# Zinc/Cobalt-Based Zeolite Imidazolate Frameworks for Simultaneously Degrading Dye and Inhibiting Bacteria

Authors: Nguyen Thi Thanh Tu; Lu Hoang Khang; Nguyen Ngoc Phuong Thao; Nguyen Thi Thu Hien; Thuy Chau To; Le Thi Hong Diep; Le Van Thanh Son; Phan Lien; Vo Thang Nguyen; Dinh Quang Khieu
Journal: Journal of Nanomaterials
 (2022)

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## Abstract

In this work, the zinc/cobalt-based zeolite imidazolate frameworks ((Co/Zn)ZIFs) were synthesized with the solvothermal method. The obtained material was characterized by utilizing scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-Vis diffusive reflectance spectroscopy, and nitrogen adsorption-desorption isotherms. XRD and SEM analyses show that (Co/Zn)ZIFs are composed of nanocrystals with polyhedral shapes of around 50–100 nm and belong to the I-43m space group as those of ZIF-8 and ZIF-67. Optical studies demonstrate a red shift in the absorbance spectrum of (Co/Zn)ZIFs compared with individual components of ZIF-67 or ZIF-8. (Co/Zn)ZIF composite was utilized as photocatalytic material to treat a model aqueous solution containing rhodamine B and bacteria. It was found that (Co/Zn)ZIFs could simultaneously degrade rhodamine B and inhibit bacteria (E. coli and S. aureus). The manufactured composite could catalyze the mineralization of rhodamine B and also exhibited good antibacterial activity against Gram-negative E. coli (93.32% inhibition rate) and Gram-positive S. aureus (90.86% inhibition rate) in the visible-light region within four hours of irradiation. Gram-negative bacteria were more resistant to (Co/Zn)ZIFs than Gram-positive bacteria. (Co/Zn)ZIFs can be used as light-driven catalysts for water and environmental detoxification from organic compounds like dyes and bacteria.


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## Body

## 1. Introduction

The rapid development of industrialization in recent years has contributed to significant economic growth. At the same time, environmental pollution is also at an alarming level, affecting the living environment of humans and animals. In particular, wastewater from industrial production facilities, especially wastewater from textile dyeing factories, poses a severe threat to the environment. The presence of dyes in wastewater is a major concern because of their toxicity and unaesthetic that can cause adverse effects on biological life, including cancer and genetic mutations [1–3]. In addition, the presence of bacteria in water also affects the environment, especially strains of enterobacteria and Gram-negative, anaerobic bacteria, including Escherichia coli (E. coli), being able to cause diseases on the digestive tract in humans and animals. Therefore, developing friendly and effective techniques to degrade dyes and, at the same time, disinfect wastewater has become a vital and urgent issue in environmental protection. Numerous techniques for separate treatment of dyes or bacteria have been reported. Physicochemical methods have been developed to treat dye wastewater, such as biochemical oxidation [4], adsorption [5–7], ion exchange [8, 9], photocatalysis [10, 11], and advanced oxidation [12]. Recently, several oxide or metal nanoparticles exhibiting antibacterial activity or both antibacterial and organic compound degradation activities have been reported. Nair et al. addressed the antibacterial activity of ZnO- and Co-doped ZnO nanoparticles on Gram-positive and Gram-negative bacteria [13]. Valerini et al. presented that Ag-coated polycaprolactone exhibited an excellent antibacterial action against E. coli [14]. In another study, Rao et al. found that Ag nanoparticles/g-C3N4 nanosheets showed efficient photocatalytic and antibacterial performance [15].ZIFs (zeolitic imidazole frameworks) are a class of metal-organic frameworks (MOFs) formed from tetrahedral divalent cations (M2+ = Zn2+ or Co2+) with imidazolate ligands [16, 17]. Most ZIFs have a large bandgap energy (Eg); for example, Eg of ZIF-67 and ZIF-8 is 4.3 and 5.3 eV [18]. Therefore, reducing Eg to improve the photocatalytic performance of ZIFs in the visible-light region is essential and has practical significance. Chen et al. [19] reported that ZnO/ZIF-8 was an effective photocatalyst for completely degrading methylene blue. Yang et al. [20] added Cu to ZIF-67 to promote the catalytic activity of Cu/ZIF-67 for methyl orange decomposition in the visible-light region. Zhou et al. [17] synthesized ZIFs based on Zn/Co for the first time with improved chemical properties compared with single ZIF-8 or ZIF-67. Taheri et al. reported the antibacterial activity of ZIF-8 for E. coli [21]. According to our best knowledge, research on the photocatalytic activity of this kind of material for simultaneously degrading dyes and inhibiting bacteria is less available. Therefore, developments of ZIFs with antimicrobial properties are of considerable interest. In the present study, we present the synthesis of (Co/Zn)ZIFs with the hydrothermal method and the photocatalytic activity toward rhodamine B. The antimicrobial activity of (Co/Zn)ZIFs against E. coli ATCC 25922 (Gram-negative) and S. aureus ATCC 25923 (Gram-positive) in aqueous solutions was also addressed.


