Aqueous Mechanical Oxidation of Zn Dust: An Inventive Technique for

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Aqueous Mechanical Oxidation of Zn Dust: An inventive Technique for Bulk Production of ZnO Nanorods Santhosh Balanand, Kunnathuparambil Babu Babitha, Mathews Jeen Maria, Abdul Azeez Peer Mohamed, and Solaiappan Ananthakumar ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01966 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 5, 2017

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ACS Sustainable Chemistry & Engineering

ACS Sustainable Chemistry & Engineering Manuscript Ref No: sc-2017-019662.R2 [Revised manuscript]

Aqueous Mechanical Oxidation of Zn Dust: An inventive Technique for Bulk Production of ZnO Nanorods

Santhosh Balanand †‡, Kunnathuparambil Babu Babitha †, Mathews Jeen Maria †, Abdul Azeez Peer Mohamed † and Solaiappan Ananthakumar †*

†

Functional Materials Section, Materials Science and Technology Division, CSIR-

National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram, Kerala – 695 019, India. ‡

Department of Industrial Engineering, University of Trento, via Sommarive 9, 38123,

Trento, Italy.

*Corresponding author. Tel.: +91-471-2515289, +91-9497271547 E-mail address: [email protected] (Solaiappan Ananthakumar)

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ABSTRACT Metal to bulk metal oxide nano particles have been successfully processed via a sustainable, facile and eco-friendly (green) approach namely ‘aqueous mechanical-oxidation’. Micron sized zinc (Zn) dust (~45 µm) was directly wet-milled using ceramic milling media for 72 h, resulting in the production of bulk monocrystalline ZnO nanorods (aspect ratio ~5.2, hydrodynamic diameter of 315 nm) and voluminous H2 gas by catalyst-free water-splitting reaction. The mill-induced surface oxidation and the chemical purity of the synthesized nano ZnO were carefully studied using the XPS analysis. Other standard analytical tools were also employed to understand the crystallinity, phase purity, morphology and surface area of the final artefact. The photocatalytic activity of these mechanically grown ZnO nanorods were ascertained from two cationic dye degradation experiments; using the dyes methylene blue and rhodamine 6G. In a nutshell, the study throws a new insight into a cost effective, zero effluent, single-step and parallel processing approach of two high value products, bulk nano ZnO and catalyst-free H2 gas from micronic Zn dust.

KEYWORDS: Micronic Zn dust, Green Synthesis, Aqueous Mechanical Oxidation, ZnO nanorods, H2 gas, Photocatalytic dye degradation.

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INTRODUCTION Top-down, solid-state mechanochemical approaches are often adopted for the synthesis of functional materials.1,2 Milling based techniques like mechanical alloying and mechanical activation are ideally opted by industries for the bulk production of nano-materials owing to the simple unit-operations involved at the various stages of processing.3-6 In fact, mechanical methods are often allied with many chemical techniques because of the ‘zero-waste’ processing and lack of harmful chemical effluents.7,8 Presently, mechano-synthesis is a proven green-technique for the production of various commercially and scientifically useful nano-materials including many nano-alloys, nanocomposites and nano-quasicrystalline materials.8-11High energy ball milling (HEBM) is a well acknowledged technique, successfully applied not only for particle size reduction operation but also to trigger atomic level surface activation and for even tailoring the chemistry of multi-functional nanomaterials.6,11-14In mechanical milling, the kinetic energy developed from the ceramic milling-media is transferred as high impact energy that creates localized high temperature regions (< 1000 oC) along with high pressure upto several GPa.15 This can induce nucleation and growth of exotic nano scale metal oxides and phase transformations.11,16,17 In fact, the sequential phase transformations during a mechanical treatment were found identical to that induced during thermal treatments.18 In the work by Delogu et al.,19 the mechanically induced phase transformation of TiO2 from anatase to TiO2 (II) and eventually to rutile during milling was described. Bodaghi et al.20 also identified the formation of α-Al2O3 seeds during the milling of metastable γ- Al2O3, which accelerated the transformation kinetics of γ- Al2O3 with milling operation. Zyryanov,21 enlisted the capability of mechano-syntheses for triggering various complex oxide formation reactions. Nano metal oxides prepared through high energy milling has reported to have high degree of structural and surface order

22,23

and uncommon

functional properties16 compared to particles that prepared through ‘soft methods’, like chemical routes.24,25 Handful reports are also there on nano metal oxide synthesis by this route at ambient temperature conditions from their nano chemical precursors.26-28 Work by Castro et al.29 also endorsed the importance of the mechanical activation step in achieving stable, conducting Bi2VO5.5 complex oxide. The effectiveness of the mechanical activation step in achieving soft PZT type piezoceramics was reported by Miclea et al.30 Lately in a 2 ACS Paragon Plus Environment


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work by Khayati et al.,31 Cuprous (Cu2O) nanoparticle formation were confirmed during solid state milling of copper in a simulated oxygen atmosphere. However, till date there are no reports on employing milling and mechano-syntheses techniques directly as a bulk production tool for nano metal oxides from metals, by aqueous milling. Amid the different metal oxides, nano zinc oxide (ZnO) has earned huge appreciation for its diverse applications.32-38 Varied synthesis techniques for ZnO, from laboratory scale to quantities in bulk industrial scale, are often discussed.17,32,33,36,39,40 Commonly adopted synthesis approaches usually starts either from various chemical precursors or from the ores of the metal as such.36,39-41 Though evaporation and direct thermal oxidation technique

