ORIGINAL PAPER

Template-Free Processing of Ag-Anchored ZnO Polyscale Sheetsand Their Application in the Photocatalytic Degradationof Organics Present in Pharmaceutical Waste

C. P. Sajan1& Amol Naik2

& H. N. Girish3& H. R. Ravi4 & Rupesh Singh5

Received: 16 January 2017 /Revised: 17 March 2017 /Accepted: 25 April 2017 /Published online: 17 May 2017# Springer Science+Business Media Singapore 2017

Abstract Template-free processing of Ag-anchored ZnOpolyscale sheets was successfully synthesized using the one-pot hydrothermal technique. The characterization of the synthe-sized samples was done using powder X-ray diffraction, Fouriertransform infrared spectroscopy, scanning electron microscopy,UV-vis spectroscopy, and X-ray photoelectron spectroscopy.The photocatalytic degradation of organics present in pharma-ceutical waste on exposure to sunlight and UV light confirms thephotocatalytic degradation efficiency of the synthesized samples.The effectiveness of the degradation was measured based on thechemical oxygen demand and percent decomposition of the or-ganics present in the pharmaceutical waste. Our studies revealedthat the photodegradation efficiency of the samples varies withadded amounts of Ag content. The maximum photocatalyticdegradation efficiency was obtained using 2% Ag-depositedZnO polyscale sheets (2AZ), where the rates of decompositionwere 77.8% under sunlight and 76% under UV light.

Keywords Photocatalysis . Hydrothermal . Ag-anchoredZnO polyscale sheets . Pharmaceutical waste

Introduction

Semiconductor photocatalysis is an emerging technique for thedestruction/removal of organic pollutants present in wastewater/industrial effluents. Among semiconductors as photocatalyst, ti-tanium dioxide (TiO2) and zinc oxide (ZnO) are used. TiO2

possesses certain unusual properties such as low-cost and long-term stability and very low toxicity which makes them an attrac-tive candidate as a photocatalyst in environmental remediation[1]. Processing of TiO2 hybrid materials, doped/deposited withmetal oxides such as cobalt oxide (CoO), nickel oxide (NiO),and iron oxide (FeO), has shown an enhanced photocatalyticproperty in the removal of organics [2–4]. However, comparedto TiO2, the quantum efficiency of ZnO is much higher, and so ithas also been extensively used as photocatalyst for the degrada-tion of various pollutants [5, 6]. Over decades, ZnO has beenused as the photocatalyst, due to its unusual properties like hav-ing a wide bandgap (3.37 eV), high exciton binding energy(60 MeV), biocompatibility, and piezoelectricity. Meanwhile,ZnO possesses certain properties similar to that of TiO2 such aslow-cost, high-purity, clear morphological crystals, and environ-ment benign, possessed by ZnO makes them an active semicon-ductor photocatalyst [7–9]. ZnO is an accomplished n-type semi-conductor having useful application in many fields such as gassensors, as a photocatalyst, optoelectronic devices, and solar cells[10–13]. ZnO nanostructures have also been extensively used assensors, actuators, nanoelectronics, optoelectronics,nanogenerators, cancer therapy, and detection, dye-sensitized so-lar cell (DSSCs), making these nanostructures as one of the mostimportant multifunctional nanomaterials having a broad range ofapplications [14–17]. Recently, processing of 2D ZnO

Electronic supplementary material The online version of this article(doi:10.1007/s41101-017-0022-6) contains supplementary material,which is available to authorized users.

* C. P. Sajansajan.saj@rediff.com

1 Department of Studies in Environmental Science, University ofMysore, Manasagangothri, Mysore 570006, India

2 Lead - Chemistry R and D VerdeEn Chemicals Pvt. Ltd, D-11,UPSIDC Industrial Area, Masoorie - Gulawati Road, Distt.,Hapur, Uttar Pradesh 201015, India

