自然科学

Nitrogen Doped Ultrathin BiOCl Nanosheets for Enhanced Photocatalytic 1

CO2 Reduction under Visible Light

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Jian Shang, Lejuan Cai, Lizhi Zhang*

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Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, Institute of Environmental 4

Chemistry, College of Chemistry, Central China Normal University, Wuhan 430079, P. R. China 5

*Corresponding author: Email: zhanglz@https://www.360docs.net/doc/81688782.html, (L.Z. Zhang)

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Introduction

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The conversion of carbon dioxide (CO2) to reusable chemical feedstock contributes to solving the 23

global energy shortage and mitigating the greenhouse effects caused by the depletion of fossil fuels, 24

therefore attracts intensive attention. Among various CO2 conversion strategies, the photocatalytic 25

CO2 reduction strategy is the most promising one because of its mild reaction condition and 26

environment-friendly progress. Unfortunately, up to now, the total photocatalytic reduction 27

efficiency is relatively low and needs further promotion for practical utilization. According to 28

previous researches, the low efficiency of photocatalytic CO2reduction is mainly caused by the 29

following two reasons: (1) the majority of photocatalysts, such as metal oxides, sulfides, and their 30

mixed solid solutions, have low efficiency in adsorption of CO2molecules; (2) the traditional 31

photocatalysts usually possess low efficiency in solar-energy harvesting and charge carrier 32

separation, that leads to dissatisfied performance in activation of CO2molecules. As such, it is 33

extremely important to fundamentally improve the efficiency of CO2 adsorption and activation of the 34

photocatalysts.

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Recently, nitrogen-doped materials have been reported to increase the efficiency in capturing and 36

activating CO2, because the unshared pair electrons of doping nitrogen atom can act as Lewis basic 37

site to bond with CO2molecule. Moreover, doping nitrogen atoms in materials can extend the 38

UV-vis light absorption range and also accelerate charge carrier separation process, thus finally 39

achieving high efficiency in CO2 reduction. However, for lack of ideal theoretical nitrogen doped 40

model, the progress in systematically investigating the effects of doping nitrogen atoms with 41

atomic-level insight on CO2 reduction is seriously hampered. Hence, developing a doping model to 42

explain the enhanced mechanism of CO2 reduction induced by doping nitrogen atoms still remains a 43

great challenge.

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It is widely accepted that the two dimensional material is an ideal platform for investigating the role 45

of active sites in catalysis due to its large surface area and extreme sensitivity to the change of 46

electronic structure. Currently, a lot of work have used two dimensional materials to explore the 47

active sites, for example, Xie’s group have used pits confined two dimensional cerium(IV) oxide to 48

study the effect of coordinately unsaturated pit-surrounding Ce sites on carbon monoxide oxidation 49

progress. Their result shows that the coordinately unsaturated pit-surrounding Ce sites reduced the 50

activation barrier and decreased the catalyst poisoning chance. Very recently, they also have 51

proposed O-vacancies confined two dimensional indium oxide porous sheet to insight into the role 52

of O-vacancies during the photocatalytic process. So in this regard, developing a nitrogen doped 53

material model with exposed unique crystal plane as well as nanoscale thickness holds the main 54

point to disclosing the role of doping nitrogen atoms in photocatalysis.

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Bismuth oxychloride (BiOCl), a typical layered ternary oxide semiconductor, may become an 56

extremely potential two dimensional material due to its open crystalline structure and weak van der 57

Waals interaction between the layers. Moreover, its stable and excellent surface structure offer a 58

great opportunity to study the model reaction of small molecular activation.Based on the above two 59

key factors, in this communication, we firstly presented a conceptually new nitrogen-doped ultrathin 60

BiOCl nanosheet as an ideal material model and disclosed the role of doping nitrogen atoms 61

theoretically and experimentally in photocatalytic CO2 reduction progress.

