TiO₂ nanotube arrays decorated with Au and Bi₂S₃ nanoparticles for efficient Fe³⁺ ions detection and dye photocatalytic degradation

1. Introduction

Green and clean energy demand has now become a mainstream trend due to environmental pollution and global energy crisis. Photocatalytic semiconductors have attracted attention as a promising strategy for solving these problems. Many researchers have developed nanostructured materials for the degradation of contaminants. Titanium dioxide (TiO₂) is widely studied due to its good photocatalytic activity, environmental safety, relatively low cost, mechanical stability and corrosion resistance [[1], [2], [3], [4], [5], [6]]. It has been applied in many fields including photocatalytic degradation of contaminants, dye-sensitized solar cells, water splitting, gas sensors and biomedical materials [[7], [8], [9], [10], [11], [12], [13]]. However, TiO₂ nanotube arrays (TiO₂ NTAs) have some shortcomings, limiting their applications in many aspects [[14], [15], [16], [17], [18], [19]]. TiO₂ has a wide band gap, which absorbs only 3%-5% of the solar energy (λ < 387 nm), which gives a low utilization rate [1]. Moreover, the separation of electrons-holes of TiO₂ NTAs is low resulting in a low photocatalytic activity.

A series of modification strategies have been carried out to overcome these disadvantages, such as anatase TiO₂ NTAs doped with metal, non-metal or semiconductor. Narrow bandgap semiconductors in particular bismuth-based semiconductors have been used to make TiO₂ NTAs susceptible to visible, maintain excellent charge transfer and photofluorescence properties [20]. So Bi₂S₃ have attracted considerable attention. Bi₂S₃ is a non-toxic photoconductive semiconductor material with nonlinear optical response and a narrow band gap (1.3 eV) [21]. As the conduction band (CB) energy level of Bi₂S₃ is 0.26 eV higher than that of TiO₂, the photogenerated electron can be shifted from Bi₂S₃ to TiO₂. In contrast, the valence band (VB) of TiO₂ is lower than the Bi₂S₃ one, so that the photogenerated hole can be shifted to Bi₂S₃. Hence, valid charge separation of electrons-holes in TiO₂ and Bi₂S₃ is possible [22,23]. Noble metal nanoparticles (Ag, Cu, Pt, Au, etc.) dispersed on the surface of TiO₂ NTAs can enhance the capture of photo-generated electrons, accelerate the separation of electrons-holes, and thus inhibit photo-generated electrons-holes recombination [[24], [25], [26], [27]]. The addition of Au NPs to TiO₂ can reduce the intermittent rate of the trap state and partially compensate for the negative effects of the surface trap sites, thereby improving the light conversion efficiency [[28], [29], [30]].

While addressing environmental energy issues, we should also pay attention to human health issues. Lots of transition metal ions, such as Cu²⁺, Mn²⁺, Ni²⁺, Co²⁺, Mo²⁺, Fe³⁺, Zn²⁺, etc., are beneficial to body function. Among these, Fe³⁺ ion is one of the most dominant and abundant in the earth [31]. Fe³⁺ ions play a crucial role in many physiological and pathological processes such as enzyme catalysis, oxygen metabolism, electron transfer, RNA and DNA replication, blood production and thermoregulation [32]. Fluorescence sensing for metal ions is charming because it has the advantages of simplicity, affordability, high sensitivity, fast response and non-perturbative measurements, compared with spectrometric techniques [33]. Most of the current scientific works of fluorescent sensing are dedicated to detecting metal ions and other contaminants [[34], [35], [36]]. Fegley et al. [37] introduced the use of a fluorescent conjugated polymer as a chemical sensor to differentiate Fe²⁺ and Fe³⁺ ions. Wang et al. [38] reported a thiophene-modified rhodamine as a new turn-on fluorescent chemosensor to detect Fe³⁺ ions in aqueous media. Sheng et al. [39] reported a fluorescent probe for Fe³⁺ ions using a rhodamine derivative (RD1) with metal chelating moiety. The findings of these literature studies proved that fluorescence spectroscopy is the most popular method. Furthermore, as far as we know, most of the reported researches of induced Fe³⁺ ions are limited to dye-based biomolecule-based fluorescence, quantum dots, chelate-based sensors, etc [40]. It is rarely reported that Fe³⁺ ions are sensed by heterostructured Bi₂S₃ -TiO₂.

