Molecular weight effects of PEG on the crystal structure and photocatalytic activities of PEG-capped TiO₂ nanoparticles

Abstract

Polyethylene glycol (PEG) was used as stabilizer to prepare water-soluble anatase titanium dioxide (TiO₂). The molecular weight effects of PEG on the crystal structure and photocatalytic activities of PEG-capped TiO₂ nanoparticles were systematically studied. The results show that the steric hindrance effect of PEG molecular chains will hinder the PEG molecules from being tied to the surface of TiO₂, resulting in the decrease of PEG molecules capped on the surface of TiO₂ with the growth of PEG molecular chains. It is significant that PEG can effectively promote the dispersion of TiO₂ nanoparticles in water, which becomes better and better with the increase of molecular weight of PEG. Moreover, the PEG-capped TiO₂ aqueous solutions can keep stable for more than two months. PEG cannot influence the crystal type and size of TiO₂, which can be well controlled by the introduction of HCl in the reaction system. The photocatalytic activity of PEG-capped TiO₂ was evaluated by monitoring the degradation of methyl orange, which is better than that of commercial P25 TiO₂ mainly due to the good dispersion in water. However, PEG molecular chains are detrimental to the transfer of photogenerated electrons and reactive oxygen species while the increasing dispersion in water with the increase of PEG molecular weight can increase the photocatalytic activity of TiO₂. As a result, PEG400 capped TiO₂ presents the best photocatalytic activity while PEG2000 capped TiO₂ exhibits the worst photocatalytic activity. The photocatalytic activity begins to increase when the molecular weight of PEG is larger than 2000.

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1. Introduction

Titanium dioxide (TiO₂), one of the most promising photocatalysts, has been widely investigated for decades since Fujishima discovered the photocatalytic splitting of water on TiO₂ electrodes.^1–4 TiO₂ nanoparticles (NPs) have multiple advantages such as good structural stability, abundance, nontoxicity and low cost.^5–7 Hence, TiO₂ NPs have been used in many areas for instance solar energy conversion,^8–10 photocatalytic self-cleaning,^11–15 sensors^16–18 and photochromic devices.^19,20

TiO₂ has three crystallographic phases: anatase, brookite and rutile.²¹ Among them, the anatase TiO₂ has been confirmed to have excellent photoactivity and optoelectronic properties.^22,23 However, the high purity anatase TiO₂ is difficult to be prepared by traditional methods, because that the hydrolytic process of many traditional methods for the preparation of TiO₂ is hard to be controlled at room temperature and titanium alkoxides are sensitive to moisture.²⁴ Nowadays, new routes have been developed to prepare high quality anatase TiO₂ NPs by using an organic media to reduce the sensitivity of precursor to moisture.^25–27 However, the surface of produced TiO₂ NPs is often hydrophobic due to the hydrophobicity of capped organic media like oleic acid, which limits the applications of TiO₂ NPs since the most applications are carried out in aqueous solution.²⁴ Recently, Wang and co-workers²⁴ have successfully prepared water-soluble anatase TiO₂ NPs using ethylene glycol as a surfactant. Yan and co-workers²⁸ have reported a new and facile route for the controllable synthesis of water-soluble anatase TiO₂ NPs using polyethylene glycol 400 (PEG400) as a solvent and a stabilizer, with phases well controlled by introduction of HCl in the reaction system. The results indicated that PEG was an effective media for the preparation of water-soluble anatase TiO₂ NPs. It is known that the capped surfactant should impact the surface activity of TiO₂ NPs, which has been thought to be correlated with its photovoltaic and photocatalytic properties.²⁴ However, to the best of our knowledge, only ethylene glycol and PEG400 have been used as stabilizers to prepare water-soluble anatase TiO₂ NPs. The effects of molecular weight of PEG on the structure and photocatalytic activity of TiO₂ NPs have not attracted enough attention, which has not been discussed in detail.

