p-AgBr heterostructured photocatalysts for dye degradation

Journal Pre-proof Preparation and photocatalytic application of ternary n-BaTiO3 /Ag/p-AgBr heterostructured photocatalysts for dye degradation Yanping Wang (Data curation) (Writing - original draft), Hua Yang (Conceptualization) (Supervision) (Writing - review and editing), Xiaofeng Sun (Data curation), Haimin Zhang (Data curation) (Validation), Tao Xian (Data curation) (Validation)

PII:

S0025-5408(19)32651-0

DOI:

https://doi.org/10.1016/j.materresbull.2019.110754

Reference:

MRB 110754

To appear in:

Materials Research Bulletin

Received Date:

12 October 2019

Revised Date:

20 December 2019

Accepted Date:

23 December 2019

Please cite this article as: Wang Y, Yang H, Sun X, Zhang H, Xian T, Preparation and photocatalytic application of ternary n-BaTiO3 /Ag/p-AgBr heterostructured photocatalysts for dye degradation, Materials Research Bulletin (2019), doi: https://doi.org/10.1016/j.materresbull.2019.110754

This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.

Preparation and photocatalytic application of ternary n-BaTiO3/Ag/p-AgBr heterostructured photocatalysts for dye degradation Yanping Wang1, Hua Yang1,*, Xiaofeng Sun2, Haimin Zhang1, Tao Xian2 1

State Key Laboratory of Advanced Processing and Recycling of Non-ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China

College of Physics and Electronic Information Engineering, Qinghai Normal University, Xining 810008, China

Fax: +86 931 2976040

E-mail: [email protected] (H. Yang)

Jo

ur

na

lP

re

Tel: +86 931 2973783

-p

*Corresponding author:

1

ro of

2

-p

ro of

Graphical abstract

re

Highlights

New ternary n-BaTiO3/Ag/p-AgBr heterostructured photocatalysts were prepared.



The ternary composite photocatalysts are constructed by coupling BaTiO3 and AgBr

lP



particles onto Ag nanowires

The ternary composite photocatalysts exhibit excellent photodegradation of RhB



The underlying photodegradation mechanism was systematically investigated and

Abstract:

ur

discussed.

na



Jo

We reported the preparation of new ternary n-BaTiO3/Ag/p-AgBr heterostructured

photocatalysts and their promising photocatalytic application for the dye degradation. The ternary n-BaTiO3/Ag/p-AgBr heterostructured photocatalysts are constructed by coupling n-type BaTiO3 and p-type AgBr particles onto Ag nanowires. Various methods including XRD, SEM, TEM, XPS, FTIR, UV–vis diffuse reflectance spectroscopy, PL spectroscopy, photocurrent response and EIS were used to characterize the as-prepared photocatalysts. The photodegradation 2

performances of the photocatalysts were assessed by using RhB as the model pollutant and simulated sunlight as the light source. It is demonstrated that the n-BaTiO3/Ag/p-AgBr ternary composite photocatalysts exhibit excellent photodegradation performances superior to bare BaTiO3 and AgBr particles. Particularly, the 20%BTO/1.2%Ag/AgBr composite can photocatalyze 99.3% removal of RhB after 12 min photoreaction, and has a photodegradation activity which is about 3.1 times larger than that of bare AgBr particles. The underlying photodegradation mechanism of the ternary composite photocatalysts was investigated and

ro of

discussed.

Key words: AgBr particles; BaTiO3 particles; Ag nanowires; BaTiO3/Ag/AgBr composites;

-p

photodegradation performance

re

1. Introduction

lP

The rapid industrialization has caused ever-increasing pollution to our surroundings. In particular, billions of tons of wastewater is generated annually from the chemical industries, which contains a huge number of harmful and carcinogenic substances, such as organic dyes. As

na

one of the famous organic dyes largely produced in the industrial wastewater, rhodamine B (RhB) is highly water-soluble, chemically stable, non-biodegradable and carcinogenic. It must be

ur

artificially removed because its toxic and carcinogenic properties pose a great threat to mankind’s

Jo

health and aquatic life. In recent years, semiconductor-based photocatalysis—a “green” technology has received much attention due to its potential application in the wastewater purification [1‒4]. This technology offers an outstanding advantage that it can decompose organic dyes into harmless inorganic substances by using solar radiation as the power source. The sunlight irradiation stimulates the generation of conduction band (CB) electrons (e−) and valence band (VB) holes (h+) in semiconductor photocatalysts. The photogenerated electrons have a reduction 3

capability and photogenerated holes have an oxidation capability, which directly or indirectly act on organic dyes to cause their degradation. However, the photogenerated electrons and holes in semiconductors are quite easy to recombine, thus limiting their photocatalytic activity. To achieve the excellent photodegradation performance of semiconductors, the photogenerated electrons and holes have to be spatially separated so that more charge carriers are available for the photocatalytic reactions [5‒11]. Nanostructured carbon materials and noble metals (e.g., carbon quantum

ro of

dots/nanotubes/graphene, noble metal nanoparticles/nanowires) have been widely used as co-catalysts or modifiers to enhance the photocatalytic performances of semiconductor

photocatalysts [12‒16]. These nanomaterials have received a great deal of interest owing to their

-p

intriguing physicochemical properties and a wide range of technological applications in biomedicine, electronic devices, sensors, wave absorption, and so on [17‒25]. In the

re

photocatalysis application, the carbon and noble metal nanomaterials can act as excellent electron

lP

sinks to prevent the recombination of photoexcited e−/h+ pairs by trapping the electrons. Further, the photoluminescence (PL) up-conversion of nanocarbons and localized surface plasmon resonance (LSPR) of nanometals can enhance the absorption and utilization of visible light during

na

the photocatalysis process [26–28]. Another important strategy for promoting the separation of photogenerated e−/h+ pairs is to construct heterostructured composites between two

ur

semiconductors [29,30]. The carrier transfer between two semiconductors is the dominant

Jo

mechanism resulting in the efficient separation of photogenerated e−/h+ pairs. However, for most of the semiconductor-semiconductor hybrid composite photocatalysts, the poor interface between the two semiconductors could influence the carrier transfer and separation. Due to their excellent conductivity, carbon and noble metal nanomaterials can be introduced at the interface between semiconductors with the aim of promoting the transfer and separation of photogenerated e−/h+ pairs. 4