42

of

zinc (Zn) to ZnO is the most opted approach being simple, cost effective and catalyst free, 43,44

the control over these types of synthesis processes are poor and the associated

evaporation losses of Zn metal still remains problematic.42,45 Furthermore, the thermal approach also entails an additional energy requirements for the melting and vaporization of the metal.46 There are a good number of reports on two-step water splitting of Zn/ZnO45,47 pair and resultant production of hydrogen,45,48 via hydrolysis of Zn using solar process heat.45,47,48 This water splitting capability of Zn can be attributed to the high oxygen looping capacity per unit mass of Zn, as reported by Weibel et al.46 Abanades et al.49 explained the thermal reduction of metal oxides using a solar reactor, to produce solar hydrogen. This technique can be an effective approach to produce solar fuels48 as a by-product along with the intended material synthesis. However, the applicability of a mechanical route for initiating direct metal to oxide syntheses has never been tried before, though the process been simple and sustainable for scale up. But, the milling procedure has been recognised well before as an activation step at various stages of ZnO synthesis.17,50,51 In a work, Salah et al.52 reported the structural modifications induced in milled ZnO powders, which actively improved its inherent antibacterial effect. Glushenkov et al.53 used the same technique for a sort of activation before the sequence of annealing steps, for producing corrugated nanowires of ZnO. Reports show that the mechanically formed crystalline imperfections can significantly enhance the activity of the ZnO material.54,55 Recently, Francavilla et al.56 reported improved photocatalytic degradation of Phenol using ZnO synthesized by dry milling of chemical precursors and polysaccharide sacrificial templates followed by a calcination step (@ 600 oC). Many of the catalyst assisted synthesis approaches use milling as an activation method, a pre thermal


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treatment and even as a facile mechanochemical synthesis route,57,58 but till date there are no reports on synthesis of photocatalytically active nano ZnO particles directly by mechanical milling of Zn metal. In this work we report an eco-friendly, aqueous, mechanical approach for direct oxidation of Zn dust to nano ZnO by systematically controlling the operating parameters and process conditions. The transformation of the raw Zn dust to nano-grade ZnO and the evolution of the valuable by-product hydrogen gas during the process were studied. Standard characterization tools were employed at each stages of the process to draw conclusions regarding the synthesis. The photocatalytic activity of these synthesized nano ZnO was studied, projecting it as an efficient, cost-effective and green routed photocatalyst. EXPERIMENTAL SECTION Materials. Zinc (Zn) dust (99.9 %) having an average particle size of ~45 µm (Binani Zinc Pvt. Ltd., India) was used as the starting material. Commercial ZnO (Sigma–Aldrich, Germany) was used for the comparative study. Wet milling was conducted in distilled water. Anatase TiO2 powder (CDH Laboratory Reagents, India), was used as a reference for the photocatalytic studies. Methylene blue (MB) (C16H18ClN3S) (SRL, India) and Rhodamine 6G (Rh6G) (C28H31ClN2O3) (S. D. Fine Chemicals Limited) dyes were used for the dye degradation studies. Mechanical milling and material preparation. The milling was carried out using a belt driven planetary type ball mill. Definite amount of the raw Zn dust was loaded in the mill chamber; propylene jars (Tarsons, India) along with Zirconia balls (Jyoti Ceramics Pvt. Ltd., India.) and distilled water as the milling medium. All the process parameters were optimised and fixed after repeated trials. The process parameters which were selected for this work are listed in Table 1. Table 1. Optimized process parameters for milling. Mill chamber volume ( mL) Milling media Mass of Zn dust charged (g) Liquid carrier, charged (mL) Speed of mill (rev/min) BPMR1 Milling time (h) 1

250 Zirconia balls (10mm dia.,~3g (each)) 20 Distilled water, 170 250 6:1 24,48,72

BPMR – Ball to Powder Mass Ratio



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The raw dusts were milled for different time periods (24, 48, and 72 h) and were labelled as Zn 24, Zn 48, and Zn 72, respectively. The milled slurry was then sonicated using an ultrasonic bath (GT Sonic, Ultra Instruments, India) for a period of 5 min to ‘peel off’ the surface oxide layer formed. The unconverted fraction of Zn was transferred back to the milling chamber for each individual experiments, while the dispersion formed were collected and centrifuged (TC 4100F, Eltek, India) to get the final product. The scheme for processing is illustrated in the Figure S1 in the supporting information. Characterizations. ZnO formation was confirmed using powder X-ray diffraction technique (XRD, Philips X’pert pro PW 1710), the Zn dust milled for different time periods were studied within the angle range 2θ = 20 to 70o using Cu Kα (λCu = 1.542 Å) radiation. The average nanocrystallite size (DXRD) of the synthesized ZnO can be calculated from the XRD data, using Debye-Scherrer’s equation:59 DXRD = 0.9λ / β cosϴ