Herein, we developed a simple and effective method to produce Au/Bi₂S₃@TiO₂ NTAs composite. Au/Bi₂S₃ NPs composites were evenly distributed on TiO₂ NTAs surface. Au NPs functioned as electron trapping sites to enhance charge separation efficiency. Thus, Au/Bi₂S₃@TiO₂ NTAs heterogeneous composite was able to absorb solar light over a wide spectrum range because of the local surface plasmon resonance (LSPR) influence of Au and the narrow bandgap of Bi₂S₃. Compared to pure TiO₂ NTAs, the prepared Au/Bi₂S₃@TiO₂ NTAs composite exhibited superior photocatalytic activity under irradiation of the UV and visible light. In addition, it proved to act as an excellent fluorescent sensor, with good sensitivity, towards the detection of Fe³⁺ ions.

2. Experimental

2.1. Preparation of two-step anodized TiO₂ NTAs

As shown in Scheme 1, the TiO₂ NTAs were constructed via a two-step anodization process. Specifically, titanium oxide was used as the anode, a platinum sheet was used as the cathode and NH₄F in ethylene glycol solution (0.5 wt% of NH₄F and 2 vol.% of water in ethylene glycol) served as the electrolytic solution. Before the anodization, the titanium foil (3.0 cm × 1.5 cm) was cleaned by ultrasonicating in 1 M sulfuric acid, acetone, and ethanol sequentially and rinsed by deionized water. The anodic oxidation was performed twice. The first anodic oxidation voltage was 55 V for 2 h, the second anodic oxidation voltage was 55 V for 8 min. The obtained TiO₂ NTAs were calcined in air at a temperature of 450 ℃ for 2 h to enhance the crystallographic form. The heating and cooling rate of calcination were both 5 ℃ min^-1. Anatase TiO₂ NTAs were obtained as a result of this final step.

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Scheme 1.   Schematic illustration of synthesis process for Au/Bi₂S₃@TiO₂ NTAs.

2.2. Preparation of Au NPs

The solution of HAuCl₄ (0.005 wt%-0.02 wt%) dissolved in deionized water was put in an oil bath heated at 130 ℃ and stirred. At boiling, 600 g of deionized water containing 1.0 wt% of sodium citrate was rapidly added to the gold solution for a reaction time of 1, 2, 3 and 4 h. So four kinds of Au NPs reduced by sodium citrate were formed as shown in Scheme 1.

2.3. Preparation of Au NPs and Bi₂S₃ composites

According to Scheme 1, 50 mL of these four Au NPs solutions were successively measured out, then 500 μL of deionized water containing 3.0 mg of thioacetamide and 3.86 mg of bismuth acetate were added respectively, and the mixture was granted to react at 80 ℃ for 10 h in the oven. Four mixed solutions of Au/Bi₂S₃ NPs was obtained where the concentration of Bi₂S₃ NPs remained unchanged.

2.4. Preparation of Au/Bi₂S₃@TiO₂ NTAs

According to Scheme 1, TiO₂ NTAs was added to 15 mL of an ethanol solution including 150 μL of 3-mercaptopropyltrimethoxysilane (MPTS) and 30 μL of NH₄OH (27%). The mixture was kept at room temperature in a dark room for 24 h. The TiO₂ NTAs were rinsed with ethanol and deionized water. As prepared TiO₂ NTAs were immersed in 15 mL of Au/Bi₂S₃ NPs solution in the centrifuge tubes at 37 ℃ for 4 h in the oven.

