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Room-temperature synthesis of BiOI with tailorable (0 0 1) facets and enhanced photocatalytic activity
Accepted Manuscript Room-temperature synthesis of BiOI with tailorable (001) facets and enhanced photocatalytic activity Rongan He, Jinfeng Zhang, Jiaguo Yu, Shaowen Cao PII: DOI: Reference:
S0021-9797(16)30371-X http://dx.doi.org/10.1016/j.jcis.2016.06.012 YJCIS 21322
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
16 April 2016 2 June 2016
Please cite this article as: R. He, J. Zhang, J. Yu, S. Cao, Room-temperature synthesis of BiOI with tailorable (001) facets and enhanced photocatalytic activity, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/ 10.1016/j.jcis.2016.06.012
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Room-temperature synthesis of BiOI with tailorable (001) facets and enhanced photocatalytic activity
Rongan Hea,b, Jinfeng Zhanga, Jiaguo Yua,c,*, Shaowen Caoa,*
a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,
Wuhan University of Technology, Wuhan 430070, PR China b
Hunan Province Key Laboratory of Applied Environmental Photocatalysis, Changsha
University, Changsha 410022, PR China c
Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589,
Saudi Arabia *Corresponding author. Fax: 0086-27-87879468; Tel.: 0086-27-87871029. E-mail: [email protected] (J. Yu); [email protected] (S. Cao)
1
Abstract:
The photocatalytic activity of bismuth oxyhalides largely depends on their
morphologies and microstructures. In this work, hierarchically structured bismuth oxyiodide (BiOI) with tunable ratios of (110) and (001) facets are fabricated through a facile route combining solid-state reaction with subsequent hydrolysis at room temperature. The hierarchical structures endow BiOI with excellent visible-light photocatalytic performance for phenol degradation. Besides, the optimal ratio of (001) and (110) surfaces also plays an important role in enhancing the photocatalytic activity of BiOI. DFT calculation demonstrates that a surface heterojunction formed between (001) and (110) surfaces can improve the separation of electrons and holes on different surfaces and thus enhance the photocatalytic activity. Keywords: bismuth oxyiodide; hierarchical structure; room-temperature synthesis; surface heterojunction; photocatalysis
2
1. Introduction Over the past decades, increasing attention has been aroused to the solution of the serious pollution and environmental crisis. One of the potential ways is photocatalysis, which is recognized as a kind of environmental friendly and effective technique for environment decontamination [1-6]. Semiconductors with a narrow band gap are considered as the promising visible-light photocatalysts for their excellent light absorption ability [7-9]. Among them, bismuth oxyhalide (BiOX, X = Cl, Br, I) is an important family. Bismuth oxyhalide is constructed by interleaved [B2O2]2+ slabs and [X2]2− slabs (X = Cl, Br, I), which lead to the formation of internal electric fields and consequently facilitate the separation of photo-generated electrons and holes [10-13]. As is well known, the photocatalytic activity of a semiconductor photocatalyst is largely dependent on its shape, size, surface area and exposed facets [4,14-19]. Generally, hierarchical structures can improve the mass transfer and light harvesting of photocatalysts; high specific surface area can increase the surface accessibility of the photocatalyst, and thus improve the utilization of photo-generated electrons and holes; appropriate exposed facets can increase the active sites on the particle surface; nano-sized particles enable shorter diffusion distance of electrons and holes as compared to their bulk counterparts. All these factors are beneficial for increasing the photocatalytic activity of the semiconductor photocatalyst. Therefore, a number of studies have been focused on the morphology and structure tuning of BiOX, to enhance their photocatalytic performance [10,20-25]. Especially, promoting the photocatalytic efficiency of BiOX by designing nanostructure with hierarchical structures and high 3
specific surface area has been well demonstrated [22,24,25]. Facet controlling as another structure tuning strategy has also played an important role in enhancing the photocatalytic performance of BiOX. Jiang et al. [26] found that BiOCl nanosheets with exposed (001) facets exhibited higher activity than those with exposed (010) facets under UV light, while the reverse trend occurred under visible light. Another study indicated that the exposed (001) facets on Bi3O4Cl could enhance the activity of Bi3O4Cl [27]. Wu et al. [28] also reported that BiOBr nanosheets with dominant (001) facets exhibited remarkably higher photocatalytic activity than those with dominant (010) facets for bacterial inactivation. However, some other investigations suggested that other facets instead of (001) facets of BiOX performed better. For example, Liu et al. [29] revealed that BiOBr with higher exposed (110) facets exhibited better performance for photocatalytic degradation of MO. Pan et al. [30] found that BiOI with more exposed (110) facets showed superior photocatalytic activity, as compared to those with more exposed (001) facets. In addition, when coupled with Ag quantum dots, the photocatalytic performance of BiOI was better in the case of less exposed (001) facets [31]. Recently, Sun et al. [32] reported the BiOI/BiOCl heterojunctions with interface of BiOCl(010) and BiOI(001) surfaces for enhanced photocatalytic activity. It was found that the synergistic action of BiOCl(010) and BiOI(001) surfaces could greatly improve the photocatalytic property, due to the faster migration of electrons from interface to top surface of BiOCl rather than the bottom surface. The above-mentioned studies indicate the effect of exposed facets is extremely significant for BiOX photocatalysts. However, the structure controlling of BiOX usually 4
needs hydrothermal or thermal treatment. Room-temperature synthesis of BiOX with tailorable structure is still a challenge. Moreover, surface heterojunction formed between different facets of a single semiconductor particle was found to strongly affect the activity of photocatalysts [33]. Hence in this work, we prepare BiOI with hierarchical structures, and tailorable ratios of (001) facets to (110) facets through a facile route combining solid-state reaction with subsequent hydrolysis. The effects of resultant surface areas and exposed facets on the photocatalytic activities of BiOI for phenol degradation were systematically investigated. Note that the room-temperature solid-state method used here is considered as a promising alternative to those widely used solution-phase reactions, hydrothermal method and calcination method for the fabrication of nanomaterials, due to its simplicity, low cost, high yield and low environmental impact [34,35].
