A controlled anion exchange strategy to synthesize Bi₂S₃ nanocrystals/BiOCl hybrid architectures with efficient visible light photoactivity

Abstract

Bi₂S₃ nanocrystals/BiOCl hybrid architectures with tunable band gaps were synthesized by a controlled anion exchange approach and they displayed highly efficient visible light photoactivities, which is associated with suitable energetics and structural topotactic relationship that can benefit the interfacial charge transfer.

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Semiconductor nanocrystals (NCs), which display a range of novel chemical/physical properties, have drawn ever growing interest for their applications in energy conversion, supercapacitors and photocatalysis.^1–3 In particular, derived from the dramatic quantum confinement, narrow band gap (NBG) semiconductor NCs (e.g., CdSe and PbS)³ with tunable band gaps have opened up new horizons for harvesting solar light that predominately concentrates in the visible (Vis) and the infrared (IR) region. Such an intriguing size-dependent feature of NBG NCs allows one to tune the light absorption and tailor the band edges; thus the photoinduced charges can be interfacially separated by coupling with other wide band gap (WBG) semiconductors such as TiO₂ with favorable energetics.

As a lamellar binary semiconductor, bismuth sulfide (Bi₂S₃) has significant applications in photovoltaics, thermoelectrics and electrochemical hydrogen storage.⁴ Moreover, the size quantization enables Bi₂S₃ nanoparticles (1.3 eV for bulk) to show tunable photosensitization and considerable photoactivity in the visible region.⁵ However, most of the reported approaches to Bi₂S₃ NCs require high reaction temperatures and capping agents involving oleylamine (OAm) as stabilizers, which increases the complexity and infeasibility. Recently, ion exchange, which permits the exchange of the component ions with the incoming species, has paved a new way to engender various inorganic nanostructures from the existing materials.⁶ Furthermore, controlled partial ion exchange gives birth to NC heterostructures, which bring about the close interconnection between the semiconductors to benefit the separation of carriers.^6e,f Therefore, developing a wet-chemical ion exchange route to Bi₂S₃ NCs sensitized hybrids, which could tailor the visible light response and facilitate the interfacial charge transfer, is highly desirable for practical applications yet it remains a challenge.

In the present work, a novel Bi₂S₃ NCs/BiOCl hybrid has been synthesized by a facile anion exchange route. In addition to providing Bi³⁺ ions, BiOCl architectures herein also play a role of a framework to construct the Bi₂S₃ NCs/BiOCl hybrid. Controlled release of S²⁻ ions from various sulfur sources to react with BiOCl by ion exchange leads to the Bi₂S₃ NCs with tunable sizes, thus regulating the visible light absorption of the hybrids. The as-prepared Bi₂S₃ NCs/BiOCl hybrid showed highly efficient visible light photocatalytic activities in decontamination of noxious pollutants, which is considered to be interrelated with the effective interfacial charge transfer.

First of all, hierarchical mesoporous BiOCl architectures with an average diameter of 1–2 μm (Fig. S1–S2, ESI†) were synthesized using Bi(NO₃)₃ and [C₁₂Mim]Cl ion liquid under solvothermal treatment (ESI†) as described elsewhere.⁷

Due to the rather lower solubility of Bi₂S₃ (K_sp = 1.0 × 10⁻⁹⁷) relative to BiOCl (K_sp = 1.8 × 10⁻³¹), BiOCl can adopt a thermodynamically favored direction to yield Bi₂S₃, and a partial anion exchange reaction could give birth to the Bi₂S₃/BiOCl hybrid. In our experiments, such a transformation process was realized through introducing the as-prepared BiOCl architectures into the appropriate sulfur source such as thiourea, cysteine and thiacetamide (TAA) solutions followed by ion exchange treatments (ESI†). XRD characterization (Fig. S3, ESI†) was carried out to identify the phase structure of the hybrids; however, no diffraction peaks assigned to Bi₂S₃ were found, which may result from its high dispersity and low content.^6e The structural information of the hybrids was further investigated by Raman spectra, which was shown in Fig. 1A. Two typical vibrational peaks at 141 and 200 cm⁻¹ are observed for the BiOCl sample, which are assigned to A_1g and E_g of Bi–X stretching mode, respectively.⁹ The broad peak regions (70–150 cm⁻¹) in both bulk Bi₂S₃ (ESI†) and Bi₂S₃/BiOCl hybrids indicate the presence of Bi₂S₃ species, which are in good agreement with the reported values.¹⁰

Fig. 1 (A) Raman spectra and (B) UV–Vis diffuse reflectance spectra of the samples: (a) BiOCl, (b–e) Bi₂S₃/BiOCl hybrids synthesized using thiourea, cysteine, TAA at room temperature and TAA at 60 °C, respectively, and (f) bulk Bi₂S₃.