(1)

where, ‘λ’ is the wavelength (in nm) of the X-ray radiation, ‘β’ is the full width at half maximum intensity (in radian) of the prominent peak and ‘ϴ’ is the half of the measured diffraction peak angle, 2ϴ (in degree). X-Ray photoelectron spectroscopic (XPS, Surface Science Labs SSX-100,USA) studies were also conducted on the milled Zn dust using monochromatic Al-Kα X-ray source (1486.6 eV) and the photoelectrons were studied and processed with the help of ESCA V2.1 software. The binding energy values are calibrated with respect to the C 1s binding energy at 284.6 eV.60 The nano growths were ascertained with the help of UV-vis absorption spectra taken within the wavelength range of 200-800 nm using a UV-vis spectrophotometer (UV-2401 PC, Shimadzu, Japan). The band gap energy was calculated from UV-vis absorption spectral data, using the equation:59 (αhυ)2 = A( hυ-Eg )

(2)

where ‘A’ is a constant and ‘α’, ‘hυ’, and ‘Eg’ represents absorption coefficient, photon energy, and optical band gap (in eV), respectively. The band gap energy (Eg) can be determined by plotting (αhυ)2 against ‘hυ’ and extrapolating the tangent of the curve obtained to (αhυ)2 = 0. Thermogravimetric analyses (TGA, Shimadzu TGA-50 Thermal Analyzer) of the milled and unmilled Zn dust was also taken in oxygen atmosphere from room temperature to a temperature of 700 oC at a heating rate of 10 oC min-1 to study the shift in the curves, if any. A Scanning Electron Microscope (SEM, Zeiss, EV018) was used to study the surface oxide growth and the morphological changes during the oxide formation. Transmission electron microscopic (TEM, FEI Tecna 30G2S–Twin Transmission) imaging was used to


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confirm the morphology and the selected area electron diffraction (SAED) patterns were used to study the crystallinity of the synthesized ZnO. The elemental analysis was also conducted using Energy-dispersive X-ray spectroscopic (TEM-EDX) techniques and the elemental percentages were calculated. The average particle size (Davg) distribution (Zeta-Size, Malvern Instrument, NANO ZS) of the nano ZnO particles formed during milling in water medium were analyzed using Dynamic light scattering (DLS) technique. The Brunauer–Emmet-Teller (BET) surface area (m2g-1) of the synthesized ZnO was detected using N2 adsorption– desorption measurements (Micromeritics Gemini 2375 Surface Area Analyzer, USA). Hydrogen evolution during the synthesis was detected using Gas Chromatographic (490 Micro GC, Agilent Technologies, UK) technique using GPA Standard 2145-09 (FPS). Photo-catalytic Dye degradation study. The photocatalytic activity of synthesized ZnO under

UV-irradiation was determined by measuring the degradation of the cationic dyes MB and Rh6G in aqueous solution. From the repeated initial experimental trials conducted, the dye concentration was optimised to 15 µM. An amount of 0.3 gL-1 of the synthesized ZnO (aqueous mechano-synthesized and microwave routed), and CDH-TiO2 were used for the whole study. The initial studies shown complete degradation of dyes within 50-60 min under UV exposure and hence the maximum time of UV exposure in the chamber was fixed as 60 min. To ensure the proper dispersion of the catalyst in the medium, initially 0.03 g of the catalyst was sonicated in 50 mL distilled water for 5 min. To this sonicated dispersion, 50 mL of 30 µM dye solution was added to form a final concentration of 0.03 g catalyst dispersed in 15 µM dye solution. In this work, the activities of three catalysts were studied; the aqueous mechanical oxidised/aqueous mechano-synthesized nano ZnO obtained through direct milling approach (H), micro-rods obtained via a microwave approach (M)17 and finally for comparison, commercial CDH TiO2. Hence, the photocatalytic activity of these 3 catalysts (H, M and CDH), on the dyes MB and Rh6G were systematically evaluated. All the suspensions were kept under continuous magnetic stirring in the chamber. The suspensions were initially stirred in dark for a period of 30 min to attain the adsorption-desorption equilibrium condition.59, 61

The solution systems were then irradiated with UV radiations having an intensity of 0.4

mWcm-2 and wavelength 200-400 nm for a period of 60 min in a UV chamber.61 Sample aliquots were collected (~5 mL) in between, at an interval of 10 min (i.e. at 10, 20, 30, 40, 50, and 60 min). The collected solutions were then centrifuged @ 3000 rpm for 10 min to remove any suspended particle from the irradiated dye solution to be studied.



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Dye degradation studies were conducted by measuring the absorbance in the range 200 to 800 nm using UV-vis spectrophotometer. According to Lambert–Beer law there exist a linear relationship between absorbance of light by a solution and its concentration.62 The relation is given as: 62 (3)

A = εbc

where, ‘A’ is the light absorbance by the solution, ‘ε’ is the molar absorptivity, ‘b’ is the path length of the sample, and ‘c’ is the concentration of the solution. The maximum absorption peak, λmax of the dyes MB and Rh6G, used in this study were reported59 and analyzed to be at 656 nm and 530 nm, respectively. The absorbance corresponding to these values were used for evaluating the dye concentration. The dye degradation was calculated from the equation:59 D (%) = [C/C0] 100

(4)

where, ‘D’ is the dye degradation in percentage,‘C0’ and ‘C’ are the dye concentrations at the initial time and after a time ‘t’, respectively in mgL-1. The photocatalytic decomposition of these organic molecules follows the Langmuir–Hinshelwood kinetics, which is usually denoted as:63 dC/dt = kappC