2. Experimental section 2.1 Preparation of bismuth oxyiodides All the regents in this paper, such as Bi(NO3)3∙5H2O (Guangdong Chemical Reagent Engineering-technological Research and Development Center), KI (Guangdong Guanghua chemical factory Co., Ltd.), ethanol (Sinopharm chemical regent Co., Ltd.) and phenol (Tianjin Guangcheng Chemical Reagent Co., Ltd.), were analytical pure and were directly used as received without further purification. The sample preparation includes grinding of solid reactant mixture [36] and 5
subsequent hydrolysis. Typically, 0.97 g (2 mmol) of Bi(NO3)3·5H2O and 0.34 g (2 mmol) KI were mixed and ground for 30 min at room temperature, till the formation of the black paste. Then 1, 5, 80 and 400 mL distilled water were well mixed with the black paste respectively by grinding or stirring. In the case of 1 and 5 mL water, the mixture was ground for 30 min and kept standing for 5 h. In the case of 80 and 400 mL water, the mixture was stirred for 5 h. The resultant products were washed with distilled water and ethanol and dried at 80 ºC. The corresponding samples were marked as BiOI-1, BiOI-5, BiOI-80 and BiOI-400, respectively.
2.2 Characterization The X-ray diffraction (XRD) patterns of the prepared samples were obtained on a Rigaku Ultima III X-ray Diffractometer (Japan) using Cu-Kα radiation. The average crystallite sizes could be calculated according to Scherrer equation. Morphology analysis was carried on a JSM-7500F field emission scanning electron microscope (JEOL, Japan). The transmission electron microscope (TEM) images of the prepared samples were obtained on a Tecnai G20 U-Twin electron microscope (FEI, USA). The nitrogen adsorption and desorption isotherms at -196
o
C were measured on a
Micromeritics ASAP 2020 adsorption analyzer, before which the samples were degassed at 150 oC for 3 h. The specific surface areas were determined according to Brunauer–Emmett–Teller (BET) model, and the pore size distributions were fitted based on Barret–Joyner–Halender (BJH) model [37]. The UV-visible diffuse reflectance spectra (DRS) of the
prepared samples 6
were recorded on a
UV-visible
spectrophotometer (UV-2600, Shimadzu, Japan).
2.3 Photocatalytic degradation Photocatalytic degradation of phenol (100 mg/L) aqueous solution under visible-light irradiation over BiOI samples were performed at ambient temperature. Briefly, 100 mg BiOI samples were coated on the bottom of a Petri dish (8 cm in diameter), using ethanol as the dispersant, which was evaporated subsequently at 80 oC. Then 20 mL phenol aqueous solution was added. The Petri dish was illuminated under a xenon lamp (300 W, Changzhou Siyu Technic Co., China) with a 420 nm cut-off filter. At given time intervals, 3 mL phenol solution was extracted, and the corresponding UV-vis spectra were measured on a UV-vis spectrophotometer (UVmini-1240, Shimadzu, Japan). The concentration of phenol was monitored utilizing the maximum absorbance at 269 nm.
2.4 Computational analysis. The density of states (DOS) plots of (001) and (110) surfaces for tetragonal-phase BiOI were calculated by CASTEP code through the plane-wave pseudopotential method [38].
And
the
generalized
gradient
approximation
(GGA)
with
the
Perdew–Burke–Ernzerhof (PBE) form was adopted as the exchange–correlation function [39,40]. The interaction between the ionic core and valence electrons was described by ultrasoft pseudopotential. The lattice parameters and atomic coordinates were relaxed through the cutoff of 380 eV and Monkhorst-pack grids of 6 × 6 × 1 7
k-points for (001) surface and 3 × 4 × 1 for (110) surface. In addition, the interaction between the adjacent (001) and (110) surfaces was eliminated by employing a model vacuum slab of 1.5 nm thick.
3. Results and discussion 3.1 Crystal structure The XRD patterns of the as-synthesized samples are shown in Fig. 1. The diffraction peaks of all samples can be assigned to the tetragonal phase of BiOI (JCPDS No. 10-0445). It is noteworthy that there is obvious difference in peak intensities at 9.7 and 31.6º, which correspond to (001) and (110) facet, respectively. Following the sequence from BiOI-1 to BiOI-400, the decrease of (001) peak and increase of (110) facet can be easily observed. The intensity ratio I(001)/I(110) increases gradually in the order of BiOI-400, BiOI-80, BiOI-5 and BiOI-1, indicating that the crystals became more and more preferable to grow along the (001) facet which is perpendicular to the nanoplates [31]. For in-depth understanding of the formation of BiOI, the XRD pattern of the black solid mixture was analyzed, as shown in Fig. 2. Most of diffraction peaks of the solid mixture can be indexed to BiI3 (JCPDS No. 76-1742), indicating that BiI3 is the main product during the grinding procedure. The peaks at 11.2 and 22.8º might belong to some bismuth basic nitrates in the black solid mixture [41,42]. The result reveals that BiOI is formed via the hydrolysis of BiI3 after the addition of H2O, in terms of the following reaction [43]. 8
Bi3+ + I¯ + H2O → BiOI +2H+
(1)
(102) (001) (200) (212)
Intensity (A. U.)
(110)
BiOI-1
BiOI-5
BiOI-80 BiOI-400 PDF#10-0445
20
40
60
80
2 (Degreees) Fig. 1. XRD patterns of BiOI-1, BiOI-5, BiOI-80 and BiOI-400.
Intensity (A. U.)
*
* BiI3
*
*
*
*
**
Black mixture
* * **
**
PDF#76-1742
20
40
60
2 (Degreees) Fig. 2. XRD patterns of the black solid mixture.
9
80
3.2 Morphology analysis The SEM images of BiOI-1, BiOI-5, BiOI-80 and BiOI-400 are displayed in Fig. 3. It can be seen that BiOI-1 and BiOI-5 are composed of individual plates. And the plates of BiOI-5 are thinner than that of BiOI-1. Interestingly, BiOI-80 and BiOI-400 are consisting of hierarchical microspheres assembled by nanoplates which are much thinner than those of BiOI-1 and BiOI-5. The microspheres of BiOI-400 are smaller and more uniform than BiOI-80, while the intercross patterns of BiOI-80 are more complicated and close-packed, indicating a more remarkable hierarchical structure as compared to that of BiOI-400.
Fig. 3. Morphologies of BiOI-1, BiOI-5, BiOI-80 and BiOI-400.