In this work, a series of PEG with different molecular weight were used as stabilizers to prepare water-soluble TiO₂ NPs. The anatase phase of TiO₂ was controlled by introduction of HCl in the reaction system. Then the crystal structures and physicochemical properties of the PEG-capped TiO₂ NPs were investigated. The photocatalytic activities of the TiO₂ NPs were evaluated by monitoring the degradation of methylene blue (MB) and methyl orange (MO). Our aim is to understand the effects of molecular weight of PEG on the crystal structure and photocatalytic activities of PEG-capped TiO₂ nanoparticles.

2. Experimental section

2.1 Materials

Titanium butoxide (≥99.0%) and polyethylene glycol (PEG) were purchased from Aladdin Industrial Corporation (Shanghai, China). Hydrochloric acid (HCl, 36.0–38.0%) was purchased from Kelong Chemical Reagent Company (Chengdu, China). Commercial P25 TiO₂ powder was purchased from Sigma-Aldrich Corporation.

2.2 Preparation of the PEG-capped TiO₂ NPs

TiO₂ NPs capped by PEG with different molecular weight were prepared according to previous method with a slight modification.²⁸ 3 mL of titanium butoxide was mixed with 6 g of PEG under magnetic stirring at 65 °C to form a white titanium alkoxide complex. Then, 1 mL of HCl was dropped into the titanium alkoxide complex. The formed transparent slight yellow solution was then transferred to a stainless poly (tetra fluoroethylene) (Teflon)-lined autoclave (50 mL) and heated in a vacuum drying oven at 150 °C for 5 h. The tawny TiO₂ products were separated by high-speed centrifugation (10 000 rpm), thoroughly washed with hot ethanol for 2–3 times and dried in the vacuum drying oven at 50 °C for 24 h. 5 types of PEG with molecular weight of 400, 2000, 4000, 6000 and 10 000 were used to prepare TiO₂ NPs. The corresponding resulting products were named as TP400, TP2000, TP4000, TP6000 and TP10000, respectively.

2.3 Characterization of the PEG-capped TiO₂ NPs

The particle size of TiO₂ NPs in aqueous solution was measured using a Malvern Zetasizer Nano – ZS ZEN3600 instrument at a detection angle of 90° at 25 °C. The concentration of TiO₂ NPs aqueous solution was 2 g L⁻¹. The mean particle sizes were obtained in the cumulant mode using the built-in Malvern software. The crystal structures of the TiO₂ NPs were identified by an 18 kW X-ray diffractometer (X'Pert Pro, Philips, Almelo, Netherlands) with a fixed CuKα radiation (λ = 1.54 Å). The scanning range of the diffraction angle (2θ) was from 10° to 80° with a rate of 2° min⁻¹. The morphology and microstructure of the TiO₂ NPs were characterized by transmission electron microscopy (TEM) (JEM-2100, JEOL, Japan) and scanning electron microscopy (SEM) (JSM-7500F, JEOL, Japan). Thermogravimetric analysis (TGA) of the samples was carried out on a NETZSCH thermal analyzer (TG209F1, NETZSCH, Germany). The samples were dried in a vacuum drying oven at 105 °C for 24 h before the test. The measurements were running from 50 °C to 700 °C at a heating rate of 10 °C min⁻¹ and under nitrogen atmosphere to avoid thermo-oxidative reactions.

2.4 Photocatalytic activity evaluation of the PEG-capped TiO₂ NPs

The photocatalytic activities of the PEG-capped TiO₂ NPs were evaluated by monitoring the degradation of methylene blue (MB) and methyl orange (MO). 30 mg of TiO₂ NPs was suspended in 15 mL aqueous solution of MB or MO (10 mg L⁻¹) to obtain the reaction slurry. The slurries were stirred for 6 h in the dark to assure the establishment of an adsorption–desorption equilibrium. Then, the slurries were irradiated by a 90 W UV lamp (maximum emission at 302 nm). The photocatalytic degradation of MB was examined by measuring its absorbance at 664 nm every 30 minutes. The photocatalytic degradation of MO was examined by measuring its absorbance at 464 nm every 30 minutes. The measurements were carried out for five times and the mean values were used.

