Size-tunable ZnO nanotapes as an efficient catalyst

Applied Catalysis A, General 562 (2018) 58–66

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Applied Catalysis A, General journal homepage: www.elsevier.com/locate/apcata

Size-tunable ZnO nanotapes as an efficient catalyst for oxidative chemoselective CeB bond cleavage of arylboronic acids Shreemoyee Phukana, Abhijit Mahantaa,b, Md. Harunar Rashida, a b

T

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Department of Chemistry, Rajiv Gandhi University, Rono Hills, Doimukh, 791 112, Arunachal Pradesh, India Department of Chemical Sciences, Tezpur University, Tezpur, 784 028, Assam, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Zinc oxide Nanotapes Heterogeneous catalysis Ipso-hydroxylation Phenol

Herein, we report a simple but effective chemical approach for the synthesis of size-tunable ZnO nanotapes by precipitation method in the presence of phytochemicals present in the flower extract of Lantana camara plant. The electron microscopic study confirmed that the size of ZnO nanotapes can be systematically controlled by varying the concentration of either flower extract or metal ions and the flower extract played the key role in controlling the growth of ZnO nanotapes. The phase and crystalline analysis was carried out by X-ray diffraction method which indicated that ZnO nanostructures are highly crystalline in nature and are free from any impurities. The synthesized ZnO nanostructures exhibited interesting optical properties as investigated by UV–vis absorption and photoluminescence spectroscopy. Further the surface functionalities affect the optical properties of ZnO nanostructures which possess relatively strong UV emissions; a blue emission and a green emission. The synthesized ZnO nanostructures showed excellent catalytic properties in the ipso-hydroxylation of different aryl/ hetero-arylboronic acid to phenol in a relatively greener reaction conditions. These catalysts are highly stable and are re-usable upto six cycles of ipso-hydroxylation without losing its catalytic properties.

1. Introduction As a semiconductor material, nanostructured zinc oxide (ZnO) has attracted considerable attention of researchers because of their fantastic physical and chemical properties, which are significantly different from their bulk counterparts. ZnO nanostructures possess excellent chemical stability, good electrical and piezoelectric properties, broad radiation absorption range, high electrochemical coupling coefficient, and high photostability [1,2]. Most importantly ZnO is biocompatible, biodegradable, and biosafe for medical and environmental applications [3]. As a result ZnO nanomaterials find wide range of applications in photocatalysis [4–9], optoelectronic devices [10,11], biosensors [10,12], solar cells [13–15], light emitting diodes [16,17], catalysis [18–20] etc. Due to the versatile applications; researchers adopted various synthetic methods; such as sol-gel [21,22], solvothermal [6,23], hydrothermal [15,24], co-precipitation [4,7,18,25], and other vapour deposition techniques for synthesis of size and shape tunable ZnO nanostructures. However, most of the synthetic approaches are either expensive or involved toxic chemicals. Hence, the development of an efficient, simple and cost effective synthetic technique for size or shape controlled synthesis of ZnO nanostructures is important to answer the demand for exploring the potential applications of ZnO nanostructures. Compared

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Corresponding author. E-mail address: [email protected] (Md. H. Rashid).

https://doi.org/10.1016/j.apcata.2018.05.037 Received 26 March 2018; Received in revised form 18 May 2018; Accepted 29 May 2018 Available online 30 May 2018 0926-860X/ © 2018 Elsevier B.V. All rights reserved.

to the different synthetic approaches, precipitation is relatively more popular method since it is easier, low-cost, and environmentally friendly, low-temperature process, suitable for large scale production and can be performed under mild reaction conditions. Fortunately, surfactants, polymers and other organic additives coupled with precipitation method are able to modify the growth kinetics and thereby control the size and shape during their formation. However, toxic additives are used in most of the cases which limits their applicability for environmental application and health hazards. In this regards, recently different research groups have utilized plant extract as green additive to tune the size or shape of ZnO nanostructures [7,26]. In this report, we adopted a facile wet chemical precipitation strategy to synthesize ZnO nanotapes in methanol using tetrabutylammonium hydroxide (TBAH) as a base in presence of flower extract of Lantana camara as additive under mild reaction conditions. Lantana camara is a popular ornamental and garden plant growing up to 2–4 m in height, with a number of flower colours viz. yellow, red, pink and white [27]. It also grows naturally at road or river side’s and is a notorious invasive weed found throughout India. Research on the chemical composition of Lantana camara plant revealed the presence of terpenoids, steroids, flavonoids, alkaloids and other polyphenols as major constituents which are responsible for the capping of metal


