**Tuneable Functionalization of Glass Fibre Membranes with ZnO**/**SnO<sup>2</sup> Heterostructures for Photocatalytic Water Treatment: E**ff**ect of SnO<sup>2</sup> Coverage Rate on the Photocatalytic Degradation of Organics**

### **Vincent Rogé 1, \*, Jo**ff**rey Didierjean 1 , Jonathan Crêpellière 1 , Didier Arl 1 , Marc Michel 1 , Ioana Fechete 2,3 , Aziz Dinia <sup>4</sup> and Damien Lenoble 1**


Received: 2 June 2020; Accepted: 30 June 2020; Published: 2 July 2020

**Abstract:** The construction of a ZnO/SnO<sup>2</sup> heterostructure is considered in the literature as an efficient strategy to improve photocatalytic properties of ZnO due to an electron/hole delocalisation process. This study is dedicated to an investigation of the photocatalytic performance of ZnO/SnO<sup>2</sup> heterostructures directly synthesized in macroporous glass fibres membranes. Hydrothermal ZnO nanorods have been functionalized with SnO<sup>2</sup> using an atomic layer deposition (ALD) process. The coverage rate of SnO<sup>2</sup> on ZnO nanorods was precisely tailored by controlling the number of ALD cycles. We highlight here the tight control of the photocatalytic properties of the ZnO/SnO<sup>2</sup> structure according to the coverage rate of SnO<sup>2</sup> on the ZnO nanorods. We show that the highest degradation of methylene blue is obtained when a 40% coverage rate of SnO<sup>2</sup> is reached. Interestingly, we also demonstrate that a higher coverage rate leads to a full passivation of the photocatalyst. In addition, we highlight that 40% coverage rate of SnO<sup>2</sup> onto ZnO is sufficient for getting a protective layer, leading to a more stable photocatalyst in reuse.

**Keywords:** photocatalysis; ZnO; SnO2; atomic layer deposition

### **1. Introduction**

Photocatalytic water treatment has been intensively described in the last two decades. The amount of polluted water is constantly increasing, making the maintenance of reserves of clean drinkable water more and more challenging [1]. Current water depollution technologies (filtration membranes, reverse osmosis, adsorption, coagulation, deep UV with H2O2, etc.) have high operating costs and consume a lot of energy [2–5]. Consequently, the development of green and energy-efficient depollution technologies is attracting much attention. Photoactive materials working under sunlight are part of them. Among the different photocatalysts studied in the literature, semiconductors like ZnO or TiO<sup>2</sup> appear to be promising candidates as they are abundant, safe, thermally stable and display high photocatalytic properties [6–9]. ZnO has a direct band gap of 3.2–3.3 eV at room temperature (≈380 nm) and an exciton binding energy of 60 meV, making it photoactive in the ultraviolet (UV) range. Nevertheless, the large-scale use of photocatalysis for water treatment is limited due to the fast recombination of photogenerated charge carriers in those materials. ≈

In order to improve photocatalytic properties of ZnO, different strategies have been proposed, for example, by using doping elements to improve the photoresponse range [10,11] or developing heterostructures (heterojunctions) with other semiconductors [12,13]. Based on their band alignment, heterostructures can be classified into three types (Figure 1): type I (symmetric), type II (staggered) and type III (broken). Type I heterostructures are often found in light emitting diode (LED) systems, as they promote the recombination of photogenerated electrons/holes [14]. Type II heterostructures are particularly interesting for photocatalytic applications, since they allow the respective delocalisation of photogenerated charge carriers. Holes are driven in the valence band maximum (VBM) of one semiconductor and electrons in the conduction band minimum (CBM) of the second one [15]. Consequently, photogenerated electrons/holes' lifetimes are increased. Type III heterostructures can be applied in tunnelling field-effect transistors [16].

ZnO-based type II heterostructures can be produced using different metal oxides or metal sulphides, like TiO<sup>2</sup> [17], CdS [18], CdSe [19], or SnO<sup>2</sup> [20]. Among those, the ZnO/SnO<sup>2</sup> heterojunction is highly attractive for photocatalytic applications, as SnO<sup>2</sup> is a thermally and chemically stable material, insoluble in water, and has a band gap of 3.6 eV (≈345 nm). This is higher than that of ZnO, and thus, it is almost transparent in the 3.2–3.6 eV range. In addition, ZnO and SnO<sup>2</sup> have different Fermi energy levels [21] and they both possess valence bands potentials around 3.0 V/ENH and 3.8 V/ENH, respectively, i.e., higher than the H2O/OH· redox couple (2.8 V/ENH). . ≈

**Figure 1.** Schematic representation of the three different possible types of heterostructures.

According to the ZnO/SnO<sup>2</sup> heterostructure band alignment (Figure 2), photogenerated holes in the space charge area are delocalised in the valence band of ZnO, and electrons drift in the conduction band of SnO2.