2.5. Characterization

The morphology and structure of Au/Bi₂S₃@TiO₂ NTAs were measured by field emission scanning electron microscopy (FESEM, Hitachi-S4800). The microstructure and composition of the Au/Bi₂S₃ NPs were determined by utilizing a transmission electron microscope (TEM, FEI Tecnai G-20 operated at 200 kV). The crystal phases were employing an X-ray diffractometer (XRD) with Cu-Kα radiation (Philips, X′pert-Pro MRD). The elements were analyzed by using an energy dispersive X-ray (EDX) spectrometer. Photoluminescence (PL) measurements were performed by utilizing fluorescence spectroscopy (HORIBA JOBIN YVON, FM4P-TCSPC) and xenon lamp as excitation sources (λ_ex = 250 nm). The chemical state was studied by using X-ray photoelectron spectroscopy (XPS, KRATOS, Axis Ultra HAS). The binding energies were standardized with respects to C 1s peak at 284.5 eV. The UV-vis diffuse reflectance spectra at 250-800 nm were measured by utilizing UV-3600 spectrophotometer.

2.6. Photoelectrochemical measurement

The electrochemical properties of Au/Bi₂S₃@TiO₂ NTAs electrodes were studied by CV measurements and chronoamperometry. The electrochemical experiments were carried out using a PGSTAT302 N electrochemical workstation (Autolab, Switzerland). Prior to all measurements, the as-prepared electrodes were dip in phosphate buffer solution (pH = 7.2) for 15 min. TiO₂ NTAs or Au/Bi₂S₃@TiO₂ NTAs used as working electrodes, Pt plates used as counter electrodes and Ag/AgCl used as reference electrodes in 0.1 M Na₂SO₄ aqueous solution in a standard three-electrode system. The working electrode was exposed to a xenon lamp (GY-10) at a distance of 15 cm. The intensity of light focused on the beaker was 100 mW/cm².

2.7. Photocatalytic measurement

The photocatalytic activity of 10 mg/L (pH = 8.5) MB contaminant solution was investigated by PS-GHX photochemical reactor. The UV and visible light sources were simulated by a mercury lamp (300 W) and a xenon lamp (300 W), respectively. The light source was 60 mm away from the quartz tube (the reactor). Before irradiation, all the electrodes were completely immersed in the 15 mL MB solution respectively in a quartz reactor in a dark room for 0.5 h to achieve adsorption-desorption equilibrium. Under ambient conditions, MB concentration was measured at 463 nm by UV-vis spectrophotometer (Hitachi, UV-1080, Japan).

2.8. Study of detection of metal ions

The prepared Au/Bi₂S₃@TiO₂ NTAs were soaked in different aqueous solutions, 10 mM of Cu⁺, Al³⁺, K⁺, Pb²⁺, Cu²⁺, Fe³⁺, Ni²⁺, Cd²⁺, NH₄⁺, Mn²⁺ and Zn²⁺ for 1 h. Then, the samples were taken out and dried. The fluorescence spectra were measured at excitation wavelength 380 nm and emission wavelength 822 nm, respectively. To study the selectivity of the composites for Fe³⁺ ions, interference experiments were studied by mingling other metal ion solutions with a constant concentration (300 M) of Fe³⁺ ions solutions.

3. Results and discussion

Fig. 1 exhibits the SEM images of TiO₂ NTAs loaded with Au/Bi₂S₃ NPs mixed together, with only Auand with only Bi₂S₃. It is possible to see that most of the Au/Bi₂S₃ NPs were on the surface of the TiO₂ NTAs with a small amount of Au/Bi₂S₃ NPs loaded into the TiO₂ NTAs structure as shown in Fig. 1(a). On the contrary, the low loading of nanoparticles was found in TiO₂ NTAs sample loaded only with Au (Fig. 1(b)) or Bi₂S₃ (Fig. 1(c)). As shown in Fig. 1(d), the TiO₂ NTAs prepared by the two-step anodization method [41] have a smooth surface and an arranged order. The diameter of the tube is about 100 nm. Fig. S1(a, b) show 0.02 wt% and 0.005 wt% Au/Bi₂S₃ NPs decorating the TiO₂ NTAs for a reaction time of 2 h. Fig. S1(a, b) indicate that the loading of Au/Bi₂S₃ NPs was not uniform at higher concentrations of Au NPs.