## 2. Experimental

### 2.1. Materials

Cobalt nitrate (Co(NO3)2·6H2O, Deajung, Korea), 2-methylimidazole (C4H6N2, Sigma, USA) (Hmim), methanol (CH3OH, Guangzhou, China), ethanol (C2H5OH, Guangzhou, China), zinc nitrate (Zn(NO3)2·6H2O, Deajung, Korea), and rhodamine B (C28H31ClN2O3, denoted RhB) were used in the study.


### 2.2. Preparation of ZIF-8, ZIF-67, and (Co/Zn)ZIFs

ZIF-8, ZIF-67, and (Co/Zn)ZIFs were synthesized according to references [22, 23]. For ZIF-67, 1.164 g of Co(NO3)2·6H2O and 1.312 g of 2-methylimidazole were dissolved in 100 mL of CH3OH. The mixture was placed in a Teflon autoclave and kept in an oven at 150°C for 5 h. The product was then centrifuged for 15 min (1500 rpm). The obtained solid was rinsed three times with ethanol. Then, the resulting purple solid was dried at 120°C for about 24 h to obtain the ZIF-67. ZIF-8 was synthesized with the same procedure with Zn(NO3)2·6H2O (1.188 g). A series of (Co/Zn)ZIFs with different molar Co/Zn ratios (1 : 9), (2 : 8), (3 : 7), (5 : 5), (6 : 4), and (7 : 3) was synthesized in a similar manner by using a mixture of Co(NO3)2·6H2O and Zn(NO3)2·6H2O. In a preliminary test, (Co/Zn)ZIFs with Co/Zn molar ratio of 6/4 exhibit high photocatalytic and antibacterial activity; therefore, this material was selected for further experiments (Table S1).


### 2.3. Apparatus

The powder X-ray diffraction (XRD) was performed on D8 Advance, Bruker (Germany). The morphology of ZIF-8, ZIF-67, and (Co/Zn)ZIFs was observed from scanning electron microscopy (SEM JMS-5300LV, Japan) images. Textural properties were investigated by means of nitrogen adsorption-desorption isotherms (Micrometrics TriStar 3000 at 77 K). The elemental composition of samples was detected by EDX mapping (JSM-IT200 InTouchScope). UV-Vis spectroscopy was measured by using a Lambda 25 Spectrophotometer-Perkin Elmer atλmax of RhB (553 nm). Total organic carbon (TOC) was measured on TOC-VCPH/CP (Shimadzu).


### 2.4. Bacterial Strains and Growing Conditions

Gram (–)Escherichia coli ATCC 25922 (E. coli) and Gram (+) Staphylococcus aureus ATCC 25923 (S. aureus) were used to study the antibacterial activity of the synthesized products. The E. coli strain culture was grown in LB (Luria–Bertani) broth, and S. aureus was grown in TS (tryptic soy) broth.


### 2.5. Visible-Light-Derived Photocatalytic Activity for RhB Degradation and Antibacterial Activity againstE. coli and S. aureus with (Co/Zn)ZIFs

0.1 g of each ZIF-67, ZIF-8, and (Co/Zn)ZIF material was placed into a 1000 mL beaker containing 500 mL of RhB solution (30 mg·L–1) and E. coli and S. aureus strains (106–107 CFU·mL–1). The beakers were sealed and placed in the dark to prevent light exposure, and the mixture was stirred with a magnetic stirrer for 120 min to reach adsorption/desorption equilibrium. Then, an Osram 160 W filament lamp (filter cutoff λ<420 nm) illuminated the resulting suspension. 5 mL of suspension was withdrawn at defined intervals and then centrifuged to remove the solid. The mineralization degree of RhB in the supernatant was estimated with the total organic carbon measurement. Each experiment was repeated three times. Simultaneously, 100 μL of the solution was taken, diluted 10,000 times, and spread on LB or TSB agar plates (incubated at 37°C; the number of colonies was recorded after 24 h to assess the antibacterial activity of the material via the plate spread method).The antibacterial rate was utilized to assess the antibacterial activity according to the following equation:(1)H%=CCS−CSSCCS×100%,where CSS and CCS are the bacterial population of the survey and control samples (CFU·mL–1).The kinetics study was performed in a similar manner on RhB solution at initial concentrations of 10, 30, and 50 ppm. The concentration of remaining RhB in the supernatant was determined with a UV-Vis adsorption spectroscope atλmax equal to 553 nm.


## 2.1. Materials

Cobalt nitrate (Co(NO3)2·6H2O, Deajung, Korea), 2-methylimidazole (C4H6N2, Sigma, USA) (Hmim), methanol (CH3OH, Guangzhou, China), ethanol (C2H5OH, Guangzhou, China), zinc nitrate (Zn(NO3)2·6H2O, Deajung, Korea), and rhodamine B (C28H31ClN2O3, denoted RhB) were used in the study.