2.5 Statistical analyses

The SPSS version 17.0 software was used for statistical analysis of experimental data. Data were assessed with one-way analysis of variance (ANOVA) by using Duncan's multiple range test. A value of p < 0.05 was considered to be significant.

3. Results and discussion

3.1 Preparation and characterization of the PEG-capped TiO₂ NPs

The photographs of the PEG-capped TiO₂ NPs aqueous solutions are presented in Fig. 1. It can be seen that the series of TiO₂ NPs capped by PEG with different molecular weight all have good dispersion in water. Furthermore, we found that the PEG-capped TiO₂ NPs aqueous solutions can maintain stable for more than two months without obvious precipitation. It is worth noting that the transparency of the TiO₂ NPs aqueous solutions is increasing from TP400 to TP10000. The result indicates that the dispersion of TiO₂ NPs in water is getting better with the increase of molecular weight of PEG. To further understand the water solubility of PEG-capped TiO₂ NPs, the particle sizes of the PEG-capped TiO₂ NPs in aqueous solutions were measured and showed in Table 1. It is obvious that the particle size of TiO₂ NPs decreases from 76.2 to 49.2 nm with the growth of PEG molecular chains. The result is agreed with previous works that PEG can promote the dispersion of TiO₂ NPs in water.²⁸ Furthermore, the dispersion of TiO₂ NPs in water can become better when capped by PEG with longer molecular chain.

Fig. 1 Photographs of the PEG-capped TiO₂ NPs aqueous solutions.

Table 1 Particle sizes of the PEG-capped TiO₂ NPs in aqueous solutions

Samples	TP400	TP2000	TP4000	TP6000	TP10000
Particle size (nm)	76.2	68.6	60.9	56.3	49.2

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To study the crystallographic phases of as-prepared PEG-capped TiO₂ NPs, the typical XRD patterns of the PEG-capped TiO₂ NPs were measured and showed in Fig. 2. There is almost no difference among the five samples. They exhibit distinctive characteristic peaks at 2 theta 25.2°, 37.8°, 47.8°, 53.9°, 62.2° and 69.7°, corresponding to the anatase phase planes of (101), (004), (200), (105), (204) and (116) according to JCPDS card no. 21-1272.²⁹ The result indicates that the produced PEG-capped TiO₂ NPs are anatase phase. Yan and co-workers have successfully prepared water-soluble anatase TiO₂ NPs under mild solution conditions using PEG400 as a stabilizer and HCl as a phase controlling reagent. The results showed that the amount of concentrated HCl in the reaction system was found to have a strong influence on the TiO₂ crystal phases. For the low HCl content, the pure anatase phase could be produced.²⁸ In our work, PEG with different molecular weight were used as stabilizers while the low HCl content was selected. The water-soluble anatase TiO₂ NPs were successfully prepared too. The result indicates that the molecular weight of PEG has no influence on the crystal structures of the TiO₂ NPs, which can be controlled by the introduction of HCl in the reaction system.

Fig. 2 XRD patterns of the PEG-capped TiO₂ NPs.

The morphology and microstructure of the PEG-capped TiO₂ NPs were characterized by TEM and SEM to further study the crystallographic phases of as-prepared PEG-capped TiO₂ NPs. Fig. 3 shows the TEM images of the PEG-capped TiO₂ NPs. It can be observed that the TiO₂ NPs exhibit almost spheroidal shapes with the average grain sizes in the range of 5–10 nm, which is consistent with the shapes of anatase TiO₂ reported in previous works.¹² Note that the grain size of the TiO₂ in Fig. 3 is much smaller than that in aqueous solutions (Table 1). Moreover, the grain sizes of the TiO₂ are basically the same while the particle sizes of TiO₂ NPs in water decrease as the growth of PEG molecular chains. The results indicate that TiO₂ is present in the form of aggregates in water. PEG can facilitate the dispersion of TiO₂ NPs in water, but cannot influence the crystal types and size of TiO₂ NPs. Fig. 4 shows the SEM images of the PEG-capped TiO₂ NPs. As seen from Fig. 4, the samples present spherical shapes, which is consistent with the results gotten from TEM images. However, the morphology of TiO₂ NPs gradually becomes unclear with the increase of the molecular weight of PEG. The reason may be that PEG capped on the surface of TiO₂ NPs with high molecular weight can make TiO₂ NPs present the morphology of polymers.