The TEM and HRTEM images of BiOI-80 were shown in Fig. 4. Fig. 4b and d 10
displayed the HRTEM images viewed from the directions perpendicular (marked area in Fig. 4a) and parallel (marked area in Fig. 4c) to BiOI nanoplates, respectively, in which clearly resolved crystalline domains could be observed. The uniform lattice distance of fringes in Fig. 4b and d are 0.29 and 0.93 nm, corresponding to the (110) and (001) facets, respectively [44,45]. Combining the TEM results with the SEM observation, it can be concluded that the frontal and lateral surfaces of BiOI nanoplates are exposed with (001) and (110) facets, respectively [44-46].
Fig. 4. TEM (a, c), HRTEM images of frontal surface (b), lateral surface (d) of BiOI-80 11
nanoplates. Obviously, the growth direction of BiOI was largely affected by the amount of introduced water. It has been demonstrated that strong acidic environment would result in similar growth rate in the directions vertical and parallel to (001) facets of BiOCl in hydrolysis synthesis, while the growth parallel to (001) facets became much faster in weak acidic environment [47]. This is consistent with our observation. As deduced from equation (1), more amount of water would lead to lower concentration of H +, and thus weaker acidic environment. This would promote the growth parallel to (001) facets. Thus, BiOI-80 and BiOI-400 prepared with more water possess thinner plates, i.e., more exposed (001) facets. We have simply investigated the hydrophilic property of two typical BiOI samples, BiOI-5 with thick plates and BiOI-80 with thin plates, by dropping the same amount of water or HNO3 solution (1 M) on the FTO glass-supported BiOI samples. The digital photos in Fig. 5 show that the hydrophilicity of BiOI-80 is just slightly better than that of BiOI-5 in the presence of pure water. However, in the presence of HNO3, the hydrophilicity of BiOI-80 is much better than that of BiOI-5. Since BiOI-80 have more exposed (001) facets than BiOI-5, the above-mentioned results imply that the hydrophilicity of (001) surface is superior in comparison with that of (110) surface in acidic environment. In this regard, the hydrolysis process with less water may not benefit the growth along the direction parallel to the nanoplates for more exposed (001) facets, since the excellent hydrophilicity of (001) surface in strong acidic condition can allow the adsorption of more dissolved Bi3+ and I– ions for the growth along the 12
direction perpendicular to the nanoplates (Scheme 1). Therefore, BiOI nucleuses prefer to grow along the direction parallel to the nanoplates in a condition with higher pH value, which assures the exposure of more (001) facets (Scheme 1).
Fig. 5. Digital photos of one-drop specific liquid on BiOI-5 and BiOI-80.
Scheme 1. Mechanism of BiOI nanoplates growing at different pH.
3.3 BET specific surface area and pore structure The N2 adsorption-desorption isotherms of as-prepared BiOI samples are shown in Fig. 6, from which the BET specific surface areas of BiOI-1, BiOI-5, BiOI-80 and 13
BiOI-400 are calculated to be 3.1, 4.5, 21.4 and 13.8 m2/g, respectively. The pore size distribution curves ranging in 2-100 nm (inset of Fig. 6) show that BiOI-80 and BiOI-400 possess mesoporous structures, while BiOI-1 and BiOI-5 have nearly non-porous structures, which are in consistent with the SEM observation. The total pore volume of BiOI-80 and BiOI-400 is 0.129 and 0.086 cm3/g, and the average pore size of BiOI-80 and BiOI-400 is 23.7 and 24.8 nm, respectively. Since higher surface energy is expected for the nanoplates of BiOI-80 and BiOI-400 due to their much smaller thickness as compared to that of BiOI-1 and BiOI-5, these thinner nanoplates tend to aggregate and assemble together to reduce the surface energy [24], resulting in the formation of hierarchical porous structures of BiOI-80 and BiOI-400. In addition, BiOI-80 possesses more pores than BiOI-400, which could be attributed to its more
4 BiOI-1 BiOI-5 BiOI-80 BiOI-400
0.010
3
dV(d) (cm /g/nm)
Quantity Adsorbed (mmol/g)
condensed intercross patterns, as seen in Fig. 3.
3
2
0.005
0.000 10
100
Pore size (nm)
1
0 0.2
0.4
0.6
0.8
1.0
P/P0 Fig. 6. N2 adsorption-desorption isotherms of BiOI-1, BiOI-5, BiOI-80 and BiOI-400.
3.4 Optical property 14
Fig. 7 shows the UV–vis DRS of BiOI samples. The absorption edges of BiOI-1, BiOI-5, BiOI-80 and BiOI-400 are 663, 660, 656 and 650 nm, respectively. Accordingly, the estimated band gaps (Eg) of BiOI-1, BiOI-5, BiOI-80 and BiOI-400 are 1.87, 1.88, 1.89 and 1.91 eV, respectively. Note that the BiOI samples exhibit increased visible-light absorption in the range of 400-600 nm in order of BiOI-80 > BiOI-400 > BiOI-5 > BiOI-1. Obviously, the hierarchical structures of BiOI-80 and BiOI-400 endow them with improved light harvesting by increasing the light traveling paths and the interaction time [19]. Moreover, the more pores in BiOI-80 result in more light traveling paths and higher absorption efficiency, and thus stronger light absorption ability as compared to BiOI-400. Consequently, stronger visible-light absorption is beneficial for the generation of more photo-induced electrons and holes and thus better photocatalytic activity. BiOI-80
Intensity (A. U.)
1.5
1.0
BiOI-400 BiOI-5 BiOI-1
0.5
0.0 300
400
500
600
700
800
Wavelength (nm)
Fig. 7. UV–vis diffuse reflectance spectra of BiOI-1, BiOI-5, BiOI-80 and BiOI-400.
3.5 Photocatalytic activity The photocatalytic degradation of phenol (100 mg/L) over the as-prepared BiOI 15
samples was evaluated under visible light (λ > 420 nm). The adsorption of phenol in dark in a time period of 3 h was found to be negligible. Also, self-photodegradation of phenol was not observed in the absence of any photocatalyst. Among the BiOI samples, BiOI-80 exhibited the highest photocatalytic activity, while BiOI-1 showed the lowest performance, as shown in Fig 8a. The temporal evolution of the UV-vis spectra of phenol solution over BiOI-80 was presented in Fig. 8b, which illustrates the quick decrease of the maximum absorption at 269 nm, corresponding to the fast photocatalytic degradation of phenol. This is mainly attributed to the hierarchical structures of BiOI-80, providing the strongest light absorption ability and highest specific surface area. However, the performance of BiOI-5 was a little better than that of BiOI-400, suggesting that some other factors also partially affect the photocatalytic activity, as the specific surface area of BiOI-5 is smaller than that of BiOI-400. This will be discussed in the following paragraphs.