(5)

where, ‘dC/dt’ denotes the rate of change of concentration with respect to the irradiation time ‘t’, ‘kapp’ is the apparent first-order reaction rate constant (min-1) and ‘C’ is the concentration of the dye (mgL-1). The ln (C0/C) value was plotted against the UV exposure time. The rate of degradation, ‘kapp’ was obtained from the slope of the linear regression of ln C0/C versus UV exposure time plot. A control photocatalysis experiment was also performed in the absence of the photocatalysts to ratify the stability of studied dyes in an aqueous solution under the continuous UV-radiation exposure. Under this condition, the dyes concentration remained unaffected even after irradiating the sample for 120 min. RESULTS AND DISCUSSION Metal to Metal Oxide via Milling. The growth of nano ZnO on the surface of Zn dust during milling in distilled water is apparent from the SEM images shown in Figure 1. The passivation of the Zn surface by an impermeable layer of ZnO during different processing steps was previously reported in several literatures.17,46 The Figure 1(a) shows a clean raw Zn dust particle with no distinctive surface growths. After about 12 h of incessant milling, idiosyncratic growths were observed on the surface of these Zn dust particles, as evidenced from the Figure 1(b). These sprouting on the surface of Zn dust results from the mechanochemical interaction of Zn dust with the milling media and environment. Hydrolysis of metal precursors with mechanical milling was previously reported by Pineda et al.64 Single 7 ACS Paragon Plus Environment

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crystalline nanorods of ZnO formed through oriented attachment (OA), a non-classical mechanism,65 from spherical particles was first reported by Pacholski et al.66 The concept also state the existence of a ‘pearl chain’ like transitional phase, finally leading to the formation of rod shaped oxide nanoparticles.66 In the present work, a flaky layer growth was seen on the Zn surface with extended milling for about 48 h, (as seen in Figure 1(c) and 1(d)). This is the mechanically induced oxide layer on Zn due to the hydrolysis reaction occurring at the interface67 and these growths formed get detached from the surface with further milling forming the ZnO seeds which grow into nanorods. Thus a direct conversion of the metal to metal oxide was perceived, caused by the mechanochemical interactions occurring at the mill chamber. The ZnO yield from Zn is calculated to be ~ 32% after 72 h of milling.

Figure 1. Growth of surface ZnO with milling of Zn dust, (a) Raw Zn dust , (b) Zn surface after 12 h of milling showing small growths, (c) Zn surface after 48 h of milling showing the flaky transitional phase, (d) enlarged portion of (c) clearly showing the flaky ZnO layer on the surface. TG analyses (Figure S2(a) in the supporting information) conducted on the Zn dust shows a clear down (left) shift of nearly 100o C in the oxidation onset temperature of the milled Zn 8 ACS Paragon Plus Environment


dust, clearly indicating the formation of ZnO seeds17 on the Zn dust during the mechanical action. The Figure S2(b) in the supporting information gives the TG curves of the Zn dusts milled for different time interval (viz 24, 48, and72 h), held at the maximum temperature (700o C) for a period of 1 h. The decreased mass gain for the Zn 72 at this extreme condition (700o C) during the held period (1 h) is quite obvious from the Figure S2 (b). This gradual decrease in mass gain from Zn 24 to Zn 72 clearly validates the increased conversion of Zn to ZnO with an increase in the milling time.17 However these observations from SEM and TG analysis cannot confirm the formation of ZnO. The formation of ZnO nanoparticles after mechanical milling was confirmed from the XRD analysis. The Figure 2(a) gives the diffraction pattern of the powder collected from the sonicated dispersion of the Zn dust milled for different time periods. The peaks sprouting at 2θ positions 31.91o, 34.45o, 36.28o, 47.60o, 56.64o, 62.82o, and 67.93o after 24 h of milling indicates the wurtzite phase ZnO crystal (JCPDS 36-1451) formation.68 The peaks are found to become more prominent with the increase in milling time, as evidenced from the Figure 2(a). Some minor peaks are also evident in the region between 23o-27o, which corresponds to the hydroxide peaks of Zn.40 However, with an increase in the milling time, these peaks become less prominent and after 72 h of milling, they completely vanish confirming the complete transformation of the hydroxide phase to the oxide phase.


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Figure 2. (a) XRD analysis, (b) UV-vis spectroscopy on the powder from the milled dispersion ( inset shows the dispersions obtained). The XRD patterns of the raw Zn dust, the starting material, having characteristic peaks at 36.28o, 39.07o, 43.33o, and 54.32o and of commercial ZnO (Sigma–Aldrich) are also presented in the Figure 2 (a). The UV-vis spectroscopic analysis conducted on the dispersion collected from the milled Zn dust shows absorption peaks in the region between 360-380 nm clearly indicating the nano ZnO formation (shown in Figure 2(b)). The dispersions used for the study are shown as inset in the Figure 2(b). The photographs clearly depict the dispersion