– − – – − The synthesis of Janus-like nanoparticles, with both ZnO and SnO<sup>2</sup> exposed to the solution to be cleaned, is one of the most described structures in the literature [22–24]. The main advantage of such heterostructures is that holes and electrons are available for both the oxidation and the reduction of water in the form of OH· and O<sup>2</sup> .<sup>−</sup> radicals, respectively. It has already been shown that OH· radicals are the most efficient ones for water treatment [25], as they are strong oxidisers (2.8 V/ENH) able to oxidise the C–C bonds of organic molecules [26–28]. O<sup>2</sup> .<sup>−</sup> radicals however, follow an indirect pathway through H2O<sup>2</sup> and then OH· . Therefore, the recombination rate of those radicals is higher than that of OH· ones, and thus their photocatalytic degradation performance is usually reduced. ZnO/SnO<sup>2</sup> heterostructures are mostly found in the literature in the form of nanoparticles [29,30], nanorods [31] or fibres [13,32]. Various synthesis methods have been reported, such as liquid phase processes (i.e., sol-gel or hydrothermal growth) [33,34], electrospinning [35] or gas phase techniques [36]. However, one of the main drawbacks of this configuration is that it requires some post-treatment filtering process. As a matter of fact, a direct contact between (photo)catalytic nanoparticles and fauna and/or flora can be extremely harmful [37]. To circumvent this problem, some recent developments proposed to have the photocatalyst directly supported on a substrate [38]. Membranes are already widely used in the water treatment; coupling their physical separation properties with the photocatalytic activity of photocatalysts appears to be a promising strategy for the development of safe-by-design supported photocatalysts. This is such a strategy being pursued in the work reported here.

**Figure 2.** Schematic representation of the two possible ZnO/SnO<sup>2</sup> morphologies envisaged in this work. On the left, one can see that ZnO is partially covered by SnO<sup>2</sup> nanoparticles. In the scheme on the right, the ZnO is fully covered with a SnO<sup>2</sup> thin film.

In this publication, we propose to study the photocatalytic properties of ZnO/SnO<sup>2</sup> structures by adjusting the coverage rate of SnO<sup>2</sup> particles grown on ZnO nanowires. Therefore, we investigate the synthesis and characterisation of a ZnO/SnO<sup>2</sup> heterostructure based on ZnO nanorods/SnO<sup>2</sup> nanoparticles supported on glass fibres membranes. The functionalisation of glass fibres by ZnO nanorods has been performed using a liquid phase hydrothermal process and SnO<sup>2</sup> nanoparticles have been deposited using a gas phase Atomic Layer Deposition (ALD) process.

As presented in Figure 2, two different strategies can be foreseen to develop the desired ZnO/SnO<sup>2</sup> heterostructure. The first one consists in a core/shell type structure [39] obtained by a full coverage of ZnO (i.e., ZnO nanorods), with a continuous SnO<sup>2</sup> thin film (Figure 2, right). The advantage of this strategy is that the ZnO will be completely protected by the insoluble and stable SnO<sup>2</sup> film. Indeed, one of the drawbacks of ZnO is its known instability in water when the pH drops below 6 or increases above 8, unlike SnO2, known to be stable and insoluble over a larger pH range. However, with the ZnO being completely covered by the SnO<sup>2</sup> thin film, photogenerated holes may be trapped inside the nanowire and not be available anymore on the surface for water oxidation (OH· formation). In this case, only reducing species will be active for the photocatalytic degradation of contaminants via O<sup>2</sup> .− radical-induced reactions. The second strategy aims at partially covering ZnO nanowires with SnO<sup>2</sup>

particles (Figure 2, left). This structure is close to a Janus one. By doing so, we expect to have a part of the ZnO surface available for photocatalytic reactions. It is yet unclear if the heterostructure formed between ZnO and SnO<sup>2</sup> will be efficient enough to balance the loss of ZnO exposed surface area. −

As we showed in a previous work that the stability of ZnO in water could be strongly improved when protected with SnO2, even at coverage rates below 100% [40]; the stability of the ZnO/SnO<sup>2</sup> over multiple reuse tests will also be studied.

