Continuous synthesis of hedgehog-like Ag–ZnO nanoparticles in a two-stage microfluidic system

Abstract

Hedgehog-like Ag–ZnO nanoparticles (NPs) were successfully prepared in a continuous two-stage microfluidic system. The first stage was the synthesis of monodispersed triangular Ag nanoprisms by the reduction of AgNO₃ with NaBH₄ in the presence of trisodium citrate, H₂O₂ and sodium dodecyl sulfate. The second stage was the growth of Zn(OH)₂ nanorods on triangular Ag nanoprisms to form a hedgehog-like shape. Hedgehog-like Ag–ZnO NPs were finally obtained by the thermal decomposition of as-prepared hedgehog-like Ag–Zn(OH)₂ NPs. The structure, morphology and chemical state of the obtained samples were characterized by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. It was found that H₂O₂, the molar ratio of NaOH to Zn(NO₃)₂ and the flow rate ratio of AgNO₃ to Zn(NO₃)₂ played a crucial role in the preparation of hedgehog-like Ag–ZnO NPs. In addition, the photocatalytic activity of hedgehog-like Ag–ZnO NPs was evaluated in the degradation of methyl orange. The results showed that the photocatalytic activity was greatly enhanced by hedgehog-like Ag–ZnO NPs in comparison to pure ZnO and spherical-like Ag–ZnO NPs. The reaction rate constant of the hedgehog-like Ag–ZnO NPs was twelve times the value of pure ZnO and twice the value of spherical-like Ag–ZnO NPs.

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1. Introduction

As an important semiconductor with a direct wide band gap (3.3 eV) and high exciton binding energy (60 meV), ZnO has great potential for application in the fields of catalysis, sensing, medicine and piezoelectricity.^1,2 In particular, ZnO has been explored as an efficient photocatalyst for the degradation of organic compounds because of its high activity, low cost and environmental friendliness. Generally, the photocatalytic activity of ZnO is closely related to the amount of electron–hole pairs excited by the photons whose energy is equal to or higher than the band gap of ZnO. Unfortunately, the proton-induced electron–hole pairs usually recombine quickly and thus decrease the photocatalytic activity of ZnO. To solve this problem, considerable efforts have been made, and one of the most effective ways is to synthesize noble metal–ZnO heterostructures.^3–5 In a typical noble metal–ZnO heterostructure, the proton-induced electrons in the conduction band of ZnO can transfer to the noble metal due to the Schottky barrier formed at the noble metal–ZnO interface, which can prevent the electrons and holes from recombination. Therefore, the noble metal can act as an electron sink, improving the charge separation efficiency and thereby enhancing the photocatalytic activity.^6,7 In addition, doping amounts of noble metal on ZnO can also result in lattice defect and impurity energy level. Consequently, it can improve the efficiency of photons and expand the range of absorbance to visible light.

