Enhanced photocatalytic removal of nitric oxide over Ag-decorated ZnSn(OH) 6 microcubes

Presently, most of the population has been facing a string of severe climate change problems that primarily come from the intensive emission of nitric oxide (NO), which requires a practical approach to sustain our living conditions. Herein, Ag nanoparticles-decorated ZnSn(OH) 6 microcubes (Ag:cZHS) photocatalysts were synthesized rapidly and used for photocatalytic NO removal under solar light activation. The properties of the newly prepared photocatalysts are comprehensively characterized by a series of routine methods. The NO removal performance over the ZnSn(OH) 6 microcubes (c:ZHS) photocatalysts was increased markedly upon being combined with Ag nanoparticles through the surface plasmon resonance effect. The contribution of e − , h + , •OH, and •O 2 was extensively investigated through trapping tests and electron spin resonance analysis (ESR). Also, the by-products and apparent quantum eciency of the cZHS photocatalysts were studied.


Introduction
In recent years, with the rapid development of advanced industries, environmental pollution has become increasingly severe. Some notable examples include the summer heat record peaked at 55°C in the USA or the water loss by the non-typhoon in Taiwan [1,2]. The mass emission of several toxic gases such as CO 2 , CH 4 , NO x , VOCs [3] from manufacturing, transportation, factory, and power plant activities represents an obvious risk to the global environment and human health. Nitrogen oxide (NO x ) is one of the leading greenhouse gases directly causing global warming. Also, NO x represents a highly toxic gas, causing extreme weather phenomena such as acid rain, ozone layer depletion, PM 2.5 [4], or photochemical smog [5].
Nitric oxide (NO) is a primary component of NO x released during combustion processes [6], together with nitrogen dioxide (NO 2 ) which is more harmful than NO [7]. Extensive research on photocatalytic NO x removal from ambient air or ue gas has been performed using various methods, such as selective catalytic reduction, selective non-catalytic reduction, bio ltration, adsorption, absorption, and absorption photocatalytic [8][9][10][11]. Among them, photocatalytic materials have attracted much attention due to their great potential, including eco-friendly, low-cost, high e ciency, and non-toxic [12,13]. A wide range of photocatalysts has been studied for NOx gas removals, such as Bi 2 O 3 [14], ZnSn(OH) 6 [15], g-C 3 N 4 [16], SnO 2 [17] and TiO 2 [18]. Among these materials, Zinc hydroxystannate (ZnSn(OH) 6 ) presents a highly promising photocatalyst, especially in addressing air pollution. However, its wide bandgap (~ 3.7 eV) remains the key challenge for the wide-spread use of ZnSn(OH) 6 in visible light [15].
Therefore, several methods have been performed to reduce the bandgap, including the combination with noble metals (Au, Ag, Cu), metal oxides (ZnO, SnO 2 ) [12,19,20], and non-metals (C, S) [21,22]. The metal decoration is one of the best ways to signi cantly enhance the photocatalytic performance for NO x gas removal through the surface plasmon resonance (SPR) effect [23,24]. Also, Ag is well-known as a good combination with other semiconductors [25] to reduce their bandgap and increase photocatalytic e ciency.
In this work, silver nanoparticles (Ag NPs) were used to modify the photocatalytic ability of ZnSn(OH) 6 microcubes (cZHS). The photocatalytic mechanism of Ag:cZHS was determined experimentally by the photodegradation of NO under solar light. In addition, the characterizations of materials were studied by various techniques. Signi cantly, the presence of SPR of Ag NPs was identi ed by diffuse re ectance spectroscopy (DRS). The results of this work promise to provide a new and effective way to address NO pollution.
A pluronic@127 (F127) solution was made by adding 1.4 g of pluronic@127 (F127, CAS: 9003-11-6, Sigma-Aldrich ) into 300 mL of deionized water and 200 mL of ethanol 98%. The synthesis process of cZHS is illustrated in Fig. 1. In this work, 7.5 mL of zinc acetate dihydrate 2 mM (98%, C 4 H 10 O 6 Zn, CAS: 5970-45-6, ACROS Organics) and 7.5 mL of tin (IV) chloride pentahydrate 2 mM (SnCl 4 .5H 2 O, CAS: 10026-06-9, Sigma-Aldrich) were mixed and stirred for 30 min until the solution becomes transparent. Next, 15 mL of F127 and 6 mL of NaOH were added into the mixture and stirred for 30 min to achieve a homogeneous solution, and a white precipitate was formed during this step. Finally, this solution was transferred into a 50 mL te on pot for the hydrothermal step at 100°C for 10 h. The samples were rinsed with deionized water and ethanol until the pH equals the pH of deionized water, followed by a drying step at 80 o C to obtain cZHS.
The preparation of Ag:cZHS is shown in Fig. 1. The te on pot for the hydrothermal process was fastly cooled down by tap water. Then, the different amounts of silver nitrate 0.01 M (AgNO 3 , CAS: 7761-88-8, Sigma-Aldrich) were added into the hot-cZHS and stirred in 30 min. The solution turned to light-gray color after the addition of AgNO 3 . The resulting solutions were rinsed with ethanol and deionized water serval times and dried at 80°C to produce Ag:cZHS with 5 wt%, 10 wt%, 15 wt%, 20 wt%, and 30 wt% of the expected Ag donated as 5% Ag:cZHS, 10% Ag: cZHS, 20% Ag:cZHS, 30% Ag:cZHS, respectively.

