Preparation of thickness-tunable BiOCl nanosheets with high photocatalytic activity for photoreduction of CO₂

Abstract

BiOCl nanosheets with different percentages of exposed {001} facets were prepared by a controlled facile hot-injection technique. The synthesis was conducted using bismuth chloride as a metal precursor, octadecene as a solvent, and oleic acid, oleylamine and octadecene as morphology-control agents. The presence of trace amounts of water in the reaction system leads to mild hydrolysis. The obtained BiOCl nanosheets with highly exposed {001} facets exhibited high activity for photocatalytic reduction of CO₂ to CH₄. Moreover, the thinner the nanosheets, the higher the production yield. The detailed mechanism has been studied. This work may afford a feasible protocol for the preparation of BiOCl nanosheets with high photocatalytic activity.

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Introduction

Heterogeneous photocatalysis assisted by semiconductors and solar energy is considered a sustainable, effective technique to convert CO₂ into energy-containing carbon fuels (such as CO and CH₄), which is a promising and economical approach to mitigate carbon emission and the energy crisis.^1,2 So far many semiconductors have been used for photocatalytic reduction of CO₂ like TiO₂,^3–5 ZnGeO₄ (ref. 6 and 7) and BiMO₄ (M = W, V).^8,9 Different products are observed over these catalysts (such as CO, CH₄ and CH₃OH), while the conversion efficiency is still very low. Several strategies have been used to improve the efficiency, such as preparation of semiconductor catalysts with different crystal structure and/or morphology,¹⁰ modification of semiconductor with noble metal,¹¹ preparation of semiconductor–semiconductor nanocomposite.¹² Recently, facet engineering exposed with nearly 100% reactive facet has drawn great attention due to the strong influence of crystal-facet characteristics (atomic arrangement, electronic structure, defects, etc.) on the photocatalytic activity.

During the past decades, bismuth oxyhalides, BiOX (X = F, Cl, Br, I), have attracted much attention due to the remarkable photocatalytic activity,^13–18 especially the photodegradation of organic dye.^13–16 The BiOX compounds have a tetragonal PbFCl-type structure (space group P4/nmm, no. 129) with unique layered structure consisting of [X–Bi–O–Bi–X] sheets stacked together by non-bonding interactions through the X atoms along the c-axis. It is believed that the internal static electric field between the [Bi₂O₂]²⁺ and anionic halogen layer can lead to more efficient separation of photogenerated electron–hole pairs, which is in favor of the photocatalytic activity of the catalysts.^19–23 Recent work indicates that the BiOX nanoplates exposed with {001} facet exhibit good photoactivity owing to a high oxygen vacancy density in the {001} facet.²⁴ Compared with the conventional doping methods, the introduction of oxygen vacancy is a self-doping approach, without introducing any impurity elements, leading to improved photocatalytic performance. Surface oxygen vacancy may be beneficial to the separation of charge carriers.

It is known that the size and exposed facets of a catalyst can affect the photocatalytic activity significantly.^25,26 It is expected that the photocatalytic efficiency can be improved via the combination of a large surface area and a high degree of reactive surface.²⁷ Thus, 2D BiOCl nanocrystals with exposed active {001} facets are highly desirable. The typical lamellar structure of BiOCl with stacking layers of [Bi₂O₂]²⁺ enables the possibility of synthesis of ultrathin nanosheets with improved percentage of exposed {001} facet. One effective approach is reducing the thickness along the [001] direction and increasing the 2D lateral size of the {001} planes as well. So far most of the BiOCl nanosheets are usually prepared via alcoholysis and hydrolysis, e.g., by using diethylene glycol²⁸ and water.^29,30 Since the alcoholysis and hydrolysis of Bi³⁺ are readily to occur in these systems, the 3D microspheres are often obtained, which may adsorb contaminants on the surface and thus lead to an inferior activity.²⁰ In addition, only few work is reported hitherto about the photocatalytic reduction of CO₂ over BiOCl, which shows a low yield of CH₄ (∼1.2 μmol g⁻¹ in 8 h).³¹

