materials Article
Synthesis and Photocatalytic Performance of ZnO/Bone Char Composite Puqi Jia 1,2,3 , Hongwei Tan 3 , Kuiren Liu 2 and Wei Gao 3, * 1 2 3
*
College of Earth and Environmental Sciences, Lanzhou University, Lanzhou 730000, China;
[email protected] Department of Nonferrous Metallurgy, School of Metallurgy, Northeastern University, Shenyang 110819, China;
[email protected] Department of Chemical and Materials Engineering, The University of Auckland, Auckland 1142, New Zealand;
[email protected] Correspondence:
[email protected]; Tel.: +64-9923-8175
Received: 23 August 2018; Accepted: 8 October 2018; Published: 15 October 2018
Abstract: ZnO/bone char (ZnO/BC) composites were successfully synthesized by the precipitation of a ZnO precursor on pyrolytic bone char. The effects of bone char size, mass ratio of ZnO to BC, and molar ratio of ZnO to triethylamine (TEA) on the microstructure, specific surface area, and light absorbance of ZnO/BC were studied. The photocatalytic property of ZnO/BC was evaluated by the degradation of methylene blue. Results show that with a uniform nano-ZnO particle layer distributed evenly on the bone char surface, ZnO/BC has the strongest light absorbance and can effectively degrade methylene blue. The photocatalytic performance of ZnO/BC is related to the light absorbance of the photocatalyst, as well as the amount and distribution state of the loaded ZnO. This study indicates that bovine bone waste can be used as a nano-photocatalyst carrier to prepare photocatalytic composites, which is not only a good way to clean wastewater but also an ideal solution to utilize animal bone waste. Keywords: ZnO; bone char; uniform layer; UV–visible absorbance; photocatalysis
1. Introduction Billions of animals are used daily to meet the dietary needs of humans and carnivores. Bone is one of the appendant byproducts. If not handled in time, it becomes solid waste, breeds bacteria, endangers human health, and pollutes the environment. Bone products are worth $4000 to the total output value after processing a ton of raw bone materials. The largest daily-output bone species in the world are beef bones, lamb bones, and pork bones. Slaughter plants usually process bones into bone grains, then sell them to agricultural and sideline product manufacturing companies for further processing. The products extracted from bones reach 7 major categories, including 300 varieties, amongst which the most common products are bone soup, bone oil, bone marrow extract, bone meal, and bone flavor. After disinfection by boiling, the bone particles can also be used as fertilizer for green plants. Bone char can be obtained after removing organics by pyrolysis; it is mainly composed of hydroxyapatite and commonly used as the raw material for the high-grade bone china of ceramics. With the advantages of thermostability, alkali resistance, water insolubility, large specific surface area, good adsorption ability, nontoxicity, and recyclability, bone char has been widely used as an adsorbent for refining sugar [1] and fish oil [2] and removing heavy metal ions [3,4] and fluoride [5]. Recently, some researchers also applied it to adsorb 17-estradiol [6], endotoxin [7], and methylene blue [8,9]. Green degradation technology uses photocatalytic oxidation to oxidize toxic organic compounds and reduce heavy metal ions [10–12]. It plays a key role in solving serious environmental problems, such as contaminated river, air, and soil. Nanosized photocatalyst powder is used as a photocatalyst;
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however, it tends to agglomerate, reducing the photocatalytic efficiency. Another shortcoming is that it is difficult to recover, resulting in secondary environmental pollution. Large-sized supported photocatalysts can be synthesized by combining large particle sizes of millimeter-scale bone char with nano photocatalytic materials. This technique not only improves the high value-added bone waste, but also solves the environmental pollution problem. It was reported that the combination of photocatalysis with adsorption by mixing nano ZnO photocatalyst and bone char (BC) particles increased the removal efficiency of a formaldehyde pollutant [13]. However, nano ZnO particles were not distributed evenly on the surface of the bone char through physical mixing. A uniform nano ZnO particle layer loaded on the surface of bone char is expected to yield perfect photocatalytic performance. In this paper, to optimize the micromorphology and photocatalytic performance of ZnO/BC, ZnO/BC was prepared by depositing ZnO precursor on bone char surface. The size of bone char, mass ratio of ZnO to BC, and molar ratio of ZnO to triethylamine (TEA) were investigated. Their effects on the microstructures, specific surface areas, and absorbance