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Fig. 1.   SEM images of 0.01 wt% Au/Bi₂S₃@TiO₂ NTAs (a), Au@TiO₂ NTAs (b), Bi₂S₃@TiO₂ NTAs (c) and TiO₂ NTAs (d).

Fig. 2(a, b) show most of the Au/Bi₂S₃ NPs was on the surface of the TiO₂ NTAs with a small amount of Au/Bi₂S₃ NPs loaded into the TiO₂ NTAs structure. Fig. 2(c) shows the EDX spectra of Au/Bi₂S₃@TiO₂ NTAs. It was found that the elements of the sample included Ti, O, Au, Bi, and S, and their atomic percentages were 34.62%, 64.49%, 0.09%, 0.30%, and 0.51%, respectively. Fig. 2(d) displays the area mapping of the elements. The relatively even colours in the images indicate that the distribution of Au, Bi, and S is uniform.

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Fig. 2.   SEM images of 0.01 wt% Au/Bi₂S₃@TiO₂ NTAs in different magnifications (a, b). Corresponding EDX spectrum (c) and mapping (d) of Au/Bi₂S₃@TiO₂ NTAs.

The TEM images, Fig. 3(a, b), further show that the Au/Bi₂S₃ NPs uniformly distribute on the outside and inside of the TiO₂ NTAs. The high resolution transmission electron microscope (HRTEM) (Fig. 3(c)) and selected area EDX mapping (Fig. 3(d)) show that the lattice spacing (d) of the TiO₂ anatase (101) plane is 0.35 nm, Au (111) plane spacing is 0.237 nm, and the interplanar spacing of Bi₂S₃ (130) is 0.36 nm [22]. As shown in Fig. S2(a), the size of Au/Bi₂S₃ NPs were about 15 nm and 20 nm, respectively. HRTEM images show that the Au/Bi₂S₃ NPs were both dispersed and combined as shown in Fig. S2(b-d).

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Fig. 3.   TEM (a, b) and HRTEM (c) images of Au/Bi₂S₃@TiO₂ NTAs. Selected area EDX mapping (d) of Au/Bi₂S₃@TiO₂ NTAs.

XPS measurements were implemented to study the chemical composition and elemental valence state of Au/Bi₂S₃ NPs on the TiO₂ NTAs. The XPS spectra (Fig. 4(a)) confirmed the presence of Au, Bi, S, C, O, and Ti. The binding energy peaks at 284.5, 458.9, and 532.4 eV refer to C 1s, Ti 2p, and O 1s, respectively [42,43]. Fig. 4(b-d) exhibit the higher resolution XPS spectra of Au 4f, Bi 4f and S 2p core level for Au/Bi₂S₃@TiO₂ NTAs. The binding energies of 83.8 eV (Au 4f_7/2) and 87.2 eV (Au 4f_5/2) were found in the XPS spectrum (Fig. 4(b)), which indicates the existence of elemental Au NPs [44]. The two strong peaks at 157.8 eV (Bi 4f_7/2) and 163.2 eV (Bi 4f_5/2) in Fig. 4(c) correspond to the standard Bi³⁺ peaks of Bi₂S₃ [45]. The XPS spectrum of sulphur was fitted into two peaks located at 159.2 eV and 164.3 eV shown in Fig. 4(d), which are designated to S 2p_3/2 and 2p_1/2 and correspond to those expected for a sulphide anion. Now, the as-synthesized Bi₂S₃ and Au were confirmed by the XPS analysis.

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Fig. 4.   XPS spectra (a) and high resolution XPS spectra of Au 4f (b), Bi 4f (c) and S 2p (d) obtained on Au/Bi₂S₃@TiO₂ NTAs samples.

XRD measurements were utilized to characterize the crystal phases of the pure TiO₂ NTAs, Au/Bi₂S₃@TiO₂ NTAs, Au@TiO₂ NTAs and Bi₂S₃@TiO₂ NTAs. As shown in Fig. S3, all diffraction peaks were ascribed to TiO₂. The diffraction peaks at 25.3°, 37.9°, 48.0° and 53.9° of pure TiO₂ NTAs correspond to the (101), (004), (200) and (105) planes of the TiO₂ anatase phase (JCPDS No. 21-1272), indicating the formation of anatase TiO₂ after annealing at 450 ℃ for 3 h. However, no obvious peaks of Bi₂S₃ and Au were observed in the Au/Bi₂S₃@TiO₂ NTAs, probably due to the smaller size and well dispersion of Bi₂S₃/Au NPs.

