Piezoelectric catalytic,photocatalytic and adsorption capability and selectivity removal of various dyes and mixed dye wastewater by ZnO nanoparticles

Abstract

The C-O functional group decorated ZnO nanoparticles with high UV absorption and VIS/NIR reflectance were synthesized by a simple wet chemistry method using various chelating agents. This study attempts to explore the internal mechanism of the piezoelectric catalytic activity, photocatalytic activity and adsorption performance of ZnO nanoparticles. The phase purity, particle size, optical band gap and photocatalytic activity of ZnO nanoparticles showed strong chelating agent - dependent behavior. The ZnO nanoparticles prepared by using EDTA as a chelating agent exhibits smallest particle size, highest photocatalytic activity for the degradation of methyl orange, methylene blue and rhodamine B, high adsorption capacity for the adsorption of Congo red and high vibration-catalytic performance for the vibration degradation of rhodamine B. The synergies mechanism among piezoelectric catalysis, photocatalysis and adsorption capacity of ZnO nanoparticles are discussed on the basis of the experimental results.

Keywords

ZnO wet chemistry method chelating agent piezoelectric catalytic photocatalytic activity

1 Introduction

With the rapid growth of the world population, the economy has taken off, but also to the environment, especially the water resources has brought serious harm [1]. The key technologies to solve water resource pollution mainly include: (1) Green semiconductor technology powered by light energy [2 –8]. (2) Semiconductor adsorption technology with electrostatic adsorption [9, 10]. (3) Ultrasonic vibration technology based on piezoelectric effect [11 –16]. A few materials have the above catalytic ability at the same time due to the selectivity of dyes and the different physical and chemical properties of semiconductor materials. Zinc oxide (ZnO) is a kind of semiconductor material with excellent performance and widespread applications in gas sensing devices, humidity sensing devices, photocatalysis, piezoelectric catalysis and adsorption fields [17 –21]. ZnO materials have become a hot material in the catalysis field due to its piezoelectric catalytic, photocatalytic and adsorption properties. Many studies on the synergistic effect of piezocatalysis and photocatalysis have been developed to achieve better performances of piezoelectric photocatalysts [22 –24]. However, the internal correlation mechanism of the piezoelectric catalysis, photocatalysis and adsorption properties of ZnO materials is still not clear, which is of great significance to further reveal the charge transport ability and separation efficiency of ZnO materials.

At present, various synthesis techniques have been used to synthesize ZnO materials, including the sol-gel method, [25] the co-precipitation method, [26] the electrospinning method, [27] the ion exchange method, [28] the hydrothermal method, [29, 30] the polyacrylamide polymer method, [31] and so on. Among these methods, the polyacrylamide gel method is a highly efficient method for preparing metal oxide nanoparticles, which has been widely used in the preparation of single-phase semiconductor materials and composite semiconductor materials [32 –36]. By using this method, the surface of metal oxide particles can be modified with C-O or N-O functional groups, which is beneficial to improve the photocatalytic activity of metal oxide semiconductor materials [37]. Therefore, the construction of C-O functional group modified ZnO materials by the polyacrylamide gel method has important research significance for improving the photocatalytic activity of ZnO materials.

In this paper, the C-O functional group decorated ZnO nanoparticles with high UV absorption and VIS/NIR reflectance were synthesized by a simple wet chemistry method using the citric acid, tartaric acid, oxalic acid or EDTA as a chelating agent. The effect of chelating agent on the structure, surface morphology, elementary composition, charge state and photocatalytic activity of C-O functional group decorated ZnO nanoparticles were investigated. The photocatalytic performance of C-O functional group decorated ZnO nanoparticles was estimated via the study on the photocatalytic degradation process of methyl orange, Congo red, methylene blue, rhodamine B, and mixed dyes of methyl orange + Congo red + methylene blue + rhodamine B in water. The adsorption capacity of C-O functional group decorated ZnO nanoparticles towards removal of Congo red with different concentrations in water were studied in detail. The piezoelectric catalytic activity of C-O functional group decorated ZnO nanoparticles were studied for the vibration degradation of rhodamine B, methyl orange and methylene blue. Based on the high dye selectivity demonstrated by the C-O functional group decorated ZnO nanoparticles, the synergies effect among piezoelectric catalysis, photocatalysis and adsorption have been investigated in detail.

2 Experimental

2.1 Synthesis of ZnO nanoparticles

A appropriate amount of zinc sulfate (ZnSO₄) was dispersed in 20 mL deionized water to obtain a final solution of 0.015 mol / L with the total cations. After the solution was transparent, a stoichiometric amount of chelating agent (citric acid, tartaric acid, oxalic acid or ethylenediamine tetraacetic acid (EDTA)) was added into above solution in the mole ratio of 1.5:1 with respect to the total cations (Zn^2 +) to complex the cations. After that, 20 g glucose, 9.5958 g acrylamide and 1.9192 g N, N’-methylene- bisacrylamide were dissolved into the solution. N, N’-methylene- bisacrylamide was used as a crosslinking agent. The resultant solution was heated at 90 °C to initiate the polymerization reaction of acrylamide and N, N’-methylene- bisacrylamide. Subsequently, a jelly-like gel was obtained. The gel was dried at 120 °C for 24 h in a thermostat drier to obtain a xerogel. The xerogel was ground into powder and some powder was calcined at 500, 600 and 700 °C for 5 h in air to prepare the ZnO nanoparticles. The preparation flow charts of ZnO nanoparticles by different chelating agents and sintering temperatures are shown in Fig. 1. The ZnO nanoparticles prepared by using citric acid as chelating agent and sintered at 500, 600 and 700 °C are labeled as Samples S1, S2 and S3, respectively. The ZnO nanoparticles prepared by using tartaric acid, oxalic acid and EDTA as chelating agent and sintered at 600 °C are labeled as Samples S4, S5 and S6, respectively.