## 2.2. Preparation of ZIF-8, ZIF-67, and (Co/Zn)ZIFs

ZIF-8, ZIF-67, and (Co/Zn)ZIFs were synthesized according to references [22, 23]. For ZIF-67, 1.164 g of Co(NO3)2·6H2O and 1.312 g of 2-methylimidazole were dissolved in 100 mL of CH3OH. The mixture was placed in a Teflon autoclave and kept in an oven at 150°C for 5 h. The product was then centrifuged for 15 min (1500 rpm). The obtained solid was rinsed three times with ethanol. Then, the resulting purple solid was dried at 120°C for about 24 h to obtain the ZIF-67. ZIF-8 was synthesized with the same procedure with Zn(NO3)2·6H2O (1.188 g). A series of (Co/Zn)ZIFs with different molar Co/Zn ratios (1 : 9), (2 : 8), (3 : 7), (5 : 5), (6 : 4), and (7 : 3) was synthesized in a similar manner by using a mixture of Co(NO3)2·6H2O and Zn(NO3)2·6H2O. In a preliminary test, (Co/Zn)ZIFs with Co/Zn molar ratio of 6/4 exhibit high photocatalytic and antibacterial activity; therefore, this material was selected for further experiments (Table S1).


## 2.3. Apparatus

The powder X-ray diffraction (XRD) was performed on D8 Advance, Bruker (Germany). The morphology of ZIF-8, ZIF-67, and (Co/Zn)ZIFs was observed from scanning electron microscopy (SEM JMS-5300LV, Japan) images. Textural properties were investigated by means of nitrogen adsorption-desorption isotherms (Micrometrics TriStar 3000 at 77 K). The elemental composition of samples was detected by EDX mapping (JSM-IT200 InTouchScope). UV-Vis spectroscopy was measured by using a Lambda 25 Spectrophotometer-Perkin Elmer atλmax of RhB (553 nm). Total organic carbon (TOC) was measured on TOC-VCPH/CP (Shimadzu).


## 2.4. Bacterial Strains and Growing Conditions

Gram (–)Escherichia coli ATCC 25922 (E. coli) and Gram (+) Staphylococcus aureus ATCC 25923 (S. aureus) were used to study the antibacterial activity of the synthesized products. The E. coli strain culture was grown in LB (Luria–Bertani) broth, and S. aureus was grown in TS (tryptic soy) broth.


## 2.5. Visible-Light-Derived Photocatalytic Activity for RhB Degradation and Antibacterial Activity againstE. coli and S. aureus with (Co/Zn)ZIFs

0.1 g of each ZIF-67, ZIF-8, and (Co/Zn)ZIF material was placed into a 1000 mL beaker containing 500 mL of RhB solution (30 mg·L–1) and E. coli and S. aureus strains (106–107 CFU·mL–1). The beakers were sealed and placed in the dark to prevent light exposure, and the mixture was stirred with a magnetic stirrer for 120 min to reach adsorption/desorption equilibrium. Then, an Osram 160 W filament lamp (filter cutoff λ<420 nm) illuminated the resulting suspension. 5 mL of suspension was withdrawn at defined intervals and then centrifuged to remove the solid. The mineralization degree of RhB in the supernatant was estimated with the total organic carbon measurement. Each experiment was repeated three times. Simultaneously, 100 μL of the solution was taken, diluted 10,000 times, and spread on LB or TSB agar plates (incubated at 37°C; the number of colonies was recorded after 24 h to assess the antibacterial activity of the material via the plate spread method).The antibacterial rate was utilized to assess the antibacterial activity according to the following equation:(1)H%=CCS−CSSCCS×100%,where CSS and CCS are the bacterial population of the survey and control samples (CFU·mL–1).The kinetics study was performed in a similar manner on RhB solution at initial concentrations of 10, 30, and 50 ppm. The concentration of remaining RhB in the supernatant was determined with a UV-Vis adsorption spectroscope atλmax equal to 553 nm.


