Ag2CO3 p-n junction photocatalyst with highly enhanced photocatalytic activity

Journal of Photochemistry & Photobiology A: Chemistry 374 (2019) 206–217

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Fabrication of a novel polyhedron-like WO3/Ag2CO3 p-n junction photocatalyst with highly enhanced photocatalytic activity Minghuan Gao, Lisha You, Linna Guo , Tiesheng Li ⁎

T

⁎

College of Chemistry and Molecular Engineering, Zhengzhou University, The Key Lab of Chemical Biology and Organic Chemistry of Henan Province, The Key Lab of Nanoinformation Materials of Zhengzhou, Zhengzhou, 450001, PR China

ARTICLE INFO

ABSTRACT

Keywords: Polyhedron-like WO3 The polyhedron-like WO3/Ag2CO3p-n junction photocatalyst 17 times higher than commercial powders (P25)

Polyhedron-like WO3 (n-type WO3) nanoparticles were synthesized by a simple hydrothermal reaction without using any template or surfactant. The Polyhedron-like WO3/Ag2CO3 p-n junction photocatalyst was first synthesized by a simple impregnation-deposition way. The obtained samples were investigated by XRD, BET, XPS, SEM, TEM etc. The separation mechanisms of photo-induced electrons (e−) and photo-induced holes (h+) of the Polyhedron-like WO3/ Ag2CO3 p-n junction samples were characterized by PL technique, EIS and determination of active sites and species in the photocatalytic reactions. In this p-n junction structure, Polyhedron-like WO3 nanoparticles adhered to the surface of the Rod-like p-type Ag2CO3 nanoparticles. The Polyhedron-like WO3/Ag2CO3 p-n junction photocatalyst showed much higher photocatalytic activity than both single materials for RhB degradation under UV–vis light irradiation, and even 17 times higher than commercial powders (P25) at the same circumstance. Even compared with the related photocatalyst, it still exhibits the highest photocatalytic ability. The highly enhanced photocatalytic of the Polyhedron-like WO3/Ag2CO3 p-n junction photocatalyst can be attributed to extended absorption, more effective separation of photogenerated charges, the transfer rate of photo-induced charge carriers and forming of p-n junction system.

1. Introduction Nowadays, environmental pollution has become a worldwide problem. Industrial wastewater discharged by various types of enterprises contains a large amount of organic dyes, and their presence will lead to a series of serious problems such as eutrophication of the water and etc, which will affect the original ecological environment [1,2]. The semiconductor photocatalyst material that uses sunlight as its driving force has attracted more and more attention from researchers because of its green, high-efficiency, and sustainable characteristics. However, wide band gap and small quantum efficiency are still the “restrictions” to the majority of photocatalysts to achieve the practical application requirement [3]. For instance, although conventional TiO2 possess excellent photocatalytic activity and stability, it can only be stimulated by the UV region light (consists of only 4% solar spectrum) which extremely restricts its application [4]. To solve these challenges above, two common strategies have been widely employed. The first one is the hybridization of another semiconductor, owning different band gap, to form heterostructures, which has been reported as an effective method. The other one is the morphology control of a photocatalyst [5]. So, we intended to use both solutions to enhance photocatalytic activity in our work [6]. As is known, WO3, as an indirect band gap photocatalyst, can absorb ⁎

light from the UV to the visible [its experimental band gap energy (Eg) of WO3 varied from 2.5 to 3.0 eV], also having good resilience towards photo corrosion and excellent electron transport properties [7]. Since the first article [8] about WO3 published in 1976, its photocatalytic properties have been widely studied. However, in recent years, numbers of reports focusing on morphology control have been published. For instance, Sadakane et al. [9] had successfully synthesized three-dimensional WO3 and found the photocatalytic activity increased up to be 30 times larger than the common one. Meanwhile, the photocatalytic activity was achieved also ca. 30 times by introducing the 3DOM structure. Ofori et al. [10] prepared WO3 nanofiber which exhibit more excellent photocatalytic property upon RhB solution. Even more, the Chen’s group [11] have successfully synthesized various hierarchical structure WO3 nanoparticles, such as spheres, dumbbells and dendrites. And each one of them exhibits higher photocatalytic performance, compared with commercial WO3 particles. However, onefold WO3 is not always a perfect semiconductor photocatalyst owing to its fast recombination rate of photo-induced charge carriers and narrow sunlight utilization [12–14]. Moreover, taking account of that the conductor band (CB) level of common WO3 materials is always less negative, compared with the potential of O2/%O2−, the conductor band (CB) level of common WO3 materials usually have no ability to transform O2 to %O2− by single-electron reduction way [15,16]. The

Corresponding authors. E-mail addresses: [email protected] (L. Guo), [email protected] (T. Li).

https://doi.org/10.1016/j.jphotochem.2019.01.022 Received 31 October 2018; Received in revised form 16 January 2019; Accepted 22 January 2019 Available online 24 January 2019 1010-6030/ © 2019 Published by Elsevier B.V.