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and allowed to dry in shade. The dried flowers were then grinded into powder in a blender and stored in an airtight container. The powdered materials (50 g) were then taken in a soxhlet apparatus and extracted the phytochemicals present in the flower in methanol (500 mL) at 60 °C for 5 h. The solvent was then evaporated in a rotary evaporator (Buchi) to isolate the solid product. The isolated solid product was dried in vacuum at 60 °C for overnight. The dried thick oily mass (11.5 g, 23%) was stored at 4 °C in a refrigerator for further use. For the synthesis purpose, an 10.0 gL−1 solution of the extract was prepared in methanol and filter through cotton to remove any suspended substance.

nanoparticles as claimed in the recent reports [7,27]. Since, this plant is readily available in the North-East region of India; it is chosen over other conventional medicinal or edible plants for the synthesis of ZnO nanostructures. In the domain of synthetic organic chemistry, phenol and its derivatives are very important building blocks for the construction of significant structural moieties such as biologically active natural products, pharmaceuticals, agrochemicals and polymers etc. [28–30]. Traditionally, phenols are prepared by the pyrolysis of the sodium salt of benzene sulfonic acid, hydrolysis of diazonium salts or nucleophilic aromatic substitution of aryl halides possessing electron-withdrawing substituent which mostly suffer from several drawbacks like harsh reaction conditions, poor functional group compatibility and less substrate scope [28,31]. In recent years, ipso-hydroxylation of arylboronic acid has emerged as an important synthetic tool for phenolic derivatives due to mild and non-toxic nature of the starting material and the availability of diverse functional groups. More importantly arylboronic acids are highly stable under air and moisture, commercially available, and cost effective [32]. ipso-hydroxylation of arylboronic acids via C−B bond cleavage with hydrogen peroxide was first reported in 1954 [29]. In this context, lots of efforts have been devoted towards hydroxylation of arylboronic acids via C−B bond cleavage utilizing different reagents/ catalysts. Some of the notable examples include N-oxide [33], biosilicaH2O2 [34], Al2O3-H2O2 [35], KOH-TBHP [36], iron(III) oxide [37], etc. Lately, Kandasamy and his co-workers reported a chemoselective hydroxylation of arylboronic acids using urea-H2O2 complex [38]. Very recently a couple of natural base promoted green methodologies have also been identified to assist hydroxylation in short reaction time [39]. Two other protocols have been published recently for ipso-hydroxylation reaction: one of the protocols used photocatalytic route [40] and the other one used silica-chloride/H2O2 [41] as an effective catalytic system. Although a majority of the reported protocols have been found to be useful for the conversion of arylboronic acids to phenols via C−B bond cleavage, use of base or acidic catalyst is essential in most of the cases. Hence, it is desirable to develop a readily accessible, air and moisture stable, inexpensive, environmentally friendly method that can be performed under mild reaction conditions for ipso-hydroxylation of arylboronic acid avoiding strong acid catalyst or base. Subsequently a few exciting improvements have been accomplished for quick ipso-hydroxylation reaction utilizing metal and metal oxide nanoparticles as catalysts such as AgNPs@K10/H2O2 [42], Cu2O NPs [43], and Fe2O3@ silica/H2O2 [44] for efficient C−B bond cleavage. These reports prompted us to design an efficient methodology for quick and clean ipso-hydroxylation of arylboronic acid via C−B bond cleavage using ZnO nanotapes as catalyst and minimum amount of hydrogen peroxide as oxidant in aqueous medium under metal-, ligand-, and base-free conditions.

