“defect engineering” has been consiered as an effective strategy to improve its photocatalytic activity. ZnO1 − x photocatalysts containing oxygen defects were prepared by “oxygen-atom capture” method in different lithium-naphthalene solution. The effect of concentration of lithium-naphthalene solution on the oxygen vacancies and photocatalytic performance of ZnO were researched comprehensively. The results indicate that ZnO photocatalysts treated in lithium-naphthalene solution show disordered structure on the material due to the presence of oxygen vacancies. Compared with W-ZnO, ZnO1 − x exhibit higher visible light absorption and enhanced photocatalytic properties. Moreover, more oxygen vacancies are introduced into ZnO−0.8, which reduce its bandgap to 3.04 eV and improve the separation efficiency and transfer speed of photo-generated carriers. Therefore, the efficiency of NO removal by ZnO−0.8 is enhanced to 54.3% under ultraviolet light irradiation, and its degradation efficiency of NO is ~ 12 times greated than that of W-ZnO. Oxygen vacancies acted as capturer of electrons, inhibiting the recombination of photogenerated electrons and holes. Thus, increasing the appropriate concentration of oxygen vacancies on the surface of the material can enhance its photocatalytic activity.
Research Article
Preparation of Gray ZnO 1-x Photocatalyst Containing Oxygen Vacancies for the Highly Efficient Removal of Nitric Oxide and Acetaminophen
https://doi.org/10.21203/rs.3.rs-4827361/v1
This work is licensed under a CC BY 4.0 License
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ZnO
Oxygen Defects
NO Removal
Oxygen-atom capture
Lithium-Naphthalene solution
In nanomaterials, decreasing the particle size will increase the surface binding energy and the number of unsaturated coordination bonds on the material surface, thereby forming a defect state on the material surface. At the same time, during the material preparation, the internal lattice of the material can be distorted because of the short-range arrangement of atoms, leading to the formation of defects, that can therefore afford special properties to materials[1]. Under normal circumstances, defects are classified into four types: point, line, surface, and body defects. Vacancy is a type of point defect in the lattice, which is formed at minimize the Helmholtz free energy to establish thermodynamic equilibrium at a specific temperature. Meanwhile, vacancy can effectively alter the charge distribution and electronic energy levels near the defect site[2]. The formation of metal oxide defects can be regulated and controlled via defect engineering. Metal-oxide-based materials are formed via a metallic bond between metal and oxygen atoms. When metal oxides are treated by the restore method, lattice oxygen atoms are removed, and an oxygen vacancy is formed. for achieving charge balance, the metal atoms would be reduced to low valence ions[3].
Oxygen vacancies are the most common anion vacancy defects that are used to frequently altered the physical and chemical properties of materials[4]. Among anion vacancy defects, the formation energy of oxygen vacancies is the lowest, and oxygen vacancies are the easiest to form. The properties of semiconducting materials can be changed by regulating the formation of oxygen vacancies[5]. For semiconductor metal oxides, ZnO, an n-type semiconductor, possesses high chemical stability, low cost, non-toxicity, suitable redox potential, and is an exceptional material for photocatalysis[6]. Nevertheless, the application of ZnO is limited due to high recombination rates of photogenerated carriers and its wide bandgap[7]. Consequently, extensive efforts are underway to enhance the photocatalytic activity of ZnO by reducing the bandgap and introducing defects such as oxygen vacancies[8–10]. Nitric Oxide (NO), a toxic emission, can lead to acid rain and photochemical smog even at low concentrations, posing adverse health effects[11]; thus, its emissions must be stringently controlled. ZnO containing oxygen vacancies is extensively utilized in the removal of nitrogen oxides, owing to its improved light absorption, charge separation, and redox capacity. The Reshalaiti Hailili research group has attempted to control the microstructure of a novel ZnO photocatalyst through solid-state synthesis with ionic liquids, altering reaction temperatures and alkalinity, and further researched its effectiveness in the light-driven removal of dilute NO. Notably, oxygen-vacancy-rich ZnO increased the NO removal rate by 4.16 times and decreased NO2 production by a factor of 2.76. These results suggest that the microstructure of ZnO, defect-related carrier separation, and enhanced surface interaction are beneficial for the generation of active substances and activation of molecular oxygen, thereby improving NO removal rates and mitigating NO2 formation. Furthermore, ZnO with various structures was synthesized and modified to intentionally create surface defects. The flux used in the synthesis process provided a reducing environment conducive to the spontaneous formation of surface vacancies with precisely controlled gradient concentrations. The characterization of these photocatalysts confirmed successful defect generation, evaluated further in solar-driven NO removal studies, which demonstrated that oxygen-enriched ZnO had removal rates 5.43 times and 1.63 times higher than those of oxygen-deficient and original ZnO, respectively[12–13]. These findings underscore that introducing oxygen vacancies is a highly effective strategy to enhance light absorption, promote the separation and rapid transport of photogenerated carriers in metal oxides. However, material preparation often requires harsh physical conditions, such as high temperature/pressure or vacuum environments, along with reducing gases, leading to increased production costs and safety risks.