2.2 Material characterization

The phase structure and purity of ZnO nanoparticles were recorded by a DX-2007BH X-ray diffractometer (XRD) with Cu Kα radiation at a wavelength of 1.5406 Å operated at 40 kV and 30 mA. Fourier transform infrared (FTIR) spectra of ZnO nanoparticles in the range 400–4000 cm^–1 were obtained using a FTIR-650 spectrometer. The charge state, chemical composition, electron level of ZnO nanoparticles were characterized by a KRATOS X SAM 800 x-ray photoelectron spectrometer. The surface morphology of ZnO nanoparticles were characterized by a SU8010 Hitachi new high resolution field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). Ultraviolet–visible (UV-Vis) diffuse reflectance spectrum of ZnO nanoparticles were measured by a UV1800 UV-Vis spectrophotometer with the measurement step of 1 nm. The electrochemical impedance spectroscopy (EIS) data were measured by the CHI760E electrochemical workstation. The transient photocurrent response was measured by the Hangzhou Ruiqing Environmental Protection Technology Co., Ltd, Smart Pro-Hi Potentiostats.

2.3 Photocatalytic experiments

The photocatalytic experiment of ZnO nanoparticles for the degradation of various mixed dyes includes methyl orange, Congo red, methylene blue, rhodamine B, and mixed dyes of methyl orange + Congo red + methylene blue + rhodamine B were performed by a UV1800 UV-Vis spectrophotometer. The irradiation light source, initial dye and mixed dye concentrations, and photocatalyst content were 500 W high-pressure mercury lamp, 5 mg / L (Each of the dye), and 1 g / L, respectively. In the configuration of these dyes, all dyes are dissolved in deionized water, except for methyl red, which uses a little alcohol (10 mL) as the solvent. Before ultraviolet light irradiation, the above solution is absorbed for half hour in the darkroom. During the photocatalytic degradation experiment, the water-jacketed reactor was cooled with water-cooling system to keep the mixed solution at ambient temperature. Subsequently, a small amount of dye solution was taken, after the elapse of a period of time, for the examination of mixed dye concentration. The absorbance of different dyes and mixed dye at different time intervals were obtained by a UV1800 UV-Vis spectrophotometer.

2.4 Adsorption experiments

The adsorption experiment of ZnO nanoparticles for the degradation of Congo red was performed by a 721 spectrophotometer. The initial Congo red concentration and catalyst content were (5, 10, 20, 30, 40 or 50) mg / L and 1 g / L, respectively. The dye solution is absorbed for 90 min in the darkroom. A small amount of dye solution was taken, after the elapse of a period of time, for the examination of Congo red concentration. In the early stage of the adsorption experiment, the liquid was taken every 10 minutes. After adsorption equilibrium is reached, the liquid is taken every half an hour. The concentration of Congo red at different time intervals were obtained by a 721 spectrophotometer.

2.5 Piezoelectric catalytic experiments

The piezoelectric catalytic experiments were carried out in rhodamine B, methyl orange and methylene blue solutions with dye concentration of 5 mg / L and catalyst content of 1 g/L. The piezoelectric catalysis experiment and the adsorption experiment are similar, but the difference is that the ultrasonic instrument with the frequency of 40 kHz and power of 180 W is used as the ultrasonic source for the piezoelectric catalysis experiment and take samples every 20 minutes.

3 Results and discussion

3.1 Phase purity and structure analysis

Phase purity and structure of ZnO nanoparticles were characterized by XRD. Figure 2(a) shows the XRD patterns of Samples S1, S2, S3, S4, S5 and S6. The ZnO nanoparticles prepared by using citric acid as chelating agent and sintered at 500 °C exhibit a mixed phase with low crystallinity including wurtzite ZnO with standard JCPDS card No. 36–1451 (a = 0.3250 nm, c = 0.5207 nm, space group: P63mc(186)) and cubic ZnSO₄ with standard JCPDS card No. 70–1254 (a = 0.7176 nm, space group: F23(196)). With the increase of sintering temperature, the diffraction peak intensity of ZnO increases gradually, and pure wurtzite ZnO nanoparticles are obtained at 600 and 700 °C. When tartaric acid, oxalic acid and EDTA were used as chelating agents, single phase ZnO nanoparticles were obtained at 600 °C. Thirteen diffraction peaks at around 31.797, 34.439, 36.240, 47.558, 56.561, 62.878, 66.397, 67.938, 69.078, 72.578, 77.015, 81.418 and 89.489° can be ascribed to the (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), (202), (104) and (203) planes, respectively.

The average crystallite size of ZnO nanoparticles can be calculated by the Debye-Scherrer equation. $D = \frac{k λ}{β cos θ}$ (1)

Where k is the shape factor of value 0.9, β is the corrected half-peak width of (101) peak, λ is the X-ray wavelength, and θ is the Bragg angle. The average crystallite size of ZnO nanoparticles are gathered in Table 1. As can be seen from Table 1, the crystallite size of ZnO nanoparticles with the citric acid used as chelating agent increases with the increasing of sintering temperature. Compared with the ZnO nanoparticles synthesized by other chelating agents, the crystallite size of ZnO nanoparticles prepared by using citric acid as the chelating agent is the largest, while the crystallite size of ZnO nanoparticles prepared by using EDTA as the chelating agent is the smallest. The values of cell parameters of all samples were larger than those of reference samples in the standard JCPDS card No. 36–1451.

Table 1

The crystallite size and cell parameters of ZnO nanoparticles

Sample	Crystallite size (nm)	Cell parameters
		a(Å)	c(Å)
S1	18.3756	3.2508	5.2110
S2	27.1114	3.2517	5.2119
S3	32.0377	3.2509	5.2067
S4	21.3585	3.2517	5.2129
S5	26.6637	3.2512	5.2106
S6	18.7460	3.2513	5.2118

Figure 2(b) shows the observed, calculated, and difference plots from the structural refinement of Sample S3. As can be seen from Fig. 2(b), the fitting results are highly consistent with the experimental results. Figure 2(c) shows the crystal structure of ZnO nanoparticles. The oxygen atom and zinc atom layers are tightly packed in a hexagonal. Figure 2(d) shows the FTIR spectra of ZnO nanoparticles. For the all samples, four characteristic peaks at 3474, 1640, 545 and 418 cm^–¹ can be observed. These peaks at 3474, 1640, 545 and 418 cm^–¹ can be ascribed to the stretching vibration of O-H, bending vibration of H-O-H, the stretching vibration of Zn–O and bending vibration of Zn-O-Zn, [38 –44] respectively. Except for sample S3, all the other samples contain a characteristic peak of 1117 cm^–1, which can be attributed to C-O functional group, [45] indicating that further increase of sintering temperature is needed to obtain high purity ZnO. The presence of C-O functional groups in these samples will have a great influence on the photocatalytic activity of ZnO nanoparticles [41, 45].

