Bismuth-based photocatalysts have aroused wide research interests due to their unique electronic structure and excellent photocatalytic performance , , , , , , , . Among these materials, BiOX (X=Cl, Br, I), (BiO)2CO3 and BiOCOOH have similar structures with the general formula [Bi2O2][Xm] (X=halogen or other groups) that usually possesses alternating [Bi2O2]2+ sheets and X slabs (m=1, 2 or 3 rarely). This unique structure benefits the certain ions that enter into the space between the layers, and replace the X−, COOH−, or CO32− ions and co-ordinate with [Bi2O2] layers. Recently, to extend the light absorption spectra of (BiO)2CO3 or BiOX into visible light region, diverse approaches have been developed, such as surface decoration, doping and heterojunction construction , , , , , , , , , .
BiOCOOH is a recently discovered photocatalyst that can only be excited by UV light due to its wide band gap , . Chen et al.  prepared BiOCOOH with different shape through a facile and template-free solvothermal process, including spherical-like, sponge-like, tremella-like and flower-like hierarchical nanostructures. The sponge-like BiOCOOH hierarchical nanostructures exhibited the highest photocatalytic activity amongst those nanostructures . Afterwards, Zhu et al.  synthesized ultralong BiOCOOH nanowires via a simple solvothermal route, which exhibited highly efficient adsorption performance for Cr(VI) and MO due to its positively charged surface, large porosity, and good dispersion in water. To improve the photocatalytic activities of pure BiOCOOH, coupling a wide band gap semiconductor with a narrow band gap semiconductor to form heterostructures with a staggered alignment of band edges could greatly extend light responsive range and significantly improve the charge seperation. Chai and Wang  fabricated the BiOI/BiOCOOOH composites by facile partial ion exchange between BiOCOOH and KI at acidic condition. To date, little is known about the effect of nonmetal doping on the properties of BiOCOOH.
In this work, we report a simple ion-exchange method for I-doped BiOCOOH nanoplates to extend the light absorption spectra of BiOCOOH into visible light region. The photocatalytic activity of the samples was evaluated for removal of NO at ppb-level under visible light. The results indicated that the I-doped BiOCOOH nanoplates exhibited highly enhanced visible light photocatalytic activity in comparison with BiOCOOH as I-doping could effectively improve the visible light absorption and charge separation. The optimized catalyst (IHB-1.00) exhibited the highest photocatalytic activity and excellent stability, which is significant for its practical application.
Synthesis of I doped BiOCOOH
All chemicals used in this study were analytical grade and were used without further purification. The synthesis of BiOCOOH nanoplates was reported in our previous work . In a typical process, 0.812 g of BiOCOOH was added into 60 mL distilled water, and then dispersed for 20 min with ultrasound treatment. Then 40 mL of KI aqueous solution (AR, KeLong, Chengdu, China) was added to the above suspension under continuous stirring for 2 h. The obtained sample was filtered, washed with water and ethanol each for two times and dried at 60°C for 12 h to get the final samples. The molar ratio of KI to BiOCOOH was controlled at 0.10, 0.25, 0.50, 1.00, 2.00 and 4.00, respectively. Accordingly, the resulted products were labeled as IHB-0.10, IHB-0.25, IHB-0.50, IHB-1.00, IHB-2.00 and IHB-4.00, respectively.
The crystal structure of the as-obtained samples was analyzed by X-ray diffraction with Cu-Karadiation (XRD: model D/max RA, Rigaku Co., Japan). The morphological structures were examined by transmission electron microscopy (TEM: JEM-2010, JEOL, Japan). X-ray photoelectron spectroscopy with Al Kα X-rays (hν=1486.6 eV) radiation operated at 150 W (XPS: Thermo ESCALAB 250, USA) was used to investigate the surface properties. The shift of the binding energy due to relative surface charging was corrected using the C1s level at 284.8 eV as an internal standard. The Brunauer-Emmett-Teller (BET: Gemini VII 2390, Micromeritics, Shanghai, China) surface area and the pore size distribution of the products were identified by a Micromeritics ASAP 2020 apparatus. All the samples were degassed at 150°C prior to measurements. The UV-vis diffuse reflection spectra were obtained from the dry-pressed disk samples using a Scan UV-vis spectrophotometer (UV-vis DRS: UV-2450, Aucy, Shanghai, China) equipped with an integrating sphere assembly, using 100% BaSO4 as reflectance sample. The solid-state photoluminescence (PL) spectra were measured with fluorescence spectrophotometer (PL: FS-2500, Hitachi, Japan) using a Xe lamp as an excitation source with optical filters.
