Engineered K-doped ZnO/borneol based hydrogel composite material for led-based photocatalytic degradation of methylene blue and evaluation of antimicrobial activity

The significant improvement of decolorization and disinfection technologies has been a hotspot in wastewater reutilization. In this study, we realized a novel construction of K-doped nano-ZnO and borneol based hydrogel composite material (K-ZnO/B-hydrogel) by low-temperature in situ sol–gel growth. The techniques such as fourier transform infrared (FTIR), X-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM) and X-ray energy dispersive spectroscopy (EDS) were applied to recognize the synthesized hydrogel. The results revealed that K-doped ZnO nanoparticles had been uniformly decorated onto the B-hydrogel. Ultraviolet-visible (UV–vis) absorption spectra showed that impurity doping of potassium element into ZnO could reduce the band gap, improving the visible light absorption efficiency. Under LED illumination, the photodegrading rate of K-ZnO/B-hydrogel was approximately 2.3 times greater than that of K-ZnO/B-hydrogel on methylene blue (MB) removal. Remarkably, aside from CO2 and H2O, no by-products were generated during the photodegradation process. In addition, the antimicrobial activities against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) of K-ZnO/B-hydrogel achieved up to 99.9%, which were at least 1.5 times higher than K-ZnO/B-hydrogel. This composite will push ahead with a closed-loop wastewater treatment system for dye and pathogenic microorganism disposal, which combines the excellent adsorption ability of hydrogel and the outstanding photocatalytic ability of ZnO nanoparticles with easy sample handling and separation, and help to eliminate secondary pollution.


Introduction
In recent years, the removal of organic contaminants and pathogenic microorganisms in wastewater has become a common global concern [1].Among the organic pollutants, dye pollution takes an important role [2] since it could lead to various illnesses including burns, jaundice, and tissue necrosis [3].Besides, the accumulation of dyes in water may inhibit sunlight penetration, thereby hindering the occurrence of photosynthesis [4].Considering dyes are high molecular weight and chemical stability, traditional disposal methods like physical chemistry and biochemistry can't achieve the anticipated effect [5].Apart from dye pollutants, pathogenic bacteria contamination is also a critical issue in sewage [6].Misuse of antibiotics in daily life increases the danger that pathogenic bacteria evolve resistance [7].To address above-mentioned problems, it is urgent to develop new multifunction material for effluent treatment.
Nanometal oxides have been widely studied [8][9][10][11][12] because of their unique physicochemical characteristics, for example, catalytic activity, magnetic performance, electronic and antibacterial properties.Facile green synthesis is favorable to obtain the metal nanoparticles with preferable size and morphology [13].Metal oxidebased hydrogel composite materials have increasingly drawn attention to water purification, owing to their strong adsorption and good reutilization character [14].For multitudinous polymer hydrogels, ZnO-based nanocomposite hydrogel appears to be the most sought-after candidate due to excellent antimicrobial activity, superior dye adsorption and photocatalytic ability [15].Mittal et al [16] reported the gum arabic-crosslinkedpoly(acrylamide)/zinc oxide nanocomposites to adsorb malachite green from aqueous solution.They found that the introduction of zinc oxide nanoparticles (ZnO NPs) was helpful in enhancing thermal stability and dye adsorption capacity for hydrogel matrix.In fact, the major factors affecting the photocatalytic performance of ZnO NPs are particle size, dispersity and illumination intensity.Unfortunately, although current nano-ZnO synthesis technologies, such as hydrothermal method [17], pyrolysis method [18], are able to produce small, stable and uniform particles, they are inevitably limited by high temperature, complex equipment, poor dispersion and agglomeration.Moreover, common ZnO NPs alway suffer from fast electron-hole pair recombination and low quantum yield under visible light [19].As an attractive substrate material, hydrogel impacts the overall properties of composites as well.Generally, hydrogel is a crosslinked polymer featuring three-dimensional network structure, which can absorb large amounts of liquids and low-molecular weight compounds through physical or chemical interactions without morphological and solubility change [20].Undoubtedly, it is easily synthesized, low-cost and high adsorption efficiency [21].However, petroleum-based hydrogel is difficult to degrade and require additional post-processing [22,23].On the other hand, the biofilm produced by bacterial adhesion will reduce antibacterial effect and cause secondary pollution for material under long-term practical application.Even ZnO in composite material can effectively kill bacteria, it can't completely prevent bacterial adhesion.Obviously, dead bacteria would likely be adhering to surface to form biofilms [24][25][26].Motivated by these concerns, the emergence of borneol breathes new life into an ideal antimicrobial surface.Borneol, a bio-based material extracted from lavender, chamomile or other plants, has potent antibacterial adhesion property attributing to its unique chiral characteristic [27][28][29][30].Research showed that borneol-based polymers generated by borneol as the backbone could inherit its outstanding anti-adhesion behavior [31].
Herein, borneol-based hydrogel behaved as the skeleton, zinc acetate and potassium chloride were served as raw material, we realized an ingenious design of K-ZnO/B-hydrogel via low-temperature in situ sol-gel method.
Such straightforward yet powerful approach can not only avoid adverse effects of high temperature process on hydrogel substrate, but also is beneficial to nano-ZnO dispersion.As expected, the three-dimensional pore structure of hydrogel inhibits nano-ZnO agglomeration.Meanwhile, impurity doping of potassium element into ZnO can extend the visible light absorption to further modify its photocatalytic activity.By means of FTIR, XRD, XPS, SEM and EDS, they offer vital insight into the structural features of K-ZnO/B-hydrogel.In depth, the related dye adsorption capacity, photocatalytic efficiency, antibacterial properties under visible light condition were evaluated.This composite material exhibited 100% photodegradation ability for MB at 30 h later without harmful by-product production, especially which was easily recovered by a simple cleaning separation and could be recycled four times with no significant loss of photocatalytic activity.Beyond that, K-ZnO/B-hydrogel was proved approximately 100% bactericidal efficacy against E. coli and S. aureus.It will open a novel perspective for water treatment.