2.3. Synthesis of ZnO nanostructures ZnO nanostructures were synthesized by precipitation method in the presence of flower extract in methanol under ambient conditions. In a typical synthesis, 10 mL of flower extract solution was diluted with 26.67 mL methanol in a stopper conical flask. The flask containing the methanolic solution of extract was placed on a hot plate-cum-stirrer at 65 °C under reflux conditions. To this extract solution, 5 mL; 0.5 M zinc acetate solution (in methanol) was added and then subjected to stirring at the same temperature for 10 min followed by drop-wise addition of 8.33 mL TBAH solution. The reaction mixture was heated at the same temperature under constant magnetic stirring for 2 h. After that, the reaction mixture was removed from the hot plate and allowed to cool down to room temperature. The solid product was then isolated by centrifugation (REMI, India) at 8000 rpm for 10 min. The isolated product was purified by washing with methanol and then centrifugation. This process was repeated for two times after which the isolated solid product was dried in vacuum at 60 °C for overnight. The sample was designated as ZnO-1 (Table 1). Similar set of reactions were carried out by either varying the concentrations of extract or Zn2+ ions in the reaction medium keeping other reaction conditions same. The details of the reaction parameters are provided in Table 1. Further to study the effect of flower extract on the size or morphology of the nanostructure, we repeated the synthesis of ZnO in the absence of flower extract (ZnO-4) keeping other reaction conditions identical. 2.4. Ipso-hydroxylation of phenylboronic acid In a 50 mL round-bottomed flask, a mixture of arylboronic acid (1 mmol), H2O2 (30% aq, 0.2 mL), ZnO nanocatalyst (5 mol%; sample ZnO-1) and 2 mL of water were stirred at room temperature under aerobic condition. The progress of the reaction was monitored by thin layer chromatography (TLC). After completion of the reaction, the reaction mixture was diluted with 20 mL of water and extracted with (3 × 20) mL of diethylether. The combined organic layer was washed with brine and dried over Na2SO4. The solvent was removed in a rotary evaporator under reduced pressure. The crude product was purified by column chromatography (hexane/ ethylacetate, 9:1) on silica (100–200 mesh) to get the desired product. The products were identified by 1H NMR and 13C NMR.

2. Experimental section 2.1. Materials and methods Zinc acetate dihydrate [Zn(CH3CHO)2·2H2O], tetrabutylammonium hydroxide (TBAH; 40% solution in methanol), methanol (EMPARTA) were purchased from Merck, India. Flowers of Lantara camara were collected from the Rajiv Gandhi University campus, Doimukh, Papumpare district of Arunachal Pradesh, India. All the reagents were used without further purification. All the glassware’s were cleaned in a bath of freshly prepared aqua–regia solution (HCl: HNO3 = 3:1, v/v) and then rinsed thoroughly with double distilled water and dried in oven. All solutions were prepared in methanol.

2.5. Characterization X–ray diffraction (XRD) study of dried ZnO powder was carried out Table 1 Reaction recipe for the synthesis of ZnO nanostructures.

2.2. Isolation of flower extract The flowers extract were isolated following the previously reported method [7]. Typically, the collected flowers were washed in tap water 59

Sample ID

[Extract] (wt%)

[Zn2+] (M)

[TBAH] (M)