2.0
a
b
1.0
Intensity (A. U.)
no BiOI
C/C 0
0.8 BiOI-1
0.6
BiOI-400
0.4 BiOI-5
0 min 20 min 40 min 60 min 90 min 120 min 150 min 210 min
1.5 1.0 0.5
0.2 0.0
BiOI-80
0.0 0
50
100
150
200
250
240
260
280
300
320
Wavelength (nm)
t (min)
Fig. 8. (a) Degradation curves of phenol on BiOI-1, BiOI-5, BiOI-80 and BiOI-400 under visible-light (λ > 420 nm) irradiation, (b) Evolution of UV-vis spectra of phenol aqueous solution over BiOI-80.
16
3.6 Photocatalytic mechanism To investigate the reactive species involved in the photo-degradation process of phenol, a series of active specie trapping experiments were carried out over the BiOI samples, employing 1 mM p-benzoquinone (BQ), 10 mM triethanolamine (TEOA) and 10 mM isopropanol (IPA) as the scavengers for ·O2¯, holes (h+) and ·OH, respectively [48,49]. The effects of these scavengers on photocatalytic degradation of phenol under visible light (λ > 420 nm) are shown in Fig. 9.、
1.0
(C 0-C)/C 0
0.8 0.6
No scavenger IPA BQ TEOA
0.4 0.2 0.0 BiOI-1
BiOI-5
BiOI-80
BiOI-400
Fig. 9. Active specie trapping experiments for BiOI-1, BiOI-5, BiOI-80 and BiOI-400 under visible-light irradiation (λ > 420 nm) for 120 min.
It can be found that the addition of TEOA remarkably suppressed the photocatalytic degradation of phenol, indicating that photogenerated h+ acts as the main active specie during the reaction. When IPA was added, only slight decrease in the photocatalytic efficiency was observed, suggesting that the contribution of ·OH to the photocatalytic reaction is negligible. This is because the VB potential of BiOI is not 17
positive enough to oxidize H2O to ·OH [50]. Another active specie is ·O2¯, as the photocatalytic efficiency also partially decreased in the presence of BQ. Note that the CB of BiOI is not negative enough for the reduction of O 2, hence the generation of ·O2¯ should be attributed to some electrons excited to a higher energy level (negative enough for the reduction of O2) [51,52]. As previously mentioned, in addition to the effect of hierarchical structure and surface area, some other factors play important roles in determining the photocatalytic activity of BiOI samples. Here we propose that the synergistic action of (001) and (110) surfaces could improve the photocatalytic performance of BiOI samples. To prove this assumption, the DOS of exposed (001) and (110) facets were calculated. As shown in Fig. 10, the energy levels of both CB and VB for exposed (110) facets are more positive than that for (001) facets, estimated on the basis of an equal Fermi level of the adjacent (001) and (110) facets. The slight difference in energy levels of the CB and VB between (001) and (110) facets can lead to the formation of a surface heterojunction [33]. Such surface heterojunction could improve the separation of photogenerated electrons and holes onto different surfaces. As shown in scheme 2a, electrons tend to migrate from CB of (001) surface to CB of (110) surface and holes will migrate from VB of (110) surface to VB of (001) surface, resulting in the spatial accumulation of electrons and holes on the (110) and (001) surfaces, respectively. Thus the recombination opportunity of electrons and holes is greatly reduced. It is noteworthy that there is an optimal ratio of (001) and (110) surfaces for the highest photocatalytic activity. As shown in Scheme 2b, the ratio of exposed (001) surfaces in thin nanoplates is higher than that in thick 18
nanoplates. However, excess exposure of (001) surface will cause an overflow effect of photogenerated electrons, due to the insufficient exposure of (110) surface [33]. Such effect will lead to the recombination of charge carriers on the (001) surface. In this regard, the ratio of (110) facet of BiOI-400 might be so low that the overflow effect occurred, thus the surface heterojunction of BiOI-400 does not act as effective as that of BiOI-5. Thus, BiOI-5 exhibited a little better performance than BiOI-400. While the ratio of (110) facet of BiOI-80 is higher than that of BiOI-400, forming a more effective surface heterojunction. Hence both hierarchical structure and surface heterojunction promote the photocatalytic reaction over BiOI-80, resulting in the best photocatalytic
DOS (electrons / eV)
activity. (001)
6 4 2 0
(110)
10 5 0 -6
-4
-2
0
2
4
6
Energy (eV)
Fig. 10. Density of states (DOS) plots for (100) and (110) surface of tetragonal BiOI.
19
Scheme 2. (a) Illustration of charge transfer across the surface heterojunction of BiOI(001) and BiOI(110) factets, (b) changes of exposed ratio and contact area of (001) and (110) surfaces.
4. Conclusion BiOI hierarchical structures with tailorable ratio of (001) and (110) surfaces are successfully fabricated via a facile room-temperature approach, combining solid-state reaction with subsequent hydrolysis, by tuning the amount of introduced water. The BiOI hierarchical structures with optimal ratio of (001) and (110) surfaces exhibited the best visible-light photocatalytic activity for phenol degradation; because (1) hierarchical structures allow for strongest light absorption ability and highest specific surface; (2) the surface heterojunction formed between (001) and (110) facets improve the separation of electrons and holes across the interface of the two different facets.
Acknowledgments 20
This work was supported by the 973 program (2013CB632402), NSFC (51472191, 21407115, 51272199, 51320105001 and 21433007), Natural Science Foundation of Hubei Province of China (2014CFB164, 2015CFA001), the Fundamental Research Funds for the Central Universities (WUT: 2015-III-034), Self-determined and Innovative Research Funds of SKLWUT (2015-ZD-1) and the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province.
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27
Graphical Abstract
Highlights 1. Hierarchical porous structured BiOI with tailorable (001) facets was fabricated. 2. Hierarchical structure enhanced visible light absorption of BiOI. 3. Surface heterojunction of (001) and (110) facets reduce the e-h recombination. 4. BiOI exhibits excellent photocatalytic activity for phenol degradation.