spectra of ZnO/BC photocatalyst were studied. The degradation of methylene blue from water was also carried out. 2. Materials and Methods Bovine bone was sawed into 2–3 cm bone slices by handsaw and washed in boiled water until fat and residual protein pieces were removed completely, then, the bone slices were crushed into bone particles with a size of 0.25–0.50 mm or 0.50–0.80 mm, according to our previous study [14]. The bone particles were pyrolyzed at 400 ◦ C for 2 h in a furnace. ZnO/BC composites were prepared as we reported before [15], namely, zinc acetate dehydrate was added to 55 ◦ C absolute ethanol solution and dissolved with a little of the capping agent triethylamine (TEA). Zinc acetate dehydrate and TEA were mixed at a molar ratio of 1:1 or 2:1 to form 0.2 mol/L of nano ZnO precursor sol (zinc hydroxide) in absolute ethanol solution. The pH value of ZnO precursor sol was 8.4. Bone char was dipped in the ZnO precursor solution to produce 1:5.5, 1:11, or 1:16.5 of m(ZnO):m(BC) for 3 h. Then, zinc hydroxide and bone char combined with each other through hydrogen bonding. The as-prepared samples were dried at room temperature through the volatilization of absolute ethanol and acetic acid, and further at 110 ◦ C in an oven for 8 h, followed by sintering at 400 ◦ C for 1 h to obtain ZnO/BC samples. Table 1 shows the sample labels. Table 1. ZnO/bone char (BC) samples obtained by different synthesis parameters. Sample 1
Particle Size of Bone Char/mm
m(ZnO):m(BC)
n(ZnO):n(TEA)
a b c d e
0.25–0.50 0.50–0.80 0.50–0.80 0.50–0.80 0.50–0.80
1:5.5 1:5.5 1:11 1:16.5 1:11
1:1 1:1 1:1 1:1 2:1
1
The difference between sample a and sample b is the different bone char particle sizes. The difference between sample b, sample c, and sample d is the different mass ratios of ZnO to bone char. The difference between sample c and sample e is the different mole ratios of ZnO to triethylamine (TEA).
The crystals structure of ZnO/BC samples were identified by using an X-ray diffractometer (Bruker, D2 Phaser, Billerica, MA, USA). The microstructures were characterized by an SEM (JEOL, JSM-7800F, Tokyo, Japan). The specific surface areas of bone char and ZnO/BC samples were analyzed by a surface area instrument (Micromeritics, 3 FLEX, Norcross, GA, USA). The ultraviolet–visible absorbance spectra of the ZnO/BC photocatalyst were measured on a UV–vis spectrophotometer (Shimadzu, UV-2550, Kyoto, Japan) using BaSO4 as the reflectance standard. The photocatalytic performance of ZnO/BC (4 g/L) was evaluated through the photocatalytic degradation of methylene blue (MB, 4.48 mg/L) under simulated solar irradiation (OSRAM, ULTRA-VITALUX, 300 W, Munich, Germany), and the reaction system was cooled by a circulating
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The photocatalytic performance of ZnO/BC (4 g/L) was evaluated through the photocatalytic Materials 2018, 11, 3 of 8 degradation of1981 methylene blue (MB, 4.48 mg/L) under simulated solar irradiation (OSRAM, ULTRA-
VITALUX, 300 W, Munich, Germany), and the reaction system was cooled by a circulating water bath to maintain room temperature. Oxygen was introduced into the reaction system at a rate of 0.4 L/min. water bath to maintain room temperature. Oxygen was introduced into the reaction system at a rate of Dark adsorption was carried out without a light illuminator. The concentrations of the MB solution 0.4 L/min. Dark adsorption was carried out without a light illuminator. The concentrations of the MB were measured using a UV–vis spectrometer (Perkin Elmer, Lambda 35, Waltham, MA, USA) at 664 solution were measured using a UV–vis spectrometer (Perkin Elmer, Lambda 35, Waltham, MA, USA) nm during the reaction. at 664 nm during the reaction. 3. Results 3. Results and and Discussion Discussion The XRD XRD patterns patterns of of the the obtained obtained ZnO/BC ZnO/BC samples 1. All peaks The samples are are shown shown in in Figure Figure 1. All detectable detectable peaks can be indexed to the hydroxyapatite and ZnO hexagonal wurtzite structure. Hexagonal wurtzite can be indexed to the hydroxyapatite and ZnO hexagonal wurtzite structure. Hexagonal wurtzite ZnO ZnO is an effective structure for photocatalysis. the massofratios of bone ZnO char to bone is an effective structure for photocatalysis. BecauseBecause the mass ratios ZnO to werechar lesswere than less than 1:5.5, the peak intensities of ZnO are much weaker than those of bone char. The 1:5.5, the peak intensities of ZnO are much weaker than those of bone char. The peak intensity of peak ZnO intensity of ZnO at 36.25 sample the highest of allthis samples sample had the at 36.25◦ (2θ) in sample a is(2) the in highest of aallissamples because samplebecause had thethis highest mass ratios highest mass ratios of ZnO to bone char.of ZnO to bone char.
