Background

SnO2 is an important n-type semiconductor with large bandgap (Eg = 3.6 eV, at 300 K), high electron mobility, and low cost, which enable it with outstanding properties in gas sensing [1], lithium ion batteries [2], optoelectronic devices, and dye sensitized solar cells [3,4,5,6,7,8]. In the past two decades, the robust SnO2 material has garnered considerable attention and various nanostructures have been reported [9, 10]. Among which, three-dimensional (3D) hierarchical structures self-assembled by nanosheets building blocks are much more interesting due to their special structure and fascinating properties [11, 12]. Nevertheless, there are only a few reports on the catalytic performance of SnO2 and the catalytic efficiency is relatively low [13,14,15]. It is thus important to synthesize hierarchical SnO2 structures and study the catalytic performance. Especially, as we know, noble metal nanoparticles (NPs) such as Au-, Ag-, Pt-, and Pd-modified 3D hierarchical structures will show much enhanced catalytic performance [16]. However, most of the processes of the syntheses of the above noble metal-modified oxides are more complicated multi-step process and usually toxic and harmful environmentally [17]. So developing facile and efficient methods to fabricate noble metal NP-modified hierarchical SnO2 and studying the catalytic performance are highly desirable.

Increased contamination of our limited water resources owing to the widespread dispersion of various industrial dyes, heavy metal ions, and other aromatic pollutants are endangering our planet [18]. The 4-nitrophenol (4-NP), a well-known toxic pollutant, is widely present in industrial effluents and agriculture wastewater [19]. Among various treatment techniques, such as membrane filtration [20], photo degradation [21], adsorption [22], and chemical reduction [23,24,25,26,27,28,29,30], the chemical reduction of 4-NP to 4-aminophenol (4-AP) is a favorable route, owing to the product (4-AP) which is an important intermediate for the manufacture of analgesic and antipyretic drugs, photographic developer, corrosion inhibitor, anticorrosion lubricant, and hair-dyeing agent [31, 32]. Hence, the reduction of 4-NP to 4-AP possesses great significance for the pollution abatement and resource regeneration [33].

In this paper, we reported a green synthesis of noble metal Ag nanoparticle (NP)-modified SnO2 hierarchical architectures by a simple one-pot hydrothermal route without the assistant of any templates and surfactants at mild temperature. The effects of reaction time on morphologies of Ag-decorated SnO2 microsphere were investigated, and a possible growth mechanism of Ag-decorated SnO2 hierarchical structures was proposed. The catalytic results indicate the as-synthesized products exhibit excellent catalytic performance for the reduction of 4-NP to 4-AP, with normalized rate constant (κ nor) of 6.20 min−1g−1L. In addition, the Ag-decorated SnO2 hierarchical structures sustain high catalytic efficiency in ten cycles and show stability after the first five cycles. This obtained Ag-decorated SnO2 hierarchical structures may have potential applications of water contaminant treatment, and this simple one-step hydrothermal route could be extended to design other noble metal NP-modified composite with a wide range of practical applications for the future.

Methods

Materials

Silver nitrate (AgNO3, 99.8%), urea (CO(NH2)2, 99%), ammonia solution (NH3·H2O, 25~28%), and potassium borohydride (KBH4, 97%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Sodium stannate rehydrate (Na2SnO3·3H2O, 98%) and 4-nitrophenol(C6H5NO3, 98%) were supplied by Aladdin Reagent Co. Ltd. All the materials were used without further purification.

Synthesis of Ag-Decorated SnO2 Microsphere

Ag-decorated SnO2 powder (mole ratio of Ag:SnO2 = 1:1) was synthesized by one-pot hydrothermal method. In a typical procedure, 2.67 g of sodium stannate rehydrates and 0.2 g of urea were dissolved in 25 mL of ultra-pure water and stirred vigorously for 30 min to form a mixture. Then, 1.69 g of silver nitrate was dispersed in 25 mL of ultra-pure water, and then, 2.4 mL ammonium hydroxide was added into the silver nitrate solution to form silver–ammonia solution. After stirring for 5 min, the freshly prepared silver–ammonia solution was added into the mixture under magnetic stirring for 1 h. Subsequently, the resulting mixture was migrated into a 50-mL Teflon-lined autoclave and heated at 150 °C for 5, 10, 24, and 36 h. After the hydrothermal procedure, the autoclave was cooled down naturally to room temperature and the SnO2/Ag product was collected by centrifugation, followed by rinsing with deionized water and ethanol and drying in a vacuum oven at 60 °C. SnO2/Ag microsphere with different mole ratios (1.5:1, 1:1, 0.5:1, 0.01:1) of Ag to SnO2 are synthesized in a similar way except for the amounts of AgNO3 and NH3·H2O. For comparison, pure SnO2 and Ag were also synthesized by the similar procedure without the addition of AgNO3 and Na2SnO3.

