Multifunctional ZnO materials prepared by a versatile green carbohydrate-assisted combustion method for environmental remediation applications

Author’s Accepted Manuscript Multifunctional ZnO materials prepared by a versatile green carbohydrate-assisted combustion method for environmental remediation applications Cristian D. Ene, Greta Patrinoiu, Cornel Munteanu, Ramona Ene, Mariana Carmen Chifiriuc, Oana Carp www.elsevier.com/locate/ceri

PII: DOI: Reference:

S0272-8842(18)32949-3 https://doi.org/10.1016/j.ceramint.2018.10.144 CERI19856

To appear in: Ceramics International Received date: 25 July 2018 Revised date: 20 September 2018 Accepted date: 17 October 2018 Cite this article as: Cristian D. Ene, Greta Patrinoiu, Cornel Munteanu, Ramona Ene, Mariana Carmen Chifiriuc and Oana Carp, Multifunctional ZnO materials prepared by a versatile green carbohydrate-assisted combustion method for environmental remediation applications, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.10.144 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Multifunctional ZnO materials prepared by a versatile green carbohydrateassisted combustion method for environmental remediation applications

Cristian D. Ene1*, Greta Patrinoiu1, Cornel Munteanu1, Ramona Ene1, Mariana Carmen Chifiriuc2, Oana Carp1*

1

“Ilie Murgulescu” Institute of Physical Chemistry, Romanian Academy, Splaiul

Independentei 202, 060021 Bucharest, Romania 2

Department of Microbiology, Faculty of Biology, University of Bucharest, Research

Institute of University of Bucharest, Aleea Portocalilor 1-3, 60101 Bucharest, Romania *

Corresponding authors: Oana Carp ([email protected]) and Cristian D. Ene ([email protected])

Abstract Nanosized ZnO has been prepared following a non-polluting, straightforward, and flexible combustion synthesis method, using as biofuels starch and cellulose, and as zinc source nitrate and acetate. If the crystallite size and the surface area of the ZnO materials are to a certain extent, sensible of the post-thermal processing temperature, the defect chemistry is dependent on reductant/oxidant ratio. The obtained ZnO materials, particularly the oxides obtained from carbohydrate-nitrate precursors present considerable photodegradation activity upon two hazardous EDCs phenolic wastes and excellent antimicrobial efficiency against planktonic Gram negative and positive strains. Both results can be explained by an optimum defects configuration of these oxides. The photocatalytic and biocidal efficiency associated with the easy and complete recovery

1

from the treated wastewaters proves the potential toward environmental remediation applications of these “two in one“ formulations.

Keywords: Carbohydrate; Zinc oxide; Defect chemistry; Photocatalytic activity; Antimicrobial efficiency.

Introduction Zinc oxide, a versatile important semiconductor, has been attracting extensive attention due to its unique combination of properties and wide range of applications. Besides structure, size, surface, and morphology, its key characteristics depend on defect chemistry: optical, sensing, photocatalytic, and antimicrobial properties are defects-sensible. Defect-engineered ZnO can be achieved either during the synthesis via an adjustment of the experimental conditions for supporting ZnO formation in kinetically controlled conditions [1,2], presence of additives [3,4], incorporation of dopants [5] or through post-synthesis designed protocols [6,7]. Definitely, synthesis methods that promote and control in situ defect formation are the simplest way of a rational introduction of defects. Considering that ZnO defective chemistry is very sensitive to the synthesis environment, a procedure that implies an adjustable redox reaction such as combustion is a convenient option [8]. The procedure implies the combustion of an oxidizer and a reductant fuel that produces the exothermicity required by the synthesis. The properties of the resulting oxides are governed by the enthalpy/temperature of the flame generated during the combustion, which depends on the thermochemical characteristics of the fuel and reductant-oxidant ratio [9,10]. The most used fuels (urea, hydrazine, amino acids, citric acid) are high-temperature flame, a