## 3. Results and Discussion

### 3.1. Characterization of Materials

Figure1 shows the XRD patterns of the ZIF-67, ZIF-8, and (Co/Zn)ZIF samples. The spatial distances d of the faces (011), (002), (112), (022), (113), (222), and (114) of ZIF-8 or ZIF-67 are in good agreement with those reported previously [17, 24–26]. Since both ZIF-67 and ZIF-8 crystallize in the crystal system I-43m [20], the diffraction peaks of (Co/Zn)ZIFs are observed at the same 2θ as ZIF-67 and ZIF-8 with an intermediate intensity between ZIF-67 and ZIF-8. The surface area and pore properties of the materials were investigated by measuring nitrogen adsorption and desorption at 77 K (Figure 1(b)). The adsorption and desorption isotherms follow the type I according to the IUPAC classification. All samples have a microporous structure. However, the isotherm of ZIF-8 rises remarkably at high relative pressures, indicating that a mesoporous capillary system was formed between the particles.Figure 1
(a) XRD diagram and (b) nitrogen adsorption and desorption isotherms of ZIF-67, ZIF-8, and (Co/Zn)ZIFs.
(a)(b)The BET-specific surface area of ZIF-67, ZIF-67, and (Co/Zn)ZIFs calculated from the N2 adsorption and desorption isotherm is 1530, 1176, and 1325 m2·g-1, respectively. The pore size distribution calculated from the Barrett-Joyner-Halenda model shows that ZIF-67 and (Co/Zn)ZIFs have almost the same pore width (around 20 Å), indicating their structural similarities. In addition, the adsorption on (Co/Zn)ZIFS is fast with saturation at a relative pressure of less than 0.1, confirming its microporous structure, analogous to ZIF.The morphology of ZIF-67, ZIF-8, and (Co/Zn)ZIFs is presented in Figure2. While ZIF-67 consists of polyhedron particles with a diameter ranging from 200 to 500 nm, (Co/Zn)ZIFs retain the polyhedral morphology of ZIF-67, yet with a smaller size (50–200 nm).Figure 2
SEM images of ZIF-67 (a), ZIF-8 (b), and (Co/Zn)ZIFs (c).
(a)(b)(c)To assess the optical absorption capacity of the as-prepared ZIFs, we calculated the bandgap (Eg) of the material by using UV–Vis diffuse reflection spectroscopy (Figure 3(a)). The bandgap of a material can be obtained from the Kubelka–Munk function formula.
(2)α×h×ν=A×h×ν–Egn/2,where α, h, ν, A, and Eg are the absorption coefficient, Plank’s constant, light frequency, proportionality constant, and bandgap energy, respectively; n=1 and 4, corresponding to direct and indirect bandgap semiconductors. Figure 3(b) presents the fitting diagram of α×h×ν2 vs. h×ν for the indirect bandgap [27]. Thus, extrapolating the linear region to the abscissa yields the bandgap energy of the material. Two bandgaps are found at 2.05 and 3.98 eV for ZIF-67, while only one bandgap is around 5.18 eV for ZIF-8. (Co/Zn)ZIFs have two bandgaps at around 2.45 and 3.05 eV. The high bandgap energy at 5.18 and 3.98 eV for ZIF-8 and ZIF-67 can mainly be attributed to the charge transfer between the ligand and the metals [28]. The low bandgap of ZIF-67 (2.05 eV), corresponding to the characteristic absorption band at 500–650 nm, is attributed to the typical d-d transition in tetrahedral cobalt [29]. Furthermore, the bandgap of (Co/Zn)ZIFs is lower than that of ZIF-8 or ZIF-67, indicating that the combination of Co and Zn reduces the energy required for optical transition and can promote its photocatalytic activity.Figure 3
UV-Vis DRS spectra (a) and Tauc’s plots of (Co/Zn)ZIFs, ZIF-67, and ZIF-8 (b).
(a)(b)The distribution of the elements in the composite was studied with EDX mapping (Figure4). As expected, the EDX spectrum shows the presence of five elements: Zn (11.78%), Co (17.49%), C (44.24%), O (0.07%), and N (26.42%). The elements randomly disperse throughout the sample surface, indicating the composite homogeneous phase.Figure 4
EDX spectrum of (Co/Zn)ZIFs (a), SEM image (b), carbon mapping (c), nitrogen mapping (d), cobalt mapping (e), and zinc mapping (f).
(a)(b)(c)(d)(e)(f)