3 Department of Studies in Earth Science, University of Mysore,Manasagangothri, Mysore 570006, India

4 PG Department of Physics and Research Centre, Bharathi College,Bharathi Nagar, Mandya 571401, India

5 Department of Chemical Engineering, IIT, Kanpur 208001, India

Water Conserv Sci Eng (2017) 2:31–41DOI 10.1007/s41101-017-0022-6

nanostructures such as nanosheets and their application, particu-larly in gas sensors, has been an interesting topic in the field ofresearch. The 2D structures have a high surface to volume ratios,where the lateral dimensions and the thickness of the surfacespace charge region are comparable [18–22]. The combined ef-fect of these two favors them useful in enhancing the efficienciesat a low-operating temperature having a fast response with highrecovery capacity [18–22]. The hot topic in the field of materialscience is the processing of polyscale ZnO sheets with differentmorphology and doping/ deposition of materials like gallium(Ga), tellurium (Te), europium (Eu), vanadium (V), silver (Ag),and their application in DSSCs, photocatalysis, nanogenerators,and the bactericidal effect [16, 23–31]. Lately, a lot of reportspublished focus on the metal oxide semiconductor modified bynoble metals like silver. These materials are applicable as therelevant material in the field of functional nanostructures[32–37]. While fabricating of these nanocrystals with distinctmorphology, most of the researchers opt for templates/cappingagents/organic or inorganic ionic additives/surfactants to controlthe morphology and growth direction of the crystal [38–40]. Inour present work, we report on the novel method applied for theprocessing of Ag-deposited polyscale ZnO sheets in the absenceof template and their enhanced efficiency in the decompositionof the organics present in the real time pharmaceutical waste.

Experimental

Materials

Ranbaxy Chemicals Co. Ltd. and Loba Chemicals Co. Ltd.supplied Zinc oxide (ZnO) and silver powder (Ag). Both thereagents were laboratory grade and used without further puri-fication. Throughout the experiment, distilled water was used.

Template-Free Processing of Ag-Anchored Polyscale ZnOSheets

In the template-free processing of Ag-anchored polyscale ZnOsheets, hydrothermal technique was employed. Commerciallyavailable ZnO was utilized as a starting material in the process-ing of Ag-anchored polyscale ZnO sheets. In a Teflon liner,2.025 g of ZnO was taken to which 25 ml of distilled waterwas added and stirred for 2 h till the formation of a whitesolution. On addition of 0.027 g Ag powder (1 wt%), the mix-ture was stirred using a magnetic stirrer for another 1 h. TheTeflon liner was closed and placed in an autoclave and heated at150 °C for 24 h. Similarly, different weight % of Ag was addedand hydrothermally treated at the set temperature. After theexperimental run, the sample inside the liner was separatedfrom the solution and washed with the double distilled waterfor three cycles and then ultrasonicated. The product extractedwas centrifuged to remove undesired components and dried at a

temperature of 35–40 °C in a dust-free environment. The ob-tained samples were labeled based on the ratio of ZnO: Ag as0AZ, 1AZ, 2AZ, 3WT, 5AZ, and 10AZ, where 0, 1, 2, 3, 5, and10 represent the weight % of Ag added.

The photocatalytic degradation of pharmaceutical waste wasachieved using the synthesized sample. The pharmaceuticalwaste (pharmaceutical influent without treatment) collectedfrom RV chemical industry located in JP Nagar industrial area,Mysore, was utilized in the photocatalytic degradation studies.Specification of the pharmaceutical waste and the limitation asper the Environment (Protection) Rules, 1986 is given inTable 1. A known amount of synthesized sample was intro-duced in a beaker containing 50 ml of the pharmaceuticalwaste. For comparison, the experiment was carried out underboth UVand sunlight. The pharmaceutical waste to which thephotocatalysts was added was placed in the UV chamber(Sankyo, Denki, Japan, 8 W) which serves as UV source.The distance between the UV light and pharmaceutical wastewas 18 cm. Photolysis of uranyl oxalate estimated the intensityof sunlight and UV light. It was determined that the intensity ofsunlight was 1.47 W/m2, and the intensity of UV was 0.52 W/m2. On exposure to light, 2–3 ml of the sample was taken andcentrifuged for 4–5 min at 1000 rpm. The extract was obtainedand further used to measure the destruction of organics presentin the pharmaceutical waste. COD test of the samples exposedto light was carried to know the organics removed/degraded inpharmaceutical waste. COD was estimated before and after thetreatment (using the K2Cr2O7 oxidation method). In the ab-sence of light and catalyst, the blank experiment was carried.The photodegradation efficiency of the synthesized samples inthe removal of organics present in the pharmaceutical wastewas estimated using the equation:

Photodegradation or decomposition

¼ initialCOD−finalCOD=initialCOD � 100

Instrumentation and Characterization

The powder X-ray diffraction (XRD) pattern of the synthe-sized samples was recorded using Rigaku Miniflex X-ray dif-fractometer, model IGC2, Rigaku Co. Ltd., Japan. The 2θrange was set between 20° and 80°. The identification of thecrystalline phase is accomplished by comparing with JCPDSusing PCPDFWin version 2.01; the cell refinement was madeusing chek cell software. The Fourier transform infrared spec-trometry (FTIR) spectra were recorded using JASCO-460Plus, Japan. The structural features of the synthesized sampleswere analyzed using a high-resolution scanning electron mi-croscope (SEM), model HITACHI S-4200. UV-visible spec-trophotometer, model UV-2550, Shimadzu, Japan, was

32 Water Conserv Sci Eng (2017) 2:31–41

adopted to record UV-visible absorbance spectra for the dry-pressed samples using BaSO4 as a standard.

Results and Discussion

XRD of Polyscale ZnO Sheets and Ag-Anchored PolyscaleZnO Sheets

The powder XRD patterns of bare ZnO sheets and Ag-anchored polyscale ZnO sheets are shown in Fig. 1. The iden-tification of the crystalline phase of these samples was finalizedby comparing with JCPDS file (PCPDFWIN-2.01). The XRDpatterns for both polyscale ZnO sheets and Ag-anchoredpolyscale ZnO sheets match with PDF: 800075 representing ahexagonal system, belonging to space group P63/M. All thediffraction peaks of the synthesized samples match with thestandard data of PDF: 800075. However, the development ofnew peaks at 37.9 and 44.0 corresponds to 111 and 200 phaseof Ag. The intensity of these peaks increases with increase inAg content. Meanwhile, there is a very slight shift in peakintensities of 0.2 angle in all the samples, suggesting the effectof the deposition of Ag onto polyscale ZnO sheets.Furthermore, minor changes in the average lattice constants aand c of ZnOwere detectedwhich increases with increase in Agcontent (Table 2). The ionic radii of Ag2+ are comparativelyhigh compared to Zn2+ (ionic radii of Ag2+ is 94 pm and Zn2+

74 pm). The incorporation of Ag2+ into ZnO crystal latticemight reduce the cell volume and might lead to the peaks shift

to the larger angles with the increase of Ag2+ content [41].Similarly, the replacement of Ag2+ in place of Zn2+ might leadto an increased volumetric expansion of the crystal. However,no much change in the cell parameters was observed furtherconfirming that silver ions have not entered into the ZnO latticebut has loaded onto the ZnO surface. No other impurities werefound in the XRD pattern of these samples suggesting that Ag-anchored polyscale ZnO sheets can be prepared in the absenceof templates by employing the one-pot hydrothermal technique.

FTIR Studies of Polyscale ZnO Sheets and Ag-AnchoredPolyscale ZnO Sheets

The FTIR spectral studies of polyscale ZnO sheets and Ag-anchored polyscale ZnO sheets were conducted to support thepresence of Ag in the synthesized samples (Fig. 2). The peaksaround the region 3400 cm−1 in the spectra represent an O–Hstretching [42], which is originated due to the addition ofwater during post treatment after the hydrothermal run. Theband within the region 1600 and 2360 cm−1 is assigned to theC–O stretching, caused by the absorption of atmospheric CO2

on the metal cations [43, 44]. The area between 470 to500 cm−1 clearly represented the Zn–O stretching vibration[45]. The band in the region 538 cm−1 corresponds to thecharacteristic stretching mode of Ag–O [46].