Evaluation of photocatalytic activity
The photocatalytic activity of the resulting samples was investigated by oxidation of NO at ppb levels in a continuous flow reactor at ambient temperature. The volume of the rectangular reactor, which was made of stainless steel and covered with Saint-Glass, was 4.5 L (30 cm×15 cm×10 cm). A 150 W commercial tungsten halogen lamp was vertically placed outside the reactor above the reactor. For each photocatalytic activity test experiment, two sample dishes (with a diameter of 12 cm) containing the photocatalyst powders were placed in the center of the reactor. The photocatalyst samples (The weight of the photocatalysts used for each dish was kept at 0.10 g) were prepared by coating an aqueous suspension of the samples onto the glass dish and then dried at 60°C. The NO gas was acquired from a compressed gas cylinder at a concentration of 100 ppm of NO (N2 balance). The initial concentration of NO was diluted to 550 ppb by the air stream supplied by purified air. The desired relative humidity (RH) level of the NO flow was controlled at 60% by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 3.3 L/min by a mass flow controller. After the adsorption-desorption equilibrium was achieved, the lamp was turned on. The concentration of NOx was continuously measured by a chemiluminescence NOx analyzer (Thermo Environmental Instruments Inc., 42i-TL), which monitors NO, NO2 and NOx (NOx represents NO+NO2) with a sampling rate of 1.0 L/min. The NO removal ratio (η) was calculated as η(%)=(1–C/C0)×100%, where C and C0 are concentrations of NO in the outlet steam and the feeding stream, respectively.
Photoelectrochemical measurements were conducted using a three-electrode quartz cell equipted on CHI-660B electrochemical system. The saturated calomel electrode (SCE) was used as the reference electrode, and platinum wire served as counter electrode. The working electrodes were BiOCOOH and IHB-1.00 films coated on ITO glass. The electrolyte was 0.1 M of Na2SO4 solution. All the photoelectrochemical measurements were carried out under visible light irradiation emitted from a 500 W Xe lamp with a 420 nm cut-off filter. The average power of visible light was controlled at 45 mW/cm2.
Results and discussion
To characterize the phase structure of the as-prepared samples, XRD was performed. Figure 1a shows the XRD patterns of BiOCOOH, BiOI and IHB-X samples. All diffraction peaks of the samples are strong and sharp, indicating the samples are well-crystallized. The diffraction peaks of the BiOCOOH and IHB-X samples can be indexed to the BiOCOOH (JCPDS-ICDD Card No. 35-0939). The peak at around 2θ=28.6° corresponds to (102) plane of BiOCOOH, and this peak intensity in each IHB-X sample is lower than that in pure BiOCOOH. Meanwhile, this peak has a slight shift to a higher angle (Fig. 1b), indicating a decreased layer distance. The decreased peak intensity and the shifting of the peak position can be ascribed to the doping of I− ions. The I− ions could replace the COOH− ions in the layers of BiOCOOH and co-ordinate with oxygen atoms of [BiO]+ layer. Note that no BiOI phase can be detected, even a large amount of I− ions were added. This result indicates that the I− ions were doped into the crystal structure of BiOCOOH. As the size of I− ions is smaller than that of COOH− ions, the partial replacement of COOH− ions with I− ions would result in a decreased layer distance. The layer distance would not decrease as the molar ratio of I− to COOH− increased from 1.00 to even 4.00 indicating that the replacement of COOH− ions with I− ions has a limit, defining as “saturate doping”.