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accumulated photo-induced electrons located on the CB of WO3 have no ability to be removed via reduction process to transform O2, existed in wasted water, into superoxide anion free radical (%O2−), which might lead to easy recombination of electrons (e−) and holes (h+). As the results, the photocatalytic activity of pure WO3 semiconductor is generally low without any modification strategy [17]. Numerous methods have been used to alter the photocatalytic of pure WO3 semiconductor, including morphology control, noble metal deposition and the hybridization etc [18–20]. And, among various modification strategies, formation of a heterojunction structure between two semiconductors is always an effective strategy to prohibit the accumulation process of photo-induced electron on the CB of a single semiconductor photocatalyst [21]. Yu et al [22]. Reported that Ag2O/Ag2CO3 hetero-photocatalysts exhibits extremely high activity and stability. Similar results were found in AgX/ Ag3PO4 (X stands for Cl, Br and I) hetero-photocatalyst [23] systems. These results indicated that formation a type of heterojunctions might be an effective way to improve the photocatalytic of single semiconductor photocatalyst. Among the hetero-photocatalysts system, there is a special one which has been synthesized based on the p-n type junction system. Usually after the contact between the two type semiconductors, an internal electric filed between the two materials will be formed and the photo-induced electron-holes will constantly transfer from two type semiconductors until the Fermi levels of them were counterpoised under conditions in thermodynamic equilibrium region [24,25]. The p-n heterojunction always has a faster transmission of photogenerated electrons and holes compared with either p-p type or n-n type heterojunctions.WO3 as a typical n-type semiconductor [26],when combined with p-type semiconductor owned a narrow band gap, may form a highly efficient p-n junction system under light irradiation in the UV–vis region. In recent years, Ag-based compounds have also attracted much attention owing to its strong photocatalytic activity. AgX (X = Cl, Br, I) [27–29] and Ag3PO4 etc. have been well studied in photocatalytic field. Very recently, Ag2CO3 due to possess excellent photocatalytic properties for decomposition of organic pollutants, has been widely reported [30]. For instance, Tonda et al [31]. prepared Ag2CO3/g-C3N4 composites with much higher photocatalytic and stability than the pure Ag2CO3 and gC3N4 semiconductor photocatalyst for the degradation of rhodamine B (RhB). Yu et al [32]. prepared heterojunction structured Ag2CO3/TiO2 which showed a highly enhanced photocatalytic activity towards methyl orange (MO) and so on. In addition, Ag2CO3 is a typical p-type semiconductor [33]. Nevertheless, there are extremely little works focusing on the fabrication and characterization about the photocatalytic activities of Ag2CO3/WO3 p-n junction materials. Up till now, in this research area, only one related article was just reported [34], and which has little detailed and clear representation about the Fermi level and p-n junction for some reason. Inspired by the work above, we decided to combine the idea of morphology controlled, hybridization and p-n junction together. In present, we have firstly successfully synthesized a Polyhedron-like WO3 nanoparticles via a simple facile hydrothermal method with heat treatment, and after coupling with proper amount p-type Ag2CO3 material by a facile precipitation method. What’s exciting has happened, the Ag2CO3/WO3 p-n junction photocatalyst has exhibited an excellent photocatalytic activity for the degradation of RhB under visible light irradiation than pure n-type WO3 or p-type Ag2CO3 semiconductor photocatalyst does. Moreover, a proper mechanism was proposed, and the highly enhanced photoactivity of Polyhedron-like WO3/Ag2CO3 composite material is attributed to the successfully formation of p-n junction system between the Polyhedron-like WO3/Ag2CO3 heterostructure and confirmed by photocurrent, PL spectra and so on.

methyl orange (MO) were purchased from Aladdin (China). Sodium bicarbonate (NaHCO3) and sulfuric acid (H2SO4, 98%) were bought from Tianjin Bohai Chemical Group (China). Deionized (DI) water was used during the experimental process, prepared using an ultra-pure purification system. 2.2. Synthesis of polyhedron-like WO3 nanoparticles Polyhedron-like WO3 (p-type WO3) nanoparticles were also obtained via a hydrothermal process. Firstly, dilute concentrated sulfuric acid (98%) to aqueous sulfuric acid (3 mol/L). Then, 3.1870 g of (NH4)6H2W12O40•xH2O was uniformly dissolved in 20 mL of deionized water with sonicating for about 10 min. Secondly, adding 6.8 mmol of aqueous sulfuric acid (3 mol/L) to the above (NH4)6H2W12O40·xH2O solution drop by drop with stirring until forming a uniform yellow transparent solution. After magnetic stirring for 30 min., the resulting precursor solution was transferred into a 50 mL Teflon-lined stainlesssteel autoclave. Then keep it in an oven at 180 ℃ for 14 h. Finally, after the container above was fully cooled down to the room temperature, the precursor was collected, washed, and dried at 60 ℃ for 12 h. Finally, the prepared precursors were calcined at 700 ℃ for 1 h. 2.3. Synthesis of polyhedron-like WO3/Ag2CO3 p-n junction photocatalysts According to the previous reports [34], the Polyhedron-like WO3/ Ag2CO3 p-n junction photocatalysts were synthesized as follows : an appropriate amount of the as-prepared Polyhedron-like WO3 nanoparticles (by the method above) was firstly dispersed in 15 mL DI (deionized) water and sonicated for another 30 min. 4 mL solution containing excess AgNO3 0.050 g was then added into the fully-dispersed WO3 suspension above and magnetic stirred in the dark reaction for about 30 min. to assure that Ag+ ions completely adsorped on the surface of WO3. Thereafter, a solution containing proper amount NaHCO3 solution (0.05 M, 5 mL) was dropwise to the above Ag+-WO3 suspended solution and stirred for another 2 h. At this point, we did not place the resulting suspension under visible light irradiation (λ > 420 nm) to make sure that all experimental procedures are carried out in the dark. (because our target products were the Ag2CO3/ WO3 p-n junction hetero-photocatalyst, we need to prohibit the formation of Ag particles). Different ratios of Ag2CO3/WO3 p-n junction hetero-photocatalysts were prepared by the same method. The samples are denoted as AWP-10, AWP-20, AWP-30 and AWP-40 respectively when the weight percentages of p-type Ag2CO3 in composites are 10%, 20%, 30%, 40%. The pure p-type Ag2CO3 was pared by the same method without any Polyhedron-like WO3 (n-type WO3) powders. 2.4. Characterization X-ray diffraction (XRD) experiments were performed with a Bruker D8 VENTURE diffractometer (Bruker, Greman) using Cu-Ka radiation. The morphology was examined by scanning electron microscopy (SEM, Hitachi S-4800). Diffuse reflectance ultraviolet-visible (UV–vis) absorption spectra were measured by a PerkinElmer Lambda 950 spectrometer in the region of 200–700 nm, while BaSO4 was used as a reference. The SBET of the samples above was also determined by multipoint BET methods (ASAP2000, Micromeritics, USA). Photoluminescence (PL) spectroscopy measurement was characterized at the excitation wavelength of 446 nm on F-7000 Fluorescence Spectrophotometer at room temperature. The photocurrent and electrochemical impedance spectropy (EIS) properties were studied in a three-electrode set-up [35] (CHI660, CH Instrument, USA) at room temperature. In these systems, ITO glass electrodes were modified with as-prepared samples (Polyhedron-like WO3, Ag2CO3 and AWP-20) as the working electrode, a saturated calomel (saturated KCL) electrode (SCE) was used as the reference electrode, an aqueous solution of Na2SO4 (0.1 M) was used as the electrode. While, when the system

