1 Introduction

In recent years, titanium dioxide (TiO2) as one of the most important photocatalytic materials has been extensively investigated due to its wide applications in environmental purification. [1–4] TiO2 has been extensively used and investigated as an excellent photocatalyst because of its exceptional properties such as nontoxicity, low cost, and long-term stability against photo and chemical corrosion. [5–9] However, its limited UV-driven activity largely inhibits its practical application and overall efficiency under natural sunlight, consisting of about 5 % UV, 43 % visible and 52 % infrared. One of the most potential solutions for enhancing its efficiency is to shift its absorption from the UV region into the visible-light region, allowing for more photons to be absorbed and utilized for decomposing the pollutants.

Noble-metal nanoparticles (NPs) show strong visible-light absorption because of size- and shape-dependent plasmon resonance (SPR), which has a wide variety of applications in photocatalysts. [10–13] SPR can dramatically amplify the absorption of visible light and is therefore utilized to develop efficient visible-light-driven plasmonic photocatalysts. Since Huang et al. introduced a new plasmonic photocatalyst Ag@AgCl which is stable under visible light irradiation. The rapid development of plasmonic photocatalysts has offered a new platform to solve the above mentioned issues. [14] Bi et al. prepared Ag/AgCl core–shell nanowires by in situ oxidation reaction which showed improved photocatalytic activity under visible light irradiation for the decomposition of methyl orange (MO) dye. [10] Hu et al. synthesized Ag/AgBr@TiO2 by deposition–precipitation for the destruction of azodyes and bacteria under visible light. This compound kept its high photocatalytic activity and stability through five cycles. [15].

In this work, anatase TiO2 nanotubes were prepared by hydrothermal synthesis and a hydrogen peroxide treatment. Then, Ag@AgCl nanoparticles were loaded onto the nanotubes by precipitation and photoreduction reactions with AgNO3 and HCl to give a visible light photocatalyst of Ag/AgCl/TiO2 nanotubes. The structure of this photocatalyst was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), transmission electronmicroscopy(TEM), Raman spectra, X-ray photoelectron spectroscopy (XPS) and UV–Vis diffuse reflectance spectroscopy. Photocatalytic performance of the photocatalyst was studied for the decolorization of Rhodamine B (RhB) under visible light irradiation.

2 Experimental

2.1 Preparation of catalyst

TiO2 powder and 35 ml of 10 mol/L NaOH solution were mixed together. After stirring for 10 min at room temperature, the mixture was heated in a Teflon-lined autoclave at 160 °C for 24 h. After the hydrothermal section, the filtered powder was washed with distilled water and 0.1 mol/L HCl aqueous solution several times, and subsequently separated from the washing solution. Then, the powder was mixed with 0.15 mol/L HCl solution and stirred for 12 h at room temperature.The filtered powder was washed alternatively with distilled water and 0.1 mol/L HCl solution several times until the pH value of the rinsing solution was 6.9. Then, the synthesized TiO2 nanotubes were treated by 20 % H2O2 aqueous solution under refluxing conditions at 40 °C for 6 h. Then, the filtrated powder was washed with distilled water and dried in an oven at 80 °C for 12 h. The dried powder was calcined at 450 °C for 2 h. Finally, anatase TiO2 nanotubes (TiO2–NTs) were obtained.

The as-prepared TiO2–NTs (1 g) was dispersed in deionized water, ultrasonicated for 15 min at room temperature, and then mixed with 10 ml of 0.2 mol/L AgNO3 solution. After stirring for 20 min at room temperature, adding 10 ml of 0.1 mol/L HCl aqueous solution. The mixture was ultrasonicated for 10 min and stirred for 25 min at room temperature. The filtered powder was washed with deionized water and dried at 90 °C for 12 h. Finally, the dried powder was irradiated with a 500 W halogen tungsten lamp for 20 min to reduce part of the Ag+ ions in the AgCl particles to Ag0 species. Finally, the plasmonic photocatalyst of anatase TiO2 nanotubes modified with Ag@AgCl nanoparticles.

2.2 Evaluation of photocatalytic activity

The photocatalytic activity of the as-prepared photocatalyst was evaluated by decomposing RhB in aqueous solution under visible light irradiation. The visible light source was a 300 W Dy lamp with an optical cut off filter to ensure the irradiation only by visible light. The luminous flux was about 22,000 lm and the reaction temperature was hold at 25 °C through air conditioner controller. The experiment was performed at room temperature as follows: aqueous suspensions of RhB (100 ml, 10 mg/L) with 0.20 g of the as-prepared catalyst were placed in the beaker. Prior to illumination, the dispersion was sufficiently stirred in the dark for 20 min. At given irradiation time intervals, reaction solution (about 6 ml) was taken out and centrifuged to remove photocatalyst particles. The concentration of residual RhB in solution was monitored by an UV–visible spectrophotometer by recording the absorbance of the characteristic peak of RhB at 554 nm. For comparison, the photocatalytic degradation of RhB by the P25, the Ag–AgCl catalyst, the TiO2 and Ag-TiO2 composite was performed using the same procedure as described above.