Lithium-naphthalene solution, a strong reducing agent (E0(THF) = -3.10 V), is frequently employed in organic reactions[14]. Therefore, oxygen-deficient ZnO photocatalyst is synthesized by the “oxygen-atom capture” method in this study. Under normal temperature and pressure, we use lithium elements in the lithium-naphthalene solution to capture oxygen atoms in ZnO surface, forming defects on the surface of ZnO materials. By modifying different concentrations of lithium-naphthalene solution, the formation of different contents oxygen vacancies on the surface of ZnO is prepared. Then, we investigate the effect of oxygen vacancies on the photocatalytic performance of ZnO semiconductor photocatalysts.
2.1 Preparation of carbon spheres
All of reagents were analytical purity and not used undergone additional purification in the experiment. The water utilized in the experiment was deionized water. First, 0.4 mol/L anhydrous glucose was weighed and added to 80 mL of deionized water, which was stirred slowly until to form a colorless transparent solution. Second, the glucose solution was transferred to a 100mL polytetrafluoroethylene, which was placed in a reaction oven at 180℃ for 20 h, then, the reactor was cooled to room temperature, and the contents were removed and washed several times via centrifugation with absolute ethanol and deionized water, and dried in an oven at 60℃ to obtain carbon spheres (CSs).
2.2 Preparation of the ZnO precursor
0.4 g of CSs were weighed and added to 60 mL of absolute ethanol. They were fully dispersed using a magnetic stirrer. Then, 2.4 g of zinc acetate was weighed and slowly added to the above solution of CSs, stirred for 4 h, and dried at 80℃. After 2 h, the solution was transferred to porcelain boat, which was placed in a tube furnace at 700℃ with the heating rate of 1℃ min− 1. The temperature of the furnace was maintained at 700℃ for 4 h to obtain white ZnO, named W-ZnO.
2.3 Preparation of lithium naphthalene solutions
41.65, 55.53 and 69.41 mg of metallic lithium and 769.1, 1025.4 and 1282 mg of organic naphthalene were weighed and dissolved in 10 mL of tetrahydrofuran solution for forming 0.6, 0.8 and 1.0 mol/L lithium-naphthalene solution, respectively.
2.4 "oxygen-atom capture" method
Oxygen atom capture technology is a method that produces oxygen vacancies via interactions between metals and oxygen atoms. During this process, reactive metals can capture oxygen atoms. When these atoms contact metal surfaces, they may be adsorbed or bonded by metal atoms, thereby creating oxygen vacancies. The development of these vacancies can significantly influence material properties.