2

high-temperature combustion being not adequate for a refined defect customizing [1113]. A solution is the partial or total replacement of these conventional fuels with low-temperature flame fuels like carbohydrates [14]. Besides a more controllable combustion, several reasons argue for their use: (i) eco-friendly uniqueness; (ii) diversity in terms of chemical functionality and reducing ability that facilitates binding to metal cations, improving the homogeneity of the precursors while enabling fine tuning of the defect chemistry. Herein, cellulose and starch are the selected fuels for scientific and economic reasons: (i) both have the same chemical formula (C6H10O5)n, hence the same heat of combustion (~-2814 kJ mol-1) [15,16], but consist of differently linked D-glucose units, allowing to investigate how this subtle feature influences the characteristics of the oxides (ii) the carbohydrates are the most abundant biopolymers. Several studies reported their use as co- or single fuel in oxides synthesis, but none has comparatively assessed their combustion behaviour and the relation between the reductant/oxidant ratio and defect chemistry [14,17-19]. Because the biopolymers are either insoluble (cellulose) or sparingly soluble (starch) in water, the synthesis of the precursors and their combustion were performed in solid state. In this paper, it is demonstrated that the change of reductant/oxidant ratio following the replacement of zinc nitrate with zinc acetate has a remarkable impact on ZnO defect chemistry, crystallite size, and, to a lesser extent, surface area. Consequently, the optical, photocatalytical, and antibacterial properties may be modulated. It was found that a lower concentration of deep-level defects led to an

3

increase in the photocatalytical and antimicrobial activity of ZnO materials, thus making them suitable for environmental remediation applications.

2. Experimental 2.1 Materials The chemicals were used as received: zinc nitrate hexahydrate [Zn(NO3)2·6H2O, Merck], zinc acetate dihydrate [Zn(CH3COO)2·2H2O (Reactivul)], soluble starch (Carl Roth GmbH), cellulose (Aldrich). 2.2 Synthesis The synthesis of ZnO materials was performed following a solid-state combustion. As sources of zinc cations, nitrate and acetate salts have been used. Nitrate also acted as oxidant, whereas starch and cellulose as fuels. The stoichiometry of metal nitrate-fuel system was chosen based on the total oxidizing (VO) and reducing (VR) valences of the raw materials, for an equivalence ratio Φe(VR/VO) = 1. The acetate system was prepared using the same zinc salt : carbohydrate molar ratio. The raw materials were mixed (30 min) in an agate mortar, than dried on P2O5. The oxides were obtained following thermal treatments at 600 oC (nitrate) or 800 oC (acetate). Samples notation: PXY and ZnOXY (X represents the carbohydrate (S = starch, C = cellulose) and Y the anion (N = nitrate, A = acetate) of the metal salt) denote the precursors and corresponding oxides, respectively. 2.3 Characterization The X-ray diffraction measurements (XRD) were carried out on a Proto X-Ray Benchtop using Ni-filtered Cu Kα radiation with an average wavelength of 1.54251 Å in the range 20-80º 2θ. The crystallite size was estimated as an average using Scherrer

4

equation for all the diffraction peaks, LaB6 (NIST SRM 660b) being used to determine the instrumental broadening contribution. The FTIR spectra (KBr pellets) were recorded on a FTIR Bruker Tensor V-37 spectrophotometer in the 4000-400 cm-1 region (in the case of ZnO, 1 mg of oxide was milled with 300 mg KBr and then pressed into a pellet). UV-Vis spectra were measured by the means of a JASCO V-670 spectrophotometer in the 200-1000 nm range. Thermal measurements were performed on a Netzsch STA 449 F1 Jupiter Simultaneous Thermal Analyzer using lid-covered alumina crucibles in dynamic air (30 mL/min), with a heating rate of 5 ºC min-1. Scanning electron microscopy (SEM) measurements were carried out on a FEI Quanta 3D FEG. Photoluminescence

