Enhanced Visible Light-Responsive Photocatalytic Properties of Ag/BiPbO2Cl Nanosheet Composites

Ag/BiPbO2Cl nanosheet composites were successfully prepared by hydrothermal synthesis and photo-reduction. The morphology, microstructure, and optical properties of the as-prepared Ag/BiPbO2Cl nanosheet composites were characterized using TEM, XRD, and UV-Vis diffuse reflection spectroscopy. The prepared Ag/BiPbO2Cl nanosheet composites with 0.5 wt% Ag exhibit favorable photocatalytic activity, which is 3.6 times that of BiPbO2Cl nanosheet. The enhanced photocatalytic properties can be attributed to the inner electromagnetic field, higher visible light response range, excellent conductivity, and lower Fermi level of Ag.


Background
In recent years, environmental pollution has become increasingly serious. In order to solve the problem of organic pollutants, semiconductor photocatalytic materials have been widely adopted due to their unique advantages [1][2][3][4]. ZnO, TiO 2 , and other wide bandgap semiconductors are popular in photocatalytic degradation of organic pollutants [5][6][7][8]. However, wide bandgap semiconductors can only absorb ultraviolet lights, which limits the application prospects of these catalysts. Therefore, it is necessary to search for photocatalytic materials that are responsive to visible lights [9,10].
Bismuth-based semiconductor photocatalysts possess rich structural characteristics and suitable valence band positions, which can satisfy the requirements of organic matter decomposition [11,12]. Among them, BiPbO 2 Cl is considered to be commendable due to its narrow band gap, built-in electric field between [BiPbO 2 ] and [Cl] plates, and hybrid band structure [13,14]. Nevertheless, the fast electron-hole recombination rate limits its application in the field of photocatalysis.
It has been reported that the combination of semiconductor photocatalytic materials with noble metals or graphene can improve their photocatalytic properties [15,16]. This is because the recombination rate of photo-generated electrons and holes decreases after compounding. Noble metals, such as Au, Ag, and Pt, have been used as electron acceptors to separate the photo-generated electron and holes [17,18].
In this paper, the Ag/BiPbO 2 Cl composite photocatalyst was synthesized by hydrothermal method and photo-reduction for improving the photocatalytic properties of BiPbO 2 Cl nanosheets. The prepared Ag/BiP-bO 2 Cl nanosheet composites with 0.5 wt% Ag exhibit favorable photocatalytic activity, which is 3.6 times that of BiPbO 2 Cl nanosheet.

Preparation of Ag/BiPbO 2 Cl Nanosheet Composites
The BiPbO 2 Cl nanosheets were prepared through one-step hydrothermal method as we used before [13]. The Ag/BiP-bO 2 Cl composites were synthesized by photo-reduction. The obtained BiPbO 2 Cl (1 mmol) was dispersed in 20 mL deionized water with the aid of magnetic stirring, and then, an appropriate amount of AgNO 3 was added. The suspension was then irradiated with a 500-W Xe lamp with stirring at room temperature for 3 h, with light being cut off below 420 nm using a cut-off filter. The resulting granules were washed with deionized water to remove residual organic matter and dried in air at 80°C for 2 h. In order to study the effect of Ag content on the photocatalytic activity of BiPbO 2 Cl, the added contents of Ag were denoted as 0.25, 0.5, and 0.75 wt%.

Photocatalytic Activities
The photocatalytic activity was characterized in a XPA series photochemical reaction instrument by a 500-W Xe lamp with a cutoff filter of 420 nm. The characterization of photocatalytic activity of samples was used by methyl orange (MO) as organic dyes. During the photocatalytic performance test, 50 mg Ag/BiPbO 2 Cl nanosheet composite powders was added into 50 mL MO aqueous solution (10 mg/L) with continuous stirring for 1 h in the dark. The absorption spectra of the solution were collected on a Shimadzu UV-2700 spectrometer.

