Introduction

Semiconductor photocatalysis is a promising green technology that can potentially resolve the increasing energy and environmental crisis using solar energy, such as for pollutant degradation and hydrogen generation from water (Khin et al. 2012; Osman et al. 2017; Patil et al. 2019; Saeed et al. 2018; Shwetharani et al. 2019; Tarkwa et al. 2019). BiOCl is one of the most popular photocatalysts due to its unique layered structure, non-toxicity, chemical stability, and highly catalytic efficiency (Li et al. 2018a, b; Seddigi et al. 2017; Zhao et al. 2013). Although BiOCl is a wideband semiconductor which responds only to ultraviolet light, it can effectively degrade dyestuffs under visible light irradiation through photosensitization (Hu et al. 2014; Song et al. 2016). Adsorption capacity is one of the main factors affecting a semiconductor’s photosensitization efficiency, and it is heavily influenced by the photocatalyst’s morphology, particle size, and crystal facets (Bello and Raman 2019; Ding et al. 2015; Wang et al. 2012). Among the different BiOCl morphologies, the 3D hierarchical nanoflowers structure is particularly useful for adsorption due to its large surface area (Chen et al. 2012; Zhao et al. 2017). In addition, the exposed {001} facets are densely covered in oxygen atoms and have a high surface energy (Peng et al. 2015), which allows BiOCl to adsorb dyes and accumulate photogenerated electrons that can bind with O2 to form reactive oxygen species, such as ·O2− (Haider et al. 2016; Li et al. 2016a, b). The controllable synthesis of flower-like BiOCl structures with highly exposed {001} facets has been accomplished by adding surfactants such as polyethylene glycol (Li et al. 2018a, b) and polyacrylamide (Li et al. 2017). However, these polymeric surfactants are difficult to remove when adsorbed to the catalyst surface, which decreases its catalytic performance. Thus, a simple and efficient synthesis of BiOCl nanoflowers with highly exposed {001} facets is desirable.

Rhodamine B is extensively used in the production of paints, paper, makeup, cloth, and plastics. Rhodamine B wastewater is toxic to living beings, and results seriously irritate the respiratory tract, eyes, and skin, which has caused the treatment of rhodamine B wastewater to receive tremendous scientific interest. In previous research, we found that BiOCl nanosheets have shown a good performance in photodegradation of rhodamine B, and furthermore, constructing heterojunctions can effectively improve its catalytic efficiency (Zhang et al. 2019). In this paper, the effects of the morphology and exposed facets on the photocatalytic performance of pure BiOCl nanomaterials are discussed. 3D hierarchical BiOCl nanoflowers with exposed {001} facets were synthesized through a rapid solvothermal method, followed by a facile water-assisted procedure at room temperature without the use of surfactants. The as-prepared BiOCl nanoflowers exhibited improved adsorption and high activity toward the photosensitized degradation of rhodamine B under visible light irradiation. This research provides new insights to better understand the role of heterogeneous dye-sensitized semiconductor systems in the treatment of organic pollutants in wastewater.

Experimental

Materials

Bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), hydrochloric acid (HCl, 37 wt%), methanol, ethanol, ethylene glycol, rhodamine B, p-benzoquinone, and isopropanol were all purchased from Sinopharm Chemical Reagent Co., Ltd. (China). All chemicals were analytical grade and used as received. The water used in this experiment is ultrapure water with a resistivity of 18.25 MΩ cm.

Preparation of BiOCl nanoflowers and nanosheets

BiOCl nanoflowers were synthesized by a novel, two-step water-assisted method. Briefly, Bi(NO3)3°5H2O (0.97 g, 5 mmol) and HCl (5 mmol) were added to 32 mL of ethylene glycol. The mixture was then stirred for 1 h and transferred to a 50-mL polytetrafluoroethylene autoclave and heated at 200 °C for 30 min. After cooling to room temperature, the obtained transparent solution was transferred to a 200-mL beaker. After 60 mL of deionized water was poured into the beaker, a solid white precipitate immediately formed due to rapid hydrolysis and was separated by filtration, washed with water and ethanol, and dried at 60 °C for 12 h. BiOCl nanosheets were directly synthesized by a similar procedure in ethylene glycol via a one-step solvothermal reaction for 90 min. The solid was separated by filtration, washed with water and ethanol, and dried at 60 °C for 12 h. The yield of BiOCl nanosheets and BiOCl nanoflowers was > 99% since no Bi3+ was detected in the filtered solvent.