Figure 1. 1. XRD XRD patterns patterns of of ZnO/BC ZnO/BC samples. Figure samples.
images of ZnO/BC. ZnO/BC. Sample Figure 2 shows the SEM images Sample aa (Figure (Figure 2a) 2a) and and sample sample bb (Figure (Figure 2b) show number of of stacked stackedZnO ZnOparticles particlesloaded loadedon onthe thebone bonechar charsurface surface due large mass ratio a large number due toto thethe large mass ratio of ◦ (2θ) of ZnO to bone char. The XRD peak intensity of ZnO at 36.25 (2) sample b was weaker than that ZnO to bone char. The XRD peak intensity of ZnO at 36.25 in in sample b was weaker than that of of sample a because some bare bonechar chardecreased decreaseda aproportion proportionofofZnO ZnOexposed exposed to to X-ray. X-ray. Reducing sample a because some bare bone the amount of ZnO precursor by half, ZnO particles with a size of about 30 nm were uniformly distributed on Further reducing thethe amount of on the the bone bonechar charsurface surfaceby bylayer layerininsample samplec c(Figure (Figure2c). 2c). Further reducing amount ZnO precursors, thethe ZnO particles in sample d (Figure 2d) were distributed sparsely on the char of ZnO precursors, ZnO particles in sample d (Figure 2d) were distributed sparsely onbone the bone surface. The The ZnOZnO precursor could notnot be be dispersed uniformly char surface. precursor could dispersed uniformlyininabsolute absoluteethanol ethanol solution solution by reducing the amount of capping agent, triethylamine; triethylamine; eventually, eventually, the the ZnO ZnO in sample sample e (Figure 2e) easily agglomerated into large particles.
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Figure 2. 2. SEM samples: (a)(a) sample a, (b) sample b, (c) c, (d) d, and Figure SEM images imagesof ofZnO/BC ZnO/BC samples: sample a, (b) sample b, sample (c) sample c, sample (d) sample d, (e) sample e. e. and (e) sample
Table22lists liststhethe specific surface areas and average poreofsizes of the ZnO/BC The Table specific surface areas and average pore sizes the ZnO/BC samples.samples. The average average pore sizes of the ZnO/BC samples were about 12–14 nm, namely mesopores. The specific pore sizes of the ZnO/BC samples were about 12–14 nm, namely mesopores. The specific surface areas surface areas of ZnO/BC Table 2than were smaller than those the bone char samples, because of ZnO/BC listed in Table listed 2 wereinsmaller those of the bone charof samples, because the loaded nano the loaded nano ZnO particles cover some of the bone char surfacesamples, area. Ofsample all ZnO/BC ZnO particles could cover some ofcould the bone char surface area. Of all ZnO/BC e had samples, e had largest exposed bone surface, so itsurface had the largest surface the largestsample exposed bonethe char surface, so it had thechar largest specific area. The specific partly exposed area. char The partly char surface and the agglomerated ZnO itparticles of bone surfaceexposed and the bone agglomerated multilayered ZnO particlesmultilayered of sample b gave a slightly sample b gave it a slightly larger specific surface area than sample a. larger specific surface area than sample a. As shown shown in in Figure Figure 2, 2, a large large amount amount of of ZnO particles particles were were supported supported on on the bone char surface As of sample a owing to bonds generated in of to the the large largemass massratio ratioofofZnO ZnOtotoBC BCand andthe thestrong stronghydrogen hydrogen bonds generated the precursor solution. However, the nano ZnO particles agglomerated after sintering, forming in the precursor solution. However, the nano ZnO particles agglomerated after forming obvious holes holes due to the large surface energy. energy. The The specific specific surface surface areas areas of of the the bone bone char char samples for obvious 2/g and 116.0 m22/g, respectively, containing the sizes sizes of of 0.25–0.50 0.25–0.50mm mmand and0.50–0.80 