Barium titanate (BaTiO3, BTO) with a perovskite-type structure is known to be an important multifunctional material exhibiting many intriguing physical properties. In addition to its famous ferroelectric and piezoelectric properties [31], BaTiO3 also shows an interesting photodegradation capability for organic dyes [32,33]. Nevertheless, the photocatalytic activity of pristine BaTiO3 is generally very poor due to the high recombination rate of photoexcited e−/h+ pairs. In contrast, silver halides AgX (X = Cl, Br, I) exhibit a photodegradation activity that is much higher than that of BaTiO3 [34]. However, AgX easily undergoes photocorrosion due to the reduction of Ag+

ro of

into Ag0 by photogenerated electrons. To overcome the limitations mentioned above, BaTiO3 and AgX have been widely incorporated with other semiconductors to form promising heterostructure photocatalysts [33–39].

-p

Herein we have assembled BaTiO3 and AgBr particles onto Ag nanowires to construct new ternary BaTiO3/Ag/AgBr heterostructure composite photocatalysts. In the ternary composite

re

photocatalysts, Ag nanowires can act as excellent conductive bridges to render facile charge transfer between BaTiO3 and AgBr particles and efficiently inhibit the recombination of

lP

photogenerated e−/h+ pairs. Moreover, this efficient charge transfer process can prevent the photocorrosion behavior of AgBr and thus increases the reusability of the ternary

na

BaTiO3/Ag/AgBr composite photocatalysts. The photodegradation performances of the as-prepared ternary composite photocatalysts were assessed by degrading RhB aqueous solution

ur

under simulated sunlight illumination. The underlying photodegradation mechanism was

Jo

investigated and discussed. 2. Experimental 2.1 Materials

P25 (mixed phase of anatase and rutile TiO2, ≥ 99.8%), silver nitrate (AgNO3, ≥ 99%) and sodium bromide (NaBr, ≥ 99%) were purchased from Shandong Xiya Chemical Industry Co., Ltd. Barium hydroxide octahydrate (Ba(OH)2∙8H2O, ≥ 98%) and polyvinyl pyrrolidone (PVP) were 5

obtained from Shanghai Aladdin Reagent Co., Ltd. Sodium hydroxide (NaOH, ≥ 96%) and sodium chloride (NaCl, ≥ 99.5%) was supplied by Guangdong Chemical Reagent Engineering-Technological Research and Development Center. Ethylene glycol was derived from Yantai Shuangshuang Chemical Co., Ltd. All the raw materials and chemical reagents, being of analytical grade, were directly used in the experiments without further purification. 2.2 Synthesis of BaTiO3 particles A hydrothermal route was used to prepare BaTiO3 particles. In a typical synthesis process,

ro of

1.0410 g of Ba(OH)2∙8H2O and 0.1 g of PVP were dissolved in 20 mL deionized water (designated as solution A). 0.2396 g of P25 was dispersed in 20 mL deionized water (designated as suspension B) and 8 g of NaOH was dissolved in 40 mL deionized water (designated as

-p

solution C). The suspension B and solution C were successively added to the solution A drop by drop, followed by 30 min of magnetic stirring. The obtained mixture solution was loaded into a

re

stainless steel autoclave with a 100 mL Teflon liner, and subjected to hydrothermal reaction at

lP

200 oC for 24 h. After the autoclave was naturally cooled to room temperature, the product was collected as BaTiO3 particles. To remove the impurities, the product was washed with 30%(v/v)

h of drying at 60 oC.

na

HNO3 solution (one time), deionized water (three times) and ethanol (three times), followed by 12

2.3 Synthesis of BTO/AgBr hybrid composites

ur

Several x%BTO/AgBr hybrid composite samples (x% = mBTO/(mBTO + mAgBr) × 100% = 10%,

Jo

20%, 30% and 40%) were prepared via a co-precipitation route. Typically, to prepare 20%BTO/AgBr, 0.3 g of as-synthesized BaTiO3 particles was dispersed in 50 mL deionized water. 1.0860 g of AgNO3 and 0.6578 g of NaBr were dissolved in the BaTiO3 suspension. After 5 h of magnetic stirring, the product was washed with deionized water/ethanol and dried at 60 oC for 12 h to obtain 20%BTO/AgBr hybrid composite. Bare AgBr particles were also prepared via the co-precipitation method without adding BaTiO3 particles. 6

2.4 Synthesis of BTO/Ag/AgBr hybrid composite A typical hybrid composite 20%BTO/1.2%Ag/AgBr was prepared as follows. 1 g of the as-prepared BaTiO3 particles were dispersed in 30 mL ethylene glycol with 30 min ultrasonic treatment. 0.1 g of PVP, 0.1 g of AgNO3 and 2 mg of NaCl were dissolved in 20, 20 and 10 mL ethylene glycol, respectively. The obtained PVP, AgNO3 and NaCl solutions were successively added to the BaTiO3 suspension slowly. After 30 min of magnetic stirring, the mixture was transferred into a Teflon-lined stainless steel autoclave (capacity: 100 mL), and subjected to heat

ro of

treatment at 160 oC for 5 h. After the autoclave was naturally cooled to room temperature, the product was obtained as BTO/6%Ag (washing with acetone and ethanol, and drying at 60 oC for 2 h). 0.3 g of the as-prepared BTO/6%Ag composite was dispersed into 50 mL deionized water. To

-p

the BTO/6%Ag suspension was added with 1.0860 g AgNO3 (magnetic stirring for 1 h) and then 0.6578 g of NaBr (magnetic stirring for 5 h). The precipitant, after washed with deionized

re

water/ethanol and dried at 60 oC, was obtained as the 20%BTO/1.2%Ag/AgBr hybrid composite,

lP

where BTO and Ag occupied 20% and 1.2% in the composite, respectively. The preparation

Jo

ur

na

process of BTO/Ag/AgBr hybrid composite photocatalyst was schematically illustrated in Fig. 1.

Fig. 1 Schematic illustration of the preparation process of BTO/Ag/AgBr hybrid composite 7

photocatalyst.