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Result and discussion

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Theoretical calculation is first used to check the possible proposed nitrogen doped ultrathin BiOCl 65

model.A perfect ultrathin BiOCl slab and nitrogen-doped ultrathin BiOCl slab model are initially 66

built and first-principle density functional theory (DFT) calculations are implemented to study the 67

effect of the doped nitrogen in ultrathin BiOCl on the electronic structure. Obviously, as shown in 68

Figure 1b, the calculated total density of states (DOS) of 5-atom-thick BiOCl nanosheet exhibited an 69

evidently increase of DOS at the conduction band minimum (CBM) with respect to the bulk BiOCl 70

counterpart, suggesting the more efficient photoinduced charge transfer and separation in ultrathin 71

BiOCl nanosheets. Moreover, as displayed in Figure 1c, after doping the ultrathin BiOCl with 72

nitrogen, an impurity band consisted of N 2p orbitals between the valance band and conduct band is 73

formed, indicating that the photo-excited electrons can not merely come from the valance band 74

consisted of Cl 3p orbitals, but also come from the impurity band, that extends the absorption edge 75

of nitrogen-doped ultrathin BiOCl nanosheet into the visible light region, thereby achieving higher 76

photoconversion efficiency. More importantly, owing to the thickness of the BiOCl, the CVM of 77

ultrathin BiOCl nanosheets and nitrogen doped ultrathin BiOCl nanosheets up-shifted by 0.2 eV 78

compared to that of bulk BiOCl, which mean the excited electrons in the CBM of ultrathin BiOCl 79

nanosheets has stronger reducing capacity. On the basis of the above results, the nitrogen doped 80

ultrathin BiOCl nanosheets would not merely show improved photo-responsivity and extend 81

absorption range, but also have stronger reduction activity.

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To further understand the roles of doping nitrogen on ultrathin BiOCl surfaces, we employ the 83

surface charge density to analyze the electron distribution and self-induced local electric field 84

interaction. From the charge density contour plots viewed from the either [Bi2O2]2+and N doped 85

[Cl2]2- slices of BiOCl (Fig. 2a and 2b), it can be seen clearly that the charge density surrounding N 86

atoms is higher than that of Cl atoms in (001) surface. According to the previous research, the 87

non-uniform charge distribution between [Bi2O2]2+ and [Cl2]2- slices in the bulk BiOCl would induce 88

the presence of internal static electric fields and the self-induced internal electric field can be 89

calculated by the following equation:

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34eff P E d πε= 91

2E d 4eP e d πε?Φ=???=

92 in which P is the intrinsic dipole moment, ε is the dielectric constant, d is the thickness of the two 93

slices, and ?Φ is the electrostatic potential difference between two slices. After a few Cl atoms 94

were replaced by N atoms, the charge of the N doped [Cl 2]2- slices will be more negative, the 95

intrinsic dipole moment P will be enhanced, that may enhance the internal static electric fields. As 96

we expected, after nitrogen doping, the electrostatic potential difference between [Bi 2O 2]2+ and N 97

doped [Cl 2]2- slices is improved 0.7 eV (Figure 2c), which is large enough to accelerate the 98

separation and transfer progress of the photoinduced charge carriers.

99 The above observations suggest that the nitrogen-doped ultrathin BiOCl would exhibit an excellent 100 photocatalytic performance. Yet, the behavior of CO 2 adsorption and activation in nitrogen-doped 101 ultrathin BiOCl is still unknown. To investigate it, we build the initial structure (Fig. 3a) and 102 optimized the initial adsorption geometric structure. From the optimized structure (Fig. 3b), it could 103 distinctly find that the C atom of the CO 2 molecule was bound to the doping nitrogen atom and the 104 adsorbed CO 2 molecule also exhibited a bent configuration, where the O-C-O angle was 127.3°. The

105 two C-O bonds length was activated to 1.27 ?, which is close to that of CO 2 ·- (1.22 ?, Fig S7). To 106

further understand the CO 2 adsorption and activation processes, the charge density difference and 107

bader charge were employed to trace the electron behavior. As shown in Fig. 3c, the exchange and 108

transfer of electrons mainly occurred among the two partially reduced Bi atoms, doping nitrogen 109

atoms and adsorption CO 2 molecules. Once CO 2 was adsorbed on the doping nitrogen atom, the 110

electrons localized between Bi1, Bi2 and doping nitrogen transferred to the CO 2 molecule, which 111

would result in activation the CO 2 to CO 2·- (Tab. S2 ).

