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Preparation and Characterization of ZnO Nanoparticles Supported on Amorphous SiO2 Ying Chen, Hao Ding * and Sijia Sun Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Wastes, National Laboratory of Mineral Materials, School of Materials Science and Technology, China University of Geosciences (Beijing), Beijing 100083, China; [email protected] (Y.C.); [email protected] (S.S.) * Correspondence: [email protected]; Tel.: +86-010-8232-2982 Received: 30 June 2017; Accepted: 24 July 2017; Published: 10 August 2017

Abstract: In order to reduce the primary particle size of zinc oxide (ZnO) and eliminate the agglomeration phenomenon to form a monodisperse state, Zn2+ was loaded on the surface of amorphous silica (SiO2 ) by the hydrogen bond association between hydroxyl groups in the hydrothermal process. After calcining the precursors, dehydration condensation among hydroxyl groups occurred and ZnO nanoparticles supported on amorphous SiO2 (ZnO–SiO2 ) were prepared. Furthermore, the SEM and TEM observations showed that ZnO nanoparticles with a particle size of 3–8 nm were uniformly and dispersedly loaded on the surface of amorphous SiO2 . Compared with pure ZnO, ZnO–SiO2 showed a much better antibacterial performance in the minimum inhibitory concentration (MIC) test and the antibacterial properties of the paint adding ZnO–SiO2 composite. Keywords: amorphous SiO2 ; load; monodisperse; ZnO nanoparticle; antibacterial

1. Introduction Antimicrobial tests and environmental toxicity tests have been widely explored in order to improve health, safety, and the environment [1–3]. Zinc oxide (ZnO), as a semiconductor material with a band gap of 3.3 eV at room temperature [4], has high chemical stability, strong photosensitivity and non-toxicity property and is widely used in antibacterial materials [5]. Compared with ordinary ZnO powder, ZnO nanoparticles have a large specific surface area and small size effect, and show wide application potential in microbial inhibition and mildew removal [6,7]. However, like most of the nanoparticles, ZnO nanoparticles are prone to forming serious agglomeration, including hard agglomeration among the particles formed via the chemical reaction of the surface groups and soft agglomeration formed by other physical effects [8]. It is difficult to depolymerize the particles involved in hard agglomeration. Therefore, the apparent grain size of the primary ZnO particles tends to increase to the micron scale and the normal performance of ZnO nanoparticles is inhibited. In the preparation process of ZnO nanoparticles, in addition to the control of the ZnO morphology and primary particle size, the agglomeration phenomenon of ZnO particles should be suppressed to obtain dispersed nanoparticles. Wang et al. synthesized the doped ZnO nanoparticles with the mixture of alcohol and water as the solvent according to a precipitation method [9]. Chen et al. prepared ZnO nanocrystals via the reaction of zinc stearate with excessive alcohol in the hydrocarbon solvent [10]. Weller et al. used the low-temperature solvent thermal method to synthesize dispersible spherical ZnO nanoparticles and nano-rods with zinc acetate as precursors in methanol [11]. However, these methods have low synthesis performance and limited control ability. Especially, the solvent thermal process [12,13] is required to deal with organic solvents and it is difficult to realize industrial production. Therefore, some nanoparticles (such as titanium dioxide, TiO2 ) [14,15] are supported on the surface or pores of the inorganic carrier. In this way, the strong interaction between the carrier surface and nanoparticles, and the forced isolation among the carrier particles Nanomaterials 2017, 7, 217; doi:10.3390/nano7080217

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strong interaction between the carrier surface and nanoparticles, and the forced isolation among the efficiently preventefficiently the agglomeration among the nanoparticles andnanoparticles improve the and dispersion effects carrier particles prevent the agglomeration among the improve the and functions. dispersion effects and functions. Amorphous aggregate of of SiO SiO22 particles particles AmorphousSiO SiO22,, commercially commercially known as white carbon black, is an aggregate (SiO 2 ·nH 2 O) and commonly used as [16,17]. The primary particle size (SiO · nH O) and commonly used as a rubber reinforcing additive The primary particle sizeof of 2 2 SiO 2 particles is generally 10–100 nm. SiO 2 particles containing rich Si–OH groups, which can form aa SiO particles is generally 10–100 nm. SiO containing rich Si–OH groups, which can form 2 2 stronginteraction interactionbetween betweenthe theSiO SiO22 carrier carrier surface and Zn–OH (precursors strong (precursors of of ZnO). ZnO).This Thisinteraction interaction reducesthe thecombination combination between between Zn–OH Zn–OH and and prevents prevents its aggregation, aggregation, thus reduces thus contributing contributing to to the the formation of monodisperse ZnO nanoparticles. In addition, small amorphous SiO 2 particles have formation of monodisperse ZnO nanoparticles. In addition, small amorphous SiO2 particles have high high dispensability can further preventagglomeration further agglomeration of2 ZnO–SiO 2 composite. Therefore, dispensability and canand prevent of ZnO–SiO composite. Therefore, amorphous amorphous SiO 2 was selected as the carrier of supported ZnO nanoparticles. SiO was selected as the carrier of supported ZnO nanoparticles. 2 Basedon onthe theabove aboveresults, results, this paper, environmentally friendly hydrothermal method Based in in this paper, thethe environmentally friendly hydrothermal method was 2+ on the surface of amorphous SiO2 and calcining 2+ was adopted to prepare ZnO–SiO 2 by loading Zn adopted to prepare ZnO–SiO2 by loading Zn on the surface of amorphous SiO2 and calcining active active products at high temperatures. Moreover, the structures and antibacterial properties of asproducts at high temperatures. Moreover, the structures and antibacterial properties of as-prepared prepared ZnO–SiO 2 were explored. ZnO–SiO2 were explored. ExperimentalProcedure Procedure 2.2.Experimental