stability with milling time. It can be seen that the dispersions become more stable with an increase in milling time indicating the transformation of dense Zn dust to ZnO nanoparticles. Figure 3(a), 3(b), and 3(c) shows the SEM microstructures of the surface characteristics of a single Zn dust particle, milled for 0 (unmilled), 48, and 72 h, respectively. It is obvious from the SEM images that the particle growth is initiated at the surface of the Zn dust with milling. Corresponding XPS analyses were conducted, to affirm the surface growth and chemical composition. The Figure 3(d) shows a low resolution scan conducted over a wide energy range. Presences of Zn and oxygen (O) along with traces of carbon (C) were observed from the spectrum. This confirms the formation of oxidic phases over the surface of the pure metal milled. These traces of C observed can come from the atmosphere. However, these peaks has less importance in the study, but as previously mentioned in the experimental part, this C 1s peak positioned at 284.6 eV is taken as the reference peak throughout the study. High resolution scans conducted for the Zn 2p and O 1s peaks are shown respectively in the Figure 3(e) and 3(f). From the narrow scan results, the peaks at 1020.9 eV and 1044.03 eV (in Figure 3(e)) for the 72 h milled sample corresponds to the Zn 2p3/2 and Zn 2p1/2, which can be assigned to the ZnO formed.69-71Some characteristic peaks of Zn (2p) were seen at 1021.4 eV and 1044.5 eV positions for unmilled Zn dust (0 h). However, these peaks can be attributed to the Zn-OH bond formation70,71 which occurs when the metal comes in contact with the atmospheric water-vapour. With the milling of the Zn dust, these peaks shows a clear shift (right shift), as evidenced from the aforementioned, 72 h milled samples. From the deconvoluted data (see supporting information, Figure S3 (d-f)), this shift in the energy values can be clearly understood. During milling, the loosely attached surface hydroxides of Zn gradually transforms to the stable oxide phase due to the diffusion of the oxygen atoms into the Zn lattice.72-74 This transformation is obvious after 48 h of milling the raw Zn dust, as the peaks of Zn 2p are seen at 1020.9 eV and 1044.03 eV positions (Figure 3(e)) (also see Figure S3 (e) in the supporting information).


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Figure 3. (a-c) SEM images showing the surface growth of ZnO on a single Zn particle after 0, 48, and 72 h of milling, (d) Low resolution wide scan on the milled samples (0, 48, and 72 h); High resolution scan for, (e) Zn 2p and (f) O 1s peaks on the milled samples (0, 48, and 72 h). The presence of hydroxide and oxide phases on Zn was further confirmed from the narrow scan peaks of O 1s. The peaks at 531.3 eV seen for the unmilled (0 h) sample can be attributed to the presence of oxygen in the hydroxide phase69,70,75 (Figure 3(f)). However, after 48 h of milling this peak get shifted to the position 529.8 eV showing the transformation 12 ACS Paragon Plus Environment


of the hydroxide phase to the oxide phase.70,71 The sample obtained after 72 h of milling showed a peak at 529.95 eV, clearly confirming the stable oxide formation.70,71 These shifts in the O 1s peaks are more evident from the deconvoluted data (supporting information, Figure S3 (a-c)). Thus, the XPS analysis result clearly evidences the formation of ZnO nanoparticles by mechanical milling of micron sized Zn dust particles in water. Distinct nano layers of ZnO are formed on the surface of Zn dust after 48 h of milling itself, which grow further deeper with increase in milling time. After 72 h of milling, these nano ZnO formations become quite perceptible. Thus from the XRD, XPS, and UV-spectroscopic results, the formation of nano ZnO via mechanical milling was confirmed. Further studies were conducted with this nano ZnO collected from the dispersion (after 72 h of milling, as optimized), henceforth referred to as ‘aqueous mechano-synthesized ZnO’ . Gas analysis: Identification of the liberated gases. The gas generated during the milling of Zn dust in water (after 72 h), was collected and analyzed using standard gas chromatographic techniques. Three sets of trails were conducted to identify the gas and the the main gas identified was hydrogen. The details of the analysis and the results are presented in the section S4 in the supporting information (also see Figure S4 in the supporting information). The gas collection method in detail is given in the section S5 of the supporting information. Morphology of ‘aqueous mechano-synthesized ZnO’. The SEM images of the aqueous mechano-synthesized ZnO collected from the dispersion (as previously shown in the Figure 2(b) (inset)) after 72 h of milling shows near rod shaped particles having aspect ratio of ~ 5.2, with length in the range 300-400 nm (see Figure 4(a)). The ZnO seeds formed on the surface of the Zn dust during the milling in due course grows into nanorods through the much believed oriented attachement (OA) technique. The mechanism was previously mentioned in different works using reflux technique, where the reflux time was testified to have a direct effect on the length of the nanorods formed.66 The hydrodynamic diameter of the particles were also measured using DLS technique, which gave an average particle size value of 315 nm (as shown in the Figure 4b)), authenticating the observation from the SEM analysis.