0.50–0.80mm mmwere were119.2 119.2mm2 /g the and 116.0 m /g, respectively, mainly mesopores (2–50 nm) [6]. The specific surface area of the bone char in in sample sample bb was was smaller smaller mainly than that that in sample a, implying that the slightly lower bone char surface energy decreased decreased the amount amount than of adsorbed adsorbed ZnO ZnO and and weakened weakened the the agglomeration agglomeration of ZnO. Meanwhile, Meanwhile, the specific surface area of of of sample b was was bigger bigger than than that that of of sample sample aa because because of of some some bare bare mesopores mesopores and and micropores micropores on on the the sample bone char charsurface. surface.The Theexposed exposedbone bonechar char surface area sample d was more of sample bone surface area of of sample d was more thanthan thatthat of sample b andb and sample butsmallest the smallest amount of ZnO supported the surface brought a relatively sample a, buta,the amount of ZnO supported on theonsurface brought aboutabout a relatively even even distribution, resulting in the smallest contribution of the nano ZnO particles to the specific surface area of the ZnO/BC composite, so sample d had the smallest specific surface area among the
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Materials 2018, 11, x FOR PEER REVIEW 5 of 8 distribution, resulting in the smallest contribution of the nano ZnO particles to the specific surface area of the ZnO/BC composite, so sample d had the smallest specific surface area among the three three samples. Thechar bonesurface char surface in sample c was smoothly and compactly covered by a uniform samples. The bone in sample c was smoothly and compactly covered by a uniform nano nano ZnO particle layer, causing it to have the smallest specific surface area. In summary, the surface specific ZnO particle layer, causing it to have the smallest specific surface area. In summary, the specific surface of the ZnO/BC samples followed the order of: sample e sample b sample sample areas of areas the ZnO/BC samples followed the order of: sample e > sample b > sample a > asample d d sample c. > sample c.
Table2.2.The Thespecific specificsurface surfaceareas areasand andaverage averagepore poresizes sizesofofthe theZnO/BC ZnO/BCsamples. samples. Table
Sample Sample a a bb cc dd ee
Specific SurfaceArea/m Area/m g1−1 2 g2− Specific Surface 105.9 105.9 106.7 106.7 100.1 100.1 103.4 103.4 108.2 108.2
Average PoreSizes/nm Sizes/nm Average Pore 12.15 12.15 12.10 12.10 13.64 13.64 12.12 12.12 13.61 13.61
Figure3 shows 3 shows UV–vis absorbance of the ZnO/BC samples. The absorbance weak light Figure the the UV–vis absorbance spectraspectra of the ZnO/BC samples. The weak light absorbance of sample b and sample e was due to the fact that the agglomerated ZnO particles the of sample b and sample e was due to the fact that the agglomerated ZnO particles on the on bone bonesurface char surface severely hinder photon energy absorption ZnOloaded loadedininthe the bottom. bottom. char couldcould severely hinder the the photon energy absorption of of ZnO Theuniformly uniformlydistributed distributednano-ZnO nano-ZnOparticles particleson onthe thebone bonechar charsurface surfaceincreased increasedthe theamount amountof ofZnO ZnO The exposedto tolight, light,enhancing enhancingthe thelight lightabsorbance absorbanceof ofsample samplec.c.The TheZnO ZnOunder underthe thehole holeand andshadow shadowof of exposed sample aamade madethe theamount amountof ofZnO ZnOexposed exposedto tolight lightless lessthan thanthat thatof ofsample samplec,c,weakening weakeningthe thelight light sample absorbance.Also, Also,the thelow-proportion low-proportioncoverage coverageof ofZnO ZnOin insample sampleddcaused causedlower lowerlight lightabsorbance. absorbance. absorbance.
Figure Figure3.3. UV–vis UV–visabsorbance absorbancespectra spectraof ofdifferent differentsamples. samples.