Fig. 1B presents the UV–Vis diffuse reflectance spectra of the as-prepared samples. The white BiOCl only exhibits UV light response with an absorption edge around 380 nm, while the black Bi₂S₃ shows intense absorption over the visible range even extending to the infrared region. The fitted direct band gap of Bi₂S₃ was determined to be 1.33 eV, which is equal to its bulk value. It is noted that the Bi₂S₃/BiOCl hybrids show tunable visible light absorption in a large range, which is attributed to the quantum confinement of Bi₂S₃ NCs. The band gaps of Bi₂S₃ NCs/BiOCl hybrids synthesized using thiourea, cysteine, TAA at room temperature and TAA at 60 °C are estimated to be about 3.35, 2.90, 2.20 and 1.60 eV, respectively (Fig. S4, ESI†). On the basis of the effective mass approximation model, the blue shift of Bi₂S₃ NCs relative to the bulk is dominated by the confinement of electrons and holes, as described by the following equation:¹¹

where ΔE_g(R) is the band gap shift for the crystal radius R, h the Planck's constant, m₀ the electron mass, while and are the effective masses of electrons and holes, respectively. From the band gap shift as a function of crystal radius (Fig. S5, ESI†), the calculated sizes of the as-prepared Bi₂S₃ NCs in the Bi₂S₃/BiOCl hybrids range from 3.1 to 8.5 nm, which are summarized in Table 1. As the relative reactivity follows the order TAA > cysteine > thiourea,⁸ the higher reactivity of the sulfur source engenders the faster growth rate of Bi₂S₃, thereby leading to the larger size of Bi₂S₃ NCs with a smaller blueshift. In addition, higher temperature (60 °C) favors the decomposition of TAA, and as a consequence a Bi₂S₃ NCs/BiOCl hybrid with larger Bi₂S₃ size was obtained. Since the size of Bi₂S₃ NCs is much smaller than its Bohr excitation radius (ca. 24 nm),¹¹ remarkable quantum size confinement is thus observed.

Table 1 The band gaps, Bi₂S₃ sizes and photodecomposition rate constants of the Bi₂S₃/BiOCl hybrids synthesized under different sulfur treatments: (b) thiourea, (c) cysteine, (d) TAA at room temperature and (d) TAA at 60 °C

Sample	E g/eV	Bi2S3 size/nm	k ^a/h^−1
a Normalized 2,4-DCP photodecomposition rate constant.
b	3.35	3.1	0.266
c	2.90	3.5	0.426
d	2.20	4.7	0.693
e	1.60	8.5	0.457

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The morphology of the hybrids was studied and a typical scanning electron microscopy (SEM) image of the Bi₂S₃ NCs/BiOCl hybrid synthesized using TAA at room temperature is illustrated in Fig. 2a, in which quasi uniform architectures are observed. It is found that during the course of anion exchange reaction, the morphology of BiOCl architectures with hierarchy was retained in the final Bi₂S₃ NCs/BiOCl hybrid with high fidelity (Fig. S6, ESI†). Chemical element mapping analysis (Fig. 2b–d) shows that the elements including bismuth, chlorine and sulfur are homogenously distributed over the hybrid, which demonstrates the nanoscale hybridization between Bi₂S₃ NCs and BiOCl architectures. The energy-dispersive X-ray spectrum (EDS, Fig. 2e) confirms the presence of Bi, Cl, O and S elements in the Bi₂S₃ NCs/BiOCl hybrid, and the relative atom ratios are 32.22%, 26.43%, 41.04% and 0.31%, respectively (Fig. S7, ESI†). Moreover, as seen from the typical transmission electron microscopy (TEM) image in Fig. 2f, the sample exhibits hierarchical architectures with a size of 1–2 μm built by nanosheets. As seen from the high resolution TEM (HRTEM) image (Fig. 2g), the adjacent lattice spacing of ca. 0275 nm is in agreement with the (110) plane of tetragonal BiOCl. It is worth noting that some nanoparticles (as illustrated) with the size in the range of 4–6 nm are anchored on the surface of BiOCl nanosheets, which corresponds to the Bi₂S₃ NCs with a calculated size value.

Fig. 2 (a) Typical SEM image, (b–d) chemical element mapping data, (e) EDS, (f) TEM image and (g) HRTEM image of the Bi₂S₃ NCs/BiOCl sample synthesized using TAA at room temperature.