2.5 Characterization methods A D8 Advance X-ray diffractometer was used for the X-ray powder diffraction (XRD) analysis of the samples. The scanning/transmission electron microscopy (SEM/TEM) observations were performed on a JSM-6701F field-emission scanning electron microscope and a JEM-1200EX field-emission transmission electron microscope. X-ray photoelectron spectroscopy

ro of

(XPS) measurement was carried out on a PHI-5702 multi-functional X-ray photoelectron spectrometer. A TU-1901 double beam ultraviolet-visible (UV-vis) spectrophotometer was

applied for the UV-vis diffuse reflectance spectroscopy measurement. A Spectrum Two FTIR

-p

spectrophotometer was available for the Fourier transform infrared (FTIR) spectroscopy

investigation. An RF-6000 fluorescence spectrophotometer was used to record the PL spectra of

lP

2.6 Photoelectrochemical measurement

re

the samples (excitation wavelength: ~315 nm).

The photoelectrochemical properties of the samples, including photocurrent response and electrochemical impedance spectroscopy (EIS), were measured on a CST 350 electrochemical

na

workstation by using a three-electrode cell configuration. The working electrode preparation and photoelectrochemical measurement procedures were described in our previous work [40]. 0.1 mol

ur

L−1 Na2SO4 aqueous solution was used as the electrolyte. Simulated sunlight emitted from a

Jo

200-W xenon lamp was used as the light source. 2.7 Photocatalytic testing procedure RhB aqueous solution with concentration of 5 mg L−1 was used as the model pollutant to

assess the photodegradation performances of the samples under irradiation of simulated sunlight emitted from a 200-W xenon lamp. The photocatalyst dosage was 0.1 g in 100 mL RhB solution (Cphotocatalyst = 1 g L−1). The adsorption of RhB onto the photocatalyst was determined by 8

magnetically stirring the mixture solution for 30 min in the dark. After that, the photodegradation experiment was initiated with turning on the 200-W xenon lamp. To monitor the residual RhB concentration during the photodegradation process, 2.5 mL of the reaction solution was sampled from the photoreactor, and its absorbance was measured on a UV-vis spectrophotometer at λRhB = 554 nm. Before the absorbance measurement, the photocatalyst was removed by centrifugation (4000 rpm, 10 min). From residual concentration of RhB (Ct), the degradation percentage (η) of RhB is derived as: η = (C0 − Ct)/C0 × 100%, where C0 is the initial RhB concentration.

ro of

3. Results and discussion To determine the crystalline structures of AgBr, BaTiO3, 20%BTO/AgBr and

20%BTO/1.2%Ag/AgBr, XRD patterns were recorded, as shown in Fig. 2. It is seen that the

-p

diffraction peaks of bare AgBr can be perfectly indexed in terms of the standard diffraction

pattern of PDF#79-0149, implying the crystallization of pure AgBr cubic phase with space group

re

Fm-3m (a = b = c = 0.5775 nm). The BaTiO3 sample shows diffraction peaks that are perfectly

lP

consistent with the standard diffraction pattern of PDF#79-2263, indicating the formation of pure BaTiO3 cubic phase with space group Pm-3m (a = b = c = 0.4006 nm). The XRD pattern of 20%BTO/AgBr presents two sets of diffraction peaks, corresponding to the cubic BaTiO3

na

(dominant phase) and cubic AgBr (minor phase). Whereas for 20%BTO/1.2%Ag/AgBr, additional diffraction peaks from Ag are detected on the XRD pattern, implying the formation of

Jo

ur

ternary BTO/Ag/AgBr hybrid composite photocatalyst.

9

Fig. 2. XRD patterns of AgBr, BaTiO3, 20%BTO/AgBr and 20%BTO/1.2%Ag/AgBr.

It is generally accepted that the physical properties of nanomaterials are highly related to their light-absorption characteristics [41‒43]. To determine the optical absorption properties of AgBr, BaTiO3, 20%BTO/AgBr and 20%BTO/1.2%Ag/AgBr, their UV-vis diffuse reflectance spectra were measured as shown in Fig. 3(a). It is seen that bare BaTiO3 has a poor optical absorption in the visible-light region with λ > 450 nm, whereas bare AgBr has a relatively higher

ro of

visible-light absorption. A strong visible-light absorption is observed for the 20%BTO/1.2%Ag/AgBr composite, which is due to the fact that Ag nanowires with strong

visible-light absorption are introduced into the composite. The visible-light absorption properties

-p

of the samples can be confirmed by their apparent colors, as depicted in Fig. 3(c). The

first-derivative curves of the UV-vis diffuse reflectance spectra were used to determine the

re

bandgap of the samples, as shown in Fig. 3(b). According to the peak on the first-derivative

lP

curves [44], the absorption edge wavelength λabs of bare BaTiO3 and AgBr is obtained as 386.7 and 458.6 nm, respectively. This suggests that bare BaTiO3 has a bandgap of 3.21 eV and bare AgBr has a bandgap of 2.70 eV. For the 20%BTO/AgBr and 20%BTO/1.2%Ag/AgBr composites,

Jo

ur

na

the bandgaps of BaTiO3 and AgBr undergo almost no change.

10

ro of

-p

Fig. 3. UV-vis diffuse reflectance spectra (a), first derivative curves of the diffuse reflectance spectra (b) and apparent colors (c) of BaTiO3, AgBr, 20%BTO/AgBr and

re

20%BTO/1.2%Ag/AgBr.

lP

Fig. 4(a)‒(c) show the SEM images of AgBr, BaTiO3 and 20%BTO/1.2%Ag/AgBr, respectively. It is seen that bare AgBr is crystallized into sphere-like particles with size of 1–2 μm

na

(average particle size: 1.5 μm), and bare BaTiO3 presents a morphology of cubic or polyhedral particles with size of 150–350 nm (average particle size: 180 nm). For the

ur

20%BTO/1.2%Ag/AgBr composite, BaTiO3 cubic/polyhedral particles are evolved into sphere-like particles due to the Ostwald ripening mechanism (i.e., dissolution-crystallization

Jo

mechanism) occurring in the hydrothermal assembling process of the BTO/Ag composite. Moreover, AgBr particles become much smaller in the composite compared to bare AgBr particles. This phenomenon implies that BaTiO3 particles and Ag nanowires can greatly inhibit the growth of AgBr particles. The SEM images clearly demonstrate that AgBr and BaTiO3 particles are coupled with Ag nanowires to form ternary BTO/Ag/AgBr hybrid heterostructures.