### **2. Results and Discussion**

In order to control the coverage rate of SnO<sup>2</sup> nanoparticles on ZnO nanorods, we used an ALD process in gas phase, with a chlorinated tin precursor. ALD deposition processes are typically used to grow conformal thin films on complex substrates. However, in some particular cases, they can be used for the synthesis of particles [41,42]. This is often attributed to the use of halogenated precursors (mainly chlorinated ones) that attack the film during the growth process, leading to particle structures [40]. This can be observed on Figure 3, where Scanning Electron Microscopy (SEM) images highlight that the growth of ZnO nanorods on each fibre of the membrane seems to be homogeneous, even on the deepest fibres. In addition, the SEM pictures point out that at a rather low number of SnO<sup>2</sup> ALD cycles (~500), small particles around 10 nm in size are observed on the ZnO nanorods. At 1000 cycles, the particles are slightly bigger, around 15 nm, and their density is much higher. After 1500 cycles, this effect is more pronounced, with particles around 25 nm in diameter. At 2000 cycles, ZnO nanorods seems to be almost completely covered with aggregated SnO<sup>2</sup> particles. Over 2500 ALD cycles, ZnO nanorods are completely covered with a granular SnO<sup>2</sup> film.

**Figure 3.** SEM images of a glass fibre membrane, ZnO nanorods grown in the glass fibre membrane, ZnO/SnO<sup>2</sup> after 500, 1000, 1500, 2000, 2500 and 3500 SnO<sup>2</sup> ALD cycles.

SnO<sup>2</sup> coverage rates have been estimated from the obtained SEM pictures using an image processing software (ImageJ software, thresholding process). A contrast is observed between SnO<sup>2</sup> particles and the ZnO underneath. Therefore, the image analysis is based on the roundness particles edge detection (SnO<sup>2</sup> particles) versus the background correction (here ZnO). Results are presented in Figure 4. It highlights that about 8% SnO<sup>2</sup> coverage is achieved for 500 ALD cycles. After 1000 cycles, around 40% of the surface of nanorods is covered. A slower deposition rate is observed after 1500 cycles, with around 70% coverage. Above 2000 ALD cycles, the coverage rate is close to 100%, with some porosity due to the structure of the SnO<sup>2</sup> film.

**Figure 4.** SnO<sup>2</sup> coverage rate as a function of the number of ALD cycles used (estimated from SEM pictures).

α α α α α α β An Energy Dispersive X-ray (EDX) analysis (Figure 5a) of the synthesized ZnO/SnO<sup>2</sup> structure covered at 70% with SnO<sup>2</sup> nanoparticles reveals the presence of oxygen (Kα = 0.52 keV), Zinc (Lα = 1.01 keV, Kα1 = 8.63 keV and Kα2 = 9.53 keV), silicon (Kα = 1.74 keV) and tin (Lα = 3.44 keV and Lβ = 3.46 keV). The EDX spectrum is in accordance with the corresponding SEM picture, as we observe an intense peak of Zn related to the ZnO being the major component of the ZnO/SnO<sup>2</sup> structure. Peaks related to Tin are weak compared to the Zn one. This is related to the relatively small overall quantity of SnO<sup>2</sup> deposited on the ZnO nanowires. Indeed, the inherent volume of interaction of EDX probing down to few micrometres leads to a higher contribution of ZnO as well as the detection of silicon due to the glass fibre membrane used as support. α α α α α α β

**Figure 5.** *Cont.*

**Figure 5.** (**a**) EDX analysis of ZnO/SnO<sup>2</sup> (70% SnO<sup>2</sup> surface coverage). (**b**) High resolution XPS spectrum of the Sn3d peak.

– – Besides the first chemical screening performed by EDX analysis, an elemental composition of the developed membrane has been determined by X-ray Photoelectron Spectroscopy (XPS) analysis, with a specific focus on the oxidation state of Sn. Figure 5b corresponds to a high-resolution analysis of the Sn3d peak. In this figure, one can see the position of the Sn3d3/<sup>2</sup> peak at 495.4 eV and the Sn3d5/<sup>2</sup> peak at 486.7 eV, distinctive of a Sn4<sup>+</sup> oxidised state of Sn in SnO<sup>2</sup> [13,43]. In addition, the coupled spin orbit splitting between the Sn3d3/<sup>2</sup> peak and the Sn3d5/<sup>2</sup> peak is exactly 8.5 eV, featuring the Sn–O bonding. The sharp shape of both peaks confirms one chemical bonding contribution: Sn–O. This further confirms that particles are composed of SnO2.