112 The as-prepared ultrathin nanosheets had been fully characterized by various techniques. Figure 4b 113

was the transmission electron microscopy (TEM) image of N-doped ultrathin BOC, revealing the 114

uniform freestanding and large-area sheet-like morphology with the size of 50–100 nm. The nearly 115

transparent feature of the nanosheets indicated its ultrathin thickness. High-resolution TEM 116

(HRTEM) (Figure 4c) revealed the highly crystalline nature of the exfoliated nanosheets. The 117

mutually perpendicular lattice fringes with an interplanar lattice spacing of 0.275 nm corresponded 118

to the (110) atomic planes. Moreover, Fast Fourier Transform image in Figure 4d also indicated the 119

single-crystalline characteristic. The angle labeled in the FFT pattern is 45°, which was in matched 120

well with the theoretical value of the angle between the (110) and (200) plane, so the set of 121

diffraction spots could be indexed as the [001] orientation of N-doped ultrathin BiOCl. On the basis 122

of the above results and the crystal structure of BiOCl, we could validate that the ultrathin 123

nanosheets were exposed with {001} facets. The thickness and size distribution of as-prepared 124

nanosheets was evaluated from the tapping mode atomic force microscopy (AFM). As shown in 125

Figure 4e,f, the diameter of the nanosheets range from 80 to 120 nm and thickness only about 0.5 nm, 126

indicating that the exfoliated nanosheets were consisted of only about one unit cell along the [001] 127

direction.

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As evidenced by X-ray diffraction (XRD) characterization, the diffraction intensity of the Nitrogen 129

doped samples had sharply decreased and the (001) diffraction peaks in the range of 2θ = 10?15° 130

slightly shifted toward a higher 2θ value compared to the bulk materials (Figure S3). Accord ing to 131

Bragg’s law, that mean the uniform bulk BOC structure had turned into nonuniform nanosheets and 132

at the same a few Cl atoms in the crystal were replaced by the N atoms, which was in good 133

agreement with the HRTEM and AFM results. To further confirm the existence, chemical 134

composition and chemical states of dopants, X-ray photoelectron spectroscopy (XPS) analysis was 135

further carried out (Figure S4). According to early XPS investigations on N-modified materials, we 136

attributed the peak at 398.8 eV to Bi-N bonds and the peak at 400.2 eV to N-H bonds. On the basis 137

of the above, we have successfully obtained the expected materials.

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The success in controlled synthesis of N-doped ultrathin BiOCl nanosheets allowed us to confirm 139

the theoretical calculation results and investigate photochemical performance systematically. The 140

UV?vis diffuse-reflectance spectroscopy (DRS) (Figure S3) shows that the two ultrathin samples 141

had the most appropriate band-gap energies about 2.6 eV, indicating that the absorption spectra of 142

the N-doped ultrathin BiOCl have successfully extended to the visible light region, enhancing the 143

absorbance of solar light, which was in accordance with the above theoretical prediction results. The 144

transient photocurrents and Photoluminescence (PL) emission were used to survey the separation 145

efficiency of the photogenerated charge carriers in a semiconductor that could directly reflect the 146

function of the internal electric field. As shown in Figure 5, the N-doped ultrathin BOC film 147

electrode exhibited a higher photocurrent than the N-doped-bulk BOC and ultrathin BiOCl film 148

electrodes, indicating the more efficient photoinduced charge separation and transfer in N-doped 149

ultrathin BOC under visible light irradiation.

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Inspired by the above interesting results and analysis, we evaluated the photoreduction performance 151

of obtained materials. Under visible-light irradiation, the photocatalytic reduction performance of 152

CO2to CO rate was estimated to be 2.76 umol?h-1per one gram of catalyst of N-doped ultrathin 153

BiOCl, which was over 3 times higher than that of the N-doped bulk BOC and ultrathin BiOCl (Fig. 154

5b). And the control sample, bulk BiOCl, almost showed no photoactivity, which was due to the 155

wide bandgap.