Preparation of hydrogel
HEMA, DMAa and AA in weight ratio of 0.7:1.5:0.05 were added to the three-necked flask, followed by adding 10% weight of 0.055 g/ml AIBN methanol solution and 0.5% weight of MBA, then keeping at 70 °C for 40 min in nitrogen atmosphere.The resultant hydrogel was soaked in deionized water for three days, changing the water daily.Finally, the product was dried at 70 °C and cut into an appropriate shape.

Preparation of B-hydrogel
The preparation of B-hydrogel was quite similar to aforementioned hydrogel.Briefly, HEMA, DMAa, BN and AA in weight ratio of 0.7:1:0.25:0.05were added to the three-necked flask.Subsequently, 10% weight of 0.055 g/ ml AIBN methanol solution and 0.5% weight of MBA was injected to the mixed solution, which was maintained at 70 °C for 40 min with nitrogen protection.The yielded B-hydrogel was immersed repeatedly with ultrapure water for three days and dried at 70 °C in air.Prior to use, B-hydrogel was tailored to a certain size.
2.4.Preparation of ZnO/B-hydrogel 14 g ml of HMTA, 50 ml of zinc acetate aqueous solution (0.14 mol L −1 ) were mixed in the round-bottomed flask, then 20 g of B-hydrogel was slowly added to the dispersion with constant stirring for 1 h at room temperature.Immediately, the mixture was heated at 95 °C for 4 h.The resulting sediment was rinsed thoroughly with distilled water several times to remove excess ZnO particles.

Preparation of K-ZnO/B-hydrogel
K-ZnO/B-hydrogel was prepared analogously to ZnO/B-hydrogel synthesis.In brief, 14 g ml of HMTA, 50 ml of zinc acetate aqueous solution (0.14 mol L −1 ), 0.3 wt% potassium chlorid were poured into the roundbottomed flask.Afterwards, 20 g of B-hydrogel was added, which was continually stirred 1 h at ambient temperature.The obtained mixture was incubated at 95 °C for 4 h.After completion of the reaction, K-ZnO/Bhydrogel were treated with a cleaning process to remove impurities.