2. Experimental 2.1. Materials Ammonium metatungstate hydrate [(NH4)6H2W12O40•xH2O], silver nitrate (AgNO3), rhodamine B (RhB, 95%), Acid Blue 93 (MB) and 207

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above works as test system for photocurrent, the working electrode need to be intermittently irradiated with visible-light during the measurements. 2.5. Photocatalytic evaluation To conduct the photocatalytic experiments, 30 mg as-obtained samples were dispersed into 30 mL of RhB (30 ppm), and a 50 mL quartz test tube was used as the photoreactor. A 350 W Xenon lamp working system (SGYⅡ, Nanjing Haobin Technology Co. Ltd. China) was used as the light source, and to ensure the UV–vis light irradiation, without using any ultraviolet cut off filter, and every light involved reaction in this literate is in the same situation. Before irradiation, the suspensions were stirred for 60 min. in the dark to achieve an equilibrium adsorption state before visible-light illumination. And as for the photocatalytic activity measurement of MO and MB organic dyes, 30 mg catalyst MO (20 ppm,30 mL) and MB (30 ppm, 30 mL) were used. The degradation of MO and MB dyes was measured according to the same method as RhB. Every 3 mL suspension volume was taken out at a selected time for subsequent analysis. After reobtaining the composites by centrifugation, we can analyse the concentration of every dye solution in the different light absorption at 554 nm/464 nm/662 nm (λmax for RhB/MO/MB solution) using a spectrophotometer (WFJ-7200, Unico, USA). The percentage of degradation was calculated by C/C0. Herein, C0 is the initial concentration of dye solution, while the remaining dye solution concentration at each irradiated time interval is denoted as C. The 2-CP concentration in the solution were detected by high-performance liquid chromatography (HPLC) using an Agilent 1260 series equipped with a UV detector at 273 nm.

Fig. 2. XRD patterns of as-prepared Polyhedron-like WO3, Ag2CO3, AWP-10, AWP-20, AWP-30, AWP-40.

No. 26-00339) and after comparing with standard cards (JCPDS No. 431035 or JCPDS No. 26-0339), we finally find that it is attributed to the (210) facet of the another p-type Ag2CO3 phase (JCPDS No. 31-1237) which belongs to Hexagonal phase Ag2CO3. The presence of the (210) facet can be further confirmed by the HRTEM (Fig. 5d). Besides, Polyhedron-like WO3/Ag2CO3 p-n junction hetero-photocatalysts with different mass ratios of Ag2CO3 were also investigated by XRD methods. As depicted in Fig. 2, the results also indicated that after coupling with p-type Ag2CO3 nanoparticles, the composites of AWP-10, AWP-20, AWP-30 and AWP-40 showed the diffraction peaks of both WO3 and Ag2CO3 phases, indicated that a coexistence of p-type Ag2CO3 and ntype WO3. Moreover, we can clearly find that the intensity of the (210) facet diffraction peak is related to the doping amount of p-type Ag2CO3 materials, as the increase of the doping amount of p-type Ag2CO3, the density diffraction peak of (210) facet is also gradually increasing.

3. Results and discussion 3.1. XRD analysis XRD patterns of as-obtained photocatalysts in Fig. 1. indicate that for pure p-type Ag2CO3, all the diffraction peaks can be indexed very well with monoclinic phase Ag2CO3 (JCPDS NO.26-0339) [36,37], and no peaks of other impurities such as metallic silver or silver oxide and so on can be detected. In addition, the Polyhedron-like WO3 (n-type WO3) nanoparticles have a very good correspondence with the monoclinic phase WO3 (JCPDS No. 43-1035) [38]. After the Polyhedron-like WO3 (n-type WO3) samples coupled with p-type Ag2CO3 materials, the composites of AWP-20 show both the diffraction peaks of n-type WO3 and ptype Ag2CO3 phases which reflect the existence of two phases. Furthermore, it is worth to be noted that a slight diffraction peak located at 30° can be clearly observed in AWP-20 sample, which is neither belong to ntype WO3 phase (JCPDS No. 43-1035) nor p-type Ag2CO3 phase (JCPDS