From the published literature dedicated to noble metal–ZnO heterostructure, various kinds of noble metal–ZnO nanostructures such as Au–ZnO and Pt–ZnO have been successfully prepared.^8–11 Among the noble metals, Ag is most widely used to modify ZnO due to its low cost and high efficiency. In the past decades, Ag–ZnO with different morphologies such as worm-like NPs, nanorods and core–shell NPs have been successfully developed.^12–15 The strategies for Ag–ZnO synthesis can be classified as three types: (1) the first step of the formation of ZnO followed by the deposition of Ag NPs on ZnO; (2) the first step of the formation of Ag seeds followed by the deposition of ZnO on Ag seeds; (3) one-pot synthesis of Ag–ZnO. In the first strategy, ZnO with different morphologies were firstly prepared and then Ag NPs were deposited on ZnO by photo-reduction or hydrothermal/solvothermal method. Zheng et al. presented a facile solvothermal method to form Ag–ZnO heterostructure where NaOH/ethanol was utilized as the solvent.¹⁶ Liang et al. reported the synthesis of porous 3D flower-like Ag–ZnO heterostructure by two-steps, namely hydrothermal and photo-chemical deposition methods.¹⁷ This 3D flower-like Ag–ZnO heterostructure showed an obviously improved activity in comparison with pure ZnO in the degradation of Rhodamine B. Chamjangali et al. prepared Ag–ZnO multipodes via a solution route/photo-reduction process and used them as an efficient photocatalyst in simultaneous methylene blue and methyl orange degradation.¹⁴ In the second strategy, Ag seeds were firstly prepared by reducing the precursors containing Ag⁺ ions with reducing agents in the presence of stabilizers such as PVP and trisodium citrate. Subsequently, ZnO were deposited on Ag seeds by precipitation or hydrothermal method. Mondragón-Galicia et al. synthesized unidirectional Ag–ZnO nanostructured brushes on the basis of the use of unidimensional Ag nanowires as precursors.¹⁸ Yin et al. prepared Ag nanoparticle/ZnO nanorods nanocomposites with a core/shell structure via a seed-mediated method.¹³ In their study, Ag NPs were firstly synthesized by a polyol synthesis and used as seeds for further growth of ZnO nanorods. To simplify the process for Ag–ZnO synthesis, one-pot synthesis was proposed. Lu et al. demonstrated the synthesis of 3D hollow microspheres Ag–ZnO NPs assisted by sodium alginate via one-pot method.¹⁹ Huang et al. reported a one-pot synthesis of branched Ag–ZnO heterojunction nanostructures with Ag NPs of 500 nm as cores.²⁰ The branched Ag–ZnO heterojunction nanostructures showed a superior photocatalytic activity over the branched ZnO. Obviously, great efforts have been put in and huge advances have been obtained so far. However, there is still plenty of room to optimize the process for Ag–ZnO synthesis. Conventionally, the preparation of Ag–ZnO including the literatures mentioned above is carried out in a batch reactor. As is well known, it is difficult for batch reactors to precisely control the NPs' particle size distribution and morphology due to the limitation of mixing and heat transfer. Additionally, the discontinuous operation of the batch process always results in a low productivity and poor reproducibility, which limits the large-scale production of nanoparticles.

Recently, microreactor has attracted much attention in the synthesis of nanoparticles, such as CdSe, PdS, Ag, TiO₂, CaCO₃ and layered double hydroxides, etc.^21–26 Additionally, some hybrid nanoparticles have been successfully synthesized in the microfluidic system including Ag–Zn₂GeO₄ nanowires, FeAl@Al_(1−x)Fe_xO_y and CoZn@Zn_(1−x)Co_xO_y, etc.^27–29 Microreactor has been considered as a potential tool to overcome the limitations associated with batch reactors in the nanoparticle synthesis. In comparison with the conventional batch reactor, microreactor has a plenty of advantages such as large surface-to-volume ratio, high heat and mass transfer rate, etc.^30–32 Therefore, microreactor can offer a uniform reaction condition which makes the NP's size and shape more controllable. In addition, the continuous operation and numbering-up of microreactor are both beneficial for large-scale production of nanoparticles. In this paper, a facile and efficient method for continuous synthesis of hedgehog-like Ag–ZnO NPs in a microfluidic system was proposed. Hedgehog-like Ag–ZnO NPs were successfully obtained by a rational design and precise parameter control. To the best of our knowledge, no microfluidic-based method has been reported for the preparation of Ag–ZnO heterostructure.

2. Experimental

2.1. Materials

Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O), sodium hydrate (NaOH), silver nitrate (AgNO₃), sodium borohydride (NaBH₄), sodium dodecyl sulfate (C₁₂H₂₅SO₄Na, SDS), trisodium citrate (Na₃C₆H₅O₇·2H₂O), hydrogen peroxide (H₂O₂) and methyl orange (C₁₄H₁₄N₃SO₃Na, MO) were purchased with available purity. All chemicals were used as received without further treatment. Deionized water was used in all experiments.

2.2. Synthesis of Ag–ZnO NPs

Ag–ZnO NPs were synthesized in a continuous two-stage microfluidic system. As shown in Fig. 1, the two-stage microfluidic system was comprised of three consecutive microreactors (T-mixer, spiral microreactor and cross-type mixer), which were all fabricated by polytetrafluoroethylene. The geometric dimensions of different microreactors were summarized in Table 1.

Fig. 1 Experimental arrangements used for the continuous preparation of Ag–ZnO NPs.