11

ro of

-p

Fig. 4. SEM images of (a) AgBr, (b) BaTiO3 and (d) 20%BTO/1.2%Ag/AgBr.

re

To reveal the microstructure of the ternary 20%BTO/1.2%Ag/AgBr hybrid composite, TEM investigation was further performed. Fig. 5(a) shows the TEM image of 20%BTO/1.2%Ag/AgBr,

lP

revealing the coupling of AgBr and BaTiO3 particles with Ag nanowires. The selected area electron diffraction (SAED) pattern in Fig. 5(b) shows obvious diffraction spots, indicating the

na

good crystallization of the 20%BTO/1.2%Ag/AgBr composite. However, these diffraction spots are irregularly and nonperiodically arranged, and moreover, some of the spots are very weak,

ur

which implies the possible existence of fine AgBr or BaTiO3 nanoparticles in the composite. The good crystallization and hybrid composite structure of 20%BTO/1.2%Ag/AgBr is further

Jo

confirmed by the high-resolution TEM (HRTEM) images as given in Fig. 5(c) and (d). In the HRTEM image of Fig. 5(c), the nanowire exhibits crystal lattice fringes with d = 0.204 nm, which correspond to the (200) facets of cubic Ag, and the particle decorated onto the Ag nanowires displays lattice fringes with d = 0.289 nm, which are ascribed to the (200) facets of cubic AgBr. In the HRTEM image of Fig. 5(d), the lattice fringes with d = 0.333 and 0.231 nm match well with the (111) facts of cubic AgBr and (111) facets of cubic BaTiO3, respectively. In addition to 12

the ternary BTO/Ag/AgBr hybrid structures, the TEM observation also suggests the possible

lP

re

-p

ro of

formation of BTO/AgBr heterostructures in the 20%BTO/1.2%Ag/AgBr composite.

na

Fig. 5. TEM images (a),SAED pattern (b) and HRTEM images (c,d) of 20%BTO/1.2%Ag/AgBr.

The 20%BTO/1.2%Ag/AgBr composite was further analyzed from the point of view of

ur

elemental composition. The energy-dispersive X-ray spectroscopy (EDS) spectrum given in Fig. 6(a) clearly shows the presence of elements Ba, Ti, O, Ag and Br in the composite. The

Jo

additionally detected Cu and C signals on the EDS spectrum could originate from the TEM microgrid holder [45]. Fig. 6(b) shows the dark-field scanning TEM (DF-STEM) image of 20%BTO/1.2%Ag/AgBr, and Fig. 6(c)‒(g) illustrate the corresponding EDS elemental mapping images. The element distribution clearly demonstrates that the composite is composed dominantly of AgBr particles, together with a relatively small quantity of BaTiO3 particles nanocuboids and Ag nanowires. Moreover, the AgBr and BaTiO3 particles are well coupled with Ag nanowires, 13

ro of

giving support to the formation of ternary BTO/Ag/AgBr hybrid heterostructures.

-p

Fig. 6. EDS spectrum (a), DF-STEM image (b) and EDS elemental mapping images (c‒g) of

re

20%BTO/1.2%Ag/AgBr.

lP

The element oxidation states in the 20%BTO/1.2%Ag/AgBr composite was elucidated by the XPS analysis. The survey scan XPS spectrum in Fig. 7(a) reveals that the elements Ba, Ti, O,

na

Ag and Br are obviously included in the composite. The binding energy was calibrated by the C 1s peak at 284.8 eV. The high-resolution XPS spectra of Ba 3d, Ti 2p, O 1s, Ag 3d and Br 3d core

ur

levels are measured as indicated in Fig. 7(b)‒(h). The Ba 3d XPS spectrum (Fig. 7(b)) presents two sharp peaks at 779.8 and 795.2 eV, which are characterized as the Ba 3d5/2 and Ba 3d3/2

Jo

core-electron binding energies of Ba2+ species, respectively [46]. On the Ti 2p XPS spectrum (Fig. 7(c)), the observed peaks at 459.3 and 465.1 eV correspond to the Ti 2p3/2 and Ti 2p1/2 core-electron binding energies of Ti4+ species, respectively [46]. Two types of oxygen species are observed on the O 1s XPS spectrum (Fig. 7(d)). The crystal lattice oxygen of BaTiO3 is confirmed by the O 1s binding energy at 530.7 eV, whereas the O 1s binding energy at 532.6 eV implies the presence of chemisorbed oxygen species on the sample [47,48]. The Ag 3d XPS 14

spectrum (Fig. 7(e)) can be deconvoluted into two sharp peaks at 367.7 (Ag+ 3d5/2) and 373.7 eV (Ag+ 3d3/2), and two weak peaks at 368.6 (Ag0 3d5/2) and 374.6 eV (Ag0 3d3/2). This suggests that Ag species exists mainly in Ag+ oxidation state from AgBr particles and minorly in Ag0 metallic state from Ag nanowires [49]. The peaks at 68.0 and 69.0 eV detected on the Br 3d XPS spectrum (Fig. 7(f)) are attributed to Br− 3d5/2 and Br− 3d3/2 core-electron binding energies, respectively

ur

na

lP

re

-p

ro of

[49].

Fig. 7.

Jo

Fig. 7. Survey scan XPS spectrum (a) and high-resolution XPS spectra of (b) Ba 3d, (c) Ti 2p, (d) O 1s, (e) Ag 3d and (f) Br 3d of 20%BTO/1.2%Ag/AgBr.