Because stable when thethe pH pH levellevel is less 5 or more [16],11 the[16], initial levelpH of BecauseZnO ZnOisisnot not stable when is than less than 5 or than more11than thepH initial the MB solution was set at 10.4. In this situation, the surface hydroxy of bone char occured deprotonation level of the MB solution was set at 10.4. In this situation, the surface hydroxy of bone char occured and then formed negatively charged surfacecharged on the bone char hydroxyapatite) could quickly deprotonation and then formed negatively surface on(mainly the bone char (mainly hydroxyapatite) adsorb positive MB ions through strong electrostatic attraction [14]. For the photocatalysis could quickly adsorb positive MB ions through strong electrostatic attraction [14]. process, For the the photogenerated holes produced by light irradiation migrate to theirradiation ZnO surface and act the photocatalysis process, the photogenerated holes produced by light migrate to with the ZnO adsorbed MBact nearby. Studies have shown that the added acting an electron surface and with the adsorbed MB nearby. Studies have oxygen, shown that the as added oxygen,acceptor, acting as produces a higher reaction rate and photocatalytic capacity than with only the photocatalyst or an electron acceptor, produces a higher reaction rate and photocatalytic capacity than with only the oxygen [17,18].orTherefore, the photocatalytic experiments in this work wereinconducted photocatalyst oxygen [17,18]. Therefore, the photocatalytic experiments this workunder were oxygen-rich conditions. The self-degradation rateself-degradation of MB under simulated for 120 solar min conducted under oxygen-rich conditions. The rate ofsolar MB irradiation under simulated was about 15.5%. irradiation for 120 min was about 15.5%. Figure Figure44shows showsthe thedark darkadsorption adsorptionand anddegradation degradationefficiencies efficienciesof of sample sample aa and and sample sample bb for for MB. MB. The adsorption efficiency of sample a for MB was larger than that of sample b due to the contribution The adsorption efficiency of sample a for MB was larger than that of sample b due to the contribution
of adsorption sites by aggregated nano-ZnO particles in a loose and hollow state, as shown in Figure 2a. The ZnO particles below the hole shadow could not participate in photocatalytic reactions, as they were unexposed to light, resulting in the lower photocatalytic efficiency of sample a.
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of adsorption sites by aggregated nano-ZnO particles in a loose and hollow state, as shown in Figure 2a. The ZnO particles below the hole shadow could not participate in photocatalytic reactions, as they Materials 2018, 11, x FOR PEER resulting REVIEW in the lower photocatalytic efficiency of sample a. 6 of 8 were unexposed to light,
Figure 4. Effect size of of ZnO/bone ZnO/bone char degradation of Figure 4. Effect of of bone bone char char size char on on the the dark dark adsorption adsorption and and degradation of methylene blue.
Figure Figure 55 shows shows the the effect effect of of mass mass ratios ratios of of ZnO ZnO to to bone bone char char on on the the dark dark adsorption adsorption and and for MB. MB. The The contribution contribution to to photocatalysis photocatalysis was related to not only only degradation efficiencies efficiencies of of ZnO/BC ZnO/BC for Sample d the light absorbance of of the the photocatalyst, photocatalyst,but butalso alsothe theamount amountand anddistribution distributionstate stateofofZnO. ZnO. Sample light absorbance had the highest adsorption efficiency and the lowest photocatalytic efficiency for MB, because sample d had the highest adsorption efficiency and the lowest photocatalytic efficiency for MB, because d had the largest of activeofbone char adsorption sites and theand smallest proportion of loaded sample d had the number largest number active bone char adsorption sites the smallest proportion of ZnO of ZnO the three samples shown (b, c, and efficiency of sample was close to loaded of the three samples shown (b,d). c, The and photocatalytic d). The photocatalytic efficiency of bsample b was that of sample c, because the extra ZnO loaded at the hole bottom could not receive light irradiation close to that of sample c, because the extra ZnO loaded at the hole bottom could not receive light (see Figure 2b). uniform nano-ZnO layerparticle in sample c distributed and made fulland use irradiation (see The Figure 2b). The uniformparticle nano-ZnO layer in sample evenly c distributed evenly of light irradiation, contributing well to photocatalysis. made full use of light irradiation, contributing well to photocatalysis.