Herein, we report the synthesis of BiOCl nanosheets with exposed {001} facet and controllable thickness by a facile hot-injection technique under mild hydrolysis. The hot-injection technique is in favor of controllable synthesis of nanomaterials with a special morphology.^32,33 The obtained BiOCl ultra-thin nanosheets exhibit a much higher production yield of CH₄ over the photoreduction of CO₂ (41.48 μmol g⁻¹ in 8 h) than the reported value.³¹

Experimental

Preparation of BiOCl nanomaterials

The synthesis was conducted by using 1-octadecene as solvent and bismuth chloride as metal precursor. Typically, 1 mmol of bismuth chloride, 16 mmol of oleic acid (OA) and 2 mmol of trioctylphosphine oxide (TOPO) were mixed with 20 mL of 1-octadecene in a 3-neck flask. The reaction vessel was first purged by nitrogen and kept at 90 °C for 5 min to remove the moisture and oxygen, and then was heated at 150 °C for 30 min until a white milky solution was obtained. After 10 mmol of oleylamine (OAm) was injected into the reaction solution, the temperature of the reaction system was further increased to 170 °C and the vessel was maintained at this temperature for different period so as to prepare the BiOCl nanosheets with different thickness (5 min for ultrathin nanosheets, 10 min for nanosheets, and 30 min for nanoplates, which were denoted respectively as BiOCl-UTNS, BiOCl-NS and BiOCl-NP). The growth of BiOCl nanosheets was terminated by removing the heating source. The obtained mixture was cooled down to room temperature and washed by hexane for several times, followed by being dried in a vacuum oven at 70 °C.

Characterization

Powder X-ray diffraction (XRD) patterns were collected by a diffractometer with Cu-Kα radiation (λ = 0.15406 nm). UV-visible absorption spectra were recorded on a Lambda 950 spectrophotometer using BaSO₄ as reference. Transmission electron microscopy (TEM), high resolution TEM (HRTEM) and selective area electron diffraction (SAED) images were measured by using Tecnai G2 F20 U-TWIN transmission electron microscopy equipped with an energy disperse X-ray spectrometer (EDX) annex. The X-ray photoelectron spectra (XPS) were recorded with ESCALAB 250 Xi. The time-resolved photoluminescence (PL) spectra were recorded at 398 nm by a multifunction steady state and transient state fluorescence spectrometer (FES920) with 330 nm excitation. The Brunauer–Emmett–Teller (BET) surface area of the BiOCl powder was calculated from N₂ adsorption isotherm obtained using an ASAP 2020 at 77 K. The photocurrent measurements were conducted on a CHI 660D electrochemical station under −0.6 V using a standard three-electrode cell with a Pt wire counter electrode, a saturated calomel electrode (SCE) reference electrode, and a solution containing 0.5 M of Na₂SO₄ as the electrolyte. The working electrode was prepared according to the following procedure. About 20 mg of the as-synthesized sample was suspended in 0.5 mL of ethanol, which was then dip-coated on a 10 mm × 10 mm fluorine doped tin oxide (FTO) glass electrode. The working electrode was obtained after being annealed at 150 °C for 30 min. The light source was provided by a 350 W Xe lamp equipped with a 420 nm narrow band filter. Electron spin resonance (ESR) spectra were collected at 100 K with an ESR spectrometer (JEOL FA-200). The g factor was corrected by using 2,2-diphenyl-1-picrylhydrazyl (DPPH) as the reference.

Evaluation of photocatalytic performance

The photocatalytic activity of obtained BiOCl nanomaterials were evaluated via photocatalytic reduction of CO₂ in a quartz reactor, which has been described previously.^34–36 The reactor was illuminated by a 300 W Xe lamp (PLS-SXE300) equipped with a 250–380 nm UVREF filter. About 20 mg of BiOCl was suspended in 80 mL of water under sonication. Pure CO₂ was bubbled into the suspended solution for at least 30 min before the irradiation so as to remove air and approach the saturation of CO₂ in the water. The suspension was magnetically stirred in dark for 30 min to achieve absorption–desorption equilibrium of CO₂ before photocatalysis. The temperature of the solution was kept at 25 ± 2 °C controlled by a thermostatic water bath. The reactor was tightly sealed during the reaction and stirred continuously so as to prevent sedimentation of the catalyst. The products were collected every half an hour and analyzed online by gas chromatograph (GC, Agilent 7890A) equipped with a flame ionization detector (FID) and a thermal conductivity detector (TCD).