Table 1 Geometric dimensions of different microreactors

Microreactor	Cross section	Channel size/mm (inner diameter × length)
T-mixer	Circle	0.6 × 10
Spiral microreactor		0.6 × 4000
Cross-type mixer		0.6 × 710

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In the first stage, monodispersed triangular Ag nanoprisms were prepared in T-mixer and spiral microreactor following a modified protocol reported by Carboni et al.³³ AgNO₃ and NaBH₄ were used as the precursor and reducing agent, respectively. In a typical experiment, two aqueous solutions were prepared firstly. One was 1 mmol L⁻¹ NaBH₄ solution with fixed pH (10–12) which was adjusted by adding adequate NaOH. The other was made by adding AgNO₃, trisodium citrate, H₂O₂ and SDS into deionized water. The concentration of AgNO₃, trisodium citrate, H₂O₂ and SDS was 0.5 mmol L⁻¹, 0.2 mmol L⁻¹, 80 mmol L⁻¹ and 10 mmol L⁻¹, respectively. The two as-prepared solutions were continuously fed into the two-stage microfluidic system with the same flow rate (V_AgNO3) by two syringe pumps.

In the second stage, the obtained suspension containing triangular Ag nanoprisms was directly fed into cross-type mixer. At the same time, Zn(NO₃)₂ aqueous solution and NaOH aqueous solution were injected into cross-type mixer with the same flow rate (V_Zn(NO3)2). The concentration of Zn(NO₃)₂ was 50 mmol L⁻¹, and the molar ratio of NaOH to Zn(NO₃)₂ varied from 2 : 1 to 20 : 1. The two stages mentioned above were both carried out at room temperature. The precipitation out of the cross-type mixer was collected by centrifugation and washed three times with deionized water and ethanol alternately. Ag–ZnO NPs were finally obtained by drying the precipitation at 100 °C for 5 h. For comparison, the precipitation was also freezing-dried and the obtained sample was denoted as Ag–Zn(OH)₂ NPs. Additionally, pure ZnO was also prepared following the same protocol in this two-stage microfluidic system. The NaBH₄ and AgNO₃ solutions were replaced by deionized water. ZnO was synthesized under a condition of a molar ratio of NaOH to Zn(NO₃)₂ equal to 2 : 1.

2.3. Characterizations

The powder X-ray diffraction patterns (XRD) of the as-prepared samples were recorded by an X-ray diffractometer (X'pert Pro) at a scanning rate of 5° min⁻¹ for 2θ ranging from 30° to 90°. The morphologies of the samples were characterized by transmission electron microscopy (JEOLJEM-2000EX) with the accelerating voltage of 120 kV. X-ray photoelectron spectroscopy (XPS) analysis was performed on an ESCALAB 250Xi system, using Al Ka radiation as the X-ray source, which was used to analyze elemental and chemical states of the samples. Inductively coupled plasma-optical emission spectroscopy (ICP-OES, PerkinElmer ICP-OES 7300DV) was employed to measure the concentration of Zn²⁺ ions in the aqueous solution.

2.4. Photocatalytic activity test

The photocatalytic performance of as-prepared Ag–ZnO NPs was evaluated by using MO as presentative dye pollutant. A 300 W Xenon lamp (Ushio-CERMAXLX300) was used as the visible light source. In a typical procedure, 30 mg catalyst was dispersed into 100 mL MO solution with the concentration of 20 mg L⁻¹. After stirring in the dark for 3 h, the suspensions were placed under the xenon lamp with a constant stirring rate. During the photoreaction, the samples were withdrawn and the photocatalyst was separated from the suspension by centrifuging. The residual MO in the solution was measured by the UV-vis spectroscopy (UV4802) to evaluate the photocatalytic degradation process.