Fig. 8 shows the FTIR spectra derived from AgBr, BaTiO3 and 20%BTO/1.2%Ag/AgBr. On the FTIR spectrum of BaTiO3, the broad absorption peak centered around 549 cm−1 is characterized as the Ti–O vibration mode, which confirms the crystallization of BaTiO3 15

perovskite-type structure [50]. No obvious absorption peaks assignable to AgBr is detected on the FTIR spectrum of AgBr, possibly due to the infrared inactivity of AgBr particles. For the 20%BTO/1.2%Ag/AgBr composite, the presence of BaTiO3 is confirmed by the observation of Ti–O vibration absorption peak at 549 cm−1. For all the samples, two obvious absorption peaks are observed at 1392 and 1633 cm−1. The absorption peak at 1392 cm−1 can be ascribed to the anti-symmetric stretching vibration of CO32− groups [50]. The absorption peak at 1633 cm−1 is caused by water molecules (H–O bending vibration) adsorbed on the surface of the samples

lP

re

-p

ro of

[51,52].

na

Fig. 8. FTIR spectra of AgBr, BaTiO3 and 20%BTO/1.2%Ag/AgBr.

The photoexcited carrier separation behavior, having an important effect on the

ur

photodegradation performance of photocatalysts, can be characterized by PL spectroscopy [53]. Fig. 9 shows the PL spectra of AgBr, 20%BTO/AgBr and 20%BTO/1.2%Ag/AgBr. An obvious

Jo

broad PL emission peak centered around 467 nm is observed for all the samples, which is induced by the recombination of photoexcited electron/hole pairs. However, the PL emission peak from the composites becomes weaker than that from bare AgBr; in particular, the 20%BTO/1.2%Ag/AgBr composite has the weakest PL emission peak. The PL spectroscopy analysis suggests an enhanced electron/hole separation efficiency in the 20%BTO/1.2%Ag/AgBr ternary composite photocatalyst. 16

ro of

Fig. 9. PL spectra of AgBr, 20%BTO/AgBr and 20%BTO/1.2%Ag/AgBr.

Photoelectrochemical measurements, including photocurrent response and electrochemical EIS, were further carried out to elucidate the photoexcited carrier separation/transfer behavior in

-p

the photocatalysts. By intermittently illuminating the photocatalysts with simulated sunlight, the

re

transient photocurrent-time curves were recorded as shown in Fig. 10(a). An obvious photocurrent response behavior with cycles of light-on and light-off is observed for all the photocatalysts. On

lP

the irradiation, the potocurrent density of the photocatalysts is obtained as 4.85 (AgBr), 0.31 (BaTiO3), 5.66 (20%BTO/AgBr) and 7.77 μA cm−2 (20%BTO/1.2%Ag/AgBr). The highest

na

potocurrent density is observed for the ternary 20%BTO/1.2%Ag/AgBr composite. Fig. 11(b) shows the EIS Nyquist plots of the samples, on which a typical semicircle is observed in the

ur

high-frequency region. The semicircle diameter is highly related with the charge-transfer resistance [54]. Among the samples, the ternary 20%BTO/1.2%Ag/AgBr composite exhibits the

Jo

smallest semicircle diameter, implying that it has the smallest charge-transfer resistance. Both the photocurrent response and EIS analyses further confirm that the composite photocatalysts, particularly the ternary 20%BTO/1.2%Ag/AgBr composite, demonstrate much enhanced electron/hole separation and interface charge transfer properties.

17

ro of -p re

Fig. 10. Transient photocurrent response curves (a) and Nyquist plots of the EIS spectra (b) of

lP

AgBr, BaTiO3, 20%BTO/AgBr and 20%BTO/1.2%Ag/AgBr.

na

The photodegradation performances of the samples were investigated by degrading RhB in aqueous solution under simulated sunlight irradiation. Fig. 11(a) illustrates the time-dependent

ur

photodegradation of RhB over the samples, together with the RhB adsorption onto the samples at 30 min of contact time in the dark. A very small RhB adsorption is observed for bare BaTiO3

Jo

particles, whereas bare AgBr particles and the composite photocatalysts have a relatively large adsorption toward the dye. It is generally accepted that an appropriate adsorption of the dye molecules onto the photocatalyst surface is beneficial to the dye photodegradation. At 12 min of photocatalytic reaction, the degradation percentage of RhB is observed to be η = 7.5% for bare BaTiO3 particles and η = 80.5% for bare AgBr particles, implying that the latter exhibits a photodegradation activity much superior to the former. When BaTiO3 and AgBr are coupled 18

together, the formed BTO/AgBr heterostructure composites exhibit an enhanced photodegradation activity. With increasing the BaTiO3 content from 10% to 40%, the optimal composite photocatalyst is observed for 20%BTO/AgBr, which photocatalyzes 90.5% degradation of RhB after 12 min photoreaction. Furthermore, a degradation percentage of η = 99.3% is achieved for the ternary 20%BTO/1.2%Ag/AgBr composite, indicating that the introduction of Ag nanowires can further improve the photodegradation performance of the composite photocatalyst.

ro of

The degradation kinetic analysis is very useful in qualitatively comparing the photodegradation activity between the samples. As shown in Fig. 11(b), the degradation kinetic plots exhibit a linear trend with irradiation time, conforming well to the pseudo-first-order kinetic

-p

equation Ln(Ct/C0) = −kappt, where kapp is the apparent first-order reaction rate constant [55]. It is seen that the value of kapp is increased from 0.13145 min−1 for bare AgBr to 0.18938 min−1 for

re

20%BTO/AgBr and 0.40304 min−1 for 20%BTO/1.2%Ag/AgBr. This implies that the

lP

20%BTO/AgBr and 20%BTO/1.2%Ag/AgBr composites have a photodegradation activity which is about 1.4 and 3.1 times as large as that of bare AgBr particles, respectively. The reusability of the 20%BTO/1.2%Ag/AgBr composite photocatalyst was examined by

na

recycling photocatalytic experiment. After each cycle of the photodegradation experiment, the photocatalyst was collected and recovered by washing with deionized water and drying at 60°C

ur

for 5 h. The recovered photocatalyst was loaded in 100 mL of fresh RhB solution and initiated the

Jo

next photodegradation cycle under the same condition. Fig. 11(c) shows the degradation percentage of RhB (irradiation time: 12 min) over 20%BTO/1.2%Ag/AgBr repeatedly used for four times. It is observed that the composite photocatalyst maintains a high photocatalytic removal of RhB at the 4th photocatalytic cycle (η = 94.7%), indicating that it exhibits a good recycling stability and reusability for the dye degradation. Reactive species trapping experiments were carried out to determine the role of hydroxyl 19

(•OH), superoxide (•O2−) and photoexcited h+ in the RhB photodegradation over the 20%BTO/1.2%Ag/AgBr composite photocatalyst. To achieve this aim, 5 mL of ethanol (scavenger of •OH), 0.1 mmol of benzoquinone (BQ, scavenger of •O2−) and 0.1 mmol of ammonium oxalate (AO, scavenger of h+) were separately added in 100 mL reaction solution (CRhB = 5 mg L−1, Cphotocatalyst = 1 g L−1) [56]. The photodegradation experiments were performed under the procedure same to that without adding scavengers. Fig. 11(d) shows the effects of ethanol, BQ and AO on the degradation percentage of RhB (irradiation time: 12 min). It is found

ro of

that the photodegradation of RhB is decreased by adding all the scavengers, implying that the photodegradation reactions are related to •OH, •O2− and h+. In particular, •O2− plays the largest

Jo

ur

na

lP

re

-p

role in the photodegradation of RhB since BQ causes the highest suppression efficiency.