The photocurrent intensity test was performed in simulated sunlight. The switch was turned on and off alternately every 30 s to analyze the photo-induced charge separation. The short-lived photocurrent responses of pure TiO₂ NTAs and Au/Bi₂S₃@TiO₂ NTAs were measured with or without visible light by chronoamperometry. The photocurrent density of all samples raised when the light was on, and reduced when the light was off. The photocurrent density of all samples will fall closer to zero in the absence of light irradiation. Measured photocurrent densities of 0.01 wt% Au/Bi₂S₃@TiO₂ NTAs, pristine TiO₂ NTAs, Au@TiO₂ NTAs and Bi₂S₃@TiO₂ NTAs were 0.75, 0.3, 0.5, 0.6 mA/cm² shown in Fig. 5(a) while those of 0.02 wt%, 0.01 wt%, and 0.005 wt% Au/Bi₂S₃@TiO₂ NTAs were 0.7, 0.75, and 0.45 mA/cm², respectively (Fig. S4(a)). The higher responsive photocurrent of Au/Bi₂S₃@TiO₂ NTAs may be ascribed to two reasons: the sensitization of Au/Bi₂S₃ NPs significantly extends the response of TiO₂ NTAs into the visible region. In addition, the high separation rate of photo-generated electrons-holes could promote the separation and shift of photo-generated electrons [46,47].

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Fig. 5.   Samples of photocurrent response (a), photofluorescence spectra excited at 380 nm (b), UV-vis diffuse reflectance absorption spectra (c).

Fig. 5(b) shows the photo luminescence emission spectra of the 0.01 wt% Au/Bi₂S₃@TiO₂ NTAs, Au@TiO₂ NTAs, Bi₂S₃@TiO₂ NTAs and pure TiO₂ NTAs catalysts under excitation of 380 nm UV light. Compared to TiO₂ with or without Au or Bi₂S₃, the Au/Bi₂S₃@TiO₂ NTAs shows a decrease in photo luminescence intensity indicating a lower electrons-holes recombination rate on Au/Bi₂S₃@TiO₂ NTAs catalyst. This is consistent with literature that photo-generated electrons shift from the CB of TiO₂ to Au NPs, which leads to the separation of electrons and holes leading to a higher photocatalytic activity when Au/Bi₂S₃@TiO₂ NTAs is used as the catalysts [48,49]. Fig. S4(b) shows the low fluorescence intensity of different concentrations of Au NPs of Au/Bi₂S₃@TiO₂ NTAs which also indicates a lower electrons-holes recombination rate. As shown in Fig. 5(c), the optical absorption performance of the prepared photocatalyst (200-800 nm) was investigated using UV-vis diffuse reflectance spectroscopy. The strong enhanced absorption from 400 to 600 nm can be attributed to the LSPR of Au NPs. Compared to the pristine TiO₂ NTAs, Au/Bi₂S₃@TiO₂ NTAs enhanced the photoelectric conversion quantum yield. As the concentration of the precursor increases, the absorption of the Au/Bi₂S₃@TiO₂ NTAs increases. However, the precursor concentration increased again leading to a decrease in the relative absorption intensity in the Au/Bi₂S₃@TiO₂ NTAs electrode (Fig. S4(c)). The relative adsorption value of 0.01 wt% Au/Bi₂S₃@TiO₂ NTAs sample in visible spectrum is the best, which is consistent with the response of photocurrent.