The photocatalytic experiment.
For the photocatalytic test, 0.2 g of the catalysts was dispersed into a petri disk (d = 14 cm) by 10 mL of deionized water by ultrasonicator for 3 min, followed by a drying step at 80°C for 30 min to eliminate water. Then, the catalyst was placed in a photocatalytic reactor tank (volume = 3 L). The reactor tank is made of stainless steel with a quartz-sealed top to allow light to penetrate the surface of the photocatalyst.
First, the NO 100 ppm (N 2 balanced, Ming Yang company) was diluted with zero air to reach a concentration of 500 ppb for the experimental process. The gas input was controlled by mass ow controllers (MFC); the pressure value of NO source and zero air source was 35 psi and 45 psi, respectively.
The humidity and ow rate adjusted at 40% and 1.5 L min -1 , respectively. The concentrations of NO, NO 2 , and NO x were recorded with a NO x analyzer (model 42c, Thermo-science). To eliminate the adsorption capacity of the material, we let the fabric and NO gas interact with each other in the dark environment for about 10 to 15 min until the concentration of the gases reached equilibrium. Then, a 300 W xenon lamp (λ > 300 nm) was turned on 15 min, as the solar power source of this experiment. The full spectra of the xenon lamp were shown in Fig. S1.
The recycling test attested to the durability of the catalyst. Here, the catalyst was washed with water and dried after each completion of a photocatalytic experiment, repeated ve times. The trapping test determined the critical factors of NO photocatalytic degradation. The potassium iodide (KI), isopropyl alcohol (IPA), and dichromate solution (K 2 Cr 2 O 7 ) scavengers were added to trap h + , •OH, and e -. [14,26].
The NO degradation, NO 2 conversion, and apparent quantum e ciency (AQE) were evaluated by Eqs.

Characterization
Fourier transform infrared spectrophotometer (FTIR) analysis was established in the range of 400-4000 cm − 1 by using Jasco FT/IR-6500 to investigate molecular vibrations. X-ray diffraction (XRD) patterns were used to determine the materials' phase composition and crystal structure. This analysis was carried out using Bunaciu, Udristioiu, and Aboul-Enein 2015 X-ray diffractometer with the Cu K radiation (λ = 0.154064 nm) and the scanning rate of 6 o min − 1 in the 2θ range of 10 o -80 o . The surface and crystal morphology of the materials were observed by scanning electron microscopy (SEM) and transmission electron microscop (TEM). The SEM images were captured using Hitachi FE-SEM S-4800N, and the TEM image was developed by the JEOL JEM 2000FXI model; samples were dispersed in ethanol and coated on a copper mesh for analysis. X-Ray photoelectron spectroscopy (XPS) analysis was also conducted for more investigation about the surface elemental composition and elemental states. Furthermore, the materials' optical property and surface area were examined using differential re ectance spectroscopy (DRS) analytic and Brunauer-Emmett-Teller (BET) analyzer, respectively. The electron spin resonance (ESR) was invested to determine the generation of radicals.