2.5 Preparation of the gray oxygen-deficient ZnO photocatalyst
0.2 g of W-ZnO was individually added to 0.6, 0.8 and 1.0 mol/L lithium-naphthalene solutions, respectively. The mixture was reacted for 5 min, then washed and dried at 60℃. The dried samples were placed in tube furnace for heat treatment in an argon atmosphere at 300℃ for 4 h at the heating rate of 5℃ min− 1. After heat treatment, the samples were washed with a dilute hydrochloric acid solution for removing the organic carbon coated on the surface of the material, followed by drying at 60℃. The products obtained at different lithium naphthalene concentrations were named ZnO-0.6, ZnO-0.8, and ZnO-1.0, respectively. The schematic of the experimental process was shown in Fig. 1.
2.5 Characterizations
The X-ray diffraction (XRD) patterns were obtained to determine the phase structures using Bruker D8-DISCOVER diffractometer with Cu Kα radiation (λ = 1.542 Å). Raman spectrum was recorded with a Renishaw InVia Raman spectrometer (λ = 785nm). Surface charges of the samples in ultrapure water were measured on a zeta potential analyzer (Zetasizer Nano ZSP, Malvern, Britain). UV-Vis diffuse reflectance absorption spectrum was recorded on Cary 5000 in the range of 200–800 nm using BaSO4 as a reference. The photoluminescence (PL) spectra and time-resolved fluorescence decay spectra were analyzed by a spectrophotometer (Edinburgh Instruments, FS5) equipped with xenon lamp source (150 W) at an excitation wavelength of 340 nm. X-ray photoelectron spectra (XPS) and valence band spectrum (VB XPS) were measured on an AXIS SUPRA using the reference of C1s (284.6eV) with an excitation source of 150 W Al Kα X-rays. Transmission electron microscopy (TEM, JEOL JEM-ARM300F) measurement was performed to check the microstructure of the samples. Vacancy defects in materials tested by electron paramagnetic resonance using Bruker EMX-plus instruments.
2.6 Photocatalytic and electrochemical test of materials
Electrochemical impedance spectroscopy measurements and Mott-Schottky curve were measured by means of three-electrode system on an electrochemical workstation (PGSTAT302N). 10 mg of samples on glassy carbon electrode (2 mm in diameter) substrate was used as working electrode. The graphite electrode and Ag/AgCl electrodes were used as counter electrode and reference electrode, respectively. The Na2SO4 aqueous solution was used as the electrolyte (0.1 mol L− 1, 150 mL).
Photocatalytic removal of NO was tested in UV light irradiation (40 W, 365 nm) by using PLR-GSPR Atmospheric Gas-Solid Phase Photocatalytic Reaction System and NO analyzer (Thermo Scientific, 42i). The experiments were performed in a continuousflow reactor with starting concentration of NO at 1 ppm levels. The cuboid reactor was made of pyrex glass with a top window made of quartz. 10 mg of samples was deposited on glass and placed in the middle of the reactor. A 40 W UV lamp was placed above the reactor window so that the incident light could irradiate directly on the sample. NO concentration was diluted to about 1 ppm by air and nitrogen, and the flow rate was controlled at 1.15 L/min by a mass flow controller.
The degradation of the acetaminophen (ACT) was tested under an 8 W LED lamp. First, the photocatalyst (10 mg) was mixed with an aqueous solution of ACT (10 mg/L, 50 mL) to achieve adsorption equilibrium, and the resulting solution used to test the photocatalytic properties.
