3D nano-macroporous structured TiO₂-foam glass as an efficient photocatalyst for organic pollutant treatment

Abstract

This work reports the preparation of 3D nano-macroporous structured TiO₂-foam glass (p-TiO₂/FG) as an efficient photocatalyst for the degradation of organic pollutants in water. Nanoporous TiO₂ film was coated on macroporous foam glass by using a simple sol–gel coating method. In this work, we used waste glass to prepare foam glass with suitable apparent density (0.98 g cm⁻³) and high open porosity, which facilitated direct light absorption by the TiO₂ film. Especially, the foam glass had a high adsorption capability for methylene blue (MB), which increased the quantity of organic pollutant to come in contact with TiO₂ through means of adsorption and hence improved the photo-degradation efficiency. The p-TiO₂/FG exhibited much higher photocatalytic activity than that of the TiO₂/FG without a 3D nano-macroporous structure, which was attributed to the increase of reaction sites on p-TiO₂/FG generated from its special 3D nano-macroporous structure. Additionally, the p-TiO₂/FG presented high stability and recyclability, retaining high photocatalytic activity even after 10 cycles, which was mainly attributed to the uniform and stable coating of TiO₂ film on the foam glass.

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Introduction

Conventional technologies for wastewater treatment are often too expensive for practical applications. TiO₂ as a photocatalyst has received much attention due to its attractive properties of being low-cost, nontoxic, photocatalytically active, chemically stable and having broad applications, especially in degrading organic pollutants in wastewater.^1–3 It is well known that TiO₂ photocatalysts can generate strong oxidant species (holes h⁺ and radical ˙OH) under sunlight irradiation which are able to decompose and to mineralize bio-recalcitrant organic pollutants into CO₂ and H₂O.⁴ TiO₂ can be applied in either suspended powder or immobilized form on a substrate as a thin film. TiO₂ powder exhibits high photocatalytic activity for the degradation of organic and bio-pollutants in wastewater due to numerous reaction sites. Practically, the commercial TiO₂ nano-powder Degussa P25, composed of anatase (80%) and rutile (20%), is the most active photocatalyst used in wastewater treatment.⁵ However, TiO₂ nanoparticles easily agglomerate in water, which reduces photonic efficiency for most degradation processes to less than 10% and incurs inappropriate and complex post-separation process for recovering micro-sized aggregated TiO₂ particles from water.⁶

Usually, there are two routes to improve the performances of TiO₂. One is to prepare TiO₂ photocatalysts supported on a variety of supports such as carbon, glass fibers, zeolite, clay minerals and beads.^7–11 This method can efficiently prevent the agglomeration of TiO₂ nanoparticles and the decrease of photocatalytic activity. But the inappropriate and complex post-separation process is still necessary for recovering TiO₂ from water. The other way is to immobilize TiO₂ film on substrate such as glass slide, which can avoid the requirement of post-separation process, but the efficiency of the overall photocatalytic process greatly decreases because the illuminated surface area and reaction sites are smaller than that of TiO₂ nanoparticles by several orders of magnitude.

Therefore, it is desirable to develop new TiO₂ photocatalysts which can avoid the requirement of the complex post-separation process, prevent the agglomeration of TiO₂ nanoparticles during wastewater treatment process, increase illumination surface area and reaction sites, and improve the efficiency of sunlight utilization. It would be promising to deposit TiO₂ photocatalyst onto foam materials to achieve the above requirements and thus improve the efficiency of the overall wastewater treatment. Foam glass is a nontoxic, chemically stable, low density and porous material, which is usually produced from waste glass. Foam glass can be adopted as the substrate for TiO₂ coating because of its high porosity (up to 95%), and the application of foam glass, produced from waste glass, in wastewater treatment is environmental friendly as it uses “waste” to treat “waste”. Recently, Obregón Alfaro et al.¹² reported the preparation of silver–TiO₂ nanocomposites deposited on foam glass by a sonochemical technique. The Ag–TiO₂/foam glass showed high photocatalytic activity for degradation of eosin yellow dye under natural sunlight irradiation, probably due to high illumination surface area and reaction sites. However, the deposition of Ag/TiO₂ nanocomposites on foam glass was non-uniform and only a small part of foam glass surface was covered by Ag–TiO₂. Moreover, the Ag–TiO₂ was physically deposited onto foam glass by using ultrasound and it was impossible to be firmly attached to foam glass. The Ag–TiO₂ nanocomposites may be easily removed by flowing water, and thus will greatly decrease the photocatalytic activity. It is well known that the adsorption capability of substrates also affects the photo-degradation rate significantly.^13,14 Matos et al. reported that activated carbon increased the photo-degradation rate by progressively allowing an increased quantity of organic pollutant to come in contact with TiO₂ through means of adsorption.¹⁵ However, the adsorption capability of foam glass has not been reported in the opening literatures.