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Figure 4. (a) SEM analysis on the synthesized ZnO, (b) Average particle size measured. However the particles seems to be much agglomerated from the SEM image. A high magnification image was required to confirm the particle morphology. The TEM analysis conducted on the same samples confirmed the rod shaped particles. The particles were found to be more isolated and evident in the TEM images. The dimensions of the particles observed from TEM were also found to be in the range 300-400 nm (as seen in Figure 5(a) and 5(b)), which is in par with the observations from the SEM and DLS analysis. The SAED pattern shows the highly crystalline nature of the particles as shown in Figure 5(c). The pattern shows single crystalline character for the synthesized ZnO.65,66 This was much expected as the oriently attached (OA) particles were mostly identified to form mono-crystalline materials.66 Thus this observation further validate the claim of OA occurance in the system. The EDS analysis conducted at selected region shows prominent peaks of Zn and oxygen confirming the formation of ZnO. Peaks corresponding to copper (Cu) seen in the spectrum corresponds to the Cu-membrane grid used in the study (Figure 5(d)). The elemental compostion calculated from the EDS also shows the equivalent contribution of Zn and oxygen in the selected area of the particle (inset in Figure 5(d)). A high resolution TEM (HRTEM) image of the synthesized ZnO was taken at the selected region, shown in the Figure 5(e).



Figure 5. (a and b) TEM images of the synthesized ZnO, (c) SAED pattern, (d) EDXspectrum and (e) HR-TEM images of the ZnO showing growth in (001) direction. Oxidation Mechanism: ‘aqueous mechano-synthesized ZnO’. The ZnO formation

mechanism has to be understood to validate the process.The Zn/ZnO redox reactions to cause water splitting is well documented in earlier reports.45,47 The milling impacts create small localised hotspots having high temperature and pressures, triggering the reaction15 between Zn dust and H2O.The reaction is exothermic in nature as reported in previous works45,48 cause the thermolysis of water to generate hydrogen (H+) and oxygen (O2-)8,45,47 leading to the hydrolysis of zinc. The chances of a gaseous phase reaction of Zn is ruled out in this case as the chamber was degased before the onset of the milling operation. The oxygen ions (O2-) generated as a result of the water splitting reaction diffuses into the Zn lattices forming ZnO.73,74 The H+ ions generated will ultimately enter the gaseous phase and the formation was confirmed from the gas analysis conducted, validating the claim. The initially developed ZnO seeds on the surface of the metal with milling 17, grows into


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scales with the advancing of the process, with time. These ZnO seeds formed would further act as a catalyst to cause the further conversion of Zn to ZnO along with the liberation of the H2 gas.8,46 The cystallization of these synthesized ZnO progresses through the oriented attachment (OA) technique, reported as a non classical mechanism of crystallization by Verges et al.76 in the case of ZnO itself, previously. In addition to these reaction initiations, the passivation caused due to the impervious ZnO layer on the Zn surface46 are also ‘skinned off’ due to the milling impacts, exposing the fresh metal surface for further oxidation, thus the entire particle are exposed to the reaction. Thus, this is the possible ZnO formation mechanism occuring during the synthesis. A brief schematic of the proposed mechanism of ZnO formation is shown in the Figure 6.

Figure 6. Schematic of the oxidation mechanism proposed for the mechanical conversion of Zn to ZnO. Photo-degradation using the aqueous mechano-synthesized ZnO. The degradation of the dyes,

MB and Rh6G using the synthesized ZnO was studied to corroborate the new catalytic system as a effective, low cost, and green routed dye degrading material. The activity of ZnO semiconductor under the action of a proper energy source is well explained in numerous



works.59,77,78 The highly reactive OH. radicals formed as a result of the photocatalytically induced e-/h+ pairs are mainly responsible for the degradation of the organic dyes.79 The detailed mechanism is well explained by Periyat et al.79 In his work, he also reported the breaking down of the organic dyes like MB and Rh6G to CO2 , NH4+ , NO3- , SO4- and H2O.

Figure 7. Photo activity studies using H ZnO, M ZnO,17 and CDH TiO2 as the catalysts; corresponding ln (C0/C) vs. Irradiation Time graph (a; b) using dye MB, (c; d) using dye Rh6G. The Figure 7 shows the results of the photodegradation studies carried out using the ZnO samples synthesized through different routes and the CDH-TiO2. It is pretty apparent from the plots 7(a) and 7(c) that the H shows quick degradation of both the dyes MB and Rh6G, in around 40 min when compared to the other two catalysts (M and CDH). The rate of reaction (kapp) for the MB and Rh6G systems were obtained from the ln (C0/C) vs. Irradiation time graph (Figure 7(b) and 7(d)). The greater kapp values, 0.091 min-1(MB) and 0.190 min-1 (Rh6G) (shown in Table 2) for the H when compared to the rest of the samples in the study clearly cites its potential in causing the rapid degradation of the tested dyes. 17 ACS Paragon Plus Environment

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Table 2. Reaction rate constant values for H, M, and CDH samples for the dyes MB and Rh6G. MB