Figure 2 XRD patterns (a), XRD partial enlarged picture (b) and Raman (c) of different materials
Figure 2a shows the XRD patterns of different materials. The XRD pattern of the CS exhibits only one diffraction peak at 20.7°, indicating that the CS is a polymer formed via the dehydration polymerization of glucose. The characteristic diffraction peaks at 2θ = 31.9°, 34.6°, 36.4°, 47.6°, 56.8°, 63.0°, 68.1°, and 69.2° in the XRD spectrum of the prepared ZnO by using CS as template, were attributed to (100), (002), (101), (102), (110), (103), (112), and (201) crystal planes, respectively (JCPDS card no. 31451)[15]. The enlarged XRD patterns of different ZnO materials are shown in Fig. 2b. When ZnO is fabricated using a 0.6 mol/L lithium-naphthalene solution, the diffraction peaks of ZnO shift from 36.45° to 36.42°, indicating that the interplanar spacing increases. When ZnO is fabricated using a 0.8 mol/L lithium-naphthalene solution, the diffraction angle further decreases from 36.42° to 36.38°, indicating that the interplanar spacing continues to increase. which is attributed to the formation of oxygen vacancies. When ZnO is fabricated using 1.0 mol/L lithium-naphthalene solution, the diffraction angle increases from 36.38° to 36.41°, indicating that the number of oxygen vacancies decreases, and a new phase is formed. The new phase is cubic ZnO (JCPDS card no. 21-1486[16]. Figure 2c shows the Raman spectrum of different ZnO samples. Hexagonal wurtzite ZnO exhibits six Raman-active phonon modes, that is, E2 (low), E2 (high), A1 (transverse optical), A1 (longitudinal optical), E1 (transverse optical), and E1 (longitudinal optical). The symmetrical E2 nonpolar phonon modes exhibit two vibration frequencies, which are the structural modes of ZnO. The high-frequency vibrational mode E2 (high) is related to the oxygen atoms in the lattice, and the low-frequency vibrational mode E2 (low) is related to the replacement oxygen and sublattice. The E1 mode is caused by defects in ZnO, such as oxygen vacancies. The Raman spectrum of ZnO exhibits five Raman vibration peaks at 203, 331, 382, 437, and 582 cm− 1, respectively. The 582 cm− 1 peak corresponds to the E1 (longitudinal optical) mode. The 382 cm− 1 peak corresponds to the A1 (transverse optical) mode. The 437 cm− 1 peak corresponds to the E2 (high) mode, and the 203 and 331 cm− 1 peaks correspond to E2 (low) and E2 (high) modes, respectively. The E2 (high) mode of ZnO-0.6 and ZnO-0.8 is weaker and peak intensity is lower than that of pure ZnO. The peak moves from 437 cm− 1 (ZnO) to 435 cm− 1 (ZnO-0.6) and 433 cm− 1 (ZnO-0.8); the peak moves from 331 cm− 1 (ZnO) to 330 cm− 1 (ZnO-0.6) and 328 cm− 1 (ZnO-0.8), indicating that the oxygen atoms in the lattice are captured. The E1 mode (582 cm− 1) of ZnO is related to vacancy defects[17]. The intensity of the E1 mode of ZnO-0.6 and ZnO-0.8 is higher than that of ZnO, indicating that increase of content of oxygen vacancies in the samples. Because of the mixed crystal structure of ZnO-1.0, leading lattice disorder anharmonic phonon–phonon interactions.
Figure 3 TEM images of ZnO (a) and ZnO-0.8 (b); HRTEM images of ZnO-0.8 (c)
Figure 3a shows that ZnO has a spherical structure with a diameter of ~ 100–200 nm. As shown in the TEM image of ZnO-0.8 (Fig. 3b), after treating with lithium naphthalene solution, The size of ZnO-0.8 basically unchange, but the spherical structure was destroyed and irregular structure was observed. In the HRTEM image of ZnO-0.8 (Fig. 3c), 2–3 nm disordered layer is observed on the surface of the material, which is attributed to the presence of oxygen vacancies on the material surface[18]. Because ZnO is immersed in the lithium-naphthalene solution, metallic lithium will rob the oxygen atoms in the ZnO material, resulting in the formation of an amorphous disordered layer on the material surface. The image shows clear lattice fringes; the lattice spacing of 0.281 nm corresponds to the (100) crystal plane of wurtzite ZnO, and the lattice spacing of 0.247 nm corresponds to the (101) crystal plane of wurtzite ZnO[19].
