Triethanolamine interface modification of crystallized ZnO nanospheres enabling fast photocatalytic hazard-free treatment of Cr(VI) ions

The hazard-free treatment of Cr(VI) ions is critical in modern industry. Photocatalytic reduction and detoxification are high-potential strategies. Photocatalysts with high efficiency, low cost, environmental friendliness, and easymass products are required. In this study, we report a facile strategy using triethanolamine (TEOA) as a capping ligand to modify the surface of ZnO nanospheres. The experimental and theoretical results reveal that the presence of TEOA on the surface was beneficial for electron enrichment on Zn and hole transfer to TEOA and can photocatalytically reduce Cr(VI) by almost 100% in 5min. The photocatalytic property of TEOA-modified ZnO could be renewed by the reabsorption of TEOA in solution. The great sedimental properties benefit photocatalyst recycling and renewal, which also provides high potential for water pollution and environmental emergency responses applications.


Introduction
Water pollution has threatened most lives on Earth. Addressing pollution by volatile organic compounds, nutrient pollutants, and heavy metals has gained the most attention. Of particular interest are heavy metals that are generally toxic and can cause major lifelong issues, especially for young people, including the hypofunction of vital organs and embryo teratism, as reported by Jomova and Valko [1]. Additionally, Islam et al. reported the severe situation of heavy-metal pollution in the surface water and sediment of developing countries [2]. Cr(VI) has been listed as the most toxic pollutant in water, and its removal has been a concern of researchers [3]. It comes extensively from electroplating, leather, pigments, metallurgy, and so on [4]. Due to its high solubility, it can be easily absorbed through the mouth, respiratory tract, or skin and can interfere with enzyme systems, even causing carcinogenic, teratogenic, and mutagenic effects, as discussed by Jaishankar et al. [5]. Fortunately, as discussed by Barrera-Diaz et al., these are the toxicities of Cr(VI) instead of Cr(III), Cr(IV), and metallic Cr [6], which avoid producing secondary pollution of hazardous wastes containing hightoxicity Cr(VI). Zhang et al. reported a novel cellulose hydrogel coating with nanoscale Fe 0 for simultaneous Cr(VI) adsorption and reduction [7]. Therefore, the key to the treatment of wastewater containing Cr is to reduce and detoxify the content of Cr(VI). The most effective route is reducing Cr(VI) into Cr(III) through a chemical reaction and further removing Cr(III) by the precipitation process.
Generally, to remove these harmful heavy metal elements, multifarious approaches have been explored including adsorption, precipitation, membrane separation, electrodialysis, and so on. These methods abate pollution while limited by efficiency or cost. In recent years, Mezzenga et al. summarized sustainable technologies for water purification and heavy-metal removal [8]. As reviewed by Wang et al., photocatalysis has attracted much interest in the hazard-free treatment of toxic heavy metal ions [9]. During the photocatalytic process, photoinduced electrons can participate in the reduction of highvalent metal ions into low-valent ions, even into metallic materials. For example, Zhang et al. realized enhanced Cr(VI) reduction with Ag-modified BiOCl nanosheets [10]. Bian et al. achieved the enhanced photocatalytic reduction of Cr(VI) to Cr(III) using carbon dot-coupled TiO 2 mesocrystals [11]. A UiO-66-NH 2 (Zr/Hf) metal-organic framework (MOF) was also used for the photocatalytic reduction of Cr(VI) and exhibited robust properties [12].
Later, a Z-scheme UiO-66-(COOH) 2 /ZnIn 2 S 4 hybrid decorated with a nonnoble MoS 2 cocatalyst was proposed for photocatalytic hydrogen production and Cr(VI) reduction [13]. Metal-free semiconductors, such as graphitic carbon nitride-based photocatalysts, also possess great properties for Cr(VI) reduction [14]. The photocatalytic treatment of Cr(VI) has been considered a high-potential route to reduce Cr(VI) pollution.
ZnO is a semiconductor having a direct band gap and good photocatalytic activity, which enables water decontamination. Therefore, as a promising photocatalyst, ZnO has been widely used for dye degradation, water splitting, and photocatalytic CO 2 reduction reactions due to its high efficiency [15]. Pung et al. proposed the mechanisms of heavy-metal ion removal by ZnO particles, confirming the low cost and high efficiency of ZnO in Cr(VI) removal [16]. Lee et al. studied the photocatalytic reduction of Cr(VI) with ZnO nanoparticles [17]. A ZnO@ZIF-8 heterostructure photocatalyst has achieved a satisfactory selective reduction of Cr(VI) [18]. Similar to the partial reaction of water splitting for H 2 production, the consumption of photoinduced electrons would enrich photoinduced holes. Then, the photoinduced electrons would be significantly suppressed. To address this issue, hole sacrificial agents, including glucose, methanol, and lactic acid, have been incorporated into the reaction system to realize reaction equilibrium [19]. However, the added pollution needs to be removed further. Mostly, the separation of photocatalysts from solution is attractive, especially for practical applications. Jiao et al. reported the hollow structured photocatalyst with high photocatalytic property and recycle property [20,21]. Briefly, highly efficient photoinduced charge separation and good recycling properties from solution should be important for industrial applications.
Herein, in the current work, spherical ZnO was used for the photocatalytic hazard-free treatment of Cr(VI). The photocatalytic property and the adsorption property of ZnO were used to reduce high-valent Cr and precipitate the reduced Cr products. Triethanolamine (TEOA) was used for the surface modification of ZnO to adjust the photocatalytic properties and settlement characteristics and to form the ZnO-TEOA composite (ZOT). Moreover, TEOA plays a great role in surface adsorption as a companion hole sacrificial agent, resulting in the durability of the photocatalytic reduction of Cr(VI). ZOT would be recovered with TEOA with great photocatalytic properties. The graphical abstract of the design is shown in Figure 1. We reported a one-pot synthesis of TEOA-modified ZnO spheres that possess high activity for the photocatalytic reduction of Cr(VI) and good recoverable properties. This design possesses great potential for the practical application of the photocatalytic hazard-free treatment of Cr(VI).

