Ball-Milling Preparation of La³⁺/TiO₂ Photocatalyst and Application in Photodegradation of PVC Plastics

Abstract

The fact that the use of a large number of plastic products has brought serious pollution to the environment has always been the focus of global attention. The development of photocatalytic degradable plastics is one of the effective ways to solve the problem of “white pollution”. In this work, La³⁺ modified TiO₂ nanoparticles were prepared by ball milling and characterized. La³⁺/TiO₂ was mixed with Polyvinyl chloride (PVC) plastic to make a photodegradable composite film, and the photodegradation performance and mechanical properties of films were evaluated. The photodegradable films were characterized by infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), and scanning electron microscopy (SEM). After 30 h UV irradiation, the weight loss rate of the PVC was only 2.12%, while that of the TiO₂/PVC reached 8.94%. The accelerating of the degradation rate was due to the mixing of TiO₂ into PVC. As for the La³⁺/TiO₂/PVC composite film, when the mass percentage of La³⁺/TiO₂ was 1.5%, the weight loss rate of La³⁺/TiO₂/PVC sample reached a maximum of 17.78%, which was eight times the degradation rate of PVC and two times the degradation rate of TiO₂/PVC. The La³⁺/TiO₂/PVC film showed good photodegradability. La is a transition metal element with a special 4f electronic structure. The reaction mechanism of photodegradation of PVC by the interaction of La³⁺ and TiO₂ were discussed.

1. Introduction

PVC is one of the five biggest general-purpose plastics, having the characteristics of non-combustibility, corrosion resistance, chemical resistance, insulation, and good mechanical properties. Recently, PVC was widely used in industry, products, daily necessities, building materials, environmental protection materials, etc. [1,2]. However, with the increasing use of one-off plastic products, the pollution resulting from the plastic waste has become more and more serious [3,4]. Plastic pollution has always been a hot spot of global concern. The treatment methods are incineration, landfill [5], recycling, chemical degradation, and the production of degradable plastics [6,7]. Photodegradable plastics have been widely studied, which is an ideal way to solve the “white pollution” [8,9].

Photocatalysis is a green-friendly technology that combines abundant solar energy resources with environmental cleanup and resource reuse and has received widespread attention [10]. TiO₂, ZnO, CdS, and Fe₂O₃ are common semiconductor materials [11]. Because of its low cost, non-toxicity and its photocatalytic activity for the decomposition of various environmental pollutants, TiO₂ is widely used in the field of photodegradable plastics [12,13]. At the present stage, the main modification method of TiO₂ is to regulate the structure and composition of the catalyst, including the regulation of TiO₂ crystal structure and defects, grain size, and energy band position. TiO₂ can be doped with metal, non-metal, precious metal, and other elements, and can also be modified by surface photosensitization and semiconductor composite [14,15]. The modification of TiO₂ is aimed at expanding the absorption range of TiO₂ to sunlight, promoting the efficiency of charge separation, inhibiting the recombination of photo-generated carriers and improving the stability of catalyst [16]. Rare earth is a type of transition metal element with a special 4f electronic structure. Lanthanum is second only to cerium in rare earth elements and is rich in resources. Therefore, rare earth doped modified TiO₂ has been widely studied to improve photocatalytic activity [17,18,19,20].

Due to its simple operation and low cost, the ball milling method is an effective method in the industry to efficiently mix solids and solid reactants by using the rotation or vibration of a ball mill [21]. The rare earth modified TiO₂ photocatalyst made by the ball milling method applied to the degradation of PVC waste plastic is of great significance for solving the problem of environmental pollution from the viewpoint of practical application.

In this paper, La³⁺/TiO₂ photocatalyst was prepared by the ball milling method. The La³⁺/TiO₂/PVC composite film was prepared by introducing La³⁺/TiO₂ into PVC plastic. After being compared with the PVC and TiO₂/PVC films, the La³⁺/TiO₂/PVC film was characterized and its photodegradability was also studied. Moreover, the reaction mechanisms of the photodegradation of PVC by the interaction of La³⁺ and TiO₂ were discussed.