Figure 4a shows the zeta potentials of different ZnO in deionized water. The zeta potentials of ZnO, ZnO-0.6, ZnO-0.8, and ZnO-1.0 samples are + 20.3, -9.1, -10.3 and − 9.6 mV, respectively. The results prove that because of the presence of oxygen vacancies, the surface of the material contains a large number of dangling bonds, carrying more hydroxyl groups. The higher the number of hydroxyl groups on the material surface, the more negative the zeta potential and the higher the photocatalytic activity of the material[20]. The ZnO-0.8 sample exhibits the largest negative zeta potential, which is beneficial for adsorbing more pollutant molecules and enhancing the photocatalytic performance. The hydrodynamic diameter reflects the aggregation state and final photocatalytic activity of the photocatalyst in an aqueous suspension. The hydrodynamic diameters of all samples are shown in Fig. 4b. The particle sizes of ZnO, ZnO-0.6, ZnO-0.8, and ZnO-1.0 are 2142, 2068, 1748, and 1768 nm, respectively. Because the smallest hydrodynamic diameter indicates the highest photocatalytic activity, ZnO-0.8 should have the best photocatalytic activity[21].
The light absorption capability of photocatalysts plays an important role in photocatalytic reactions[22]. Figure 5a presents the UV-Vis diffuse reflectance absorption spectra of different ZnO materials. Compared with pure ZnO, the light absorption range of ZnO treated with the lithium-naphthalene solution is red-shifted and the light absorption intensity is higher, which is attributed to the formation of oxygen vacancies[23]. During the formation of oxygen vacancies, a disordered layer appears on the surface of the material. Oxygen vacancies and disordered layers can be regarded as trapping sites, which prevent the recombination of photogenerated carriers, thereby facilitating electron transfer and enhancing photocatalytic reactivity[24]. Figure 5b illustrates the bandgap energy of different ZnO materials. It demonstrates that the bandgaps of ZnO, ZnO-0.6, ZnO-0.8, and ZnO-1.0 samples are 3.17, 3.14, 3.04, and 3.12 eV, respectively. The presence of oxygen vacancies in ZnO1 − x plays a significant role in expanding its visible light absorption capability. The narrowing of the bandgap is directly associated with the presence of oxygen vacancies[25]. The carrier lifetime is calculated by fitting the time-resolved fluorescence decay curve (Fig. 5c). The average lifetime (τav) can be calculated using Eq. 1, and the relevant parameters are given in Table 1, where B and τ are the relative amplitude and decay lifetime, respectively. The short lifetime (τ1) is the time for electrons to be trapped in the shallow state, and the long lifetime (τ2) is the time for electrons to recombine with holes in the valence band[26].
(1)
The order of short lifetime (τ1) is ZnO-0.8 (0.23 ns) < ZnO-1.0 (0.41 ns) < ZnO-0.6 (0.44 ns) < ZnO (0.82 ns), indicating that electrons can be quickly captured on the surface of the material for promoting the photocatalytic reaction. The order of long lifetime (τ2) is ZnO-0.8 (8.28 ns) < ZnO-1.0 (8.88 ns) < ZnO-0.6 (15.88 ns) < ZnO (21.11 ns), which indicates that the electrons migrated from the valence band to the conduction band rapidly are combined with molecules adsorbed on the surface to form reactive oxygen species[27]. Similarly, τav of ZnO-0.8 (4.2 ns) <τav of ZnO-1.0 (4.3 ns) <τav of ZnO (5.7 ns)<τav of ZnO-0.6 (6.9 ns), showing that in the presence of oxygen vacancies, photogenerated electrons and holes are rapidly separated, thereby ensuring effective photocatalytic performance[28].