Morphology

ZnO-1 ZnO-2 ZnO-3 ZnO-4

0.2 0.5 0.5 0

0.05 0.05 0.02 0.02

0.25 0.25 0.25 0.25

Tapes of mixed lengths Short tapes Nearly spherical Long tapes/ agglomerated


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The sharpness of the XRD peaks indicates the high crystallinity of the synthesized samples. In order to study the morphological evolution and the size of the ZnO nanostructures, TEM micrographs of the as-prepared ZnO samples were recorded. Fig. 2a shows the TEM images of sample ZnO-1 prepared with 0.2 wt% of extract and 0.05 M of Zn2+ ions. The TEM micrographs show the formation of mostly tape-like nanostructures. The lengths of the tape-like ZnO nanostructures are not measurable due to the overlapping of the particles. However, widths of such tape-like nanostructures are within 9 nm. The high resolution TEM (HRTEM) micrograph recorded from tape-like ZnO nanostructures (Fig. 2b) shows the presence of perfectly aligned lattice fringes with interplane spacing of 0.26 nm. This corresponds to the (002) crystal planes in ZnO. This observations suggest that the preferential growth along the [001] crystallographic direction (c-axis) results in the formation of tape-like structures [45]. Also it is evident from the HRTEM image that the surfaces of such nanotapes are rough indicating the presence of large number of surface defects on the surface. The SAED pattern (Fig. 2c) recorded from such nanotapes is dominated by the bright spots superimposed on a circular ring pattern. This is due to the overlapping to the particles from where the diffraction took place resulting in ring pattern. Due to compactness in the pattern, the crystal planes could not be identified properly. To study the effect of extract concentration on the size of the tapelike ZnO nanostructures, we increased the extract concentration from 0.2 to 0.5 wt% keeping the Zn2+ ion concentration and other reaction parameters identical (sample ZnO-2; Table 1). The TEM micrograph (Fig. 3) shows the formation of again tape-like ZnO nanostructures. However, in this case the nanotapes are shorter and the length of these tape-like ZnO nanostructures is varied from 9 nm to 27 nm. While the average width of such tape-like nanostructures is 7 nm. As the width and length are almost of similar sizes, some of the nanostructures are matching well with square shape morphology as can be seen in the micrographs. The HRTEM micrograph of a short tape- like ZnO nanostructure (Fig. 3b) shows the presence of perfectly aligned lattice fringes with interplane spacing of 0.258 nm. This corresponds to the (002) crystal planes as also observed in the previous sample. To study the effect of metal ion concentration on the size of ZnO nanostructures, we repeated the synthesis by decreasing the concentration of Zn2+ ions from 0.05 to 0.02 M (sample ZnO-3) keeping the concentration of extract and other reaction parameters identical with sample ZnO-2. The TEM image (Fig. 4a) recorded from this sample shows the formation of nearly spherical uniform sized ZnO nanoparticles. The average size of such nanoparticles is 6 nm. The HRTEM image shown in Fig. 4b again shows the presence of aligned lattice fringes with interplane spacing value of 0.28 nm corresponding to (100) lattice plane. It is known that additive plays an important role in controlling the size and shape of ZnO nanoparticles. In the current study, to ascertain the role of plant extract we prepared ZnO in the absence of plant extract (sample ZnO-4). The TEM micrographs recorded from this sample (Fig. 5a) show the presence of mostly tapes of different sizes. Also the tapes are agglomerated to form clusters. The HRTEM image of such nanostructures further confirmed the presence of overlapped tapes (Fig. 5b). The sizes of such nanostructures could not be measured due to their overlapping fashion. But it seems that the lengths of some of the tape-like ZnO are close to 60 nm. The micrographs clearly confirmed the role of extract played in the formation of tape-like ZnO nanostructures. Further the HRTEM image indicated the presence of aligned lattice fringes with interplane spacing of 0.247 nm which corresponds to the (101) lattice plane in ZnO. The chemical compositions of synthesized ZnO nanostructures were analyzed by energy dispersive X-ray (EDX) spectroscopy recorded during TEM analysis. The representative EDX spectra of samples ZnO-1 and ZnO-4 are shown in Fig. S1 in the Electronic Supplementary Information (ESI). These spectra revealed the presence of peaks due to elemental zinc and oxygen confirming that the synthesized materials

in a Phillips X’pert Pro multipurpose diffractometer at an accelerating voltage of 40 kV using Cu Kα radiation (λ = 1.54 Å) as X–ray source. For transmission electron microscopic (TEM) studies, a drop of methanolic suspension of ZnO was cast on a carbon coated copper grid. The excess solutions were soaked with a tissue paper followed by drying in air. The micrographs were then recorded in a high–resolution JEOL electron microscope (JEM 2100 EM) at an accelerating voltage of 200 kV. ImageJ software (National Institute of Health) was used to measure the size and interplane spacing values of the nanostructures. For UV–vis spectroscopic analysis, a small quantity of dried ZnO powder was dispersed in methanol under ultrasonic vibration and the requisite volume of the colloidal ZnO suspension was transferred into a quartz cuvette of path length 1.0 cm and the spectrum was recorded in an Agilent Cary60 spectrophotometer in the wavelength region of 300–800 nm against methanol as blank. The photoluminescence (PL) spectra of the same suspension of ZnO were recorded in a Cary Eclipse fluorescence spectrophotometer at an excitation wavelength of 330 nm. Fourier transforms infrared spectra (FTIR) of the powder samples were collected in a Thermo Scientific Nicolet iS5 spectrophotometer in the range of 4000 – 400 cm−1. The pellets for recording the FTIR spectra were prepared by mixing the powder sample with dried KBr in the weight ratio of 1:100. Thermogravimetric analyses (DT-TGA) of the dried samples were carried out on a simultaneous thermal analyzer (Perkin Elmer, STA 8000) at a heating rate of 10 °C/min under nitrogen atmosphere. The nitrogen (N2) gas adsorption–desorption isotherms of sample ZnO-1 was recorded at 77 K (Quantachrome Nova 1000 Instrument) after degassing the powder samples at 150 °C for 4 h in an inert atmosphere. BET specific surface area was determined from the adsorption-desorption isotherms following the well-known BarrettJoyner-Halenda (BJH) method.