Sample Characterizations

The crystalline phase of the as-prepared samples were investigated by X-ray powder diffraction (XRD, Cu Kα radiation (λ = 1.5418 Å)). The scanning electron microscopy (SEM) measurements were performed on a SU-70 field emission SEM microscope with an acceleration voltage of 20 kV. Transmission electron micrograph (TEM) and high-resolution transmission electron microscopy (HRTEM) were taken on a Tecnai G2 F20 S-TWIN transmission electron microscope with an accelerating voltage of 200 kV. X-ray photo-electron spectroscopy (XPS) was performed to identify surface chemical composition and chemical states of the catalysts on a MARK II X-ray photoelectron spectrometer using Mg Kα radiation. The specific surface area of sample was evaluated by the Langmuir model and Brunauer–Emmett–Teller (BET) model based on the nitrogen adsorption isotherm obtained with a V-sorb X2008 series, while the pore size distribution was estimated by Barrett–Joyner–Halenda (BJH) theory.

Catalytic Activity of Ag-Decorated SnO2 Microsphere

The reduction of 4-NP with KBH4 solution was used as a model reaction to study the catalytic activity of Ag-decorated SnO2 composites. The catalytic reduction process was carried out in a standard quartz cell with a 1-cm path length and about 4 mL volume with 0.3 mL of freshly prepared aqueous solutions of 4-NP (20 mg/L) and KBH4 (1.5 mg). The high molar ratio of KBH4 to 4-NP ensured an excess amount of the former, and hence, its concentration remained essentially constant during the reduction reaction. Upon the addition of KBH4 into the 4-NP solution, its color changed immediately from light yellow to dark yellow due to the formation of 4-nitrophenolate ion (formed from the high alkalinity of KBH4). Later, the dark yellow color faded with time (due to the conversion of 4-NP to 4-AP) after the addition of 1.5 mg of Ag-decorated SnO2 hybrids. The UV–Vis absorption spectra were recorded by an UV–Vis spectrometer in a scanning range of 250–500 nm at room temperature at time interval of 1 min. Several consecutive reaction rounds were measured to determine the stability of the catalyst.

Results and Discussion

Characterization of Ag-Decorated SnO2 Microsphere

The composition and phase structure of the synthesized Ag-decorated SnO2 powders for different times were investigated by XRD, and the corresponding patterns are shown in Fig. 1. It can be seen that the characteristic diffraction peaks match well with the tetragonal rutile phase SnO2 (JCPDS file no. 41-1445, a = 4.738Å and c = 3.187 Å) and face centered cubic (fcc) phase Ag (JCPDS file no. 04-0783). No diffraction peaks from any other impurities were detected indicating that the powders are the mixture of pure SnO2 and Ag. For the sample reacted for 5 h, the characteristic diffraction peaks at 38.12° and 44.2°, corresponding to the (111) and (200) planes of Ag, are relatively weak. With increasing hydrothermal time, the peak intensities of Ag increase and the full widths of diffraction peak decrease as well, indicating the enhanced crystallinity of Ag nanoparticles or the increased weight of Ag. This can be further verified by the XRD patterns of the samples obtained under different temperatures and different mole ratios of Ag and SnO2 (Additional file 1: Figure S1).