3.2 XPS analysis

The qualitative, semi-quantitative, electron energy level and elemental composition information of ZnO nanoparticles were characterized by XPS. Figure 3(a) shows the XPS full-scan spectra of Samples S2, S4, S5 and S6. The XPS full-scan spectrum of ZnO nanoparticles suggested the presence of only Zn and O components, as well as the C element from C-O functional group and XPS instrument itself. Figure 2(b) shows the Zn 2p electron energy level of Samples S2, S4, S5 and S6. The Zn 2p electron energy level spectrum has two characteristic peaks found to be at 1045.49 and 1022.31 eV corresponding to Zn 2p1/2 and Zn 2p3/2 of + 2 oxidation states for the ZnO nanoparticles, [46] respectively.

The high resolution O 1s spectra of Samples S2, S4, S5 and S6 as shown in Fig. 3(c)-(f). The O 1s spectrum for the ZnO nanoparticles can be divided into three peaks by Avantage software. These peaks at 529.52–530.24, 530.63–531.59 and 531.41–532.29 eV are the lattice oxygen of ZnO nanoparticles, C-O functional group, and the oxygen vacancy on the surface of ZnO nanoparticles [23 , 47–49], respectively. Table 2 shows the XPS parameters of high resolution O 1s spectra and the area ratio of O_v/O_L for the Samples S2, S4, S5 and S6. The highest area ratio of O_v/O_L for the Sample S6 means that the Sample S6 has a high oxygen vacancy concentration. The C-O functional group and oxygen vacancy have great influence on the catalytic activity of ZnO nanoparticles, which need to be confirmed by detailed catalytic experiments.

Table 2

XPS parameters of high resolution O 1s spectra for the Samples S2, S4, S5 and S6

Sample	Lattice oxygen O_L	C-O	Oxygen vacancy O_V	O_V/O_L
S2	529.52	530.63	531.41	0.67
S4	530.24	531.38	532.29	0.72
S5	529.71	531.03	531.81	0.65
S6	530.17	531.59	532.28	0.77

3.3 Surface morphology analysis

Figure 4(a)-(d) shows the SEM images of Samples S2, S4, S5 and S6. For all samples, the particles were spherical, and there were a small amount of adhesions and agglomerations among the particles. The ZnO nanoparticles prepared with citric acid as chelating agent has the largest average particle size, while the ZnO nanoparticles prepared with EDTA as chelating agent has the smallest average particle size. The variation trend of average particle size is consistent with the results of XRD calculation. Figure 4(e) shows the TEM image of Sample S6. The average particle size of Sample S6 is about 25 nm, which is consistent with the results of XRD calculation. The high resolution transmission electron microscopy (HRTEM) image of Sample S6 as shown in Fig. 4(f). The lattice spacing of 0.2475 nm corresponds to the d-spacing of (101) plane can be ascribed to the ZnO nanoparticles with the standard JCPDS card No. 36–1451.

3.4 Optical properties analysis

Figure 5(a) shows the UV-Vis-NIR diffuse reflectance spectra of Samples S2, S4, S5 and S6. Below 400 nm, the reflectivity of all samples almost does not change with wavelength. When the wavelength reaches 400 nm, the reflectivity increases sharply, and then increases slowly after 600 nm. Sample S5 has the highest reflectivity in the wavelength range of 190–1100 nm, while Sample S2 has the lowest reflectivity in the wavelength range of 190–400 nm, and Sample S4 has the lowest reflectivity in the wavelength range of 400–1100 nm.

Figure 5(b) shows the UV-Vis-NIR absorption spectra of Samples S2, S4, S5 and S6 on the basis of the Kubelka-Munk (K-M) equation (2). $F (R) = \frac{α}{S} = \frac{{(1 - R_{\infty})}^{2}}{2 R}$ (2)

Where, R is the reflectance, α is the optical absorption coefficient and S is the scattering coefficient. For all samples, the ZnO nanoparticles exhibits high UV absorption and the obvious absorption is ascribed to the band gap of ZnO [50]. In the visible light range, the ZnO nanoparticles also exhibit weak absorption, indicating that C-O functional groups have an effect on the optical properties of ZnO.

The relationship between (αhn)² and hn of ZnO nanoparticles can be calculated by the equation (3). $(α h ν)^{n} = A (h ν - E_{g})$ (3)

Where, hn, A and Eg represents the photon energy, constant and the optical band gap of ZnO nanoparticles, respectively. The (αhν)² versus hν extrapolated to α= 0 gives the Eg values of Samples S2, S4, S5 and S6 as shown in Fig. 5(c) - (f). The estimated Eg values of Samples S2, S4, S5 and S6 are found to be about 3.05, 2.97, 3.01 and 3.18 eV, respectively. Generally, the Eg value for the semiconductor materials decreases with the increasing of particle size. This phenomenon was not observed in this experiment, which may be caused by the presence of a small amount of C-O functional groups in ZnO nanoparticles.

3.5 Photocatalytic activity

3.5.1 Effect of different dyes on photocatalytic activity of ZnO nanoparticles

To study the effect of different dyes on photocatalytic activity of ZnO nanoparticles, the methyl orange, Congo red, methylene blue and rhodamine B were selected as the target degradation dye. The characteristic wavelengths of methyl orange, Congo red, methylene blue and rhodamine B were 465, 515, 665 and 555 nm, respectively. The catalyst content and dye concentration were 1 g/L and 5 mg/L, respectively. Figure 6 shows the degradation percentage of methyl orange, Congo red, methylene blue and rhodamine B in the presence of ZnO nanoparticles under near ultraviolet light irradiation. Before the photocatalytic experiment, the four dyes were adsorbed for half an hour respectively. The results showed that the adsorption of ZnO on methyl orange, methylene blue or rhodamine B was less, while the degradation percentage of Congo red reached more than 70 % after half an hour of adsorption. With the increase of illumination time, the absorbance decreased, indicating that the dye had been greatly degraded. When exposed to near ultraviolet light for 90 min, the degradation percentages of methyl orange, Congo red, methylene blue and rhodamine B reached 90 %, 75 %, 80 % and 78 %, respectively. The results also showed that the optimal dye concentration of methyl orange degraded by ZnO nanoparticles was 5 mg/L.