Crystalline structures of functionalized photocatalytic membranes have been investigated by X-Ray Diffraction (XRD). Resulting diffractograms are presented in Figure 6 With a 70% SnO<sup>2</sup> surface coverage rate, the hexagonal wurtzite structure of ZnO is detected. The three main diffraction planes of the ZnO wurtzite structure, at 31.75◦ , 34.45◦ and 36.25◦ , corresponding to the (100), (002) and (101) diffraction planes, respectively, are intense and sharp. ZnO is well crystallised, which is a critical feature for efficient water treatment by photocatalysis [44]. In this sample, no SnO<sup>2</sup> crystalline structure can be identified. This may be due to the excessively small amount of material deposited, to the fact that the SnO<sup>2</sup> could have grown in an amorphous state or to very small crystallite sizes. The XRD diffractogram recorded for the sample completely covered with a SnO<sup>2</sup> film exhibits the same ZnO hexagonal wurtzite structure, but weak diffraction peaks characteristic of the tetragonal cassiterite structure of SnO<sup>2</sup> are also visible. They correspond to the (100), (101) and (211) diffraction planes at 26.54◦ , 33.89◦ and 51.78◦ respectively. The presence of those peaks confirms that the SnO<sup>2</sup> is crystalline. Nevertheless, the relative amount deposited compared to ZnO is too low to see some intense and well-defined peaks. Considering the configuration of the ZnO/SnO<sup>2</sup> heterostructure grown in a glass fibre membrane (with circular fibres), the probing of the surface with a grazing angle XRD analysis in very challenging to set up and not fully representative of the global structure.

Crystalline ZnO has a direct optical and electronic band gap of approximately 3.2 eV. It absorbs UV light and show photoluminescent properties with a near band edge (NBE) emission around 380 nm (3.2 eV), corresponding to the excitonic radiative recombination. This emission band is usually sharp and intense for highly crystalline ZnO materials. A second band is often observed in the visible region, centred in the green zone around 530 nm (2.33 eV), corresponding to deep level emission (DLE), due to defects in the ZnO matrix [45,46].

**Figure 6.** XRD diffractograms of the ZnO/SnO<sup>2</sup> heterostructure synthesized in glass fibre membranes after different SnO<sup>2</sup> coverage rate.

The optical properties of the synthesized ZnO nanorods and ZnO/SnO<sup>2</sup> structures after 70% and 100% coverage rate are presented in Figure 7. An intense and sharp peak is observed at 384 nm for ZnO nanorods. The emission peak related to defects in this case is relatively weak. This suggests that ZnO nanorods are highly crystalline with low defects, further reinforcing the conclusion drawn from the XRD analysis.

**Figure 7.** Photoluminescence spectra of ZnO nanorods, ZnO/SnO<sup>2</sup> with 70% coverage rate and ZnO/SnO<sup>2</sup> with 100% coverage rate.

When ZnO nanorods are covered by SnO2, partially or totally (70% or 100%), the NBE emission is lowered in intensity, but still present. This lowering of intensity can be assigned to the presence of the heterostructure between ZnO and SnO2, which stabilises photogenerated carriers, and thus, limits the radiative recombination and consequently the NBE emission intensity. Interestingly, the NBE emission intensity is in the same order of magnitude for the ZnO covered at 70% by SnO<sup>2</sup> and for the ZnO fully covered by SnO2.

This reveals two important features when considering photocatalytic applications. Firstly, SnO<sup>2</sup> remains transparent to UV light, so that the ZnO underneath can still be excited, generating electron/hole carriers, even when fully covered. Secondly, a 70% coverage rate of SnO<sup>2</sup> is enough to prevent charge carriers recombination. The presence of the broad peak in the visible range can be attributed to remaining defects in the ZnO or SnO2, particularly oxygen vacancies. The annealing treatment of ZnO, as well as the growth temperature of SnO2, were limited to 300 ◦C because of the substrate instability above this temperature. Thus, it is likely that all oxygen vacancies were not completely eliminated [47,48].

The photocatalytic degradation properties of ZnO/SnO<sup>2</sup> structures have been investigated with standard methylene blue (MB). MB is a well-known and widely used chemical probe for the simple investigation of photocatalytic efficiencies of metal oxide photocatalysts. Five samples have been tested: ZnO nanorods without SnO2, ZnO/SnO<sup>2</sup> with 8% coverage rate, ZnO/SnO<sup>2</sup> with 40% coverage rate, ZnO/SnO<sup>2</sup> with 70% coverage rate and ZnO/SnO<sup>2</sup> with 100% coverage rate. Results are presented in Figure 8.