Synthesis of ZnO nanospheres with TEOA
ZnO nanospheres were synthesized using a simple hydrolysis process [22]. Typically, 0.5 g of zinc acetate was added to 100 mL of purified water, and then, 5 mL of TEOA was added to the solution. After magnetic stirring for half an hour, the solution was placed in a water bath at 90°C for 2 h. The obtained ZnO was washed with deionized water and then with ethanol 3 times successively. After drying at 60°C for 12 h, the ZnO-TEOA (ZOT) sample was obtained. After being calcined at 350 and 500°C for 2 h, ZOT-350 and ZOT-500 samples were obtained.

Photocatalytic hazard-free treatment of Cr(VI)
In a typical Cr reduction experiment, the K 2 Cr 2 O 7 concentration was 30 mg/L, and a 300 W Xe simulated sunlight lamp (100 mW/cm 2 ) was used as the light source. Different amounts of ZnO sample were mixed with 50 mL of salt solution, and then, the solution was kept in the dark to reach absorption equilibrium in ∼10 min. The concentrations of Cr(VI) in the solution were measured by UV-Vis spectrophotometry before and after photocatalysis for a certain time. The characteristic absorption peak at 375 nm of Cr(VI) was used to measure concentrations.

Characterization
The X-ray diffraction (XRD) patterns of all samples were obtained by a Bruker D8 Advance powder X-ray diffractometer (German) with Ni-filtered Cu Ka radiation, and data were collected in the 2θ range between 10°and 90°w ith 0.04°step intervals. Scanning electron microscopy (SEM) images were taken by a Hitachi S-4800 field-emission scanning electron microscope (Japan). The particle diameter distribution was analyzed by Nano Measurer 1.2 software (China). High-resolution transmission electron microscopy (HRTEM) was performed using a JEOL JEM 2100 microscope (Japan) operated at 200 kV. Fourier transform infrared (FTIR) spectra were recorded by a Thermo Nicolet NEXUS 670 spectrometer (USA). A spherical aberration correction transmission electron microscope (FEI Themis Z, USA) was operated at 200 kV to obtain HADDF STEM images. X-ray photoelectron spectroscopy (XPS) was performed with a Thermo ESCALAB 250XI spectrometer (USA), and electron paramagnetic resonance (ESR) data were recorded by a JEOL JES-X320 instrument (Japan). The UV-Vis spectrophotometry used for the photocatalytic property study is UV-6100 from Shanghai Metash Instruments Co., Ltd (China).