3. Results and discussion The phase structures of the synthesized ZnO nanostructures were studied using powder XRD technique. Fig. 1 shows the XRD patterns of the products prepared by varying the different reaction parameters (Table 1). The diffractograms show the presence of diffraction peaks at 2θ = 32, 34.6, 36.4, 47.7, 56.8, 63.10, 68.1, 69.2, 72.7, and 77.2°. These peaks correspond to the reflection from planes (100), (002), (101), (102), (110), (103), (200), (112), (201) and (202) respectively [7]. The diffraction angles and their corresponding reflection planes in all the powder samples matches completely with the reported values for hexagonal phase ZnO (JCPDF No. 36-1451). No other peak due to any impurities was noticed indicating the formation of pure crystalline ZnO.

Fig. 1. Powder XRD patterns of different ZnO samples. 60


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Fig. 2. (a) TEM image of ZnO nanostructures recorded from sample ZnO-TBAH-1prepared in presence of 0.2 wt% flower extract and 0.05 M zinc precursor, (b) HRTEM image of tape-like ZnO nanostructures and (c) SAED pattern recorded from tape-like ZnO nanostructures.

surface faces and thereby promoting the growth parallel to the [001] crystallographic direction (c-axis). This results in the formation of tapelike structures. In the presence of extract, polyphenolic molecules which are believed to act as capping agent might get adsorbed on the growing crystal. The competitive adsorption of the different molecules results in the capping of the growing crystal and thereby limits the length of the tape like ZnO nanostructures. This is the reason we observed shorter tape-like ZnO in the micrograph of sample ZnO-1 prepared in presence of 0.2 wt% extract (Fig. 2). At further higher extract concentration [0.5 wt%; sample ZnO 2], efficient capping by polyphenolic molecules resulted in the formation of very short tape-like ZnO nanostructures (Fig. 3). Whereas in sample ZnO-3 [0.5 wt% extract; Zn2+ = 0.2 M], the relative higher extract concentration in comparison to Zn2+ ions effectively capped the growing nuclei inducing the formation of smaller sized nearly spherical ZnO nanostructures (Fig. 4). The optical properties of the ZnO nanostructures were studied by recording the UV–vis absorption spectra from their methanolic suspension (Fig. 6a). Sample ZnO-1 exhibits strong absorption peak at 333 nm while samples ZnO-2 and ZnO-3 show the absorption peak at 328 nm along with a small hump at around 395 nm. Whereas the absorption band for sample ZnO-4 prepared in the absence of extract shifted to 346 nm and is quite wide compared to the absorption peak of other samples. The strong absorption band is attributed to the band edge absorption of wurtzite hexagonal ZnO which is blue shift relative to its bulk (380 nm). The blue shift denotes the decrease in size of particle and increase in band gap energy. In general, the excitonic photoluminescence (PL) signal mainly results from surface oxygen vacancies and defects of semiconductors. Fig. 6b shows the room temperature PL spectra of ZnO nanostructures recorded from their methanolic suspension at an excitation wavelength

are composed of zinc and oxygen only. No other peak due to any impurity was detected in the spectra. This observation is in good agreement with the XRD results, which confirmed the phase purity of ZnO nanostructures. From the above microscopic analysis, it is evident that the most striking feature of this study is the dependence of the size and morphology of ZnO nanostructures on the concentration of flower extract. It is believed that the formation of ZnO in alkaline hydrolysis took place via the following chemical reactions[18]-

Zn2 + + 2OH− → Zn (OH )2

(1)

Zn (OH )2 + 2OH− → [Zn (OH )4]2 −

(2)

[Zn (OH )4]2 − → ZnO ↓ + H2 O + 2OH−

(3)

Structurally, wurtzite ZnO consists of tetrahedrally coordinated zinc and oxygen atoms that are stacked alternately along the c-axis. Such a structural feature results in a spontaneous polarization of the {001} surfaces and a divergence in surface energy [46]. The Zn2+ terminated {001} and {101} surfaces are exposed and are more susceptible to bonding with anion additives. Whereas, cation additives can bind with the O2− terminated {00−1} surfaces and thereby inhibits its growth [46]. It is reported that positively charged tetrabutylammonium (TBA+) ion can be selectively adsorbed on such planes and thereby restraining the growth of lateral faces [47]. This fact favors the growth of nanostructures along the c-axis resulting in rod-like/ tape-like ZnO. Based on the previous reports, we claimed that in the absence of plant extract, reaction between TBAH and Zn2+ ions, constituent growth units, [Zn(OH)4]2− are formed in the initial stages (eq. 2). During the growth TBA+ undergo selective adsorption on the reactive