2.3 Characterization

The morphologies of the samples were measured by a JSM-6510LV scanning electron microscopy (SEM) and an FEI Tecnai G2 S-Twin transmission electron microscope (TEM). The elements were confirmed by the Oxford ISIS-300 Energy dispersive spectrometer (EDS). Powder X-ray diffraction (XRD) measurements were carried out on a Rigaku D/max 2500PC single-crystal diffractometer using CuKα radiation (λ = 0.15405 nm). X-ray photoelectron spectroscopy (XPS, ESCLAB250) was used for the purpose of determining the surface composition of samples. UV/Vis absorption spectra were obtained by a UV/Vis spectroscope (Hitachi, U-4100). The Raman analysis of the obtained samples was performed on a Renishaw InVia Confocal Raman Microprobe using 633 nm (HeeNe laser) excitation at room temperature.

3 Results and discussion

Figure 1a presents a typical SEM image of the TiO2–NTs. This hollow structure has a large surface area for the adsorption of small organic and inorganic nanoparticles inside the nanotubes, and provides a good platform for the loading of Ag@AgCl nanoparticles. Shown in Fig. 1b is a SEM image of the Ag/AgCl NPs deposited into TiO2–NTs, indicating that well-ordered pores structure still exists. Ag/AgCl NPs deposited into TiO2–NTs were further examined using TEM. Figure 1c is a TEM image of the TiO2–NTs, showing clearly that the sample has an ordered array tubular structure. Figure 1d is a TEM image of Ag/AgCl/TiO2, showing that nanoparticles have deposited into the pore of the TiO2–NTs. These NTs have a uniform width size distribution around 20 nm.

Fig. 1
figure 1

A typical top view SEM image of TiO2 NTs (a) and Ag/AgCl/TiO2 NTs (b); TEM image of TiO2 NTs (c) and Ag/AgCl/TiO2 NTs (d)

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Figure 2 shows the EDS spectrum of the Ag/AgCl/TiO2 NTs, it can be seen that Ag, Ti, O and Cl elements were present in the samples. Emental analysis also reveals that there are no other elements in the prepared photocatalyst, which confirms the high purity of the prepared products. The proportion of Ag/AgCl/TiO2 NTs was 31.34, 48.26 and 20.40 % respectively.

Fig. 2
figure 2

Corresponding EDS pattern of Ag/AgCl/TiO2 NTs

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X-ray diffraction (XRD) was used to determine the phase structure of samples. Quantitative analysis of this pattern indicates that all peaks in the pattern can be indexed using the anatase phase of TiO2 (JCPDS file No: 21-1272), the cubic phase of AgCl with lattice constant a = 5.5491Ǻ (JCPDS file No: 31-1238) and metallic Ag (JCPDS file No: 65-2871), which are marked with A, B and C in Fig. 3, respectively. The broad diffraction peaks suggest that the sizes of the AgCl NPs exist, and this is consistent with the above SEM and TEM observations.

Fig. 3
figure 3

XRD patterns of Ag/AgCl/TiO2 NTs

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The elemental and chemical compositions of the samples were further analyzed by X-ray photoelectron spectroscopy (XPS). Figure 4a shows that the sample consists mainly of the elements Ti, Ag, O, Cl and C. No other impurity peaks were detected, evidencing the high purity of the resulting sample. The C1s is attributed to the adventitious hydrocarbon from the XPS instrument itself. As shown in Fig. 4b, the Ag 3d spectra consists of two individual peaks approximately at 374 and 368 eV, which are due to Ag 3d3/2 and Ag 3d5/2 binding energy, respectively. Figure 4c shows the Cl 2p peak, which is deconvoluted into two peaks (197 and 199 eV), which are assigned to the Cl 2p3/2 and Cl 2p1/2, respectively. This result suggests that the Cl elements are mainly in the form of Cl− [16, 17]. In Fig. 4d, the Ti 2p XPS spectra has two peaks centered at 458.9 and 464.7 eV, which are attributed to the Ti 2p3/2 and 2p1/2 spin–orbital splitting photoelectrons in Ti4+ [18].