3.5.2 Effect of different particle sizes on photocatalytic activity of ZnO nanoparticles

To study the effect of particle size on the photocatalytic activity of ZnO nanoparticles, Fig. 7 shows the degradation percentage of methyl orange in the presence of Samples S2, S4, S5 and S6 under near ultraviolet light irradiation. The catalyst content and dye concentration were 1 g/L and 5 mg/L, respectively. For all the samples, the degradation percentage increases with the increasing of illumination time. After 40 min of illumination, the degradation percentage of ZnO nanoparticles decreases with the increasing of crystallite size. Compared with other samples, Sample S2 degrades methyl orange more slowly, while Sample S6 degrades methyl orange more quickly.

3.5.3 Effect of mixed dye concentration on photocatalytic activity of ZnO nanoparticles

To investigate the effect of ZnO nanoparticles (Sample S6) on the degradation behavior of mixed dyes of methyl orange + Congo red + methylene blue + rhodamine B, Fig. 8 shows the UV-Vis absorption spectra of mixed dyes of methyl orange + Congo red + methylene blue + rhodamine B in the presence of Sample S6 under near ultraviolet light irradiation. When the above mixed dyes with different concentrations were adsorbed for half an hour, the absorbance decreased obviously mainly due to the strong adsorption capacity of ZnO nanoparticles for the Congo red. With the increase of dye concentration, the time required for complete degradation of the four dyes increased continuously.

3.5.4 Adsorption of CR

In Fig. 6, the ZnO nanoparticles shows a high adsorption capacity for the Congo red dye. To further analyze the adsorption capacity of ZnO nanoparticles, different concentrations of Congo red were used to conduct adsorption experiments. Figure 9(a) shows the adsorption kinetics of Congo red adsorption on the surface of ZnO nanoparticles. The amount of Congo red adsorbed by the ZnO nanoparticles was calculated from: $q = \frac{(C_{0} - C_{t}) \times V}{m_{catalyst}}$ (4)

where q represents the amount adsorbed (mg/g) of Congo red, C₀ represents the initial Congo red concentration (mg/L), C_t represents the concentration at time t of Congo red, V represents the volume of dye solution (L) and m_catalyst represents the dosage of ZnO nanoparticles (g). The higher correlation coefficient (R²), the equilibrium adsorption capacity (q_e) and adsorption rate constant (k₂) of ZnO nanoparticles can be fitted by origin 8.0, the pseudo-first-order and second-order models [39]. The adsorption equilibrium capacity of ZnO nanoparticles can be obtained as follow: $\frac{d q_{t}}{dt} = K_{2} (q_{e} - q_{t})^{2}$ (5)

where q_e represents the adsorption capacity at equilibrium (mg/g_catalyst) and q_t represents the adsorption capacity at time t (mg/g_catalyst). When t = 0 to t = t and q_t = 0 to q_t = q_t, the equation (5) can be integrated as follows: $\frac{t}{q_{t}} = \frac{1}{{kq}_{e}^{2}} + \frac{t}{q_{e}}$ (6)

A linear relationship between t/q_t and t for the ZnO nanoparticles are presented in Fig. 9(b). The R², the adsorption rate constant (K₂) and q_e from the pseudo-second-order kinetics for the adsorption of Congo red on ZnO nanoparticles as shown in Table 3. The pseudo-second-order model fits the observed value very well for the ZnO nanoparticles with an initial Congo red concentration of 5 to 50 mg/L. The value of R² is close to 1, indicating that the fitting results are in good agreement with the experimental results. When the initial Congo red concentration was 50 mg/L, the ZnO nanoparticles showed the best adsorptivity, ca. 54.2915 mg/g_catalyst, along with the lowest k₂ value about 0.01792 g/mg/min. The results showed that the ZnO nanoparticles exhibited the highest adsorption capacity for the Congo red.

Table 3

The adsorption rate constant (K₂) and the adsorption capacity at equilibrium (q_e) value from the pseudo-second-order kinetics for the adsorption of Congo red with different initial concentrations on ZnO nanoparticles

	Initial concentration
	5 mg/L	10 mg/L	20 mg/L	30 mg/L	40 mg/L	50 mg/L
R²	0.99751	0.9985	0.99862	0.99793	0.99874	0.99971
K₂ (g/mg/min)	1.56722	0.83313	0.11028	0.05361	0.04794	0.01792
q_e (mg/g)	0.6156	1.1876	8.2868	18.1098	20.192	54.2915

3.5.5 Piezoelectric catalytic degradation of different dyes

To study the piezoelectric catalytic activity of ZnO nanoparticles, Rhodamine B, methyl orange and methylene blue, which are difficult to be adsorbed and degraded, were selected as the target degradation dyes. Figure 10(a)-(c) shows the UV-Vis absorption spectra of rhodamine B, methyl orange and methylene blue solutions in the presence of ZnO nanoparticles under different vibration time. It can be seen from the figure that the ultrasonic vibration has a strong degradation efficiency for the rhodamine B, and can partially degrade methyl orange, but hardly degrade methylene blue. Figure 10(d) shows the real photos of rhodamine B solutions under different vibration time. After 140 min of ultrasonic vibration, most of the rhodamine B was degraded. The rhodamine B changes color from its initial pink to white. Figure 10(e) shows the time-dependent vibration-catalytic degradation of rhodamine B in the presence of ZnO nanoparticles. After 140 min of ultrasonic vibration, the degradation percentage reached about 70%. Figure 10(e) insets shows the ln(C_t/C₀) versus vibration time for the rhodamine B, where C₀ is the initial rhodamine B concentration, C_t is the instantaneous concentration of rhodamine B at vibration time t. The slope between ln(C_t/C₀) and vibration time t is expressed in terms of k, which is considered to be the pseudo first order rate constant. The rate constant of ZnO nanoparticles for vibration degrade rhodamine B is estimated to be 0.00807 min^–1. Figure 10(f) shows the time-dependent vibration-catalytic degradation of methyl orange in the presence of ZnO nanoparticles. The vibration degradation percentage of methyl orange by ZnO nanoparticles was about 20 % for the vibration degradation of 140 min. The rate constant of ZnO nanoparticles for vibration degrade methyl orange dye is estimated to be 0.00166 min^–1 as shown in Fig. 10(f) inset. The results show that the ZnO nanoparticles is highly selective for the vibration degradation of organic dyes.