(PL)

analysis

was

performed

on

a

JASCO

FP

6500

spectrophotometer using a 350 nm excitation line of xenon light. Nitrogen sorption isotherms at -196oC were recorded on a Micromeritics ASAP 2020 automated gas adsorption system. 2.4 Photocatalytical tests The photocatalytic degradation of phenol and resorcinol was investigated using stationary quartz reactors (5 mL) with a distance between the light source and reaction tube of 8 cm with UV (60W, filter at λ = 365 nm) and visible (60 W, λ > 380 nm) lamps, at 20oC, using 0.05 g of catalyst and 3 mL of pollutant solution 2 × 10−3 M. Prior to irradiations, the suspensions were magnetically stirred in dark for 30 min in order to reach adsorption- desorption equilibrium. After 6 hours, the reaction products were filtered through Millipore membrane filters and analysed on a DANIGC 1000 gas chromatograph connected to a FID detector. The experiments were carried out at the natural pH of the suspension, without any adjustment. 2.4 Antimicrobial activity assay

5

The antimicrobial activity of the materials was determined against ATCC reference strains, i.e.: Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853, Staphylococcus aureus ATTC 25923, and Enterococcus faecalis ATCC 29212, from which microbial suspensions were prepared in sterile saline, with a density of 1.5 x 108 CFU mL-1 (corresponding to the 0.5 McFarland nephelometric standard). The materials were dispersed both in water and DMSO at 5000 μg mL-1 concentration. The antimicrobial activity was assayed by a quantitative, liquid medium microdilution method, in 96 multi-well plates, in which serial two-fold dilutions of the tested materials ranging from 2500 to 1.95 µg mL-1 were performed in a 200 L volume of nutrient broth. The minimal inhibitory concentration (MIC) values corresponded to the lowest concentration which inhibited the microbial growth [20]. The anti-biofilm activity was investigated by the microtiter assay, establishing the minimal biofilm eradication concentration (MBEC) defined as the lowest binary concentration which inhibited the development of the microbial biofilm on the plastic walls.

3. Results and discussions 3.1 Characterization of the precursors The infrared spectra of the ZnO precursors differing in the zinc salt are plotted together with the one corresponding to the neat polysaccharide they contain, (Figures 1 and S1 from ESI). A thorough list of the wavenumber values and intensities of the occurring absorption bands including their assignments is presented in Table S1 (ESI). Such an approach enables the identification of specific supramolecular forces that might emerge by intimately mixing the two raw materials, i.e. hydrogen bonds established between the metal salt anions (nitrate/acetate) and the hydroxo groups from

6

carbohydrate fragments (see Figure 1 for starch as well as the detailed analysis embedded in ESI). The thermal behaviour of the precursors is anion dependent (Figure 2). Nitrates are strong oxidants and entail exothermicity on combustion, governing the oxidative degradations of the carbohydrates, whereas acetates significantly decrease the final degradation temperature. The latter effect is explained by the structural destabilization of the carbohydrate matrix induced by acetate elimination as a result of the alteration in supramolecular associations. Through their different thermal stability, the carbohydrates mark only acetates decomposition: dissimilarities between the thermal behaviour of starch and cellulose precursors can be identified at temperatures above 300 oC, due to the sensibly higher thermal stability of cellulose (Figures S2 and S3 from ESI). Within a low and narrow temperatures range (~50-150 oC), the nitrate systems undergo one endothermic process corresponding to water elimination which is partially covered by two strong exothermic decompositions associated with the most significant mass loss (~55/56 %, cellulose/starch). As for the acetate precursors, only two endothermic events assigned to water evolving are observed within the same temperature interval. The thermal stability of the subsequent mass loss plateau is shifted according to the zinc anion, toward higher (nitrates) and lower (acetates) temperatures. The existence of this plateau up to 270 oC for nitrates demonstrates the formation of a carbonaceous intermediate more stable than the source carbohydrate. The subsequent exothermic process, characterized by a relatively small mass loss (~5-7%), corresponds to the degradation of this intermediate. As for the acetate precursors, the disappearance of the plateau at ~190 oC is determined by the elimination of this counterion at around 240 oC [21]. The process is followed by three/one large (cellulose/starch) exothermic

7

processes assigned to the carbohydrate combustion (with a mass loss of ~58% in both cases). The final decomposition temperature of the precursors is lower in comparison with the neat carbohydrates, with 120/70oC and 120/30oC (cellulose/starch systems) for nitrates and acetates, respectively. After the completion of their decomposition, the samples present an exothermic phase transformation, more pronounced for acetate systems, assigned to both nucleation and crystal growth of ZnO crystallites, these processes being superimposed due to the relatively low heating rate [22].