Fig. 4
figure 4

XPS spectra of Ag/AgCl/TiO2 NTs: a survey, b Ag 3d peaks, c Cl 2p peak, and d Ti 2p peaks

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Figure 5 shows the scatting raman spectra for the TiO2 and the Ag/AgCl/TiO2 NTs. The spectrum intensity of the Ag/AgCl/TiO2 NTs increased as compared to the TiO2–NTs. The changes in intensity and position of the scattering peaks were probably associated with distortions and defects in the TiO2 crystal lattice induced by the loading of Ag/AgCl nanoparticles. The Ag/AgCl/TiO2 NTs shows an enhanced Raman active mode at 151 cm−1 when compared to TiO2 nanotubes, this can be attributed to the SERS (surface enhancement of raman signals) effect induced by the surface plasmon resonance of the silver particles in Ag/AgCl/TiO2 NTs. The details shown as the surface plasmon modes of Ag on Ag/AgCl/TiO2 NTs are excited and produce a strong local electromagnetic field around the Ag particles, and thus the intensity of the raman bands assigned to the raman active vibration modes of TiO2 is evidently [19, 20].

Fig. 5
figure 5

Raman spectrum of the Ag/AgCl/TiO2 NTs and TiO2–NTs

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The UV–Vis diffuse reflectance spectra of the samples are presented in Fig. 6. As can be seen, the absorption edges of TiO2 and was near 200–350 nm. After loading Ag/AgCl particles, the absorption intensities of Ag/AgCl/TiO2 NTs were slightly improved. In contrast to TiO2, Ag/AgCl/TiO2 NTs displayed a marked absorption enhancement in the visible light region from 400 to 800 nm. This can be attributed to the plasmon resonance of Ag particles in Ag/AgCl/TiO2 NTs. Therefore, the as-prepared composite may have the potential to achieve high photocatalytic activity in the whole light region. In a word, the as-prepared photocatalyst is expected to have excellent photocatalytic activity for the degradation of organic contaminants.

Fig. 6
figure 6

UV–Vis reflectance spectra of Ag/AgCl/TiO2 NTs and TiO2–NTs

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The temporal evolution of the absorption spectral changes during the photocatalytic degradation of RhB by the Ag/AgCl/TiO2 nanocomposites was shown in Fig. 7. As shown in the Fig. 7, the peak intensity decreases rapidly at wavelengths of 554 nm within 20 min. Moreover, with increasing irradiation time, not only did the main absorbance in the visible region decrease rapidly, but the peak intensities in the UV region also decreased. This indicates that both dye chromophore and aromatic ring can be destroyed by the Ag/AgCl/TiO2 photocatalyst.

Fig. 7
figure 7

UV–Vis spectra changes in the degradation of RhB on the photocatalysts

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As can be seen from Fig. 8, using the Ag/AgCl/TiO2 nanocomposite to photodegrade RhB dyes, the total degradation rate is almost 100 % within 20 min irradiation. Meanwhile, the adsorption of RhB by the photocatalyst in the dark was also checked. In dark within 20 min, the adsorption reached equilibrium, and about 25 % of RhB was adsorbed onto Ag/AgCl/TiO2. The Ag/AgCl/TiO2 composite exhibits much higher photocatalytic activity than the well-known commercial TiO2 photocatalyst P25, the TiO2, the Ag–TiO2, the AgCl–TiO2 and the Ag–AgCl composite, which indicating the excellent photocatalytic activity of the as-prepared photocatalyst.

Fig. 8
figure 8

Photodegradation of RhB under visible light irradiation

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Recently, the recycling use of the catalyst from aqueous solution after experiment has been a critical issue for large-scale application in industry. The main reason is the high cost of separation and the complicated separation process, which restricts the development of environmental technology of Ag/AgCl/TiO2. Based on the above consideration, the stability of the Ag/AgCl/TiO2 nanotubes has further investigated by cycling experiments. Recycling experiments are conducted to assess the durability of Ag/AgCl/TiO2 NTs, and the corresponding results are presented in Fig. 9. Ag/AgCl/TiO2 NTs are used repeatedly to degrade RhB solution under visible light irradiation for 5 times. As observed, after 5 cycles of photodegradation of RhB, the catalyst does not exhibit any loss of activity, indicating that the catalysts are relatively stable during the photocatalytic reaction. Such an important and useful property for the Ag/AgCl/TiO2 NTs will greatly promote their practical application in a scale-up for industrial water remediation.

Fig. 9
figure 9

Recycling test of Ag/AgCl/TiO2 NTs in the photocatalytic degradation of RhB solution under visible light irradiation

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4 Conclusion

In this paper, we successfully fabricate stable and highly efficient direct visible light plasmonic photocatalyst Ag/AgCl/TiO2 NTs. The diffuse reflectance spectra of Ag/AgCl/TiO2 NTs indicate strong absorption in both UV and visible light region. Under direct visible light irradiation, the Ag/AgCl/TiO2 NTs exhibit high activity and stability for the photodegradation of organic pollutants, e.g. RhB. The photocatalytic activity of the as-prepared Ag/AgCl/TiO2 NTs still maintained a high level, even though it was used 5 times for dyes degradation, indicating its sufficient stability for environment purification.