Fig. 3. (a) TEM image of ZnO nanostructures recorded from sample ZnO-2 prepared in presence of 0.5 wt% flower extract and 0.05 M zinc precursor and (b) HRTEM image recorded from a short tape-like ZnO nanostructure. 61


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Fig. 4. (a) TEM image of ZnO nanostructures recorded from sample ZnO-3 prepared in presence of 0.5 wt% flower extract and 0.02 M zinc precursor and (b) HRTEM image of a nearly spheroidal ZnO nanoparticle.

participation of biomolecules in capping the ZnO nanostructures. The absorption peak at 1431 cm−1 appeared in the spectra of all the samples can be attributed to the bending of C–H of CH3–. The peak at 1040 cm−1 assigned to the CeN stretching mode in aliphatic amines which might appear due to either TBAH or nitrogen containing phytochemicals from extract. The presence of aromatic rings containing compounds in the extract was confirmed from the peaks at 1580 cm−1 arising due to the stretching vibration of C]C bond. The FTIR results confirmed the presence of biomolecules and TBA+ on the surface of ZnO which played the active role in the growth and formation of tapelike ZnO of different sizes and morphologies. Further to confirm the involvement of extract on the growth of ZnO nanostructures, we have carried out TG-DTA on ZnO nanostructures prepared both in the presence of extract (sample ZnO-1) and in the absence of extract (sample ZnO-4). The TGA thermogram (Fig. S2a in the ESI) of sample ZnO-1 exhibited weight loss of about 15 and 12 wt% in the temperature ranges from 50 to 200 °C and 200 to 500 °C. The decomposition temperatures are 174 and 291 °C. Whereas sample ZnO4 prepared in absence of flower extract shows sharp decomposition at 154 °C (Fig. S2b). The corresponding weight loss took place in the region 50 to 200 °C is about 12.5 wt%. These results clearly indicated that phytochemicals present in the extract are anchored on the surface of ZnO nanostructures during their growth which ultimately controlled the growth of the tape-like ZnO nanostructures. ZnO nanostructures are known to exhibit catalytic activities towards organic transformation. So to assess the catalytic activity of the tapelike ZnO nanostructures, we intended to carry out ipso-hydroxylation of arylboronic acid to afford phenol in presence of safe and eco-friendly ZnO nanocatalyst. For initial investigation, we chose phenylboronic

of 330 nm. It is reported that ZnO can exhibit a number of emission peaks in the visible spectral region which is attributed to the defect emission [48]. The PL spectra shown in Fig. 6b consist of four emission bands in the regions 410–415, 448–450, 485–487, and 529 nm. The ultraviolet blue emission at 410–415 nm is very consistent with the energy interval between the valence band and the level of VZn, while the blue emission centered at 448–450 nm is assigned to the transition of the electrons from shallow levels to the acceptor levels above the valence band as reported by Lai et.al [49]. The emission at 485–487 nm might originate from the electron transition from the level of the ionized oxygen vacancies to the valance band. Green emission at 529 nm occurs due to the recombination of photogenerated hole with a highly ionized oxygen vacancy band for the ZnO NPs [50]. In order to ascertain the role of biomolecules present in the flower extract in the formation of tape-like ZnO nanostructures, FTIR spectroscopy was employed. The spectra of the ZnO nanostructures exhibit a series of absorption peaks as shown in Fig. 7. The absorption band at 3425 cm−1 observed in sample ZnO-1 and ZnO-4 corresponds to the stretching vibration of the –OH group indicating the presence of surface adsorbed water molecules in the samples [26]. Whereas the peaks at 2982 cm−1 in sample ZnO-1 and ZnO-4 is due to the stretching vibration of aliphatic –CH– group present in the extract as well as in the TBAH. However FTIR spectrum of pure extract shows the absorption band due to stretching vibration of aliphatic –CH– group at 2940 cm−1. So peak shifting is observed due to the involvement of extract in capping the ZnO nanostructures. Besides when the FTIR spectra of extract and sample ZnO-1 prepared in presence of extract is compared, either a significant disappearance of several peaks or shifting of some major peaks in the spectra of flower extract is noticed. This signifies the active

