aqueous solution of methylene blue (MB) dye under a 200 W tungsten filament lamp as visible light source. The apparent rate constants were also calculated for degradation processes, and it has been observed that 1 and 3 wt % Au-ZnO nanocomposites respectively have 2.27 and 3.2 times higher photoactivity, compared to pure ZnO. This enhanced photoactivity of biogenic Au-ZnO composite materials was resultant from formation of stable and effective Schottky contact between Au metal and ZnO surfaces. KEYWORDS: nanocomposites, Au-ZnO, cumin seeds, biogenic synthesis, plant extract

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INTRODUCTION The significant applications of semiconductor photocatalysts toward the treatment of wastewater and remediation of environmental pollutants has been widely studied.1 For example, UV light driven photocatalytic degradation of organic waste involving semiconductors such as TiO2, SnO2, and ZnO, etc. has attracted remarkable attention from research groups.2−4 ZnO has a wider band gap of around 3.32 eV and, thereby, can be considered as a suitable candidate in photocatalysis and dyesensitized solar cells.5 However, because of the higher band gap value, its capacity of light absorption is restricted to a very small region of the solar spectrum.6 Moreover, it also suffers from a major limitation of reunion of the photogenerated hole and electron pair which leads to deceleration of photocatalytic activity. In comparison to a single semiconductor photocatalyst, composite semiconductors driven photocatalysis has also been broadly investigated to amplify the photoresponse in the visible region.7−9 In the recent past, alloying of noble metal NPs and semiconducting nanomaterials to make plasmonic photocatalysts has affectionate researchers from both pure and applied © XXXX American Chemical Society

sciences and together they have opened unique doors of science and technology to surpass the confined efficiency of photocatalysts and photovoltaic devices.10 Surface modification/ decoration of semiconductors photocatalysts with noble metal nanocrystals dramatically enhances the photocatalytic efficiency of these catalysts toward decaying of toxic organic dyes11 and pollutants.12 These plasmonic metal−semiconductor nanocomposites exhibiting significant properties such as surface plasmon resonance (SPR)13−16 and Schottky defects, synergistically produce new tracks, which depend upon the interfacial interconnection involved between the metal and the semiconductor.17 In another context, the metal nanoparticles raise their nanoreceivers alluring photoactivation and boosting effective charge segregation and hence increase absorption of light in the visible region. This finally results in the complete transport of charge carriers that enhance photocatalysis.18 Received: February 17, 2018 Accepted: March 28, 2018 Published: March 28, 2018 A

DOI: 10.1021/acsanm.8b00272 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials Scheme 1. Plausible Mechanism for the Preparation of Au-ZnO Nanocomposites

Among various noble metal−semiconductor photocatalysts, Au-ZnO nanocomposites exhibit outstanding physicochemical properties such as rapid transfer of resonance energy,19 the nonlinear optical property,20 and have found multitudinous applications in photocatalysis,21 sensing,22 and dye-sensitized solar cells.10 Therefore, there are immense opportunities to explore some simple and low-cost synthesis procedures to produce Au-ZnO nanocomposites. In recent years, the fabrication of Au-ZnO nanocomposites have been achieved by employing various physical and chemical processes such as simple hydrolysis,23 microwave irradiation,24 chemical reduction,25 hydrothermal process,26 solvothermal technique,27 seed mediated growth processes,28 and electrophoretic processes,29 etc. All above-mentioned synthetic techniques are highly expensive and utilize various hazardous materials. Some of the processes also require expert manual skills for their operation. For instance, hydrothermal technique requires maintaining of high temperature and pressure, whereas, in the chemical reduction process, toxic materials such as hydrazine, citrate,or borohydride are used as reducing agents and some organic compounds such as glutathione,25 oleylamine, and tetraline,28 etc., are utilized as capping/stabilizing agents that pose a potential risk to biological as well as environmental system. Moreover, removal of theses organic molecules from products becomes a difficult task and may result in the formation of insulating layers around nanomaterials, hence restricting the formation of efficient Schottky contacts. To the best of our knowledge, a facile, low-cost, and green preparation of the Au− ZnO nanocomposites with highly extended photocatalytic efficacy under irradiation of visible light has not been reported so far. Therefore, there is a growing need for the development of some advance ecofriendly methods through biomimetic approaches. In this report, for the first time we demonstrate a simple, costeffective, and environmentally friendly phytogenic synthetic procedure for the fabrication of Au-ZnO nanocomposites using cumin seeds extract (CSE) as reducing as well as a stabilizing agent. This phytogenic procedure for the fabrication of stable AuZnO composites is simple, cheap, and environmentally friendly compared to common reported protocols. Cumin is a small flowering herbaceous plant belonging in the Apiaceae family in the genus Cuminum. Its scientific name is Cuminum cyminum (C. cyminum). The seeds are native to the Middle-East Asian region and popular for their distinctive spice flavor. The dried seeds of cumin are commonly featured ingredients in Middle-Eastern, Western Chinese, North African, North Mexican, and Indian