2.1. 2.1.Materials Materials InInthis Henan Jiaozuo Jiaozuo Fluoride Fluoride New New Energy Energy thisstudy, study,amorphous amorphous SiO SiO22 was was purchased purchased from Henan TechnologyCo., Co.,Ltd Ltd(Jiaozuo, (Jiaozuo,Henan, Henan, China). China). The properties of amorphous amorphous SiO Technology SiO22 are are described describedas as follows:SiO SiO 2 content of 96.63%, whiteness of 96.76%, average aggregate size of 20 μm, primary follows: content of 96.63%, whiteness of 96.76%, average aggregate size of 20 µm, primary particle 2 particle size nm, of 20–30 nm, andsurface specificarea surface aream of2 /g. 59.54 m2/g. Zinc (Zn(NO nitrate (Zn(NO 3)2·6H O) as the size of 20–30 and specific of 59.54 Zinc nitrate as 2the source 3 )2 ·6H2 O) 2+ 2+ of Zn was fromYili Beijing Fine Chemical Co., LtdChina). (Beijing,Sodium China).polyacrylate Sodium polyacrylate ofsource Zn was from Beijing Fine Yili Chemical Co., Ltd (Beijing, (PAAS) as as awas dispersant supplied byRun Changzhou Run Yang Chemical Co., LtdJiangsu, (Changzhou, a(PAAS) dispersant suppliedwas by Changzhou Yang Chemical Co., Ltd (Changzhou, China). Jiangsu, China). Pure ZnO, as an antibacterial agent, was compared with the ZnO–SiO 2 composite in Pure ZnO, as an antibacterial agent, was compared with the ZnO–SiO2 composite in antibacterial antibacterial performance. It was produced by the Xi Long Chemical Co., Ltd (Guangzhou, performance. It was produced by the Xi Long Chemical Co., Ltd (Guangzhou, Guangdong, China) Guangdong, and thewas sizeabout of the200 particles was about 200SEM nm. images Figure 1ofshows SEM images of and the size ofChina) the particles nm. Figure 1 shows amorphous SiO2 and amorphous SiO 2 and pure ZnO. pure ZnO.

Figure and (b) (b) pure pureZnO. ZnO. Figure1.1.Micrographs Micrographsof of(a,c) (a,c) amorphous amorphous SiO SiO22 and

2.2.ZnO–SiO ZnO–SiO22Precursor Precursor 2.2. AmorphousSiO SiO22,, sodium sodium polyacrylate polyacrylate (1% of the weight of SiO22)) and Amorphous and H H22OO were were mixed mixedand and stirredtotoprepare prepare suspension with solid content of 18%. Ceramic polishing (diameter: 1–3 stirred thethe suspension with solid content of 18%. Ceramic polishing ballsballs (diameter: 1–3 mm) mm)added were added intosuspension the suspension according to the proportion 50%ofofthe thesolid solid content content and were into the according to the proportion of of 50% andthen then stirredatataaspeed speedofof1000 1000r/min r/min for for 11 h h to to prepare prepare the the depolymerized depolymerized amorphous stirred amorphous SiO SiO22slurry. slurry.AAzinc zinc nitratesolution solution(0.09 (0.09 wt wt %) %) was was added added into into the the slurry slurry and nitrate and the the pH pH of of the the mixture mixture was was respectively respectively

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◦ adjusted 5.0and and7.0 7.0by byadding adding 66 mol/L mol/LNaOH NaOH and and 66 mol/L HNO33.. The The mixture mixture was adjusted toto5.0 mol/L HNO was stirred stirredatat60 60 C were obtained obtained after after suction suctionfiltration, filtration,washing, washing,and anddrying dryingand and denoted °Cfor for11h. h. The The precursors precursors were denoted asas Zn–SiO (the precursor was prepared with pH value 5.0) and Zn–SiO (the precursor 2 -pH5.0 2 -pH7.0 Zn–SiO 2-pH5.0 (the precursor was prepared with pH value at at 5.0) and Zn–SiO 2-pH7.0 (the precursor was prepared with pH value at 7.0) respectively. The preparation process is shown in Figure was prepared with pH value at 7.0) respectively. The preparation process is shown in Figure 2.2.

2.3. Preparation ZnO–SiO 2.3. Preparation of of ZnO–SiO 2 2 ◦ C for 1 h to obtain the The precursors Zn–SiO and Zn-SiO were calcined 400 2 -pH5.0 2 -pH7.0 The precursors Zn–SiO 2-pH5.0 and Zn-SiO 2-pH7.0 were calcined at at 400 °C for 1 h to obtain the composite particles ZnO andSiO SiO denoted ZnO–SiO (the composite obtained 2 and 2 -pH5.0 composite particles ofof ZnO and 2 and denoted asasZnO–SiO 2-pH5.0 (the composite obtained byby calciningthe the precursor precursor which with pH value at 5.0)atand ZnO–SiO composite 2 -pH7.0 2(the calcining whichwas wasprepared prepared with pH value 5.0) and ZnO–SiO -pH7.0 (the obtained by calcining the precursor which was prepared with pH value at 7.0). The loads of ZnO were composite obtained by calcining the precursor which was prepared with pH value at 7.0). The loads 4.51 and 11.26%, respectively. of ZnO were 4.51% and 11.26%, respectively.

Figure 2. Preparation of composite particles of ZnO–SiO2 precursor. Figure 2. Preparation of composite particles of ZnO–SiO2 precursor.