Figure 5. 5. Effect Effectofof mass ratio of ZnO to char boneofchar of ZnO/bone char photocatalyst on:adsorption (a) dark mass ratio of ZnO to bone ZnO/bone char photocatalyst on: (a) dark adsorption and degradation, and (b) degradation forblue. methylene blue. and degradation, and (b) degradation behavior forbehavior methylene
Figure 66 shows samples Figure shows the the dark dark adsorption adsorption and and degradation degradation efficiencies efficiencies of of MB MB by by the theZnO/BC ZnO/BC samples based on different molar ratios of ZnO to TEA. The adsorption efficiency of sample e for methylene based on different molar ratios of ZnO to TEA. The adsorption efficiency of sample e for methylene blue was was higher higher than than that that of of sample sample cc due due to to the the barer barer bone bone char char surface, surface, i.e., i.e., more more adsorption adsorption sites, sites, blue as shown in Figure 2e. After the deduction of adsorption, the lower photocatalytic efficiency of sample as shown in Figure 2e. After the deduction of adsorption, the lower photocatalytic efficiency of e was related its weak light absorbance, and the growth agglomeration of nano-ZnO particles, sample e was to related to its weak light absorbance, and theand growth and agglomeration of nano-ZnO which inhibited effective separation photogenerated electrons andelectrons holes to some extent. particles, which the inhibited the effective of separation of photogenerated and holes to some
extent.
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Figure6.6.Effect Effectof ofmolar molarratio ratioof ofZnO ZnOto toTEA TEAon onthe thephotocatalytic photocatalyticperformance performanceofofZnO/bone ZnO/bone char. char. Figure
4.4. Conclusions Conclusions Bone Bone char charcontaining containing mainly mainly hydroxyapatite hydroxyapatite (HAP) (HAP) was was produced produced by bypyrolyzing pyrolyzingbovine bovinebone bone ◦ C. ZnO/BC samples were synthesized using a process to adsorb ZnO precursor onto the waste at 400 waste at 400 C. ZnO/BC samples were synthesized using a process to adsorb ZnO precursor onto surface of bone effects bone char size and content of ZnO loading theloading photocatalytic the surface of char. boneThe char. Theofeffects of bone charthe size and the content of on ZnO on the performance were studied. The light absorbance and the distribution of ZnO on the bone char were photocatalytic performance were studied. The light absorbance and the distribution of ZnO on the investigated to explain their effects on the photocatalytic efficiency. The experimental results indicate bone char were investigated to explain their effects on the photocatalytic efficiency. The experimental that the indicate sample with relatively char sizelarge and suitable ZnO loading (sampleZnO c) possesses results that athe samplelarge withbone a relatively bone char size and suitable loading a(sample uniformc)nano-ZnO particle layer and the strongest UV–visible absorbance and can effectively degrade possesses a uniform nano-ZnO particle layer and the strongest UV–visible absorbance and methylene blue.degrade Overall, methylene this project blue. indicates that bone char can be used that as a bone good char nano-photocatalyst can effectively Overall, this project indicates can be used as supporter for the efficient degradation of environmental pollutants. a good nano-photocatalyst supporter for the efficient degradation of environmental pollutants. Author Contributions: Methodology, P.J.; Investigation, P.J.; Formal analysis, H.T.; Data curation, H.T.; Author Contributions: Methodology, P.J.; Investigation, P.J.; Formal H.T.; Data H.T.; Writing—original draft preparation, P.J.; Writing—review and editing, W.G.; analysis, Supervision, W.G. andcuration, K.L.; Funding Writing—original draft preparation, P.J.; Writing—review and editing, W.G.; Supervision, W.G. and K.L.; acquisition, P.J. Funding acquisition, P.J. Funding: This research was funded by the China Scholarship Council. Funding: This research funded by the China Scholarship Council. Acknowledgments: Thewas authors would like to thank the financial support from China Scholarship Council, and the assistance ofThe theauthors Department Chemical Materialssupport Engineering, the University Auckland, Acknowledgments: wouldof like to thankand the financial from China ScholarshipofCouncil, and Auckland 1142, New Zealand. the assistance of the Department of Chemical and Materials Engineering, the University of Auckland, Auckland Conflicts ofZealand. Interest: The authors declare no conflicts of interest. 1142, New
Conflicts of Interest: The authors declare no conflicts of interest.
References
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