Theoretical simulation
The experiment results performed great photocatalytic properties with the surface modification of TEOA. Therefore, we took a theoretical simulation to analyze the effect on the photocatalytic improvement of the interaction between ZnO and TEOA. In this study, we employed a (2 × 2) supercell with the facet of a three-atomic layer to represent bulk ZnO (101). The vacuum gap along the z-direction to the surface was 21 Å thick to prevent periodic interference of the adsorption structure and electron properties in the z-direction. The structural model had 72 atoms, including 24 Zn atoms and 24 O atoms. The DMol3 module in Materials Studio (Accelrys Inc.) was used for structure optimization and property calculations of the ZnO (101) facet [23]. The Perdew-Burke-Ernzerh (PBE) version of the general gradient approximation (GGA) was used as an exchange-correlation functional [24]. The related calculation parameters were set for the calculation of atomic charge and electrostatic potential properties. A double numerical plus polarization (DNP), as the basis set, was used to describe the valence orbital of all the atoms. A k-point mesh of 4 × 5 × 4 dimensions was chosen and employed in all the calculations. The self-consistent field (SCF) density convergence was 1 × 10 −6 Hartree, and a smearing setting value of 0.005 Ha was also used for acceleration.
3 Results and discussion 3.1 Morphology and structure of the spherical ZOT The morphology and structures of the as-obtained ZOT were studied with SEM and TEM images (Figure 2). The as-synthesized ZOT particles are spherical with diameters around 350-550 nm analyzed by Nano Measurer 1.2 from the SEM image. The surface of the sphere is not smooth, which implies the assembled structure of the ZOT sphere. The TEM images ( Figure 2c and the inset) show a morphology consistent with the SEM image (Figure 2a and b). The spheres are the aggregation of nanoparticles in sizes of ∼20-30 nm. As is well known, the nanoparticles possess high surface energy which will drive them to assemble. The electrical double layer of ZnO is negative while the TEOA modification would reduce the electrical double layer. Therefore, the ZnO nanoparticles would prefer to assemble into large spheres with TEOA. Note that the HRTEM image (Figure 2d) shows good crystallinity inside the nanoparticles. The observed d-spacings of 2.476 and 2.814 Å can be assigned to the (101) and (100) faces of wurtzite ZnO. There is a layer of defects at the edge of the nanoparticles. The structure of the sample was further characterized using XRD, as shown in Figure 2e. All the peaks can be assigned to the diffraction peaks of the hexagonal wurtzite phase of ZnO (JCPDS No. . There is no crystalline diffraction peak corresponding to other phases or compounds. The crystalline particle size is approximately 26 nm, as determined by the Scherrer formula.

Modification of TEOA in the ZnO spheres
The FTIR spectra and bonding energy of ZOT were analyzed to study the effect of TEOA ( Figure 3). As shown in the FTIR spectra (Figure 3a), the band at ∼3,370/cm is assigned to the surface hydroxyl groups for all the samples. The higher intensity of the ZOT sample should be due to the hydroxyl groups from TEOA [25]. Moreover, the peaks located at ∼2,966 and 2,845/cm are related to the C-H symmetric and asymmetric stretching vibrations of methylene. The peak at ∼1,400/cm is the C-H bonding vibration of methylene, and the peaks at ∼1,071 and 1,025/cm are from C-O stretching vibrations in TEOA. Compared with that of individual TEOA, the C-H symmetric and asymmetric stretching vibrations redshift by more than 11/cm, and the C-O stretching vibrations blueshifted by more than 3/cm, which would be induced by the coordination interaction between the lone pair electrons of N atoms in TEOA and Zn atoms as well as steric hindrance [26]. No C-H stretching vibration of methylene or C]O stretching vibration is detected from ZOT-350 and ZOT-500. This indicates that the bonded TEOA was either thermally desorbed or decomposed. As shown by the XPS spectrum, a weak peak at ∼399 eV corresponding to N 1s is observed (Figure 3b). TEOA is the only material with N in this system, thus indicating the presence of TEOA in the ZOT sample. Moreover, for the O 1s XPS spectrum (Figure 3c), the peak at ∼529.29 eV attributed to Zn-O bonds is dominant. More peaks are analyzed at ∼530.2, 531.2, and 532.0 eV, which can be ascribed to C-OH, Zn-OH, and C]O, respectively. The XPS spectrum of Zn 2p is shown in Figure 3d. A couple of peaks at 1021.9 and 1044.9 eV are present, consistent with the reported values for Zn 2+ [27]. The presence of no other detectable peaks indicates the low defect concentration in the ZOT sample.