2. Experimental

2.1. Materials

TiO₂ (Analytical reagent, the average particle size = 19.8 nm) was purchased from Komiou Chemical Reagent Co., Ltd., Tianjin, China. The PVC polymer was produced by Aladdin (K-Value: 62–60). La₂O₃ was provided by Hunan Rare Earth Metal Materials Research Institute. N,N-dimethylformamide (AR), abbreviated as DMF, was produced by the Damao Reagent Company, Tianjin, China. Hydrochloric acid (HCl, 36.0–38.0%) was provided by Fusheng Industry Co., Ltd., Shanghai, China.

2.2. Characterization

Using Shimadzu UV-2600 ultraviolet-visible diffuse reflectance instrument, with BaSO₄ as a reference, the ultraviolet-visible absorption spectrum of the films was measured. Under the condition of vacuum 5 × 10⁻⁴ Pa, a JEOL Ltd. JSM-7100F thermal field emission scanning electron microscope (SEM, Electronics Co., Ltd., Japan) was used to test the surface morphology of the films, and the voltage was 5.0 kV. The samples were characterized by Thermo Scientific Nicolet 6700 Fourier transform infrared spectroscopy (FT-IR, Thermo Fisher Instruments Co., Waltham, MA, USA). The crystal phase of the materials was detected on an X-ray diffractometer (XRD, Pruck instruments co., Germany) with Cu/K radiation, and the diffraction intensity was recorded in the 2θ range of 10–80° at a scanning speed of 5°/min. The digital display tensile testing machine was used to test the mechanical properties of the film, and purchased from Guangdong Zhongye Jingke Instrument Equipment Co., Ltd., Guangdong, China. The film was cut into a rectangle of 50 mm × 10 mm and fixed on the digital tensile testing machine with an initial spacing of 20 mm, running at a 50 mm/min. The thickness of the film was measured 5 times with the NSCING electronic digital micrometer, and the average value was taken.

2.3. Preparation of the La³⁺/TiO₂ Photocatalyst

The La³⁺/TiO₂ photocatalyst was synthesized by a ball milling method, according to the following procedure: Firstly, 5 g of TiO₂, six Φ 10 and twenty Φ 6 agate balls were added to the agate jar. After being ground for several minutes, La₂O₃ with the corresponding mass percentage of 0.6% and a small amount of deionized water were added and heated to 80 °C. Then, the hydrochloric acid with the volume ratio of 1:1 was slowly added into the above solution and stirred until the solid was dissolved completely. Then, the deionized water was added to the total volume of 10 mL. The speed of the ball mill was set to 500 r/min, and the ball milling time was 4 h. The ball-milled slurry was washed with water, centrifuged, dried, and ground to obtain the La³⁺/TiO₂ photocatalyst.

2.4. Preparation of Film and Photocatalytic Experiments

The preparation process and photocatalysis experiment of the La³⁺/TiO₂/PVC film are shown in Figure 1. All raw materials need to be vacuum dried. The doping content of the La³⁺/TiO₂ photocatalyst in the film were as a mass percentage of 0.1, 0.5, 1, 1.5, and 2, respectively. Hence, 2 g of PVC and 15 g of organic solvent DMF were added separately, heated, and stirred for 1 h. The mixed solution was ultrasonicated for 10 min so that the photocatalyst could be better mixed with the polymer. At room temperature, the mixed solution was dropped on the automatic film scraping machine and run at constant speed to prepare a film with a thickness of 150 μm. The flat plate was removed from the automatic film scraping machine and immersed in deionized water to obtain a composite film. The composite film was placed in the XPA-System photochemical reactor for light irradiation. The light source was a 300 W medium-pressure UV lamp with main wavelength of 365 nm. and the distance between the light source and the sample was 20 cm. The sample was taken out every 5 h, and the total illumination time was 30 h. The sample was taken out for storage and performance testing. The weight loss calculation formula is presented in Equation (1):

Weightloss (%) = W₀ − W_t/W₀ × 100%

(1)

where W_t is the weight of film at after illumination and W₀ is the initial weight of film.