Samples

Rh6G

Kapp

R

H

0.0936

M CDH

2

2

Kapp

R

0.9937

0.211

0.9773

0.007

0.993

0.004

0.961

0.048

0.985

0.051

0.942

Meanwhile the M showed the least rated degradation on the tested dyes, 0.007 min-1 (MB) and 0.004 min-1 (Rh6G). This markable variation in the activity is quite evident from the photocatalytic plots (7(a) and 7(c)). It is also worth stating that, the CDH-TiO2, which was included as a comparision catalyst also gives better rate constant values than M. However, the values are lower than that of the H sample. The rate constant values given by them, 0.048 min-1 (MB) and 0.051 min-1 (Rh6G) are two to four fold lower than the H sample. Anas et al.80 in their work identified that the higher crystallinity of ZnO increases the catalytic activity. In addition to this, the electron-hole recombinations, surface area, phases and catalyst particle size were also determined to be key factors affecting activity.63,81 In different works by Jaimy et al.63,81 on TiO2 particles, the presence of more surface hydroxyl groups and having a greater band gap energy value (blue shift in UV absorption curve) are found to increase the photoactivity. The greater band gap reduces the electron-hole recombination chances, thus prolonging the excited state of the catalyst, increasing the activity.81 The near nanorods of ZnO (300-400 nm (hydrodynamic diameter), aspect ratio of ca. 5.2) synthesized via the aqueous mechano-route (H) showed enhanced activity compared to microrods obtained through the microwave approach (M).17 This can be clearly attributed to the increased surface area in the case of ZnO nanoparticles obtained via the H-route, hence possessing more number of reactive OH. radicals on the surface.The BET analysis also confirms the greater surface area of H (~ 13.23 m2/g) samples when compared to M (~ 1.74 m2/g ) samples. The crystallite size of the ZnO sourced through H route was calculated to be ~ 21.22 nm, while that synthesized via the M route was ~ 55.56 nm (see Table S1 in the supporting information). This smaller crystallite size in the case of H ZnO samples helps the 18 ACS Paragon Plus Environment


electrons to reach the valence band quickly on excitation. The results of the UV absorbtion studies conducted on the H and M samples are shown in Figure 8 (b). A blue shift observed in the case of the H samples is a clear indication of the reduction in particle size to nano range. The band gap energy calculated for M and H (inset in Figure 8(b)) gives values 2.66 eV and 3.0 eV, respectively. This increased band gap value for H is obvious with the observed blue shift occuring during the UV absorption study. This increase in band gap energy value in the case of H samples will reduce the electron-hole recombination tendency when compared to the M, thus increasing the catalytic activity.81 The Dye degradation mechanism is depicted in Figure 8(a).

Figure 8. (a) Dye degrading mechanism in nano ZnO, (b) UV-vis spectrum of the synthesized ZnO (H and M), inset showing the bang gap energy variation in the two systems (H and M), (c) Percentage (%) of MB and Rh6G dye removal by the different samples (H, M, and CDH) after 60 min, photographs showing the degradation of the dye solutions (15 µM), (d) Rh6G system, (e) MB system; using 0.03 g of H ZnO, after 60 min of UV irradiation. The commercial grade catalytic system CDH TiO2, which are reported to be aggregates of spherical particles having size in the range 50-250 nm61 also shows comparatively reduced


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catalytic activity than the H samples, acclaiming the potentiality of these ZnO samples as an effective substitute for the specific application. Reports clearly states the structural refinement and crystal size reduction that can be achieved on a material with ball milling.15 The distinctiveness in the structure of the oxides obtained through various mechanical and milling approaches from other chemical routed oxides is previously cited elsewhere.16,18,21-23 Milling also found to keenly increase the surface reactive sites, that of OH groups, which can cause notable improvement in the photocatalytic performance of the material.16 The mechanism in the case of TiO2 is well explained in various works.61,63,81 However, it was seen that mill sourced TiO2 was found to show an initial decline in the activity, which can be due to the some defect induced surface amorphization occuring on them.82 Hopefully, this effect was not marked in the case of ZnO synthesized from Zn dust. Thus these observations certainly upholds and elucidates the claim of improved photoactivity seen in H samples. The organic dyes are degraded to CO2 and H2O mainly, under the action of active energy source. The Figure 8 (c) shows the % of dye removed after 60 min of activity in the different sample systems studied. The H samples shows nearly 100% degardation of both the dyes tested within this test period. At the same time, the M shows the least removal within this time constraint and CDH shows moderate degradation (70-80%). The photographs in Figure 8 (d) and 8 (e) shows the photocatalytic degradation occuring in the dye solutions (Rh6G and MB respectively) with H as the catalyst after 60 min (test period) of UV irradiation. Thus this synergetic effects of particle size, crystallinity, surface area, delayed electron-hole recombination tendency and mechanical distortions together can be accounted for the enhanced photocatalytic activity in the case of the aqueous mechano-synthesized ZnO (H) samples. CONCLUSIONS Thus ZnO nanorods (aspect ratio c.a 5.2) were synthesized through a green, aqueous mechanical approach by employing simple balling milling. The Zn metal (~ 45 µm) was the starting material chosen and water been the only active reactant needed for the synthesis, the process can be listed as a green technique. The process was well optimised (formation of ZnO after 72 h) and the formations were confirmed using standard characterization techniques. The oxide seeds formed on the metal surface with milling was ascertained from the XRD 20 ACS Paragon Plus Environment