3.2 Characterization of the ZnO materials The XRD patterns of all the calcined samples of ZnO precursors (Figure S4 from ESI) indicate the formation of a pure ZnO single-phase with a hexagonal wurtzite structure. Indexing was performed according to the ZnO diffraction data obtained from single-crystal measurements at ambient pressure (ICDD card No. 01-074-9939) [23]. The lattice parameters and the crystallite size values are listed in Table 1. It can be noticed that, even though both lattice constant decrease for all the obtained ZnO samples, the c/a ratio hardly varies and it is closed to the one corresponding to ICDD card No. 01-074-9939. However, the diffraction lines of each ZnO XRD pattern are shifted towards larger 2θ values with respect to this reference (see Table S2 in ESI for the

first

three

{hkl}

family

planes)

following

the

sequence

ZnOSN˂ ZnOCN˂ ZnOCA˂ ZnOSA (Figure S5 from ESI). As previously reported [24,25], an increase in this deviation correlates with a higher concentration of defects inside the formed ZnO lattice. In line with the conclusions arising from the other

8

spectroscopic methods included herein, the acetate-containing precursors are able to generate more defects within the resulting ZnO materials.

Table 1. Structural, morphological, and optical data of the ZnO samples Sample

ZnOSN ZnOCN ZnOSA ZnOCA ICDD No. 01-074-9939

a

c

(Å)

(Å)

3.2395 3.2340 3.2189 3.2227 3.2495

5.1920 5.1846 5.1603 5.1659 5.2055

c/a

V (Å3)

D (nm)

SBET (m2 g-1)

1.6027 1.6031 1.6031 1.6030 1.6020

47.19 46.96 46.30 46.46 47.60

40.0 41.3 56.1 56.0

4.93 5.70 2.19 1.35

Band gap (eV) 3.23 3.23 3.17 3.18

The FTIR spectra of ZnO samples exhibit the adsorption features of water molecules/hydroxyl groups (3770-3000 cm-1), acetate (1700-1500 cm-1), and nitrate (1400-1340 cm-1) anions adsorbed onto surface together with the distinctive, strong ZnO (600–400 cm-1) bands (Figure 3). The intensity, position, and number of ZnO bands depend on the chemical composition, defects, and geometrical shape, the last two of which have the foremost input into Zn-O bond strength [26]. Wurtzite ZnO belongs to the C6V (P63mc) space group, and A1 and E1 Raman active polar modes are also IRactive. In the case of spherical ZnO particles, both calculated and measured spectra present one absorption at 464 cm-1. This band splits progressively when the geometrical shape evolves toward a cylindrical one [27]. Thus, the presence of two maxima (varying within the 493-483 and 447-440 cm-1 intervals, Figure 3 - inset) and one shoulder at around 525 cm-1 can be explained by a superposition of absorptions arising from at least two types of particles with different degrees of spherical shape alteration. As the shoulder is more pronounced for the nitrate derivatives, it is expected that their rod-like crystallite content to be slightly higher. This shoulder can be associated with the LO

9

phonons of the E1 mode, whereas the two maxima should originate to a greater extent from their A1 mode. The presence of oxygen defects are mainly responsible for the strong red shift of these bands, but a correlation between defect concentration and absorption values is difficult to establish due to the interference of geometrical factor. SEM analysis (Figure 4) shows that the size, morphology, and dispersion degree of the ZnO materials are precursor-dependent, the anions representing the determining factor. Thus, the nitrate precursors produce loosely packed, mainly rod-like crystallites, although sporadic quasi-spherical morphologies (see marked zones) are also identified; the average size of the aggregates is smaller (up to ~200 nm) than the acetate counterparts. The acetate precursors lead to polyhedral, mostly hexagonal, semisintered, larger structures (up to ~500 nm), which are highly agglomerated. The morphology of the oxides is not significantly altered by the biofuel nature, but cellulose determines the formation of oxides with higher sizes irrespective of the used anion. The higher complexing capability of starch that permits a better dispersion of Zn2+ into the carbohydrate matrix and the higher thermal stability of cellulose (primarily manifested for acetate precursors) are potentially responsible for this effect. As anticipated by their morphology, the specific surfaces of the oxides are quite low depending on used anion (the nitrate derived oxides possess a higher surface), but are basically independent of the carbohydrate (Table 1). The electronic spectra (Figure S6, ESI) of the ZnO samples are very similar, with a strong absorption below 370 nm and a highly transparent mode in the visible region. The band gap energy (Eg) ranges between 3.17-3.23 eV (Table 1) as estimated from the Tauc plots (inset of Figure S6 and details in ESI). The highest band gap values