Fig. 1
figure 1

XRD patterns of the Ag-decorated SnO2 microspheres prepared at 150 °C for different times (a) 5 h, (b) 10 h, (c) 24 h, and (d) 36 h

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The SEM images in Fig. 2 show the interesting morphological evolution of samples prepared at different hydrothermal times from 5 to 36 h. Sample prepared for 5 h was irregular microsphere, and the enlarged view of the surface of microspheres in the inset illustrating the microsphere is assembled by nanoparticles (Fig. 2a). With increasing hydrothermal time, the microsphere became more regular. Upon the hydrothermal time increased to 24 h (Fig. 2c), the microsphere grew larger at the expense of the smaller nanoparticles and the surface nanoparticles self-assembled into nanosheets. These nanosheets assembled to form a hierarchical microsphere structure. When further increasing the hydrothermal time to 36 h, the coarse nanosheets became smoothed and the microspheres with diameters ranging from 2 to 4 μm are more uniform. Further increase of the hydrothermal time led no obvious change of the morphology and crystalline (not shown in this paper). The morphology of the sample prepared for 36 h was further observed via TEM and HRTEM. As shown in Fig. 2e, the obtained SnO2/Ag is of microsphere morphology with diameter of ~5 μm and the microsphere is assembled from nanosheets. In the typical HRTEM image (Fig. 2f), Ag NPs with an average size of about 5 nm were formed and homogeneously distributed to SnO2. The lattice fringes of d = 0.26 nm spacing can be assigned to the Ag (111) planes while the lattice fringes of d = 0.33 nm can be assigned to the (110) plane of SnO2, respectively. To further illustrate the uniform distributions of Ag nanoparticles in the microsphere, element mapping analysis of the SnO2/Ag microsphere was performed (Fig. 3). As shown in the Fig. 3, the map of Ag, Sn, and O elements are fit into the sample morphology, indicating that Ag nanoparticles are uniformly dispersed in the microspheres.

Fig. 2
figure 2

Representative FESEM images and TEM images of the Ag-decorated SnO2 microspheres prepared at 150 °C for different hydrothermal times a 5h, b 10h, c 24h, and d 36h and e, f low-magnification TEM image and high-resolution TEM (HRTEM) of the sample prepared for 36 h

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Fig. 3
figure 3

EDS element mapping of SnO2/Ag microspheres. a SEM image and element maps of b Sn, c O, and d Ag

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The N2 adsorption–desorption isotherms of samples and their corresponding pore size distribution are illustrated in Fig. 4. All of the samples exhibited type IV isotherms with H3 hysteresis loop, signifying typical mesoporous structures of uniform pore size [34]. The BET-specific surface areas were determined as 21.8, 22.4, 24.6. and 25.7 m2 g−1, respectively. Inset depicts the pore size distributions of samples. The pore size distribution is mono-modal for all the samples. The average pore diameter is ~2 nm for the as-hierarchical Ag-decorated SnO2 powders. It is noted that the calculated BET surface area and mean pore diameter has no obvious change with increasing hydrothermal time.

Fig. 4
figure 4

Typical nitrogen adsorption–desorption isotherm of the prepared SnO2/Ag microspheres prepared at 150 °C for different hydrothermal times a 5 h, b 10 h, c 24 h, and d 36 h

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XPS was used to examine the chemical states and surface composition of Ag-decorated SnO2 microspheres. Wide survey scans were recorded first followed by a detailed scanning of the edges of each element such as Sn 3d, Ag 3d, and O 1s (Fig. 5). It may be mentioned that the charging effect on the sample was corrected by setting the binding energy of the carbon (C 1s) at 284.6 eV and this carbon peak was used as a reference position for scaling all other peaks. As shown in Fig. 5b, the peak appears as a spin–orbit doublet at 369.1 eV (Ag 3d5/2) and 375.2 eV (Ag 3d3/2) for Ag0 [35, 36] in the product. The two satellite peaks at 366.5 and 372.3 eV can be account for Ag 3d in Ag-decorated SnO2 nanocomposites [37]. Furthermore, two XPS peaks located at 488 and 496.7 eV are relevant to Sn 3d5/2 and Sn 3d3/2, indicating the presence of Sn4+ in SnO2. And the peaks around 485.7 and 494.7 eV may be caused by the binding between Sn and Ag [38, 39]. The slightly binding energy shift of these elements in Ag-decorated SnO2 microsphere means electrons may transfer between Ag and SnO2, demonstrating strong interaction between Ag nanoparticles and SnO2 nanosheets rather than simply physical contact. The strong interaction is advantageous for the electron transfer among the adjacent particles, which can improve the catalytic activities and be beneficial to some similar phenomenon, which was observed in other literatures [38,39,40]. In Fig. 5d, O 1s spectra at 530.5 eV corresponded to the lattice oxygen while the peak at 532.6 eV corresponds to chemisorbed oxygen or hydroxyl ions such as O−, O2 −, or OH− at the surface of SnO2 [41,42,43,44].