3. Results and discussion

3.1. The characterization results of hedgehog-like Ag–ZnO NPs

According to the synthesis conditions of the hedgehog-like Ag–ZnO NPs, the residence time of the solution in T-mixer, spiral microreactor and cross-type mixer was calculated to be 0.2 s, 67.8 s and 7.8 s, respectively. The total residence time was equal to 75.8 s, while the conventional batch method always took more than one hour to prepare Ag–ZnO NPs with different shapes. This indicated that the continuous synthesis of Ag–ZnO NPs in the two-stage microfluidic system was highly efficient. Fig. 2 depicts the XRD patterns of hedgehog-like Ag–Zn(OH)₂ NPs and Ag–ZnO NPs. Two crystalline phases of Zn(OH)₂ and ZnO are detected in the XRD pattern of Ag–Zn(OH)₂ NPs. It can be observed that the intensity of the diffraction peaks of ZnO is much smaller than that of Zn(OH)₂. This indicates that Ag–Zn(OH)₂ NPs are mainly consisted of Zn(OH)₂. The existence of Ag cannot be identified due to the overlapping of the characteristic peaks of Zn(OH)₂ and Ag at 38.384°, 44.202°, 64.482°, 77.264° and 81.359°. For Ag–ZnO NPs, the diffraction peaks of Ag are clearly observed. In addition, the diffraction peaks at 31.772°, 34.420° and 36.256° can be indexed as the characteristic diffraction peaks of ZnO. No diffraction peaks corresponding to Zn(OH)₂ are observed. This implies that Zn(OH)₂ can be totally decomposed into ZnO at 100 °C. To visually observe the morphology of the obtained samples, transmission electron microscopy (TEM) was performed. Fig. 3 shows the TEM images of Ag seeds, hedgehog-like Ag–Zn(OH)₂ NPs and Ag–ZnO NPs. As shown in Fig. 3A, Ag seeds are mainly consist of triangular Ag nanoprisms with tip sniping. These triangular Ag nanoprisms are uniformly dispersed with an edge length between 20 nm and 50 nm. Ag–Zn(OH)₂ NPs and Ag–ZnO NPs both exhibit a hedgehog-like morphology with an average size of 500 nm (Fig. 3B and C). Each individual nanoparticle is composed of several branches, and the size of each branch is 100 nm approximately. Obviously, the obtained Ag–ZnO NPs maintained the shape of Ag–Zn(OH)₂ NPs, indicating that this hedgehog-like morphology was stable in the thermal decomposition of Zn(OH)₂ to ZnO. A similar morphology was also obtained in the studies of Huang et al.²⁰ and Fan et al.³⁴ They suggested that ZnO nanorods preferred to grow on the basis of Ag (1 1 1) facets because of the good lattice and symmetry match between ZnO and Ag in the corresponding planes. As discussed above, hedgehog-like Ag–Zn(OH)₂ NPs and Ag–ZnO NPs were both successfully obtained in this study. It was well known that Ag nanoprisms were oriented along the (1 1 1) facets. Therefore, it can be concluded that Zn(OH)₂ nanorods besides ZnO nanorods can also grow on the (1 1 1) facets of triangular Ag nanoprisms.

Fig. 2 XRD patterns of hedgehog-like (A) Ag–Zn(OH)₂ NPs (B) Ag–ZnO NPs. 2V_AgNO3 = V_Zn(NO3)2 = 1.0 mL min⁻¹, molar ratio of NaOH/Zn(NO₃)₂ = 20 : 1.

Fig. 3 TEM images of (A) Ag seeds (B) hedgehog-like Ag–Zn(OH)₂ NPs (C) hedgehog-like Ag–ZnO NPs. 2V_AgNO3 = V_Zn(NO3)2 = 1.0 mL min⁻¹, molar ratio of NaOH/Zn(NO₃)₂ = 20 : 1.

The XPS analysis was carried out to investigate the surface composition and chemical state of the as-prepared hedgehog-like Ag–ZnO NPs. Fig. 4A represents the scan survey spectra for the hedgehog-like Ag–ZnO NPs. All of the peaks on the curve can be attributed to Ag, Zn, O and C elements. The presence of C element is mainly derived from the hydrocarbon contaminants inherently existing in the XPS survey. It can be further confirmed that the sample is merely composed of Ag, Zn and O elements, which is in good agreement with the XRD results. The high-resolution spectrum of Ag element in the hedgehog-like Ag–ZnO NPs is presented in Fig. 4B. The two peaks centered at 365.1 eV and 371.0 eV can be ascribed to Ag 3d_5/2 and Ag 3d_3/2, respectively. It can be seen that there is a splitting of 5.9 eV between Ag 3d_5/2 and Ag 3d_3/2, demonstrating the metallic nature of Ag element. In comparison with the bulk Ag (the standard binding energies for Ag 3d_5/2 and Ag 3d_3/2 are 368.2 eV and 374.2 eV, respectively), the binding energies of Ag 3d_5/2 and Ag 3d_3/2 shift to the lower value remarkably, indicating a smaller electron density of Ag in hedgehog-like Ag–ZnO NPs than that of bulk Ag. This phenomenon can be attributed to the strong interaction between Ag and ZnO.^35,36 Since the work function of ZnO was higher than that of Ag, the electrons would transfer from Ag to ZnO the moment Ag interacted with ZnO to equilibrate the Fermi level. As shown in Fig. 4C, the binding energies of Zn 2p_1/2 and Zn 2p_3/2 are 1019.3 eV and 1042.2 eV, respectively. These values are similar to those of pure ZnO,³⁷ suggesting that Zn element mainly exists as Zn²⁺ ions. As Fig. 4D showed, the XPS peak of O 1s is asymmetric, indicating that there are two kinds of oxygen species on the sample surface. The peaks centered at 528.0 eV and 529.6 eV can be ascribed to the lattice oxygen of ZnO and oxygen of the hydroxyl species chemisorbed on the surface.³⁸ The hydroxyl species were found to be of crucial importance for the high efficiency of photocatalysis. They can not only trap the photoinduced electrons and holes but also generate the active hydroxyl radicals.^39–42