10

belong to the nitrate derived oxides, as a result of their decreased sizes, typical dependence found in nanoparticles due to quantum confinement effect [28-30]. A defect analysis was carried out by photoluminescence measurements (Figure 5). Typically, ZnO exhibits emission bands both in UV (attributed to the band edge transition or exciting combination) and visible region, known to be deep-level (associated with intrinsic or extrinsic deep-level defects) regions [31]. Taking into account that the combustions of the acetate and nitrate precursors occur differently and the ZnO defect chemistry is extremely sensible to the thermal processing, distinct photoluminescent behaviour was expected for the two systems. Indeed, the two systems are characterized by different ratio of UV to defect emissions. The nitrate-generated oxides show a stronger UV emission in comparison to the small, visible ones, suggesting a low concentration of deep-level defects, whereas the oxides derived from acetates present strong blue and green emissions, and only ZnOCA presents a weak UV luminescence of free exciton recombination (an increase in the defect concentration might suppress the intensity of excitonic recombination through charge trapping or non-radiative recombination at the defect centers) [31]. Although there is no consensus on the origin of visible emissions, by considering the reducing (carbohydrate-acetate systems) and oxidant (carbohydrate-nitrate systems) environments in which ZnO materials form, the recorded visible emissions could be reasonable assigned. In the case of acetate systems, the two strong emission peaks at 492 and 537/534 nm (starch/cellulose) coupled with a shoulder at 475 nm are generated by oxygen deficiency and by single and double ionized oxygen vacancy and interstitial zinc, respectively [31,32]. The low-intensity violet (446 nm) and blue (463, 469, 474, and 487 nm) emissions of ZnOSN and ZnOCN are related to zinc vacancies (VZn) and

11

interstitial zinc (Zni) [24], respectively, while the yellow (588 nm) and red ones (631 nm) to interstitial oxygen [33]. The formation of a combination VZn and Zni defects is plausible due to their formation energy in an oxygen-rich environment [34]: low migration energy barrier of interstitial zinc allows its diffusion into zinc vacancy, decreasing the intensities of the corresponding emissions. The photoluminescent behaviour of ZnO materials is practically independent of the carbohydrate biofuel. Moreover, it is not the calcination temperatures, but the fuel/oxidant ratio that dictates the defect chemistry of the materials.

3.3 Photocatalytical activity ZnO can be considered as a viable alternative to TiO2 due to similar optoelectronic properties, superior quantum efficiency, absorbance over larger fraction of the solar spectrum, and lower cost, while its superior photocatalytic activity has been already confirmed in certain cases [35]. The photocatalytic activities of the ZnO materials under UV and visible irradiation were investigated on two representative phenolic pollutants, phenol and resorcinol. Several reasons underpin this choice: (i) both are used in a plethora of industries and, unfortunately, are contained in their wastewaters discharges [36,37]; (ii) both are potent endocrine disrupting chemicals and may undergo natural reductive anaerobic degradation yielding potentially carcinogenic aromatic intermediates [38]; (iii) their degradation has been achieved so far mostly under UV irradiation in the presence of TiO2-based catalysts [39] and only few studies report photocatalytic oxidation in visible domain [4,40], and (iv) resorcinol is one of the intermediates of phenol photodegradation [41].