Characterization
FTIR spectra were conducted at Bruker EOUINOX55 spectrometer (Rheinstetten, Germany).XRD measurements were collected in the range of 5°-65°on MAC Science/MXP X'pert diffractometer with a step size of 0.04°, operated at 40 kV and 30 mA.XPS was carried out on ESCA 5600 photoelectron spectrometer with Mg Kα as the excitation source.Thermo-gravimetric analysis (TGA) was performed on NETZSCH TG 209 thermal analyzer under argon shield from 27 °C to 800 °C with heating rate of 10 °C min −1 .Differential scanning calorimetry (DSC) test was detected by Shimadzu DSC-60A (Japan) under nitrogen condition.The temperature range of samples was 0∼800 °C, and the heating/cooling rate was 5 °C min −1 .Morphologies of all the thin films were taken on a JEOL JSM-7800F SEM at an acceleration voltage 10 kV.The chemical composition analysis was measured by EDS using 15 kV acceleration voltage.The absorbances of the film samples in the wavelength range of 200-800 nm were monitored by UV-vis spectrometer (PerkinElmer Lambda 35) with a scanning resolution of 10 nm.Prior to metal elements analysis, the sample was dissolved in dilute hydrochloric acid for 12 h.Zinc diluted 500000 times was tested in atomic absorption spectroscopy (AAS, PerkinElmer, PinAAcle 900 T) and potassium diluted 250 times was analysed with flame photometer (Sherwood, Model 410).The quantitative samples were added into 20 ml of MB aqueous solution (10 mg/l).A 30 W Philips lamp as visible light source was installed about 10 cm away from the suspended reaction vessel.The photodegradation efficiency was characterized by measuring absorbance change at 664 nm in regular intervals with UV-vis spectrophotometer.After 30 h, the photodegradable solution was detected by Agilent 7000D triple quadrupole gas chromatography-mass spectrometry (GC-MS).The chromatographic column is Agilent DB-5MS (30 m, 0.25 μm).Transfer-line and injector temperatures were kept at 230 °C and 250 °C, respectively.The scan range was from m/z 30 to 320 (250 ms).
E. coli and S. aureus strains were cultured on 50 ml of sterile liquid medium at 37 °C overnight with shaking (200 rpm).Whereafter, the bacteria solution was diluted to 10 6 CFU/ml with sterile phosphate buffered saline (PBS).The antimicrobial activity of materials to E. coli and S. aureus were tested.Firstly, the samples were submerged in 10 6 CFU/ml of bacterial liquid for 4 h under visible LED light irradiation at 37 °C.During this process, let the bacterial liquid fully contact with the sample.After the incubation, 0.1 ml of the solution was evenly spread on the solid medium, further put upside down in an incubator at 37 °C for 24 h.For comparison, 0.1 ml of bacterial solution was uniformly applied on the agar medium and cultivated at 37 °C for 4 h as the blank group.To guarantee the reliability of data, all samples were run in triplicates [31,32].