Results and discussion

Crystal structure and morphology

As shown in Fig. 1, the XRD pattern could be well indexed to the tetragonal phase of BiOCl with lattice parameters of a = b = 0.3891 nm and c = 0.7369 nm (JCPDS no. 06-249). The intense and sharp diffraction peaks of (101), (110) and (102) planes suggest that the as-synthesized products are well crystallized. Moreover, it is noted that the full width at half-maximum (FWHM) of (001) peak of the obtained BiOCl-NP, BiOCl-NS and BiOCl-UTNS is 0.184°, 0.391° and 0.439°, respectively. So the BiOCl-UTNS has the largest FWHM of the (001) peak, indicating a lower value of parameter c, while the smallest FWHM of the (001) peak for BiOCl-NP implies a larger value of c.^20,37

Fig. 1 XRD patterns of as-synthesized BiOCl-NP, BiOCl-NS and BiOCl-UTNS.

TEM, high-resolution TEM (HRTEM) images and fast Fourier transform (FFT) pattern for typical BiOCl nanosheets (BiOCl-NS) are represented in Fig. 2. Obviously, the obtained products are square-like nanosheets with good dispersion and with a size ranging from 70 to 100 nm. The HRTEM image exhibits good crystalline and clear lattice fringes with an interplanar lattice spacing of 0.27 nm and an angle of 90° (Fig. 2b), which matches well with the (110) plane of the tetragonal BiOCl crystal. As depicted in the inset of Fig. 2b, the corresponding FFT pattern displays a spot pattern, implying the single-crystalline characteristic of the obtained nanosheets. The angle of spots labelled in the FFT pattern is 45°, which is identical to the theoretical value of the angle between the (110) and (200) planes of tetragonal BiOCl. Based on the above results and the symmetry of tetragonal BiOCl, the bottom and top surfaces of the BiOCl samples are identified as {001} facets.

Fig. 2 (a) TEM and (b) HRTEM image of typical BiOCl-NS. Inset is the corresponding FFT pattern of BiOCl-NS.

Moreover, the thickness of obtained BiOCl nanomaterials can be tailored by adjusting the reaction time. The BiOCl-UTNS can be obtained when a shorter reaction time was used, while the BiOCl-NP is obtained with a longer reaction time. Similar to the BiOCl-NS, it is noted that both BiOCl-UTNS and BiOCl-NP are comprised of sheet-like, high quantity nanostructures (Fig. 3). The respective thickness of obtained BiOCl-UTNS, BiOCl-NS and BiOCl-NP is about 25, 40 and 95 nm, determined from the TEM images (Fig. 4). On the basis of the above structural information, the percentage of highly reactive (001) facets is estimated to be 70%, 49% and 25% for BiOCl-UTNS, BiOCl-NS and BiOCl-NP, respectively.³⁸ As a matter of fact, the thickness of the obtained BiOCl reflects the periodic structure of the BiOCl crystal along the c axis. A thicker BiOCl nanosheet corresponds to a larger size of the BiOCl nanocrystal along the c axis. The variation of the thickness of the as-synthesized BiOCl is consistent with the above FWHM variation of the (001) peak in the XRD patterns.

Fig. 3 TEM images of (a) BiOCl-NP and (b) BiOCl-UTNS.

Fig. 4 TEM images of different BiOCl samples for the determination of thickness, (a) BiOCl-UTNS, (b) BiOCl-NS and (c) BiOCl-NP.