12

As shown in Figure 6, the conversions of both pollutants reached after 6 hours of irradiation are superior under UV illumination, the phenol photodegradation being higher than the resorcinol one. The tested contaminants display a similar conversion order, being dependent on both the anion and the carbohydrate nature: the oxides derived from nitrates and starch are more active than those obtained from acetates and cellulose. The relatively large number of studies on phenol photodegradation point out that, besides photocatalyst characteristics, ZnO efficiency is greatly dependent on experimental parameters (pH, light intensity and wavelength, reaction time, catalyst loading, concentration of pollutant, etc.) [36]. Thus, in the absence of a test reaction standardization, a valid analysis of our conversion values partially loses its relevance. Overall, although conversions of 100% have been reported [42], the obtained results are among the best concerning phenol photodegradation (especially for nitrate-derived ZnO) [43,44]. The assertion is valid even in comparison with our previous studies performed under the same conditions using ZnO catalysts with hierarchical morphologies [4,35,45,46]. When compared to phenol, the investigations reported on resorcinol removal are remarkably fewer. Although the total resorcinol conversions reached with oxides obtained from nitrates are very high, ranging between 77-84% (UV) and 71-80% (visible), several studies claim the achievement of total conversion. These experiments were carried out under more favourable conditions (lower/higher concentration of resorcinol respective catalyst, modification of reaction pH) [47,48] or using more sophisticated catalysts (e.g. S-ZnO [47], Ag2O/ZnO composite [49]). However, the

13

visible activity of ZnOSN is superior to ZnO hierarchical porous micro/nanospheres under UV irradiation [50]. Typically, higher photocatalytic activities are correlated with larger surface areas, optimum configuration defects, and superior surface energy (nonfaceted vs. faceted particles) [51,52]. According to a simple photocatalytic mechanism model, an optimized photocatalyst should satisfy two requirements that are generally conflicting each other, namely large surface area for the absorbing substrates and high crystallinity for reducing the photo-excited electron–hole recombination rate [53]. From this point of view our samples are atypical as the oxides obtained at lower temperature exhibit both a larger surface area and a higher separation efficiency of electron-hole pairs, thus minimizing the energy waste. An additional asset of these ZnO materials is their large dimensions that make possible an easy and complete recovery from the treated wastewater by a common filter paper.

3.4 Antimicrobial activity The antimicrobial efficiency of the ZnO spherical structures was tested against planktonic and biofilm-embedded reference strains (traceable to ATCC) either Gram negative and positive. The MIC and MBEC values are listed in Table 2. The DMSO dispersions exhibited a higher antimicrobial activity than the water ones, probably due to a better dispersibility of ZnO materials in DMSO. A brief analysis of the obtained results reveals that the oxides derived from nitrate system presents a higher biocidal effect than those derived from acetates and, as expected, the antimicrobial efficiency against planktonic strains is superior to that against biofilm.

14

Table 2. MIC and MBEC values (µgmL-1) of the ZnO materials Microbial strains

Dispersion

MIC (µg mL-1)

MBEC (µg mL-1)

ZnO

ZnO

medium

Gram negative strains

Gram positive strains

Escherichia coli ATCC 25923 Pseudomonas aeruginosa ATCC 27853 Staphylococcus aureus ATTC 25923 Enterococcus faecalis ATCC 29212

SN

CN

SA

CA

SN

CN

SA

CA

DMSO

9

9

39

19

water

19

19

156

39

DMSO

19

19

39

39

2500

2500

2500

2500

water

19

19

312

39

>5000

>5000

>5000

>5000

DMSO

19

19

78

78

78

39

39

39

water

39

19

312

78

78

39

156

39

DMSO

19

19

78

78

5000

5000

5000

2500

water

19

19

625

156

2500

2500

5000

5000

For both Gram negative strains, the MIC values obtained for the materials derived from nitrates are two to four times smaller than those corresponding to the acetate systems (even 8 times for ZnOSA dispersed in water). In the case of Escherichia coli, the MIC value of 9 µgmL-1 (ZnOSN and ZnOCN, DMSO) is one of the best reported in literature for bare [54-56], doped [55], and ZnO composites [57,58]. Excellent activity was also obtained for the oxides generated by acetate systems (MIC values of 39 and 19 µgmL-1 for starch and cellulose-derived oxides, respectively). For Pseudomonas aeruginosa ATCC 27853 strain, the MIC values (19 and 39 µgmL-1 for nitrate respective acetate derived oxides, DMSO and water, except ZnOSA water) are lower than those reported for alternative ZnO materials [54,56,59,60] indicating an antimicrobial efficiency superior even to several antibiotics [61]. As for the Gram positive strains, the ZnO materials lead to similar MIC values (19/78 µg ml-1 for nitrate/acetate derived oxides, respectively, DMSO), which, although