### 3.2. Rhodamine B Degradation and Antibacterial Activity of ZIF-67, ZIF-8, and (Co/Zn)ZIFs

The elimination of RhB with ZIF-67, ZIF-8, and (Co/Zn)ZIFs might occur in two steps: adsorption and photocatalytic degradation. The ability to eliminate RhB was evaluatedvia the decrease of TOC, which is expressed as elimination efficiency (F)
(3)F=100∗C0−CtC0,where C0 is the initial TOC and Ct is TOC at time t.The adsorption was carried out in the dark for 120 min (to ensure that the adsorption/desorption equilibrium was reached), and then, the light turned up. The TOC data show that ZIF-67, (Co/Zn)ZIFs, and ZIF-8 adsorb a significant amount of RhB in the aqueous solution. The adsorption efficiency of ZIF-67, (Co/Zn)ZIFs, and ZIF-8 is 79.8, 68.7, and 45.6%, respectively (Figure5(a)). However, ZIF-67 and ZIF-8 do not exhibit any photocatalytic activity under the visible light region. The decreasing TOCs of the solutions treated with ZIF-8 and ZIF-67 are due only to adsorption, whereas (Co/Zn)ZIFs catalyze the degradation of RhB up to 95.5% under illumination. Figure 5(a) shows that TOC decreases from 20.66 to 0.93 mg·L–1 for the RhB solution treated with (Co/Zn)ZIFs, in which the decrease of 14.19 mg·L–1 is due to adsorption and 5.54 mg·L–1 due to photocatalytic degradation.Figure 5
(a) Visible light-driven photocatalytic degradation of RhB on different ZIF materials (experimental condition:V=1000 mL; the initial concentration is from 30 mg·L–1; the adsorbent mass is 0.1 g) and (b) kinetics of decolorization on (Co/Zn)ZIFs (experimental condition: V=1000 mL; the initial concentration is from 10, 30, and 50 mg·L–1; the adsorbent mass is 0.1 g). (c) Effect of photogenerated carrier scavenger on photodegradation of RhB. (d) Leaching (experimental conditions: V=1000 mL, mass of catalyst=0.1 g, dark adsorption in 150 min, and photodegradation in 340 min).
(a)(b)(c)(d)The leaching experiment was also performed where the catalyst was filtered after 150 min of reaction; the decolorization of RhB almost stopped despite further illumination (Figure5(c)). This halt indicates that (Co/Zn)ZIF is a heterogeneous catalyst in the photocatalytic degradation of RhB. Because heterogeneous catalysts usually leach the metal ions to the solution, the presence of Zn and Co in the supernatant was detected by using AAS. The results show that a small amount of zinc is present, but cobalt is detected a very small amount in the solution (Table S2). It is possible that the Co-imidazole bond (Co–N) is stronger than the Zn-imidazole bond (Zn–N) because cobalt possesses higher electronegativity (1.88) than zinc (1.65).The kinetics of the decomposition of RhB on (Co/Zn)ZIFs was studied by using the Hinshelwood–Langmuir model for heterogeneous catalysis at different concentrations in the reduced form as follows:(4)lnCtC0=−kr×t,where kr is the decomposition rate constant (min–1) and C0 and Ct are the concentration of RhB at equilibrium in the dark and at time t of photocatalysis. The results (Figure 5(b)) show that the decomposition rate coefficient (kr) decreases gradually from 0.028 to 0.004 min–1 when the RhB concentration increases from 10 to 50 mg·L–1. RhB at a higher concentration might prevent light from interacting with the catalyst, reducing the photocatalytic efficiency and thus the degradation rate. It is difficult to compare the decomposition rate coefficient because the data are derived from different reaction conditions. However, the kr value of RhB decomposition in this study is comparable to that of other previously reported studies if the initial concentration is concerned (Table 1).Table 1
A comparison of rate coefficient of (Co/Zn)ZIFs with other previously reported materials.
CatalystCRhB (mg·L–1)kr (min–1)ReferenceBiVO450.00102[30]CuO–ZrO2100.0006[31]ZnO50.0028[32]CuO50.0203[32]ZnO/CuO50.0326[32](ZnO/CuO)/rGO50.0627[32]ZnO50.0203[33]ZnO/g–C3N450.0284[33](Co/Zn)ZIFs100.028This work(Co/Zn)ZIFs300.016This work(Co/Zn)ZIFs500.004This workThe antibacterial activity of ZIF materials in an aqueous solution was evaluated by comparing the number of viable bacterial cells after contact with the material with the control sample (without ZIF). Figure6 shows that ZIF-67 and ZIF-8 have poor antibacterial performance on both S. aureus and E. coli, with cell survival rates ranging from 87 to 96% with or without light illumination. However, (Co/Zn)ZIFs have much higher antibacterial activity against both S. aureus and E. coli without light. Specifically, in the presence of light, (Co/Zn)ZIFs exhibit excellent antibacterial activity on S. aureus and E. coli strains with a survival rate of 9.14 and 6.68%. The photographs of Petri dishes obtained from E. coli and S. aureus culture without or with (Co/Zn)ZIFs in the dark and under illumination are shown in Figures 7 and 8. As expected, the E. coli and S. aureus cultures in the absence of (Co/Zn)ZIFs, i.e., the control samples, continue to grow within four hours (Figures 7(a), 7(e), 8(a), and 8(e)) regardless of lighting or not. The results also show that Gram-negative bacteria E. coli are more sensitive to the (Zn/Co)ZIFs than Gram-positive bacteria S. aureus because of the difference in their cell membrane structure [34].Figure 6
Variation of logarithm of bacterial population ofE. coli (a) and S. aureus (b) with different treatment materials in the dark and under visible-light illumination at different times (each experiment in triplicate).
(a)(b)Figure 7
Cell density of bacterialE. coli (a) in the dark (A: control sample, B: 60 min, C: 120 min, and D: 240 min). Cell density of bacterial E. coli (b) under illumination (E: control sample, F: 60 min, G: 120 min, and H: 240 min).Figure 8
Cell density of bacterialS. aureus (a) in the dark (A: control sample, B: 60 min, C: 120 min, and D: 240 min). Cell density of bacterial S. aureus (b) under illumination (E: control sample, F: 60 min, G: 120 min, and H: 240 min).Table2 shows that (Co/Zn)ZIFs have antibacterial activities against Gram-positive and Gram-negative bacteria equivalent to some previously reported materials.Table 2
A comparison of antibacterial activity of (Co/Zn)ZIF with other materials reported previously.
MaterialsBacterial strainsAntibacterial activityReferencesRFP&o-NBA@ZIF-8E. coliInhibition rate of 95% againstE. coli under light condition during 12 hours[35]MOF-53(Fe)/VanS. aureusInhibition rate of 99.3% againstS. aureus at concentration 200 μg·mL–1 of MOF-53(Fe)/Van[36]HKUST-1/OCBsE. coliGrowth inhibition of bacteria: 90.2%[37]CP/CNF/ZIF-67E. coliAntibacterial rate reached 80%[38]WO3-XE. coli and S. aureusInhibition rate of 100% againstE. coli and S. aureus at 100 μg·mL–1 after six hours of contact[39](Co/Zn)ZIFsE. coli and S. aureusInhibition rate 90.86% againstS. aureus and 93.8% against E. coli after four hours of contact under visible light conditionThe present studyThe (Co/Zn)ZIF material is a potential photocatalyst, capable of decolorizing RhB and also inhibiting microorganisms. ZIF-8 and ZIF-67 have largeEg and therefore are not excited under visible light. Combining Zn with Co creates a catalyst with photochemical degradation ability under visible light. This ability of (Co/Zn)ZIFs may result from the addition of Zn to the intermediate band formation between the conduction (CB) and valence (VB) regions of ZIF-67 and ZIF-8. Visible light can excite electrons from VB, and an energy state adds to the Co3d level to CB, causing electron formation (e–) and photogenerated hole (h+). The e–/h+ pair initiates the reduction or oxidation of O2 and H2O in the solution to generate free radicals. These free radicals oxidize organic pollutants to form mineral products. The photocatalytic reaction can occur as follows:
(5)CoZnZIFs+hν⟶CoZnZIFsh+e–e–+O2⟶O2∗–CoZnZIFsh++H2O⟶˙OH+H++CoZnZIFsh+˙OHO2∗–+RhB⟶DecompositionproductsThe free radicals˙OH/O2∗– can oxidize and damage cell membranes, organelles, and bacterial DNA, resulting in the destruction of bacteria. This pathway is also considered one of many materials’ principal bactericidal mechanisms, as reported in previous studies [8–12, 34, 40–43].


