Hole Doping to Enhance the Photocatalytic Activity of Bi₄NbO₈Cl

Abstract

An increase of carrier concentration is one of the most important routes for enhancing the catalytic performance of semiconductor photocatalysts. In this study, the Sillén–Aurivillius oxychloride Bi₄NbO₈Cl with hole doping was successfully prepared by a solid-state reaction method. X-ray powder diffraction (XRD), scanning electron microscopy (SEM), ultraviolet–visible diffuse reflectance spectra (UV–vis DRS), X-ray photoelectron spectrometry (XPS) and photoluminescence spectra (PL) were used to characterize and analyze the prepared samples. The experimental results and density functional theory calculations demonstrate that hole doping can be formed in Bi₄NbO₈Cl by inserting zinc into the niobium site, and the photocatalytic activity can be improved by introducing additional holes into Bi₄NbO₈Cl. The photogenerated hole (h⁺) is considered to be the main active species to degrade trypan blue (TB) through trapping experiments. The optimal photocatalyst of Bi₄Nb_0.8Zn_0.2O₈Cl exhibits excellent photocatalytic activity in degradation of trypan blue under visible light irritation. Moreover, a possible photocatalytic degradation mechanism is discussed according the experimental and analytical results.

1. Introduction

Recently, environmental pollution and energy shortages have become two main challenges for human beings. In particular, the discharge of various organic wastewaters has seriously caused irreversible damage to the environment and human beings [1,2,3]. Among the reported remediation methods, semiconductor photocatalysis has been regarded as an effective route to eliminate contaminants. In particular, the development of visible light-driven photocatalysts has attracted increasing attention from the perspective of solar energy conversion [4,5].

In recent years, a novel bismuth-based photocatalyst Bi₄NbO₈Clhas has attracted much attention. It has a layered Sillén–Aurivillius perovskite structure and consists of single-layer NbO₄ perovskite blocks that are separated by (Bi₂O₂)₂Cl blocks, which is beneficial for the efficient separation and migration of the photogenerated charge carriers. Moreover, the valence band maxima (VBM) of Bi₄NbO₈Cl is mainly composed of O-2p orbital rather than Cl-3p orbits, so its VBM level is more negative than that of typical oxides [6,7,8]. Therefore, Bi₄NbO₈Cl is considered to be a stable visible light-response photocatalyst with narrow bandgap and which is usually applied to degrade organic pollutants. Shi et al. [9] prepared a layered Bi-based oxychloride Bi₄NbO₈Cl, and found that the visible light-driven photocatalytic activities for degrading methyl orange (MO) over different catalysts follow the decreasing order of Bi₄NbO₈Cl > Bi₃O₄Cl > anatase TiO₂; Sundaram et al. [10] synthesized Bi₄NbO₈Cl nanoparticles by a solution combustion method and the mineralization efficiency of Congo red dye can reach 75% in 80 min.

However, in order to further improve the photocatalytic performance of Bi₄NbO₈Cl, many methods have been developed such as semiconductor recombination [11,12], deposition of noble metals [13,14], deposition of non-noble metals [15] and metal ions doping [16]. Among these methods, transition metal ions doping is considered to be one of the most promising, which can increase carrier concentration and improve the charge carrier transport [17]. Thus, transition ion doping can effectively improve the photocatalytic performance of Bi₄NbO₈Cl. Shangguan et al. [16] utilized yttrium-doped Bi₄NbO₈Cl to synthetize Bi_4−xY_xNbO₈Cl and enhance photocatalytic activity. As common dopants, doping of transition metals tungsten (W) and zinc (Zn) is considered to be an effective method to improve the photocatalytic activity of semiconductors [18,19].

However, through our investigation, there are few reports preparing tungsten- or zinc-doped Bi₄NbO₈Cl material and studying their photocatalytic performance. In this work, a series of W-doped and Zn-doped Bi₄NbO₈Cl materials with different dopant concentrations were synthesized via a solid-state reaction method. The photocatalytic activities of the W-doped and Zn-doped Bi₄NbO₈Cl materials were investigated by degradation of trypan blue (TB) under visible light. Finally, the possible hole-doping mechanism of photocatalytic degradation of TB was also discussed.