Fig. 11. (a) Time-dependent photodegradation curves of RhB over the as-prepared samples. (b) Kinetic plots of the RhB degradation. (c) Photodegradation of RhB over 20%BTO/1.2%Ag/AgBr repeatedly used for four times (irradiation time: 12 min). (d) Effects of ethanol, BQ and AO on 20

the RhB degradation over 20%BTO/1.2%Ag/AgBr (irradiation time: 12 min).

Mott–Schottky method was used to determine the CB and VB potentials of AgBr and BaTiO3 [57,58]. Fig. 12(a) and (b) shows the Mott–Schottky plots of AgBr and BaTiO3, respectively, derived by the electrochemical measurements at different frequencies (3000 and 5000 Hz). The flat band potential (VFB) of the semiconductors can be obtained by extrapolating the linear portion of the Mott–Schottky plots to the x-axis. As seen from Fig. 12, AgBr and

ro of

BaTiO3 particles have a VFB value of +2.18 and −0.93 V vs standard calomel electrode (SCE), respectively. Correspondingly, the flat band potential of the two semiconductors is obtained as VFB,AgBr = +2.84 V and VFB,BaTiO3 = −0.28 V with reference to normal hydrogen electrode (NHE)

-p

according to the relationship V(NHE) = V(SCE) + 0.059 × pH(=7) + 0.242 [58]. AgBr is

determined to be a p-type semiconductor from the negative slope of its Mott–Schottky plot,

re

whereas BaTiO3 behaves as an n-type semiconductivity due to the positive slope of its

lP

Mott–Schottky plot. It is generally accepted that the flat band potential can be approximately equal to the CB edge potential of the n-type semiconductor and VB edge potential of the p-type semiconductor [57]. As a result, the CB/VB potentials of AgBr and BaTiO3 are derived as

Jo

ur

na

+0.14/+2.84 and −0.28/+2.93 V vs NHE, respectively.

Fig. 12. Mott–Schottky plots of (a) AgBr and (b) BaTiO3.

21

When BaTiO3 particles, AgBr particles and Ag nanowires are coupled to form ternary BTO/Ag/AgBr heterostructures, electrons will flow from n-type BaTiO3 to Ag, and holes will diffuse from p-type AgBr to Ag, consequently leaving behind positive charge centers at the interface of BaTiO3 and negative charge centers at the interface of AgBr, as schematically depicted in Fig. 13. The created internal electric field (i.e., potential barrier) will prevent the migration of the charge carriers, and finally the ternary BTO/Ag/AgBr heterojunctions reach a thermal equilibrium state. Under irradiation by simulated sunlight, both BaTiO3 and AgBr are

ro of

photoexcited to generate electrons in their CB and holes in their VB. Driven by the internal electric field, the CB electrons of AgBr and VB holes of BaTiO3 will migrate to Ag nanowires and undergo geminate recombination. This carrier transfer and recombination process is very

-p

similar to the Z-scheme mechanism [59]. As a result, more CB electrons of BaTiO3 and VB holes of AgBr can be available for the photodegradation reactions. This is the dominant mechanism for

re

the enhanced photodegradation performances of the ternary BTO/Ag/AgBr heterostructured

lP

photocatalysts. Moreover, this charge transfer process can prevent the photocorrosion behavior of AgBr and thus improve the photocatalytic stability of the ternary composite photocatalysts. In addition, there are also some other factors that are beneficial to the photodegradation

na

performances of the composites. For example, the LSPR of Ag nanowires could induce locally enhanced electromagnetic field and stimulate the generation and separation of electron/holes pairs

ur

in BaTiO3 and AgBr, and moreover, the LSPR-induced electrons in Ag nanowires could take part

Jo

in the photodegradation reactions. •OH, •O2− and h+ are confirmed, by the reactive species trapping experiments, to be the

dominant reactive species causing the RhB photodegradation over the BTO/Ag/AgBr heterostructured photocatalysts (Fig. 11(d)). It is noted that •OH radicals are generated mainly through the reactions between h+ and OH−/H2O. This implies that the main role of the photoexcited h+ in the VB of AgBr is to react with OH− or H2O species to produce •OH radicals. 22

These reactions are thermodynamically feasible due to the sufficiently positive VB potential of AgBr (+2.84 V vs NHE) when compared with E0(OH–/•OH) = +1.99 V and E0(H2O/•OH) = +2.38 V vs NHE [60]. The CB potential of BaTiO3 (−0.28 V vs NHE) is more negative than the redox potential of O2/•O2− (E0(O2/•O2−) = −0.13 V vs NHE [61), indicating that the photoexcited e− in

re

-p

ro of

the CB of BaTiO3 can react with adsorbed O2 species to produce •O2− radicals.

Fig. 13. Schematic illustration of the photocatalytic mechanism of the BTO/Ag/AgBr

4. Conclusions

na

lP

heterostructured photocatalysts.

ur

In this work we have assembled n-BaTiO3 and p-AgBr particles onto Ag nanowires to form n-BaTiO3/Ag/p-AgBr ternary heterostructured photocatalysts. The as-prepared ternary composite

Jo

photocatalysts manifest a promising application in the photodegradation of dyes under simulated sunlight illumination. At 12 min of photoreaction, 99.3% of RhB is observed to be degraded over the 20%BTO/1.2%Ag/AgBr composite. Based on the degradation kinetic analysis, it is concluded that the photodegradation activity of 20%BTO/1.2%Ag/AgBr is about 3.1 times larger than that of bare AgBr. The dominant mechanism resulting in the enhanced photodegradation activity of the ternary composite photocatalysts can be explained as follows. Under action of the internal 23

electric field, photoexcited CB electrons of AgBr and VB holes of BaTiO3 will migrate to Ag nanowires and are recombined in pairs. This spatial separation of charge carriers leads to more CB electrons of BaTiO3 and VB holes of AgBr available for the photodegradation reactions.