Results and discussion
In order to precisely identify possible functional group, FTIR spectra of hydrogel, B-hydrogel, ZnO/B-hydrogel and K-ZnO/B-hydrogel are shown in figure 1.All samples exhibit two hydrogel characteristic peaks at 1728 cm −1 and 3435 cm −1 , which are severally corresponded to C=O stretching vibration of the ester group and -OH stretching vibration of adsorbed water molecules [33,34].Compared to hydrogel and B-hydrogel, there is an absorption peak around 1348 cm −1 in ZnO/B-hydrogel and K-ZnO/B-hydrogel, attributing to C-OH group from the interaction of carbonyl groups with Zn atoms and O-H deformation.The appearance of above peak indicates that ZnO particles were grown in situ in the hydrogel successfully.
To study the structural properties of commercial ZnO, hydrogel, B-hydrogel, ZnO/B-hydrogel and K-ZnO/B-hydrogel, XRD patterns were recorded in the range of 5°-65°, as depicted in figure 2. Commercial ZnO (figure 2(a)) displays a high degree of purity and crystallinity.The obvious diffraction peaks at 2θ of 31°, 34°, 36°, 47°, 56°and 62°, and 67.96°were observed, which are exactly indexed to (100), (002), (101), (102), (110) and (103) of hexagonal wurtzite ZnO phase (JCPDS 65-3411) [35], respectively.For hydrogel (figure 2(b)) and B-hydrogel (figure 2(c)), we have only seen a broad peak of the amorphous structure.Though a (100) crystal face of ZnO appears in the spectrum of ZnO/B-hydrogel (figure 2(d)), the intensity is much less than commercial ZnO, ascribing to high dispersion of the ZnO particles in the hydrogel.Kim et al [36] have described similar XRD results in their reasearh.The remaining diffraction peaks in figure 2(d) may be caused by the impurities in the composite.In virtue of potassium introduction and polymer synergistic effect, these can destroy the crystallization effect of ZnO, so it is difficult to detect apparent ZnO peaks.Only a broad amorphous peak is found in the K-ZnO/B-hydrogel spectrum (figure 2(e)).The thermal stabilities of B-hydrogel and K-ZnO/B-hydrogel are examined by TGA analyses (see figure 4) [40].The decomposition of B-hydrogel proceeded in two steps, the first stage was in the temperature interval of 100 °C-289 °C, which was mainly due to the breakage of groups on the side chains of the polymer (for instance, ester groups, amides, carboxylic acids).The second stage was between 289 °C and 489 °C, which was attributed to the degradation of the main chain and cross-linked system.The pyrolys process of K-ZnO/B-hydrogel bears remarkable resemblance to that of B-hydrogel, but the temperature at maximum degradation rate is slightly different.The addition of K-ZnO particles is helpful to improve thermal stability of the hydrogel, so K-ZnO/Bhydrogel takes a higher temperature of 239 °C to decompose.It can be seen that the prepared hydrogel composites retain good thermal stability below 100 °C, which is completely satisfied for dye degradation in water., the hydrogel and B-hydrogel possess a similar layered interconnected porous structure with microporous morphology in layered connected cross-section.When ZnO nanoparticles were embedded in the B-hydrogel, it still maintained a layered inter-connected porous structure and had bulged ZnO particles with homogeneous dispersion near micropores (figure 5(c)-(d)).To our knowledge, carboxyl groups of the polymer chains were distributed evenly throughout the hydrogel.In-situ growth of ZnO occurred in the hydrogel, it would happen the interaction of ZnO nuclei and carboxyl groups, leading to uniform distribution of newly generated ZnO crystals.With the aid of physical barrier for pore structure at hydrogel surface, it avoids the agglomeration of ZnO nanoparticles.For K-ZnO/B-hydrogel  (figure 5(e)-(f)), layered structure had collapsed and only porous structure were left.Abundant K-doped ZnO nanoparticles were visible at the top of the porous structure.
To further analyze the dispersion effect of ZnO in hydrogel composites, EDS test was used in figure 6.There is no doubt that EDS pattern of ZnO/B-hydrogel in figure 6(a) proves the existence of ZnO nanoparticles and their homogeneous distribution over the surface, which are in line with the XPS and SEM data.Apart from uniform formation of ZnO in the surface, the EDS images of K-ZnO/B-hydrogel have returned local enrichment for ZnO .It is observed that zinc content in K-ZnO/B-hydrogel is far higher than ZnO/B-hydrogel (figure 6(b)).With the help of AAS and flame photometer analyses, both zinc and potassium are detectable, which the former is dominated (188 mg L −1 ) and doped with 1.47% potassium.Unquestionably, above results verify that K-doped ZnO nanoparticles has been successfully anchored in B-hydrogel.
For a better overview of electronic interactions in commercial ZnO, ZnO/B-hydrogel and K-ZnO/Bhydrogel near the optical band gap region, UV-vis diffuse reflectance spectra were applied.As presented in figure 7, all samples exhibit preferable ultraviolet absorption.Additionally, commercial ZnO has almost no photocatalysis capability in the visible region.Once ZnO nanoparticles are combined with B-hydrogel, the resultant composites have improved visible light absorption ability to some extent, which thanks to the occurrence of polyacrylic acid impurities, electrons, etc in the polymer matrix.When K-ZnO was introduced to B-hydrogel, the product shows noticeable absorption enhancement in the range of 400-700 nm.This is mostly because potassium ion doping forms defects in the ZnO lattice, including oxygen defects and hydrogen defects, which are favorable for hole-electron pairs separation to improve photocatalytic performance [41].At the same time, K-ZnO particles display excellent dispersing property in B-hydrogel (figure 6(b)), the quantum effect makes the band gap decreases as well, thus upscaling the photogenerated carrier extraction efficiency.
According to UV-vis diffuse reflectance spectra data (figure 8), the optical band gap of commercial ZnO, ZnO/B-hydrogel and K-ZnO/B-hydrogel were calculated by Tauc′s equation as follows: where α is the absorption coefficient, A and n are constants.If material is a semiconductor with direct band gap, n is equal to 0.5.The band gap value was estimated from extrapolation of a linear part of the curve to intersect horizontal axi.The result show that band gap energy of commercial ZnO is 3.22 eV, which in agreement with the data in the literature [42].With regard to ZnO/B-hydrogel and K-ZnO/B-hydrogel, their band gap energy are  .Probably, the hydrogel matrix owns some adsorption properties by swelling, to some extent, it allows the dye molecules access to contact with K-doped ZnO.Benefiting from these advantages, K-ZnO/B-hydrogel performs the optimum degradation ability.Besides high photocatalytic activity, K-ZnO/B-hydrogel also has a strong stability.After four-cycle degradation, the photocatalytic activity of K-ZnO/B-hydrogel does not almost change (figure 10).The study by Chen and Liu et al  also found that the composite of zinc oxide with other materials can achieve dye photodegradation under visible light.If these catalysts are loaded on the hydrogel, especially the catalyst containing copper oxide, additional release antibacterial effect may be achieved, and it is more conducive to recovery [43,44].To investigate structures of degraded dye products, we have conducted a GC-MS experiment.Apparently, blank spectrum in figure 11 reveals a complete degradation process.Except for CO 2 and H 2 O, there are no harmful by-products generation like organic acids.
Microbial contaminant treatment plays a crucial role in preventing the spread of infectious diseases, and the antibacterial activity of materials can reflect their ability to treatment microbial contaminant [45][46][47].The antimicrobial activity of ZnO/B-hydrogel and K-ZnO/B-hydrogel were assessed in visible light condition at water environment.As summarized in figure 12, the antibacterial rate of K-ZnO/B-hydrogel achieves more than 99.9% against S. aureus as well as E. coli, which were higher than those of ZnO/B-hydrogel against E. coli and S. aureus (65.9% and 62.1%).During the photocatalytic sterilization, the photocatalyst was activated by light.The generated electron-hole pairs would react with water molecules or hydroxide ions to produce reactive oxygen species, which can degrade the cellulose and molecules on the cell membrane of bacteria (e.g.phospholipids, proteins, etc), thus leading to their inactivation.Refer to the relevant literature, structural differences between gram-negative and gram-positive bacteria might arouse disparity in inactivation efficiency.By contrast, gram-negative bacteria with thinner cell wall is inactivated faster and easier than gram-positive  bacteria [48].Relying on good dispersion of ZnO particles in the hydrogel, it endows ZnO-based materials larger contact area with the bacterial solution and stronger antibacterial properties.In view of visible absorption superiority for K-ZnO/B-hydrogel, its antibacterial activity was tremendously improved than ZnO/B-hydroge.