2. Results

2.1. Synthesis of Bi₄Nb_1−xW_xO₈Cl and Bi₄Nb_1−xZn_xO₈Cl (x = 0.1, 0.2, 0.3) Powder

The photocatalysts were synthesized by a solid-state reaction. Firstly, 2 mM Bi(NO₃)₃·5H₂O was dissolved in 20 mL ethylene glycol, subsequently 10 mL KCl solution (0.2 mol/L) gradually added to obtain the precursor. After filtration and washing, the precursor was dried at 333 K for 12 h to obtain BiOCl. Secondly, the stoichiometric amount of prepared BiOCl, Bi₂O₃ (Aladdin, 99.9%), Nb₂O₅ (Aladdin, 99.9%) and WO₃ or ZnO powders (Aladdin, 99.9%) were adequately ground and calcined in a muffle furnace at 973 k for 24 h. Finally, Bi₄Nb_1−xW_xO₈Cl and Bi₄Nb_1−xZn_xO₈Cl (x = 0.1, 0.2, 0.3) series powders were prepared. WO₃ or ZnO was used to replace the component of Nb₂O₅, the x value of the very small part being 0, 0.1, 0.2, 0.3, respectively. The samples with the atomic ratio of Zn to Nb (0, 1:9, 2:8, 3:7) were labeled as BNO, BNZ-1, BNZ-2 and BNZ-3, respectively. Those of W to Nb (1:9, 2:8, 3:7) were labeled as BWZ-1, BWZ-2, BWZ-3, respectively.

2.2. Characterization

An X-ray diffractometer (XRD) with Cu Kα radiation (X’Pert³ Powder, PANalytical, Almelo, the Netherlands, λ = 0.15406 nm) was used to carry out of the phase identification. The morphologies of the samples were studied by Nova a Nano 450 (FEI, Hillsboro, OR, USA) field-emission scanning electron microscope (FESEM), X-ray photoelectron spectroscopy (XPS) was performed by an ESCALAB 250xi XPS system (Thermo Fisher Scientific, Carlsbad, CA, USA). A Shimadzu UV-2550 ultraviolet UV–visible diffuse reflectance spectrum (Shimadzu, Kyoto, Japan) was used for the test of the spectral absorption curves of the samples, with a wavelength scanning range of 200–800 nm. The photoluminescence (PL) spectra were collected by a Hitachi F-4600 spectrometer (Hitachi, Tokyo, Japan).

2.3. Photocatalytic Activity

Photocatalytic activities of Bi₄Nb_1−xW_xO₈Cl and Bi₄Nb_1−xZn_xO₈Cl series were evaluated by degrading TB under a 500 W Xe Lamp (420 nm cut off filter) illumination. In the study of degradation, 30 mg prepared photocatalyst was dispersed evenly in 30 mL TB solution (10 mg/mL) at room temperature. The mixture was magnetically stirred in the dark for 30 min before illumination. Then the TB solution was exposed to visible light illumination with stirring, and 3 mL suspension was collected every 15 min, and then centrifuged at 12,000 rpm for 5 min to remove the photocatalyst. Finally, the concentration of TB was determined by a spectrophotometer.

2.4. Density Functional Theory (DFT) Calculation

Here, in this paper, we studied the electronic structures of pure and doped Bi₄NbO₈Cl systems by performing first-principles calculations based on density functional theory (DFT). The Wien2k program package [20] was employed and the Perdew-Burke-Ernzerh (PBE) version of the generalised gradient approximation (GGA) [21] was adopted. We used a 13 × 13 × 2 momentum grid, and R_MT*K_max = 7.0 with a muffin-tin radius R_MT =1.84, 2.30, 1.67 and 2.5 a.u. for Nb, Bi, O and Cl atoms, respectively.

The crystal structure presented in Section 3.6 is adopted for calculations. Electron and hole doping are approximately achieved by applying virtual crystal approximation (VCA) implemented in Wien2k. We treat the valence states of W and Zn as 6s²5d⁴ and 4s², respectively. Considering the valence state of Nb is 5d¹4d⁴, hence 20% W and 20% Zn doping induces 0.2 electron and 0.6 hole doping per Nb, respectively.