**Figure 8.** Photocatalytic degradation of Methylene blue under UV light (365 nm, 8 W) over ZnO or ZnO/SnO<sup>2</sup> photocatalysts. Percentages in bracket indicate the coverage rate of SnO<sup>2</sup> nanoparticles around ZnO.

In a first step, membranes were exposed to the solution in the dark for 90 min in order to stabilise the adsorption/desorption of MB on membranes. This process is crucial for a reliable determination of the photocatalytic degradation kinetics. If not taken into account, this may induce errors due to the uncertainty of distinguishing between adsorption and degradation phenomena. The control membrane (glass fibre only without any photocatalyst) showed high adsorption properties of the organic methylene Blue in the dark, as the measured concentration in solution decreased drastically from 5 mg·L <sup>−</sup><sup>1</sup> down to 1 mg·L −1 . The membrane functionalized with ZnO nanorods also showed good adsorption properties toward MB, but less than glass fibres, as the concentration dropped from 5 mg·L <sup>−</sup><sup>1</sup> down to 2.7 mg·L −1 . ZnO/SnO<sup>2</sup> heterostructure-based membranes exhibited lower affinity for MB, as less than 1 mg·L <sup>−</sup><sup>1</sup> was adsorbed (5 mg·L <sup>−</sup><sup>1</sup> down to more than 4 mg·L −1 ), independently of the coverage rate. UV light irradiation on the control membrane (which corresponds to t = 0) had no visible effect on its behaviour toward MB. The picture in Figure 9 confirms that the MB is just adsorbed on the glass fibres, and not degraded. Indeed, the control membrane on the left-hand side of Figure 9 is completely blue after the photocatalytic test, which is not the case of the ZnO/SnO<sup>2</sup> photocatalytic membrane on the right-hand side of the picture. The ZnO functionalised membrane showed a peculiar behaviour compared to the control after exposure to UV light. A decrease in the MB concentration following pseudo first order kinetics can be observed. In less than 200 min, the solution has been completely decoloured. Surprisingly, all ZnO/SnO<sup>2</sup> membranes revealed very low photocatalytic properties, independently from the SnO<sup>2</sup> coverage rate. The adsorption process in the dark revealed a poor affinity with all surfaces, which could explain the slow degradation of MB. Another hypothesis could be that impurities trapped in the film or at the surface may act as scavengers for photogenerated carriers (e−/h <sup>+</sup>) or photogenerated OH· radicals. Considering the growth process of SnO<sup>2</sup> with the chlorinated precursor SnCl4, chlorine could be present in/on the ZnO/SnO<sup>2</sup> structure and strongly affect the resulting photocatalytic properties [49]. In order to investigate the role of surface defects or residual chlorine, a model structure has been prepared by growing a ZnO/SnO<sup>2</sup> thin film on a silicon wafer, covered with 2–3 nm native oxide. Depth profiling of the sample was performed by Secondary Ions Mass Spectrometry (SIMS) analysis. − − − − − − − −

Figure 10 presents the uncalibrated concentration vs. depth of five elements tracked during the SIMS analysis: silicon, tin, zinc, chlorine and oxygen. The stack SnO2/ZnO is featured by a high contribution of tin in the first 400 s of pulverisation followed by the zinc contribution from 500 to 1200 s. The oxygen contribution remains stable along the depth profile (pulverisation time from 0 s to 1200 s). After 1200 s of pulverisation, the zinc and oxygen contributions disappear in favour of silicon corresponding to the substrate contribution. Interestingly, a high contribution of chlorine is detected in the SnO<sup>2</sup> film. When the ZnO film is formed, the contribution of chlorine is lowered drastically. However, some chlorine is still visible in the ZnO film. This clearly evidences the fact that Cl is trapped within the SnO<sup>2</sup> films during its growth and that it slightly diffuses into the ZnO underneath. No chlorine is detected in the substrate level.

**)**

**Figure 10.** Depth profile SIMS analysis of SnO<sup>2</sup> grown by ALD on ZnO (a mirror-polished silicon wafer was used as substrate).

In order to remove defects like oxygen vacancies or chlorine, as-prepared ZnO/SnO<sup>2</sup> membranes have been cleaned under a UV/ozone atmosphere (254 nm, 20 W) for 30 min. Compared to plasma or thermal post-treatments, the dry UV/ozone (also called UVO, for ultra-violet ozone) post-treatment has been favoured for its ability to generate, at room temperature, clean and well-oxidised metal oxide structures with very low impact on their morphologies [50]. Also, this technique is known for being able to effectively remove chlorine defects from SnO<sup>2</sup> structures [51]. Photocatalytic degradation tests of MB have been performed again. They are presented on Figure 11.