Author Statement

ro of

Yanping Wang: Data curation, Writing- Original draft preparation; Hua Yang: Conceptualization, Supervision, Writing- Reviewing and Editing; Xiaofeng Sun: Data curation; Haimin Zhang and Tao Xian: Data curation, Validation.

-p

Declaration of interests We declare that we have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

re

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant No.

lP

51662027) and the HongLiu First-Class Disciplines Development Program of Lanzhou University

na

of Technology.

References

326–336.

ur

[1] X.X. Zhao, H. Yang, H.M. Zhang, Z.M. Cui, W.J. Feng, Desalin. Water Treat. 145 (2019)

Jo

[2] Z.M. He, B. Tang, J.B. Su, Y.M. Xia, J. Mater. Sci.-Mater. Electron. 29 (2018) 19544–19553. [3] S. Wang, C. Chen, Y. Li, Q. Zhang, Y. Li, H. Gao, J. Electron. Mater. 48 (2019) 6675–6685. [4] C. Berberidou, V. Kitsiou, D.A. Lambropoulou, A. Antoniadis, E. Ntonou, G.C. Zalidis, I. Poulios, J. Environ. Manag. 195 (2017) 133–139. [5] X.X. Zhao, H. Yang, Z.M. Cui, Z. Yi, H. Yu, J. Mater. Sci.-Mater. Electron. 30 (2019) 13785–13796. 24

[6] Z. Yi, X. Li, H. Wu, X.F. Chen, H. Yang, Y.J. Tang, Y.G. Yi, J.Q. Wang, P.H. Wu, Nanomaterials 9 (2019) 1254. [7] Y.M. Xia, Z.M. He, K.J. Hu, B. Tang, J.B. Su, Y. Liu, X.P. Li, J. Alloy. Compd. 753 (2018) 356–363. [8] S.Y. Wang, H. Yang, X.X. Wang, W.J. Feng, J. Electron. Mater. 48 (2019) 2067–2076. [9] M. Ahmadi, H.R. Motlagh, N. Jaafarzadeh, A. Mostoufi, R. Saeedi, G. Barzegar, S. Jorfi, J. Environ. Manag. 186 (2017) 55–63.

ro of

[10] S.F. Wang, H.J. Gao, Y. Wei, Y.W. Li, X.H. Yang, L.M. Fang, L. Lei, CrystEngComm 21 (2019) 263–277.

[11] C. Cai, S.B. Han, W. Liu, K. Sun, L. Qiao, S. Li, X.T. Zu, Appl. Catal. B-Environ. 260 (2020)

-p

118103.

[12] H.J. Gao, F. Wang, S.F. Wang, X.X. Wang, Z. Yi, H. Yang, Mater. Res. Bull. 115 (2019)

re

140–149.

lP

[13] L.J. Di, T. Xian, X.F. Sun, H.Q. Li, Y.J. Zhou, J. Ma, H. Yang, Micromachines 10 (2019) 503.

[14] A. Esmaeili, M.H. Entezari, J. Colloid Interf. Sci. 466 (2016) 227-237.

na

[15] M. Karmaoui, L. Lajaunie, D.M. Tobaldi, G. Leonardi, C. Benbayer, R.Arenal, J.A. Labrincha, Appl. Catal. B-Environ. 218 (2017) 370–384.

ur

[16] T. Xian, L.J. Di, X.F. Sun, H.Q. Li, Y.J. Zhou, H. Yang, Nanoscale Res. Lett.

Jo

(https://doi.org/10.1186/s11671-019-3206-5). [17] C.P. Liang, Y.B. Zhang, Z. Yi, X.F. Chen, Z.G. Zhou, H. Yang, Y. Yi, Y.J. Tang, W.T. Yao, Y.G. Yi, Results Phys. 15 (2019) 102635. [18] P.Q. Yu, X.F. Chen, Z. Yi, Y.J. Tang, H. Yang, Z.G. Zhou, T. Duan, S.B. Cheng, J.G. Zhang, Y.G. Yi, Opt. Mater. 97 (2019) 109400. [19] Y.Y. Wang, F. Qin, Z. Yi, X.F. Chen, Z.G. Zhou, H. Yang, X. Liao, Y.J. Tang, W.T. Yao, Y.G. 25

Yi, Results Phys. 15 (2019) 102711. [20] C.P. Liang, Z. Yi, X.F. Chen, Y.J. Tang, Y. Yi, Z.G. Zhou, X.G. Wu, Z. Huang, Y.G. Yi, G.F. Zhang, Plasmonics (DOI: 10.1007/s11468-019-01018-4). [21] X.X. Wang, J.K. Zhu, X.L. Wen, X.X. Wu, Y. Wu, Y.W. Su, H. Tong, Y.P. Qi, H. Yang, Opt. Mater. Express 9 (2019) 3079–3088. [22] H. Tong, Y.Q. Xu, Y.W. Su, X.X. Wang, Results Phys. 14 (2019) 102460. [23] X.X. Wang, X.L. Bai, Z.Y. Pang, H. Yang, Y.P. Qi, X.L. Wen, Acta Phys. Sin. 68 (2019)

ro of

037301. [24] S.B. Han, Y.M. Zhu, C. Cai, J.K. Zhu, W.B. Han, L. Chen, X.T. Zu, H. Yang, M. Gu, Appl. Phys. Lett. 114 (2019) 113901.

