Facile Synthesis of CeO2 Blended CoWO4 Nanocomposites For High Photocatalytic Performance And Investigation of Their Antimicrobial Activity


 In this work, novel CeO2/CoWO4 hetero structured nanocomposites (NCs) was synthesized via a hydrothermal method. X-ray diffraction (XRD), high-resolution transmission electron microscopy(HRTEM), UV-VisDRS and photo luminescence (PL) spectroscopy were categorized herein to gain the crystal structure, deep morphology, optical assets, and charge separation of the as-produced photocatalysts (PCs)respectively. Related by the pristine CoWO4,CeO2 and CeO2/CoWO4 photocatalyst was considered by enriched activity of them ethylene blue (MB)aqueous dye photodegradation under visible light exposure. Chiefly, the photodegradation efficacy of as-attained CeO2/CoWO4 photocatalyst exposed the premier decomposition ratio (92.5%) of MB dyein105 min among all samples, which was noticeably 1.8 and 2.2 folds over the pristine CeO2 (43 %) and CoWO4(60 %), separately. Likewise, the CeO2/CoWO4 PCs sustained satisfactory activity even after 4sequentialrecycling runs, signifying its great photocatalytic steadiness and robustness. Hence the ensuing superior PCs preferred the further efficient charge (e−-h+) separation, solid visible light fascination, and worthy interfacial energy transfer leads amid CoWO4 and CeO2nanoparticles (NPs).A likely mechanism liable for photodegradation was eventually projected. The synergistic things of antibacterial motion via CeO2/CoWO4 NCs were probed by the weld diffusion scheme. Thus, effort finding deals with a novel avenue for the growth of stable and proficient visible-light active PCs for environmental purification.


Introduction
As photocatalysis deliberatedexistence one of the utmostcapable ways to resolve the growingenvironmental weakening fuel and energy problems,semiconductors (SCs) centeredphotocatalysts (PCs) consumebroadlyfascinatedinto several multidisciplinary domains of materials, environment, chemistry, and copiouscourtesyfor thesevererejection of riskywater toxins (Chowdry et al. 2019;Shu et ak. 2017) Until now, ithas recognized that ecological effluence owed to the speedy industrial and population evolution is a kind of utmost vital tasks facing entire leaving existences in wide-reaching. Still, none of these semiconductor PCs couldcertainlyfulfillfor entiresupplies of realuses, for occurrence, greatexploitation of solar energy, greateffectiveness, high steadiness and low cost (Alshehri et al. 2017;Ke et al. 2018). For case, one of the utmostimperative and low-priced colorants, methylene blue (MB) remainstypicallyused in fabrics as a pigment, photographic and printing trades, also retains the carcinogenic, mutagenic in wildlife,chronic noxiousnessandcould be even bio-accumulate in the food chains towards hominids/animals (Ke et al. 2018;Subhan et al. 2015) .
Accordingly, to eliminatethe MB dye since the ecological wastewater byvisible-light-driven photocatalytic progressionstandsanactualcapableand conservationresponsivepolicy to exploit solar energy, togethertoresolve the ecologicaldifficulties (Rashad et al. 2014).Lately, the photocatalytic action of transition metal oxides(TMO)and Amidseveral lanthanides, CeO2 nanostructures (NSs)resourceshave fascinated the concern of distinctivepossessionsand the former decade owing to their extensiveclaims in catalysis, gas sensors, ultraviolet ray detectors, and eco-friendly dyestuffs, etc., (Cui et al. 2019;Wetchakun et al. 2012). In CeO2 is a chemically steady oxide nanomaterial, series of its discreteassets such as great optical transparency in the visible province, constancy at high temperature great hardness and proficiency of provided that oxygen species to easy source or relief oxygen alterationamong Ce 3+ and Ce 4+ oxidation states of their lattice oxygen assemblyin oxygen vacancies (Vo).The cobalt tungstate (CoWO4) as a uniquetransition metal tungstate photocatalyst, retain a narrow band-gappedp-type semiconductor(Eg =~2.8 eV). Sincegreat stability, situationsociability and low-cost assembly, CoWO4 has been used in sensors, colorantessences,supercapacitors, photovoltaic electrochemical cells, and photocatalystdeprivation (Jothivenkatachalam et al. 2015).Yet, owing to the shortefficiency of (e --h + ) separation, the photocatalytic action of pristine CoWO4nanomaterials (NMs)was quite low. As is fine-recognized by agents, the assemblyof heterojunctionassemblyamongSCs couldseriouslyimprove the photocatalytic movement of pristine NMs by refiningtheir charge separation capability and enlargementof the light captivationseries (Ahmadi et al. 2016 C16H18ClN3S) dye from SD Fineand was used as received. All the chemicals stayed of analytical/methodical reagent (A.R) grade and used without extrarefinement, deionized water (D.I) was used in wholeinvestigates.