**Figure 11.** Photocatalytic degradation of methylene blue under UV light (365 nm, 8 W) over ZnO or ZnO/SnO<sup>2</sup> photocatalysts after UV/ozone treatment. Percentages in brackets indicate the coverage rate of SnO<sup>2</sup> nanoparticles around ZnO.

After cleaning, the affinity with the surface in the dark is not enhanced, but the photocatalytic degradation properties under UV light have been greatly improved for ZnO/SnO<sup>2</sup> heterostructures with 8%, 40% and 70% coverage rates. Among them, the heterostructure with 40% coverage rate is the quickest at cleaning the solution of MB according to the steep slope (starting from t = 0). Concerning the ZnO/SnO<sup>2</sup> heterostructure with 100% coverage rate, the cleaning had absolutely no impact on its poor photocatalytic degradation properties. The disappearance of MB remains slow compared to other synthesized heterostructures.

The photocatalytic degradation of MB over ZnO and ZnO/SnO<sup>2</sup> photocatalysts seems to follow pseudo-first order degradation kinetics, as usually observed when considering the photocatalytic degradation of pollutants in water [52,53]. Consequently, from the results obtained on Figure 8 (before cleaning) and Figure 11 (after cleaning), we determined the first order degradation rate constant k (in min−<sup>1</sup> ) of the photocatalysts by using the following equation: −

$$\frac{\mathbf{C}\_0}{\mathbf{C}} = \mathbf{c}^{kt} \tag{1}$$

where *C*<sup>0</sup> is the initial concentration after 90 min in dark (mg·L −1 ), C the concentration (mg·L −1 ) at the time *t* (min). Calculated *k*, extracted from a plot of ln(*C*0/*C*) versus *t*, are reported on Figure 12. − −

− **Figure 12.** First order degradation rate constant k (min−<sup>1</sup> ) of ZnO and ZnO/SnO<sup>2</sup> photocatalysts before and after UV/Ozone cleaning.

− − − − − − − − − − − − − − − − − − − The first order degradation rate constant of ZnO nanorods is found to be 7 × 10−<sup>3</sup> min−<sup>1</sup> . When covered by SnO<sup>2</sup> (even partially) without any cleaning, the rate drops to 1 <sup>×</sup> <sup>10</sup>−<sup>3</sup> – 2 <sup>×</sup> <sup>10</sup>−<sup>3</sup> min−<sup>1</sup> for all samples. Chlorine defects seems to inhibit completely the photocatalytic activity of the material. However, after cleaning, degradation rates increased from 1 × 10−<sup>3</sup> min−<sup>1</sup> up to 4 × 10−<sup>3</sup> min−<sup>1</sup> for the ZnO/SnO<sup>2</sup> heterostructure with 8% SnO<sup>2</sup> coverage rate, 2 <sup>×</sup> <sup>10</sup>−<sup>3</sup> min−<sup>1</sup> up to 6 <sup>×</sup> <sup>10</sup>−<sup>3</sup> min−<sup>1</sup> for 40% SnO<sup>2</sup> coverage rate, 1 <sup>×</sup> <sup>10</sup>−<sup>3</sup> min−<sup>1</sup> up to 3 <sup>×</sup> <sup>10</sup>−<sup>3</sup> min−<sup>1</sup> for 70% SnO<sup>2</sup> coverage rate. With 100% SnO<sup>2</sup> coverage rate, however, no significant improvement is observed. Those results highlight an optimum coverage rate of ZnO by SnO<sup>2</sup> of around 40%, with a maximum rate value obtained of 6 × 10−<sup>3</sup> min−<sup>1</sup> . As discussed above in Figure 2, a trade-off exists between the ZnO surface

availability for the photocatalytic degradation and the number of SnO<sup>2</sup> nanoparticles available at the surface for the heterostructure-based stabilisation of charge carriers. In the case of a very low SnO<sup>2</sup> nanoparticle coverage (8%), the loss of specific surface area of ZnO free for the photodegradation is more impactful than the presence of the heterostructure. For a coverage of 70% and beyond, the heterostructure delocalises the photogenerated holes in the core of the nanorod and inhibits the photocatalytic performance of the material. Around 40% coverage, the loss of specific surface area is compensated by the effect of the heterostructure on the surface. The ZnO/SnO<sup>2</sup> heterostructure developed at 40% coverage rate shows a photocatalytic efficiency close that of ZnO alone. Although this is not a strong improvement, it is still interesting as the SnO<sup>2</sup> acts as a protective coating, preventing the dissolution of ZnO in water, even when not completely covering the surface. We demonstrated this tendency in a previous work [40], where SnO<sup>2</sup> protected ZnO inside mesoporous anodic aluminium oxide membranes.