A possible growth mechanism of the BiOCl nanosheets is briefly discussed here. Normally, the formation of nanocrystals can be divided into nucleation and following crystal growth from the nuclei. The solution with transparent color is obtained at low temperature before the injection of OAm, which can be attributed to the formation of the complexes between Bi³⁺ and OA and/or TOPO. Once the OAm are injected into the system, the mixed solution turns into white color, implying the formation of BiOCl crystal seeds due to the opposite electric polarity of amine group to the carboxylic and phosphine oxide group, followed by the formation of BiOCl-UTNS. Here the presence of trace amount of water in the reaction system gives rise to mild hydrolysis of Bi³⁺, resulting in the controllable growth. In addition, the OA, OAm and TOPO play an important role in the formation of homogeneous BiOCl nanosheets since they can cat as surfactant during the crystal growth. The thickness of BiOCl increases with increasing reaction time owing to the further growth of crystal, resulting in the formation of BiOCl-NS and, eventually, BiOCl-NP.

XPS analysis was used to determine the chemical states and surface composition of the as-synthesized BiOCl nanomaterials. The XPS spectra are corrected by referencing the C 1s peak of 284.60 eV. Here BiOCl-NS is used as the example. The peaks of Bi, O, Cl, and C elements can be well identified in the survey spectrum, as shown in Fig. 5a. Two peaks with binding energy of 157.6 and 164.9 eV appear in the Bi 4f XPS spectrum (Fig. 5b), corresponding respectively to the Bi 4f_7/2 and Bi 4f_5/2 of Bi³⁺. The peaks at 196.5 eV and 197.6 eV can be assigned to Cl 2p_3/2 and 2p_1/2 (Fig. 5b), respectively. The atomic ratio of Bi/O/Cl is determined to be approximately 1 : 1 : 1 according to quantification of the peak area of Bi 4f, O 1s and Cl 2p in the survey spectrum, together with the sensitivity factor of Bi, O and Cl. This agrees well with the stoichiometric ratio of BiOCl. The Bi, O and Cl elements can also be observed in the EDX spectrum (Fig. 5c), which has the same atomic ratio of Bi/O/Cl as the XPS results. The observed Cu and C signals in the EDX arise from the copper grid used for the measurements.

Fig. 5 XPS spectra of (a) survey, (b) Bi 4f and Cl 2p, as well as (c) EDX spectrum of BiOCl-NS.

Photocatalytic activity

The results of photocatalytic reduction of CO₂ over the BiOCl samples in aqueous solution under UV-light irradiation are shown in Fig. 6a. Different control experiments were carried out, i.e., the BiOCl aqueous suspension under illumination but without CO₂ and BiOCl suspension with CO₂ but without illumination. No photoreduction product like CH₄ can be observed in these control experiments, implying that the light irradiation is critical for the reduction reaction and the CO₂ is indeed the carbon source. Moreover, the BiOCl nanocatalysts exhibit high selectivity to the formation of CH₄, as no other products can be observed. The CH₄ yield increases with irradiation time, which reaches 41.48, 27.36 and 16.64 μmol g⁻¹ after 8 h for BiOCl-UTNS, BiOCl-NS and BiOCl-NP, respectively. The CH₄ yield for BiOCl-UTNS is about 1.5 times of that for BiOCl-NS and about 2.5 times of that for BiOCl-NP.

Fig. 6 (a) Photocatalytic evolution of CH₄ from the BiOCl nanocatalysts for a period of 8 h illumination, (b) three repeated runs of photocatalytic production of CH₄ on the BiOCl-UTNS, (c) XRD patterns of the BiOCl-UTNS before and after the photocatalysis.

The stability of the BiOCl sample was tested by repeating the catalytic experiments and XRD measurements (Fig. 6b and c). Here the BiOCl-UTNS is used as the example. No significant change in the photocatalytic activity and crystalline structure can be observed after three cycles, though oxygen vacancies can be created after photocatalysis as discussed later. Thus, the BiOCl-UTNS is quite stable upon photocatalysis.