2.4. Characterization 2.4. Characterization The X-ray diffraction (XRD) was measured using a D/max-Ra X-ray diffractometer (Ouyatu, The X-ray diffraction (XRD) was measured byby using a D/max-Ra X-ray diffractometer (Ouyatu, ◦ (2θ) with a step of 0.02◦ (2θ). Scherer Japan, Cu Kα radiation = 1.54 Å) in an angular range of 10–80 Japan, Cu Kα radiation = 1.54 Å) in an angular range of 10–80° (2θ) with a step of 0.02° (2θ). Scherer Equation [18] used calculate the average grain size ZnO nanoparticles: Equation [18] is is used toto calculate the average grain size ofof ZnO nanoparticles: Kλ,, (1)(1) Dcosθ 00 where K is the shape factor constant (0.94); λ is X-ray wavelength; D is the grain size; θ is the where K is the shape factor constant (0.94); λ is X-ray wavelength; D is the grain size; θ is the diffraction diffraction angle; β is the diffraction peak half width. angle; β is the diffraction peak half width. An X-ray fluorescence spectrometer (XRF Shimadzu-1800, Kyoto, Japan) was used to analyze An X-ray fluorescence spectrometer (XRF Shimadzu-1800, Kyoto, Japan) was used to analyze the the oxide content of samples. The particle size and size distribution of the composite particles of ZnO oxide content of samples. The particle size and size distribution of the composite particles of ZnO and SiO2 were characterized by transmission electron microscopy (TEM FEI Tecnai G220, Portland, and SiO were characterized by transmission electron microscopy (TEM FEI Tecnai G220, Portland, OR, USA).2 Scanning electron microscopy (SEM) was used to explore the morphology of ZnO–SiO2 OR, USA). Scanning electron microscopy (SEM) was used to explore the morphology of ZnO–SiO2 by by a Hitachi field emission scanning electron microscope (Hitachi S4800, Tokyo, Japan) under the a Hitachi field emission scanning electron microscope (Hitachi S4800, Tokyo, Japan) under the voltage voltage of 10 kV. The Fourier transform infrared spectroscopy (FTIR, Madison, WI, USA) of 10 kV. The Fourier transform infrared spectroscopy (FTIR, Madison, WI, USA) measurement was measurement was carried out to explore the changes in functional groups of ZnO-SiO2 by Nicolet carried out to explore the changes in functional groups of ZnO-SiO2 by Nicolet IS50. The samples IS50. The samples were finely pulverized and then diluted in dried KBr to form a homogeneous were finely pulverized and then diluted in dried KBr to form a homogeneous mixture according to the mixture according to the sample-KBr ratio of 1/200. The X-ray photoelectron spectroscopy (XPS, sample-KBr ratio of 1/200. The X-ray photoelectron spectroscopy (XPS, Manchester, UK) measurement Manchester, UK) measurement was conducted on an Axis Ultra spectrometer with monochromatic was conducted on an Axis Ultra spectrometer with monochromatic Mg Kα (1253.6 eV) radiation to Mg Kα (1253.6 eV) radiation to investigate the valence state of Zn. investigate the valence state of Zn.

= β=

2.5. Antimicrobial Test 2.5. Antimicrobial Test The antimicrobial ability of ZnO–SiO2 under dark conditions was investigated through The antimicrobial ability of ZnO–SiO2 under dark conditions was investigated through antibacterial tests [19–21]. Different concentrations of ZnO–SiO2-pH5.0, ZnO–SiO2-pH7.0, and pure antibacterial tests [19–21]. Different concentrations of ZnO–SiO2 -pH5.0, ZnO–SiO2 -pH7.0, and pure ZnO were added to the agar medium, and then E. coli (CGMCC 1.2385) was inoculated on the medium to observe the growth of bacteria and determine the minimum inhibitory concentration (MIC) [22].

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ZnO were added to the agar medium, and then E. coli (CGMCC 1.2385) was inoculated on the medium 4 of 12 to observe the growth of bacteria and determine the minimum inhibitory concentration (MIC) [22]. The antimicrobial coating was obtained by mixing 12 wt % styrene-acrylic emulsion, 34 wt % The antimicrobial coating was obtained by mixing 12 wt % styrene-acrylic emulsion, 34 wt % of of H2 O, 50 wt % of the filler (0–8 wt % of ZnO–SiO2 -pH7.0), and 4 wt % of paint additive. H2O, 50 wt % of the filler (0–8 wt % of ZnO–SiO2-pH7.0), and 4 wt % of paint additive. The The antibacterial property of ZnO–SiO2 was evaluated by testing the antibacterial property of the antibacterial property of ZnO–SiO2 was evaluated by testing the antibacterial property of the coating. coating. The antibacterial rate of the coating was tested according to Chinese national standard The antibacterial rate of the coating was tested according to Chinese national standard GB/T21866GB/T21866-2008 [23]. The antibacterial rate (R) is calculated as: 2008 [23]. The antibacterial rate (R) is calculated as: Nanomaterials 2017, 7, 217