Fig. 5. (a) TEM image of ZnO nanostructures recorded from sample ZnO-4 prepared in absence of extract and (b) HRTEM image of tape-like ZnO nanostructures. 62


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Fig. 6. (a) UV-vis absorption and (b) room temperature photoluminescence spectra of different samples of ZnO nanostructures recorded from their aqueous suspension.

acid (1 mmol) as a model substrate, 2 mL of H2O2 as oxidant and 10 mol % of ZnO nanocatalyst (Table 2, entry 1). All substrates were mixed in an open reaction vessel and allowed to progress the reaction. To our delight, the reaction completed within very short reaction time (0–5 min) with 95% of isolated yield. To check the effectiveness of the ZnO nanocatalyst, we performed the reaction in the absence of the catalyst which afforded 75% of the isolated yield in 7 h (Table 2, entry 2). Inspired by this satisfactory result, in the next assessment, we tried to carry out ipso-hydroxylation in presence of ‘water’ (2 mL) as solvent by minimizing the amount of H2O2 from 2 mL to 1 mL, 0.5 mL and 0.2 mL respectively, using 10 mol% of the catalyst (Table 2, entries, 35). Noticeably we found similar results for the ipso-hydroxylation in all the cases. This implies that 0.2 mL of the oxidant is enough for the required transformation with 10 mol% the catalyst (Table 2, entry 5). So, we fixed 0.2 mL of peroxide as oxidant and 2 mL of water solvent as optimized condition for the desired transformation. Proceeding with these pleasing results, we planned to optimize the amount of catalyst loading for the reaction protocol. For this, we performed the reaction with 5 and 3 mol% of the catalyst respectively (Table 2, entries 6 and 7) keeping other conditions identical. As summarize in Table 2, entry 6, 5 mol% catalysts offered 95% of isolated yield of phenol which is comparable to the yield obtained for 10 mol% catalyst (Table 2, entry 5). But with 3 mol% catalyst, the reaction provided 82% of isolated yield in 4 h (Table 2, entry 7). So, we fixed 5 mol% of the catalyst, 0.2 mL H2O2 as oxidant and 2 mL of water as solvent for optimized reaction condition. To check the cause of enhanced catalytic activity of the tape-like ZnO nanostructures, we have performed the nitrogen gas adsorption-desorption study on sample ZnO-1. From the BET isotherm (Fig. S3 in the ESI), the specific surface area was measured to 135 m2/g. The high BET surface area might be the reason of such high catalytic activity of tape-like ZnO nanostructures. Further, we repeated the catalysis reaction with optimized conditions using other ZnO nanocatalyst prepared both in presence of extract (sample ZnO-3) and in the absence of extract (sample ZnO-4). Surprisingly, we found exactly similar activities as found with sample ZnO-1. With the optimized reaction condition in hand, the generality of the newly developed protocol was expanded to wide array of sterically hindered, substituted with electron withdrawing and electron donating groups at ortho, meta and para positions of arylboronic acids such as –CN, -OMe, -Me, -tBu, and -Cl as discussed (Table 3, entries 7, 3, 2, 10, 8) with moderate to excellent yields. Interestingly, we didn’t notice any significant difference in yield for electron withdrawing and electron donating substituent’s but there was some solubility problem for substrates with substituent’s –CN, and tBu (Table 3; entries 7 and 10)

Fig. 7. FTIR spectra of ZnO NPs (sample ZnO-1 and ZnO-4) and pure flower extract. Table 2 Optimization of reaction condition for oxidant, catalyst loading and solventa.

Entry

Catalyst (mol%)

H2O2 (mL)

Water (mL)

Time

Yield (%)b

1 2 3 4 5 6 7

10 – 10 10 10 5 3

2 2 1 0.5 0.2 0.2 0.2

– – 2 2 2 2 2

0-5 min 7h 0-5 min 0-5 min 0-5 min 0-5 min 4h

95 75 95 95 95 96 82

a b

Reaction condition: 1 mmol of phenylboronic acid. Isolated yield.

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Table 3 Substrate scope of the reaction protocola.

a

Isolated yield. 1 mL of ethanol was added.

b

Table 4 Reusability of the catalyst in ipso-hydroxylation of arylboronic acid.

Entry

Run

Time (min)

Yieldb(%)

1 2 3 4 5 6

1st 2nd 3rd 4th 5th 6th

2 2 2 2 2 2

95 95 95 95 95 93

Fig. 8. (a) TEM image and (b) XRD pattern of recovered ZnO nanostructures.