cuisine, and nowadays, this spice is grown all over the world for its pleasantly aromatic seeds. The seeds of C. cyminum have been reported to exhibit antioxidant,30 antidiabetic, antimicrobial, and immunomodulatory activities.31,32 Further, these prepared composite materials have been successfully investigated as efficient plasmonic photocatalysts for visible light driven mineralization of aqueous solution of methylene blue (MB) dye.

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EXPERIMENTAL SECTION

Preparation of Cumin Seeds Extract. For the preparation of aqueous cumin seeds extract (CSE), first, seeds were washed thrice with distilled water and dried in air for 48 h. A 5 g amount of cumin seeds was packed in a thimble made of Whatman filter paper, placed in a Soxhlet extractor, and extracted with methanol for 6 h. The green methanol extract was filtered over anhydrous Na2SO4, and the solvent was evaporated over a rotary evaporator, and 1.30 g of a brown sticky compound was obtained. A 1 g amount of this compound was dissolved in 100 mL of double distilled water and used for further experiment. Biogenic Preparation of Au-ZnO Nanocomposites. The biogenic preparation of Au-ZnO nanocomposites follows a facile surfactant free phytochemical reduction route. In this simple method, 0.2 g of ZnO nanomaterial (Sigma-Aldrich; particle size < 100 nm) was ultrasonically dispersed in a 15 mL solution of water and ethanol (2:1 (v/v); H2O:C2H5OH) in a test tube. Then a calculated amount of 1 mM HAuCl4·3H2O (Sigma-Aldrich) solution was introduced slowly through a hypodermic syringe. After 30 min of stirring, the aqueous extract of cumin seeds (8:2 (v/v); HAuCl4:CSE) was added and stirred for 6 h for complete conversion of Au3+ ions into Au0 on ZnO nanosurfaces. During this period, the mixture solution turned to violet indicating the reduction of Au3+ to Au0. Then suspension was centrifuged at 5000 rpm for 5 min and washed 4 times with double distilled water and twice with absolute ethanol for complete removal of chloride ions and excess of biomolecules. The catalysts were dried at 80 °C for 12 h and labeled as xAu-ZnO, where x is the weight percent of Au in ZnO. Characterization of Prepared Catalysts. The samples for transmission electron microscopy (HRTEM; FEI, TECNAI G2 20 TWIN, 200 kV) were prepared by suspending the composite materials in ethanol and sonicated for 5 min. A drop of very dilute solution was placed on the carbon coated copper grid (Ted Pella, USA, 200 mesh), and the grid was allowed to stand for a few minutes. The surplus solution was separated with filter paper, and the grids were dried before measurement. The chemical constitution was evaluated using energy dispersive X-ray spectroscopy (EDX). The powder X-ray diffraction patterns of ZnO and Au-ZnO nanocomposites were obtained (XRD; PANalytical X’PERT−PRO) at a voltage of 45 kV and a current of 40 mA, using Cu Kα (λ = 1.54 Å) incident radiation over a 2θ range of 10− 80°. The surface elemental and chemical constituents were examined using X-ray photoelectron spectroscopy (XPS; FEI, PHI 5000 Versa Probe II). The photoluminescence spectra were taken with an excitation wavelength of 350 nm (PL; PerkinElmer LS-55 spectrophotometer). B