3. Results and Discussion

3.1. Characterization of La³⁺/TiO₂ Photocatalyst

Figure 2A shows the XRD spectrum of La³⁺/TiO₂ and raw TiO₂ after ball milling. Compared with TiO₂, the 3 mol% La³⁺/TiO₂ still maintained the anatase crystal after doping, and no other peaks of rutile crystal was observed. This suggested that La³⁺ doped TiO₂ could inhibit the appearance of rutile crystal by ball milling method [22]. Moreover, no obvious characteristic peaks of the La³⁺ were found, possibly due the small content of La³⁺ doped into the TiO₂ lattice. Furthermore, the ionic radius of La³⁺ was larger than that of Ti⁴⁺, so these doped ions might have difficulty in entering the TiO₂ lattice. The peak position of the (101) crystal plane in La³⁺/TiO₂ appeared red-shifted to 25.4°, indicating that La³⁺ could be highly dispersed on the TiO₂ surface in a free state [23]. As shown in Figure 2B, the EDX spectra indicate that the sample was mainly formed of Ti, O, and La, suggesting that La³⁺ successfully doped to TiO₂. La³⁺ was dispersed on the surface of TiO₂, and it had a squeezing effect on the TiO₂ lattice, resulting in more crystal defects and distortions, which had a certain effect on the TiO₂ crystal [24,25].

Figure 3 shows the FT-IR spectrum of raw TiO₂ and 0.6 wt% La³⁺/TiO₂. A strong characteristic peak between 2700 and 3600 cm⁻¹ was attributed to -OH in the H₂O molecule, while the corresponding bending vibration peak appeared near 1630 cm⁻¹ [26]. The characteristic peaks between approximately 500 and 900 cm⁻¹ belonged to the anatase O-Ti-O bond [27]. After La³⁺ doping, the strong peaks of the defective -OH groups obviously weakened, as the OH groups on the surface of TiO₂ could be covalently bonded to La³⁺ [28]. A decrease in the characteristic peak of the anatase O-Ti-O bond was attributed to the covalent band between La³⁺ and TiO₂ particles, further indicating that La³⁺ was successfully doped to TiO₂.

Figure 4A shows the UV-visible absorption spectra of TiO₂ and La³⁺/TiO₂. Compared with TiO₂, the UV absorption of La³⁺/TiO₂ prepared by the ball milling method had little change except a slight redshift. However, the absorption of La³⁺/TiO₂ slightly enhanced in the visible region. The red shift of the absorption band was one of the important factors that made the sample have good photocatalytic performance under visible light. The reduced band gap energy of La³⁺/TiO₂ could excite and produce more photo-generated electron-hole pairs, thus effectively improving the photocatalytic efficiency [29]. As shown in Figure 4B, the light absorption properties of PVC, TiO₂/PVC, La³⁺/TiO₂/PVC composite films were also investigated. The PVC film had little absorption in both ultraviolet and visible region. The absorption of the TiO₂/PVC film significantly enhanced in the near ultraviolet region. The absorption band of La³⁺/TiO₂/PVC appeared red-shifted and could absorb light energy in the visible region. The red shift was mainly because part of La³⁺ was embedded on TiO₂ to form defect energy levels, which reduced the energy level spacing during electron transition [30]. Therefore, it was of great significance to use La³⁺/TiO₂ with the higher photocatalytic activity to enhance the degradation performance of PVC.