Fig. 4 (A) XPS survey spectrum of the as-prepared hedgehog-like Ag–ZnO NPs (B) Ag 3d spectra (C) Zn 2p spectra and (D) O 1s spectra.

3.2. The effects of the operation parameters

3.2.1. H₂O₂. To validate the effect of H₂O₂, Ag seeds and Ag–ZnO NPs were prepared without H₂O₂, while other conditions remain unchanged. The TEM images of Ag seeds and Ag–ZnO NPs synthesized in the absence of H₂O₂ are depicted in Fig. 5. It can be clearly seen that no triangular Ag nanoprisms but Ag nanospheres are obtained in the absence of H₂O₂. Correspondingly, Ag–ZnO NPs exhibit spherical-like shape. A similar result was also found by Huang et al. in their study.²⁰ Obviously, H₂O₂ played a crucial role not only in the preparation of Ag triangular nanoprisms but also hedgehog-like Ag–ZnO NPs. It was acknowledged that H₂O₂ was essential for the synthesis of triangular Ag nanoprisms. H₂O₂ could remove the less stable Ag seeds and induce to generate Ag seeds with stacking faults which finally grew into triangular Ag nanoprisms. In the absence of H₂O₂, Ag⁺ ions were prone to grow into Ag nanospheres by the reduction of NaBH₄ in the presence of trisodium citrate and SDS. For the synthesis of hedgehog-like Ag–ZnO NPs, it was supposed that H₂O₂ also worked as an etchant and helped produce triangular Ag nanoprisms with stacking faults, which were beneficial for the growth of Zn(OH)₂ nanorods.

Fig. 5 TEM images of (A) Ag seeds (B) Ag–ZnO NPs. 2V_AgNO3 = V_Zn(NO3)2 = 1.0 mL min⁻¹, molar ratio of NaOH to Zn(NO₃)₂ = 20 : 1 and no H₂O₂ was used.