Preparation of CeO2/CoWO4nanomaterials
Concisely,0.03 mol of Ce(NO3)3·6H2Owas ultrasonically dissolved in 100 ml of D.I water. NH4OH was gradually dropped directly into the above solution for pH extended at ~ 12. Lastly, the composed precipitants  (Yan et al. 2017). Conferring to this scheme, the pristine CoWO4NPs was also attainedviaperformance withoutaccumulation of CeO2NMs.

Characterization of the as-prepared samples
Depiction ofall the X-ray diffraction (XRD) pattern wasacquired on anX-ray diffractometer (Rigaku Miniflex-II; X-ray diffractometer) over CuKα radiation.FTIR revisions were done by the Perkin Elmer RX-1 FTIR spectrophotometer. Surface morphologies of as given NMs were scrutinized viahigh resolution scanning electron microscope (HR-SEM; HITACHI S-3000 H) inquiry. The deep morphology,shape and particle size of the sample were recorded byhigh-resolution transmission electronmicroscopy(HR-TEM)imagesusing JEM-2011; JEOL-Japan.
The optical belongingsof the attained samples were considered by a UV-Vis DRS spectrophotometer(UV2550 model, Shimadzu -Japan). The optical absorption of dye degradation samples was performed via a UV-Vis spectrophotometer (UV; Perkin-Elmer Lambda-19).The photoelectron transfer of the catalyst was studied by photoluminescence (PL) spectra on a Perkin-Elmer-LS 45Spectrometerat an excitation sequence of ~ 321 nm.

Photocatalytic activity of MBdye degradation
The photodegradationenactment of as-obtained samples (50 mg) was measured by the degradationof MB (20 ppm; 100 mL solution; 10 mg/L) under visible-light exposure(300 W Xe lamp byλ>420 nm cutoff filter in a Pyrex photocatalytic vessel (Madhan et al. 2015). Preceding toexposure, the suspensions stayedconstant magnetically stirred for around30 min in the dark to certify that the dyesmightextend theabsorption-desorption balance on the photocatalyst superficialand dyes. Successively, the suspension (about 2.5 mL) was

Antibacterial activity
To resolvetheantibacterial activity of CeO2/CoWO4 NCs against pathogenic bacterial strainsviz.Gram-ve (G -)of (Escherichia coli; E. coli) and Gram +ve (G + ) of (Staphylococcus aureus; S. aureus)bacteria,was expert by agar weld diffusion method. The CeO2/CoWO4 NCs dispersed in DMSO to final concentration solutionswere filtered by Millipore filters for sterilization. The weldimpregnated with 25, 50and 75µg/mLof sample solutions, hencestandard antibiotics DMSO (as negative control) and Ampicillin (positive control) were placed and against the pathogenic bacterial strains on the inoculated agar (Nasir et al. 2017). Antibacterial activity was quantified throughcomputing the distance of circular clear inhibition zones (IZ) on the imperviouscontextual of microbialgrowingafterwardcultivationon 37 • C for 36 h in bacterial strains and average values of both assayswere intended also tabularized.

XRD analysis
The crystalline structure and phase purityof as-obtained pristine CoWO4, CeO2, andCeO2/CoWO4NCs are considered by the XRDpattern. As displayed in Figure. 1,the pristine CoWO4sample displaysthespecificXRD

FTIR spectral analysis
FTIR analysis was supported to conquer a better visionof the chemical structures, functional groups informationof pristine CeO2, CoWO4 andCeO2/CoWO4NCsover the frequency range of 4000-400 cm -1 as revealed in

Surface morphology with the elemental outline
The surface morphology of as-ascribed pristine CeO2, CoWO4, and CeO2/CoWO4 NMs samples was resolute by HR-SEMimageries as exposed in Figures  HRTEM method was used to moresurveyto approvethe features of deepnessmorphology, crystalline nature and particle size of as-obtained NCs. As exposed inFigure 5(a) apparentlyexposesthe high magnification of CeO2/CoWO4NCs, it was observed that quasi-spherical NPs were noticeablydispersed andaffixedover each other (Alsheri et al. 2017;Cui et al. 2019). The as-organizedCeO2/CoWO4NCsby the average sizesin the range of virtually25-29 nm in attendance of an effective hydrothermal heating manner. Likewise, the selected area electrondiffraction (SAED) outlinesthe inset of Figure. 5(b)exposes that the vibrantlattice fringe fixedmutually, which atteststhegoodconstruction of interfacial assemblyamong CoWO4and CeO2NPs in thecomposites also specifying random directions, whereas it is well-consistent by the XRD outcome. Henceforth it hasfavorable for fact that the photoexcited charge carrier (e --h + ) effortamid the CeO2/CoWO4NCs (Balavi et al. 2013).