Characterization of the catalysts

The crystallinity of as-prepared samples was detected via powder X-ray diffraction using a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 0.15405 nm) at 40 kV and 30 mA. General morphologies were inspected by a Hitachi S-4800 scanning electron microscope. Transmission electron microscopy and high-resolution transmission electron microscopy images were obtained using a Hitachi H 9000 NAR field emission electron microscope. The concentrations of Bi3+ in the filtered solvent were measured by inductively coupled plasma mass spectrometry (Shimadzu, ICPMS-2030). Brunauer–Emmett–Teller (BET) surface areas of the samples were recorded by a Micromeritics ASAP 2460 analyzer using the nitrogen adsorption–desorption technique. X-ray photoelectron spectroscopy spectra were obtained using a Thermo ESCALAB 250 X-ray photoelectron spectrometer. Diffuse reflectance ultraviolet–visible absorbance spectroscopy was recorded on a Perkin-Elmer Lambda 950 spectrometer.

Photocatalytic decomposition activity evaluation

The photocatalytic properties of as-prepared samples were studied by observing the aqueous-phase degradation of rhodamine B dye under visible light irradiation. The photodegradation of rhodamine B was evaluated using a 500 W xenon lamp with a 420 nm cutoff filter. More precisely, 25 mg of the catalyst was mixed with a solution of rhodamine B (50 mL, 20 mg/L), and the mixture was stirred in the dark for 60 min to achieve an adsorption–desorption equilibrium. During visible light irradiation, 2 mL samples of the suspension were extracted at given time intervals. The supernatant was obtained by centrifugation, and the remaining rhodamine B concentration was determined using an ultraviolet–visible spectrophotometer (Hach, DR3900) at 553 nm.

Results and discussion

Characterization of catalysts

Scanning electron microscope micrographs show that BiOCl synthesized via a one-step solvothermal method was composed of evenly distributed nanosheets with thicknesses of ~ 60 nm, while BiOCl synthesized via water-assisted method has 3D nanoflower structure which consisted of self-assembled nanosheets (Fig. 1). The average nanoflower size was ~ 600 nm, and the thickness of each nanosheet was estimated to be 50 nm. Many fractures were observed at the edges of BiOCl nanoflowers, possibly due to rapid hydrolysis.

Fig. 1
figure 1

Scanning electron microscope images of BiOCl nanosheets (a, b). The thickness of BiOCl nanosheet was estimated to be 60 nm. Scanning electron microscope images of BiOCl nanoflowers (c, d). The 3D hierarchical structure of BiOCl nanoflowers consisted of self-assembled nanosheets, and the thickness of each nanosheet was estimated to be 50 nm. HV, higlt voltage; WD, work distance; Det, detector; SE, secondary electron; MAG, magnification

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The BiOCl nanoflowers and BiOCl nanosheets peaks in the X-ray diffraction spectra in Fig. 2a were all well-indexed with the tetragonal BiOCl form (JCPDS No. 06–0249). The BiOCl nanosheets peaks were more intense and shaper than those of BiOCl nanoflowers, indicating that the rapid aqueous synthesis produced BiOCl nanoflowers with a low crystallinity. Moreover, the intensities of the BiOCl nanosheets and BiOCl nanoflowers diffraction peaks were markedly different. The intensities of the (001), (002), and (003) diffraction peaks were drastically lower in BiOCl nanoflowers, implying that the thickness along the [001] crystallographic direction was lower than that of BiOCl nanosheets (Ye et al. 2011). At the same time, the (110) diffraction peak increased and had the highest intensity, indicating that BiOCl nanoflowers grew along the (110) direction, which led to thinner slabs of BiOCl nanoflowers than BiOCl nanosheets (Liu et al. 2018). In contrast, BiOCl nanosheets grew in random directions. Results from X-ray diffraction were also fitted via the Rietveld method with a primary model of BiOCl with an ICSD Card No. 74502, and the refinement results were plotted in Fig. S1. The fitting curves matched closely with the experimental diffraction intensities against 2θ, with relatively smooth difference spectra, indicating a good fit for the two samples. Crystal sizes were estimated using Debye–Scherrer’s equation, with a value of 25.66 nm for BiOCl nanosheets and 9.74 nm for BiOCl nanoflowers. The BiOCl nanosheets cell parameters were slightly smaller than those of BiOCl nanoflowers, which resulted from lattice expansion due to nanoscale effect (Table S1). Atomic coordinates for the two samples were nearly identical, except for a small shift of Bi and Cl along the z-axis, as well as the Uiso value, which also resulted from the lattice expansion of the nanosized BiOCl nanoflowers particles (Table S2).