Photocatalytic hazard-free treatment of Cr(VI) with ZOT spheres
The photocatalytic performance of the as-synthesized samples was evaluated in 50 mL of 30 mg/L K 2 Cr 2 O 7 solution ( Figure 4). Figure 4a shows that Cr(VI) can be hazardfree treated in 5 min at a catalyst concentration of 1 mg/mL.  Figure 4b. There were linear relationships between the two factors, with a correlation coefficient R 2 better than 0.9. This indicates that the reaction conforms to the pseudo-first-order kinetics model. The kinetic rate constant (k) values are 0.04, 0.13, 0.22, and 0.60 with 10, 20, 30, and 50 mg of ZOT catalyst, respectively. These results confirm that the reduced concentration of Cr(VI) in the solution is highly related to the ZOT photocatalyst. Moreover, after the photocatalytic treatment, the XPS spectrum of Cr on the ZOT spheres was obtained (Figure 4c). The peak at 574.4 eV indicates the existence of metallic Cr, while the peak located at 576.3 eV belongs to Cr(III) from Cr 2 O 3 . The peak centered at 577.7 eV could be assigned to Cr(III) from Cr(OH) 3 [28].
No more Cr(VI) is detected on the surface of the ZOT photocatalyst, which indicates the complete reduction of Cr(VI) during the photocatalytic treatment. EDS mapping images show that Cr ions are adsorbed on the surface of ZOT nanospheres (Figure 4d). Therefore, the sample nanospheres could not only reduce heavy metals into low-valent states but also potentially remove heavy metals by their adsorption properties. TEOA works as a hole sacrificial agent during photocatalysis. The TEOA in ZOT would be consumed and shows a decrease in the photocatalytic property. As shown in Figure 5a, the amount of Cr(VI) photocatalytically reduced with ZOT decreases from ∼100 to ∼20% in 5 min. The sample with TEOA in the solution shows a stable property of ∼90% in 5 min. This confirms the effect of TEOA as a hole sacrificial agent, which can significantly improve the photocatalytic reduction property of ZnO. The nanoparticle-assembled ZnO spheres benefit the absorption of TEOA. Therefore, the photocatalytic property of ZOT would be renewed by the absorption of TEOA. As shown in Figure 5b, after 5 cycles, the reduction rate of Cr(VI) decreases from ∼90 to ∼10%. Then, the recycled ZOT was dispersed in the TEOA solution at the same concentration of the synthesis. Then, the ZOT was filtrated and used for the next photocatalytic treatment of Cr(VI). The reduction rate of Cr(VI) was renewed to ∼60% and then decreased to ∼45 and ∼30% for the next 2 cycles. Moreover, after 5 cycles, the Cr(VI) reduction rate of the renewed ZOT with TEOA is ∼10% again (Figure 5c). Fortunately, after renewal with TEOA again, the photocatalytic property recovers to ∼60%, which is the same as that of the first renewal process. It possesses the same property in that the photocatalytic property decreases from ∼60 to ∼45% and ∼30%. The results indicate that the property can simply be renewed by the absorption of TEOA. Moreover, the authors compared this photocatalytic property with other works as shown in Table 1. The ZOT performs the best property. Figure 5d shows the TG curve of ZOT. There is ∼1.94% weight loss between 147 and 386°C, which is assigned to the absorbed TEOA in ZOT. ZOT was calcined at 350 and 500°C to remove the TEOA in ZOT, according to the TG curve. Figure 5e shows the photocatalytic reduction of Cr(VI) with ZOT, ZOT-350, and ZOT-500. ZOT possesses the best property, while ZOT-500 possesses the worst property. The photocatalytic reduction kinetics were analyzed by the plots of ln(C/C 0 ) versus time in Figure 5f. The kinetic rate constant k is 0.354/min for ZOT, which is twice or three times the corresponding values of ZOT-350 and ZOT-500, 0.176 and 0.103/min, respectively. These results indicated that the