Fig. 5
figure 5

Representative XPS spectra of SnO2/Ag microspheres prepared at 150 °C for 36 h. a XPS full spectra. High-resolution spectra of the elements b Ag, c Sn, and d O

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Catalytic Reduction of 4-NP

The reduction of 4-NP by KBH4 in the presence of catalyst is a well-studied green chemical reaction and was chosen as the model reaction to study the catalytic activity of the as-prepared Ag-decorated SnO2 composites. The UV–Vis absorption spectrum with a maximum absorption at 400 nm is formed due to the nitro compound. With the Ag-decorated SnO2 catalyst added, the absorption peak at 400 nm, ascribed to nitro compounds, decreased sharply in 1 min and a new peak at 300 nm corresponding to 4-AP appeared, indicating the catalytic reduction of 4-NP had proceeded successfully (Fig. 6a). Considering of the excess KBH4, its concentration can be assumed to be a constant during the reaction. Therefore, a pseudo first-order kinetic equation can be applied to evaluate the catalytic rate. The kinetic equation of the reduction can be written as follows:

Fig. 6
figure 6

a, b Time-dependent UV–Vis absorption spectra and the plot of ln(C t /C 0) versus reaction time for the reduction of 4-NP of the sample prepared at 150 °C for 36 h

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$$ \frac{dC}{dt}={\kappa}_{\mathrm{app}}{C}_t\; or\;\mathrm{In}\left(\frac{C_t}{C_0}\right)=\mathrm{In}\left(\frac{A_t}{A_0}\right)=-{\kappa}_{\mathrm{app}} t $$
(1)

where the ratios of 4-NP concentrations C t (at time t) to its initial value C 0 (t = 0) were directly given by the relative intensity of the respective absorbance At/A0, κ app corresponds to the apparent rate constant. The apparent rate constant, κ app, was calculated as 3.10 min−1 for the reduction of 4-NP of the prepared Ag-decorated SnO2 microsphere at 150 °C for 36 h (Fig. 6b). In order to further assess the catalytic performance of the Ag-decorated SnO2, all the samples prepared for different hydrothermal time were carried out to catalytic reduction of the 4-NP. The UV–Vis absorption spectra of the reduction are shown in Additional file 1: Figure S2–S5, and the corresponding plots of ln(C t /C 0) versus time are shown in Fig. 7. It is clear that almost 100% of 4-NP can be reduced within 1 min of the first cycle. With the increase of cycle times, the time is longer. Nevertheless, over 80% of 4-NP can be reused within 8 min. It can be observed that ln(C t /C 0) values show good linear correlation with the reaction time for all catalysts, indicating that the reduction follows a first-order reaction law. The calculated apparent rate constants κ app of different cycles for all samples are shown in Table 1.

Fig. 7
figure 7

Plot of ln(C t /C 0) versus reaction time in the presence of Ag-decorated SnO2 microspheres prepared for different hydrothermal times a 5 h, b 10 h, c 24 h, and d 36 h

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Table 1 The apparent rate constants κ app of different cycles for all samples
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As shown in Fig. 7 and Table 1, the apparent rate constants (κ app) increase with the extension of the hydrothermal time and decrease with the cycle times, especially for the first and second cycles. The decreases of rate constant may due to the peeling off and coagulation of Ag NPs from the microsphere during the centrifugation. In order to prove the stability of the sample prepared in the work, the separated catalyst (prepared for 36 h) was reused to catalytic reduction of 4-NP for more than five cycles. The time-dependent UV–Vis absorption spectra of the sixth cycle to tenth cycle are shown in Additional file 1: Figure S6. The corresponding apparent rate constants (κ app) shown in Fig. 8 show there is only a slight decrease in the κ app value with the increasing of successive cycles, indicating that after the first five cycles, the catalysts are much more stable than the freshly prepared samples. This proves that the as-prepared Ag-decorated SnO2 samples possesses good stability for the catalytic reduction of 4-NP to p-AP by KBH4 and can be used as an alternative active and stable catalyst for the catalytic reduction of 4-NP.

Fig. 8
figure 8

a, b Plot of ln(C t /C 0) versus reaction time of the sixth to tenth cycles for the 36 h sample

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Also, the FTIR spectra of the catalyst before and after five cycles and ten cycles of catalytic reduction were shown in ESI. As shown in Additional file 1: Figure S7, after five and ten cycles of catalytic reduction, the main peaks of the samples were almost the same with the as-prepared sample and this illustrates that the catalysts are very stable.