3.6 Synergistic interaction between piezoelectric catalysis, photocatalysis and adsorption mechanisms

Combined with the photocatalytic experiments, adsorption experiments and ultrasonic vibration experiments, it can be seen that the C-O functional group decorated ZnO nanoparticles is selective in the degradation of dyes. It shows high photocatalytic activity for methyl orange, methylene blue and rhodamine B, high adsorption capacity for Congo red, and high piezoelectric catalytic performance for rhodamine B. The mechanism of photocatalysis is that the electron transition of valence band of ZnO is excited to conduction band by the energy driven by light energy, and then the hole is left in valence band of ZnO as shown in Fig. 11. In this experiment, although the band gap value of ZnO is large, the C-O functional group acts as the carrier of electron transport, making it easy for electrons to transition to the conduction band of ZnO. The valence band hole and conduction-band electron of ZnO can react with OH^–/H₂O or H₂O and O₂/H₂O₂ or O₂ to generate hydroxyl radicals (•OH) or superoxide radicals (•O ^2–) due to the redox potentials of H₂O/•OH and OH^–/•OH are +2.72 and +1.89 V versus NHE and the redox potential of O₂/•O ^2– is –0.13 V, [51, 52] respectively.

$h_{VB}^{+} + {OH}^{-} \to • OH$ (7) $h_{VB}^{+} + H_{2} O \to • OH + H^{+}$ (8) $• OH + dye \to degradation products$ (9) $H^{+} + dye \to degradation products$ (10) ${e_{CB}}^{-} + O_{2} \to • O^{2 -}$ (11) $• O^{2 -} + 2 H^{+} + {e_{CB}}^{-} \to H_{2} O_{2}$ (12) $2 {e_{CB}}^{-} + O_{2} + 2 H^{+} \to H_{2} O_{2}$ (13) ${e_{CB}}^{-} + H_{2} O_{2} \to • OH + O H^{-}$ (14) $• O^{2 -} + H_{2} O_{2} \to • OH + O H^{-} + O_{2}$ (15)

•OH mainly attack the -C-N- bond, -N = N- bond and -C-N = bond of dye molecules, and eventually degrade into non-toxic and harmless products [53 –57]. Figure 12 shows the EIS spectra and photocurrent response of Samples S4 and S6. The results confirm that the C-O functional group decorated ZnO nanoparticles have high charge transfer and migration ability and high separation efficiency. When only vibration occurs, the ZnO nanoparticles also generate •OH and •O ^2– through a series of reactions [58]. The •OH attacks the carboxyl group and xanthene ring for the rhodamine B, the -N = N- bond for the methyl orange, thus degrading the dye into non-toxic and harmless products [58]. Generally, the adsorption properties of semiconductor materials are mainly caused by electrostatic interaction, ion exchange, pore filling and π - π interaction [39 , 59–62].

Figure 13 shows the N₂ adsorption-desorption isotherms of Sample S6. The isotherms of ZnO nanoparticles is type-I isotherm, indicating that the ZnO nanoparticles have not porous structure. Based on the experimental results, the hole filling can be ignored for the adsorption of Congo red by zinc oxide smart materials. In this case, the high adsorption performance of ZnO nanoparticles for Congo red dye may be caused by the synergistic effect of electrostatic interaction, ion exchange and π - π interaction. Due to the difference of the bonds contained in each dye molecule, ZnO nanoparticles is selective in the process of dye degradation.

4 Conclusion

A simple wet chemistry method has been used to synthesize the C-O functional group decorated ZnO nanoparticles with high UV absorption and VIS/NIR reflectance by using the citric acid, tartaric acid, oxalic acid or EDTA as a chelating agent. The phase structure, functional group, surface morphology, charge state, optical properties, photocatalytic activity, adsorption capacity and vibration-catalytic performance of C-O functional group decorated ZnO nanoparticles were characterized by various tests. The photocatalytic activity, adsorption capacity and vibrational catalytic performance of C-O functional group decorated ZnO nanoparticles demonstrate high particle size dependence and dye selectivity. The C-O functional group decorated ZnO nanoparticles can high efficiently degrade simulated dye wastewater mixed with various dyes including methyl orange, Congo red, methylene blue and rhodamine B. The C-O functional group decorated ZnO nanoparticles prepared by using EDTA as a chelating agent exhibits high photocatalytic activity for the degradation of methyl orange, methylene blue and rhodamine B, high adsorption capacity for the adsorption of Congo red and high vibration-catalytic performance for the vibration degradation of rhodamine B. Based on the experimental results, the synergies mechanism among piezoelectric catalysis, photocatalysis and adsorption capacity of C-O functional group decorated ZnO nanoparticles are discussed in detail.

Competing interests

The authors declare that they have no competing interests.

Footnotes

Acknowledgments

This work was supported by the Science and Technology Research Program of Chongqing Education Commission of China (KJZD-K202001202, KJQN201901), the Chongqing Key Laboratory of Geological Environment Monitoring and Disaster Early-warning in Three Gorges Reservoir Area (No. ZD2020A0401), the NSAF joint Foundation of China (U2030116), the Chongqing Natural Science Foundation (cstc2019jcyj-msxmX0310), Project 2019DB02 supported by NPL, CAEP, the Talent Introduction Project (09924601) of Chongqing Three Gorges University and the Scientific Research Fund of Sichuan Provincial Science and Technology Department (2020YJ0137).