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To disclose the role of nitrogen atoms in affecting the adsorption and activation of CO2and 157

investigate the inner mechanism during the photocatalytic reduction progress of N-doped ultrathin 158

BOC, in situ Fourier Transform Infrared Spectrometer (FT-IR) were used to analysis the 159

time-dependent change reaction intermediates in photocatalysis. As shown in Fig. 4c, two weak 160

intense bands at about 1570 and 1680 cm?1were observed after the CO2being passed into the 161

system. According to the previous work, the broad band at 1570 cm-1could be assigned to the a 162

carboxylate CO2·- and the broad band at 1670 cm-1 could be related to a carbonate CO3= (H2CO3) 163

like species. To further investigate the surface reaction of N-doped ultrathin BiOCl under CO2 164

atmosphere, the visible light were introduced to the system. After been lighted on, several absorption 165

bands gradually increased with the irradiation time. (红外数据不好，后期重新补做)。

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Discussion

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In conclusion, the N-doped ultrathin BiOCl nanosheets with a thickness of a unit cell have been first 168

presented as an ideal material to disclose the role of doping nitrogen during the photocatalysis 169

progress. By analysis the theoretical results, we have demonstrated that the doping nitrogen atoms 170

not merely enhance the inner electronic field, but also act as an active site for CO2 adsorption and 171

reduction. Moreover, the N-doped ultrathin BiOCl nanosheets have been successfully prepared by a 172

thermal exfoliation route from bulk BiOCl for the first time, providing the opportunity to shed light 173

on the deep understanding of doping nitrogen, experimentally. Compared with bulk BiOCl, N-doped 174

bulk BiOCl and ultrathin BiOCl, the N-doped ultrathin BiOCl nanosheets show enhanced 175

photocurrent and photoreduction performance, which confirm the theoretical results. Further study 176

the detected intermediate products by in site FT-IR also confirm the reaction mechanisms. Briefly, 177

this work not only develops a thermal exfoliation strategy for fabricating nitrogen doped ultrathin 178

nanosheets but also theoretically/experimentally demonstrates atomic-level insights into the role of 179

doping nitrogen atoms in promoting reduction CO2 to CO efficiency.

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Acknowledgments

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This work was supported by National Basic Research Program of China (973 Program) (Grant 185

2013CB632402), and National Science Foundation of China (Grants 21073069, 91023010, 186

21177048, and 21377044).

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Figure 1. (a) Schematic illustration for Bulk BiOCl, Ultrathin BiOCl and Nitrogen doped Ultrathin 191

BiOCl; (b) calculated density of states (DOS) of ultrathin BiOCl and bulk BiOCl, (c) calculated 192

density of states (DOS) of the N-doped Ultrathin BiOCl and ultrathin BiOCl.

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Figure 2. Charge density plots of (a) the N doped [Cl2]2- slices and (b) [Bi2O2]2+ slices. (c) The 196

calculated electrostatic potential difference of [Bi2O2]2+ and N doped [Cl2]2- slices

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Figure 3. (a) The exfoliation process from bulk BiOCl to N-doped ultrathin BiOCl nanosheets. (b) 200

TEM image, (c) HRTEM image and (d) SEAD pattern of the N-doped ultrathin BiOCl nanosheets. 201

(e) AFM image of the as-prepared N-doped ultrathin BiOCl and (f) the corresponding height 202

measurement.

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Figure 4. (a) Schematic illustration of the photocatalytic CO2 reduction model. (b) Comparison of 207

photoreduction CO2to CO by N-doped ultrathin BiOCl nanosheets, ultrathin BiOCl nanosheets, 208

N-doped bulk BiOCl and bulk BiOCl under visible light irradiation. (c) Photocurrent responses of 209

the as-prepared samples in 0.5 M Na2SO4 aqueous solutions under visible light irradiation. (d) In situ 210

FTIR spectra recorded during the reduction progress over N-doped ultrathin BiOCl. (e) Calculated 211

potential energy profile for CO2 adsorption and reduction over N-doped ultrathin BiOCl.

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