Conclusion
In this paper, a fine K-ZnO/B-hydrogel were succeeded in fabrication on basis of low-temperature in situ sol-gel approach.Such K-doped ZnO nanoparticles accommodated in B-hydrogel render composite with a favorable morphology, accessible active sites, good thermal stability and excellent visible light-harvesting ability.Meawhile, the composite material present a double advantage, namely, superior photocatalytic degradation and pathogen inactivation.It turned out that MB degradation rate could reach 100% within 30 h and bactericidal activities against E. coli and S. aureus were up to 99.9%.Hence, K-ZnO/B-hydrogel effectively overcomes the shortcomings of traditional powder photocatalysts and has a wide application prospect in the field of water treatment.

Figure 5
Figure 5 reflects cross-sectional morphologies of the synthesized hydrogel composites at different magnifications.As shown in figure 5(a)-(b), the hydrogel and B-hydrogel possess a similar layered interconnected porous structure with microporous morphology in layered connected cross-section.When ZnO nanoparticles were embedded in the B-hydrogel, it still maintained a layered inter-connected porous structure and had bulged ZnO particles with homogeneous dispersion near micropores (figure5(c)-(d)).To our knowledge, carboxyl groups of the polymer chains were distributed evenly throughout the hydrogel.In-situ growth of ZnO occurred in the hydrogel, it would happen the interaction of ZnO nuclei and carboxyl groups, leading to uniform distribution of newly generated ZnO crystals.With the aid of physical barrier for pore structure at hydrogel surface, it avoids the agglomeration of ZnO nanoparticles.For K-ZnO/B-hydrogel

3.18 eV and 3 .
09 eV, respectively.In keeping with the outcomes of UV-vis diffuse reflectance spectra, K-doped ZnO can narrow bandgap of pure ZnO.To explore photocatalytic degradation efficiency of hydrogel, B-hydrogel, ZnO/B-hydrogel and K-ZnO/Bhydrogel for MB (10 mg /L, 20 ml) under visible light, we analyzed MB absorption change at 664 nm with different times, namely, C t /C 0 versus time in figure 9. C 0 represents the initial concentration of MB (mg/L), C t represents the concentration of MB (mg/L) at time t.Self-degradation activity of MB solution without hydrogel composite under visible light acted as negative control.In the absence of light, the degradation rate of MB was very slow for all samples.When exposed to visible light, dye degradation effect of hydrogel and B-hydrogel was not ideal enough, while it was substantially increased for ZnO/B-hydrogel and K-ZnO/B-hydrogel.Notably, the order of photodegradation efficiency of MB after 30 h visible illumination is shown below: K-ZnO/Bhydrogel (100%) > ZnO/B-hydrogel (44.1%) > B-hydrogel (41.8%)≈ hydrogel (41.2%) > MB self-degradation (38.6%).The potassium doping of ZnO can achieve narrowest band gap energy to realize visible light degradation, what's more, surface ZnO content and local effective ZnO enrichment in K-ZnO/B-hydrogel proved to be much higher than ZnO/B-hydrogel (figure 6(b))

Figure 10 .
Figure 10.Cycle runs of K-ZnO/B-hydrogel for the MB degradation.

Figure 11 .
Figure 11.GC-MS data of the MB degradation solution using K-ZnO/B-hydrogel at 30 h later.