References

Simonovic

S.P.

, Systems approach to management of water resources—Toward performance based water resources engineering, Water 12(4) (2020), 1208.

Cheng

, Gao

, Liu

, Pu

, Wang

, Yi

, Wu

and Yang

, Preparation of core-shell heterojunction photocatalysts by coating CdS nanoparticles onto Bi₄Ti₃O₁₂ hierarchical microspheres and their photocatalytic removal of organic pollutants and Cr (VI) ions, Colloid, Surface A 633 (2022), 127918.

Kaewmanee

, Phuruangrat

, Thongtem

and Thongtem

, Solvothermal synthesis of Mn–Zn Ferrite(core)@SiO₂(shell)/BiOBr_0.5Cl_0.5 nanocomposites used for adsorption and photocatalysis combination, Ceram Int 46(3) (2020), 3655–3662.

Yang

, A short review on heterojunction photocatalysts: Carrier transfer behavior and photocatalytic mechanisms, Mater Res Bull 142 (2021), 111406.

Wang

, Li

, Sun

, Xian

, Yi

and Yang

, Photocatalytic application of Ag-decorated CuS/BaTiO₃ composite photocatalysts for degrading RhB, J Electron Mater 50 (2021), 2674–2686.

Chen

and Xiao

, Construction of Z-Scheme CdS/WO₃ Heterostructure on photonic crystals with high-efficiency photocatalysis performance, J Electron Mater 49 (2020), 4995–5002.

Wang

, Sun

, Xian

, Liu

and Yang

, Photocatalytic purification of simulated dye wastewater in different pH environments by using BaTiO₃/Bi₂WO₆ heterojunction photocatalysts, Opt Mater 113 (2021), 110853.

Gao

, Yu

, Wang

, Yang

, Wang

, Tang

, Yi

and Li

, A novel photoluminescence phenomenon in a SrMoO₄/SrWO₄ micro/nano heterojunction phosphors obtained by the polyacrylamide gel method combined with low temperature calcination technology, J Lumin 243 (2022), 118660.

Lü

, Ma

, Ke

, Zhang

, Qi

and Zhao

, Synthesis of gallic acid functionalized magnetic hydrogel beads for enhanced synergistic reduction and adsorption of aqueous chromium, Chem Eng J 408 (2021), 127327.

10.

Shamsudin

M.S.

, Azha

S.F.

, Sellaoui

, Badawi

, Al-Ghamdi

Y.O.

, Bonilla-Petriciolet

and Ismail

, Fabrication and characterization of a thin coated adsorbent for antibiotic and analgesic adsorption: Experimental investigation and statistical physical modelling, Chem Eng J 401 (2020), 126007.

11.

Cheng

, Gao

, Li

, Wang

, Yi

and Yang

, Flexoelectricity-induced enhancement in carrier separation and photocatalytic activity of a photocatalyst, Appl Surf Sci 566 (2021), 150669.

12.

Bagchi

, Hoque

N.A.

, Janowicz

, Das

and Tiwari

M.K.

, Re-usable self-poled piezoelectric/piezocatalytic films with exceptional energy harvesting and water remediation capability, Nano Energy 78 (2020), 105339.

13.

Cheng

, Gao

, Sun

, Xian

, Wang

, Yi

, Liu

, Wang

and Yang

, An excellent Z-scheme Ag₂MoO₄/Bi₄Ti₃O₁₂ heterojunction photocatalyst: Construction strategy and application in environmental purification, Adv Powder Technol 32(3) (2021), 951–962.

14.

, Wang

, Dong

, Zhang

, Han

, Luo

, Huang

, Feng

, Chen

, Jia

and Zhang

, Recent advances in noncontact external-field-assisted photocatalysis: from fundamentals to applications, ACS Catal 11 (2021), 4739–4769.

15.

Guo

, Chen

, Ren

, Li

, Zhang

, Wu

, Li

, Liu

, Li

, Fu

and Ma

, Morphology engineering of type-II heterojunction nanoarrays for improved sonophotocatalytic capability, Ultrason Sonochem 81 (2021), 105849.

16.

Liu

, Zhai

, Xiao

, Tang

, Sun

, Bowen

C.R.

, Luo

and Zhang

, Piezo-photoelectronic coupling effect of BaTiO₃@TiO₂ nanowires for highly concentrated dye degradation, Nano Energy 92 (2022), 106702.

17.

Liang

Y.C.

and Chang

Y.C.

, The effect of Ni content on gas-sensing behaviors of ZnO-NiO p-n composite thin films grown through radio-frequency cosputtering of ceramic ZnO and NiO targets, CrystEngComm 22(13) (2020), 2315–2326.

18.

Sahoo

, Mohanty

, Biswas

and Nayak

, Role of hexamethylenetetramine in ZnO-cellulose nanocomposite enabled UV and humidity sensor, Mater Sci Semicond Proc 105 (2020), 104699.

19.

Saad

A.M.

, Abukhadra

M.R.

, Ahmed

S.A.K.

, Elzanaty

A.M.

, Mady

A.H.

, Betiha

M.A.

, Shim

J.J.

and Rabie

A.M.

, Photocatalytic degradation of malachite green dye using chitosan supported ZnO and Ce–ZnO nano-flowers under visible light, J Environ Manage 258 (2020), 110043.

20.

Venugopal

, Thangavel

, Vasudevan

and Zoltán

, Efficient visible-light piezophototronic activity of ZnO-Ag₈S hybrid for degradation of organic dye molecule, J Phys Chem Solids 143 (2020), 109473.

21.

Mukwevho

, Gusain

, Fosso-Kankeu

, Kumar

, Waanders

and Ray

S.S.

, Removal of naphthalene from simulated wastewater through adsorption-photodegradation by ZnO/Ag/GO nanocomposite, J Ind Eng Chem 81 (2020), 393–404.

22.

, Ren

, Wu

, Li

, Liu

, Li

, Xing

, Ma

, Wang

and Xue

, Direct Z-scheme heterojunction of ZnO/MoS₂ nanoarrays realized by flowing-induced piezoelectric field for enhanced sunlight photocatalytic performances, Appl Catal B 285 (2021), 119785.