3. Discussion

3.1. X-Ray Diffractogram (XRD) Patterns Analysis

The XRD patterns of the Bi₄NbO₈Cl, Bi₄Nb_1−xW_xO₈Cl and Bi₄Nb_1−xZn_xO₈Cl series are shown in Figure 1. As shown in Figure 1a, the diffraction peaks observed are well indexed to the Bi₄NbO₈Cl (JCPDS NO.84-0843), and no other diffraction peaks can be found. Which indicates that either W or Zn doping does not affect the crystal structure of Bi₄NbO₈Cl. It is noteworthy that the diffraction peak at about 29.6° slightly shifts to higher angles with the increase of dopant content (Figure 1b). This phenomenon can be attributed to the ionic radius of Zn²⁺ (0.74 Å) and W⁶⁺ (0.62 Å) both being smaller than that of Nb⁵⁺ (0.78 Å), the incorporation of W or Zn ions in Bi₄NbO₈Cl lattice via substituting Nb ions consequently induce distortion in the crystal lattice of Bi₄NbO₈Cl [18,22,23]. This means that W or Zn elements were successfully doped to the Bi₄NbO₈Cl lattice.

3.2. Scanning Electron Microscopy (SEM)

The morphologies of Bi₄NbO₈Cl and Bi₄Nb_0.8Zn_0.2O₈Cl are shown in Figure 2. It can be seen that Bi₄NbO₈Cl is composed of a large number of nanoplates (Figure 2a,b); After Zinc doping, the morphology of the sample has changed. Bi₄Nb_0.8Zn_0.2O₈Cl has a lamellar stacking structure with the thickness of about 100 nm (Figure 2a,b). The morphology of Bi₄Nb_0.9Zn_0.1O₈Cl and Bi₄Nb_0.7Zn_0.3O₈Cl are presented in Figure S1. It can be seen that the morphologies of the catalysts change little with the different amount of zinc doping.

3.3. X-Ray Photoelectron Spectroscopy

The surface chemical element states of Bi₄NbO₈Cl and Bi₄Nb_0.8Zn_0.2O₈Cl were investigated by X-ray photoelectron spectroscopy. Figure 3a shows the existence of C, N, Bi, Nb and Cl element in Bi₄NbO₈Cl. Compared with Bi₄NbO₈Cl, the characteristic peak of Zn is added in the Bi₄Nb_0.8Zn_0.2O₈Cl spectrum, indicating that the Zn element has been successfully doped in Bi₄NbO₈Cl. As shown in Figure 3b,d, the Bi 4f spectrum of Bi₄NbO₈Cl with the peaks at 164.17 eV and 158.86 eV are indexed to Bi 4f_7/2 and Bi 4f_5/2, respectively. This indicates that the valence state of Bi is trivalent [24,25]. The peaks of Nb 3d_5/2 and Nb 3d_3/2 appear at 206.23 eV and 208.98 eV for Bi₄NbO₈Cl, indicating the existence of Nb⁵⁺ state (Figure 3b). However, the two peaks for Bi₄NbO₈Cl located at 198.3 eV and 199.9 eV, belonging to Cl 2p_3/2 and Cl 2p_1/2, respectively (Figure 3c). Notably, the Bi 4f, Nb 3d and Cl 2p peaks for Bi₄Nb_0.8Zn_0.2O₈Cl all display a slight migration towards lower binding energy compared to Bi₄NbO₈Cl, suggesting that the chemical circumstances of Bi, Nb and Cl elements have changed since the zinc is doped [16,26]. The O 1s peaks for Bi₄NbO₈Cl (Figure 2d) can be divided into three peaks at 529.55 eV, 530.89eV, and 531.32 eV, which separately belong to the crystal lattice oxygen, Bi–O bonds and Nb–O bonds. However, when the zinc is doped, the characteristic O 1s peaks change. This may be attributed to the formation of Nb–O–Zn–O bond [27]. Zinc signals are detected after Zn²⁺ doping, as shown in Figure 3b, the peaks located at 1023.52 eV and 1046.65 eV are contributed to Zn 2p_3/2 and Zn 2p_1/2, respectively [28].

3.4. Ultraviolet–Visible (UV–Vis) Diffuse Reflectance Spectrum

Figure 4a shows the ultraviolet–visible (UV–vis) diffuse reflectance spectrum of Bi₄NbO₈Cl, Bi₄Nb_1−xW_xO₈Cl and Bi₄Nb_1−xZn_xO₈Cl series. It can be seen that all the photocatalysts have strong absorption in the visible light region and exhibit almost the same absorption edges with either W or Zn doping. The band gap energy (E_g) is determined by the following equation using the data of optical absorption vs. wavelength near the band edge [29,30]:

$$\alpha h\upsilon =A{\left(h\upsilon -E_g\right)}^{2/\mathrm{n}}$$

(1)

where A indicates a constant, α, hν, and E_g are respective to absorption coefficient, photon energy and band gap energy, respectively. In this work, n is 1 due to the BiNbO₈Cl being a kind of indirect gap semiconductor [31]. The band gap energy (E_g) value can be obtained by extrapolating the linear portion of the hν − (αhν)^1/2 curve. As shown in Figure 4b, the band gap of BNO is about 2.45 eV and there are no significant changes after either W or Zn doping, indicating that the absorption of light is caused by the band gap transition rather than the transition of the impurity energy level. The results show that a new photocatalyst with visible light response has been successfully prepared, which may provide the excellent photocatalytic activity.