## 3.1. Characterization of Materials

Figure1 shows the XRD patterns of the ZIF-67, ZIF-8, and (Co/Zn)ZIF samples. The spatial distances d of the faces (011), (002), (112), (022), (113), (222), and (114) of ZIF-8 or ZIF-67 are in good agreement with those reported previously [17, 24–26]. Since both ZIF-67 and ZIF-8 crystallize in the crystal system I-43m [20], the diffraction peaks of (Co/Zn)ZIFs are observed at the same 2θ as ZIF-67 and ZIF-8 with an intermediate intensity between ZIF-67 and ZIF-8. The surface area and pore properties of the materials were investigated by measuring nitrogen adsorption and desorption at 77 K (Figure 1(b)). The adsorption and desorption isotherms follow the type I according to the IUPAC classification. All samples have a microporous structure. However, the isotherm of ZIF-8 rises remarkably at high relative pressures, indicating that a mesoporous capillary system was formed between the particles.Figure 1
(a) XRD diagram and (b) nitrogen adsorption and desorption isotherms of ZIF-67, ZIF-8, and (Co/Zn)ZIFs.
(a)(b)The BET-specific surface area of ZIF-67, ZIF-67, and (Co/Zn)ZIFs calculated from the N2 adsorption and desorption isotherm is 1530, 1176, and 1325 m2·g-1, respectively. The pore size distribution calculated from the Barrett-Joyner-Halenda model shows that ZIF-67 and (Co/Zn)ZIFs have almost the same pore width (around 20 Å), indicating their structural similarities. In addition, the adsorption on (Co/Zn)ZIFS is fast with saturation at a relative pressure of less than 0.1, confirming its microporous structure, analogous to ZIF.The morphology of ZIF-67, ZIF-8, and (Co/Zn)ZIFs is presented in Figure2. While ZIF-67 consists of polyhedron particles with a diameter ranging from 200 to 500 nm, (Co/Zn)ZIFs retain the polyhedral morphology of ZIF-67, yet with a smaller size (50–200 nm).Figure 2
SEM images of ZIF-67 (a), ZIF-8 (b), and (Co/Zn)ZIFs (c).
(a)(b)(c)To assess the optical absorption capacity of the as-prepared ZIFs, we calculated the bandgap (Eg) of the material by using UV–Vis diffuse reflection spectroscopy (Figure 3(a)). The bandgap of a material can be obtained from the Kubelka–Munk function formula.
(2)α×h×ν=A×h×ν–Egn/2,where α, h, ν, A, and Eg are the absorption coefficient, Plank’s constant, light frequency, proportionality constant, and bandgap energy, respectively; n=1 and 4, corresponding to direct and indirect bandgap semiconductors. Figure 3(b) presents the fitting diagram of α×h×ν2 vs. h×ν for the indirect bandgap [27]. Thus, extrapolating the linear region to the abscissa yields the bandgap energy of the material. Two bandgaps are found at 2.05 and 3.98 eV for ZIF-67, while only one bandgap is around 5.18 eV for ZIF-8. (Co/Zn)ZIFs have two bandgaps at around 2.45 and 3.05 eV. The high bandgap energy at 5.18 and 3.98 eV for ZIF-8 and ZIF-67 can mainly be attributed to the charge transfer between the ligand and the metals [28]. The low bandgap of ZIF-67 (2.05 eV), corresponding to the characteristic absorption band at 500–650 nm, is attributed to the typical d-d transition in tetrahedral cobalt [29]. Furthermore, the bandgap of (Co/Zn)ZIFs is lower than that of ZIF-8 or ZIF-67, indicating that the combination of Co and Zn reduces the energy required for optical transition and can promote its photocatalytic activity.Figure 3
UV-Vis DRS spectra (a) and Tauc’s plots of (Co/Zn)ZIFs, ZIF-67, and ZIF-8 (b).
(a)(b)The distribution of the elements in the composite was studied with EDX mapping (Figure4). As expected, the EDX spectrum shows the presence of five elements: Zn (11.78%), Co (17.49%), C (44.24%), O (0.07%), and N (26.42%). The elements randomly disperse throughout the sample surface, indicating the composite homogeneous phase.Figure 4
EDX spectrum of (Co/Zn)ZIFs (a), SEM image (b), carbon mapping (c), nitrogen mapping (d), cobalt mapping (e), and zinc mapping (f).
(a)(b)(c)(d)(e)(f)