28
S0021-9797(16)30371-X http://dx.doi.org/10.1016/j.jcis.2016.06.012 YJCIS 21322
To appear in:
Journal of Colloid and Interface Science
Received Date: Accepted Date:
16 April 2016 2 June 2016
Please cite this article as: R. He, J. Zhang, J. Yu, S. Cao, Room-temperature synthesis of BiOI with tailorable (001) facets and enhanced photocatalytic activity, Journal of Colloid and Interface Science (2016), doi: http://dx.doi.org/ 10.1016/j.jcis.2016.06.012
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Room-temperature synthesis of BiOI with tailorable (001) facets and enhanced photocatalytic activity
Rongan Hea,b, Jinfeng Zhanga, Jiaguo Yua,c,*, Shaowen Caoa,*
a
State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,
Wuhan University of Technology, Wuhan 430070, PR China b
Hunan Province Key Laboratory of Applied Environmental Photocatalysis, Changsha
University, Changsha 410022, PR China c
Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589,
Saudi Arabia *Corresponding author. Fax: 0086-27-87879468; Tel.: 0086-27-87871029. E-mail: [email protected] (J. Yu); [email protected] (S. Cao)
1
Abstract:
The photocatalytic activity of bismuth oxyhalides largely depends on their
morphologies and microstructures. In this work, hierarchically structured bismuth oxyiodide (BiOI) with tunable ratios of (110) and (001) facets are fabricated through a facile route combining solid-state reaction with subsequent hydrolysis at room temperature. The hierarchical structures endow BiOI with excellent visible-light photocatalytic performance for phenol degradation. Besides, the optimal ratio of (001) and (110) surfaces also plays an important role in enhancing the photocatalytic activity of BiOI. DFT calculation demonstrates that a surface heterojunction formed between (001) and (110) surfaces can improve the separation of electrons and holes on different surfaces and thus enhance the photocatalytic activity. Keywords: bismuth oxyiodide; hierarchical structure; room-temperature synthesis; surface heterojunction; photocatalysis
2
1. Introduction Over the past decades, increasing attention has been aroused to the solution of the serious pollution and environmental crisis. One of the potential ways is photocatalysis, which is recognized as a kind of environmental friendly and effective technique for environment decontamination [1-6]. Semiconductors with a narrow band gap are considered as the promising visible-light photocatalysts for their excellent light absorption ability [7-9]. Among them, bismuth oxyhalide (BiOX, X = Cl, Br, I) is an important family. Bismuth oxyhalide is constructed by interleaved [B2O2]2+ slabs and [X2]2− slabs (X = Cl, Br, I), which lead to the formation of internal electric fields and consequently facilitate the separation of photo-generated electrons and holes [10-13]. As is well known, the photocatalytic activity of a semiconductor photocatalyst is largely dependent on its shape, size, surface area and exposed facets [4,14-19]. Generally, hierarchical structures can improve the mass transfer and light harvesting of photocatalysts; high specific surface area can increase the surface accessibility of the photocatalyst, and thus improve the utilization of photo-generated electrons and holes; appropriate exposed facets can increase the active sites on the particle surface; nano-sized particles enable shorter diffusion distance of electrons and holes as compared to their bulk counterparts. All these factors are beneficial for increasing the photocatalytic activity of the semiconductor photocatalyst. Therefore, a number of studies have been focused on the morphology and structure tuning of BiOX, to enhance their photocatalytic performance [10,20-25]. Especially, promoting the photocatalytic efficiency of BiOX by designing nanostructure with hierarchical structures and high 3
specific surface area has been well demonstrated [22,24,25]. Facet controlling as another structure tuning strategy has also played an important role in enhancing the photocatalytic performance of BiOX. Jiang et al. [26] found that BiOCl nanosheets with exposed (001) facets exhibited higher activity than those with exposed (010) facets under UV light, while the reverse trend occurred under visible light. Another study indicated that the exposed (001) facets on Bi3O4Cl could enhance the activity of Bi3O4Cl [27]. Wu et al. [28] also reported that BiOBr nanosheets with dominant (001) facets exhibited remarkably higher photocatalytic activity than those with dominant (010) facets for bacterial inactivation. However, some other investigations suggested that other facets instead of (001) facets of BiOX performed better. For example, Liu et al. [29] revealed that BiOBr with higher exposed (110) facets exhibited better performance for photocatalytic degradation of MO. Pan et al. [30] found that BiOI with more exposed (110) facets showed superior photocatalytic activity, as compared to those with more exposed (001) facets. In addition, when coupled with Ag quantum dots, the photocatalytic performance of BiOI was better in the case of less exposed (001) facets [31]. Recently, Sun et al. [32] reported the BiOI/BiOCl heterojunctions with interface of BiOCl(010) and BiOI(001) surfaces for enhanced photocatalytic activity. It was found that the synergistic action of BiOCl(010) and BiOI(001) surfaces could greatly improve the photocatalytic property, due to the faster migration of electrons from interface to top surface of BiOCl rather than the bottom surface. The above-mentioned studies indicate the effect of exposed facets is extremely significant for BiOX photocatalysts. However, the structure controlling of BiOX usually 4
needs hydrothermal or thermal treatment. Room-temperature synthesis of BiOX with tailorable structure is still a challenge. Moreover, surface heterojunction formed between different facets of a single semiconductor particle was found to strongly affect the activity of photocatalysts [33]. Hence in this work, we prepare BiOI with hierarchical structures, and tailorable ratios of (001) facets to (110) facets through a facile route combining solid-state reaction with subsequent hydrolysis. The effects of resultant surface areas and exposed facets on the photocatalytic activities of BiOI for phenol degradation were systematically investigated. Note that the room-temperature solid-state method used here is considered as a promising alternative to those widely used solution-phase reactions, hydrothermal method and calcination method for the fabrication of nanomaterials, due to its simplicity, low cost, high yield and low environmental impact [34,35].