| Parameter Sample | B1 | τ1 (ns) | B2 | τ2 (ns) | τav (ns) |
|---|---|---|---|---|---|
| ZnO | 0.322 | 0.82 | 0.004 | 21.11 | 5.7 |
| ZnO-0.6 | 0.449 | 0.44 | 0.009 | 15.88 | 6.9 |
| ZnO-0.8 | 0.666 | 0.23 | 0.018 | 8.28 | 4.2 |
| ZnO-1.0 | 0.412 | 0.41 | 0.016 | 8.88 | 4.3 |
| State Sample | Lattice Oxygen (OL) (peak area / ratio) | Hydroxyl Oxygen (OOH) (peak area / ratio ) | Adsorbed Oxygen (OAds) (peak area / ratio ) |
|---|---|---|---|
| ZnO | 530.0 eV (19610.9 / 50.66%) | 530.7 eV (8914.3 / 23.03%) | 531.8 eV (10187.1 / 26.31%) |
| ZnO-0.8 | 530.2 eV (16620.8 / 38.93%) | 530.7 eV (13461.3 / 31.35%) | 531.7 eV (12615.8 / 29.54%) |
The chemical composition and bond valence information of W-ZnO and ZnO-0.8 were analyzed via XPS. Figure 6a shows the Zn 2p XPS spectra of different photocatalysts. In the Zn 2p XPS spectrum of ZnO, the peaks of Zn 2p3/2 and Zn 2p1/2 are observed at binding energies of 1021.7 and 1044.6 eV, respectively. The difference between the two binding energies is 22.9 eV, which indicates that Zn ions are mainly present as Zn2+ [29]. In the Zn 2p XPS spectrum of ZnO-0.8, because of the presence of oxygen vacancies, the peaks of Zn 2p3/2 and Zn 2p1/2 are blue-shifted to 1021.5 and 1044.5 eV, respectively. Figure 6b and Table 2 show the O 1s XPS spectra and the calculation results of different oxygen peaks of different photocatalysts. In the O 1s XPS spectrum of ZnO, the peak at 530.0 eV corresponds to lattice oxygen (Zn2+-O bond), the peak at 530.7 eV corresponds to hydroxyl oxygen (Zn-OH), and the peak at 531.8 eV corresponds to adsorbed oxygen[30]. The proportions of lattice oxygen, hydroxyl oxygen, and adsorbed oxygen in ZnO are 50.66%, 23.03%, and 26.31%, respectively. Because of the presence of oxygen vacancies, the lattice oxygen content decreases from 50.66–38.93%. At the same time, oxygen vacancies can adsorb a large number of groups, resulting in a significant increase in the content of surface hydroxyl and adsorbed oxygen species, and the ratios of hydroxyl oxygen and adsorbed oxygen increase to 31.53% and 29.55%, respectively. Surface hydroxyl groups and adsorbed oxygen play a very important role in the photocatalytic reaction[31]. Hydroxyl groups can capture photogenerated holes to form hydroxyl radicals with strong oxidative properties, and adsorbed oxygen species can capture photogenerated electrons to form superoxide radicals or singlet oxygen. The abundant formation of these reactive species significantly enhances photocatalytic reactivity. Figure 6c shows the EPR spectra of different samples. EPR spectroscopy was used to provide material defects,The EPR spectra of W-ZnO and ZnO-0.8 show a single lorentz line with g-value of ~ 2.002, indicating the presence of oxygen vacancy defects in both materials[32]. The resonance peak intensity of ZnO-0.8 is higher than that of W-ZnO, indicating that ZnO-0.8 has more oxygen vacancy defects. The bandgap is decreased, the light absorption ability is enhanced, and the position of the “color center” is changed of ZnO-0.8. These changes cause the color of ZnO change from white (W-ZnO) to gray (ZnO-0.8), the optical photograph of ZnO is shown in Fig. 6d.