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ACS Applied Nano Materials The optical absorbance was documented with T-90+ UV−visible spectrometer (PG Instruments, England). The diffuse reflectance spectra (DRS) were recorded in the range of 300−700 nm, with an integrating sphere accessory (IS 19-1). Photocatalytic Efficacy of Prepared Catalysts. To evaluate the photocatalytic efficacy of prepared Au-ZnO nanocomposite materials, MB was selected as representative dye pollutant. A 100 mL aliquot of 10 μM MB and 20 mg of catalyst were collected in a photoreactor, and before irradiation, the mixture was stirred for 30 min under dark to achieve the adsorption−desorption equilibrium of MB on the surface of the photocatalysts. A 200 W tungsten filament lamp (Philips) was used as a source for visible light, and the mixture solution was placed under the lamp for photodegradation. To avoid the heating of the mixture solution, the temperature of the photoreactor was maintained to 30 °C by circulating cold water. For monitoring of the dye degradation rate, the samples were collected at regular intervals of 30 min and the catalysts were separated by centrifugation (6000 rpm for 10 min). Using UV−vis spectrophotometer, the concentration of dye pollutant was measured before and after degradation.

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Figure 1. FT-IR spectra of the pure ZnO and Au-ZnO nanocomposites.

RESULTS AND DISCUSSION Synthesis and Characterization of Au-ZnO Composites. The CSE was employed as an innovative material for the fabrication of highly stable Au-ZnO nanocomposites, which is an ecofriendly, surfactant free, and cost-effective method. Scheme 1 presents the plausible reaction mechanism for the anchoring of Au 0 on ZnO nanosurfaces. The biomolecules such as polyphenolic acids and (or) flavonoids present in cumin seeds, which exhibits significant antioxidant properties, provide a bunch of electrons and are responsible for reduction of Au3+ ions to Au0 and they themselves resonance stabilized to corresponding quinones.33 Then these stabilized quinones may coordinate to AuNPs and stabilize them on the surface of ZnO. This seed extract mediated preparation and stabilization of Au-ZnO nanocomposites highlights the advantage of this approach for the fabrication of numerous stable nanocomposites materials with various heterostructures. Another advantage of this synthetic procedure to fabricate the Au-ZnO nanocomposites by using plant extracts may facilitate the formation of effective transfer of charge carriers through Schottky junctions.34,35 Generally, cationic/anionic surfactants or other organic molecules are used for the fabrication of noble metal−semiconductor nanocomposites, and removing excess organic coordinating molecules is a difficult task and therefore may serve as insulating layers and, hence, prevent the formation of Schottky junctions.35 Moreover, plant extract assisted preparation does not employ high-energy intake36 and the process occurs normally in aqueous medium at ambient temperature conditions, suggesting it is an efficient and cost-effective technique for hybrid nanocomposite synthesis. FT-IR Study. The FT-IR spectra of pure ZnO and Au-ZnO nanocomposite materials were acquired to obtain the structural features of prepared nanomaterials. As found in these spectra (Figure 1), the peaks around 3400−3450 cm−1 are ascribed to vibration due to the stretching mode of surface hydroxyl groups (-OH). The small peaks around 1645.16 cm−1 in the case of 3 wt % Au-ZnO and 1650.21 cm−1 in 1 wt % Au-ZnO composite materials can arise due to stretching vibrations of aromatic CO groups of quinones, which further reflect the contributing role of carbonyl groups in the stabilization of gold nuclei on the ZnO surface. Further, small blue shiftings in wavenumber were noticed for these distinguishing peaks of Au-ZnO hybrid structures as compared with pure ZnO nanomaterial, signifying the existence of pronounced interaction between AuNPs and ZnO material. Hence, these FT-IR investigations effectively

reveal that AuNPs have been strongly accumulated on the ZnO nanostructures. X-ray Diffraction Study. The XRD pattern of Au-ZnO nanocomposites with two different gold contents (1 and 3 wt %) and pure ZnO is presented in Figure 2. The peaks corresponding

Figure 2. XRD patterns of the pure ZnO and Au-ZnO nanocomposites.