As the quantum confinement of Bi₂S₃ NCs enables Bi₂S₃/BiOCl hybrids to display tunable photoabsorption in the visible region, the visible light photocatalytic activities of the hybrids were carried out by decomposition of 2,4-dichlorophenol (2,4-DCP) solution, which eliminates the influences such as self-photodegradation. As shown in Fig. 3a, bulk Bi₂S₃ and BiOCl could decompose 40.8% and 47.1% of 2,4-DCP under irradiation for 150 min, respectively. As a reference photocatalyst, N-doped P25 could only decompose 12.1% of 2,4-DCP molecules. While in the case of the Bi₂S₃ NCs/BiOCl hybrid synthesized using TAA as the sulfur source at room temperature, the photocatalytic decomposition efficiency can reach as high as 82.3% (Fig. S8, ESI†). Given the pseudo-first-order photodecomposition of the pollutants, the 2,4-DCP decomposition rate over the Bi₂S₃ NCs/BiOCl hybrid (0.693 h⁻¹) is faster than that of N-doped P25 (0.0516 h⁻¹) by a factor of thirteen. Besides, the correlation between the Bi₂S₃ quantum sizes and the photocatalytic performances of Bi₂S₃ NCs/BiOCl hybrids was also investigated (Fig. 3b). It is found that the 4.7 nm Bi₂S₃ NCs/BiOCl hybrid exhibits the best photocatalytic performance. However, Bi₂S₃ NCs with larger (8.5 nm) or smaller (3.1 and 3.5 nm) sizes render the Bi₂S₃ NCs/BiOCl hybrids with inferior photocatalytic performances. Apart from decomposing 2,4-DCP, the Bi₂S₃ NCs/BiOCl hybrid also exhibits highly photocatalytic efficiency in methyl orange (MO) degradation (ESI†).

Fig. 3 (a) Photocatalytic decomposition of 2,4-DCP over different samples under visible light irradiation (λ ≥ 400 nm), and (b) the influence of Bi₂S₃ size on the photocatalytic efficiencies of the Bi₂S₃ NCs/BiOCl hybrid.

It is of great interest to explore the plausible reaction mechanism of the Bi₂S₃ NCs/BiOCl hybrid, as is shown in Fig. 4. Through ion exchange reaction between BiOCl architectures and sulfur sources, Bi₂S₃ was in situ anchored on the surface of BiOCl nanosheets. The structural topotactic relationship,^4c originating from the close matching of lattice constants of tetragonal BiOCl (a = b = 0.388 nm) and orthorhombic Bi₂S₃ (c = 0.398 nm), could favor the formation of Bi₂S₃/BiOCl heterostructures and facilitate the interfacial charge transfer. Moreover, band position calculations (ESI†) suggest that BiOCl and Bi₂S₃ have the staggered energy potentials that can reduce the recombination of the photogenerated carriers between the two semiconductors. As the sizes of Bi₂S₃ NCs decrease, the quantum confinement makes the Bi₂S₃ NCs/BiOCl hybrids absorb less visible light; while the conduction band (CB) of Bi₂S₃ shifts to more negative potentials to result in the larger CB energy difference (ΔE_C) between BiOCl and Bi₂S₃, which favors the electrons transfer.⁵ Upon the interrelationship of the two above mentioned factors, the optimized condition is observed for a 4.7 nm Bi₂S₃ NCs/BiOCl hybrid, which shows the superior photocatalytic performance. Under visible light irradiation, Bi₂S₃ NCs are excited and the as-photoinduced electrons migrate to the surface of BiOCl, which could be further trapped by molecular oxygen in solution to form O²⁻ and other oxidative species.¹² As an extension, a Bi₂S₃ NCs/BiOBr hybrid was also prepared by a partial anion exchange reaction and showed highly efficient visible light photocatalytic activity (ESI†).

Fig. 4 Schematic illustration of band energy positions and the charge transfer process of the Bi₂S₃ NCs/BiOCl hybrid under visible light irradiation.

In summary, a novel Bi₂S₃ nanocrystals/BiOCl hybrid with tunable visible light response was synthesized by a controlled anion exchange approach. The Bi₂S₃ nanocrystals/BiOCl hybrid, which involves suitable energetics and structural topotactic relationship that can benefit the interfacial charge transfer, displays highly efficient visible light photoreactivity. The work illustrates a facile controlled anion exchange access to isogenous “mother–children” Bi₂S₃ NCs/BiOCl hybrids with tunable photoresponse and photoreactivity, which could allow general modifications to other semiconductor materials.

This work was financially supported by the National Natural Science Foundation of China (Nos. 20973102, 51021062, 51002091), the National Basic Research Program of China (No. 2007CB613302) and Graduate Independent Innovation Foundation of Shandong University (GIIFSDU, 11250071613044).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental section, band calculations and Fig. S1–S14. See DOI: 10.1039/c1cc15487g

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