Photocatalytic activity
The photocatalytic NO removal of cZHS and Ag:cZHS materials under solar light are shown in Fig. 2a.
The photocatalytic e ciency was signi cantly increased upon being combined with Ag NPs, indicating the effect of the SPR of Ag NPs. The photocatalytic e ciency of 20% Ag:cZHS reached 88.4% after only 7 min and remained unchanged until the end of the test. In contrast, the photocatalytic e ciency of cZHS increased slowly during the reaction and reached the highest e ciency of 60% at the end of the test. Thus, the 20% Ag:cZHS is the most e cient sample. Practically speaking, the reusability of materials is important, which was evaluated by a recycling test. As shown in Fig. 2b, the photocatalytic e ciency of 20% Ag:cZHS was decreased from 87.30-77.55% after 5 times recycling at the same condition. The result showed that the durability of the sample was promising. Besides, the conversion of NO to NO 2 and green products was also calculated and shown in Fig. 2c. The apparent quantum e ciency (AQE) was calculated by Eq. (S3) to understand the effect of photons on the photocatalytic ability of Ag:cZHS and cZHS (Fig. 2d). The AQE (10 − 4 %) value of cZHS, 1% Ag:cZHS, 5% Ag:cZHS, 10% Ag:cZHS, 20% Ag:cZHS, 30% Ag:cZHS were 3.81, 4.59, 4.60, 5.56, 7.11, and 6.51, respectively. Figure 3a shows the FTIR spectra of 1%, 5%, 10%, 20% and 30% of Ag:cZHS and cZHS, the tracing region in the range from 400-4000 cm − 1 . The wide peak observed in 3800 cm − 1 to 2750 cm − 1 shows OH bending and stretching vibrations [12]. Besides, the sharp peak appears at 1170 cm − 1 , attributed to Sn -OH deformation vibration [28]. The stretching vibration of Sn-O-Sn is observed at 779 cm − 1 and 536 cm − 1 [29]. The results obtained con rm the success of the cZHS synthesis, but the signal of Zn and Ag were di cult to detect by the FTIR analysis. Therefore, the XRD diffraction was carried out to con rm the structure and crystallization of cZHS.

Materials characterizations
The XRD patterns of all samples are shown in Fig. 3b  planes of the Ag NPs in the Ag:cZHS, respectively [30]. While other studies have been in the direction of peak expansion, peak intensity decreased or even peak material loss.

Morphology of materials
The SEM images in Fig. 4 show the cube shape of the sample before and after Ag NPs loading. As shown in Fig. 4a, the cZHS were synthesized with precise cube shapes, smooth surfaces. The cZHS were dispersed well in the solvent, without any conglomeration. It can be seen in Fig. 4b that the original morphology of cZHS was changed when Ag NPs were loaded onto the surface. The shape and structure of the Ag:cZHS become distinctive. However, the change is not much, which proves that this synthesis method does not change the morphology of the substrate material. Such change in the morphology demonstrated that the Ag NPs successfully loaded onto the cZHS surface. Figure 5 shows the morphology and structure of cZHS and 20% Ag:cZHS. As shown in Fig. 5a, b, the TEM images of the pristine cZHS with the precise edges and a length range from 300 nm -600 nm match well with SEM images. It is observed in Fig. 5c, d that the presence of Ag NPs covered on the surface of cZHS cubes with a diameter less than 10 nm. Furthermore, the dots appearing in Fig. 5d are predicted to be Ag NPs. In contrast, in Fig. 5b, the dots of Ag NPs do not appear; instead, the nanopores were created by the F127. These nanopores were lled with Ag NPs in 20% Ag: cZHS.

Speci c surface area of the materials
The N 2 adsorption isotherms of cZHS and 20% Ag:cZHS are shown in Fig. 6. The quantity adsorbed of cZHS is higher than that of the 20% Ag:cZHS. In addition, as shown in Table 1, the BET surface area of cZHS and 20% Ag:cZHS is 20.24 m 2 g − 1 and 34.93 m 2 g − 1 , respectively. The total pore volume and average pore width of cZHS and 20% Ag:cZHS are 0.034 cm 3 g − 1 , 6.702 nm, and 0.0267 cm 3 g − 1 , 3.967 nm, respectively. These results con rm that the higher total pore volume and average pore width of cZHS are due to the presence of F127, which increases the porosity of the sample [31]. The hypothesis that Ag NPs lled the nanopores of cZHS has been determined by the lower total pore volume and average pore width of 20% Ag:cZHS. Table 1 BET surface area, total pore volume, and average pore width of cZHS and 20% Ag: cZHS.