Fig. 3 TEM images of the PEG-capped TiO₂ NPs. (a) TP400, (b) TP2000, (c) TP4000, (d) TP6000 and (e) TP10000.

Fig. 4 SEM images of the PEG-capped TiO₂ NPs. (a) TP400, (b) TP2000, (c) TP4000, (d) TP6000 and (e) TP10000.

TGA was used to evaluate whether PEG was capped on the surface of TiO₂ NPs according to the published method.³⁰ Fig. 5 shows the TGA and DTG curves of the PEG-capped TiO₂ NPs. The weight loss of the PEG-capped TiO₂ NPs at 700 °C and the ratio of weight loss to molecular weight of PEG were calculated and showed in Table 2. It is found that the weight losses for TP400, TP2000, TP4000, TP6000 and TP10000 at 700 °C are 24.70, 17.31, 18.03, 23.57 and 27.61 wt%, respectively. The weight losses are mainly attributed to the decomposition of organic groups on the surface of TiO₂, which verifies that PEG has been successfully capped on the surface of TiO₂ NPs. Table 2 shows that the weight losses of TiO₂ NPs at 700 °C are irregular. However, it is interesting that the ratio of weight loss to molecular weight of PEG is decreasing with the increase of molecular weight of PEG. The result indicates that the number of PEG molecules capped on the surface of TiO₂ is reduced with the growth of PEG molecular chains. The reason may be that the growth of PEG molecular chain leads to the increase of steric hindrance to result in the decrease of reaction site for PEG molecules.

Fig. 5 TGA (A) and DTG (B) curves of the PEG-capped TiO₂ NPs.

Table 2 Weight loss of the PEG-capped TiO₂ NPs at 700 °C and the ratio of weight loss to molecular weight of PEG

Samples	Weight loss (%)	Weight loss/molecular weight of PEG
TP400	24.70	0.062
TP2000	17.31	0.009
TP4000	18.03	0.005
TP6000	23.57	0.004
TP10000	27.61	0.003

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3.2 Photocatalytic activity of the PEG-capped TiO₂ NPs

The photocatalytic activities of the PEG-capped TiO₂ NPs were evaluated by monitoring the degradation of methylene blue (MB) and methyl orange (MO). Fig. 6 shows the photocatalytic degradation of MB in the presence of PEG-capped TiO₂ under UV light irradiation and the corresponding photographs versus irradiation time. It shows that the degradation of MB takes place induced by PEG-capped TiO₂ NPs. The reaction slurry of TP400 becomes colorless after the UV-irradiation for 30 minutes. However, it is interesting that the color begins to recover in 150 minutes. The same phenomenon can be observed for TP2000. The phenomenon of reversible color change of MB has been reported by previous works, which is attributed to the reduction–oxidation behavior of MB.^31,32 The oxidized form of MB presents blue and the reduced form is colorless. The oxidation–reduction potential of MB is lower than the conduction band edge of TP400 and TP2000. Hence, the photogenerated electrons transferred from TP400 and TP2000 to the MB molecules reduce the MB molecules from blue to the colorless state. However, the reversible color phenomenon cannot be observed for TP4000, TP6000 and TP10000. It may be because that the conduction band edge of TP4000, TP6000 and TP10000 is lower than the oxidation–reduction potential of MB.

Fig. 6 Photocatalytic degradation of MB in the presence of PEG-capped TiO₂ under UV light irradiation (A) and the corresponding photographs versus irradiation time (B).