In current work, we report a simple and “green” coating method for the preparation of 3D nano-macroporous structured TiO₂-foam glass as an efficient photocatalyst under UV illumination. The nanoporous TiO₂ film was coated on macroporous foam glass, which exhibited significantly increased reaction sites. The macroporous structure allows the scattering and refraction of UV light in the foam glass, thus the UV light can be efficiently utilized. Moreover, the TiO₂ film was attached to foam glass firmly and it was very stable during the wastewater treatment process. In addition, we report that the foam glass can be good absorber for dye in wastewater which can help the efficient photo-degradation of dye due to direct contact of dye with reaction sites. Owing to the direct contact of dye with reaction sites and efficient light utilization generated from the special 3D nano-macroporous structure, the photocatalytic activities of the TiO₂-foam glass were significantly enhanced.

Experimental

Materials

Titanium(IV) chloride (purity > 99%), ammonia solution and sodium silicate (7.5–8.5% Na₂O and 25.5–28.5% SiO₂) were purchased from Merck. Hydrogen peroxide (30%) was purchased from AnalaR. Poly-ethylene glycol (PEG) (MW = 2000) and methylene blue solution (0.05 wt%) were purchased from Sigma-Aldrich. The chemicals were used without further purification.

Glass powder was prepared by ball-milling soda lime waste glass at a speed of 350 rpm for 7 min. The final average glass particle size was around 75 μm.

Preparation of foam glass

A modified method was developed to prepare foam glass by replicating the structure of a lattice-honeycomb polymer, poly-urethane foam (PUF).¹⁶ Typically, 50 wt% of glass powder and 50 wt% of sodium silicate solution were mixed together uniformly. Then the PUF was immersed into the mixture for 5 min. After that, the mixture in the pores of PUF was slightly squeezed out remaining some mixture attached to the surface of PUF. Finally, the sample was dried at 80 °C for 3 h, heat-treated at 400 °C for 2 h and 750 °C for 0.5 h with a heating rate of 5 °C per min, subsequently. The obtained foam glass was labeled as FG in this manuscript and the properties of this foam glass were shown in Table 1.

Table 1 The properties of foam glass prepared from waste glass

Properties	Apparent density (g cm^−3)	Bulk density (g cm^−3)	Open cell (%)	Close cell (%)	Pore size (mm)
Foam glass	0.98	0.30	69.4	18.6	0.1–3.0

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Preparation of peroxotitanate aqueous solution (PTA)

The method for preparation of PTA solution was reported elsewhere.¹⁷ In a typical preparation, 3.6 mL of TiCl₄ was added drop-wise into 300 mL DI H₂O in an ice-water bath with strong magnetic stirring. After 30 min, the pH of the solution was adjusted to 7 by drop-wise addition of diluted ammonia solution (0.1 M). After stirring for 24 h, the obtained white precipitates were filtered and washed thoroughly with DI H₂O repeatedly until Cl⁻ was not detected. Thereafter the precipitates were ultrasonic dispersed in 80 mL DI H₂O. H₂O₂ (28 mL) was added drop-wise into this mixture in an ice-water bath with strong magnetic stirring. Appropriate amount of PEG (the weight ratio of PEG to TiO₂ was around 20%) was dissolved into the PTA solution (p-PTA). Finally, the concentration of titania in the resulting transparent yellow solution was adjusted to 2.0 wt%.