UV-Vis DRS absorption spectra
As is well recognized, the photocatalytic action of a photocatalyst is determined via the light absorption NPs leads to the slighter of the bandgapenergysimilarly increasing the visible-light absorptionaptitude (Wang et al. 2018). It was itemized that the well-suitedoverlying band-structure in the composite couldquicken the separation/charge-transfer of electron-hole (e --h + )sets, thusproposing that CeO2/CoWO4NCs mightbe refining the photocatalytic motionfor the exclusion of organic impurity under visible light revelation (Pourmasoud et al. 2017).

Photoluminescence (PL) property
To categorize the excitation stuff, trapping, charge migration, and separation effectivenessof photomotivated chargecarriersof the as-attained samples was moreappraisedviaPLemission spectrascrutiny (Yang et al. 2019). The room hotnessPL spectra of the CeO2/CoWO4heterojunction NCs,pristine CeO2,andCoWO4 NPs were publicized in Figure. 7.Since Figure 7, it was originated that the PL emission intensity of CeO2/CoWO4was depressed than pristine CeO2andCoWO4 NPs. Speciously the outline of CoWO4NPsindicated a strong emission peak at placed~ 428, 446 and 478 nm, the CeO2/CoWO4sample exposedknowinglylessened PL intensity, which expressesthat the heterojunction reservedvia blending with CeO2 and CoWO4 NPs effects in a strangelyhinderedthe recombination rate of photoexcited(e --h + ) couples,henceaugmenting the photocatalytic enactment (Singaram et al. 2017). The effectsalso signifying the upgradedcharge separation adeptnessowing to the transference of photoelectrons (e -)across the boundary of CeO2 and CoWO4pairing materials.

Photocatalytic performance under visible light treatment
The photocatalytic actions of the as-obtained samples were estimated via the aqueous MB dye photodegradation by visible-light exposure. Figure. Additionally, bythe purpose of better compare/defend the photocatalyticdoingsandphotodegradationratekinetics of MB dye consideredover as-inventedsamplessurveyedthe pseudo-first-order kinetic model ( Figure. 10 (a)). Also,lnC0/Ct=kt, where k symbolized the first-order rate constantplanned from a graph, C0symbolizes the initial attention and Ctdesignates the concentration of MB dye afterward an exposure time t of visible light (Kumar et al. 2016 Qiao et al. 2018). Lastly, this photo-excited eand h + couldrespond with H2O to generateOH •radicals and oxygen species rather than to decay the dye molecules.

Reusability test
The reusability and steadinessof photocatalyst NMs are a furtheressentialaspect in real-timeapplications.
The succeeding runs of aqueous MB dye over the CeO2/CoWO4active PCs under visible light were discoveredtoevaluate itsstability trials( Figure. 10 (b)). Respectively cyclic runsrecuperate well again the photocatalyst viacentrifugationwithfurther washed throughD.I water in numeroustimes and dried, hereafter the recovered PCs remainedusedfor the successivecatalytic runs .As directed inFigure. 10 (b), there is no deceptivereduction of photodegradation effectiveness during fourreusability paths, likewise,theratio of the effectiveness was faintlydeclined from 92.5 % in the first run to 88 % in the fourth run. Thislessening in photodegradation efficacystaysascribed to the photo-dissolution and photo-corrosion of theas-obtained catalyst. In accumulation, theXRD patterns and FTIR spectra of the CeO2/CoWO4NC before and after fourrepeated runs were explored in Figure. 11 (A andB).Hence, there was no evidentvarianceamid the intact PCs, establishing that the coupling of CoWO4 and CeO2NMs might availably constrain therecombination of photoexcited (e --h + ) charges to a convinced degree, authorizing the good stability and robustness of CeO2/CoWO4PCs.Thus, the reusability oftheseoutcomesvalidates that the CeO2/CoWO4photocatalyst ownsgreat reusable steadiness and prospectivefor practical photocatalytic applications (Magdalane et al. 2017;Yuan et al. 2017 andZi-ya et al. 2018). Besides, the photodegradation and photoactivity of the CeO2/CoWO4 catalyst wereequated with several nanocomposite materials (Wei et al. 2014;Rajendran et al. 2016;Hamrouni et al. 2013;Luo et al. 2012;Yang et al. 2012) and it originated in Table. 1.