## 3.2. Rhodamine B Degradation and Antibacterial Activity of ZIF-67, ZIF-8, and (Co/Zn)ZIFs

The elimination of RhB with ZIF-67, ZIF-8, and (Co/Zn)ZIFs might occur in two steps: adsorption and photocatalytic degradation. The ability to eliminate RhB was evaluatedvia the decrease of TOC, which is expressed as elimination efficiency (F)
(3)F=100∗C0−CtC0,where C0 is the initial TOC and Ct is TOC at time t.The adsorption was carried out in the dark for 120 min (to ensure that the adsorption/desorption equilibrium was reached), and then, the light turned up. The TOC data show that ZIF-67, (Co/Zn)ZIFs, and ZIF-8 adsorb a significant amount of RhB in the aqueous solution. The adsorption efficiency of ZIF-67, (Co/Zn)ZIFs, and ZIF-8 is 79.8, 68.7, and 45.6%, respectively (Figure5(a)). However, ZIF-67 and ZIF-8 do not exhibit any photocatalytic activity under the visible light region. The decreasing TOCs of the solutions treated with ZIF-8 and ZIF-67 are due only to adsorption, whereas (Co/Zn)ZIFs catalyze the degradation of RhB up to 95.5% under illumination. Figure 5(a) shows that TOC decreases from 20.66 to 0.93 mg·L–1 for the RhB solution treated with (Co/Zn)ZIFs, in which the decrease of 14.19 mg·L–1 is due to adsorption and 5.54 mg·L–1 due to photocatalytic degradation.Figure 5
(a) Visible light-driven photocatalytic degradation of RhB on different ZIF materials (experimental condition:V=1000 mL; the initial concentration is from 30 mg·L–1; the adsorbent mass is 0.1 g) and (b) kinetics of decolorization on (Co/Zn)ZIFs (experimental condition: V=1000 mL; the initial concentration is from 10, 30, and 50 mg·L–1; the adsorbent mass is 0.1 g). (c) Effect of photogenerated carrier scavenger on photodegradation of RhB. (d) Leaching (experimental conditions: V=1000 mL, mass of catalyst=0.1 g, dark adsorption in 150 min, and photodegradation in 340 min).
(a)(b)(c)(d)The leaching experiment was also performed where the catalyst was filtered after 150 min of reaction; the decolorization of RhB almost stopped despite further illumination (Figure5(c)). This halt indicates that (Co/Zn)ZIF is a heterogeneous catalyst in the photocatalytic degradation of RhB. Because heterogeneous catalysts usually leach the metal ions to the solution, the presence of Zn and Co in the supernatant was detected by using AAS. The results show that a small amount of zinc is present, but cobalt is detected a very small amount in the solution (Table S2). It is possible that the Co-imidazole bond (Co–N) is stronger than the Zn-imidazole bond (Zn–N) because cobalt possesses higher electronegativity (1.88) than zinc (1.65).The kinetics of the decomposition of RhB on (Co/Zn)ZIFs was studied by using the Hinshelwood–Langmuir model for heterogeneous catalysis at different concentrations in the reduced form as follows:(4)lnCtC0=−kr×t,where kr is the decomposition rate constant (min–1) and C0 and Ct are the concentration of RhB at equilibrium in the dark and at time t of photocatalysis. The results (Figure 5(b)) show that the decomposition rate coefficient (kr) decreases gradually from 0.028 to 0.004 min–1 when the RhB concentration increases from 10 to 50 mg·L–1. RhB at a higher concentration might prevent light from interacting with the catalyst, reducing the photocatalytic efficiency and thus the degradation rate. It is difficult to compare the decomposition rate coefficient because the data are derived from different reaction conditions. However, the kr value of RhB decomposition in this study is comparable to that of other previously reported studies if the initial concentration is concerned (Table 1).Table 1
A comparison of rate coefficient of (Co/Zn)ZIFs with other previously reported materials.