2. Experimental section 2.1 Preparation of bismuth oxyiodides All the regents in this paper, such as Bi(NO3)3∙5H2O (Guangdong Chemical Reagent Engineering-technological Research and Development Center), KI (Guangdong Guanghua chemical factory Co., Ltd.), ethanol (Sinopharm chemical regent Co., Ltd.) and phenol (Tianjin Guangcheng Chemical Reagent Co., Ltd.), were analytical pure and were directly used as received without further purification. The sample preparation includes grinding of solid reactant mixture [36] and 5
subsequent hydrolysis. Typically, 0.97 g (2 mmol) of Bi(NO3)3·5H2O and 0.34 g (2 mmol) KI were mixed and ground for 30 min at room temperature, till the formation of the black paste. Then 1, 5, 80 and 400 mL distilled water were well mixed with the black paste respectively by grinding or stirring. In the case of 1 and 5 mL water, the mixture was ground for 30 min and kept standing for 5 h. In the case of 80 and 400 mL water, the mixture was stirred for 5 h. The resultant products were washed with distilled water and ethanol and dried at 80 ºC. The corresponding samples were marked as BiOI-1, BiOI-5, BiOI-80 and BiOI-400, respectively.
2.2 Characterization The X-ray diffraction (XRD) patterns of the prepared samples were obtained on a Rigaku Ultima III X-ray Diffractometer (Japan) using Cu-Kα radiation. The average crystallite sizes could be calculated according to Scherrer equation. Morphology analysis was carried on a JSM-7500F field emission scanning electron microscope (JEOL, Japan). The transmission electron microscope (TEM) images of the prepared samples were obtained on a Tecnai G20 U-Twin electron microscope (FEI, USA). The nitrogen adsorption and desorption isotherms at -196
o
C were measured on a
Micromeritics ASAP 2020 adsorption analyzer, before which the samples were degassed at 150 oC for 3 h. The specific surface areas were determined according to Brunauer–Emmett–Teller (BET) model, and the pore size distributions were fitted based on Barret–Joyner–Halender (BJH) model [37]. The UV-visible diffuse reflectance spectra (DRS) of the
prepared samples 6
were recorded on a
UV-visible
spectrophotometer (UV-2600, Shimadzu, Japan).
2.3 Photocatalytic degradation Photocatalytic degradation of phenol (100 mg/L) aqueous solution under visible-light irradiation over BiOI samples were performed at ambient temperature. Briefly, 100 mg BiOI samples were coated on the bottom of a Petri dish (8 cm in diameter), using ethanol as the dispersant, which was evaporated subsequently at 80 oC. Then 20 mL phenol aqueous solution was added. The Petri dish was illuminated under a xenon lamp (300 W, Changzhou Siyu Technic Co., China) with a 420 nm cut-off filter. At given time intervals, 3 mL phenol solution was extracted, and the corresponding UV-vis spectra were measured on a UV-vis spectrophotometer (UVmini-1240, Shimadzu, Japan). The concentration of phenol was monitored utilizing the maximum absorbance at 269 nm.
2.4 Computational analysis. The density of states (DOS) plots of (001) and (110) surfaces for tetragonal-phase BiOI were calculated by CASTEP code through the plane-wave pseudopotential method [38].
And
the
generalized
gradient
approximation
(GGA)
with
the
Perdew–Burke–Ernzerhof (PBE) form was adopted as the exchange–correlation function [39,40]. The interaction between the ionic core and valence electrons was described by ultrasoft pseudopotential. The lattice parameters and atomic coordinates were relaxed through the cutoff of 380 eV and Monkhorst-pack grids of 6 × 6 × 1 7
k-points for (001) surface and 3 × 4 × 1 for (110) surface. In addition, the interaction between the adjacent (001) and (110) surfaces was eliminated by employing a model vacuum slab of 1.5 nm thick.
3. Results and discussion 3.1 Crystal structure The XRD patterns of the as-synthesized samples are shown in Fig. 1. The diffraction peaks of all samples can be assigned to the tetragonal phase of BiOI (JCPDS No. 10-0445). It is noteworthy that there is obvious difference in peak intensities at 9.7 and 31.6º, which correspond to (001) and (110) facet, respectively. Following the sequence from BiOI-1 to BiOI-400, the decrease of (001) peak and increase of (110) facet can be easily observed. The intensity ratio I(001)/I(110) increases gradually in the order of BiOI-400, BiOI-80, BiOI-5 and BiOI-1, indicating that the crystals became more and more preferable to grow along the (001) facet which is perpendicular to the nanoplates [31]. For in-depth understanding of the formation of BiOI, the XRD pattern of the black solid mixture was analyzed, as shown in Fig. 2. Most of diffraction peaks of the solid mixture can be indexed to BiI3 (JCPDS No. 76-1742), indicating that BiI3 is the main product during the grinding procedure. The peaks at 11.2 and 22.8º might belong to some bismuth basic nitrates in the black solid mixture [41,42]. The result reveals that BiOI is formed via the hydrolysis of BiI3 after the addition of H2O, in terms of the following reaction [43]. 8
Bi3+ + I¯ + H2O → BiOI +2H+
(1)
(102) (001) (200) (212)
Intensity (A. U.)
(110)
BiOI-1
BiOI-5
BiOI-80 BiOI-400 PDF#10-0445
20
40
60
80
2 (Degreees) Fig. 1. XRD patterns of BiOI-1, BiOI-5, BiOI-80 and BiOI-400.
Intensity (A. U.)
*
* BiI3
*
*
*
*
**
Black mixture
* * **
**
PDF#76-1742
20
40
60
2 (Degreees) Fig. 2. XRD patterns of the black solid mixture.
9
80
3.2 Morphology analysis The SEM images of BiOI-1, BiOI-5, BiOI-80 and BiOI-400 are displayed in Fig. 3. It can be seen that BiOI-1 and BiOI-5 are composed of individual plates. And the plates of BiOI-5 are thinner than that of BiOI-1. Interestingly, BiOI-80 and BiOI-400 are consisting of hierarchical microspheres assembled by nanoplates which are much thinner than those of BiOI-1 and BiOI-5. The microspheres of BiOI-400 are smaller and more uniform than BiOI-80, while the intercross patterns of BiOI-80 are more complicated and close-packed, indicating a more remarkable hierarchical structure as compared to that of BiOI-400.
Fig. 3. Morphologies of BiOI-1, BiOI-5, BiOI-80 and BiOI-400.