Preparation of 3D nano-macroporous structured TiO₂-foam glass (p-TiO₂/FG)

Fresh prepared foam glass with dimension of 6.0 × 2.5 × 0.5 cm was cooled down to room temperature and immersed into PTA solution immediately for 2 min. Then the foam glass was withdrawn and centrifuged (15 min at 200 rpm) to prevent excessive impregnation. Finally, the foam glass was dried at 100 °C (p-PTA/FG) for 1 h and further heated at 500 °C for 0.5 h. The final sample was labeled as p-TiO₂/FG. The weight of foam glass before (m₁) and after (m₂) TiO₂ loading were measured, and the loading amount of TiO₂ on foam glass was calculated by m₂ − m₁. Based on the results, the loading amount of TiO₂ on foam glass was determined to be around 3.8 ± 0.2 mg cm⁻³ of foam glass.

For comparison, TiO₂ supported on foam glass (TiO₂/FG) was also prepared without adding PEG into the PTA solution and its photocatalytic activity was evaluated.

The scheme of the preparation of 3D nano-macroporous structured p-TiO₂/FG film is illustrated in Fig. 1.

Fig. 1 Scheme of the preparation of FG, TiO₂/FG and p-TiO₂/FG.

Characterization

X-ray diffraction (XRD) analysis was carried out using a Philips PW1010 X-ray diffraction meter with Cu K_α radiation. XRD pattern was recorded with a scan step of 1° min⁻¹ (2θ) in the range from 20 to 70°. Surface morphology of the p-TiO₂/FG was evaluated by stereo zoom microscope (Nikon SMZ745T) and JEOL field emission electron microscope (FESEM) JSM-6700F. Surface species of these samples were analyzed by Fourier Transform Infrared Spectroscopy (FTIR, Digilab FTS 3100). Lastly, porosity of foam glass was investigated by using pycnometer (Model: Ultrapyc 1200e from Quanta chrome instrument).

Evaluation of photocatalytic activities

For the evaluation of photocatalytic activities of the FG, TiO₂/FG and p-TiO₂/FG, an 8 W UV lamp (OSRAM DULUX@ S 9/78) held at 15.0 cm above the sample was used as the light source. Photocatalytic activity measurements were performed using methylene blue (MB) as the model organic compound. Each sample with dimension of 6.0 × 2.5 × 0.5 cm was immersed into 80 mL of MB solution (10 ppm) in a beaker. The samples could float at the top of the MB solution. Before light irradiation, the mixture was kept in the dark for 6 h to achieve absorption/desorption equilibrium between photocatalyst and MB. Then the mixture was irradiated with UV light. At different intervals (0, 1, 2, 3, 5 and 10 h), 2.5 mL of MB solution was collected. The concentration of MB was determined (according to the concentration-absorbance at λ = 664 nm) by using a UV-vis spectrophotometer.

Actual wastewater treatment experiments were conducted using dye wastewater collected from local industry as the model wastewater. The p-TiO₂/FG photocatalyst was immersed into 100 mL of dye wastewater in a beaker. Then the mixture was irradiated under solar light during 10 am to 3 pm without continuous stirring, which could simulate the actual wastewater treatment process. The TOC of the dye wastewater before and after solar light irradiation were measured. The determination of the total organic carbon (TOC) was made as function of time using a TOC-Vcsn analyzer (Shimadzu).

To investigate photocatalytic stability and recyclability of the p-TiO₂/FG, the p-TiO₂/FG was separated easily from MB solution after each cycle of the photocatalytic activity evaluation. Then the p-TiO₂/FG was illuminated under UV light for 48 h to degrade the residual MB. After that, the photocatalytic activity of the same p-TiO₂/FG was evaluated again.