The photocatalytic activity of the as-obtained Au/Bi₂S₃@TiO₂ heterostructure was investigated by UV and simulated visible light photo-degradation of MB dye as representative waste water contaminant. Prior to the photocatalytic reaction, the photocatalyst required an adsorption-desorption experiment to achieve an adsorption-desorption equilibrium and we found that the sample reached equilibrium after 1 h. Fig. 6(a, b) show the degradation rate of MB under UV and visible light irradiation, respectively. C_t represents the concentration of MB after a certain period of reaction, C₀ represents the initial concentration of MB. Distinctively, the degradation rate of MB by using the pristine TiO₂ NTAs electrode shows just about 10.5% with UV light irradiation for 120 min (Fig. 6(a)). But the photocatalytic activity was improved due to the decoration of Au/Bi₂S₃ NPs with a degradation rate of approximately 70%. By contrast, the degradation rate of Au@TiO₂ NTAs and Bi₂S₃@TiO₂ NTAs electrodes were only about 45% and 62% under the same condition. When the light source became simulated visible light (Fig. 6(b)), the MB exhibited about 38% degradation after 120 min for the pristine TiO₂ NTAs electrode. The degradation of MB for the Au/Bi₂S₃@TiO₂ NTAs electrode was 60%, which is lower than that under the UV light [50,51].

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Fig. 6.   Degradation of MB under UV light (a) and visible light (b) irradiation for 120 min.

In order to evaluate the stability of the Au/Bi₂S₃@TiO₂ NTAs electrode, four cycles of photocatalytic degradation of contaminants were carried out under the same environment. The last degradation efficiency of the Au/Bi₂S₃@TiO₂ NTAs electrode under UV and visible light exposure did not display any significant deviation shown in Fig. 7(a, b), which indicates the stability of the electrode.

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Fig. 7.   The reusability of Au/Bi₂S₃@TiO₂ NTAs under visible light (a) and UV (b) irradiation for 8 h. The influences of different scavengers (c) on the degradation of MB over Au/Bi₂S₃@TiO₂ NTAs.

In order to further understand the catalytic mechanism, we performed a free radical capture experiment. The influences of several radical scavengers on the photocatalytic degradation of MB were detected. Ammonium oxalate (AO), isopropanol (IPA) and 1, 4-benzoquinone (BQ) served as h⁺, ^•OH and O₂^- scavengers respectively in the reaction system. As shown in Fig. 7(c), η represents the photodegradation efficiency. When BQ was added, the degradation efficiency was high in the absence of the action of O₂^-, and thus O₂^- plays a secondary role. Similarly, when AO and IPA were added, the degradation effect was significantly reduced, indicating that ^•OH and h⁺ play a major role during photocatalytic degradation.

Due to the high electron affinity of Au, we propose that the interface between Bi₂S₃/Au NPs formed a Schottky junction, and the electric field of the Schottky junction was enhanced to promote the separation of electrons-holes. A schematic synergetic mechanism of MB photodegradation is illustrated in Scheme 2(A). The surface of the photogenerated electrons can react with the absorbed oxygen molecules, resulting in O₂^-. Bi₂S₃ surface can also absorb H₂O molecules to produce ^•OH. Another part of the enhanced photocatalytic activity is that the n-type TiO₂ is close to CB, and the Fermi level of the Bi₂S₃ and TiO₂ form a p-n heterojunction [52,53], and the Fermi level of the p-type Bi₂S₃ is close to VB. When the n-type TiO₂ and the p-type Bi₂S₃ form a p-n heterojunction, it generates an internal electric field between the surface of TiO₂ and Bi₂S₃. The electrons are transferred from Bi₂S₃ to TiO₂, and the holes are transferred from TiO₂ to Bi₂S₃ until the system reaches an equilibrium. Under irradiation with UV, the surface of the n-type TiO₂ produces photo-generated electron holes drifting in the different direction of the electric field. The holes transfer to the negative field, when the electrons flow to the positive field. Hence the electrons move from the CB of Bi₂S₃ to the CB of TiO₂, and accordingly, the holes move from the VB of TiO₂ to the VB of Bi₂S₃. So the reduction of electrons-holes recombination is reduced [54]. The photocatalytic degradation mechanism of MB is as follows:

Bi₂S₃+hυ→Bi₂S₃(e^-+h⁺) (1)

Bi₂S₃(e^-)+TiO₂→TiO₂(e^-)+Bi₂S₃ (2)