3.5. Photocatalytic Activity

In the dye degradation experiment, TB was chosen as the dye model under visible irradiation at pH ≈ 7. Figure 5a shows the degradation profiles under visible irradiation. It can be seen that the degradation ratio of BNO is about 83.5% within 90 min. However, the photocatalytic degradation efficiency of BNW-1, BNW-2 and BNW-2 are lower than that of BNO indicating that W doping could not effectively improve the degradation efficiency of TB. In contrast, it can be found that all of the Zn-doped samples show higher photocatalytic degradation abilities than the pure Bi₄NbO₈Cl photocatalyst. The BNZ-2 photocatalyst displays the highest photocatalytic activity and the removal rate can reach 96% in 90 min. The results confirm that the introduction of Zinc in Bi₄NbO₈Cl could effectively enhance photocatalytic activity.

In order to quantitatively study the reaction kinetics of the degradation process, a first-order model is used to fit the reaction data. The formula is as follows [32,33]: ln (C₀/C) = kt + b, where k is the apparent first-order rate constant, and ln (C₀/C) has a linear relationship with catalytic reaction time. As can be seen in Figure 5c, the pseudo first-order rate constants (k) for BNZ-2 reaches 0.03085 min⁻¹, which is about 1.5 times as high as that of BNO (k = 0.02019) and about 3 times that of BNW-2, respectively. Figure 5d shows the temporal evolution of the UV–visible absorption spectral changes during TB degradation process by BNZ-2 photocatalyst. As time goes on, the characteristic absorption peaks between 500 and 800 nm region have reduced gradually, which indicates that the molecular structure of organic pollutants is destroyed.

3.6. Computational Details

Using the first-principles methods, we calculated the band structures of Bi₄NbO₈Cl and doped systems. As the chemical valence is two for zinc and six for tungsten, this means hole doping for Bi₄Nb_0.8Zn_0.2O₈Cl and electron doping for Bi₄Nb_0.8W_0.2O₈Cl system. From the band structures in Figure 6, we can clearly see the tungsten doping will introduce the additional electron carriers, while the hole carriers will increase by zinc doping. Therefore, it is not too difficult to understand the reason why enhancement of photodegradation properties of Bi₄Nb_0.8Zn_0.2O₈Cl, in which the hole carriers play a role for the photocatalytic mechanism.

3.7. Possible Photocatalysis Mechanism

It is noteworthy that all of the W-doped Bi₄NbO₈Cl photocatalysts have lower photocatalytic activity than that of pure Bi₄NbO₈Cl. In contrast, the Zn-doped Bi₄NbO₈Cl photocatalysts have higher photocatalytic activities than that of pure Bi₄NbO₈Cl. This phenomenon can be attributed to the Zn²⁺ ions being incorporated into the Bi₄NbO₈Cl via replacing the Nb⁵⁺ lattice sites, and the positive charge of Zn²⁺ ion being less than that of Nb⁵⁺, thus forming a negative charge center at the position of Zn²⁺. In order to maintain the charge balance, Zn²⁺ ions will fetter the hole (h⁺) and make it difficult to move freely [34]. Therefore, the increase of the concentration of holes (h⁺) around zinc ion leads to the formation of hole doping in Bi₄NbO₈Cl, which has strong oxidation performance and is beneficial to photocatalytic degradation. In addition, hole fettering limit the effective recombination of photogenerated electrons and holes. The photoluminescence technique is used to analysis the separation efficiency of photoelectron-hole pairs. As shown in Figure 7, BNO exhibits higher emission peak than BNZ-2, indicating that the recombination of photoelectron-hole pairs is faster [35,36]. Therefore, the doping of zinc can promote the separation of photogenerated electrons and holes, improving the photocatalytic efficiency.

