A novel preparation of Ag-doped TiO₂ nanofibers with enhanced stability of photocatalytic activity

Abstract

Hierarchical structures with high densities of secondary Ag nanoparticles grown on primary TiO₂ fibers have been produced by a facile and low-cost way, by combining electrospinning and hydrothermal techniques. The photocatalysis properties were verified by degradation of Rhodamine B (RhB), and exhibited high efficiency and stability under UV-light illumination.

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Titanium dioxide has attracted lots of attention in past decades since the discovery of photocatalytic splitting of water under ultraviolet light by Honda and Fujishima in 1972.¹ To date, TiO₂ has been proven to be a promising semiconductor catalyst and an excellent photocatalyst for the degradation of organic contaminants in the environment^2–4 due to the significantly low energy consumption, friendly operation conditions, high oxidizing power, low cost and nontoxicity in the process of applications.^5,6 Therefore, it has been applied in many areas, such as solar cells,⁷ gas sensors,⁸ and other fields. However, there are also some deficiencies for the application of TiO₂. The main drawbacks of TiO₂ photocatalysts are the lack of visible light and the high rate of recombination of hole–electrons for the large band gap of TiO₂^{9, 10} (3.2 eV in anatase phase). Numerous scientific researchers have made great progress towards the enhancement of photocatalytic efficiency to overcome these drawbacks, but the methods are complex, high energy-cost, and hard to apply in industrial production.¹¹ All of the research to enhance the activity of TiO₂ can be divided into three aspects. First, some research was conducted to improve the quantum yield and noted that the crystal structure,¹² hydroxyl groups on the surface, exposed facets and oxygen deficiencies have a significant influence on it.^13–18 Second, attempts to extend light absorption from the UV region into the visible spectrum have made some progress and enhanced the photocatalytic activity greatly. Third, some of the work intended to suppress the recombination of electron–hole pairs in TiO₂, such as by depositing noble metals and coupled semiconductor heterostructures.^19–21 In our work, a facile method was put forward to produce the heterostructure of silver and TiO₂ in a cost-effective way, by combining electrospinning with a hydrothermal process. The electrospinning technique has gained increasing appeal as a versatile technique for fabricating nano- and sub-micron fibers possessing the feature of a large surface-to-volume ratio.²² The first preparation of TiO₂ fibers through electrospinning was reported by Li and Xia.²³ Since then, there have been lots of studies about the fabrication of TiO₂ fibers by employing sol–gel electrospinning, including a dopant of silver for enhancing the properties of TiO₂ in applications^24–26 and producing a higher reactivity than pure TiO₂. Driven by these studies, lots of innovative approaches have been developed to synthesize TiO₂-based heterosturctures.^27–29 Unfortunately, there is hardly any research to ensure the large demand for industrial production. It is worth noting that the exploration of nano-sized Ag to modify TiO₂ is of great interest in photocatalytic applications, because Ag could serve as an electron scavenging center to separate electron (e⁻)–hole (h⁺) pairs because its' Fermi level is below the conduction band of TiO₂. Additionally, small silver particles have the ability to generate surface plasmon resonance with TiO₂,³⁰ thus leading to the markedly enhanced photocatalytic activity of TiO₂ fibers and an increased quantum yield for the photocatalytic processes.

In this communication, we firstly report an effective three-step route by the combination of electrospinning and hydrothermal methods to synthesize Ag/TiO₂ hierarchical structures, which probably contribute to enhancement of electron–hole separation and interfacial charge transfers. Firstly, we produced anatase TiO₂ fibers (after calcination) by electrostatic spinning. Next, the TiO₂ fibers were put into AgNO₃ solution under UV illumination for 5 min. Finally, the TiO₂ fibers treated by AgNO₃ solution were mixed with specific concentrations of Ag⁺ and HMTA (C₆H₁₂N₄) aqueous solutions in a reaction kettle under 90 °C for 10 h. The first step is to produce the TiO₂ fibers, which have a large specific surface area and provide a platform for the deposition of a Ag crystal nucleus in the second step. After the second step, the Ag crystal nucleus would provide the growth direction of silver during the hydrothermal process.