to a typical structure of wurtzite ZnO (JCPDS File No. 36-1451) were observed in all three patterns. In the case of Au-ZnO nanocomposites, two mixed sets of XRD peaks were observed. One additional peak at 38.37 in 1 wt % Au-ZnO and two peaks (indicated with an asterisk (*)) at 38.37 and 44.78 were observed in 3 wt % Au-ZnO, which are the distinctive peaks of metallic AuNPs (JCPDS Card No. 04-0784). Further, it can be clearly seen that there was no enormous displacement in the position of peaks, which substantiates that the as-prepared nanocomposite catalysts were comprised of ZnO and Au phases. Transmission Electron Microscopy and Energy Dispersive X-ray Study. Transmission electron microscopy metaphors of pure ZnO and Au-ZnO nanocomposites are revealed in Figure S-1 (Supporting Information) and Figure 3a− d, respectively. TEM pictures of Au-ZnO showed well dispersed spherical AuNPs, appended to the edges as well as surfaces of the ZnO nanostructures. The particle size distributions of AuNPs are shown in (insets) Figure 3a,c. The average particle sizes of gold nanoparticles, as measured using ImageJ software, are approximately15 ± 3 nm (in 1 wt % Au-ZnO) and 10 ± 2 nm (in 3 wt % Au-ZnO). The small proliferation in the AuNPs size in the case of C


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ACS Applied Nano Materials

Figure 3. TEM images (a, b) for 1 wt % Au-ZnO and (c, d) for 3 wt % Au-ZnO. EDX spectra (e and f) for 1 and 3 wt % Au-ZnO, respectively.

1 wt % Au-ZnO can be owed to the coagulation of Au particles on the semiconductor surfaces. Further, the HRTEM image (Figure 3d) showed the formation of a strong interfacial contact and continuity of lattice fringes between AuNPs and ZnO. The lattice fringes with d-spacing (d = 0.27 nm) matches with the (002) crystallographic plane of ZnO. The d-spacing (0.24 nm) resembles the (111) plane of the face centered cubic structure of AuNPs. The selected area electron diffraction (SAED) pattern of 3 wt % Au-ZnO showed (Supporting Information, Figure S-2) a ring pattern, indicating the crystalline nature of prepared samples. Some limited designated planes in the SAED patterns were marked, with the findings fairly consistent with the XRD.

Further, the quantitative estimation of gold in the samples was confirmed with EDX spectroscopy (Figure 3e,f).37 It was clear from the TEM and EDX analysis that AuNPs were effectively anchored on the surface of ZnO by green phytochemical reduction method. X-ray Photoelectron Spectroscopy Study. For further evaluation of the chemical and elemental state of biogenically prepared Au-ZnO nanocomposites using CSE, XPS analysis was carried out on 3 wt % Au-ZnO nanocomposite. The corresponding results of 3 wt % Au-ZnO and pure ZnO are shown in Figure 4a. The XP spectrum of Au-ZnO showed only peaks corresponding to C, O, Zn, and Au. No other peak had D


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Figure 4. XPS spectra (a) complete survey spectra of pure ZnO and 3 wt % Au-ZnO nanocomposite. High resolution spectra of (b) Zn, (c) O, and (d) Au.

Diffuse Reflectance Spectroscopy Study. The results corresponding to electronic spectra of pure ZnO and as-prepared samples of 1 and 3 wt % Au-ZnO, using DRS technique are shown in Figure 5. A strong absorption edge at around 390 nm

been perceived, which verifies that the biogenically prepared AuZnO nanocomposite was pure and only constituted of three elements, i.e., Zn, O, and Au, which was further in reliable agreement with the above-described results of EDX and XRD. Moreover, examination of XPS results again validates that synthesized Au nanoparticles were successfully adsorbed on ZnO nanosurfaces. High-resolution spectra of Zn 2p are shown in Figure 4b. It was observed that the binding energy for 2p1/2 and 2p3/2 remained the same, i.e., 1043.4 and 1020.2 eV respectively in pure ZnO and Au-ZnO nanocomposite. The 23.2 eV difference between the two binding energies further confirms that Zn was present in the Zn2+ oxidation state in both samples. Further, as compared to pure ZnO, no change in chemical state of O was observed in the Au-ZnO nanocomposite, which can be clearly seen in Figure 4c. The asymmetric peak of the 1s level of O can be fitted into two symmetric peaks, i.e., 529.1 and 530.5 eV. The first peak at 529.1 eV denotes the 1s level of O in the ZnO, which is enveloped by Zn atoms in the lattice, and the second peak at 530.5 eV is ascribed to chemisorbed oxygen of surface hydroxyls. The Au 4f (Figure 4d) shows two small peaks positioned at 86.51 and 82.2 eV, which were ascribed to Au 4f5/2 and Au 4f7/2, respectively. The locations of Au 4f peaks were surprisingly shifted to lesser binding energies related to those of bulk Au (Au 4f5/2, 87.14 eV; Au 4f7/2, 83.04 eV).38 This shift might be due to the accumulation of negative charge on Au nanoparticles, during interaction with ZnO. In other words, the donor level of ZnO is almost equivalent to the Fermi level of Au (5.4 eV);39 therefore, there might be an opportunity for electron transport from ZnO to the Au, which can lead to an increase in the charge density on the surface of AuNPs.40