3.2.2. The molar ratio of NaOH to Zn(NO₃)₂. The effect of the molar ratio of NaOH to Zn(NO₃)₂ on the synthesis of hedgehog-like Ag–ZnO NPs was also investigated. It can be seen from Fig. 6 that when the molar ratio of NaOH to Zn(NO₃)₂ equals to the stoichiometric ratio of 2, irregular aggregated Ag–ZnO NPs are obtained. As the molar ratio of NaOH to Zn(NO₃)₂ increases to 15 : 1 and 20 : 1, irregular aggregated Ag–ZnO NPs disappear and hedgehog-like Ag–ZnO NPs are found. It is evident that the molar ratio of NaOH to Zn(NO₃)₂ plays a crucial role in the synthesis of hedgehog-like Ag–ZnO NPs. Generally, the morphology of nanoparticles was dependent on the nucleation and growth kinetics. When the molar ratio of NaOH to Zn(NO₃)₂ was equal to 2, a large number of Zn(OH)₂ nuclei were formed once NaOH and Zn(NO₃)₂ contacted. These Zn(OH)₂ nuclei grew into large particles and agglomerated gradually. Thus, irregular aggregated Ag–ZnO NPs were finally obtained. When the molar ratio of NaOH to Zn(NO₃)₂ was much larger than 2, Zn(OH)₂ quickly dissolved into the solution containing excess NaOH to form Zn(OH)₄²⁻. In previous studies, ZnO with different morphologies were successfully synthesized with the molar ratio of NaOH to Zn(NO₃)₂ much larger than 2. Zn(OH)₄²⁻ could be transformed to ZnO under the hydrothermal conditions. For example, Li et al. synthesized one-dimensional needle-like ZnO nanowhiskers by aging a solution of Zn(OH)₄²⁻ and SDS at 85 °C for 5 h.⁴³ Ge et al. reported that sisal-like 3D ZnO nanostructures were fabricated via the assemble of CTA⁺ (C₁₆H₃₃(CH₃)₃N⁺) and Zn(OH)₄²⁻ in the temperature range from 120 to 220 °C.⁴⁴ In this study, a high molar ratio of NaOH to Zn(NO₃)₂ was also found to be essential for the synthesis of hedgehog-like Ag–ZnO NPs. Taking the molar ratio of NaOH to Zn(NO₃)₂ of 20 as an example, Zn(OH)₂ was the major component of the freeze-dried sample according to the XRD results. Evidently, this phenomenon was interesting because Zn(OH)₂ could not exist at this strong basic condition. To investigate the mechanism of the transformation of Zn(OH)₄²⁻ to Zn(OH)₂, a transparent solution containing Zn(OH)₄²⁻ was prepared by adding Zn(NO₃)₂ into NaOH aqueous solution with the molar ratio of NaOH to Zn(NO₃)₂ of 20. Trisodium citrate, SDS and NaBH₄ were added into this transparent solution respectively and no precipitant was found, implying that the transformation of Zn(OH)₄²⁻ to Zn(OH)₂ was closely related to the presence of Ag seeds. The suspension out of the cross-type mixer was directly centrifuged, and the concentration of Zn²⁺ ions in the supernatant liquid was measured by ICP-OES. The concentration of Zn²⁺ ions in the supernatant liquid was 2.72 mmol L⁻¹, which was equal to 5% of initial concentration of Zn²⁺ ions. This implied that most Zn²⁺ ions existed in the form of Zn(OH)₂. Unfortunately, the reason why Ag seeds could result in the transformation of Zn(OH)₄²⁻ to Zn(OH)₂ was not clear and the relative study was under way. In addition, ZnO was also found in the freeze-dried sample out of the cross-type mixer. As is well known, Zn(OH)₂ can transform to ZnO under strong basic condition even at room temperature.⁴⁵ Therefore, it was anticipated that a small portion of Zn(OH)₂ at the outer surface of the nanoparticles transformed to ZnO and thus protected Zn(OH)₂ from being dissolved under this strong basic condition.

Fig. 6 TEM images of Ag–ZnO NPs synthesized at different molar ratio of NaOH to Zn(NO₃)₂ (A) 2 : 1 (B) 5 : 1 (C) 15 : 1 (D) 20 : 1. 2V_AgNO3 = V_Zn(NO3)2 = 1 mL min⁻¹.

3.2.3. The effect of the flow rate ratio of Ag(NO₃)₂ to Zn(NO₃)₂. For the two-stage microfluidic system, the Ag content can be easily adjusted by changing the flow rate ratio of Ag(NO₃)₂ to Zn(NO₃)₂. Therefore, it was very necessary to investigate the effect of the flow rate ratio of Ag(NO₃)₂ to Zn(NO₃)₂ on the morphology of Ag–ZnO NPs. The TEM images of Ag–ZnO NPs synthesized at different flow rate ratio of Ag(NO₃)₂ to Zn(NO₃)₂ are shown in Fig. 7. As shown in Fig. 7, hedgehog-like Ag–ZnO can be successfully prepared with the flow rate ratio of Ag(NO₃)₂ to Zn(NO₃)₂ increasing from 0.5 : 1 to 1 : 1. In addition, the width of branches increases to some extent with the increase in the flow rate ratio of Ag(NO₃)₂ to Zn(NO₃)₂.

Fig. 7 TEM images of Ag–ZnO NPs synthesized at different flow rate ratio of Ag(NO₃)₂ to Zn(NO₃)₂ (A) 0.5 : 1 (B) 0.75 : 1 (C) 1 : 1. Molar ratio of NaOH/Zn(NO₃)₂ = 20 : 1.