Optical properties of materials
The optical absorption properties of cZHS and 20% Ag:cZHS are shown in Fig. 7. In Fig. 7 (a), the 20% Ag:cZHS could re ect photons with a wavelength in the range of UV light (300 nm > λ > 400 nm). At the wavelength from 300 nm to 400 nm, an SPR peak appears, which was generated by Ag NPs. Besides, the formation of a peak of SPR in the range of UV (200 nm < λ < 300 nm) and the DRS tail of the 20% Ag:cZHS does not coincide with the Ox axis in the range of IR. In contrast, in the cZHS, there is a peak present at around 360 nm. The morphology of this peak is bizarre and attributed to the manipulation of the DRS operator. However, after repeated measurements of DRS, we concluded that the appearance of this peak of the cZHS sample was due to the in uence of F127 on the surface of the material. As shown in Fig. 7b, Kubelka-Munk plots of DRS reveals the band structure of cZHS is an indirect transition with the bandgap of 5.34 eV. The 20% Ag:cZHS has a smaller bandgap (5.1 eV), which is expected to have better photocatalytic activity.
The surface chemistry of materials is shown in Fig. 8. Figure 8a shows that the peaks around are assigned to Sn 3d, O 1s, and Zn 2p of cZHS. Furthermore, the peak around 370 eV of 20% Ag:cZHS corresponds to the Ag 3d. The HR-XPS of Sn 3d5, O 1s, and Zn 2p3 are shown in Fig. 8a, b and c, respectively. The peak at 485.3 eV, 493.8 eV, 531.8 eV, and 1020 eV corresponds to Sn 3d 5/2 , Sn 3d 3/2 , O 1s, and Zn 2p 3/2 , respectively. The HR-XPS of Ag 3d is shown in Fig. 8 (e); there are two peaks of Ag 3d at 367.7 and 373.6, corresponding to Ag 3d 5/2 and Ag 3d 3/2 , respectively. In addition, XPS is a directly used tool to measure the valence-band maximum (VBM) of the materials. In Fig. 8f, the VBM of cZHS is 2.55 eV, and 20% Ag:cZHS is 4.08. The VBM of 20% Ag:cZHS is higher than the cZHS is 1.53 eV, electrons provided by Ag enhance electron transport.
3.6. Photocatalytic mechanism over the materials.
The trapping test considered the critical factors of photocatalytic degradation of NO, as shown in Fig. 9.
In the trapping test, the h + , e -, and •OH were trapped by adding the KI, K 2 Cr 2 O 7 , and IPA scavengers, respectively. The trapping results indicate that eand •OH contributed equally to the photocatalytic activity of 20% Ag:cZHS. The photocatalytic e ciency of the 20% Ag:cZHS was decreased dramatically by adding KI. The ESR was invested in determining the generation of radicals of 20% Ag:cZHS. Figure 9b shows that the 20% Ag:cZHS generate more •O 2 radicals than •OH. These results explained that in the 20% Ag:cZHS, the ereact with O 2 before forming •OH radicals.
A proposed mechanism on the photocatalytic NO removal of the Ag:cZHS under solar light is presented in Fig. 10. Under light activation, the electrons and holes are generated in the valence band of the cZHS. The photo-generated electrons spontaneously move to the conduction band of the cZHS and are isolated by the interface between the cZHS and the Ag NPs [32]. These electrons can reduce adsorbed O 2 on the surface to produce •O 2 , as evidenced from the ERS results. These •O 2 radicals could get more electrons to produce •OH species. In the meantime, the photo-generated holes in the VB oxidized adsorbed water on the surface of cZHS to •OH radicals. The •O 2 and •OH radicals assist in the removal of NO [33]. Thus, the introduction of Ag NPs drastically enhances photocatalytic NO removal of the cZHS through the surface plasmonic effect, resulting in a better photoresponse and excellent electron-hole pairs separation.

Conclusions
In this work, the Ag:cZHS nanocomposite photocatalysts are prepared by a rapid and straightforward physical mixing process. The as-prepared Ag:cZHS nanocomposite photocatalysts show a remarkable improvement of NO removal under solar light through the surface plasmonic effect from Ag NPs. With 20% of Ag loading, a photocatalytic performance of 87.3% is achieved, much higher than that of the bare cZHS (52.14%). The green product yield produced from the 20% Ag:cZHS is also the highest (87.26%). This sample also has an excellent photostability and recycling ability with a photocatalytic performance of approximately 77% even after ve repetitive runs. This study presents the promise of Ag:cZHS nanocomposite photocatalysts for addressing air pollution.  Figure 1 The synthesis process of cZHS and Ag:cZHS.

Figure 2
The photocatalytic activity (a), the recycling test (b), NO conversion (c), apparent quantum e ciency (d) of the materials.       Schematic of photocatalytic reactions of the Ag:cZHS to treat NO gas.

Supplementary Files
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