Fig. 2
figure 2

X-ray diffraction patterns of BiOCl nanoflowers and BiOCl nanosheets (a). Compared to BiOCl nanosheets, the (110) diffraction peak of BiOCl nanoflowers increased and had the highest intensity, indicating that BiOCl nanoflowers grew along the (110) direction. High-resolution transmission electron microscopy image of BiOCl nanoflowers (b) and its corresponding fast Fourier transform image (c). Results indicated that the exposed surface of BiOCl nanoflowers was [001] facet. a.u., arbitrary unit

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More detailed information about the morphology and structure of BiOCl nanoflowers was obtained from high-resolution transmission electron microscopy image and its corresponding fast Fourier transform image. A clear lattice fringe was observed by high-resolution transmission electron microscopy (Fig. 2b), with an interplanar lattice spacing of 0.275 nm at an angle of 90°, which perfectly matched the [110] atomic planes of tetragonal BiOCl (Xu et al. 2018). The angle marked in Fig. 2c in the corresponding FFT image was 45°, which is consistent with the theoretical angle between the (110) and (200) planes (Jiang et al. 2012). The exposed surface can thus be considered to be the [001] facet of tetragonal BiOCl. Based on the X-ray diffraction and high-resolution transmission electron microscopy results, it is clear that BiOCl nanoflowers grew along the (110) direction, leading to highly exposed {001} facets (Fig. S2).

Nitrogen adsorption–desorption analysis was used to measure the Brunauer–Emmett–Teller (BET) surface area. As shown in Fig. 3a, each sample displayed typical type IV isotherms, demonstrating the formation of mesoporous materials (Li et al. 2016a, b). The BET surface area of BiOCl nanosheets was 15.41 m2/g, while that of BiOCl nanoflowers was 33.42 m2/g. As illustrated in Barrett–Joyner–Halenda (BJH) curve of Fig. S3, the pore size distribution of BiOCl nanosheets and BiOCl nanoflowers ranged from 2 to 60 nm, indicating that BiOCl nanosheets and BiOCl nanoflowers had porous structures. The mean pore diameters of BiOCl nanosheets and BiOCl nanoflowers were 11.07 and 11.55 nm, respectively. As shown in Fig. 3b, c, the BiOCl nanosheets had smooth surfaces, whereas the surfaces of the BiOCl nanoflowers were densely pitted. Thus, it can be concluded that the pronounced increase in the specific surface area of BiOCl nanoflowers was caused by its 3D hierarchical structure due to the cross-stacking of ultrathin nanosheets and its unique surface structure.

Fig. 3
figure 3

Nitrogen adsorption–desorption isotherms (a). The Brunauer–Emmett–Teller (BET) surface area of BiOCl nanoflowers was much larger than that of BiOCl nanosheets. Transmission electron microscopy images of BiOCl nanosheets (b) and BiOCl nanoflowers (c). Rough surface may be one of the factors that increase the BET surface area of BiOCl nanoflowers

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Photocatalytic experiment and photosensitized degradation mechanism

The adsorption capacity and photocatalytic ability of as-prepared samples were evaluated via photodegradation of rhodamine B under visible light irradiation (Fig. 4a). BiOCl nanosheets showed an rhodamine B adsorption of 20.77%, which corresponded to an adsorption capacity of 8.3 mg/g. BiOCl nanoflowers adsorbed 68.77% of rhodamine B, corresponding to an adsorption capacity of 27.5 mg/g, which was 3.31 times higher than the adsorption capacity of BiOCl nanosheets. The outstanding adsorption capacity of BiOCl nanoflowers was due in part to the increase in its specific surface area and also to the highly exposed {001} facets (He et al. 2015).