To better analyze the charge transfer ability of photocatalysts, electrochemical impedance spectroscopy (EIS) was performed, and the results of different materials are shown in Fig. 7a. The high-frequency arc in the Nyquist plot represents the charge transfer process, and the radius of the quarter circle represents the charge transfer resistance[33]. resulting that the interface resistance of ZnO-0.6, ZnO-0.8, and ZnO-1.0 is lower than that of W-ZnO, and ZnO-0.8 has the lowest resistance. When the content of oxygen vacancies increases, oxygen vacancies may become electron–hole recombination centers, thereby increasing material resistance. This indicates that an appropriate amount of oxygen vacancies can increase the charge separation efficiency, reduce the interface resistance, and facilitate the rapid separation of photogenerated electrons and holes[34]. The valence band edges of different materials were determined via VBXPS spectroscopy. Figure 7b shows that the valence bands of W-ZnO, ZnO-0.6, ZnO-0.8, and ZnO-1.0 are 2.60, 2.53, 2.58, and 2.39 eV, respectively. According to the measured bandgap values of the materials, the conduction band positions of W-ZnO, ZnO-0.6, ZnO-0.8, and ZnO-1.0 are − 0.57, -0.61, -0.46, and − 0.73 eV, respectively. The energy band structures of different materials are shown in Fig. 7c. Compared with W-ZnO, the conduction band of ZnO-0.8 has a positive shift by ~ 0.1 eV. It shows that the conduction band of ZnO-0.8 with oxygen defects is bent downward, and the potential barrier is smallest overcome in the photocatalytic reaction. ZnO-0.8 with surface oxygen vacancies exhibits better photocatalytic performance, and increasing the concentration of oxygen vacancies on the material surface within an appropriate range can enhance the reactivity of the photocatalyst [35].
Figure 8a shows the NO degradation curves of W-ZnO, ZnO-0.6, ZnO-0.8, and ZnO-1.0 under ultraviolet light irradiation. The degradation rate of W-ZnO is only 4.4% under ultraviolet light irradiation. When oxygen vacancies are introduced into the material, the photocatalytic activity of the material increases. The NO degradation rate of ZnO-0.6 is 11.2%, and the NO degradation rate of ZnO-0.8 is 54.3%. When the concentration of lithium-naphthalene solution is 1.0 mol/L, a new phase is generated in the product obtained after the precursor was treated, resulting in a decrease in photocatalytic activity, and the NO degradation rate of ZnO-1.0 is 14.5%. The efficiency of degrading NO in the oxygen-deficient ZnO-0.8 sample is ~ 12 times that of W-ZnO, and the introduction of oxygen vacancies significantly improves the NO degradation effect. As shown in Fig. 8c, the ACT degradation rate within 2 h was only 1% by the absence of photocatalysts, whereas those of W-ZnO, ZnO-0.6, ZnO-0.8, and ZnO-1.0 within 2 h were 54.2%, 60.4%, 78.4% and 67.4%, respectively. The tested ZnO-0.8 was washed with water and centrifuged and then continued to be used in the photocatalytic performance test. It was found that it still maintained good photocatalytic degradation performance and cycle stability(Fig. 8b and 8d). Compared with W-ZnO, the material with surface oxygen vacancies exhibits higher photocatalytic efficiency, and oxygen vacancies play a role of electrons trap, inhibiting the recombination of photogenerated electrons and holes. The moderate concentration of oxygen vacancies improves the photocatalytic activity of the sample.
The photostability of ZnO-0.8 was evaluated through a recovery study of photocatalytic NO removal across five consecutive cycles, as illustrated in Fig. 8b and detailed in the experimental section. After each cycle, the photocatalyst was repeatedly washed with distilled water and alcohol, centrifuged, reclaimed, and dried at 60℃ for four hours. Following the fifth cycle, an XRD analysis of ZnO-0.8 was conducted (refer to Fig. 9). The results verified the photochemical stability of ZnO-0.8, indicating no significant alteration in its crystal structure.
Figure 10 shows the schematic of the energy band structure of the gray oxygen-deficient ZnO, the conduction band, valence bands and bandgap of ZnO-0.8 is -0.46, 2.58 and 3.04 eV, respectively. The VB of ZnO-0.8 is higher than the standard redox potential of ·OH/H2O (+ 2.27 eV vs. NHE)[36]and ·OH/OH− (+ 1.99 eV vs. NHE)[37]. The CB of ZnO-0.8 is more negative than the standard redox potential of O2/·O2− (-0.28 eV vs. NHE)[38]. Therefore, a potential mechanism is that O2 combineswith e− to form ·O2−, and water molecules and hydroxide ions are oxidized by h+ to form hydroxyl radicals. Under the action of ·OH and·O2−, NO and ACT are ultimatelyoxidized and degraded into NO2, NO3− and CO2, H2O. The probable photocatalytic conversion pathway of NO is represented in Eqs. (2)-(8).