3.2. Photodegradation of La³⁺/TiO₂/PVC

In order to obtain a film with the best degradation effect, La³⁺/TiO₂ was mixed with PVC to make the composite films with different mass percentages, and the photodegradation rates of the above La³⁺/TiO₂/PVC composite films were compared with those of PVC and TiO₂/PVC after irradiation (Figure 5). After light irradiation, the weight loss rate of the PVC was only 2.12%, while that of the TiO₂/PVC reached 8.94%. The accelerating of the degradation rate was due to the mixing of TiO₂ into PVC. As for the La³⁺/TiO₂/PVC composite film, when the mass percentage of La³⁺/TiO₂ was 1.5%, the weight loss rate of La³⁺/TiO₂/PVC sample reached a maximum of 17.78%, which was eight times the degradation rate of PVC and two times the degradation rate of TiO₂/PVC. Obviously, the photocatalytic activity of La³⁺/TiO₂/PVC film increased greatly. Rare earth La³⁺ modified TiO₂ could inhibit the recombination of photo-generated electrons and holes and accelerate the formation of active oxides [31]. Moreover, the ball milling method was beneficial to the uniform distribution of La³⁺ and suppressed the TiO₂ agglomeration. After repeated grinding, the TiO₂ particles could be made smaller and its surface area increased [32,33], which supplied more photocatalytic active sites and further accelerated the degradation rate of PVC.

Figure 6A compares the FT-IR spectrum of pure PVC, TiO₂/PVC, La³⁺/TiO₂/PVC samples after being UV irradiated for 30 h with that of the original samples. As shown in

Table 1. The characteristic peak of infrared spectrum structure.

Serial Number	Wavenumber/cm−1	Characteristic Peak
1 2 3	2973, 2917, 1427	C-H
	698, 624	C-Cl
	1097	C-C
4	478	Ti-O-Ti
5	1720	C=O

, The characteristic peaks of C-H for the original PVC film appeared at 2973 cm⁻¹, 2917 cm⁻¹, and 1427 cm⁻¹ [34]. The characteristic peaks at 698 cm⁻¹ and 624 cm⁻¹ belonged to the C-Cl, and the characteristic peaks at 1097 cm⁻¹ belonged to the C-C of the PVC film [35]. Moreover, characteristics peak of TiO₂ could be seen in the spectrum of TiO₂/PVC and La³⁺/TiO₂/PVC samples. The stretching vibration peak at 478 cm⁻¹ belonged to Ti-O-Ti [36]. Obviously, a new band of at 1720 cm⁻¹ was assigned to the C = O structure after illumination. The intensity of the characteristic peak of the C = O structure of the La³⁺/TiO₂/PVC film increased gradually after 0 h,10 h, 20 h, and 30 h of irradiation (Figure 6B), indicating that the film was degraded with the continuous irradiation of light source. Due to the fact that La³⁺/TiO₂ produces active oxides under UV irradiation. O₂⁻ or OH free radicals attacked the PVC molecular chain, and O₂ also participated in the reaction to generate oxygen-containing groups and caused the photodegradation of PVC [37]. La³⁺ can effectively separate the hole-electron pair of TiO₂ to generate more free radicals to attack PVC, which can increase the degradation rate of plastics.

Figure 7 shows the XRD patterns of PVC, TiO₂/PVC, and La³⁺/TiO₂/PVC films before and after UV irradiation. There was no obvious sharp characteristic diffraction peak of PVC, which clearly illustrated that PVC polymer was an amorphous structure, and the peak distribution was wide and low [38]. The characteristic diffraction peaks of TiO₂ appeared in the TiO₂/PVC film. After light illumination, the peaks of TiO₂/PVC and La³⁺/TiO₂/PVC films disappeared at 13°~20°, while the pure PVC did not change. This was due to the destruction of the PVC structure in the photocatalyst/UV system. A new peak appeared at 32.5° in all composite films after light illumination, because several new small molecules appeared in the process of breaking PVC, which corresponded to the FT-IR spectrum.