R= × ×(A(A−−B)/A, R 100% = 100% B)/A,

(2) (2)

where where A A and and BB are are the the average average number number of of colonies colonies of of the theblank blankcontrol control plate plateand andantibacterial antibacterial coating coating plate plate after after 24 24 h. h. 3. 3. Results Resultsand andDiscussion Discussion 3.1. Structure Structure and and Characterization Characterization of of ZnO–SiO ZnO–SiO22 3.1. 3.1.1. Phase Phaseand and Chemical Chemical Constitution Constitution of of ZnO–SiO ZnO–SiO22 3.1.1. Figure 33 shows shows the the XRD XRD patterns patterns of of ZnO–SiO ZnO–SiO22. .Table Table11shows showsthe theXRF XRFresults resultsof ofeach eachsample. sample. Figure ◦ The XRD XRDpattern patternofofthe the SiO shows a strong bread of indicating 23 , indicating that the 2 carrier The SiO 2 carrier shows a strong bread peakpeak nearnear 2θ of2θ 23°, that the main main phase is an amorphous phase corresponding to amorphous SiO . In the XRD patterns 2 phase is an amorphous phase corresponding to amorphous SiO2. In the XRD patterns of ZnO–SiOof 2ZnO–SiO and ZnO–SiO -pH7.0, in to the above-mentioned peak the pH5.0 and2 -pH5.0 ZnO–SiO 2-pH7.0, in 2addition to addition the above-mentioned peak reflecting thereflecting amorphous ◦ , and 47.5◦ correspond to the ZnO diffraction amorphous phase, peaks at 36.3°, 31.8◦ , and 34.5◦47.5° , 36.3correspond phase, the peaks at the 31.8°, 34.5°, to the ZnO diffraction peak [24,25], 2+ 2+ peak [24,25], indicating that Zn has been transformed into ZnO after the thermal with indicating that Zn has been transformed into ZnO after the thermal reaction with thereaction SiO2 carrier the SiO The intensity ZnO diffraction intensity of ZnO–SiO significantly 2 carrier and 2 -pH7.0 was and calcination. The calcination. ZnO diffraction of ZnO–SiO 2-pH7.0 was significantly larger than that larger than 2that of ZnO–SiO dueloadings to the different of ZnO. Thein contents of 2ZnO in 2 -pH5.0 of ZnO–SiO -pH5.0 due to the different of ZnO. loadings The contents of ZnO ZnO–SiO -pH5.0 ZnO–SiO -pH5.0 and ZnO–SiO -pH7.0 are respectively 4.51 and 11.26% (Table 1). The SiO content in 2 2 2 and ZnO–SiO 2-pH7.0 are respectively 4.51 and 11.26% (Table 1). The SiO2 content in ZnO–SiO 2-pH5.0 ZnO–SiO than2-pH7.0. that in ZnO–SiO The results are the consistent with the XRD results. 2 -pH5.0 2 -pH7.0. is lower than that is inlower ZnO–SiO The results are consistent with XRD results.

Figure 3. XRD of (a) amorphous SiO22;; (b) (b) ZnO–SiO ZnO–SiO22-pH5.0; -pH5.0;and and(c) (c)ZnO–SiO ZnO–SiO22-pH7.0. -pH7.0.

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Table 1. XRF of amorphous SiO2 , ZnO–SiO2 -pH5.0 and ZnO–SiO2 -pH7.0. Table 1. XRF of amorphous SiO2, ZnO–SiO2-pH5.0 and ZnO–SiO2-pH7.0. Samples SiO2 /% ZnO/% Na2 O/%

Samples amorphous SiO2 ZnO–SiO2-pH5.0 ZnO–SiO2-pH7.0

SiO2/%

amorphous SiO2 96.63 ZnO–SiO2 -pH5.0 ZnO–SiO292.78 -pH7.0

84.22

96.63 92.78 84.22

ZnO/% 0 0 4.51 4.51 11.26 11.26

0.98 1.05 2.68

Na2O/% 0.98 1.05 2.68

According -pH7.0was wascalculated calculatedtotobe be According to tothe the XRD XRD data data in in Figure Figure 3, 3, the the grain grain size size of of ZnO–SiO ZnO–SiO22-pH7.0 3.63 3.63nm nmaccording accordingto tothe theScherer Scherer Equation. Equation. 3.1.2. Microstructure of ZnO–SiO2 3.1.2. Microstructure of ZnO–SiO2 Figure 4 shows the distribution of three elements (O, Si, and Zn) in ZnO–SiO2 -pH5.0 and Figure 4 shows the distribution of three elements (O, Si, and Zn) in ZnO–SiO2-pH5.0 and ZnO– ZnO–SiO2 -pH7.0. The distributions of these three elements are consistent with the distribution SiO2-pH7.0. The distributions of these three elements are consistent with the distribution of ZnO– of ZnO–SiO2 particles. The distribution densities of O and Si are larger than that of Zn. The Zn density SiO2 particles. The distribution densities of O and Si are larger than that of Zn. The Zn density in the inelemental the elemental distribution of ZnO–SiO 2 -pH7.0 is greater than that of ZnO–SiO2 -pH5.0. The results distribution of ZnO–SiO 2-pH7.0 is greater than that of ZnO–SiO2-pH5.0. The results indicate are SiO SiO22.. The Thecontent contentofofZnO ZnOisislow, low,but butevenly evenly indicate that that the the main main components components of of ZnO–SiO ZnO–SiO22 are distributed on the surface of SiO particles. distributed on the surface of SiO22 particles. Figure showsthethe TEM images the amorphous SiO2ZnO–SiO carrier, 2-pH5.0, ZnO–SiO and 2 -pH5.0, Figure 55 shows TEM images of theofamorphous SiO2 carrier, and ZnO–SiO 2ZnO–SiO -pH7.0. At a small scale, all the samples are regular particle aggregates. The particle 2 pH7.0. At a small scale, all the samples are regular particle aggregates. The particle size is about 20– size is about 20–30these nm. particles Although these particles overlap each other, the overall effect 30 nm. Although overlap each other, the overall dispersion effect isdispersion good. These unitis good. These particles are obviously particles. thethe scale of 10morphology nm, the surface particles areunit obviously amorphous SiO2 amorphous particles. AtSiO the2 scale of 10At nm, surface of morphology of SiO particles in the SiO carrier is uniform, indicating that no other material is loaded. 2 SiO2 carrier is uniform, 2 SiO2 particles in the indicating that no other material is loaded. At the scale At scale of homogeneous 2 nm, only homogeneous non-crystal phase observed. Dark spots of the 2 nm, only non-crystal phase particles are particles observed.are Dark spots with a size of with 3–8 anm sizeare of uniformly 3–8 nm aredistributed uniformlyin distributed in ZnO–SiO ZnO–SiO -pH7.0 at the 2 -PH5.0 and ZnO–SiO2-PH5.0 and ZnO–SiO 2-pH7.0 at the 2scale of 10 nm.scale Theseof 10 nm.spots Theseare dark spotsphase are crystal phase particles the larger magnification. The stripe can dark crystal particles at the largerat magnification. The stripe spacing canspacing reflect the reflect lattice size. The stripeofspacing of2-pH5.0 ZnO–SiO and2-pH7.0 ZnO–SiO -pH7.0 are respectively latticethe size. The stripe spacing ZnO–SiO and ZnO–SiO are2 respectively measured 2 -pH5.0 measured be 2.45 2.59 nm.(101) ZnO plane (101) plane spacing (002) plane spacing arerespectively respectively to be 2.45toand 2.59and nm. ZnO spacing and and ZnOZnO (002) plane spacing are measured measuredtotobe be2.45 2.45and and2.59 2.59Å, Å,which whichare arealmost almostconsistent consistentwith withstandard standardZnO ZnO(101) (101)plane planespacing spacingof 2.47 Å and ZnO (002) plane spacing ofof 2.60 data indicate indicatethat thatthese these of 2.47 Å and ZnO (002) plane spacing 2.60ÅÅ(ICDD (ICDDcard card##89-7102). 89-7102). These data ZnO monodispersedlyloaded loadedon onthe theSiO SiO surface. The size of ZnO particles ZnOnanoparticles nanoparticles were monodispersedly 2 surface. The size of ZnO particles is 3–is 2 3–8 nm, which is consistent with the average particle size of 3.63 nm obtained in the XRD test [18]. 8 nm, which is consistent with the average particle size of 3.63 nm obtained in the XRD test [18].