23.

, Wang

, Zhao

, Zhang

, An

, Bai

, Ren

, Wu

, Li

, Liu

, Li

and Ma

, Synthesis of ternary ZnO/ZnS/MoS₂ piezoelectric nanoarrays for enhanced photocatalytic performance by conversion of dual heterojunctions, Appl Surf Sci 556 (2021), 149695.

24.

, Ren

, Guo

, Li

, Liu

, Li

, Wu

and Ma

, Piezotronics boosted plasmonic localization and hot electron injection of coralline-like Ag/BaTiO₃ nanoarrays for photocatalytic application, J Mater Chem C 9 (2021), 12596–12604.

25.

Perveen

, Shujaat

, Qureshi

, Nawaz

, Khan

M.I.

and Iqbal

, Green versus sol-gel synthesis of ZnO nanoparticles and antimicrobial activity evaluation against panel of pathogens, J Mater Res Technol 9(4) (2020), 7817–7827.

26.

Prabakar

, Muthukumaran

and Raja

, Structural, magnetic and photoluminescence behavior of Ni/Fe doped ZnO nanostructures prepared by co-precipitation method, Optik 202 (2020), 163714.

27.

Zhang

, Zhang

, Wang

, Liu

, He

, Zhang

and Li

, Preparation of Ga₂O₃/ZnO/WO₃ double S-scheme heterojunction composite nanofibers by electrospinning method for enhancing photocatalytic activity, J Mater Sci Mater Electron 32(6) (2021), 7307–7318.

28.

Heidari

, Alizadeh

, Ebadi

, Oturan

and Oturan

M.A.

, Efficient photocatalytic degradation of furosemide by a novel sonoprecipited ZnO over ion exchanged clinoptilolite nanorods, Sep Purif Technol 242 (2020), 116800.

29.

ElFaham

M.M.

, Mostafa

A.M.

and Mwafy

E.A.

, The effect of reaction temperature on structural, optical and electrical properties of tunable ZnO nanoparticles synthesized by hydrothermal method, J Phys Chem Solids 154 (2021), 110089.

30.

Rosowska

, Kaszewski

, Witkowski

, Kuryliszyn-Kudelska

and Godlewski

, The effect of iron content on properties of ZnO nanoparticles prepared by microwave hydrothermal method, Opt Mater 109 (2020), 110089.

31.

X.X.

, Wu

, Dong

Z.F.

and Yang

, Enhanced solar light photocatalytic properties of ZnO nanocrystals by Mg-doping via polyacrylamide polymer method, J Photoch. Photobio A 356 (2018), 681–688.

32.

Wang

, Tang

, Gao

, Fang

, Hu

, Sun

, Chen

, Yu

, Liu

and Pan

, Modified polyacrylamide gel synthesis of CeO₂ nanoparticles by using cerium sulfate as metal source and its optical and photoluminescence properties, J Mater Sci Mater Electron 32(8) (2021), 10820–10834.

33.

Wang

S.F.

, Chen

X.Y.

, Gao

H.J.

, Fang

L.M.

, Hu

Q.W.

, Sun

G.A.

, Tang

S.N.

, Liu

, Yu

and Pan

X.D.

, A Comparative Study on the Phase Structure, Optical and NIR Reflectivity of BaFe₁₂O₁₉ Nano-Pigments by the Traditional and Modified Polyacrylamide Gel Method, J Nano Res 67 (2021), 1–14.

34.

Sankaranarayanan

, Shailajha

, Mubina

M.S.K.

and Anilkumar

C.P.

, Effect of Zn^2 + ions on structural, optical, magnetic, and impedance response of Zn_x@ Ni_1–xFe₂O₄ core materials prepared by two-step polyacrylamide gel method, J Mater Sci Mater Electron 31 (2020), 11833–11846.

35.

Gao

, Wang

, Gao

, Pan

, Wang

, Yang

, Chen

, Wang

, Fang

and Yi

, Phase evolution and photoluminescence behavior of MMoO₄ (M=Mg, Ca, Sr) phosphors, Optik 241 (2021), 167040.

36.

Wang

, Wang

, Gao

, Li

, Fang

, Yu

, Tang

, Zhao

and Sun

, Synthesis and characterization of BaAl₂O₄: Ce and Mn-Ce-co-doped BaAl₂O₄ composite materials by a modified polyacrylamide gel method and prediction of photocatalytic activity using artificial neural network (ANN) algorithm, Optik 221 (2020), 165363.

37.

Wang

, Gao

, Wei

, Li

, Yang

, Fang

and Lei

, Insight into the optical, color, photoluminescence properties, and photocatalytic activity of the N–O and C–O functional groups decorating spinel type magnesium aluminate, CrystEngComm 21(2) (2019), 263–277.

38.

Aboorvakani

, Vethanathan

S.J.K.

and Madhu

K.U.

, Influence of Zn concentration on zinc oxide nanoparticles and their anti-corrosion property, J Alloys Compd 834 (2020), 155078.

39.

Wang

, Gao

, Fang

, Hu

, Sun

, Chen

, Yu

, Tang

, Yu

, Zhao

, Sun

and Yang

, Synthesis of novel CQDs/CeO₂/SrFe₁₂O₁₉ magnetic separation photocatalysts and synergic adsorption-photocatalytic degradation effect for methylene blue dye removal, Chem Eng J Adv 6 (2021), 100089.

40.

Rammah

Y.S.

, El-Agawany

F.I.

, Mahmoud

K.A.

, El-Mallawany

, Ilik

and Kilic

, FTIR, UV–Vis–NIR spectroscopy, and gamma rays shielding competence of novel ZnO-doped vanadium borophosphate glasses, J Mater Sci Mater Electron 31(12) (2020), 9099–9113.

41.

, Wang

, Sun

, Gao

, Yu

, Tang

, Zhao

, Yi

, Wang

and Wei

, Facile preparation of MgAl₂O₄/CeO₂/Mn₃O₄ heterojunction photocatalyst and enhanced photocatalytic activity, Mater Today Chem 19 (2021), 100390.