Detection of reactive species and the photocatalytic mechanism
Thatinspectthe photocatalytic mechanisms of CeO2/CoWO4 heterojunctionphotocatalyst, the dissimilar scavengers were presented to recognize the influences of one or furtherintermediaryreactive/oxidative species (•OH -, O2•and h + ) in a photocatalytic manner. As illustratedin Figure. 12, once BQ was added, the degradation competence of MBaqueous colorant was evidentlydeclinedfrom 88.5 % to 23 %, signifying the • O2played adynamicpart of active species tophotodegradation (Soltan et al. 2017) . Equally, a momentous loss of photodegradation (45 %), whereasthe adding of IPA, proving the • OHpathway influenced a decisive role in the degradation progression. Moreover,whileEDTA-2Na wasadded inaphotocatalytic way, the degradation efficacy of dye was faintly depressed,specifying that littleholes (h + ) wereonly involved also it has not a crucialprovider in the photodegradationprogressionof MB dye subtraction. As aneffect, it might be resolved that • O2and • OHwere centraldonatedroles of reactive species in the photocatalytic routein visiblelight revelation (Cardillo et al. 2016;Zhu et al. 2018) On the source of the upstairs experimental grades and the theoretical scrutiny, a projectedenergy band structure illustration of the CeO2/CoWO4 PCs has established schematically in Figure. 13. The CB and VB edge potential (vs. NHE) (Hao et al. 2017) of CeO2/CoWO4 PCs at the point of zero charges could be resoluteconsumingMulliken electronegativity equations (1) and (2): Wherever χ is the absolute electronegativity of the semiconductor (χ is 5.56, 6.32 eV for CeO2 and CoWO4, singly). Eestands thefree electron energy of the hydrogen scale (4.5 eV) values and Eg is the bandgap of the obtained semiconductor (Deng et al. 2012). The intendedECB and EVB of CeO2 were -0.40 and 2.52 eV, and of CoWO4 were -0.405 and 3.23 eV, separately. Allowing to UV-DRS results, when the CeO2/CoWO4PCs system was exposed under visible light (>400 nm), togetherCeO2 and CoWO4 could be stimulated since the energy gap of CeO2 and CoWO4perceived in this revision were 2.92 and 2.83 eV.
To realize the purposeof this enrichment, ananticipated tentative photocatalytic appliance for the photodegradation of MB dye over the CeO2/CoWO4PCs is obtainable in Figure. 13. Since the forgoing inquiry, one couldeffortlesslyinvention that the CeO2 not only plays sustenance but similarlydoings as the ligands, which would be supportive to increase the separation efficacy of photoexcited(e --h + )sets (Liyanage et al. 2014 Consequently, centered on the photocatalytic reactivity, the CeO2/CoWO4compositeindicateseffectivesegregation of photoexcited(e --h + )charge-carrier, thus developedphotosensitization and photocatalytic reactivity could be realized positively

Antibacterial accomplishments
Antibacterial events of CeO2/CoWO4NCs were partitioned against pathogenic bacterial strains, for instance, Gof E. coli and G + of S. aureusbacteria, was proficientby the agar well diffusion system. Figure. 14visibly shows the zoneof inhibition (ZOI) by different concentrations (25, 50and 100 µg/mL) of CeO2/CoWO4,CoWO4, and CeO2with respect to the typical control which directed the antibacterial outcome. Antibacterial action of as-obtained catalystson the pathogenic bacterial strains by different attention is publicized in Table 2. The superiorantibacterial doings of the as-attained CeO2/CoWO4NCs onthe concentration of 75µg/mLconsumed a capable antibacterial influencebesideG + of S. aureusbacteriathrough a ZOI value of ~6.5-9 mm on equatingby the typical drug, DMSO (as negative control). Moreover, the united CoWO4/CeO2was obviouslyindicated that the NCs collectedround the surface membranes, which mightrespond with the microbial membranes and produced internalization of the NCs in the microbe's cells (Magdalane et al. 2016). The mechanism for the bactericidal motion of NPs appearancessubstantialefficacy for the responsive/reactive oxygen species (ROS) and its bactericidalstrains actualpart in its metallic NPs and their complexesin the existenceof oxygen species and accordingly (Gasmalla et al. 2019), donate to the superior mechanical damage for the utilities of microbes, and enriched bactericidal influence of unifiedCoWO4/CeO2nanostructured materials.Moreover, thegreaterattentiveness of all these managed NCs materials are destructive to together the consumers and microorganisms, but still, nano-level attention of these NMs are further applications for the destruction of microbes (Sundaram et al. 2017).

Conclusion
In summary, a novel CeO2/CoWO4heterostructured NCs photocatalyst was effectivelydeveloped through a facile hydrothermal assisted process.

Compliance with ethical standards
Conflict of interest The authors state that their research has no interest or financial conflicts of interest.

Ethical standard
No studies or animal experiments are conducted in this article by any of the authors.