Au+hυ→Au³⁺+3e^-(TiO₂) (3)

Au³⁺+3e^-(Bi₂S₃)→Au (4)

TiO₂(e^-)+O₂→TiO₂+·$O_{2}^{-}$ (5)

·$O_{2}^{-}$+H₂O→HO₂·+OH^- (6)

Bi₂S₃(h⁺)+OH^-→OH·+Bi₂S₃ (7)

OH·+MB→Degradation (8)

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Scheme 2.   Mechanism for MB photodegradation (A) and fluorescence detecting Fe³⁺ ions (B) over Au/Bi₂S₃@TiO₂ NTAs.

Recently, heavy attention to environmental protection and human health has urged active research on the underlying effects of transition metal ions. Therefore, scientists have made tremendous efforts to synthesize effective sensors for detecting transition metal ion [[55], [56], [57], [58], [59], [60], [61], [62]]. In our sensing experiment, the sample was immersed in solutions (10 mM) of different metal ions (Cu⁺, Al³⁺, K⁺, Pb²⁺, Cu²⁺, Fe³⁺, Ni²⁺, Cd²⁺, NH₄⁺, Mn²⁺ and Zn²⁺) for 1 h to study the selectivity of the sample to metal ions. As can be seen in Fig. 8, the Fe³⁺ ion has the highest fluorescence intensity change among the 11 kinds of the metal ions. Compared with other metal ions, the fluorescence quenching efficiency of Fe³⁺ ions is greatly improved which clearly demonstrates the high sensitivity and excellent selectivity of Au/Bi₂S₃@TiO₂ NTAs to Fe³⁺ ions.

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Fig. 8.   Fluorescence intensity changes (I₀/I) of the Au/Bi₂S₃@TiO₂ NTAs after being immersed into Cu⁺, Al³⁺, K⁺, Pb²⁺, Cu²⁺, Fe³⁺, Ni²⁺, Cd²⁺, NH₄⁺, Mn²⁺ and Zn²⁺ aqueous solution.

UV-Vis spectroscopy is extensively used, and the characteristics of the absorption peak can be used for simple structural analysis. As shown in Fig. 9(a), the Au/Bi₂S₃@TiO₂ NTAs with different Fe³⁺ ion concentrations have obvious absorption peaks at 353 nm, and with the increase of Fe³⁺ ion concentration, the absorption peaks are increasing. The increase indicates that the Fe³⁺ ions have strong interaction with the Au/Bi₂S₃@TiO₂ NTAs, further explaining that the Au/Bi₂S₃@TiO₂ NTAs and Fe³⁺ ions are formed by the ground state complex, which results from the surface of the Au/Bi₂S₃@TiO₂ NTAs absorbed by Fe³⁺ ions [63,64]. As can be seen from Fig. 9(b), the fluorescence intensity of the Au/Bi₂S₃@TiO₂ NTAs is affected by the concentration of Fe³⁺ ions, and as the concentration of Fe³⁺ ions increases, the fluorescence intensity decreases. There is a linear relationship between them as shown in Fig. 10(a). When the Fe³⁺ ion concentration is beyond 0.01 M, the fluorescence intensity is completely quenched. Therefore, it can be used to determine Fe³⁺ ions. In order to study whether the fluorescence response of Fe³⁺ ions is interfered in the presence of other metal ions, Cu⁺, Al³⁺, K⁺, Pb²⁺, Cu²⁺, Fe³⁺, Ni²⁺, Cd²⁺, NH₄⁺, Mn²⁺ and Zn²⁺ are added to the Au/Bi₂S₃@TiO₂ NTAs/Fe³⁺ system, respectively. It can be seen from Fig. 9(c) that the fluorescence intensity of the Au/Bi₂S₃@TiO₂ NTAs and Fe³⁺ ions is substantially unchanged in the presence of other metals, indicating that the fluorescence sensor has no effect in detecting Fe³⁺ ions in a solution mixed with other metal ions. The pH of the solution also showed an effect on the fluorescence properties, so we investigated the influence of pH on the fluorescence response of Au/Bi₂S₃@TiO₂ NTAs by adjusting the pH of the solution while ensuring the same Fe³⁺ ion concentration. As can be seen from Fig. 9(d), the fluorescence intensity first increases with increasing pH and then decreases with increasing pH, so the optimum pH is 3.07.