The morphology and microstructure of the obtained samples were characterized by SEM, TEM, HRTEM and EDS mapping. In Fig. 2a, it can be been that the pure BiOCOOH consists of numerous two-dimensional (2D) nanosheets in different sizes. The HRTEM image shows the lattice distance of the crystal plane, corresponding to the (012) plane. Compared with the pure BiOCOOH nanosheets, the morphology of IHB-1.00 is irregular (Fig. 3a). The observed lattice spacing of 0.312 nm can be well assigned to the (012) plane of BiOCOOH. The EDS mapping (Fig. 3c–f) indicates the elemental distribution of C, O, Bi and I, thereby demonstrating that the doped I is uniformly distributed across the sample.
XPS measurement was used to determine the chemical composition of BiOCOOH, IBH-0.25, IBH-1.00, and IHB-4.00 samples. The peaks (Fig. 4a) with binding energies at 159.1 and 164.2 eV can be ascribed to Bi4f7/2 and Bi4f5/2 , , , , respectively. Note that there appear new peaks around 161.1 and 166.8 eV. As the BiOCOOH has a layered crystal structure, the I− ions could enter into the space between the layers, and then replace the COOH− ions and co-ordinate with Bi and O atoms. Thus the new peaks of 161.1 and 166.8 eV can be ascribed to the formation of I–Bi–O bond. Figure 4b shows I3d spectra with two peaks at 618.3 and 631.2 eV, attributed to the presence of I− , , . The binding energy has witnessed a positive chemical shift with the increasing of I− ions contents. From XPS result, the content of doped I is determined to be 6.62, 11.53, 11.55 and 11.56%, for IHB-0.50, IHB-1.00, IHB-2.00 and IHB-4.00, respectively. This result confirms that the I− ions content reaches the “upper limit” or saturation point of doping for IHB-1.00.
Bet surface areas and pore structure
Figure 5 shows the nitrogen adsorption-desorption isotherms and the corresponding pore size distribution curves of the samples. According to the Brunauer-Deming-Deming-Teller (BDDT) classification, the majority of physisorption isotherms can be classified into six types. Typically, the BiOCOOH and IHB-X have an isotherm of type IV. The IHB-1.00 displays a stronger absorption at relatively high pressure comparing with others, suggesting the presence of more mesopores in this sample. As no mesopores are formed in the nanosheets, the observed mesopores should be the void space between the stacked nanosheets. This result is also demonstrated by the pore-size distribution curves. Figure 5b shows that the four samples have the peak pore sizes at 1.2 and 2.6 nm. The SBET, pore volume and peak pore diameter are summarized in Table 1. The sample with high surface area and pore volume would facilitate the exposure of active sites and promote the photocatalysis efficiency.
Light absorption, charge separation
The pure BiOCOOH presents a white color, while the samples after I-doping change from pale to dark yellow with the increase of I-doping. Seen from the UV-vis DRS spectra (Fig. 6a), the pure BiOCOOH displays an absorption edge around 380 nm. With the increase of I-doping from 0.25 to 1.00, the absorption edge shows a red-shift from 380 to 600 nm and the light absorption spectra are broadened to 650 nm in visible light region. The enhanced visible light absorption should be ascribed to I-doping resutling in the band gap narrowing. However, even with more amount of I− ions added, the absorption edge and light absorption spectra of IHB-2.00 and IHB-4.00 do not change obviously. As shown in Fig. 6b, the band gap energies of IHB-0.25, IHB-0.50, IHB-1.00, IHB-2.00 and IHB-4.00 samples are estimated to be 2.78, 1.99, 1.61, 1.61 and 1.61 eV, respectively, which is lower than that of the pure BiOCOOH with a value of 3.2 eV. It also was found that the band gap enengies do not change anymore when the molar ratio of KI to BiOCOOH reaches at 1.00. The XPS and UV-vis DRS results indicate that the I-doping could achieve a satuarate point when the interlayer spaces among the layers have been fully interted with I− ions.
To measure the recombination rate of the photogenerated electron-hole pairs, PL spectroscopy is applied. A low PL intensity indicates a high charge separation efficiency . As can be seen in Fig. 7a, pure BiOCOOH gives a high peak intensity. With the increase of I-doping, the PL peak intensity significantly decreases, indicating a largely suppressed recombination of photoinduced electron-hole pairs. The result implies that the I-doping can improve separation of photogenerated electron-hole pairs and thus enhance the photocatalytic performance.