Figure4. 4. SEM SEM images images of of ZnO–SiO ZnO–SiO22 and Figure and corresponding correspondingmapping mappingresults. results.

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Figure (a)(a) amorphous SiO2SiO ; (b) (c) ZnO–SiO at different scales. Figure 5.5.TEM TEMmaps mapsofof amorphous 2; ZnO–SiO (b) ZnO–SiO 2-pH5.0; (c) ZnO–SiO 2-pH7.0 at different 2 -pH5.0; 2 -pH7.0 scales.

3.1.3. Formation Mechanism of ZnO–SiO2 3.1.3. Formation Mechanism of ZnO–SiO2 Figure 6 shows the XPS pattern between 1015 and 1050 eV of the ZnO–SiO2 . The peaks of Figure 6 shows XPS pattern between 1050 of the ZnO–SiO . The peaksthe of peaks ZnO– ZnO–SiO andthe ZnO–SiO at 1046.01015 andand 1045.5 eVeV correspond to Zn2p21/2 orbital; 2 -pH5.0 2 -pH7.0 SiO 2 -pH5.0 and ZnO–SiO 2 -pH7.0 at 1046.0 and 1045.5 eV correspond to Zn2p 1/2 orbital; the peaks at at 1023.0 and 1022.5 eV correspond to the Zn2p3/2 orbital [26]. These peaks are equivalent to the 1023.0 and2p 1022.5 eV correspond to the Zn2p 3/2 orbital peaks are equivalent the 2p1/2 2p peaks (1044.2 and 1021.2 eV) of[26]. ZnOThese [27]. Moreover, the energy to difference 1/2 and 3/2 energy and 2p3/2Zn2p energy peaks (1044.2 and 1021.2 eV) of ZnO [27]. Moreover, the energy difference between between and Zn2p orbitals is 23 eV, which is the same as that of ZnO. Therefore, it can be 1/2 3/2 Zn2p 1/2 and Zn2p 3/2 orbitals is 23 eV, which is the same as that of ZnO. Therefore, it can be determined determined that the valence of Zn in ZnO–SiO2 -pH5.0 and ZnO–SiO2 -pH7.0 is +2, which is consistent that the results valenceofofXRD, Zn inXRF, ZnO–SiO 2-pH5.0 and ZnO–SiO2-pH7.0 is +2, which is consistent with the with and TEM. results of XRD, XRF, TEM. of the amorphous SiO2 carrier, composite precursors (pH 5.0 and 7.0) Table 2 shows theand percentages Table 2 shows the on percentages of the amorphous SiO2 carrier,SiO composite precursors 5.0 and and ZnO–SiO XPS. Compared with the amorphous the Zn2+(pH composite 2 based 2 carrier, 2+ 7.0) and ZnO–SiO 2 based on XPS. with amorphous carrier, The the Zn composite precursors show the increasing ratioCompared of O/Si with thethe increase in the SiO Zn 2content. change may be precursors show the increasing ratio of O/Si with the increase in the Zn content. The change be interpreted as follows. Hydroxyl groups generated by the hydrolysis of Zn form hydrogen bondsmay on the interpreted follows. Hydroxyl groupsin generated by of the Zn the form hydrogenZnO–SiO bonds on SiO thus resulting in an increase the amount O.hydrolysis Comparedof with precursors, 2 surface,as 2 the SiO2 surface, thus resulting an because increasethe in amount the amount of O. Compared thedehydration precursors, products show a decreased O/Si in ratio of oxygen is decreasedwith by the ZnO–SiO2 products a decreased O/Si on ratio theprecursors amount of oxygen is decreased the condensation reactionshow among the –OH bonds thebecause surface of during calcination. The by above dehydration condensation reaction among –OHonbonds on the surface of precursors during analysis suggests that ZnO nanoparticles wasthe loaded the surface of amorphous SiO2. 2+ composite calcination. above suggestsofthat ZnO nanoparticles was Zn loaded on the surface of Figure 7The shows the analysis infrared spectra the amorphous SiO2 carrier, precursor amorphous SiO 2. (Zn–SiO2 -pH5.0 and Zn–SiO2 -pH7.0), and the final products (ZnO–SiO2 -pH5.0 and ZnO–SiO2 -pH7.0). 