In the present case, we highlight the same protective behaviour of the ZnO/SnO<sup>2</sup> heterostructure through reusability photocatalytic tests and SEM pictures. In Figure 13, the reusability of both ZnO (a) and ZnO/SnO<sup>2</sup> (40%) (b) membranes after five photocatalytic degradation tests is presented. For both systems, the photocatalytic performance is slightly improved after several reuse tests. The reason for this improvement is not yet known, but it is likely that after exposure to UV for several hours, membranes surfaces get cleaner (degradation/removal of adsorbed surface carbon) and thus more reactive toward MB. Those reusability tests demonstrate the excellent performance of ZnO and ZnO/SnO<sup>2</sup> membranes for water depollution over time.

**Figure 13.** Reusability of ZnO (**a**) and ZnO/SnO<sup>2</sup> (40%) (**b**) functionalized membranes. Five photocatalytic tests have been performed.

If the excellent stability of the two membranes has been determined over five successive tests, the surface state of ZnO and ZnO/SnO<sup>2</sup> after those reusability tests is really different. Figure 14 presents SEM images for both ZnO and ZnO/SnO<sup>2</sup> functionalized membranes after the five photocatalytic degradation tests. On Figure 14a, we can clearly see that the ZnO nanorod structure has been damaged. On some glass fibres, the ZnO nanorods have completely disappeared. The remaining ZnO nanorods are shorter in length and diameter than before photocatalysis. In addition, an organic matrix, most likely some remaining traces of methylene blue, is present on their surface. This indicates an incomplete degradation of the pollutant. This hypothesis is confirmed by the presence of an intense carbon peak on the EDX spectrum (no carbon was detected before the photocatalytic degradation tests). In the case of the ZnO/SnO<sup>2</sup> heterostructure with 40% SnO<sup>2</sup> coverage rate (Figure 14b), the morphology of the photocatalyst is the same after five runs as before. The ZnO nanorods are undamaged and SnO<sup>2</sup> nanoparticles at the surface are still visible. Moreover, the surface seems to be clean, without any organic traces. The EDX analysis of the photocatalyst confirms that no carbon is detected on the surface. In addition, the peak related to the tin element attests to the presence of SnO<sup>2</sup> nanoparticles on the surface of the photocatalyst. Those results clearly highlight that over several degradation cycles, the ZnO photocatalytic properties will inexorably be lowered. Conversely, the ZnO/SnO<sup>2</sup> membrane appears to be more stable as a function of the degradation time. We demonstrate here the huge potential of the heterostructure, both as an efficient photoactive material and a stable heterojunction, due to the protective role of the SnO<sup>2</sup> around the ZnO.

**Figure 14.** SEM images and EDX analysis of (**a**) ZnO nanorods and (**b**) ZnO/SnO<sup>2</sup> (40%) heterostructure, after five photocatalytic degradation tests.

### **3. Experimental Section**

### *3.1. Materials and Experimental Processes*

All chemicals were purchased from Sigma Aldrich (St. Louis, Missouri, United States), and used as received. Glass fibres membranes (APFB, 1 µm pore, 25 mm diameter without binder) were provided by Merck Millipore (Darmstadt, Germany). ZnO nanorods were synthesized in liquid phase by using zinc acetate (99.999%) and 98% anhydrous hydrazine in ultrapure water. Typically, a 25 mM zinc acetate solution was prepared with ultra-pure water in a flask equipped with a reflux system. Then, 25 mM of anhydrous hydrazine was added under vigorous steering (400 rpm). After homogenisation, the glass fibre membrane was dipped in the solution using a home-made holder. The reaction temperature was set to 80 ◦C for 2 h, under stirring and reflux. At the end of the reaction, the membrane was cleaned in ultra-pure water, dried, and annealed at 300 ◦C under an air atmosphere. The annealing process allows for the elimination of defects in the ZnO and enhances the crystallinity of ZnO rods, leading to a photocatalytic performant material. The SnO<sup>2</sup> growth on ZnO nanorods has been achieved by a gas phase ALD process, in a TFS200 instrument (BENEQ®, Espoo, Finland). Tin (IV) chloride (SnCl4) precursor was used as the tin source and water as the oxidant. Precursors were stored at room temperature and low pressure in canisters. All precursors were introduced into the reaction chamber without any carrier gas. The reaction was performed between 1 to 5 mbar with nitrogen as carrier and purging gas. The chamber temperature was set at 300 ◦C during the reaction. A repeated number of SnCl4/purge/H2O/purge cycles allowed the control of the coverage rate of SnO<sup>2</sup> around ZnO nanorods. A typical ALD cycle can be described as follows (based on preliminary studies not shown here): 300 ms pulse of SnCl4/2 s purge with nitrogen/300 ms pulse of H2O/2 s purge with nitrogen. The number of cycles has been fixed between 500 and 3500 in order to investigate different SnO<sup>2</sup> particle densities, defined as coverage rate.