SEM Studies of Polyscale ZnO Sheets and Ag-AnchoredPolyscale ZnO Sheets

The micrographic study reveals the morphology and the struc-tural features of the synthesized samples (Fig. 3). The synthe-sized sample extracted was found to be thin sheets, having athickness of approximately 100 nm (Fig. 3b), exhibiting hexag-onal shape and the size ranging from few nanometer to millime-ter (Fig. 3a). Furthermore, in a sample containing Ag (10AZ),small tiny crystals ranging from 10 to 20 nm were observed

Fig. 1 XRD powder diffraction pattern of polyscale ZnO sheets and Ag-anchored polyscale ZnO sheets

Table 1 Specification of pharmaceutical waste used in present work

Parameters Color pH E.C (μS/cm) Tur. (NTU) TDS (mg/l) TSS (mg/l) COD (mg/l) BOD (mg/l)

Standards/limitation (The Environment(Protection) Rules, 1986)

– 6.0–8.5 – – – 100 250 30

Pharmaceutical waste in present work Colorless 5.96 3698 49.94 498.5 70.45 886 224

Table 2 Cell parameters of polyscale ZnO sheets and Ag-anchoredpolyscale ZnO sheets

Compound a-axis Å b-axis Å c-axis Å Cell volume Å3

OAZ 3.2501 3.2501 5.2100 47.662

2AZ 3.2502 3.2502 5.2088 47.653

5AZ 3.2508 3.2508 5.2077 47.661

10AZ 3.2530 3.2530 5.2090 47.737

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(Fig. 3b, c), confirming the anchoring of Ag particles ontopolyscale ZnO sheets.

UV-vis Diffuse Reflectance Spectra

The UV absorption spectral studies were carried out for 0AZ,1AZ, 2AZ, 5AZ, and Ag samples (Fig. 4). Except for Ag sam-ple, the absorption edge for rest of the sample is at ∼399 nm,which is reliable with the intrinsic bandgap absorption of ZnO(∼3.1 eV) [47]. Our study further confirms that with an increasein Ag content, there is a major increase in the absorption of thesamples, beyond wavelength 390 nm. There are two possiblereasons for the increase in absorption on the amount of Agadded. Firstly, the improvement in the absorption of the sampleswith an increase in Ag content is related to the localized surface

plasmon resonance (LSPR) caused due to the spatially confinedelectrons in Ag particles [48, 49]. Secondly, the black bodyabsorption caused due to the addition of Ag powder [50, 51].However, with an increase in Ag content, no much shift in theabsorption was observed confirming that the Ag has not intrudedinto the ZnO lattice, but has been effectively deposited onto theZnO sheets which are in agreement with the SEM studies.Furthermore, to understand the role of Ag in tuning the bandgapenergy of ZnO, the bandgap energy of the samples mentionedabove was estimated using Tauc plot and the extrapolation of thelinear slope of photon energy (Fig. 4b) [52, 53]. The bandgap of0AZ was 3.12 eV, whereas the bandgap of 1AZ, 2AZ, and 5AZwas determined as 3.16, 3.14, and 3.14, respectively. Based onthe bandgap studies, one can clearly conclude that the depositionof Ag has caused slight changes in the bandgap energy of ZnO.When the silver particles are deposited on the surface of ZnO, aheterojunction is formed, where the silver particles behave as anelectron acceptor. In the absence of silver particles (0AZ),photogenerated electrons are stored near the conduction bandcausing charge recombination which in turn reduces the photo-catalytic performance of the sample. However, the slight differ-ence in the bandgap of 0.04 eV further leads to the conclusionthat the Ag has not intruded into the crystal lattice of ZnO, buthas been effectively deposited onto ZnO sheets.

XPS Analysis

To confirm the anchoring of Ag particles onto the surface ofZnO sheets and to determine the chemical status of Ag ele-ment, the samples were analyzed by XPS. The survey spectra(Fig. 5a) of the samples 0AZ and 2AZ indicate the presence of

Fig. 2 FTIR spectra of polyscale ZnO sheets and Ag-anchored polyscaleZnO sheets

Fig. 3 SEM images of apolyscale ZnO sheets, b high-resolution image of polyscaleZnO sheets showing its thickness,c Ag-anchored polyscale ZnOsheets, and d high-resolutionimage of Ag-anchored polyscaleZnO sheets