42.

Nadeem

M.S.

, Munawar

, Mukhtar

, Rahman

M.N.U.

, Riaz

, Hussain

and Iqbal

, Hydrothermally derived co, Ni co-doped ZnO nanorods; structural, optical, and morphological study, Opt Mater 111 (2021), 110606.

43.

Prabhu

, Pudukudy

, Sohila

, Harish

, Navaneethan

, Ramesh

and Hayakawa

, Synthesis, structural and optical properties of ZnO spindle/reduced graphene oxide composites with enhanced photocatalytic activity under visible light irradiation, Opt Mater 79 (2018), 186–195.

44.

Wang

, Gao

, Liu

, Li

, Zhang

, He

and Sun

, Facile fabrication of ZnO nanorods modified Fe₃O₄ nanoparticles with enhanced magnetic, photoelecrochetmical and photocatalytic properties, Opt Mater 111 (2021), 110608.

45.

Wang

, Gao

, Li

, Wang

, Chen

, Yu

, Tang

, Zhao

, Sun

and Li

, Comparative study of the photoluminescence performance and photocatalytic activity of CeO₂/MgAl₂O₄ composite materials with an n-n heterojunction prepared by one-step synthesis and two-step synthesis methods, J Phys Chem Solids 150 (2021), 109891.

46.

Singha

M.K.

and Patra

, Highly efficient and Reusable ZnO microflower photocatalyst on stainless steel mesh under UV–Vis and natural sunlight, Opt Mater 107 (2020), 110000.

47.

Hegazy

I.M.

, Geioushy

R.A.

, El-Sheikh

S.M.

, Shawky

, El-Sherbiny

and Kandil

A.H.T.

, Influence of oxygen vacancies on the performance of ZnO nanoparticles towards CO₂ photoreduction in different aqueous solutions, J Environ Chem Eng 8(4) (2020), 103887.

48.

Heuer

J.K.

and Stubbins

J.F.

, An XPS characterization of FeCO₃ films from CO₂ corrosion, Corros Sci 41 (1999), 1231–1243.

49.

Guan

, Mori

, Kato

and Zhao

, Improved crystalline quality of ZnO with inserting multi-buffer layers, J Mater Sci Mater Electron 31(13) (2020), 9982–9988.

50.

Kiomarsipour

and Razavi

R.S.

, Hydrothermal synthesis of ZnO nanopigments with high UV absorption and vis/NIR reflectance, Ceram Int 40(7) (2014), 11261–11268.

51.

Tachikawa

, Fujitsuka

and Majima

, Mechanistic insight into the TiO₂ photocatalytic reactions: Design of new photocatalysts, J Phys Chem C 111(14) (2007), 5259–5275.

52.

Arai

, Yanagida

, Konishi

, Iwasaki

, Sugihara

and Sayama

, Efficient complete oxidation of acetaldehyde into CO₂ over CuBi₂O₄/WO₃ composite photocatalyst under visible and UV light irradiation, J Phys Chem C 111(21) (2007), 7574–7577.

53.

Kaur

and Singhal

, Facile synthesis of ZnO and transition metal doped ZnO nanoparticles for the photocatalytic degradation of methyl orange, Ceram Int 40(5) (2014), 7417–7424.

54.

Güy

, Çakar

and Özacar

, Comparison of palladium/zinc oxide photocatalysts prepared by different palladium doping methods for congo red degradation, J Colloid Interface Sci 466 (2016), 128–137.

55.

Khazaee

, Mahjoub

A.R.

, Khavar

A.H.C.

, Srivastava

and Sillanpää

, Synthesis of layered perovskite Ag, F-Bi₂MoO₆/rGO: A surface plasmon resonance and oxygen vacancy promoted nanocomposite as a visible-light photocatalyst, J Photoch Photobio A 379 (2019), 130–143.

56.

Hejazi

, Mahjoub

A.R.

, Khavar

A.H.C.

and Khazaee

, Novel visible-light-responsive rGO-ZnO@Bi₂MoO₆ nanocomposite with enhanced light harvesting and Z-scheme charge transfer for photodegradation and detoxification of RhB, Solid State Sci 95 (2019), 105934.

57.

Wang

, Gao

, Wang

, Fang

, Chen

, Yu

, Tang

, Liu

, Yi

and Yang

, Facile synthesis of BaMoO₄ and BaMoO₄/BaWO₄ heterostructures with type-I band arrangement and enhanced photoluminescence properties, Adv Powder Technol 32 (2021), 4186–4197.

58.

, Qin

, Lin

, Yuan

, Kang

and Bao

, Synthesis of Bi₄Ti₃O₁₂ decussated nanoplates with enhanced piezocatalytic activity, Nanoscale 11(44) (2019), 21128–21136.

59.

Ayati

, Shahrak

M.N.

, Tanhaei

and Sillanpää

, Emerging adsorptive removal of azo dye by metaleorganic frameworks, Chemosphere 160 (2016), 30–44.

60.

Abbas

, Ali

, Rizwan

, Zaheer

I.E.

, Malik

, Riaz

M.A.

, Shahid

M.R.

, Rehman

M.Z.

and Al-Wabel

M.I.

, A critical review of mechanisms involved in the adsorption of organic and inorganic contaminants through biochar, Arab J Geosci 11 (2018), 448.

61.

Gao

H.J.

, Wang

S.F.

, Fang

L.M.

, Sun

G.A.

, Chen

X.P.

, Tang

S.N.

, Yang

, Sun

G.Z.

and Li

D.F.

, Nanostructured spinel-type M(M=Mg, Co, Zn)Cr₂O₄ oxides: novel adsorbents for aqueous Congo red removal, Mater Today Chem 22 (2021), 100593.

62.

Liu

, Wang

, Gao

, Yang

, Wang

, Chen

, Fang

, Tang

, Yi

and Li

, A simple polyacrylamide gel route for the synthesis of MgAl₂O₄ nanoparticles with different metal sources as an efficient adsorbent: Neural network algorithm simulation, equilibrium, kinetics and thermodynamic studies, Sep Purif Technol 281 (2022), 119855.