Analysis of photocatalytic mechanism

The photocatalytic activity of a photocatalyst is closely related to its energy band structure. Fig. 7a shows the UV-vis diffuse reflection spectra (DRS) of BiOCl-NS, BiOCl-UTNS and BiOCl-NP. Obviously, the DRS of all the obtained BiOCl presents a similar absorption edge between 370 and 420 nm, indicating that the BiOCl samples mainly absorb UV light. The bandgap of the as-synthesized samples can be calculated by using (αhυ)ⁿ = κ(hυ − E_g), where α is the absorption coefficient, κ is the parameter related to the effective mass associated with the valence and conduction bands, n is 1/2 for an indirect transition, hυ is the absorption energy, and E_g is the bandgap energy. The extrapolated intercept of curve of (αhυ)^1/2 versus hυ based on the optical response corresponds to the bandgap value. The bandgap is thus determined to be about 2.7–2.9 eV for the obtained BiOCl nanosheets. In addition, the valence band maximum (VB) of the obtained BiOCl is determined to be about 1.8–1.9 eV by XPS valence band spectra (Fig. 7b). Thus, no big difference is observed in the alignment of energy levels among the BiOCl-UTNS, BiOCl-NS and BiOCl-NP. Since the work function of XPS instrument is 4.62 eV, the VB for the obtained BiOCl is calculated to be about 1.9–2.0 V (vs. SHE, provided 4.5 eV vs. vacuum level is 0 V vs. SHE).³⁹ The conduction band minimum (CB) is thus calculated to be around −0.9 to −1.1 V for the obtained BiOCl nanosheets, which is much more negative than the redox potential of CO₂/CH₄ (−0.24 V). So the VB is around 1.7–1.8 V (vs. SHE), which is positive enough to oxidize water. Thus, it is possible for the CO₂ to be photoreduced into CH₄ and water to be oxidized by the photogenerated holes.

Fig. 7 (a) UV-vis diffuse reflectance spectra and (b) XPS valence spectra of BiOCl-NP, BiOCl-NS and BiOCl-UTNS. Inset of (a) shows the determination of bandgap by the curve of (αhυ)^1/2 versus photon energy.

Though the detailed mechanism of CO₂ photoreduction into CH₄ is still unclear, it can be understood briefly as given below. The electrons are excited to the CB of BiOCl upon irradiation with light energy higher than its bandgap energy, left the holes behind in the VB. The holes can oxidize H₂O to form ˙OH, which subsequently results in the formation of H⁺ and O₂. The CO₂ molecules can be activated on the catalyst surface to form negative ˙CO₂⁻ radicals via the reduction of electrons in the CB,⁴⁰ which can be converted into ˙C radicals, and then the ˙CH, ˙CH₂ and ˙CH₃ radicals via a series of reactions.⁴¹ Eventually the CH₄ is produced. It is noted the photocatalytic reduction of CO₂ may also go through other mechanism, which definitely needs further study.

Since no signal from OA, OAm and TOPO can be observed in the FT-IR spectra, they are considered mainly the morphology-control agents. Thus the BiOCl reported here exhibits much higher production yield of CH₄ than the literature is ascribed mainly to the single-crystalline characteristics as well as good dispersion of the BiOCl nanomaterials, as shown by the above TEM images.

As the thickness of the BiOCl nanosheets decreases, the surface-to-volume ratio and percentage of the surface atoms with dangling bonds increase dramatically. This can provide more active sites, resulting in improved catalytic activity. This is confirmed by the measurements of BET specific surface area with nitrogen adsorption–desorption method. The BET specific surface area is about 36.62, 34.45 and 25.13 m² g⁻¹ for BiOCl-UTNS, BiOCl-NS and BiOCl-NP, respectively. That is to say, the BiOCl-UTNS has the largest BET specific surface area and thus the highest photocatalytic activity.