34 Water Conserv Sci Eng (2017) 2:31–41

Ag, Zn, and O elements and their resultant binding energies at22.6 (Zn3d), 96.9 (Zn3p), 149.2 (Zn3s), 122.5 (Zn2p3),1043.3 (Zn2p1), 367.8 (Ag3d5), and 530.4 (O1s).The peakin the region 295.1 corresponds to C1s. The presence of Celement is due to the absorption of atmospheric CO2 ontothe metal cation or might be due to adventitious hydrocarbonfrom XPS instrument. XPS spectrum of Zn2p (Fig. 5b) showsthe peaks with binding energy at 1043.6 and 1017.09 eV cor-responding to Zn2p1/2 and Zn2p3/2, confirming the presenceof Zn in Zn2+ form. In XPS spectra of Ag3d (Fig. 5c), thepeaks centered at 373.3 and 367.4 correspond to Ag3d3/2and Ag3d5/2. On the other hand, there is a variation of5.9 eV between the binding energies of Ag3d3/2 and Ag3d5/2

peaks, which further confirm that the silver present in thesample is metallic nature. Generally, in Ag3d XPS spectra ofmetallic silver, the binding energies of Ag3d3/2 and Ag3d5/2are 374.2 and 368.2 eV, respectively. However, in our presentwork, there is a shift in Ag3d peak position to a lower bindingenergy caused due to the movement of electrons frommetallicAg to ZnO sheets. Furthermore, the transfer of electrons frommetallic Ag to ZnO sheets led to an inference that there is aclose association among the metallic Ag and ZnO sheets head-ing to the formation of a heterojunction between Ag and ZnO.In O1s, XPS spectrum (Fig. 5d) represents the deconvolutedpeaks. The peak positioned at 529.1 eV is attributed to theoxygen anions of ZnO lattice, whereas the peak positioned

Fig. 4 UV-vis absorption spectra of polyscale ZnO sheets and Ag-anchored polyscale ZnO sheets

Fig. 5 XPS spectra of polyscaleZnO sheets and Ag-anchoredpolyscale ZnO sheets

Water Conserv Sci Eng (2017) 2:31–41 35

at 530.4 eV can be ascribed to the chemically absorbed oxy-gen onto the surface of ZnO sheets.

Photodegradation Studies of Pharmaceutical Waste UsingPolyscale ZnO Sheets and Ag-Anchored Polyscale ZnOSheets

The photocatalytic degradation of pharmaceutical waste ex-posed to sunlight and UV light was done using polyscale ZnOsheets and Ag-anchored polyscale ZnO sheets. In the degra-dation process, the amount of sample used was 30 mg. In a100-ml beaker, 50 ml of the pharmaceutical waste and 30 mgof the synthesized sample were taken. The beaker containingpharmaceutical waste and the sample was kept stirred using amagnetic stirrer. Soon after the addition of sample, 2–3 ml ofthe pharmaceutical waste was extracted and centrifuged, andtest for CODwas carried out. This experiment was consideredas 0-h experiment. In a similar way, after the addition of sam-ple, the pharmaceutical waste was placed in the dark chamberfor 1 h and was kept stirred. This experiment was considered

as a dark experiment. Similarly, a set of experiments wascarried out at a different time interval (Figs. 6 and 7).

The photodegradation profile for the degradation ofpharmaceutical waste using 0AZ, 1AZ, 2AZ, 3AZ, 5AZ,and 10AZ samples exposed to sunlight and UV light isgiven by C/Co, where BC^ stands for the initial COD ofthe pharmaceutical waste, and BCo^ represents final CODobtained after exposed to sunlight/UV at different time in-tervals. The degradation profile further reveals that the deg-radation of pharmaceutical waste is higher when 2AZ sam-ple is used which further increases with respect to time(Fig. 6a, b). Furthermore, the reaction kinetics for thephotodegradation of the pharmaceutical waste was estimat-ed using the equation –In (C/Co) = kt, where C is theconcentration of pharmaceutical waste after exposing tosunlight/UV at the time (t), Co is the initial concentrationof pharmaceutical waste, Bk^ is the apparent reaction rateconstant, and Bt^ is the reaction time. The linear plot of –In(C/Co) verses t was plotted to obtain the obvious rate con-stant of the reaction (Fig. 6c, d). The estimation of thephotocatalytic efficiency for the prepared samples was

Fig. 6 a, b Degradation profile of pharmaceutical waste exposed to sunlight and UV light using 0AZ, 1AZ, 2AZ, 3AZ, 5AZ, and 10AZ samples. c, dKinetic studies of pharmaceutical waste degradation exposed to sunlight and UV light using 0AZ, 1AZ, 2AZ, 3AZ, 5AZ, and 10AZ samples

36 Water Conserv Sci Eng (2017) 2:31–41

obtained based on the reaction kinetics of the photocatalyticdegradation of the pharmaceutical waste.