3.3. Photocatalytic activity test

In order to examine the photocatalytic performance of hedgehog-like Ag–ZnO NPs, the photocatalytic degradation of MO as representative experiment was launched. It is well known that MO is widely used in the textile industry, which is a typical organic pollutant and always discharged into the industrial effluent. Herein, the activity of hedgehog-like Ag–ZnO NPs (prepared in Section 3.1), spherical-like Ag–ZnO NPs (prepared in Section 3.2.1) and pure ZnO for the photocatalytic degradation of MO were compared and the results are shown in Fig. 8, where C represents the concentration of MO remaining in the solution after irradiation and C₀ represents the initial concentration of MO. It can be seen that pure ZnO exhibits a low photocatalytic activity for the degradation of MO under this condition. Only 10% MO is degraded within 90 min. The addition of Ag is found to significantly improve the performance of pure ZnO. Nearly 50% and 70% MO are degraded within 90 min over the spherical-like Ag–ZnO NPs and hedgehog-like Ag–ZnO NPs, respectively. Noticeably, the hedgehog-like Ag–ZnO NPs show the highest photocatalytic activity. Generally, the photocatalytic degradation kinetic equation could be described as

ln(C₀/C) = kt (1)

where k and t represent the pseudo-first-rate kinetic constant and irradiation time, respectively. Therefore, the slope of linear curve in Fig. 8B could be considered as the reaction rate constant. As summarized in Table 2, the reaction rate constant of the hedgehog-like Ag–ZnO NPs is twelve times the value of pure ZnO and twice the value of spherical-like Ag–ZnO NPs. So far, numerous researches have demonstrated that Ag–ZnO heterostructure showed a superior photocatalytic activity than ZnO by inhibiting the recombination of the photoinduced electrons and holes. The better performance of spherical-like Ag–ZnO NPs and hedgehog-like Ag–ZnO NPs in this study could be ascribed to the similar reason. Furthermore, the hedgehog-like Ag–ZnO NPs could offer a much larger opportunities for the interaction of MO to ZnO than spherical-like Ag–ZnO NPs. As a consequence, the catalytic activity of hedgehog-like Ag–ZnO NPs was higher than that of spherical-like Ag–ZnO NPs.

Fig. 8 (A) Photocatalytic activity and (B) kinetics of the as-prepared hedgehog-like Ag–ZnO NPs, spherical-like Ag–ZnO NPs and pure ZnO for the degradation of MO under visible light irradiation.

Table 2 Reaction rate constant (k) for the photocatalytic degradation of MO under visible light irradiation

Catalyst	Blank	ZnO	Spherical-like Ag–ZnO NPs	Hedgehog-like Ag–ZnO NPs
k (min^−1)	6.7 × 10^−4	1.0 × 10^−3	6.4 × 10^−3	1.2 × 10^−2

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The mechanism of the photocatalytic degradation of MO over hedgehog-like Ag–ZnO NPs is presented in Fig. 9. Under the visible light irradiation, ZnO could be photoactivated, and the electrons in the valence band (VB) could be excited to the conduction band (CB) of ZnO. Meanwhile, there left the same amount of holes in the VB of ZnO. Since the bottom energy level of the CB of ZnO was higher than the new equilibrium Fermi energy level of Ag–ZnO, the photoinduced electrons in the CB of ZnO transferred to the metallic Ag NPs. As a result, the recombination of electrons and holes was inhibited which prolonged the lifetime of photo-electron pairs. During the photocatalytic degradation of MO, the electrons in the CB of ZnO or trapped by Ag NPs could be captured by the adsorbed oxygen on the surface to form superoxide radicals anion (˙O₂⁻), and the holes left in the VB of ZnO could be trapped by the surface hydroxyl to form the hydroxyl radicals (˙OH) which was an extraordinary strong oxidant for the degradation of MO.

Fig. 9 Schematic diagram of the electron–hole pairs separation and catalysis mechanism of hedgehog-like Ag–ZnO NPs under visible light irradiation.

4. Conclusion

In summary, hedgehog-like Ag–ZnO NPs were continuously synthesized in a two-stage microfluidic system for the first time. It was found that H₂O₂, the molar ratio of NaOH to Zn(NO₃)₂ and the flow rate ratio of Ag(NO₃)₂ to Zn(NO₃)₂ had a significant effect on the synthesis of hedgehog-like Ag–ZnO NPs. Furthermore, hedgehog-like Ag–ZnO NPs was found to exhibit a better photocatalytic activity than spherical-like Ag–ZnO NPs and pure ZnO in the degradation of MO, which could be ascribed to the unique morphology as well as the strong interaction between Ag and ZnO. In addition, the mechanism of photocatalytic degradation of MO over hedgehog-like Ag–ZnO NPs was proposed. This brand-new method on basis of the microfluidic technology was continuous, highly efficient and easy to scale up.

Acknowledgements

The authors gratefully acknowledge the financial support from National Natural Science Foundation of China (No. 21406226, 21225627).

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