2+ composite precursor Figure 7 shows infrared spectra ofand the 1062 amorphous SiO 2 carrier, Zn The absorption peaksthe of each sample at 450 cm−1 are respectively ascribed to symmetrical − (Zn–SiO 2 -pH5.0 and Zn–SiO 2 -pH7.0), and the final products (ZnO–SiO 2 -pH5.0 and ZnO–SiO 2-pH7.0). and antisymmetric stretching vibration of Si–O–Si. The absorption peak at 799 cm 1 corresponds −1 The absorption peaks reflecting of each sample at 450 and 1062 cm 2 [28]. are respectively ascribed to bending vibration, the characteristics of SiO As shown in Figure to 7, symmetrical the infrared −1 corresponds − 1 and antisymmetric stretching vibration of Si–O–Si. The absorption peak at 799 cm spectra of Zn2+ composite precursors (Zn–SiO -pH5.0 and Zn–SiO -pH7.0) at 3317 and 3319 cm to 2 2 bending vibration, reflecting thevibration. characteristics of SiO2 [28]. As shown in Figure 7, the spectra correspond to Zn–OH stretching The absorption peaks of hydroxyl groups ininfrared zinc hydroxide 2+ −1 − 1 of Zn composite precursors 2-pH5.0 and Zn–SiO at 3317 and 3319 cm product, correspond (Zn(OH) 1347 cm(Zn–SiO reflect the bridging effect2-pH7.0) of the hydroxyl group in the and 2 ) at 1345 and −1zinc to Zn–OH stretching vibration. Theinabsorption peaks of hydroxyl (Zn(OH) 2) the Si–OH bending vibration peak the two products moves fromgroups 958 cmin (the hydroxide vibration peak of the −1 reflect the bridging − 1 at 1345 and 1347 cm effect of the hydroxyl group in the product, and the Si–OH raw material SiO2 ) to 952 and 954 cm , respectively. The changes indicate that in the hydrothermal −1 (the vibration peak of the raw 2+ cm bendingduring vibration in the process two products moves from Zn 958 reaction the peak preparation of Zn–SiO forms a complex of Zn(OH)2 , which 2 -pH5.0, −1, respectively. The changes indicate that in the hydrothermal material SiO 2 ) to 952 and 954 cm yields hydrogen bonds with Si–OH on the surface of amorphous SiO2 [29–31]. In addition, the –OH reaction vibration during the preparation ofon Zn–SiO 2-pH5.0, Zn2+ forms a complex of Zn(OH) 2, which bending peaks of water process adsorbed the surface of Zn–SiO and Zn–SiO occur 2 -pH5.0 2 -pH7.0 − 1 −1 –OH yields bonds with Si–OH The on the surface amorphous 2 [29–31]. addition, the at 1413hydrogen and 1411 cm , respectively. –OH shear of vibration peaksSiO occur at 1580Inand 1579 cm [32]. bending vibration peaks of water adsorbed on the surface of Zn–SiO 2 -pH5.0 and Zn–SiO 2 -pH7.0 The Zn–OH and Si–OH of calcined products and the –OH bond of adsorbed water disappeared occurthe at 1413 and 1411 respectively. The –OH the shear vibration peaks occur at 1580resulted and 1579 after calcination of cm the−1,precursors. Therefore, high-temperature calcination incm the−1 [32]. The Zn–OH and Si–OH of calcined products and the –OH bond of adsorbed water disappeared after the calcination of the precursors. Therefore, the high-temperature calcination resulted in the

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evaporation of SiO surface water and the dehydration condensation reaction between Si–OH and evaporation of of SiO SiO222 surface surface water water and and the the dehydration dehydration condensation condensation reaction reaction between between Si–OH Si–OH and and evaporation Zn–OH, and yielded Si–O–Zn chemical bond. Zn–OH,and andyielded yieldedSi–O–Zn Si–O–Znchemical chemicalbond. bond. Zn–OH,

Figure6. 6.XPS XPSof of(a) (a)ZnO–SiO ZnO–SiO222-pH5.0; -pH5.0;(b) (b)ZnO–SiO ZnO–SiO222-pH7.0. -pH7.0. Figure Figure 6. XPS of (a) ZnO–SiO -pH5.0; (b) ZnO–SiO -pH7.0. Table2. 2.Element Elementanalysis analysisbased basedon onXPS XPSresults results Table Table 2. Element analysis based on XPS results

Samples Samples

C1s(%) (%) C1s

Samples

AmorphousSiO SiO22 2.74 Amorphous 2.74 Amorphous SiO2 Zn–SiO22-pH5.0 -pH5.0 6.18 Zn–SiO 6.18 Zn–SiO2 -pH5.0 ZnO–SiO 2 -pH5.0 3.48 ZnO–SiO2-pH5.0 3.48 ZnO–SiO2 -pH5.0 Zn–SiO22-pH7.0 -pH7.0 6.90 Zn–SiO 6.90 Zn–SiO2 -pH7.0 ZnO–SiO2 -pH7.0 ZnO–SiO22-pH7.0 -pH7.0 5.96 ZnO–SiO 5.96

Zn2p(%) (%) Zn2p

C1s (%) 2.74 6.18 3.48 6.90 5.96

Zn2p (%)