In the photocatalytic degradation of the pharmaceuticalwaste, the rate constant k in min−1 is obtained by drawing aslope. The photodegradation of pharmaceutical waste exposedto sunlight and UV light follows pseudo first-order kineticreaction. The reaction rate constant k (min−1) for the synthe-sized samples in the photocatalytic degradation of pharmaceu-tical waste was anticipated from the slope of the kinetic plot(Fig. 7a, b). The correlation coefficient and the rate constantobtained were extracted from the plots which are tabulated inTable 3.

Among the synthesized samples to know their efficiency, acomparison study on the photocatalytic decomposition ofpharmaceutical waste was performed under both sunlightand UV light (Fig. 7c). Maximum efficiency of 77.8% undersunlight and 76% under UV light was observed using 2AZsample. It should be noted that among the sunlight striking theearth’s surface, 52 to 55% is infrared (above 700 nm), 42 to

43% is visible (400 to 700 nm), and 3 to 5% is ultraviolet(below 400 nm). In the present work, the intensity of thesunlight was more compared to that of UV light (based onphotolysis of uranyl oxalate) due to which the efficiency of

Fig. 7 a, b Kinetic studies of pharmaceutical waste degradation exposedto sunlight and UV light using 0AZ, 1AZ, 2AZ, 3AZ, 5AZ, and 10AZsamples. c Comparison of the photocatalytic decomposition

pharmaceutical waste using 0AZ, 1AZ, 2AZ, 3AZ, 5AZ, and 10AZsamples exposed to sunlight and UV light. d Cyclic decomposition ratecurve of pharmaceutical waste exposed under UV light using 2AZ sample

Table 3 Kinetic parameters obtained for the degradation ofpharmaceutical waste on exposure to sunlight and UV using differentsamples

Samples Sunlight UV light

k × 102 (min−1) R2 k × 102 (min−1) R2

0AZ 6.477 0.96038 6.198 0.96492

1AZ 6.586 0.95796 6.364 0.96212

2AZ 6.693 0.95504 6.527 0.95874

3AZ 6.467 0.95907 6.256 0.96337

5AZ 6.27 0.96288 6.087 0.96641

10AZ 6.107 0.96648 5.883 0.96986

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the samples is more under sunlight compared to UV light.Even though the samples were exposed to sunlight, there isno expected role of visible light in the present work, sincethere are no much changes in the bandgap of the sampleswhich is in agreement with the UV studies. Furthermore, thefree movement of electrons from Ag particles to the surface ofZnO favors in reducing the electron-hole recombination. Thisresults in enhancing the degradation efficiency of thephotocatalyst. However, 10AZ sample shows less efficiencycompared to rest of the samples. There are two possibilities forthe reduction in the photocatalytic efficiency. The increase inAg content leads to the formation of charge recombinationcenters, which favors the recombination of the electron-holepairs. On the other hand, when the ZnO sheets with more Agcontent (10AZ, 5AZ) were added to the pharmaceutical wastewhich is colorless, the photocatalyst gets dispersed in thepharmaceutical waste. The color of the pharmaceutical wastechanges from colorless to gray/ash color and also increases theturbidity of the pharmaceutical waste. The increase in the tur-bidity and change in the color of the pharmaceutical wasteretards the penetration of light through it which in turn reducesthe absorption of light by the photocatalyst/the intensity oflight which further reduces their efficiency. Furthermore, toknow the stability of the photocatalyst, the % decompositionperformance of the pharmaceutical waste in a cycling photo-catalytic run for three cycles was performed using 2AZ underUV light. After every cycle, the samples were extracted bycentrifuging at 1000 rpm and allow them to settle for a deten-tion period of 2 h such that the tiny particles which are stillsuspended after centrifuging get settled down. The samplesare extracted washed using double distilled water, centrifuged,dried, and reused. The 2AZ sample showed good

photocatalytic property for three cycles. The efficiency of2AZ after three cycles has reduced from 76 to 75.3% whichis considerably less (Fig. 7d). Based on this study, it can beconcluded that the 2AZ sample possesses good stability in thephotodegradation process. It should be noted that the stabilityand recycling performance are among the crucial factors thathas to be considered while selecting a photocatalyst for prac-tical application.