3.2. Morphology characterization As shown in Fig. 3a, pure p-type Ag2CO3 nanoparticles exhibit rod shape with uniform size and favorable dispersion. Clearly, p-type Ag2CO3 nanorod with an axial length of 400 nm and radial length of 1.75–2 μm, respectively. In the meantime, it can be seen that numerous Polyhedronlike WO3 nanoparticles (n-type WO3) are stacked together to form hierarchical WO3 nanoflowers and the corresponding SEM images are shown in Fig. 3b and c. The average axial length and radial length of each Polyhedron-like WO3 nanoparticle (n-type WO3) is about 300 nm and 500 nm (as shown in Fig. 3c). In addition, we can easily observe that there are several active crystals faces in the surface of each Polyhedron-like WO3 nanoparticles (n-type WO3), which may provide more reaction sites in the photocatalytic degradation process [38]. And, as for the AWP-20 samples, it can be obviously observed that the composites are mainly made up of rod-like Ag2CO3 nanoparticles (p-type Ag2CO3) and Polyhedron-like WO3 nanoparticles (n-type WO3), respectively, and tiny Polyhedron-like WO3 nanoparticles (n-type WO3) were anchored on the surface of the Ag2CO3 (as shown in Fig. 3e). That all clearly proved Polyhedron-like WO3/ Ag2CO3 p-n junction hetero-photocatalysts were synthesized successfully. Moreover, TEM and HR-TEM are further applied to examine the regular morphology and specific crystal structure-especially facet information, which are consistent with the XRD analysis (as shown in Figs. 1 and 2). The test results from TEM measurements are also related with the SEM images, in short, as shown in Fig. 4a, the TEM image of pure p-type Ag2CO3 exhibits rod-like shape and the TEM image of pure WO3 displays Polyhedron-like shape (Fig. 4b). And, from the Fig. 4c, it

Fig. 1. XRD patterns of as-prepared Ag2CO3, Polyhedron-like WO3 and AWP-20. (inset the figure, the ♣ represents the crystal faces of the monoclinic phase Ag2CO3 and ♦ represents the crystal faces of Hexagonal phase Ag2CO3). 208

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Fig. 3. SEM images of as-prepared samples (a) Ag2CO3, (b), (c) and (d) Polyhedron-like WO3, (e) AWP-20.

can be vividly seen that tiny n-type WO3 nanoparticles were adhered on the surface of p-type Ag2CO3 nanoparticles which in line with the SEM images (Fig. 3e). Moreover, Fig. 4d displays the HR-TEM image of AWP20 p-n junction photocatalyst. It can be observed that the lattice spacing of 0.297 and 0.48 nm correspond to the (210) facet of p-type Ag2CO3 phase (JCPDS No. 31-1237) and the (020) facet of p-type Ag2CO3 phase (JDPDS No. 26-0339) respectively, the (112) facet are corresponding with Polyhedron-like WO3 (n-type WO3) phase (JCPDS No. 43-1035) has also been found. The above results fully illustrated the coexistence of p-type Ag2CO3 and n-type WO3 phases, which is in conformity with the result of XRD (Fig. 1) and XPS (Fig. 6). The above results further demonstrate that Ag2CO3/WO3 p-n junction hetero-photocatalyst is successfully formed through a facile precipitation method at room temperature.

desorption isotherms were implemented (Fig. 5). We used BET and BJH methods to evaluate the detailed surface areas and specific pore sizes data of the as-prepared catalysts. Surprisingly, when the two samples are combined with each other (Polyhedron-like n-type WO3 and p-type Ag2CO3), the BET surface area (SBET) of the AWP-20 p-n junction samples, showed a slight rise comparing with the pure p-type Ag2CO3, Polyhedron-like WO3 (n-type WO3) respectively (See Table. S1 in Supporting information). In addition, the pore volume of AWP20 is slightly increased, these results above might be in line with the fact that the tiny n-type WO3 particles which major size is about 500 nm (See Fig. S1 in Supporting information) are adhered on the surface of the large p-type Ag2CO3 particles which major size is about 2 μm (See Fig. S1 in Supporting information) with high dispersion, forming some new surface pores between the two particles in the sites of heterojunction [39,40]. The average SBET is calculated to be 2.6065 m2 g−1, 2.6141 m2 g−1 and 3.4062 m2 g−1 (See Table. S1 in Supporting information) for p-type Ag2CO3, Polyhedron-like WO3 (ntype WO3) and AWP-20, respectively. It is well known that the larger specific surface areas, the more reaction sites [41,42]. The larger SBET

3.3. BET tests To investigate the specific surface area and distribution about every pore size towards the as-obtained samples, the nitrogen and

Fig. 4. TEM micrographs of (a) Ag2CO3, (b) Polyhedron-like WO3, (c) AWP-20, (d) HRTEM images of AWP-20. 209

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Fig. 5. N2 adsorption–desorption isotherms (a) and the corresponding pore size distribution (b) of the as-prepared samples.

Fig. 6. XPS spectra of the Ag2CO3, Polyhedron-like WO3 and AWP-20 composite. (a) Survey of the sample; (b) Ag 3d; (c) W 4f; (d) C 1 s and (e) O 1 s.

of AWP-20 might endow the more reaction sites, which leading to more organic molecules in pollutant water being adsorbed on its surface, finally facilitating its photocatalytic ability. The pore-size distribution of as-pared samples is in the range 2–50 nm, which are all belong to mesopores materials. In addition, the average pore diameter pore size of p-type Ag2CO3, Polyhedron-like n-type WO3 and AWP-20

is computed to be 10.1578 nm, 16.5076 nm and 10.2285 nm by BJH test methods (Fig. 5b). Compared with the pure n-type WO3 test results, the pore size of the AWP-20 sample is significantly smaller. This might be due to the formation of the p-n junction structure covering the surface pores of the Polyhedron-like WO3 (n-type WO3) nanoparticles to a certain extent. 210

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3.4. XPS analysis

irradiation light which wavelength lower than λmax can excite the semiconductor and finally be used in photocatalytic reaction. To thoroughly understand the photocatalytic mechanism about AWP20 p-n junction system, the valence band (VB) of two semiconductor materials was also calculated in our work, by introduced the concepts of electronegativity. At the point of zero charge, the VB edge potential of a semiconductor can usually be computed by the empirical equation below [33]:

XPS is always a common method to investigate the surface elemental compositions about the experimental test samples. Fig. 6a exhibits the full survey spectrum of various test samples. Clearly, the AWP-20 sample consist of O, Ag, C and W elements. In addition, to get more information about explicit chemical states of each element, we also use the typical XPS spectra in high-resolution to detect Ag 3d, W 4f, C 1 s and O1 s, respectively. The test results are shown in Fig. 6b–e. Firstly, as depicted in Fig. 6b, it can be observed clearly that the high-resolution spectrum of Ag 3d for not only pure p-type Ag2CO3 but also the AWP-20 sample all shows two typical peaks, located at 367.6 eV/368.1 eV and 374.1 eV/373.6 eV (Fig. 6b), which are corresponding to the binding energies of Ag 3d5/2 and 3d3/2, while these two peaks above are attributed to the Ag+ of Ag2CO3 [43]. Fig. 6c shows the W 4f spectrum of AWP-20 and Polyhedron-like WO3 (n-type WO3) samples. Obviously, it can be seen the peaks of W 4f7/2 (35.2 eV/35.5 eV) and W4f5/2 (37.4 eV/37.6 eV) (Fig. 6c), which is assigned to W6+ from n-type WO3 [38], indicating the formation of p-n junction structure. In the C 1 s spectrum of AWP-20 and pure p-type Ag2CO3, there are two peaks located at binding energy of 284.9 eV/ 284.7 eV and 288.9 eV/288.7 eV (Fig. 6d), which can be ascribed to the adventitious carbon species and the peak of carbon from CO32− [44,45]. Finally, The O 1 s spectrum of AWP-20 and Ag2CO3 exhibits the binding energy at 530.0 eV and 531.1 eV (Fig. 6e), respectively [45]. The above characteristic peaks are all ascribed to the lattice oxygen in the p-type Ag2CO3 and n-type WO3 [34]. Furthermore, by comparing with the test results, we can find some obvious shifts of peak which located at the region of W4f, C1s and O1s spectrum, indicating the successfully formation of Polyhedron-like WO3/Ag2CO3 p-n junction photocatalyst.

ECB = X

0.5Eg

EVB = ECB + Eg where EVB stands for the edge potential of VB, X stands for the electronegativity of the semiconductor, EƟ stands for the energy of free electrons on the hydrogen scale, which is always about 4.5 eV and finally, Eg stands for the band gap energy of the semiconductor. Furthermore, EVB (the edge potential of conduction band) can be determined by EVB = ECB + Eg. Herein, the X value for WO3 is 6.59 and the X value for p-type Ag2CO3 is 6.10, therefore, the ECB of Polyhedron-like WO3 (n-type WO3) and p-type Ag2CO3 is calculated to be 3.42 eV and 2.67 eV, respectively. And the ECB of samples above is 0.76 eV and 0.53 eV, respectively. The results are in conformity with the similar reports [11,49,50]. 3.6. Photocatalytic properties The photocatalytic activity of the AWP-10, AWP-20, AWP-30 and AWP-40 p-n junction samples were estimated by photodegradation of some characteristic organic molecules such as rhodamine B (RhB) under visible-light irradiation. The photocatalytic performance of ptype Ag2CO3 and Polyhedron-like WO3 (n-type WO3) were also presented under same identical experimental conditions for contrast. As displayed in Fig. 8a and b, compared to pure p-type Ag2CO3 and Polyhedron-like WO3 (n-type WO3), all binary Ag2CO3/WO3 p-n junction hybrids exhibited strengthened photocatalytic performance, implying the existence of a synergistic effect between Ag2CO3 and WO3 materials. Firstly, as for the AWP p-n junction samples, it is worth to be noted that the photocatalytic performance heightens gradually as the increase amount of n-type WO3 in the p-n junction system, finally when the typical amount of n-type WO3 is located at 20 wt%, the p-n junction photocatalyst shows the best photocatalytic activity. The best sample is the AWP-20 and the degradation time is about only 20 min. To further investigate the photocatalytic activity of the AWP-20 sample, we also conducted more precise photodegradation experiments, pure p-type Ag2CO3, pure n-type WO3 (Polyhedron-like WO3) and P25 were used as the photocatalyst in the photocatalytic decomposition removal of RhB (Fig. 8b and c), In Fig. 8b, it is obvious that the photocatalytic activity of the experimental samples falls down as follows: AWP-20 > P25 > Ag2CO3 > Polyhedron-like WO3. In order to have more intuitionistic understanding of the degradation rate towards the RhB by various samples, Fig. 8c exhibits the linear relationships between ln(C/C0) and T (mins) irradiation time [51]. As the relationships were all linear, the photocatalytic degradation curves fit with first-order reaction ideally. Furthermore, the rate constants of AWP-20, Ag2CO3, Polyhedron-like WO3 and P25 were 0.2451, 0.00196, 0.00138 and 0.01456 min−1, respectively. Within the above catalysts, the AWP-20 p-n junction sample showed the highest rate constant, which was approximately 125 times higher than pure p-type Ag2CO3, 177 times higher than Polyhedron-like WO3 (n-type WO3) and 17 times higher than commercial powder (P25) under same conditions. In short, the results above revealed that the AWP-20 structures exhibit excellent photocatalytic performance towards RhB. In Table. S2 (See information in Supporting information), the photocatalytic activity of the Polyhedron-like WO3/Ag2CO3 p-n junction photocatalyst is also in contrast to other similar Ag2CO3-based photocatalysts reported previously [31,34]. Considering influences of different experimental conditions, the photocatalytic activity of the various heterojunction photocatalysts