Figure 5. DRS of the pure ZnO and Au-ZnO nanocomposites.

was observed, which is assigned to ZnO semiconductor because it shows absorption only in the UV region. On comparing with pure ZnO, the Au-ZnO nanocomposites showed another absorption band in the visible range (approximately at 540 nm), which had been assigned to the surface plasmon resonance (SPR) absorption of gold. Moreover, due to this SPR absorption of AuNPs on the ZnO surface, the photocatalytic activity can be remarkably enhanced. Furthermore, with the increase in percentage of Au, the intensity of absorption in the visible E


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excitation wavelength of 350 nm at room temperature, for pure ZnO and Au-ZnO nanocomposites with different amounts of Au. A strong UV emission peak located at around 381 nm corresponds to near-band-edge emission of the wide band gap of ZnO, and two weak bands in the range of 410−510 nm were observed in the PL spectrum of pure ZnO as well as those of 1 and 3 wt % Au-ZnO nanocomposite samples, which could be respectively assigned to constrained excitons and imperfections situated on the surface of nanosized ZnO. The stronger intensity suggests the presence of a large number of defect sites in ZnO. The intensities of all three bands were observed to decrease in the case of noble metal modified ZnO nanoparticles, which is in agreement with the Stern−Volmer quenching.44,45 This described that the electron−hole pairs were well segregated and the reunion process was restricted. The relatively low intensity peaks in the case of Au-ZnO nanocomposite samples proved the inhibition of reunion of electrons and holes and the decrease in the number of surface defects, respectively. Photocatalytic Performance of Au-ZnO Nanocomposites. The photocatalytic efficacy of the prepared Au−ZnO nanocomposites under visible light treatment was investigated for the mineralization of MB dye as model pollutant. UV−vis spectra for the mineralization of MB dye in the presence of pure ZnO and 1 and 3 wt % Au-ZnO nanocomposites are shown in Figure S-3a−c (Supporting Information), respectively. Panels a and b of Figure 7 depict the photocatalytic degradation (Ct/Co) rate and degradation kinetics (ln[Co/Ct] vs time) of MB, respectively. Here, Ct is the absorption corresponding to concentration of dye solution at all intervals of time during irradiation, and Co is the absorption corresponding to the initial concentration (time 0). Figure 7a depicts the comparative concentration of MB dye residing in the suspension as a function of the time without a catalyst, with 3 wt % Au-ZnO nanocomposite, but not irradiated with visible light (dark conditions), with pure ZnO and Au-ZnO (1 and 3 wt %) nanocatalysts. The results for the mineralization of MB with 3 wt % Au-ZnO showed 8% decrease in the absorbance under dark condition. This might be because of the surface adsorption of dye onto the catalyst. On the other hand, when the MB dye solution was exposed to a visible light source, without any catalyst, 20% degradation of dye was observed, which can be ascribed to photolysis of MB dye molecules. Moreover, a higher degradation rate was observed with Au-ZnO compared to pure ZnO nanopowder. Approximately, 88% and 76% degradation of MB under visible light irradiation for 3 h was noticed using 3 and 1 wt