ZnO → ZnO* (e− CB/h+ VB) (2)
O2 (ads) + e− CB→ * O2− (3)
* O2−+H+→ HO2 (4)
H2O (ads)/OH− (ads) + h+ VB → *OH (ads) (5)
NO (ads)+*OH (ads) → HNO2 (ads) (6)
HNO2 (ads)+ *OH (ads) → NO2(ads) (7)
NO2(ads) + *OH (ads) → HNO3 (ads) ↔ NO3–(ads) (8)
Prior to the reaction initiation, when the NO concentration stabilized at 1 ppm, an adsorption-desorption equilibrium was achieved between the mixture and the photocatalyst. Upon exposure of all photocatalyst powders to light, there was a sharp decrease in the NO concentration. Influenced by various active species, the product NO2 desorbed from the photocatalyst surface, rejuvenating its activity and initiating a new photocatalytic cycle. The products NO3− and HNO3 progressively accumulated, occupying the active sites of the catalytic reaction and causing a decline in the degradation rate until the catalyst deactivated. The NO removal capability of the prepared oxygen-deficient ZnO photocatalyst was compared with previous studies and summarized in Table 2, showing that the oxygen-containing defect ZnO exhibits a strong purification capacity for NO.
| Photocatalysts | Light source | Photocatalyst Dosage (mg) | Efficiency (%) | Ref. |
|---|---|---|---|---|
| gray ZnO | 365 nm | 10 | 54.3 | this work |
| ZnO | 365 nm | 80 | 40.5 | 39 |
| ZIF−8-derived ZnO | LED | 50 | 36 | 40 |
| Bi4O5I2/CuFe2O4 | Xenon lamp | 150 | 44 | 41 |
| TiO2/Ti3C2 MXene/g-C3N4 | Xenon lamp | 100 | 56 | 42 |
| Au@oxygen-vacancies-rich Bi4Ti3O12 | Xenon lamp | 100 | 48 | 43 |
| 2D/2D α-Fe2O3/g-C3N4 | Xenon lamp | 100 | 60.8% | 44 |
In this paper, ZnO1 − x photocatalysts containing rich surface-oxygen-defect were successfully prepared by “oxygen-atom capture” method, and the effect of the concentration of the reducing agent on the performance of the photocatalyst was studied. W-ZnO and ZnO−0.8 exhibit the color of white and gray, respectively. TEM, XPS, and EPR results prove the formation of oxygen vacancies and a disordered layer on the surface for ZnO−0.8. In the presence of oxygen vacancies, the photoresponse range is enhanced. and the separation efficiency of photogenerated electrons and holes is improved. Therefore, ZnO−0.8 shows the efficiency of NO removal is ~ 12 times greated than that of W-ZnO. Studies have reported that surface-oxygen-vacancy-rich materials exhibit higher photocatalytic efficiency than materials without surface defects. Oxygen vacancies, acted as capturer of electrons, inhibiting the recombination of photogenerated electrons and holes. Increasing the appropriate concentration of oxygen vacancies on the surface of the material can enhance its photocatalytic activity.
Author Contribution
Xiang BI , Gao-Hui DU and Li-Zhong WANG wrote the main manuscript text and Dong ZHAO and Hao-Yu XU prepared figures 8-10. Yue QIU and Le DAI prepared literature research and conclusion verification. All authors reviewed the manuscript.
Acknowledgement
This research was funded by General Research Project of Basic Science (Natural Science) in Higher Education Institutions of Jiangsu Province (No. 23KJB430035) and excellent teaching team in the Blue Project of Jiangsu Province universities.
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No competing interests reported.