By contrast, the positive charge of the W⁶⁺ ion is greater than that of the Nb⁵⁺, the W⁶⁺ will capture the electrons to maintain the charge balance, and forming the electrons (e⁻) doping in Bi₄NbO₈Cl. However, the W⁶⁺ can capture the conduction band electrons (e⁻) and hinder the formation of super oxide radicals (•O₂⁻), which leads to the decrease of photocatalytic degradation efficiency [37]. Therefore, the doping of W in Bi₄NbO₈Cl leads to the decrease of photocatalytic activity. The above mechanism can be also used to understand the photocatalytic activity in Bi₄Ti_0.5W_0.5O₈Cl system, which is almost the same with Bi₄NbO₈Cl [38].

The trapping experiments were used to detect the main active species in the process of photocatalytic degradation. The Tert-butanol (t-BuOH, an •OH scavenger), p-benzoquinone (BQ; an •O₂⁻ scavenger), and disodium ethylenediamine tetraacetate (EDTA-2Na, h⁺ scavenger) were used as quenchers [39]. As shown in Figure 8a,b, after adding the t-BuOH, the photocatalytic degradation of TB is slightly decreased for both BNO and BNZ-2, indicating hydroxyl radicals (•OH) have little impact on the photocatalytic degradation process. When the BQ and EDTA-2Na are added, the photocatalytic degradation rate of TB decreased sharply, suggesting that the hole (h⁺) and super oxide radicals (•O₂⁻) are the main active species in the TB degradation process. In contrast, it suggests that the effect of hole (h⁺) on BT degradation is more critical than that of super oxide radicals (•O₂⁻) over BNZ-2 (Figure 8b).

Based on the above experiments and analysis, a mechanism of TB degradation by Zn-doped Bi₄NbO₈Cl photocatalysts under visible light is proposed. As mentioned, the Zn-doped Bi₄NbO₈Cl photocatalysts can be excited and generate the photogenerated electrons (e⁻) and holes (h⁺) pairs under visible light. The electrons(e⁻) react with adsorbed O₂ to generate superoxide radical (•O₂⁻), which is a strong oxidant and can degrade TB. Because of the hole (h⁺) doping in Bi₄NbO₈Cl, the hole is far more oxidative than •OH radical. Therefore, holes (h⁺) play a main role in the photocatalytic degradation of TB. The photogenerated holes (h⁺) of Zn-doped Bi₄NbO₈Cl can directly transform TB molecules into H₂O, CO₂ and mineral [40]. Finally, the organic pollutants are photocatalytically degraded by this novel hole-doped photocatalyst. The possible charge carrier transfer path is as follows:

$${\mathrm{Bi}}_4{\mathrm{Nb}}_{0.8}{\mathrm{Zn}}_{0.2}{\mathrm{O}}_8\mathrm{Cl}\stackrel{\mathrm{visible}\mathrm{light}}{\to }{\mathrm{h}}^{+}{+\mathrm{e}}^{-}$$

(2)

$${\mathrm{e}}^{-}{+\mathrm{O}}_2\to •{\mathrm{O}}_2^{-}$$

(3)

$${\mathrm{h}}^{+}+\mathrm{TB}\to {\mathrm{CO}}_2{+\mathrm{H}}_2\mathrm{O}+\mathrm{mineral}$$

(4)

$$•{\mathrm{O}}_2+\mathrm{TB}\to {\mathrm{CO}}_2{+\mathrm{H}}_2\mathrm{O}+\mathrm{mineral}$$

(5)

4. Conclusions

A novel layered photocatalyst of Bi₄NbO₈Cl doped with zinc was successfully synthesized by a solid-state reaction method. It was found that the introduction of zinc ions into Bi₄NbO₈Cl can construct hole (h⁺) doping in Bi₄NbO₈Cl and promote the separation of photogenerated electrons. Compared to Bi₄NbO₈Cl, Bi₄Nb_1−xZn_xO₈Cl (x = 0.1, 0.2, 0.3) series photocatalysts show significantly higher photocatalytic activities for trypan blue degradation. In particular, when the atomic ratio of Zn to Nb is 2:8, the photocatalyst showed the highest photocatalytic activity and the removal ratio of trypan blue could reach 96% in 90 min. Furthermore, hole (h⁺) was considered as the effective species in the photocatalytic degradation process. On the basis of the experimental results and density functional theory calculations, the hole carriers play a role in the photocatalytic mechanism. This material may have potential application prospects in the degradation of organic pollutants from wastewater.