region was observed to be increased, which could be ascribed to the rise of defect locations in the crystal structure of the catalyst which can prevent the reunion of the charge transporters and improves the photocatalytic efficiency of the photocatalyst.41 The pure ZnO and synthesized Au-ZnO (1 and 3 wt % Au-ZnO) nanocomposites exhibited absorption edge (λ) at 388.7, 395.0, and 399.7 nm, respectively. The band gap energy (Eg) has been calculated using the following equation. 1239.8 Eg /eV = λ /nm The energy band gaps calculated for pure ZnO, 1 wt % AuZnO, and 3 wt % Au-ZnO catalysts were found to be 3.19, 3.14, and 3.10 eV, respectively. Photoluminescence Spectroscopy Study. Since photocatalytic activity is entirely dependent on the lifespan of photogenerated electron−hole pairs, therefore the segregation and reunion of photogenerated holes and electrons are two simultaneous competitive routes in a photocatalytic reaction.42 To further examine photochemical and electronic characteristics of prepared photocatalyst samples, photoluminescence spectra were evaluated, from which the efficacy of transportation of charge and simultaneous entrapping of charge carriers can be obtained.43 Figure 6 shows the PL spectra acquired with an

Figure 6. Room temperature PL of the pure ZnO and Au-ZnO nanocomposites.

Figure 7. (a) Comparison of photodegradation of MB, with pure ZnO and Au-ZnO nanocomposites and (b) kinetic study of photocatalytic degradation of MB. F


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surface plasmon resonance of gold nanoparticles and result into the electron−hole pair separation in AuNPs. These migrated electrons in the ZnO conduction band are further entrapped by the oxygen (O2) dissolved in water and result in the formation of highly oxidative species such as superoxide radical anion (•O2−) and hydroxyl radicals (OH•).51 Further, the holes created in AuNPs also help in the formation of hydroxyl radicals (OH•). This visible light assisted production of •O2− and OH• radicals might be responsible for mineralization of dye pollutants.52,53 Furthermore, compared to pure ZnO, charge segregation was also shown by decrease in the photoluminescence intensity of Au-ZnO composite materials. In a broad sense, a photoexcited electron can react with oxygen molecules adsorbed on the AuZnO surfaces to generate (•O2−) radicals. Consequently, AuZnO shows extended visible light induced photoactivity compared to pure ZnO. Thus, it can be said that photocatalytic efficacy has a constructive relationship with the rate of radical formation; i.e., the faster the formation of radicals, the higher will be the photocatalytic efficacy of the photocatalyst.51,54 In total, these outcomes suggest that AuNPs in contact with ZnO helps in increase formation of reactive radical species and, hence, facilitate the mineralization of dyes. Stability and Reusability of the Au-ZnO Nanocomposites. The steadiness or firmness of the composite materials, such as Au-ZnO nanocomposite, is a key issue of interest. Therefore, the stability of the Au-ZnO nanocomposites was examined by sonicating the suspension of 1 and 3 wt % Au-ZnO nanocomposite in water for 1 h. Then after centrifugation, solutions were analyzed for the discharge of AuNPs using UV− visible spectrophotometer and no band corresponding to Au0 SPR was perceived in the spectra (Supporting Information, Figure S-4a,b). The reusability of the prepared nanocomposites was investigated using the 3 wt % Au-ZnO nanocomposite for three times. Catalyst collected by centrifugation after the first reaction was washed 3−4 times with distilled water and reused (Figure 8). Approximately 90% of the initial photocatalytic

% Au-ZnO as catalysts, respectively. These results further signify that biogenic deposition of 3 wt % of Au onto ZnO surface is the best deposition amount of gold. The change in the MB concentration (Figure 7b), for the pure ZnO and 1 and 3 wt % Au-ZnO nanocomposites revealed pseudo-first-order kinetics rendering to the equation presented elsewhere.46 The rate constant (k) values of pure ZnO and 1 and 3 wt % AuZnO nanocomposites for the mineralization of dye solution were depicted in Table 1. The k values for the visible light driven Table 1. Band Gap and Rate Constant Values of Various Employed Photocatalyst for Photocatalytic Degradation of MB Dye Solution sample no.

catalyst

band gap (eV)

rate constant × 10−3 (min−1)

R2a

1 2 3

pure ZnO 1 wt % Au-ZnO 3 wt % Au-ZnO

3.19 3.14 3.10

3.61 8.15 11.5

0.994 0.993 0.994

a 2

R = Correlation coefficient.