3.5. Optical properties Before testing the photocatalytic activity, the optical properties of asprepared p-type Ag2CO3, Polyhedron-like WO3 (n-type WO3) particles, and AWP-20 p-n junction composites were examined using UV–vis DRS. Referring to relative reports [65–67], we had known that the characteristic absorption peak of WO3 located at short-wavelengths (from 200 to 520 nm) is attributed to charge transfer (CT) process of W-O [65]. And, as for Ag2CO3 material, it usually shows a wide and strong light absorption in the solar spectrum (200–800 nm), which is often related to excellent photocatalytic performance. As displayed in Fig. 7a, Pure Polyhedron-like WO3 (n-type WO3) sample is only able to absorb irradiation light with a wavelength shorter than about 470 nm, the results were in agreement with previous studies [34]. While after combining with p-type Ag2CO3 material, the absorption edge of AWP-20 sample shifted to about 500 nm. Besides, this observation also shows that asprepared AWP-20 p-n junction material has the better ability to work with visible light due to higher absorbance in the region of 500–700 nm. Base on the Tauc plots, the band gap energy of a semiconductor material can be computed by the equation below [43,47]:

hv = A (hv

E

Eg ) n/2

Where α, v, Eg and A are respectively stand for the absorption coefficient, light frequency, band gap, and a constant. The integer n is determined by the characteristics of the optical transition (n = 1, 2, and 4), According to the related reports, n value of p-type Ag2CO3 and ntype WO3 was 1 and 4 respectively [11,30,38,43,48]. The Eg of WO3 was determined from a plot of (αhv) 2 versus energy (hv), and after computing the experimental date, the band gap of n-type WO3 was about 2.66 eV. Due to the consideration of a plot of (αhv)1/2 versus energy (hv) (Fig. 8b), the Eg of p-type Ag2CO3 was computed to be 2.14 eV. The computed band gap of n-type WO3 sample is similar to previously reported results [11] and the calculated band gap of p-type Ag2CO3 is in the range of related reports [49,50]. According the equation: Eg = 1240/λmax [63,64], we can also calculate the maximum absorption wavelength (λmax) of the as-prepared samples, only 211

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Fig. 7. (a) UV–vis diffuse reflectance spectra of pure Ag2CO3, Polyhedron-like WO3 and AWP-20; (d) the band gaps (Eg) of pure Ag2CO3 and Polyhedron-like WO3. Fig. 8. (a) The photocatalytic activities of Ag2CO3, Polyhedron-like WO3 and AWP samples for 30 ppm RhB degradation under UV–vis light irradiation (Xenon lamp); (b) The photocatalytic activities of AWP-20, Ag2CO3, Polyhedron-like WO3 and P25 for 30 ppm RhB degradation; (c) First-order kinetic plots in Fig. 8b for the photodegradation of RhB; (d) The photocatalytic activities of AWP-20 sample for the degradation of RhB, MB and MO (30 ppm).

is demonstrated by enhancement factor contrast to pure Ag2CO3 material (all the Ag2CO3 in related reports were synthesized by the same method [31–34]) degradation of the same organic pollutant (RhB) under UV–vis light irradiation, Evidently, the Polyhedron-like WO3/ Ag2CO3 p-n junction photocatalyst exhibits an extremely high activity among related Ag2CO3-based photocatalysts. In addition, other organic dyes such as MO and MB were also selected to further investigate the photocatalytic performance of optimal AWP-20 p-n junction sample. Fig. 8d. showed that only 24.76% of MO and 0% of MB could be degraded after adding AWP-20 sample after 18 min visible light irradiation under the same conditions. In this study, it is worth to point out that as-prepared Polyhedron-like WO3/Ag2CO3 p-n junction catalysts have unique strong photodegradation ability towards RhB. That might be caused by the respective structure of various organic dyes [52]. (RhB, MO and MB). To eliminate the influence of the organic dyes sensitization, colorless organic dye: 2-CP (o-Chlorophenol) is selected as the model containment [59,60] to evaluate the photocatalytic activity of AWP-20 sample. As shown in Fig. 9a, the degradation of 2-CP after 60 min irradiation reaches to 86.48%, 36.84% and 1.03% for Ag2CO3, Polyhedron-like WO3 and AWP-20 catalysts, respectively. The analysis and comparison of the

HPLC profiles (Fig. 9b) also shows that the excellent degradation ability of AWP-20 sample towards 2-CP. These results above are fully evidences to assure that AWP-20 sample also exhibits an enhanced photocatalytic performance even when degrading colorless organic dyes which don’t own sensitization. However, it is worth to be noted that AWP-20 sample has a better photocatalytic activity when degrading colored organic dye (Fig. 8), which indicates that the organic dyes sensitization to some extent promotes the photodegradation reaction. 3.7. Photostability of as-prepared photocatalysts Most importantly, to demonstrate the stability of the photocatalytic activities of the AWP-20 p-n junction photocatalyst, the samples were evaluated repeated four times by degrading the RhB. The experimental result was shown in Fig. 10a. After three cycles, the AWP-20 p-n junction still showed better stability. However, in the fourth cycle, there was a significant decrease in the RhB degradation rate. That might be caused by the loss of p-type Ag2CO3 nanoparticles on the n-type WO3 surface during the cycle procession. To further investigate the main reason about the sharply decrease in the photocatalytic activity after 4 cycles of recycling, XRD patterns was measured. As depicted in Fig.10b, 212

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Fig. 9. (a) The photocatalytic activities of AWP-20, Ag2CO3, Polyhedron-like WO3 for 10 ppm 2-CP (10 ppm) degradation under UV–vis light irradiation (Xenon lamp); (b) The comparison of HPLC degradation profiles of AWP-20 in complete spectrum sunlight exposure.

we can clearly see that the diffraction peak of pure Ag2CO3 is significantly reduced, and a small diffraction peak located at 38.1ﾟ which is attributed to the (111) facet of pure Ag nanoparticles (JCPDS NO. 040783) are formed. The above results shows that during the photocatalytic reaction, as the RhB is constantly being consumed, the electrons accumulated on the CB of Ag2CO3 semiconductor cannot be transferred out, and photo-corrosion occurs (part of the Ag2CO3 nanoparticles have decomposed to Ag), thereby decreasing the photocatalytic of as-prepared photocatalyst.