Fig. 4
figure 4

a Adsorption and photocatalytic capacity of BiOCl nanosheets and BiOCl nanoflowers. In the blank test without the photocatalyst, rhodamine B was very stable during visible light illumination. BiOCl nanoflowers showed superior rhodamine B adsorption and photodegradation to BiOCl nanosheets; b pseudo-first-order kinetic fitting and the determined apparent rate constants, k. The corresponding rate constant of BiOCl nanosheets and BiOCl nanoflowers was calculated to be 0.0128 min−1 and 0.0452 min−1, respectively. (Ct is the residual rhodamine B concentration in the solution after reaction time t, C is the initial concentration of rhodamine B, C0 is the concentration of rhodamine B after adsorption–desorption equilibrium)

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Rhodamine B was very stable during visible light illumination in the absence of a photocatalyst, suggesting that self-degradation was unlikely (Fig. 4a). BiOCl nanoflowers showed superior rhodamine B photodegradation to BiOCl nanosheets, eliminating 98.7% rhodamine B after 60 min of visible light irradiation, whereas BiOCl nanosheets degraded only 64.8% of rhodamine B in 60 min and 91.7% in 120 min. The corresponding rate constant of BiOCl nanoflowers was calculated to be approximately 0.0452 min−1 (Fig. 4b), which was 3.53 times higher than the rate constant of BiOCl nanosheets (0.0128 min−1).

To explore the photodegradation mechanism, optical characterization and trapping experiments were carried out for active species. As illustrated in Fig. S4, the absorption spectra of BiOCl nanoflowers and BiOCl nanosheets exhibited absorption edges at 358 and 360 nm, respectively. Accordingly, the band gap energies (Eg) of BiOCl nanoflowers and BiOCl nanosheets were about 3.46 and 3.44 eV, respectively (Fig. S5). These results indicate that the as-prepared BiOCl samples had wide band gaps incapable of being excited by visible light, and morphology had a negligible effect on the band gap width. Valence band X-ray photoelectron spectroscopy was used to locate the valence band (VB) of BiOCl samples. The estimated positions of the BiOCl nanoflowers and BiOCl nanosheets valence bands were at 2.14 and 2.02 eV, respectively (Fig. S6). From the empirical equation ECB = Eg − EVB, the position of BiOCl nanoflowers and BiOCl nanosheets conduction bands (CB) was estimated at − 1.32 and − 1.42 eV, respectively. The experimental values imply that electrons in the conduction band of BiOCl nanoflowers and BiOCl nanosheets could reduce the adsorbed O2 to produce ·O2− radicals, since the conduction band potential of BiOCl nanoflowers and BiOCl nanosheets was more negative than that of the O2/·O2− redox pair (−0.33 eV vs. NHE) (Cai et al. 2018).

In order to identify the principal photoactive species governing the photodegradation of rhodamine B, trapping experiments using scavengers were carried out. In these experiments, methanol, isopropyl alcohol, and p-benzoquinone were used to scavenge holes (h+), ·OH, and ·O2−, respectively. As illustrated in Fig. S7, the degradation efficiency of BiOCl nanoflowers was slightly lower in the presence of isopropyl alcohol and methanol, indicating that ·OH and h+ were minor active species. However, the degradation efficiency was almost completely inhibited upon adding p-benzoquinone, indicating that ·O2− was a dominant active species. Moreover, when the experiment was performed after purging dissolved oxygen in the solution using nitrogen, the efficiency of the system was significantly lower, further confirming that O2 played an important role in the sensitized degradation of dyes by BiOCl under visible light irradiation.

Lastly, a possible photosensitization degradation mechanism was proposed and schematically presented in Fig. S8. Electrons were easily excited from the highest occupied molecular orbital (HOMO) of rhodamine B to the lowest unoccupied molecular orbital (LUMO) of rhodamine B, producing the singlet and triplet excited states (rhodamine B*). Then, the excited electrons from rhodamine B were transferred to the conduction band of BiOCl (Chen et al. 2016, 2018). The electrons in the conduction band of BiOCl reacted with adsorbed O2 to generate ·O2− radicals which efficiently oxidized rhodamine B to harmless molecules.

Conclusion

In summary, 3D hierarchical BiOCl nanoflowers with exposed {001} facets were successfully synthesized via a novel, simple, and rapid water-assisted method. Compared with BiOCl nanosheets, the unique hierarchical morphology and exposed {001} facets of BiOCl nanoflowers markedly enhanced the adsorption and photodegradation properties. The BiOCl nanoflowers exhibited 3.31 times higher adsorption and 3.53 times superior photosensitized degradation of rhodamine B than that of BiOCl nanosheets under visible light irradiation. The advantages of its simple preparation and high photocatalytic activity give it promising applications in water treatment. In addition, this research has shed light on the photosensitized degradation behavior and has also provided an efficient method to synthesize other highly efficient bismuth oxyhalide photocatalysts.