-p

[25] F.A. Zhao, H.Y. Xiao, X.M. Bai, X.T. Zu, Phys. Chem. Chem. Phys. 21 (2019) 16108–16119.

re

[26] H.J. Gao, C.X. Zheng, H. Yang, X.W. Niu, S.F. Wang, Micromachines 10 (2019) 557.

lP

[27] A.S. Jonathan, L.K. Ai, A.D. Jennifer, Nature 483 (2012) 421–428. [28] X.X. Wang, Z.Y. Pang, H. Yang, Y.P. Qi, Results Phys. 14 (2019) 102446. [29] C.X. Zheng, H. Yang, Z.M. Cui, H.M. Zhang, X.X. Wang, Nanoscale Res. Lett. 12 (2017)

na

608.

[30] Y. Xia, Z. He, J. Su, K. Hu, J. Mater. Sci.-Mater. Electron. 30 (2019) 9843–9854.

ur

[31] F. Cordero, J. Appl. Phys. 123 (2018) 094103.

Jo

[32] A.A. Yadav, Y.M. Hunge, V.L. Mathe, S.B. Kulkarni, J. Mater. Sci.-Mater. Electron. 29 (2018) 15069–15073.

[33] S. Kumar, M. Sharma, S. Powar, E.N. Kabachkov, R. Vaish, J. Eur. Ceram. Soc. 39 (2019) 2915–2922. [34] H. Maimaitizi, K. Kadeer, A. Reheman, D. Talifu, Y. Tursun, A. Abulizi, Environ. Sci. Pollut. Res. 26 (2019) 26883–26892. 26

[35] K.L. Wei, B.L. Wang, J.M. Hu, F.M. Chen, Q. Hao, G.N. He, Y.Z. Wang, W. Li, J.M. Liu, Q.Y. He, RSC Adv. 9 (2019) 11377–11384. [36] T.A. Kurniawan, Y.Y. Lin, O. Tong, A.B.Albadarin, G. Walker, Mat. Sci. Semicon. Proc. 73 (2018) 42–50. [37] Z.M. He, Y.M. Xia, J.B. Su, RSC Adv. 8 (2018) 39187–39196. [38] A.I. Vaizogullar, Photochem. Photobiol. 94 (2018) 1225–1233. [39] S. Zhao, Y.W. Zhang, Y.M. Zhou, K.B. Qiu, C. Zhang, J.S. Fang, X.L. Sheng, J. Photochem.

ro of

Photobiol. A 350 (2018) 94–102. [40] Y.X. Yan, H. Yang, Z. Yi, T. Xian, R.S. Li, X.X. Wang, Desalin. Water Treat. (doi: 10.5004/dwt.2019.24747).

-p

[41] Z. Yi, Y. Zeng, H. Wu, X.F. Chen, Y.X. Fan, H. Yang, Y.J. Tang, Y.G. Yi, J.Q. Wang, P.H. Wu, Results Phys. 15 (2019) 102609.

re

[42] Z.Y. Pang, H. Tong, X.X. Wu, J.K. Zhu, X.X. Wang, H. Yang, Y.P. Qi, Opt. Quant. Electron.

lP

50 (2018) 335.

[43] D.J. Li, X.T. Zu, D.Y. Ao, Q.B. Tang, Y.Q. Fu, Y.J. Guo, K. Bilawal, M.B. Faheem, L. Li, S. Li, Y.L. Tang, Sensor. Actuat. B-Chem. 294 (2019) 55–61.

na

[44] L.J. Di, H. Yang, T. Xian, X.J. Chen, Micromachines 9 (2018) 613. [45] S. Wang, H. Gao, C. Chen, Q. Li, C. Li, Y. Wei, L. Fang, J. Mater. Sci.-Mater. Electron. 30

ur

(2019) 15744–15753.

Jo

[46] L. Gomathi Devi, P.M. Nithya, J. Environ. Chem. Eng. 6 (2018) 3565–3573. [47] O. Dehghani Dastjerdi, H. Shokrollahi, H. Yang, Ceram. Int. 46 (2020) 2709–2723. [48] S.Y. Wang, J.Y. M, Z.J. Li, H.Q. Su, N.R. Alkurd, W.L. Zhou, L. Wang, B. Du, Y.L. Tang, D.Y. Ao, S.C. Zhang, Q.K. Yu, X.T. Zu, J. Hazard. Mater. 285 (2015) 368–374. [49] Q. Zhu, W.S. Wang, L. Lin, G.Q. Gao, H.L. Guo, H. Du, A.W. Xu, J. Phys. Chem. C 2013, 117, 5894−5900. 27

[50] S. Utara, S. Hunpratub, Ultrason. Sonochem. 41 (2018) 441–448. [51] S.F. Wang, H.J. Gao, L.M. Fang, Y. Wei, Y.W. Li, L. Lei, Z. Phys. Chem. 233 (2019) 1161–1181. [52] D.J. Li, Y.L. Tang, D.Y. Ao, X. Xiang, S.Y. Wang, X.T. Zu, Int. J. Hydrogen Energ. 44 (2019) 3985–3992. [53] Y.X. Yan, H. Yang, Z. Yi, T. Xian, Catalysts 9 (2019) 795. [54] S. Wang, H. Gao, C. Chen, Y. Wei, X. Zhao, J. Sol-Gel Sci. Technol. 92 (2019) 186–199.

ro of

[55] Y.X. Yan, H. Yang, Z. Yi, X.X. Wang, R.S. Li, T Xian, Environ. Eng. Sci. (http://doi.org/10.1089/ees.2019.0284).

[56] Y.C. Ye, H. Yang, H.M. Zhang, J.L. Jiang, Environ. Technol.

-p

(https://doi.org/10.1080/09593330.2018.1538261).

[57] S.J. Hong, S. Lee, J.S. Jang, J.S. Lee, Energ. Environ. Sci. 4 (2011) 1781–1787.

re

[58] S.Y. Wang, H. Yang, Z. Yi, X.X. Wang, J. Environ. Manag. 248 (2019) 109341.

lP

[59] Y.X. Yan, H. Yang, Z. Yi, T. Xian, X.X. Wang, Environ. Sci. Pollut. Res. 26 (2019) 29020–29031.

[60] L.J. Di, H. Yang, T. Xian, X.Q. Liu, X.J. Chen, Nanomaterials 9 (2019) 399.

Jo

ur

na

[61] T. Xian, X.F. Sun, L.J. Di, Y.J. Zhou, J. Ma, H.Q. Li, H. Yang, Catalysts 9 (2019) 1031.

28