CatalystCRhB (mg·L–1)kr (min–1)ReferenceBiVO450.00102[30]CuO–ZrO2100.0006[31]ZnO50.0028[32]CuO50.0203[32]ZnO/CuO50.0326[32](ZnO/CuO)/rGO50.0627[32]ZnO50.0203[33]ZnO/g–C3N450.0284[33](Co/Zn)ZIFs100.028This work(Co/Zn)ZIFs300.016This work(Co/Zn)ZIFs500.004This workThe antibacterial activity of ZIF materials in an aqueous solution was evaluated by comparing the number of viable bacterial cells after contact with the material with the control sample (without ZIF). Figure6 shows that ZIF-67 and ZIF-8 have poor antibacterial performance on both S. aureus and E. coli, with cell survival rates ranging from 87 to 96% with or without light illumination. However, (Co/Zn)ZIFs have much higher antibacterial activity against both S. aureus and E. coli without light. Specifically, in the presence of light, (Co/Zn)ZIFs exhibit excellent antibacterial activity on S. aureus and E. coli strains with a survival rate of 9.14 and 6.68%. The photographs of Petri dishes obtained from E. coli and S. aureus culture without or with (Co/Zn)ZIFs in the dark and under illumination are shown in Figures 7 and 8. As expected, the E. coli and S. aureus cultures in the absence of (Co/Zn)ZIFs, i.e., the control samples, continue to grow within four hours (Figures 7(a), 7(e), 8(a), and 8(e)) regardless of lighting or not. The results also show that Gram-negative bacteria E. coli are more sensitive to the (Zn/Co)ZIFs than Gram-positive bacteria S. aureus because of the difference in their cell membrane structure [34].Figure 6
Variation of logarithm of bacterial population ofE. coli (a) and S. aureus (b) with different treatment materials in the dark and under visible-light illumination at different times (each experiment in triplicate).
(a)(b)Figure 7
Cell density of bacterialE. coli (a) in the dark (A: control sample, B: 60 min, C: 120 min, and D: 240 min). Cell density of bacterial E. coli (b) under illumination (E: control sample, F: 60 min, G: 120 min, and H: 240 min).Figure 8
Cell density of bacterialS. aureus (a) in the dark (A: control sample, B: 60 min, C: 120 min, and D: 240 min). Cell density of bacterial S. aureus (b) under illumination (E: control sample, F: 60 min, G: 120 min, and H: 240 min).Table2 shows that (Co/Zn)ZIFs have antibacterial activities against Gram-positive and Gram-negative bacteria equivalent to some previously reported materials.Table 2
A comparison of antibacterial activity of (Co/Zn)ZIF with other materials reported previously.
MaterialsBacterial strainsAntibacterial activityReferencesRFP&o-NBA@ZIF-8E. coliInhibition rate of 95% againstE. coli under light condition during 12 hours[35]MOF-53(Fe)/VanS. aureusInhibition rate of 99.3% againstS. aureus at concentration 200 μg·mL–1 of MOF-53(Fe)/Van[36]HKUST-1/OCBsE. coliGrowth inhibition of bacteria: 90.2%[37]CP/CNF/ZIF-67E. coliAntibacterial rate reached 80%[38]WO3-XE. coli and S. aureusInhibition rate of 100% againstE. coli and S. aureus at 100 μg·mL–1 after six hours of contact[39](Co/Zn)ZIFsE. coli and S. aureusInhibition rate 90.86% againstS. aureus and 93.8% against E. coli after four hours of contact under visible light conditionThe present studyThe (Co/Zn)ZIF material is a potential photocatalyst, capable of decolorizing RhB and also inhibiting microorganisms. ZIF-8 and ZIF-67 have largeEg and therefore are not excited under visible light. Combining Zn with Co creates a catalyst with photochemical degradation ability under visible light. This ability of (Co/Zn)ZIFs may result from the addition of Zn to the intermediate band formation between the conduction (CB) and valence (VB) regions of ZIF-67 and ZIF-8. Visible light can excite electrons from VB, and an energy state adds to the Co3d level to CB, causing electron formation (e–) and photogenerated hole (h+). The e–/h+ pair initiates the reduction or oxidation of O2 and H2O in the solution to generate free radicals. These free radicals oxidize organic pollutants to form mineral products. The photocatalytic reaction can occur as follows:
(5)CoZnZIFs+hν⟶CoZnZIFsh+e–e–+O2⟶O2∗–CoZnZIFsh++H2O⟶˙OH+H++CoZnZIFsh+˙OHO2∗–+RhB⟶DecompositionproductsThe free radicals˙OH/O2∗– can oxidize and damage cell membranes, organelles, and bacterial DNA, resulting in the destruction of bacteria. This pathway is also considered one of many materials’ principal bactericidal mechanisms, as reported in previous studies [8–12, 34, 40–43].


## 4. Conclusion

The ZIF-8, ZIF-67, and (Zn/Co)ZIF materials synthesized with the solvothermal method have uniform morphology with a large surface area. Under visible light, (Zn/Co)ZIFs exhibit high photocatalytic activity toward the RhB degradation and, simultaneously, inhibitE. coli and S. aureus, while ZIF-8 and ZIF-67 do not. The synergistic system ZIF-67/ZIF-8 for simultaneous dye degradation and bacteria inhibition suggests a new approach for developing multifunctional materials for environmental treatments.



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