15

twice as big as those obtained for Gram negative ones, they still prove a high biotoxicity against the planktonic population of these strains. Accordingly, the antibacterial activity recorded toward planktonic Staphylococcus aureus is superior to that reported for other ZnO materials [62] and different antibiotics [61,63]. The MICs obtained on Enterococcus faecalis strain are significantly lower than those reported for other ZnO [64], ZnO composites [65,66], and antibiotics such as kanamycin [67]. A ubiquitous presence in nature, the microbial biofilms are sessile, monospecific or polyspecific communities of microbial cells attached to different substrata and protected by a self-secreted matrix of extracellular polymeric substances, exhibiting a modified phenotype. Microbial cells grown in biofilms possess a much higher resistance to different stressors, including antimicrobial substances, as compared to their planktonic counterparts [68]. Therefore, the efficiency of the obtained ZnO materials was comparatively tested also on biofilm cells. Only Staphylococcus aureus biofilm development was inhibited by the ZnO materials, for which the MBEC values indicated a very good anti-biofilm activity, superior to that reported for antibiotics like chloramphenicol and ciprofloxacin [61]. An important finding that emerged from the antibacterial and photocatalytic activity investigations has to be highlighted: the antibacterial activity of the ZnO materials follows the same trend as their photocatalytic behaviour, suggesting that both activities are higher when the concentration of the deep-level defects is lower. Studies for establishing the mechanisms of antibacterial activity, a screening against a larger spectrum of reference and clinical microbial strains, and a biocompatibility assay of the ZnO materials are in progress.

16

Conclusions Herein we report the successful fabrication of ZnO materials by an environmentally friendly, versatile, and low cost combustion synthesis method. The zinc salt anion is the parameter that governs the combustion reaction, which dictates the properties of the resulting oxides. The ZnO materials originating from nitrates exhibit lower sizes, higher surfaces, and reduced defect density. The use of cellulose as fuel leads to oxides with higher sizes irrespective of the used anion. The obtained ZnO materials possess remarkable photocatalytic activity in degradation of EDCs phenolic contaminants and notable bactericidal potential against Gram positive and negative bacterial strains. The photocatalytic and antimicrobial activity are correlated, a fact that indicates the involvement of a common mechanism; in both assays, the nitrate derived oxides are the most effective. In summary, the extremely facile carbohydrate-assisted combustion synthesis may constitute a notable tool for engineering materials defect chemistry that handles, in its turn, many important properties of these materials. Therefore, the defect induced properties can be considered as a new route for tailoring the ZnO functionalities and represent a sustainable solution provided by materials science that covers design, synthesis, and product with vital applications as green environmental pollution management.

Acknowledgement This work was financially supported by the research project 234 PED-2017 of the Romanian National Authority for Scientific Research.

17

Declarations of interest: none.

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Figure captions

Figure 1. FTIR spectra for starch (a) and its zinc ion-based derivatives, PSA (b) and PSN (c) Figure 2. Thermal curves (TG, DTG, and DSC) of the precursors: PSN (a), PSA (b), PCN (c), and PCA (d) Figure 3. FTIR spectra of ZnOCN (a), ZnOSN (b), ZnOCA (c), and ZnOSA (d) (inset: magnification of the 600-400 cm-1 domain) Figure 4. SEM micrographs of ZnOSN (a), ZnOCN (b), ZnOSA (c), and ZnOCA (d) Figure 5. Normalized room-temperature photoluminescence spectra of ZnOCN (a), ZnOSN (b), ZnOCA (c), and ZnOSA (d) Figure 6. Effects of ZnO material and irradiation conditions on the degradation of phenol (A) and resorcinol (B) (6 hours, natural pH of the suspensions; commercially available ZnO (Merck) was used as control sample)

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Figure 1.

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Figure 2.

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Figure 3.

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Figure 4.

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Figure 5.

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Figure 6.

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