Sample Characterization
The powder's X-ray diffraction (XRD) patterns were measured on a PANalytical X'Pert Pro X-ray diffractometer with Cu Kα radiation (1.54178 Å). The surface morphology was obtained on the scanning electron microscope (SEM, Hitachi S-4800). Transmission electron microscope (TEM) morphology was measured on a JEOL JEM-2011 TEM. The UV-vis diffuse reflectance spectra were measured on Shimadzu UV-2450. The X-ray photoelectron spectroscopy (XPS) was measured on a Pekin Elmer PHI-5300 XPS. The photoluminescence (PL) emission spectra were measured on a Shimadzu RF-5301 with excitation wavelength at 320 nm.

Results and Discussion
Photocatalytic activity of the BiPbO 2 Cl and Ag/BiP-bO 2 Cl composites has been evaluated with degradation of MO under illumination of visible light (> 420 nm). The concentration of the MO liquid is characterized by the relative absorption strength at 464 nm. Figure 1a shows the visible light photocatalytic activity of the BiPbO 2 Cl and Ag/BiPbO 2 Cl composites. Prior to degradation, the MO solution containing the photocatalyst was stirred for 1 h in the dark environment to achieve adsorption equilibrium. From Fig. 1a, it can be concluded that the photocatalytic efficiency of the BiPbO 2 Cl composites increases with the increase of Ag content, reaching a maximum when the Ag content is 0.5 wt%. This may be due to the absorption of photo-generated electrons by Ag, resulting in a decrease in the photo-generated electron-hole recombination rate, thereby increasing its photocatalytic activity. As the Ag content further increases, its photocatalytic efficiency decreases. When the content of Ag further increases, the content of BiPbO 2 Cl correspondingly decreases, resulting in a decrease in the number of photo-generated carriers and so as the photocatalytic activity. Figure 1b shows the photocatalytic reaction kinetics of the BiPbO 2 Cl and Ag/BiPbO 2 Cl composites. From Fig. 1b, we can draw that the MO degradation rate over Ag/BiPbO 2 Cl composites (0.0158 min −1 ) is about 3.6 times that of the BiPbO 2 Cl (0.0044 min −1 ).
In order to study the morphology and microstructure, SEM, TEM, and XRD were adopted for studying the BiP-bO 2 Cl and Ag/BiPbO 2 Cl composites. From Fig. 2a, one can see that BiPbO 2 Cl is featured as nanosheets, with thickness of about 12 nm. Figure 2b shows the SEM morphology of 0.5 wt% Ag/BiPbO 2 Cl composites; the silver nanoparticles are randomly distributed on the surface of the nanosheet BiPbO 2 Cl. The diameter of Ag particles is about 10 nm. The HRTEM (Fig. 2c) images also reveal the existence of Ag. The existence of Ag is further evidenced by XPS. Figure 2d shows the XRD of BiPbO 2 Cl and 0.5 wt% Ag/BiPbO 2 Cl composites. Compared with the XRD pattern of the BiPbO 2 Cl, the pattern of Ag/BiP-bO 2 Cl composites has no obvious changes, which may result from the low amount of Ag. The compositional analysis is measured by EDS (Fig. 3). Bi, Pb, O, Cl, and Ag In order to study the surface chemical state of the sample, the XPS analysis was adopted for studying the Ag/BiPbO 2 Cl composites. As shown in Fig. 4a, the presence of Bi, Pb, O, Cl, and Ag could be observed in the XPS spectrum. As shown in Fig. 4b, the peaks of Bi 4f 7/2 and Bi 4f 5/2 are located at 159.1 and 164.5 eV, respectively, which are consistent with the characteristic of Bi 3+ [19,20]. The peaks of Pb 4f 7/2 and Pb 4f 5/2 are located at 137.9 and 142.8 eV (Fig. 4c), which are consistent with the characteristic of Pb 2+ [21]. The peak of O 1s is located at 529.8 eV, which belongs to O 2− from the Bi-O bond (Fig. 4d). As displayed in Fig. 4e, two peaks of Cl 2p are at 197.8 and 199.4 eV, which are consistent with the characteristic of Cl 1− [22]. As shown in Fig. 4f, two peaks of 368.1 and 374.3 eV are observed, which correspond to Ag 3d 3/2 and Ag 3d 5/2 , respectively. According to the results reported by Zhang et al. [23], the peaks at 368.6 and 374.6 eV can be attributed to Ag 0 .