00 0 2.24 2.24 2.24 2.472.47 2.47 6.246.24 6.24 10.55 10.55 10.55

Si2p(%) (%) Si2p Si2p (%)

31.27 31.27 31.27 27.55 27.55 27.55 30.03 30.03 30.03 24.38 24.38 24.38 25.18 25.18 25.18

O1s(%) (%) O1s

O/Si O/Si

65.84 65.84

2.11 2.11 2.25 2.25 2.12 2.12 2.50 2.50 2.31 2.31

O1s (%)

65.84 62.04 62.04 62.04 63.68 63.68 63.68 60.84 60.84 60.84 58.39 58.39 58.39

O/Si 2.11 2.25 2.12 2.50 2.31

Figure 7.7. FTIR FTIR -pH5.0, Zn–SiO -pH7.0, and 22,2, ,Zn–SiO Figure 7. FTIR spectra spectraof ofamorphous amorphousSiO SiO Zn–SiO22-pH5.0, 2-pH5.0, -pH5.0,ZnO–SiO ZnO–SiO222-pH5.0, -pH5.0, Zn–SiO Zn–SiO222-pH7.0, -pH7.0, and and Figure spectra of amorphous SiO Zn–SiO ZnO–SiO ZnO–SiO -pH7.0. 2 ZnO–SiO22-pH7.0. -pH7.0. ZnO–SiO 2+on the 2+ Figure888shows showsaaaschematic schematicdiagram diagramof ofthe thesynthesis synthesisprocess processof ofZnO–SiO ZnO–SiO222by byloading loadingZn Zn2+ Figure synthesis process of ZnO–SiO loading Zn Figure shows schematic on the amorphous SiO 2 carrier via the hydrothermal reaction and composite precursor calcination. Due to amorphous SiO22 carrier via the hydrothermal reaction and composite composite precursor precursor calcination. calcination. Due to to thelarge largenumber numberof ofSi–OH Si–OHbonds bondson onthe thesurface surfaceof ofSiO SiO22and andthe thestrong strongactivity activityof ofSi–OH Si–OHbonds, bonds,the the the

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the large number of Si–OH bonds on the surface of SiO2 and the strong activity of Si–OH bonds, the interaction interaction between between Si–OH Si–OH and and Zn–OH Zn–OH is is greater greater than than the the interaction interaction in in Zn–OH Zn–OH(multinuclear (multinuclearions). ions). 2+ 2+ Therefore, Zn is immobilized on the surface of SiO 2 and dispersed ZnO nanoparticles are formed Therefore, Zn is immobilized on the surface of SiO2 and dispersed ZnO nanoparticles are formed after after the the calcination calcination of of precursors. precursors.

Figure 8. Synthesis of the ZnO–nSiO22 Complex Complex

3.2. 3.2. Antibacterial Antibacterial Properties Properties of of ZnO–SiO ZnO–SiO22 ZnO ZnO has has antibacterial antibacterial activity activity under under light light and and dark dark conditions conditions and and is is mostly mostly applied applied under under dark dark conditions. In order to investigate the antibacterial properties of ZnO–SiO 2 under dark conditions, conditions. In order to investigate the antibacterial properties of ZnO–SiO2 under dark conditions, ZnO–SiO and pure ZnO were respectively prepared. Figure 9 shows the ZnO–SiO22-pH5.0, -pH5.0,ZnO–SiO ZnO–SiO2-pH7.0, 2 -pH7.0, and pure ZnO were respectively prepared. Figure 9 shows the bacterial growth profiles obtained by the plate plate test. test. Obvious bacterial growth profiles obtained by the Obvious colonies colonies were were formed formed in in the the blank blank control antimicrobial material material (Figure (Figure 9a). 9a). When When the the concentration concentration of of ZnO–SiO ZnO–SiO22-pH5.0 control without without the the antimicrobial -pH5.0 was colonies were observed on the plate;plate; whenwhen the concentration of ZnO– was 10 10mg/mL, mg/mL,obvious obvious colonies were observed on culture the culture the concentration of SiO 2 -pH5.0 was 20 mg/mL, the number of colonies decreased but colonies did not completely ZnO–SiO2 -pH5.0 was 20 mg/mL, the number of colonies decreased but colonies did not completely disappear; concentration of of ZnO–SiO ZnO–SiO22-pH5.0 to 36 36 mg/mL, mg/mL, no disappear; when when the the concentration -pH5.0 was was increased increased to no colony colony was was formed (Figure 9b). The concentrations of ZnO–SiO 2 -pH7.0 and pure ZnO required for colony-free formed (Figure 9b). The concentrations of ZnO–SiO2 -pH7.0 and pure ZnO required for colony-free results respectively 19 19 mg/mL mg/mL and mg/mL (Figure results were were respectively and 20 20 mg/mL (Figure 9c,d). 9c,d). Based Based on on the the above above results, results, the the minimum minimum inhibitory inhibitory concentration concentration (MIC) (MIC) of of each each sample sample was was determined determined and and converted converted into into the the minimum inhibitory concentration of ZnO based on the content of ZnO in the composite (Table minimum inhibitory concentration of ZnO based on the content of ZnO in the composite (Table 3). 3). The MIC values values of of ZnO–SiO ZnO–SiO22-pH5.0 and ZnO–SiO ZnO–SiO22-pH7.0 and 2.14 2.14 mg/mL, mg/mL, The MIC -pH5.0 and -pH7.0 were were respectively respectively 1.60 1.60 and which equivalent to to 10% 10% of of the the MIC MIC of of pure pure ZnO ZnO (20 (20 mg/mL), mg/mL), indicating which were were equivalent indicating that that the the antimicrobial antimicrobial ability of ZnO nanoparticles loaded on the SiO 2 surface was about 10 times that of pure ZnO. ability of ZnO nanoparticles loaded on the SiO2 surface was about 10 times that of pure ZnO. Obviously, Obviously, theofformation dispersed nanoparticles (3–8 on nm) loaded onSiO amorphous SiO2 greatly the formation dispersedofnanoparticles (3–8 nm) loaded amorphous 2 greatly improved its improved its antimicrobial performance. antimicrobial performance. Table 3. MIC of ZnO–SiO2 -pH5.0, ZnO–SiO2 -pH7.0, and pure ZnO. MIC (mg/mL)