In order to compare our results with other catalysts in the literature, we evaluated the catalytic ability of Ag-decorated SnO2 by normalizing the κ app values to κ nor [45, 46]. The normalized rate constant κ nor (κ nor = κ app/c cat, where c cat is the concentration of the catalyst) is a key indicator for estimating catalytic activity. The normalized rate constants κ nor were calculated to be 6.20, 0.64, and 0.54 min−1g−1L of the first cycle, fifth cycle, and tenth cycle for the SnO2/Ag microsphere reacted for 36 h, respectively. The comparison of κ nor of the SnO2/Ag (36 h) and other catalysts in literature are shown in Table 2. From Table 2, it is obvious that the normalized apparent rate constant κ nor of the sample in this work is much higher than that of some reported catalysts in literature [47,48,49,50,51,52,53,54,55,56,57,58], such as core-shell Ag@Pt (0.92 min−1g−1L), AgNPs/GR-G3.0PAMAM (0.78 min−1g−1L), rGO/Fe3O4/Au (0.52 min−1g−1L). Moreover, for the fifth and tenth cycles, the calculated κ nor (0.64 and 0.54 min−1g−1L) are even higher than these catalysts [51,52,53,54,55,56,57,58]. All these results illustrate that the prepared SnO2/Ag microsphere can be taken as a potential efficient catalyst for the reduction of 4-NP.

Table 2 Comparison of normalized rate (κ nor) of different catalysts for the reduction of 4-NP (at room temperature)
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Based on the previous results and the traditional theory about the catalytic reduction of p-NP by noble metals, the formation mechanism and the origin of the excellent catalytic efficiency of hierarchal Ag-decorated SnO2 microsphere were speculated and the schematic is shown in Figs. 9 and 10. In the facile one-pot hydrothermal method, the Ag and SnO2 NPs were formed simultaneously in the solution and the freshly born surfaces are inclined to bond with each other. With the increase in hydrothermal time, the SnO2 nanoparticles assembled into nanosheets [59] and Ag nanoparticles dispersed in the microsphere. During the catalytic reduction, the Ag nanoparticles start the catalytic reduction by relaying electrons from the donor BH4 − to the acceptor 4-NP on the adsorption sites of the samples, which was accelerated by the intimate bond between SnO2 and Ag NP. Moreover, the dispersed Ag NPs in the microsphere can avoid agglomeration during the catalytic reaction owing to the steric hindrance effect. Furthermore, the synergistic effect of Ag NPs and SnO2 nanosheets co-contribute to the excellent catalytic activity of Ag-decorated SnO2 composites. In order to verify the assumption, pure SnO2 and Ag NPs were synthesized by the similar procedures without the addition of AgNO3 and Na2SnO3, respectively, and then served for the catalytic reduction of 4-NP. The time-dependent UV–Vis spectra and corresponding plots of ln(C t /C 0) versus time for SnO2 and Ag NPs are shown in Additional file 1: Figure S8 and Figure S9. It can be observed the reduction also follows a first-order reaction law. The rate constant (κ app) values calculated from the slope of the linear region were found to be 1.24 min−1, and 1.16 min−1 for SnO2 and Ag, which is lower than that of SnO2/Ag. So the excellent catalytic activity of SnO2/Ag may arise from the synergistic effect between Ag nanoparticles and SnO2 nanosheets. However, the accurate mechanism needs to be further explored.

Fig. 9
figure 9

Schematic illustrations of the synthesis of Ag-decorated SnO2 microsphere

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Fig. 10
figure 10

Schematic illustrations of the catalytic reduction of 4-NP to 4-AP over Ag-decorated SnO2 microsphere

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Conclusions

In conclusion, hierarchal Ag-decorated SnO2 microsphere with uniform Ag nanoparticles and SnO2 nanosheets has been successfully prepared by a facile one-pot method. The catalysts prepared by this simple but effective method exhibit excellent catalytic performance for the reduction of 4-NP to 4-AP with κ nor of 6.20 min−1g−1L. Furthermore, the catalyst can sustain high catalytic performance after the first five cycles and could be expected to act as high-efficiency catalysts for the reduction of 4-NP. Moreover, we believe this method can be used as a new strategy to prepare other metal particle-modified semiconductor composites.