Photocurrent generation was carried out for BiOCOOH and I-doped BiOCOOH (IHB-1.00) electrodes to evaluate the charge separation efficiency. Figure 7b shows that the steady and prompt photocurrent generation is obtained during on and off cycles of visible light illumination. The pure BiOCOOH shows certain photocurrent responses. The charge transport in this as-prepared sample proceeds quickly, making the changes of “on” and “off” currents nearly vertical. The photocurrent of the I-doped BiOCOOH electrode is much higher than that of the pure BiOCOOH electrode shown in Fig. 7b. The photocurrent enhancement of I-doped BiOCOOH can be ascribed to the enhanced photo-generated electrons/holes separation.
Photocatalytic activity and stability
NO with concentration at ppb level is one of the dominant air pollutants, which is very stable and cannot be photolyzed under light irradiation without photocatalysts. Figure 8a shows the variation of NO concentration (C/C0%) with irradiation time over the as-prepared samples under visible light irradiation. As previous reported, BiOCOOH nanosheets exhibit no visible light activity due to the large band gap. All the samples after I-doping exhibit decent visible light photocatalytic activity for NO removal. When the molar ratio of I− to BiOCOOH is increased to 1.00, the NO removal ratio is increased to 49.7%. Further increasing the molar ratio to 2.00 and 4.00, the NO removal ration is decreased to 45.3% for IHB-2 and 44.5% for IHB-4, which can be ascribed to decreased surface areas (Table 1). Note that the NO removal is gradually decreased due to the occupation of the surface with the final products. An ideal photocatalyst should maintain photochemical stability and durability under repeated irradiation so that it can be reused , , . Figure 8b shows the repeated photocatalytic activity of IHB-1.00 under visible light irradiation, which does not have obvious deactivation even after five recycles. As demonstrated in Figs. 6 and 7, I-doping could narrow the band gap and enhance the charge separaion of BiOCOOH, which in turn directly contribute to the highly promoted visible light photocatalytic activity.
A simple ion-exchange method was developed for synthesis of I-doped BiOCOOH nanosheets via the replacement of COOH− ions with I− ions. The concept of saturate-doping in layered photocatalyst was proposed. After I-doping, the light absorption spectra of BiOCOOH can be extended into the visible light region. The I-doped BiOCOOH exhibited highly enhanced visible-light photocatalytic removal of NO in air, which can be ascribed to the increased visible light absorption and the promoted charge separation. The concept of promoting visible-light photocatalysis through anion-exchange could also be extended to other layered semiconductor photocatalysts.
This research is financially supported by the National Natural Science Foundation of China (21501016, 51478070, 51108487 and 21777011), the Innovative Research Team of Chongqing (CXTDG201602014), the Natural Science Foundation of Chongqing (cstc2016jcyjA0481, cstc2015jcyjA0061, cstc2017jcyjBX0052), and the Chongqing Education Commission (KJ1500601, Kj1500637).
S. Tokunaga, H. Kato, A. Kudo. Chem. Mater. 13, 4626 (2001). Google Scholar
C. Zhang, Y. F. Zhu. Chem. Mater 13, 3537 (2005). Google Scholar
L. Q. Ye, J. Y. Liu, C. Q. Gong, L. H. Tian, T. Y. Peng, L. Zan. ACS Catal. 8, 1677 (2012). Google Scholar
L. Q. Ye, J. Y. Liu, Z. Jiang, T. Y. Peng, L. Zan. Appl. Catal. B: Environ. 1, 142 (2013). Google Scholar
Y. S. Xu, W. D. Zhang. Appl. Catal. B: Environ. 140, 306 (2013). Google Scholar
About the article
Published Online: 2017-11-14
Published in Print: 2018-02-23
Citation Information: Pure and Applied Chemistry, Volume 90, Issue 2, Pages 353–361, ISSN (Online) 1365-3075, ISSN (Print) 0033-4545, DOI: https://doi.org/10.1515/pac-2017-0509.http://creativecommons.org/licenses/by-nc-nd/4.0/.