Mechanism of the various photocatalytic properties of ZOT spheres
The photocatalytic mechanism was investigated based on the ESR spectra ( Figure 6). As shown in Figure 6a, there was no detected ESR peak for the ZOT sample in the dark. With light irradiation, there is an obvious peak with a g value of 1.9560 (g 1 ). This should be attributed to the oxygen vacancy (Vo+) [35] because it is a light-sensitive center and can only be observed under light irradiation. A similar phenomenon has been detected in other photocatalytic processes [36,37]. Notably, the ESR peak intensity increased with continuous light irradiation (Figure 6b). The reason for this Vo+ increase under light irradiation would be due to the existence of TEOA. Because TEOA is a reductant, it would benefit the formation of Vo+ during the photocatalytic process. More photoinduced electrons would be released and benefit the photocatalytic reduction process. After removing TEOA by calcination, the ESR spectra are recorded (Figure 6c). A new peak with a g value of 1.9981 (g 2 ) is detected in both the ZOT-350 and ZOT-500 samples and can be assigned to the unpaired electron trapped in a surface oxygen ion vacancy or the Zn + ion [38]. Notably, the g 1 /g 2 intensity ratios of the two samples are different. With increasing calcination temperature, g 1 becomes more significant. According to the TG curve of ZOT, TEOA quickly oxidizes at temperatures over 380°C. The reduction of ZnO at 350°C is weaker than that at 500°C. Therefore, the surface defect concentration of the particles is lower at 350°C than at 500°C. This also implies that the TEOA in the as-synthesized ZOT is at the interfaces between particles. Interestingly, no surface defect g 1 ESR signal was observed from ZOT, even under light irradiation, which was probably due to the capping effect of TEOA. As shown in the XRD pattern (Figure 1e), the main crystal plane is the (101) plane. Therefore, the electrostatic potentials between TEOA and the (101) plane of ZnO were analyzed by the DMol3 module ( Figure 7). The Zn atom possesses a +0.934 eV Mulliken charge under the initial conditions of the ZnO surface, as shown in Figure 7a. During light irradiation, there were oxygen vacancies in ZnO. The existence of a single oxygen  vacancy can influence the Mulliken charge on the Zn atom, decreasing it to +0.851 eV (Figure 7b). This confirms the effect of light irradiation on the electron structure of ZnO. When a N atom from TEOA is adsorbed at the oxygen vacancy, the Mulliken charge of Zn further decreases to +0.773, as shown in Figure 7c. The coordination interaction between an unshared electron pair of a N atom in TEOA and surface Zn atoms can affect the electron density distribution around the bonded Zn atom and the charge state on each atom. The change in Mulliken charge confirms the interaction between TEOA and ZnO under light irradiation. This result can support the discussion of the photocatalytic process with TEOA. Obviously, the transfer of holes to TEOA also makes the hole sacrificial agent more efficient, improving the photocatalytic reduction of Cr(VI) with ZOT.

Conclusion
ZnO nanospheres modified by TEOA were synthesized via a simple hydrolysis process. The as-prepared ZnO sample was modified by TEOA. It possessed a high photocatalytic reduction efficiency for the hazard-free treatment of Cr(VI) ions in water. The full reduction of Cr(VI) required only ∼5 min. The photocatalytic property of TEOA-modified ZnO could be renewed by the reabsorption of TEOA in solution. Surface modification with TEOA promoted the electron density enhancement of atoms adjacent to the TEOA molecule, which makes the hole sacrificial agent effect of TEOA work efficiently. The great sedimental properties benefit photocatalyst recycling and renewal, which also provides great potential for practical applications. A system that can realize the quasi-continuable photocatalytic hazard-free treatment of Cr(VI) ions with ZOT should be designed based on the advantages and properties of ZOT spheres.