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Fig. 9.   UV-vis spectra of the Au/Bi₂S₃@TiO₂ NTAs with different concentrations of Fe³⁺ from 4000 μM to 30 μM (a). Fluorescence quenching of the Au/Bi₂S₃@TiO₂ NTAs with Fe³⁺ ions (0-400 mM) (b). Fluorescence response of the Au/Bi₂S₃@TiO₂ NTAs in the existence of 600 μM Fe³⁺ with/without multifarious metal ions in aqueous media (c). The fluorescence properties of the Au/Bi₂S₃@TiO₂ NTAs at different pH range (1-11) (d).

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Fig. 10.   Linear relation (a) of the Au/Bi₂S₃@TiO₂ NTAs by adding different concentrations of Fe³⁺ when excited at 380 nm. Double logarithm regression plot (b) between log [(I_o - I)/I] vs log [C].

The electron transfer process arouses the transition metal center to undergo fluorescence quenching [65]. The metal center Fe³⁺ ions have a half full level, and its energy must be between VB and CB of the fluorophore in order to realize electron transfer to achieve fluorescence quenching [40]. This is achieved by TiO₂ NTAs modified with Au/Bi₂S₃ NPs. Then two-electron exchange occurs, which transforms the fluorescent group into a matrix, resulting in non-radiative D-D decay in the metal center and fluorescence quenching, as shown in Scheme 2(B).

Fluorescence quenching of the Au/Bi₂S₃@TiO₂ NTAs involves static quenching. The binding constants and the amount of binding sites were calculated by log [(I_o-I)/I] vs log[C] double logistic regression curves (Fig. 10(b)). The formulas are as follows:

$\log[\frac{I_{0}-I}{I}]=\log K+n\log[C]$ (9)

where K is the binding constant and [C] is the concentration of the quencher agent. The amount of binding sites of the Au/Bi₂S₃@TiO₂ NTAs per Fe³⁺ ion was 1.41, and the calculated value of "K" was 1862 M^-1. The higher values of K and n demonstrate a preferable interaction of the Au/Bi₂S₃@TiO₂ NTAs and Fe³⁺ ions [66]. The detection limit is 0.221 μM according to this formula (3σ/S), which is much lower than the ones found in literature, as shown in Table 1 [[67], [68], [69], [70]]. Furthermore, the method of synthesizing fluorophores is simple and cheap compared to the previous methods [[71], [72], [73], [74]].

4. Conclusion

In summary, a simple method was developed to prepare Au/Bi₂S₃@TiO₂ NTAs. Au/Bi₂S₃ NPs were evenly distributed on the TiO₂ NTAs. The presence of Au NPs has the function of electron traps which enhances the charge separation efficiency. Au/Bi₂S₃@TiO₂ NTAs composite exhibits effective visible light activation and good electron transfer ability due to the LSPR effect. Compared with the non-functionalised TiO₂ NTAs, the prepared Au/Bi₂S₃@TiO₂ NTAs composite exhibits increased photocatalytic activity for the photo-degradation of MB under the irradiation of UV and visible light. Moreover, Au/Bi₂S₃ NPs are uniformly dispersed on TiO₂ NTAs, exhibiting high fluorescence emission at 822 nm. Due to the better binding affinity of Bi₂S₃, TiO₂ and Fe³⁺ ions, the synthesized nanocomposites show high selectivity to Fe³⁺ ions. The amount of binding sites of Au/Bi₂S₃@TiO₂ NTAs was estimated to be 1.41 from the double logarithmic regression method. The calculated value of “K” was 1862 M^-1. Fluorescence emission intensity decreases with increasing concentration (30 μM to 5000 μM). The best pH was 3.07. The detection limit of the synthesized sensor is 0.221 μM.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Nos. 51972063 and 21501127), the 111 Project (No. D17005) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

Appendix A. Supplementary data

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jmst.2019.04.043.