ZnO–SiO2 -pH5.0

ZnO–SiO2 -pH7.0

ZnO

E. coli E. coli (ZnO)

36 1.60

19 2.14

20 20

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Figure 9. Antimicrobial teststests of (a) control, (b)(b) different concentrations -pH5.0in in agar Figure 9. Antimicrobial of blank (a) blank control, different concentrationsofofZnO–SiO ZnO–SiO22-pH5.0 medium, (c) different concentrations of ZnO–SiO -pH7.0 in agar medium, (d) different concentrations 2 agar medium, (c) different concentrations of ZnO–SiO2-pH7.0 in agar medium, (d) different of pure ZnO in agar concentrations of medium. pure ZnO in agar medium. Table 3. MIC of ZnO–SiO 2-pH5.0, ZnO–SiO2-pH7.0, and pure ZnO. Figure 10 shows the antibacterial rate and colony growth conditions of E. coli on the plates added with different amounts of ZnO–SiO -pH7.0 coating. The antibacterial rate of ZnO the coating without MIC (mg/mL) ZnO–SiO 2-pH5.0 ZnO–SiO 2-pH7.0 2 ZnO–SiO2 -pH7.0 of colonies were19formed on the plate,20indicating that the E. was coli 0 and a large number 36 E. coli 1.60 When the addition 2.14of ZnO–SiO2 -pH7.020in the coating was coating showed no(ZnO) antibacterial property. only 2%, the antibacterial rate was increased above 70%, showing a good antibacterial effect; when Figure shows theincreased antibacterial growth conditions E. coli onto the added the dosage was10 gradually to rate 8%,and thecolony antibacterial rate of theofcoating E.plates coli was 90.48%, with different amounts of ZnO–SiO2-pH7.0 coating. The antibacterial rate of the coating without which met the requirements of the antibacterial effect of antibacterial coating in Chinese national ZnO–SiO2-pH7.0 was 0 and a large number of colonies were formed on the plate, indicating that the standard GBT21866-2008. The increasing antibacterial rate of the paint indicated less colonies and a Nanomaterials 2017, 7, no 217 antibacterial property. When the addition of ZnO–SiO2-pH7.0 in the coating10 of 12 coating showed was betteronly antibacterial effect. 2%, the antibacterial rate was increased above 70%, showing a good antibacterial effect; when

the dosage was gradually increased to 8%, the antibacterial rate of the coating to E. coli was 90.48%, which met the requirements of the antibacterial effect of antibacterial coating in Chinese national standard GBT21866-2008. The increasing antibacterial rate of the paint indicated less colonies and a better antibacterial effect. The antimicrobial properties of the ZnO nanoparticles supported uniformly and dispersedly on the surface of SiO2 were greatly improved, due to the large specific surface area and surface activity of ZnO nanoparticles compared with the pure ZnO of large particles, and the contact and inhibition with microbes is stronger. This should be considered as one of the means to enhance the function of ZnO.

Figure additiveson onantibacterial antibacterial rate coli). Figure10. 10.Effects Effectsof ofdifferent different additives rate (E.(E. coli).

4. Conclusions ZnO–SiO2 composite was prepared by an environmentally friendly hydrothermal method and high-temperature calcination. In this composite, ZnO nanoparticles with a particle size of 3–8 nm were uniformly and dispersedly loaded on the surface of amorphous SiO2. The size of the ZnO

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The antimicrobial properties of the ZnO nanoparticles supported uniformly and dispersedly on the surface of SiO2 were greatly improved, due to the large specific surface area and surface activity of ZnO nanoparticles compared with the pure ZnO of large particles, and the contact and inhibition with microbes is stronger. This should be considered as one of the means to enhance the function of ZnO. 4. Conclusions ZnO–SiO2 composite was prepared by an environmentally friendly hydrothermal method and high-temperature calcination. In this composite, ZnO nanoparticles with a particle size of 3–8 nm were uniformly and dispersedly loaded on the surface of amorphous SiO2 . The size of the ZnO particles used in the industry is about 500 nm, and there is a certain degree of agglomeration among particles. According to the analysis of relevant tests, the strong interaction between the SiO2 carrier surface and Zn–OH (precursors of ZnO) reduced the combination between Zn–OH, prevented its aggregation and formed monodispersed nanoparticles. Compared with pure ZnO, ZnO–SiO2 showed much better antibacterial performance in the MIC test and the characterization test of paint properties. In general, ZnO nanoparticles loaded uniformly and depressively on the surface of amorphous SiO2 greatly enhanced its antibacterial function. Acknowledgments: This work was supported by the Fundamental Research Funds for the Central Universities of China (No. 2652016160) and the Project Commissioned by Shandong Private Enterprises (No. 2015-KY19-139 20151224). Author Contributions: Ying Chen and Hao Ding conceived and designed the experiments; Ying Chen performed the experiments; Ying Chen, Hao Ding and Sijia Sun anamyzed the date; Hao Ding contributed reagents/materials/analysis tools; Ying Chen wrote the paper. Conflicts of Interest: The authors declare no competing financial interest.

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