in the degradation progress and they play the chief element for degradation of RhB. The influencing degree is %OH− > h+ > %O2−. 3.8.2. Proposed mechanisms of the AWP-20 p-n junction photocatalyst It is well known that, when p-type semiconductor and n-type semiconductor are coupled, a p-n junction between two different type semiconductors will be formed [24,25]. In a reaction system that does not contain colored organic dyes, the photocatalytic mechanism of the n-type WO3/p-type Ag2CO3 p-n junction system was shown in Fig. 12. Before contact n-type WO3 and p-type Ag2CO3 semiconductors ordinary have each different position of the Femi levels. According to the above results, we can know that the positions of CB and VB edge potentials of n-type WO3 (Polyhedron-like WO3) were 0.76 eV and 3.42 eV, respectively. For p-type Ag2CO3 were 0.53 eV and 2.67 eV, respectively. As depicted in Fig. 12a, we can clearly see the CB edge of n-type WO3 and p-type Ag2CO3 was all lower than the reduction potential of O2/%O2−, which means that both catalysts above don’t have the ability to transform O2 to %O2−, when the two semiconductors were before contact. After contact, the Fermi level of p-type Ag2CO3 was moved up, and in the meantime the Fermi level of n-type WO3 (Polyhedron-like WO3) was moved down until an equilibrium state was formed (Fig. 12b). Under these circumstances, when exposing to the visible light, the VB electrons of both p-type WO3 and n-type Ag2CO3 could be irradiated. The charge transfer at the interface between the two semiconductors may follow the Type Ⅱ or Z-scheme transfer mode [16,46]. If the Type Ⅱ mechanism is assumed, the electron in the conduction band (CB) of ptype Ag2CO3 will transfer to the conduction band (CB) of n-type WO3 (Polyhedron-like WO3), and holes in the valence band (VB) of n-type WO3 (Polyhedron-like WO3) will migrate to the valence band (VB) of ptype Ag2CO3. Finally, the holes left on the removed VB of p-type WO3 still have adequate energy to degrade organic pollutants directly or form ·OH radicals, compared to the oxidation potential of %OH/H2O

3.8. Reaction mechanisms of the AWP-20 p-n junction photocatalysts 3.8.1. Determination of active sites and species in the photocatalyst It has been reported that the %O2−, %OH and h+ are the three main reactive species in the reduction and oxidation progress of photocatalyst. In order to clearly understand the photocatalytic mechanism about the AWP20 p-n junction photocatalyst, various type scavengers were selected to investigate the actual reactive species among the reaction process. After adding about 1 mM various type scavengers into reaction solutions to eliminate the related reactive species, referring to the previous report [36]. In this study, the triethanolamine (TEOA) was used to eliminate hole (h+), isopropyl alcohol (IPA) was used for removing %OH species and p-benzoquinone (BQ) was used for clearing up %O2− species. The results are exhibited in Fig. 11a, the addition of BQ has minimal effects to degradation rate of photocatalyst, implying that %O2− active species was not the main reactive group in the degradation progress. Nevertheless, the photocatalytic performance was violently inhibited in the presence of TEOA, and as depicted in Fig. 11b and c (to be more clearly present the contrast of the rate constant), the rate constant (K) dropped to 0.00 min-1. The addition of IPA also had a considerable effect on the photocatalytic of the AWP-20 sample, and the rate constant (k) decreased from 0.245 min-1 to 0.072 min-1. Herein, we can assure that hole (h+) and ·OH should be the two main active species

Fig. 10. (a) Cycling runs in photocatalytic degradation of RhB in the presence of AWP-20 composite photocatalyst under UV–vis light irradiation (Xenon lamp); (b) XRD pattern of fresh AWP-20 and used AWP-20 catalyst after 4 recycling runs. 213

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Fig. 11. (a) Photocatalytic performance for the degradation of RhB over AWP-20 composite with different sacrificial agents under UV–vis light irradiation. (■, No scavenger; ●,1 mM IPA; ▲, 1 mM BQ; ▼,1 mM TEOA). (b) Rate constant of AWP-20 composite with different sacrificial agents under UV–vis light irradiation. (■, No scavenger; ●,1 mM IPA; ▲, 1 mM BQ; ▼,1 mM TEOA).

[53]. In the meantime, the accumulated electrons in removed CB of ptype WO3 have no ability to deoxidize O2 existed in wastewater to yield % O2− due to relatively low reduction potential compared to the one of O2/%O2−. While, if Z-scheme is accepted, electrons transferred to the CB of Ag2CO3 are highly likely to reduce O2 to %O2− due to the movement of the Fermi level and the special transmission of electrons in Z-scheme mode mechanism. Herein, it is proved that whether or not the %O2− group is generated in catalytic reaction seems to become a key criterion in determining the mechanism is Z-scheme or not.

Fortunately, by combining the results of the capture reactive species experiment (Fig. 10a), we can ensure the actual mechanism is like the Z-scheme p-n junction structure [58] (as shown in Fig. 12b). Last but not the least, in view of analyses above, a proper enhancement mechanism is presented as exhibited holonomic in Fig. 12c. That is, when n-type WO3 and p-type Ag2CO3 are combined with each other, p-n juncture structure will be formed. And, in the meantime, an electronic transfer channel will be formed between the surfaces of two materials. When under the visible light irradiation, both n-type WO3

Fig. 12. Schematic diagram of photoexcited electron-hole separation (a), (b) and (c) [in Fig. 11b, (1) and (2) represent the two possible charge transfer mode of AWP20 p-n junction photocatalyst.]. 214