The surface structure of the film could be clearly observed by SEM. As shown in Figure 8, the surface of pure PVC, TiO₂/PVC and La³⁺/TiO₂/PVC film had a certain size of hole before illumination. After UV irradiation, the size of the holes in PVC film increased slightly, while that in the TiO₂/PVC film significantly increased in terms of porosity and appeared to be crispy, indicating that a photo-oxidation reaction occurred on the film surface. The La³⁺/TiO₂/PVC composite film could be seen to gradually undergo photooxidation after 10 h, 20 h, and 30 h UV exposure. Under 30 h illumination, the hole diameter increased to approximately 2–8 μm, and the La³⁺/TiO₂ particles could be observed on the La³⁺/TiO₂/PVC sample (Figure 8C4). As shown in the inset in Figure 8C4, the film appeared to fracture and the lanthanum ion-doped TiO₂ photocatalyst was observed at the place of the break, which indicated that the photo-oxidation was caused by the exposed La³⁺/TiO₂. Therefore, the photodegradable film prepared by this method had certain application prospects for environmental friendliness.

3.3. The Performance Characterization of La³⁺/TiO₂/PVC

One of the important reasons why the additive degradable plastics have not been widely used was the poor mechanical properties of plastics. Therefore, it is of great significance to study the tensile properties of the La³⁺/TiO₂/PVC plastics. There were two important factors influencing the mechanical properties of materials: (1) the shape, orientation, and distribution of the additive, (2) the interaction between the additive and the matrix [39,40]. As shown in

Table 2. Mechanical properties of these kinds of different films.

Sample	Thickness (mm)	Tensile Strength (MPa)	Elongation at Break (%)
PVC	0.12 ± 0.01	2.10 ± 0.20	31 ± 0.08
TiO2/PVC (TiO2 1 wt%)	0.11 ± 0.01	1.70 ± 0.10	26 ± 0.10
La3+/TiO2/PVC (La3+/TiO2 1.5 wt%)	0.13 ± 0.01	1.71 ± 0.17	31 ± 0.04

, after mixing PVC and TiO₂ or La³⁺/TiO₂, the tensile strength and elongation of PVC decreased slightly. The slight decrease in mechanical properties might be due to the mixture of amorphous and crystalline TiO₂ in the PVC chain, which reduced the polymer interaction between chains [41]. However, the La³⁺/TiO₂/PVC degradable was made by this method and belonged to an environment-friendly composite material.

The thermal degradation of PVC was mainly divided into two stages (Figure 9). The first stage was the removal of HCl from PVC, where most of the degradation products in the second stage were cyclic compounds [42]. Compared with pure PVC film, the initial degradation temperature of PVC composite films had no obvious change. However, the overall degradation temperature shifted to a higher temperature, and the thermal stability of PVC composite film was improved. The mass of films did not decrease around 100 °C, indicating that the films did not contain water molecules. At 800 ℃, the PVC achieved complete thermal degradation.