This reversible color phenomenon of MB is very interesting, but is not convenient for the present purpose. To solve this problem, methyl orange (MO) was used as a probe to evaluate the photocatalytic activities of the PEG-capped TiO₂ NPs again. Fig. 7 shows the photocatalytic degradation of MO in the presence of PEG-capped TiO₂ under UV light irradiation and the corresponding photographs versus irradiation time. It is obvious that the photocatalytic activity of all the PEG-capped TiO₂ NPs is better than that of commercial P25 TiO₂. The result illustrates that the presence of PEG on the surface of TiO₂ NPs can increase the photocatalytic activity of TiO₂ mainly due to the resulting good dispersion in aqueous solution. The same results have been reported by previous work.²⁸ Fig. 7 shows that TP400 presents the best photocatalytic activity and MO can be absolutely degraded in about 60 minutes. TP2000 exhibits the worst photocatalytic activity and it needs 300 minutes to completely degrade MO. The photocatalytic activity of the PEG-capped TiO₂ NPs begins to increase with the growth of PEG molecular chain when the molecular weight of PEG is larger than 2000.

Fig. 7 Photocatalytic degradation of MO in the presence of PEG-capped TiO₂ under UV light irradiation (A) and the corresponding photographs versus irradiation time (B).

Fig. 8 shows the tentative photodegradation mechanism diagram of MB and MO on the surface of PEG-capped TiO₂ NPs. The electrons of TiO₂ NPs are stimulated under the UV-irradiation and then transfer from the valence band (VB) to the conduction band (CB).³³ Then the photogenerated electrons are captured directly by O₂ and formed superoxide anion (O²⁻). Meanwhile, the valence band hole can oxidize water or OH⁻ to form ˙OH radicals.³⁴ These reactive oxygen species would decompose organic dye to CO₂ and water. The results in Fig. 7 show that the PEG molecular chains capped on the surface of TiO₂ are detrimental to the transfer of photogenerated electrons and reactive oxygen species. Previous works have supposed that the strongly bonded surfactant should passivate the surface activity of TiO₂ NPs, resulting in the decrease of the photovoltaic and photocatalytic properties.³⁵ So the TP400 capped by PEG with the shortest molecular chain presents the best photocatalytic activity. But then, the photocatalytic activity of TiO₂ NPs is not only dependent on the photogenerated electrons, but also dependent on the particle size and surface area.³² The smaller the particles, the larger will be its specific surface area and the higher the photocatalytic activity. So the photocatalytic activity of the PEG-capped TiO₂ NPs begins to increase when the molecular weight of PEG is larger than 2000, which is mainly due to the increasing dispersion of the PEG-capped TiO₂ NPs in aqueous solutions.

Fig. 8 Tentative photodegradation mechanism diagram of MB and MO on the surface of PEG-capped TiO₂ NPs.

4. Conclusions

PEG-capped TiO₂ nanoparticles have good dispersion in water, which becomes better and better with the increase of molecular weight of PEG. Moreover, the PEG-capped TiO₂ aqueous solutions can maintain stable for more than two months without obvious precipitation. PEG cannot influence the crystal type and size of TiO₂, which can be well controlled as anatase phase by introduction of HCl in the reaction system. PEG molecular chains will hinder the PEG molecules from being tied to the surface of TiO₂, resulting in the decrease of PEG molecules capped on the surface of TiO₂ with the increase of molecular weight of PEG. Moreover, the PEG molecular chains are detrimental to the transfer of photogenerated electrons and reactive oxygen species. So the TiO₂ capped by PEG with the shortest molecular chain presents the best photocatalytic activity. But then, the increasing dispersion of PEG-capped TiO₂ in water with the increase of PEG molecular weight can increase the photocatalytic activity of TiO₂. So the photocatalytic activity of the PEG-capped TiO₂ begins to increase when the molecular weight of PEG is larger than 2000.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (NNSF) (21276166) and Fundamental Research Funds for the Central Universities of China (2015SCU04A34).

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