Moreover, the BiOCl nanosheets with exposed {001} facets exhibit high photoexcitation activity due to cooperative effect between the surface atomic structure and suitable internal electric field.^42,43 According to the calculation results, the unique surface atomic structure of BiOCl facilitates the formation of oxygen vacancies under UV light because of the low energy and long length of the Bi–O bond. Indeed, the ESR signal of oxygen vacancy can be observed after photocatalysis, while no such signal is observed before the photoreduction of CO₂ (Fig. 8). Accordingly, the color of the catalyst suspension turns to dark gray, as shown in the inset of Fig. 8. It is noted that the oxygen vacancies can facilitate the adsorption of CO₂ and induce visible-light absorption.^44,45 Moreover, molecular oxygen can be adsorbed onto the oxygen vacancies too in the {001} facets and form an end-on structure with two nearest sublayer of Bi atoms that extract electrons from the redistributed surface charges to generate ˙O₂⁻ radicals, which is in favor of suppressing the recombination of photogenerated electrons and holes. The ultrathin BiOCl nanosheets have the highest percentage of exposed {001} facets, which endows it more CO₂ and ˙O₂⁻ species available during the photocatalytic reduction and, thereby, the highest photocatalytic activity. Specifically, it has been reported that the predominant defect of the active {001} facets is in the BiOCl ultrathin nanosheets, which can lead to significantly promoted photocatalytic activity.¹⁴

Fig. 8 ESR spectra of BiOCl-UTNS before and after photoreduction of CO₂. Inset is the corresponding optical image of the suspension.

In addition, a thin layer can facilitate the charge transfer, leading to suppression of the recombination of charge carriers. This can be confirmed by the measurements of transient photocurrent response under UV-light illumination with the same wavelength range used in the photocatalytic reactions, which is shown in Fig. 9a. The photocurrent increases sharply once upon the illumination for all of the samples, while it decreases rapidly once the light is switched off. The BiOCl-UTNS sample has the highest photocurrent, followed by BiOCl-NS, while the BiOCl-NP shows the lowest photocurrent. This is consistent with the order of photocatalytic activity. Obviously, a higher photocurrent indicates more efficient suppression of the recombination of charge carriers, i.e., more efficient separation of the photogenerated electron–hole pairs. This is further confirmed by time-resolved PL spectra (Fig. 9b). All of the samples display exponential decay. The BiOCl-UTNS shows the slowest decay kinetics among all the products, followed by BiOCl-NS, while the BiOCl-NP shows the fastest decay. The decay curves can be fitted by using the tri-exponential decay kinetics.⁴⁶ The results are listed in Table 1. The lifetime can be divided into three different processes, non-radiative process (τ₁), radiative process (τ₂), and energy transfer process (τ₃).^47,48 The recombination of photogenerated electrons and holes directly relates with the radiative process. It is found that the BiOCl-UTNS displays the longest lifetime among all the samples for all the three processes, indicating the highest separation efficiency of electrons and holes.

Fig. 9 (a) Transient photocurrent responses and (b) time-resolved PL spectra of BiOCl-UTNS, BiOCl-NS and BiOCl-NP.

Table 1 Lifetime of photogenerated charge carriers derived from time-resolved PL spectra for BiOCl-UTNS, BiOCl-NS and BiOCl-NP

Sample	Lifetime (ns) τ1	Lifetime (ns) τ2	Lifetime (ns) τ3
BiOCl-UTNS	1.95	16.38	0.98
BiOCl-NS	1.93	13.42	0.65
BiOCl-NP	1.85	12.82	0.32

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Conclusions

In summary, we have developed a facial hot-injection method for the preparation of thickness-tunable BiOCl nanosheets. The obtained BiOCl nanosheets exhibit high photocatalytic activity for the conversion of CO₂ into CH₄. Moreover, the thinner the nanosheets, the higher the production yield is. This is ascribed to that the obtained BiOCl nanosheets show good dispersion, good single-crystalline characteristics, large surface-to-volume ratio and high percentage of exposed {001} facets. Moreover, plenty of oxygen vacancies can be generated upon UV-light illumination, which can facilitate the adsorption of CO₂ and molecular oxygen, leading to high activity for photocatalytic reduction of CO₂. This work not only presents a novel strategy to controllable synthesis of BiOCl nanomaterials, but also provides insight into the correlation between the structure and photocatalytic activity.

Acknowledgements

This work was supported by the Ministry of Science and Technology of China (2015DFG62610).

Notes and references

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