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Scheme 1. Proposed reaction mechanism of ZnO catalyzed ipso-hydroxylation of arylboronic acid.

The synthesized ZnO nanostructures are highly crystalline and are free from any impurities. Microscopic results confirmed that the additive plays the major role in controlling the size or morphology of tape-like ZnO nanostructures. The optical properties of the as-synthesized ZnO nanostructures are size and shape-dependent which verified that the size and morphology of the products are greatly affected by introducing the capping agent. The as-synthesized tape-like ZnO nanostructures exhibits excellent catalytic activity towards the ipso-hydroxylation of aryl/ heteroarylboronic acid to produce phenol. ZnO being non-toxic materials provides a greener pathway to produce phenol from arylboronic acid. Also the tape-like ZnO nanostructures are highly stable and can be reused for successive batches of reaction without losing its catalytic activity.

respectively and 1 mL of ethanol had to be added for better conversion. Moreover, hetero-arylboronic acid moieties (Table 3, entries 12, 13 and 14) also underwent good transformation under this reaction condition to give hetero phenolic compounds. During substrate study, it was observed that reaction of responsive group functionalized arylboronic acids underwent conversion easily to corresponding phenols with high chemoselectivity (Table 3, entries 11, 13). It is known that in the presence of oxidizing agents, such as H2O2/ catalyst, m-CPBA, oxone etc. heterocycles such as sulphides and pyridyls undergo oxidation towards sulphoxides and N-oxide. But, with our experimental conditions, selectively ipso-hydroxylation product was obtained without any by product formation. Reusability is one of the most attractive properties of a heterogeneous catalyst from the green chemistry point of view. We wanted to investigate the reusability of ‘ZnO nanocatalyst’ under the current reaction conditions. To perform this, we have chosen phenylboronic acid as the substrate (3 mmol) and the reactions were carried out by using 0.6 mL of the oxidant, 15 wt% of the catalyst and 6 mL of water at room temperature. For easy recovery issue, we carried out the reaction taking 3 times scale of the model reaction. After the completion of the first run of the reaction, the nano catalyst was filtered, washed with ethyl acetate followed by water and then water was allowed to evaporate in a vacuum oven overnight at 60 °C. The dried catalyst was used for further reaction of ipso-hydroxylation of phenylboronic acid. Surprisingly, the catalyst remained efficient and the reaction delivered excellent yields up to the sixth run (Table 4). In order to investigate the fate of the catalyst during the organic reaction, the isolated ZnO nanocatalyst was analyzed by TEM. The TEM micrograph (Fig. 8a) shows the presence of tape-like nanostructures. Further the crystallinity and phase-structure of ZnO is also maintained after catalysis as confirmed from XRD analysis (Fig. 8b). These results revealed that the tape-like ZnO nanocatalysts are stable enough to survive in the reaction condition without undergoing any changes in the morphology and crystallinity. Although the exact mechanism of ipso-hydroxylation reaction of arylboronic acid is unknown, yet we propose a plausible mechanistic pathway (Scheme 1) based on literature and our observation [20,34,35]. As mentioned in previous section, the catalytic activity of tape-like ZnO nanostructures prepared both in absence and in presence of extract are same. Also we found that the size of the ZnO nanostructures does not have any influence of the reaction time or yield. This confirmed that the reaction is not just surface catalyzed but ZnO might formed some sort of bond with the reactant during catalysis. So, we assumed that at the first step, the ZnO nanostructures react with H2O2 to form a peroxo compound, which in the next step reacts with phenylboronic acid to form an adduct (A). The adduct then undergoes a rearrangement and subsequent lose water to produce another adduct (B). Finally, the adduct (B) undergoes hydrolysis to form the desired phenol product.

Conflict of interest The authors declare no conflict of interest. Acknowledgements This work was supported by Science and Engineering Research Board, India (SERB, Project No. EMR/ 2015/ 001912). We also thank SAIF, GU and SAIF, NEHU for extending instrumental facilities to us. We are thankful to Prof. Pankaj Das, DU, Dibrugarh for help with DTTGA measurements. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.apcata.2018.05.037. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

4. Conclusion

[15] [16]

In summary, a facile and efficient solution route was employed to fine tune the size and morphology of ZnO nanostructures using flower extract of Lantana camara plant as capping cum shape-directing agent.

[17]

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