Results and discussion

To utilize sunlight efficiently, it is desirable that photocatalyst can float at the top of wastewater and absorb sufficient sunlight. However, for the TiO₂ supported on common glass slide or ceramic, the density is much higher than that of water and the photocatalyst will sink to the bottom, which can greatly decrease sunlight utilization efficiency and photocatalytic activity. Foam glass can be used as the substrate for the preparation of TiO₂ coating as photocatalyst due to its attractive advantage of being highly porous and low density. The properties of the obtained foam glass are listed in Table 1. The bulk and apparent densities of the foam glass are 0.30 g cm⁻³ and 0.98 g cm⁻³, respectively. The apparent density of the foam glass is slightly lower than that of water, so it can float on the surface of wastewater. In this experiment, the apparent density of foam glass is one of the critical parameters for the preparation of p-TiO₂/FG. If the apparent density of foam glass is higher than 1.0 g cm⁻³, the p-TiO₂/FG will sink to the bottom, which will lead to low sunlight utilization efficiency and photocatalytic activity. However, if the apparent density of foam glass is much lower than 1.0 g cm⁻³, the upper part of p-TiO₂/FG will not direct contact with MB solution, which will definitely cause the decrease of photocatalytic activity. Therefore, foam glass with apparent density slightly lower than 1.0 g cm⁻³ is considered as suitable substrate for the preparation of this photocatalyst. As shown in Fig. S1,† the surface of water and the top surface of p-TiO₂/FG photocatalyst are almost in the same level, which can improve the light absorption by TiO₂. Moreover, the foam glass is highly porous with pore size of 0.1–3.0 mm and most of the pores are inter-connected, which can facilitate the coating of TiO₂ and utilization of light. Herein, the TiO₂ photocatalyst can absorb sufficient direct light to generate large amount of strong oxidant species (holes h⁺ and radical ˙OH), which are able to decompose and mineralize bio-recalcitrant organic pollutants into CO₂ and H₂O.

The presence and crystalline phases of TiO₂ in the p-TiO₂/FG were determined by XRD and the results were shown in Fig. 2a. Compared with that of FG, the XRD pattern of the p-TiO₂/FG shows distinctive peaks at 2θ = 38.0° and 48.1° attributed to the anatase TiO₂ structure, which indicates successful coating of TiO₂ on the surface of foam glass. The TiO₂ coating mainly consists of anatase phase. FTIR analysis of FG, PTA/FG, p-PTA/FG and p-TiO₂/FG (Fig. 2b) were performed. In Fig. 2b, the bands at 1630 cm⁻¹ and the wide bands at 3100–3700 cm⁻¹ are resultant from O–H bending of adsorbed water molecules.¹⁸ Compared with that of the FG, the bands at 1420 cm⁻¹ in PTA/FG and p-PTA/FG are attributed to the stretching vibrations of the N–H bonds originated from NH₃ ¹⁷ which proves the successful coating of PTA on FG. For the p-PTA/FG, the band at 2920 cm⁻¹ is assigned to the stretching vibration of the C–H bond in PEG, which evidences the presence of PEG in p-TiO₂/FG before calcination.¹⁹ For the p-TiO₂/FG, it is obvious that the bands at 1420 cm⁻¹ and 2920 cm⁻¹ disappear, which evidences the decomposition of peroxotitanate and PEG at 500 °C.^17,20 The bands at 770 cm⁻¹, 1060 cm⁻¹ and 2340 cm⁻¹ were attributed to the reference FTIR spectrum of FG as indicated in Fig. 2b.

Fig. 2 (a) XRD patterns of the FG and p-TiO₂/FG and (b) FTIR spectra of the FG, PTA/FG, p-PTA/FG and p-TiO₂/FG.