mineralization of MB using 1 and 3 wt % Au-ZnO nanocomposites were found to be 2.27 and 3.2 times respectively greater, compared to pure ZnO. Further, it has also been observed that these biogenically prepared Au-ZnO nanocomposites are highly photoactive compared to reported AuZnO nanocomposites synthesized by chemical methods.25,47,48 Scheme 2 presents the mechanistic outline of the visible light induced charge segregation, transference, and mineralization Scheme 2. Proposed Mechanism for the Visible Light Driven Degradation of MB Using the Au-ZnO Nanocomposites

process. The enhanced spotted photocatalytic efficacy of AuZnO can be discussed, based on the work function of Au and ZnO. Noble metal−metal oxide nanocomposites can be utilized as extremely effective photocatalysts because there is synergistic effect between surface defects on metal oxide and the surface plasmon resonance effect of the noble metals.13,17 Usually, a Schottky junction is developed during alloying of two substances with dissimilar work functions, and there occurs a migration of electrons from the material of the lower work function to the material with the higher work function until the two states acquire an equilibrium position to create a new Fermi energy level.34,35,38,49,50 This equilibrium position of newly created Fermi level between two different materials generates an incorporated electric field in the territory near the interface, which enhances the splitting of photogenerated electron−hole pairs and, therefore, improves the photocatalytic efficacy.11,13,21,26,45 When illuminated with a visible light source, the electrons are migrated into the conduction band of ZnO via

Figure 8. Reusability of 3 wt % Au-ZnO catalyst for the degradation of MB for three consecutive cycles.

efficiency was preserved after recycling for three times, which discloses the good reusability of biogenically fabricated composite materials as a photocatalyst. The outcomes showed that the photocatalytic efficacy of pure ZnO was significantly improved by biogenically anchoring the AuNPs onto the ZnO nanostructures, which can be attributed to G


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Panjab University, Chandigarh; and Thapar University, Patiala for providing the TEM, XPS, XRD, and EDX facilities, respectively.

development of close Au-ZnO Schottky contact resulting from new unconventional strategy.

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CONCLUSIONS Here, we have described a clean, low-cost, and environmentally friendly methodology for the fabrication of highly stable Au-ZnO nanocomposites using a green biosynthetic tool, employing aqueous cumin seeds extract, which served a dual role of reducing and capping agent. Since the modification of semiconductor surfaces plays an important role in the photoresponse toward visible light, therefore, a strong Schottky contact must be made at the interface between the plasmonic metal and semiconductors. Traditionally, chemical or hydrothermal treatments are employed for the formation of a heterojunctions between the two, which may affect the photoactivity of prepared materials in one way or the other. The Au−ZnO heterojunctions prepared using the aqueous extract of cumin seeds exhibited an efficient Schottky junction formation and tremendous stability as confirmed by various techniques employed in this study. Moreover, the as-prepared biogenic catalyst exhibited highly enhanced visible light driven photocatalytic activity toward the mineralization of methylene blue dye, compared to pure ZnO and other reported metal oxide semiconductors. Furthermore, the materials (cumin seeds) used in the synthesis of nanocomposites are not harmful for the humans as well as the environment. Therefore, the biomimetic approach presented in this work can be continued in the facile preparation of other multifaceted semiconductor heteroassemblies with various significant applications.

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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/acsanm.8b00272. Figures S-1 to S-4 showing TEM images of pure ZnO, SAED pattern of 3 wt % Au-ZnO nanocomposite, UV−vis spectra of photodegradation, and stability test results (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*(M.K.C.) E-mail: [email protected]. Tel.: +91-1628257097; +91-814-6556-719. Fax: +91-1628-256731. *(S.S.) E-mail: [email protected]. Tel.: +91-172-2534121; +91-987-2688-577. ORCID

Manoj Kumar Choudhary: 0000-0002-5658-8814 Shweta Sharma: 0000-0002-1936-2947 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are highly grateful to the Northern Regional College Bureau, University Grants Commission (UGC), New Delhi, India for providing financial assistance under a minor research project scheme (Grant F.8-4(107)/2015(MRP/NRCB)). M.K.C. is also grateful to the Principal and Management of Guru Nanak National College, Doraha, India for their constant support and encouragement and for providing necessary work space and laboratory facilities for fulfillment of this research project. We are also thankful to the SAIFs at AIIMS-New Delhi; IIT-Kanpur; H


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ZnO Heterojunctions with Enhanced Photocatalytic Activity

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