Fig. 1A shows a SEM image of the pure TiO₂ fibers, which were fabricated using electrospinning equipment followed by calcination at 520 °C (Fig. S1, ESI†) for 4 hours.²⁰ The fibers were sub-micron or nanoscale in diameter and it can be clearly seen that the TiO₂ fibers (Fig. 1A and C) have a relatively smooth surface without secondary structures. Simultaneously, the pores between the fibers made the uniform growth of silver possible.³¹ Then, the pure TiO₂ fibers were immersed in AgNO₃ solution for 4 h at room temperature to get a stable equilibrium for absorbing and desorbing Ag⁺. After that, the fibers treated by Ag⁺ were irradiated with ultraviolet light for 5 minutes (Fig. S2A, ESI†). Lastly, a hydrothermal reaction was carried out at 90 °C to obtain the hierarchical Ag/TiO₂ nanostructures (Fig. 1B) with different concentrations of Ag⁺ in the reaction solutions. It is worth noting that the AgNPs (Ag nanoparticles) adhered on the surface tightly, and would not come off during the ultrasonic process. As we can see from the SEM images of the as-prepared products (Fig. S2B and C, ESI†), with an increasing concentration of Ag⁺, a higher density of secondary Ag nanoparticles would adhere on the TiO₂ substrates uniformly. It is interesting that we cannot obtain the hierarchical nanostructures without the second step; the Ag crystal nucleus provided the orientations for the growth of Ag.³² As we know, the properties of Ag/TiO₂ structures depend highly on the densities of Ag, such as the electrical properties, photocatalytic properties etc. In principle, the large size of the particle’s surface energies is lower than that of the small size particles of Ag. So, the small Ag nanoparticles would be inclined to grow larger, with the concentrations of reactants and the reaction time continued via the Ostwald ripening procedure.³³ Therefore, by tuning the reactant concentrations, the morphology of the products can be further controlled and the Ag/TiO₂ heterojunctions, with different particle sizes of Ag, will afford more opportunities for their potential applications. Fig. 1D shows the representative TEM (left side) and high-resolution transmission electron microscopy (HRTEM, right side) micrographs of the Ag/TiO₂ heterostructures. As shown in the TEM image, a large quantity of Ag nanocrystals are adhered on the surface of the TiO₂ fibers and it can be seen that the size distribution of Ag was mainly contracted at 5–35 nm, found by calculations via the TEM image. Furthermore, the HRTEM image of the Ag/TiO₂ fibers shows a clear lattice fringe of d = 0.35 nm (matching the anatase (101) crystallographic plane) and d = 0.24 nm (the cubic phase of the Ag (111) plane) on the right side of Fig. 1D. Simultaneously, the border of Ag and TiO₂ is invisible, which demonstrates that the composition fusion may occur between the interface of TiO₂ and Ag, as shown in the HRTEM image of Fig. 1D.

Fig. 1 (A) Pure TiO₂ fibers; (B) Ag/TiO₂ films were in a hydrothermal process with concentrations of 0.01 M; (C) TEM image of TiO₂ fibers; (D) TEM (left) and HRTEM (right) images of the Ag/TiO₂ fibers (0.01 M AgNO₃ solution).

The energy dispersive X-ray (EDX) spectrometry clearly shows the presence of Ti and O, and Ti, O, and Ag on the surface of the pure TiO₂ films (Fig. 2B) and the Ag/TiO₂ films (Fig. 2C), respectively. Fig. 2A shows the XRD patterns of the samples. As shown in the results, all of the diffraction peaks can be readily indexed to the anatase phase TiO₂ (Fig. 2a and b), and those main diffraction peaks have been indexed to JPCDS#21-1272.²⁰ The corresponding crystal face indexes have been marked in the XRD patterns. Interestingly, the TiO₂ diffraction peaks would be disappeared with the increasing load of Ag particles on the TiO₂ surface. Line e (Fig. 2A) indicated barely noticeable anatase diffraction peaks.

Fig. 2 (A) The XRD patterns of the samples ((a) pure TiO₂ fibers; (b) treated by UV illumination in AgNO₃ solution; (c, d and e) were in a hydrothermal process with concentrations of 0.01 M, 0.03 M and 0.05 M AgNO₃ respectively); (B) the EDS microanalysis on selected areas of the TiO₂ nanofibers; (C) the EDS microanalysis of selected Ag/TiO₂ (0.01 M AgNO₃ solution) heterostructures.