In Fig. 3a and b, the low-magnification microscope images show the macroporous structure of the FG and p-TiO₂/FG, respectively. Compared with these two microscope images, no obvious difference observed on the surfaces of both samples, which probably because the p-TiO₂ coating on the foam glass is thin. Fig. 3c and d show the uniform coating of p-TiO₂ and TiO₂ on the surfaces of foam glass, respectively. High-magnification FESEM images can be used to further investigate the micromorphology of the p-TiO₂/FG and TiO₂/FG surfaces. As shown in Fig. 3c (inset), the p-TiO₂ coating is nanoporous with nanoparticle size of 16–20 nm which is consistent with our previous report.¹⁷ The nanoparticles also further confirm the formation of p-TiO₂ coating on the foam glass. While in Fig. 3d (inset), the TiO₂ coating is flat without obvious porous structure observed. The nanoporous structure of p-TiO₂/FG is originated from the gas generated by the decomposition of PEG, which is consistent with the results reported by other researchers.^21,22 The low-magnification and high-magnification images of the p-TiO₂/FG together can prove the 3D nano-macroporous structured TiO₂-foam glass photocatalyst has been prepared successfully. The macroporous structure of foam glass can facilitate the scattering and refraction of light in the foam glass, which will greatly enhance the light utilization efficiency. Meanwhile, the nanoporous structure of TiO₂ coating can provide more reaction sites to degrade MB. Consequently, the photocatalytic activity should be greatly enhanced due to the high light utilization efficiency and the increase of reaction sites originated from the special 3D nano-macroporous structure.

Fig. 3 Microscopes of (a) FG and (b) p-TiO₂/FG, and FESEM images of the (c) p-TiO₂/FG and (d) TiO₂/FG, inset: high-magnification images.

Fig. S2a and b† show the EDX patterns of the FG and p-TiO₂/FG. Only O, Si, Na, Al and Na elements are observed in the EDX patterns of the FG, which are attributed to the normal elements in glass. However, there are obvious peaks assigned to Ti element in the EDX pattern of p-TiO₂/FG, which definitely proves the formation of TiO₂ coating on FG. Additionally, Fig. S2c and d† show the energy-dispersive X-ray elemental mapping images taken from the area in the SEM image, which displays that the elements of Si and Ti are uniformly distributed over the whole p-TiO₂/FG photocatalyst. These results can further prove that TiO₂ film is uniformly coated on the surface of foam glass.

The photocatalytic activities of the FG, TiO₂/FG and p-TiO₂/FG under UV light irradiation were evaluated and shown in Fig. 4a. In Fig. 4a, all of the FG, TiO₂/FG and p-TiO₂/FG absorb around 70% of MB in the solution. Additionally, Fig. 4b shows the pictures of the p-TiO₂/FG photocatalyst before and after the absorption/desorption equilibrium between photocatalyst and MB, respectively. It is obvious that the color of p-TiO₂/FG photocatalyst changes from light yellow to purple after the absorption/desorption equilibrium between photocatalyst and MB, which is attributed to the absorption of MB on the surface of p-TiO₂/FG photocatalyst. Herein, both Fig. 4a and b can intuitively prove that the FG, TiO₂/FG and p-TiO₂/FG are good absorbers for MB. It is well known that the absorption capability of photocatalyst can significantly affect the photocatalytic activity, usually higher absorption capability leads to higher photocatalytic activity by progressively allowing an increased quantity of organic pollutant to come in contact with TiO₂.^13–15 Even though the preparation of Au–TiO₂/foam glass as photocatalyst has been reported, the high absorption capability of foam glass and its correlation with photocatalytic activity have not been reported yet. The absorbed MB on the TiO₂/FG and p-TiO₂/FG is close to the reaction sites, which facilitates the degradation of MB by the photogenerated strong oxidant species. Under UV light irradiation, the photo-generated electrons can be transferred to the surface-adsorbed oxygen molecules and form superoxide anions, which could further transform to ˙OH and initiate the degradation of the absorbed MB on both the TiO₂/FG and p-TiO₂/FG.^23–25 The photo-degradation rate of the p-TiO₂/FG is much higher than that of the TiO₂/FG, which is due to more reaction sites on p-TiO₂/FG generated from its special 3D nano-microporous structure. For the p-TiO₂/FG, the concentration of MB solution is decreased to 1 ppm after 10 h irradiation. Whereas, for the FG, there is no obvious decrease of the MB concentration in solution observed after 10 h UV irradiation. Herein, the foam glass itself cannot degrade MB and the photocatalytic activities of the TiO₂/FG and p-TiO₂/FG are attributed to the TiO₂ coating. The TOC removal efficiency of the MB aqueous solution is also measured to further evaluate the photocatalytic activity of the p-TiO₂/FG photocatalyst. As shown in Fig. S3,† the TOC removal efficiency of MB aqueous solution after 10 hours illumination is around 88.1%, which further proves that the p-TiO₂/FG photocatalyst can photo-degrade MB efficiently under UV light illumination.