results and its development into surface scales were observed from the morphological and XPS analysis conducted. The evolution of the hydrogen gas during the synthesis confirmed the ‘water splitting reaction’ occurring simultaneously, which in fact actively supplied O2- for the oxide formation. The ZnO crystallization in the medium is much purported to occur by the oriented attachment (OA) technique. A formation mechanism was also proposed based on the experimental results. The photocatalytic dye degradation capability of these aqueous mechano-synthesized ZnO particle (H) were studied using the cationic dyes, MB and Rh6G. Accelerated degradation of the dyes by the H sample was observed, within 40 min. The photoactivity of these samples was also compared with active microwave synthesized ZnO (M) and CDH TiO2 samples. Hence, a new approach to synthesize photocatalytically active ZnO through a green and mechanical route, with immense scale up possibilities is suggested in this work. The work also hints the sourcing of H2, produced as a by-product during the synthesis. Acknowledgments The authors sincerely thank The Director, CSIR-NIIST, Thiruvananthapuram for providing the lab facility for the work. The author sincerely acknowledges M/s Binani Zinc Pvt. Ltd. for the raw zinc dust which was used in this work. The author is also grateful to AIMS, Kochi for the XPS analysis, Mr Kiran Mohan for TEM analysis, Dr. K. P. Prathish and Mr. V.K. Shajikumar for the gas analysis. The author S.B also would like to personally thank Mr. K.V. Mahesh, Mrs. Suyana Panneri, and Mrs. Linsha Vazhayal for their helps in completing this work. Supporting Information: Details of the experiment, figures and tables related to (i) The green processing route, (ii) TGA on different Zn dust samples, (iii) XPS deconvoluted peaks, (iv) Gas analysis results, with process explanation in section, (v) Gas collection method with TEDLAR bags, and (vi) Tabulated crystallite sizes of ZnO. References (1) Liu, X.; Xu, T.; Wu, X.; Zhang, Z.; Yu, J.; Qiu, H.; Hong, J.-H.; Jin,C.-H.; Li, J.-X.; Wang, X.-R. Top-down fabrication of sub-nanometre semiconducting nanoribbons derived from molybdenum disulfide sheets. Nat. Commun. 2013, 4, 1776. (2) Yu, H.D.; Regulacio, M. D.; Ye, E.; Han, M.Y. Chemical routes to top-down nanofabrication. Chem. Soc. Rev. 2013, 42, 6006-6018.


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Synopsis Photocatalytically active ZnO nanorods were synthesized via aqueous mechanical milling approach from micron sized Zn dust.


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Graphical Abstract



Figure 1. Growth of surface ZnO with milling of Zn dust, (a) Raw Zn dust , (b) Zn surface after 12 h of milling showing small growths, (c) Zn surface after 48 h of milling showing the flaky transitional phase, (d) enlarged portion of (c) clearly showing the flaky ZnO layer on the surface.


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Figure 2. (a) XRD analysis, (b) UV-vis spectroscopy on the powder from the milled dispersion ( inset shows the dispersions obtained).



Figure 3. (a-c) SEM images showing the surface growth of ZnO on a single Zn particle after 0, 48, and 72 h of milling, (d) Low resolution wide scan on the milled samples (0, 48, and 72 h); High resolution scan for, (e) Zn 2p and (f) O 1s peaks on the milled samples (0, 48, and 72 h).


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Figure 4. (a) SEM analysis on the synthesized ZnO, (b) Average particle size measured.



Figure 5. (a and b) TEM images of the synthesized ZnO, (c) SAED pattern, (d) EDXspectrum and (e) HR-TEM images of the ZnO showing growth in (001) direction.


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Figure 6. Schematic of the oxidation mechanism proposed for the mechanical conversion of Zn to ZnO.



Figure 7. Photo activity studies using H ZnO, M ZnO,17 and CDH TiO2 as the catalysts; corresponding ln (C0/C) vs. Irradiation Time graph (a; b) using dye MB, (c; d) using dye Rh6G.


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Figure 8. (a) Dye degrading mechanism in nano ZnO, (b) UV-vis spectrum of the synthesized ZnO (H and M), inset showing the bang gap energy variation in the two systems (H and M), (c) Percentage (%) of MB and Rh6G dye removal by the different samples (H, M, and CDH) after 60 min, photographs showing the degradation of the dye solutions (15 µM), (d) Rh6G system, (e) MB system; using 0.03 g of H ZnO, after 60 min of UV irradiation.



Table 1. Optimized process parameters for milling. Mill chamber volume ( mL) Milling media Mass of Zn dust charged (g) Liquid carrier, charged (mL) Speed of mill (rev/min) BPMR1 Milling time (h) 1

250 Zirconia balls (10mm dia.,~3g (each)) 20 Distilled water, 170 250 6:1 24,48,72

BPMR – Ball to Powder Mass Ratio


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Table 2. Reaction rate constant values for H, M, and CDH samples for the dyes MB and Rh6G. MB

Samples

Rh6G

Kapp

R

H

0.0936

M CDH

2

2

Kapp

R

0.9937

0.211

0.9773

0.007

0.993

0.004

0.961

0.048

0.985

0.051

0.942



Figure 1 170x98mm (300 x 300 DPI)


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Figure 2 131x199mm (300 x 300 DPI)



Figure 3 170x194mm (300 x 300 DPI)


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Figure 4 127x68mm (300 x 300 DPI)



Figure 5 169x134mm (300 x 300 DPI)


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Figure 6 169x91mm (300 x 300 DPI)



Figure 7 150x123mm (300 x 300 DPI)


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Figure 8 160x109mm (300 x 300 DPI)



Table of contents(TOC) image 83x47mm (150 x 150 DPI)


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Aqueous Mechanical Oxidation of Zn Dust: An Inventive Technique for

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