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and p-type Ag2CO3 can be stimulated, subsequently generating the electron-holes pairs. The photo-induced electrons (e−) will transmit to the higher energy levels of semiconductor (conductor band, CB), leaving holes (h+) on the lower one (valance band, VB). As the results of the existence of electronic transfer channel, the electrons left behind on the CB of n-type WO3 prefer migrating to the VB of p-type Ag2CO3, and at this circumstance, the VB of n-type WO3 and CB of p-type Ag2CO3 will become one of the main reactions sites in the reaction progression. Herein, the photo-induced hole (h+) accumulated at the VB of n-type WO3 surface can degrade organic pollutants by direct or indirect way, while the photogenerated electrons(e−) located at the CB of Ag2CO3 have the ability to transform O2 to %O2− (Fig. 12b), afterward degrading organic pollutants by an indirect way. The primary degradation equilibrium is as follows: + H2 O + h vb

O2 + ecb

H2 O+

H2 O + H+ (E vb

O2 (Ecb

H2 O + organic pollutant

O2 + organic pollutant + h vb

+ organic pollutant

+ 1.99 eV )

0.33 eV ) Degraded pollutant

Degraded pollutant Degraded pollutant

(1) (2)

Fig. 14. Photoluminescence emission spectra of pure-Ag2CO3, Polyhedron-like WO3 and AWP-20.

(3) (4)

forming a Z-scheme p-n junction structure directly.

(5)

3.8.3.2. Photocurrent measurement and EIS tests. The photoelectrochemical properties of Polyhedron-like WO3 (n-type WO3), ptype Ag2CO3 and AWP-20 samples were studies in a three-electrode setup. As depicted in Fig. 15a, visible light would stimulate the semiconductor materials to generate photocurrent signal. Among all prepared photocatalysts, AWP-20 sample exhibit higher photocurrent density than not only the pure Ag2CO3 but also the pure Polyhedronlike WO3 (n-type WO3). Normally, higher photocurrent intensity signifies better separation efficiency of photo-charges [54,57] under light irradiation. From above test results, we can conclude that the AWP-20 sample own higher capacity in separation of photo-charges, which is also in good agreement with the photocatalytic properties. In order to further investigate these results, the electrochemical impedance spectroscopy (EIS) spectra of the Polyhedron-like WO3 (ntype WO3), p-type Ag2CO3 and AWP-20 samples were examined according to their Nyquist plots. As depicted in Fig. 15b, it can be observed that the size of the arc radius for the electrode modified with the AWP-20 species is evidently smaller than that of the electrode modified with the Polyhedron-like WO3 (n-type WO3) and p-type Ag2CO3. This result indicated a decrease of the charge transfer resistance on the AWP20 p-n junction photocatalyst surface or in solid-state interface layer. On the basis of test results above, we might imply that highly improved photocatalytic performance of AWP-20 is attributed to better separation efficiency of photo-charges and lower charge transfer resistance among photocataysts.

Fig. 13 exhibits schematic diagram of AWP-20 p-n junction structure, when considering the organic dyes photo-sensitization. In the diagram, we can clearly observe that when under the visible light irradiation, in addition to a number of RhB molecules degraded by the AWP-20 sample, there is still a part of photo-excited electrons (e−) from LUMO of RhB [61] flow into CB of Ag2CO3, then subsequently enhances the electrons transfer [62] and photodegradation ability. 3.8.3. Evidences of mechanism 3.8.3.1. PL emission spectra. To ensure the combination and separation rate in the photo-induced carriers which played a primary role in photocatalytic reactions, we usually use PL emission spectra. Photocurrent intensity can imply the recombination speed of photoexcited electron-hole pairs. Generally, higher photocurrent intensity means faster combination of the photo-induced electron (e−) and hole (h+), and lower photocurrent intensity indicates a lower recombination rate of photo-induced electron (e−) and hole (h+) [54–56]. Fig. 14 shows the PL spectra of the p-type Ag2CO3, Polyhedron-like WO3 (n-type WO3) and AWP-20, respectively. From the picture, it can be observed that the PL intensity of the AWP-20 p-n junction photocatalyst exhibits weaker emission compared with pure Polyhedron-like WO3 (n-type WO3), suggesting lower recombination rate and higher separation probability of the photo-induced electron-holes on the AWP20 photocatalyst surface, which provides an evidence supporting the claim that the process of charge recombination is impact inhibited by

4. Conclusions In summary, a novel Polyhedron-like WO3/Ag2CO3 p-n junction photocatalysts have been synthesized by a facile impregnation-deposition process. Uniform assembly of Polyhedron-like WO3 (n-type WO3) nanoparticles were evenly adhered on the p-type Ag2CO3 nanoparticles surface. The obtained Polyhedron-like WO3/Ag2CO3 p-n junction composites all exhibit more efficient photocatalytic performance than pure p-type Ag2CO3 and Polyhedron-like WO3 (n-type WO3) for degradation the same water pollutant molecules (RhB) under UV–vis light region. The 20 wt% Polyhedron-like WO3/Ag2CO3 p-n junction system (AWP-20 sample) performed the highest degradation rate which is 17 times higher than commercial powder (P25) and it still has the highest photocatalytic activity compared with other related photocatalysts [31,34].When the excellent photocatalyst performance is attributed to the synergistic effects below: (1) extended absorption in region of for the p-n junction photocatalyst; (2) smaller solid-state interface layer

Fig. 13. Schematic diagram of photo-excited electron-hole separation electronhole separation in above diagram represent the typical charge transfer mode of AWP-20 p-n junction photocatalyst in reaction system. 215

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Fig. 15. (a). Typical transient photocurrent responses of the ITO electrodes modified with the Polyhedron-like WO3, Ag2CO3 and AWP-20 samples during intermittent UV–vis light irradiation (Xenon lamp); (b). Typical EIS spectra of the ITO electrodes modified with the Polyhedron-like WO3, Ag2CO3 and AWP-20 samples.

resistance and the charge transfer resistance; (3) Z-scheme p-n junction system, which enhances the separation of photogenerated carriers and suppress the invalid charge recombination on the surfaces of single semiconductor materials.

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