Compared with the yellow BiPbO 2 Cl nanosheets, the color of the Ag/BiPbO 2 Cl composites becomes darker with the increase of Ag content. The UV-vis absorption spectra of BiPbO 2 Cl and Ag/BiPbO 2 Cl composites are shown in Fig. 5a. The strong absorption below a wavelength of 600 nm is associated with the optical band gap of BiPbO 2 Cl. After loading Ag on the surface of BiP-bO 2 Cl, the absorbance at the range of 450-800 nm is higher than that of pure BiPbO 2 Cl, which is due to the absorption characteristic of surface plasmon caused by the composite of Ag and BiPbO 2 Cl [24]. As a result, after the loading of Ag on the surface of BiPbO 2 Cl, the visible light response range of BiPbO 2 Cl is increased. The band gap calculated from Fig. 5a is shown in Fig. 5b. After compounding with Ag, the band gap of BiPbO 2 Cl decreases from 2.05 to 1.68 eV. In addition, the photoluminescence emission spectra of BiPbO 2 Cl and Ag/BiP-bO 2 Cl composites are carried out to reflect the recombination rate of photo-generated electrons and holes. As shown in Fig. 5c, the PL intensity is decreased drastically after the loading of Ag on the surface of Fig. 2 The SEM of BiPbO 2 Cl (a) and 0.5 wt% Ag/BiPbO 2 Cl composites (b). c High-resolution TEM image of the 0.5 wt% Ag/BiPbO 2 Cl composites. d XRD of samples BiPbO 2 Cl, which is attributed to the fast transfer of photo-generated electrons from BiPbO 2 Cl to Ag, leading to reduction of recombination rate of photo-generated electrons and holes [25].
The principle of high photocatalytic activity for Ag/BiPbO 2 Cl composites is as follows. First of all, the visible light response range is increased by the composition of Ag and BiPbO 2 Cl. Secondly, the loading of Ag on the surface of BiPbO 2 Cl could generate the inner electromagnetic field. When the BiPbO 2 Cl semiconductor surface is in contact with Ag, redistribution of carriers is realized. Since the Fermi level of Ag is lower than that of BiPbO 2 Cl [26], the photo-excited electrons transfer from the BiPbO 2 Cl to Ag particles until their Fermi level is the same, thus forming the built-in field, as shown in Fig. 6b. The photo-generated electron will transfer quickly from BiPbO 2 Cl to Ag with the help of the inner electromagnetic field and excellent conductivity of Ag. Thirdly, as shown in Fig. 6a, the electrons generated by BiPbO 2 Cl will reduce the molecular O 2 to form the O 2 • active species [27]. On the other side, the photo-generated holes tend to remain on the surface of BiPbO 2 Cl. And then, these holes will

Conclusions
In summary, highly efficient Ag/BiPbO 2 Cl composites were prepared by hydrothermal synthesis and photo-reduction. The obtained 0.5 wt% Ag/BiPbO 2 Cl nanosheet composite material has better photocatalytic activity, which is 3.6 times that of BiPbO 2 Cl nanosheets. After BiPbO 2 Cl nanosheets and Ag are composited, the visible light response range increases and the electron-hole recombination rate decreases, thus improving the visible light photocatalytic properties. The excellent photocatalytic property of Ag/BiPbO 2 Cl composites are attributed to the inner electromagnetic field, higher visible light response range, excellent conductivity, and lower Fermi level of Ag.
Abbreviations DRS: Diffuse reflection spectroscopy; MO: Methyl orange; TEM: Transmission electron microscope; XRD: X-ray diffraction Fig. 5 The UV-vis absorption spectra (a) and photoluminescence emission spectra (b) of BiPbO 2 Cl and 0.5 wt% Ag/BiPbO 2 Cl composites (c) Fig. 6 a Mechanistic illustration of Ag/BiPbO 2 Cl composites for photocatalytic activity. b Band structure at the interface of Ag and BiPbO 2 Cl. The data used for BiPbO 2 Cl are from reference [26]