The TEM and HRTEM images of BiOI-80 were shown in Fig. 4. Fig. 4b and d 10
displayed the HRTEM images viewed from the directions perpendicular (marked area in Fig. 4a) and parallel (marked area in Fig. 4c) to BiOI nanoplates, respectively, in which clearly resolved crystalline domains could be observed. The uniform lattice distance of fringes in Fig. 4b and d are 0.29 and 0.93 nm, corresponding to the (110) and (001) facets, respectively [44,45]. Combining the TEM results with the SEM observation, it can be concluded that the frontal and lateral surfaces of BiOI nanoplates are exposed with (001) and (110) facets, respectively [44-46].
Fig. 4. TEM (a, c), HRTEM images of frontal surface (b), lateral surface (d) of BiOI-80 11
nanoplates. Obviously, the growth direction of BiOI was largely affected by the amount of introduced water. It has been demonstrated that strong acidic environment would result in similar growth rate in the directions vertical and parallel to (001) facets of BiOCl in hydrolysis synthesis, while the growth parallel to (001) facets became much faster in weak acidic environment [47]. This is consistent with our observation. As deduced from equation (1), more amount of water would lead to lower concentration of H +, and thus weaker acidic environment. This would promote the growth parallel to (001) facets. Thus, BiOI-80 and BiOI-400 prepared with more water possess thinner plates, i.e., more exposed (001) facets. We have simply investigated the hydrophilic property of two typical BiOI samples, BiOI-5 with thick plates and BiOI-80 with thin plates, by dropping the same amount of water or HNO3 solution (1 M) on the FTO glass-supported BiOI samples. The digital photos in Fig. 5 show that the hydrophilicity of BiOI-80 is just slightly better than that of BiOI-5 in the presence of pure water. However, in the presence of HNO3, the hydrophilicity of BiOI-80 is much better than that of BiOI-5. Since BiOI-80 have more exposed (001) facets than BiOI-5, the above-mentioned results imply that the hydrophilicity of (001) surface is superior in comparison with that of (110) surface in acidic environment. In this regard, the hydrolysis process with less water may not benefit the growth along the direction parallel to the nanoplates for more exposed (001) facets, since the excellent hydrophilicity of (001) surface in strong acidic condition can allow the adsorption of more dissolved Bi3+ and I– ions for the growth along the 12
direction perpendicular to the nanoplates (Scheme 1). Therefore, BiOI nucleuses prefer to grow along the direction parallel to the nanoplates in a condition with higher pH value, which assures the exposure of more (001) facets (Scheme 1).
Fig. 5. Digital photos of one-drop specific liquid on BiOI-5 and BiOI-80.
Scheme 1. Mechanism of BiOI nanoplates growing at different pH.
3.3 BET specific surface area and pore structure The N2 adsorption-desorption isotherms of as-prepared BiOI samples are shown in Fig. 6, from which the BET specific surface areas of BiOI-1, BiOI-5, BiOI-80 and 13
BiOI-400 are calculated to be 3.1, 4.5, 21.4 and 13.8 m2/g, respectively. The pore size distribution curves ranging in 2-100 nm (inset of Fig. 6) show that BiOI-80 and BiOI-400 possess mesoporous structures, while BiOI-1 and BiOI-5 have nearly non-porous structures, which are in consistent with the SEM observation. The total pore volume of BiOI-80 and BiOI-400 is 0.129 and 0.086 cm3/g, and the average pore size of BiOI-80 and BiOI-400 is 23.7 and 24.8 nm, respectively. Since higher surface energy is expected for the nanoplates of BiOI-80 and BiOI-400 due to their much smaller thickness as compared to that of BiOI-1 and BiOI-5, these thinner nanoplates tend to aggregate and assemble together to reduce the surface energy [24], resulting in the formation of hierarchical porous structures of BiOI-80 and BiOI-400. In addition, BiOI-80 possesses more pores than BiOI-400, which could be attributed to its more
4 BiOI-1 BiOI-5 BiOI-80 BiOI-400
0.010
3
dV(d) (cm /g/nm)
Quantity Adsorbed (mmol/g)
condensed intercross patterns, as seen in Fig. 3.
3
2
0.005
0.000 10
100
Pore size (nm)
1
0 0.2
0.4
0.6
0.8
1.0
P/P0 Fig. 6. N2 adsorption-desorption isotherms of BiOI-1, BiOI-5, BiOI-80 and BiOI-400.
3.4 Optical property 14
Fig. 7 shows the UV–vis DRS of BiOI samples. The absorption edges of BiOI-1, BiOI-5, BiOI-80 and BiOI-400 are 663, 660, 656 and 650 nm, respectively. Accordingly, the estimated band gaps (Eg) of BiOI-1, BiOI-5, BiOI-80 and BiOI-400 are 1.87, 1.88, 1.89 and 1.91 eV, respectively. Note that the BiOI samples exhibit increased visible-light absorption in the range of 400-600 nm in order of BiOI-80 > BiOI-400 > BiOI-5 > BiOI-1. Obviously, the hierarchical structures of BiOI-80 and BiOI-400 endow them with improved light harvesting by increasing the light traveling paths and the interaction time [19]. Moreover, the more pores in BiOI-80 result in more light traveling paths and higher absorption efficiency, and thus stronger light absorption ability as compared to BiOI-400. Consequently, stronger visible-light absorption is beneficial for the generation of more photo-induced electrons and holes and thus better photocatalytic activity. BiOI-80
Intensity (A. U.)
1.5
1.0
BiOI-400 BiOI-5 BiOI-1
0.5
0.0 300
400
500
600
700
800
Wavelength (nm)
Fig. 7. UV–vis diffuse reflectance spectra of BiOI-1, BiOI-5, BiOI-80 and BiOI-400.
3.5 Photocatalytic activity The photocatalytic degradation of phenol (100 mg/L) over the as-prepared BiOI 15
samples was evaluated under visible light (λ > 420 nm). The adsorption of phenol in dark in a time period of 3 h was found to be negligible. Also, self-photodegradation of phenol was not observed in the absence of any photocatalyst. Among the BiOI samples, BiOI-80 exhibited the highest photocatalytic activity, while BiOI-1 showed the lowest performance, as shown in Fig 8a. The temporal evolution of the UV-vis spectra of phenol solution over BiOI-80 was presented in Fig. 8b, which illustrates the quick decrease of the maximum absorption at 269 nm, corresponding to the fast photocatalytic degradation of phenol. This is mainly attributed to the hierarchical structures of BiOI-80, providing the strongest light absorption ability and highest specific surface area. However, the performance of BiOI-5 was a little better than that of BiOI-400, suggesting that some other factors also partially affect the photocatalytic activity, as the specific surface area of BiOI-5 is smaller than that of BiOI-400. This will be discussed in the following paragraphs.