The effective decomposition of pharmaceutical waste usingAg-anchored ZnO sheets under the influence of sunlight andUV light is strongly influenced by some factors. The 2D struc-ture possessed by the photocatalyst plays a key role in enhanc-ing the efficiency of the photocatalyst. The thickness of theZnO sheets is of few nanometers which contribute to the highsurface-to-volume ratios which also play a vital role in en-hancing the efficiency of the photocatalyst. Normally, inZnO 1D structures such as nanowires and nanorods, the sur-face area of (0001) is very low [54–56]. This is because the(0001) plane of the ZnO having highest surface energy pro-motes the growth of the crystal towards c-axis. However, inour present work, the growth rate of the six symmetric non-polar (1010) planes which move towards the top (0001) direc-tion and a basal polar oxygen (0001) plane direction are sup-pressed due to which more (0001) planes are formed. Thepresence of enormous (0001) plane plays a significant rolein enhancing the degradation rate of the photocatalyst [57].The ZnO sheets having larger specific surface area favor theadsorption organics present in the pharmaceutical waste.Meanwhile, on exposure to light, the light-generated chargecarriers which are generated within the ZnO sheets are trans-ferred towards the surface which further reacts with the or-ganics adsorbed onto the surface of the sheets. In the case of

Fig. 8 Proposed band structuresof the Ag-anchored ZnO sheets

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Ag-anchored ZnO sheets, the bandgap of metallic Ag-anchored onto the ZnO sheets is higher than that of ZnOsheets. However, it should be noted that the work functionof ZnO is (5.2 eV) [58] which is larger than that of Ag(4.2 eV) [59], due to which the fermi energy level of ZnO islower than the fermi energy level of Ag. In the absence oflight, in order to maintain the bandgap equilibrium (Ef), theelectrons generated in Ag flow towards the conduction bandof ZnO sheets [60].

It is well known that when ZnO is exposed to light higherthan its bandgap energy, the photoexcited electrons are formedin the conduction band (CB), and holes are formed in thevalence band (VB) of ZnO. Comparing the energy level ofthe CB, the energy level of ZnO CB is higher than the Ag, dueto which, the electrons tend to move/transfer freely from theCB of ZnO sheets to the Ag particles which are anchored ontothe ZnO sheets leaving behind the holes in VB (Fig. 8).Meanwhile, the Ag particles act as the electron sink whichfurther reduces the rate of photogenerated electron and holerecombination which in turn increases the life span of theelectron-hole pair. On to the ZnO sheets, so formed electronand holes react with the oxygen and water molecules, [60]adsorbed onto the sheets leading to the formation of highlyreactive species such as superoxide radical and hydroxyl rad-ical which are the important candidates in the photocatalyticprocess. These active species further participate in the decom-position of toxic organics present in the pharmaceutical wasteinto environment-friendly by-products.

Conclusion

Template-free processing of Ag-anchored ZnO polyscalesheets was carried out under mild hydrothermal condition.Characterization studies clearly revealed that Ag has beenanchored onto the ZnO sheets. The photocatalytic degradationof pharmaceutical waste further demonstrated that the photo-catalytic reaction kinetics depend on a variety of parameterssuch as light source, type of photocatalyst used, duration ofexposure, and ratio of metallic compound deposited onto thephotocatalyst. It can be concluded that the addition Ag in rightproportion enhances the rate of photocatalysis of ZnO sheets.This is due to the reduction in charge recombination due to theanchoring of Ag onto the ZnO sheets. The stability test con-ducted confirms that the as-prepared photocatalysts can bereused for a number of cycles, which in turn will decreasethe cost of operation. In our present work, both sunlight andUV light were used as the source of light. The use of sunlightis considered extremely cost-effective source, safe, and envi-ronment-friendly. The reduction in COD clearly depicts thedestruction/decomposition of organics present in the pharma-ceutical waste. The overall study leads to the conclusion thatthe use of Ag-anchored ZnO polyscale sheets in

photocatalysis is economic, eco-friendly, and an effectivemethod for the removal of organics from industrial waste.

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