The mechanism for the synthesis of Ag/TiO₂ is simple, as shown in Fig. 3. The fibers immersed in Ag⁺ solution absorb the Ag⁺ on the TiO₂ surface according to the formula (1).²⁴ Then the UV irradiation (254 nm) procedure was sustained for 5 min and the films were then washed with ultrapure water and dried under a vacuum drying oven. We can see the nanoparticles adhering on the fibers. We knew that TiO₂ would produce the electrons and holes with the UV light and the electrons would reduce the silver ions into Ag on the surface of the TiO₂ fibers. The silver nanoparticles could provide the direction for Ag growth by a photocatalytic reduction method.³⁴ After that, the TiO₂ fibers treated by the second step were put in an autoclave containing AgNO₃ and C₆H₁₂N₄. With an increasing temperature, the hexamine gradually decomposes into ammonia and formaldehyde,³⁵ following formula (2). Then, ammonia combines with Ag⁺ and transforms into Ag(NH₃)²⁺ ions (formula (3)).

Ag⁺ + Ti–OH → Ti–OH⋯Ag⁺ (1)

C₆H₁₂N₄ + H₂O → NH₃ + CH₂O (2)

Ag⁺ + NH₃·H₂O → Ag(NH₃)₂⁺ (3)

Ag(NH₃)₂⁺ + HCHO → (NH₄)₂CO₃ + Ag + NH₃ + H₂O (4)

Fig. 3 The schematic of the synthesis process for Ag/TiO₂.

Moreover, the formaldehyde (CH₂O), which was from hexamine, would reduce the silver ammonia complex ions into pure Ag growing along the same direction of the lattice as the silver crystal nucleus.

To confirm the photocatalysis properties, we chose RhB as the degradation material under UV-vis light illumination (the main wavelength of the lamp was about 365 nm, 125W, Philips). As we can see from Fig. 4A, there is no obvious shift of λ max in the photocatalysis process. The mechanism of the degradation process for RhB is discussed briefly in the ESI³⁶ (Fig. S4, ESI†). Meanwhile, we also investigated the stabilities of the Ag/TiO₂ (0.01 M AgNO₃) and pure TiO₂ fibers. The results showed that the Ag/TiO₂ membrane exhibited greater stability than pure TiO₂ fibers after being reused five times. Simultaneously, we have compared two kinds of morphology after reuse and the Ag/TiO₂ still remained good. As we know, Ag is a noble metal which is more stable in the reaction process than other materials, so during the photocatalytic degradation, it played an important role in corrosion resistance.³² As we can see from Fig. 4B, the photocatalytic efficiency of the Ag/TiO₂ heterostructure (Fig. S3, ESI†) is slightly lower than that of the pure TiO₂ films. This is because some Ag particles on the surface of TiO₂ may block ultraviolet absorption and reduce the photogenerated electron–hole pairs, and then lower the TiO₂ fibers photoactivity.³⁷ In other cases, the size of the Ag particles also plays a great role in the photocatalytic activity. The Ag would occupy the active sites on the surface of the TiO₂ fibers and cause a decrease in the activity of TiO₂. Simultaneously, the photogenerated electrons on the silver sites attract holes and recombine together.³⁸ In this case, the Ag particles adhering on the surface of the TiO₂ fibers would be the e⁻–h⁺ pair recombination centers.³⁹ At the same time, the probability of the hole-capture would be increased by the excessive coverage of silver particles, which decreases the probability of holes reacting with adsorbed RhB on the TiO₂ fiber surface.⁴⁰ However, the deposition of Ag nanoparticles on the TiO₂ photocatalyst can also highly improve its photocatalytic efficiency through the Schottky barrier conduction band electron trapping and the consequently longer electron–hole pair lifetimes.⁴¹

Fig. 4 Curves of the photocatalytic degradation of RhB under UV light: (A) in the presence of the pure TiO₂ fibers; (B) the diagram of the degradation ratio of RhB versus reuse times ((B) indicates the pure TiO₂ fibers and (C) represents Ag/TiO₂).

Conclusions

In summary, we have successfully synthesized Ag/TiO₂ heterojunctions with novel hierarchical architectures by a combination of an electrospinning method and a hydrothermal process. Moreover, the morphologies of the secondary Ag nanoparticle size could be facilely tuned by adjusting the experimental parameters. We believe that this methodology provides a new avenue that offers a relatively mild and environmentally benign approach for the large-scale preparation of various one-, two-, and three-dimensional heterojunctions with structural complexity, thus enabling various functions. The special heterojunction possesses great potential for applications in photocatalysts, photovoltaics and supercapacitors in the future.

Acknowledgements

This work was financially supported by ChangChun Science and Technology plan projects (13NK01) and the National Nature Science Foundation (Grant 51403076).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra01353d

‡ Zhiqiang Cheng and Fanli Zhang contributed to the work equally.

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