2.0
a
b
1.0
Intensity (A. U.)
no BiOI
C/C 0
0.8 BiOI-1
0.6
BiOI-400
0.4 BiOI-5
0 min 20 min 40 min 60 min 90 min 120 min 150 min 210 min
1.5 1.0 0.5
0.2 0.0
BiOI-80
0.0 0
50
100
150
200
250
240
260
280
300
320
Wavelength (nm)
t (min)
Fig. 8. (a) Degradation curves of phenol on BiOI-1, BiOI-5, BiOI-80 and BiOI-400 under visible-light (λ > 420 nm) irradiation, (b) Evolution of UV-vis spectra of phenol aqueous solution over BiOI-80.
16
3.6 Photocatalytic mechanism To investigate the reactive species involved in the photo-degradation process of phenol, a series of active specie trapping experiments were carried out over the BiOI samples, employing 1 mM p-benzoquinone (BQ), 10 mM triethanolamine (TEOA) and 10 mM isopropanol (IPA) as the scavengers for ·O2¯, holes (h+) and ·OH, respectively [48,49]. The effects of these scavengers on photocatalytic degradation of phenol under visible light (λ > 420 nm) are shown in Fig. 9.、
1.0
(C 0-C)/C 0
0.8 0.6
No scavenger IPA BQ TEOA
0.4 0.2 0.0 BiOI-1
BiOI-5
BiOI-80
BiOI-400
Fig. 9. Active specie trapping experiments for BiOI-1, BiOI-5, BiOI-80 and BiOI-400 under visible-light irradiation (λ > 420 nm) for 120 min.
It can be found that the addition of TEOA remarkably suppressed the photocatalytic degradation of phenol, indicating that photogenerated h+ acts as the main active specie during the reaction. When IPA was added, only slight decrease in the photocatalytic efficiency was observed, suggesting that the contribution of ·OH to the photocatalytic reaction is negligible. This is because the VB potential of BiOI is not 17
positive enough to oxidize H2O to ·OH [50]. Another active specie is ·O2¯, as the photocatalytic efficiency also partially decreased in the presence of BQ. Note that the CB of BiOI is not negative enough for the reduction of O 2, hence the generation of ·O2¯ should be attributed to some electrons excited to a higher energy level (negative enough for the reduction of O2) [51,52]. As previously mentioned, in addition to the effect of hierarchical structure and surface area, some other factors play important roles in determining the photocatalytic activity of BiOI samples. Here we propose that the synergistic action of (001) and (110) surfaces could improve the photocatalytic performance of BiOI samples. To prove this assumption, the DOS of exposed (001) and (110) facets were calculated. As shown in Fig. 10, the energy levels of both CB and VB for exposed (110) facets are more positive than that for (001) facets, estimated on the basis of an equal Fermi level of the adjacent (001) and (110) facets. The slight difference in energy levels of the CB and VB between (001) and (110) facets can lead to the formation of a surface heterojunction [33]. Such surface heterojunction could improve the separation of photogenerated electrons and holes onto different surfaces. As shown in scheme 2a, electrons tend to migrate from CB of (001) surface to CB of (110) surface and holes will migrate from VB of (110) surface to VB of (001) surface, resulting in the spatial accumulation of electrons and holes on the (110) and (001) surfaces, respectively. Thus the recombination opportunity of electrons and holes is greatly reduced. It is noteworthy that there is an optimal ratio of (001) and (110) surfaces for the highest photocatalytic activity. As shown in Scheme 2b, the ratio of exposed (001) surfaces in thin nanoplates is higher than that in thick 18
nanoplates. However, excess exposure of (001) surface will cause an overflow effect of photogenerated electrons, due to the insufficient exposure of (110) surface [33]. Such effect will lead to the recombination of charge carriers on the (001) surface. In this regard, the ratio of (110) facet of BiOI-400 might be so low that the overflow effect occurred, thus the surface heterojunction of BiOI-400 does not act as effective as that of BiOI-5. Thus, BiOI-5 exhibited a little better performance than BiOI-400. While the ratio of (110) facet of BiOI-80 is higher than that of BiOI-400, forming a more effective surface heterojunction. Hence both hierarchical structure and surface heterojunction promote the photocatalytic reaction over BiOI-80, resulting in the best photocatalytic
DOS (electrons / eV)
activity. (001)
6 4 2 0
(110)
10 5 0 -6
-4
-2
0
2
4
6
Energy (eV)
Fig. 10. Density of states (DOS) plots for (100) and (110) surface of tetragonal BiOI.
19
Scheme 2. (a) Illustration of charge transfer across the surface heterojunction of BiOI(001) and BiOI(110) factets, (b) changes of exposed ratio and contact area of (001) and (110) surfaces.
4. Conclusion BiOI hierarchical structures with tailorable ratio of (001) and (110) surfaces are successfully fabricated via a facile room-temperature approach, combining solid-state reaction with subsequent hydrolysis, by tuning the amount of introduced water. The BiOI hierarchical structures with optimal ratio of (001) and (110) surfaces exhibited the best visible-light photocatalytic activity for phenol degradation; because (1) hierarchical structures allow for strongest light absorption ability and highest specific surface; (2) the surface heterojunction formed between (001) and (110) facets improve the separation of electrons and holes across the interface of the two different facets.
Acknowledgments 20
This work was supported by the 973 program (2013CB632402), NSFC (51472191, 21407115, 51272199, 51320105001 and 21433007), Natural Science Foundation of Hubei Province of China (2014CFB164, 2015CFA001), the Fundamental Research Funds for the Central Universities (WUT: 2015-III-034), Self-determined and Innovative Research Funds of SKLWUT (2015-ZD-1) and the Aid Program for Science and Technology Innovative Research Team in Higher Educational Institutions of Hunan Province.
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Graphical Abstract
Highlights 1. Hierarchical porous structured BiOI with tailorable (001) facets was fabricated. 2. Hierarchical structure enhanced visible light absorption of BiOI. 3. Surface heterojunction of (001) and (110) facets reduce the e-h recombination. 4. BiOI exhibits excellent photocatalytic activity for phenol degradation.
28