Fig. 4 (a) Photocatalytic activities of the FG, TiO₂/FG and p-TiO₂/FG under UV light illumination; (b) pictures of the p-TiO₂/FG photocatalyst before and after the adsorption/desorption equilibrium between photocatalyst and MB.

To evaluate the potential value for realistic application, the actual wastewater treatment experiment was conducted using local dye wastewater as the model wastewater. After solar light irradiation, the TOC removal efficiency of the dye wastewater is around 78.2%, which proves that the p-TiO₂/FG photocatalyst can be a potential photocatalyst for actual wastewater treatment.

As it well known, the low photocatalytic stability of TiO₂ nanoparticles in water and the complex post-separation process for recovering TiO₂ nanoparticles from water limit the wide application of TiO₂ nanoparticles in wastewater treatment. In our experiments, the photocatalytic stability of the p-TiO₂/FG was evaluated over 10 cycles and the results are shown in Fig. 5. At 2nd and 3rd cycles, the photocatalytic activity of p-TiO₂/FG photocatalyst is only slightly lower than that of fresh p-TiO₂/FG. Even after 10 cycles, the p-TiO₂/FG photocatalyst retains high photocatalytic activity, which proves that the 3D nano-macroporous structured p-TiO₂/FG is reasonably stable. The high stability of p-TiO₂/FG is attributed to two main reasons: (1) TiO₂ nanoparticles are firmly coated on foam glass by this simple sol–gel method, which prevents the agglomeration of TiO₂ nanoparticles during the wastewater treatment process; (2) the 3D nano-macroporous structured p-TiO₂/FG is separated and recovered from water easily, which avoids the weight loss of photocatalyst during the complex post-separation process. The XRD and FTIR spectra of the used p-TiO₂/FG samples after wastewater treatment were shown in Fig. S4.† The characteristic peaks of the XRD and FTIR spectra of the used p-TiO₂/FG samples are identical to that of the fresh samples, respectively. These results further prove that the p-TiO₂/FG photocatalyst is stable.

Fig. 5 Photocatalytic activities of the p-TiO₂/FG at different cycle.

Conclusions

In this work, foam glass with suitable apparent density produced from waste glass was exploited as a substrate for the preparation of p-TiO₂/FG photocatalyst for wastewater treatment. The foam glass was macroporous with low density, which could float at the top of wastewater easily and facilitate direct light absorption by TiO₂. Additionally, the foam glass had high absorption capability for methylene blue, which led to the increase of the quantity of organic pollutant (MB) to come in contact with TiO₂ through means of absorption and hence the enhancement of photo-degradation efficiency. The high photocatalytic activity of the p-TiO₂/FG was mainly attributed to the enhanced light utilization efficiency and the increase of reaction sites, which generated from the special 3D nano-macroporous structure. Besides the high photocatalytic activity, the p-TiO₂/FG was stable and it retained high photocatalytic activity even after 10 cycles, which was mainly attributed to the uniform and stable coating of TiO₂ film on the foam glass. Unlike normal TiO₂ nanoparticles, the application of p-TiO₂/FG photocatalyst in wastewater treatment avoided the expensive and complex post-separation process. Therefore, the p-TiO₂/FG can be considered as an efficient and promising photocatalyst for the degradation of organic pollutant in water. The method is highly environmental friendly. It provides a sustainable solution to treat waste and sets an excellent model for the concept of “using waste to treat waste”.

Acknowledgements

This work is supported by the Fundamental Research Funds for the Central Universities (Xiamen University, Grant No. 20720150015, 20720150016 and 20720150017), the National Natural Science Foundation of China (Grant No. 11502221, 21503175 and 21401153), the 111 Project (B16029), National Nature Science Foundation of China (No. U1405226), Fujian Provincial Department of Science & Technology (2014H6022) and the 1000 Talents Program from Xiamen University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra08768j

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