3.4. Mechanism Discussion

La³⁺ doping modified TiO₂ has important significance for the increase of the TiO₂ photocatalytic activity. The reaction mechanism is shown in Figure 10. La³⁺ doped TiO₂ could inhibit the growth of TiO₂ grains and cause lattice distortion by the ball milling process. The grain refinement increased the specific surface area of TiO₂, provided more photocatalytic active sites, and improved the photocatalytic activity of TiO₂. When La ions were doped into the TiO₂ lattice, La³⁺ could capture photo-generated electrons to form La²⁺ (Formula (2)), and La²⁺ could easily form superoxide radicals with O₂ (Formula (3)), effectively inhibiting photo-generated carrier recombination (Formula (4)). Under the sufficient light energy, the valence band electrons of TiO₂ were excited to the conduction band. The outer electronic structure of the La element contained only one electron in the d orbit and the completely empty f orbit. Therefore, the empty orbits provided the transfer orbits for electrons (Formula (5)), which could be used as shallow traps to capture the photogenerated electrons, as well as facilitate the separation of photogenerated electron hole pairs. OH was formed by the reaction of photogenerated holes with H₂O on TiO₂ (Formula (6)). The photogenerated electrons reacted with O₂ to produce O₂⁻ and OH (Formulas (7)–(10)) [43]. During the photodegradation process of high-molecular polymers, the PVC molecular chains were attacked by O₂⁻ and OH to generate a free radical chain. According to the chemical bond energies of C-Cl, C-C, and C-H, 328 kJ/mol, 348 kJ/mol, and 413 kJ/mol, respectively, C-Cl was more easily destroyed by the free radicals generated under UV light (Formulas (11) and (12)). Then, the free radical chain continued to react with O₂ to form oxygen-containing compounds, which were decomposed into small molecules under the action of UV light, and finally completely mineralized (Formulas (13) and (14)). Moreover, La³⁺ doping modified TiO₂ could inhibit the growth of TiO₂ grains and cause lattice distortion by ball milling process. The grain refinement increased the specific surface area of TiO₂, supplying more photocatalytic active sites and improving the photocatalytic activity of TiO₂. The La³⁺ doping would also change the band gap of TiO₂, so that La³⁺/TiO₂ had absorption in the visible region [44]. Therefore, it is of great significance to use La³⁺/TiO₂ with the higher photocatalytic activity to enhance the degradation performance of PVC.

La³⁺ + e⁻ → La²⁺

(2)

La²⁺ + O₂ → La³⁺ + •O₂⁻

(3)

2 •O₂⁻ +2 H⁺ + e⁻ → •O₂H + OH⁻ + O₂

(4)

TiO₂ + hν → TiO₂ (e⁻ + h⁺)

(5)

h⁺ + H₂O → •OH

(6)

e⁻ + O₂ → •O₂⁻

(7)

•O₂⁻ + H₂O → •O₂H + OH⁻

(8)

2 •O₂H → H₂O₂ + O₂

(9)

H₂O₂ + hν → 2 •OH

(10)

−(CH₂CHClCH₂CHCl)− +·OH + hv → −(CHCHCH₂CH)− + H₂O + 2·Cl

(11)

−CH·CHCH₂CH·CH₂− + 2·Cl → −CHCH = CHCH = CH− + 2 HCl

(12)

−CH = CH-CH₂− + ¹O₂ → −(HOO)CH-CH = CH−

(13)

−(CHCH = CHCH)− + O₂ → →CO₂ + H₂O

(14)

4. Conclusions

An La³⁺/TiO₂ photocatalyst was successfully prepared and characterized by the ball milling method. The characterization results showed that La³⁺ successfully doped modified TiO₂. La³⁺ was dispersed on the surface of TiO₂, increasing the surface oxygen vacancies and lattice distortion. Compared with TiO₂, the La³⁺/TiO₂ photocatalyst showed higher photocatalytic activity. La³⁺/TiO₂ was applied to photocatalytic degradation of PVC plastic. After 30 h of UV light irradiation, the weight loss rate of the PVC and TiO₂/PVC were 2.12% and 8.94%, respectively, while that of the La³⁺/TiO₂/PVC (La³⁺/TiO₂ 1.5 wt%) reached 17.78%, which was eight times the degradation rate of PVC and two times the degradation rate of TiO₂/PVC. The La³⁺/TiO₂/PVC composite film showed higher photodegradability. After illumination, the FT-IR spectrum showed a new peak at 1720 cm⁻¹, which belonged to C = O, and the XRD patterns showed a new peak at 32.5°. The surface pore size of the La³⁺/TiO₂/PVC sample increased to 2~8 μm. These results indicated that the La³⁺/TiO₂/PVC film had photodegradability. Predictably, La³⁺/TiO₂/PVC has a good application prospect, and it is an environment-friendly composite material.