### *3.2. Characterisation Techniques*

High-resolution Scanning Electron Microscopy (SEM) images were obtained on a Helios Nanolab 650 microscope (FEI, Eindhoven, The Netherlands), at an acceleration voltage of 2 kV and a current of 25 mA. Energy Dispersive X-ray (EDX) analyses were performed with a 50 mm<sup>2</sup> Xmax spectrometer (Oxford Instruments, Abington, UK), connected to a Helios Nanolab SEM. The working acceleration voltage was set to 10 kV for a current of 50 mA. XPS analyses were fulfilled with an Axis Ultra DLD X-ray spectrometer from Kratos Analytical (Manchester, UK), working with an Al Kα X-ray source (λ = 0.8343 nm, hυ = 1486,6 eV) at 150 W. The crystallographic structures of ZnO/SnO<sup>2</sup> photocatalysts were studied by X-ray diffraction (XRD) in a Brüker D8 (Billerica, MA, USA) Discover diffractometer, with a Cu Kα X-ray source (λ = 0.1542 nm) in θ-2θ mode. The photoluminescent properties of ZnO/SnO<sup>2</sup> were determined with an Infinite M1000 pro spectrometer (TECAN, Männedorf, Switzerland), at an excitation wavelength of 280 nm and a detection range from 300 nm to 700 nm. The secondary ion mass spectrometry (SIMS) technique has been used to determine the different elements present in the photocatalysts. Experiments were performed in a SC Ultra system (CAMECA, Gennevilliers, France), with Cs ions accelerated at an energy of 1 keV. To perform this analysis, the ZnO/SnO<sup>2</sup> structure (20 nm thick SnO<sup>2</sup> film on 50 nm thick ZnO) has been grown on (100) one side polished silicon wafers covered with a 2–3 nm native oxide layer.

### *3.3. Photocatalytic Degradation of Methylene Blue*

The photocatalytic characterisation of ZnO/SnO<sup>2</sup> heterostructures on methylene blue (MB) was carried out in 6-well plates (from Greiner, Kremsmünster, Austria, 35 mm well diameter, 2 mm height, 15 mL maximum volume), with 5 mL of a solution of MB at 5 mg·L −1 , under homogeneous stirring. The samples to be analysed were placed in separated wells. Like in many publications dealing with photocatalytic materials' performances, methylene blue has been used as a chemical probe to determine the degradation kinetic induced by different ZnO/SnO<sup>2</sup> photocatalysts [54–56]. MB concentration

during the photocatalytic degradation process was determined from optical absorption measurements at 666 nm using the TECAN Infinite M1000UV-visible spectrometer. Photocatalysts were irradiated with a tubular UV lamp (from Hitachi, Chiyoda City, Tokyo, Japan, F8T5, 8 W) working at 365 nm, with a measured power density of 2.28 mW·cm−<sup>2</sup> .

### **4. Conclusions**

ZnO/SnO<sup>2</sup> heterostructures have been synthesised in macroporous glass fibres membranes using a hydrothermal process to grow ZnO nanorods along with a gas phase ALD process for the SnO<sup>2</sup> growth. We show that by adjusting the number of ALD cycles, it is possible to synthesize SnO<sup>2</sup> particles with a fine control of the coverage rate. Those functionalised membranes have been tested for the photocatalytic degradation of methylene blue under UV light. It has been shown that an optimum coverage rate of approximately 40% led to the most efficient photocatalytic activity against MB. Indeed, it appeared that the exposure of the ZnO surface to the solution to be cleaned is an important parameter for efficient photocatalysts, and that higher coverage rates inhibit the ZnO/SnO<sup>2</sup> structures' activity. We also point out that the ZnO/SnO<sup>2</sup> heterostructure with 40% coverage rate was highly stable in water after many reuse tests, whereas ZnO nanorods alone were damaged.

**Author Contributions:** Conceptualization, V.R., D.A., M.M., I.F., D.L.; methodology, V.R., J.D., J.C.; validation, I.F., A.D., D.L.; formal analysis, V.R., J.D., J.C.; writing—original draft preparation, V.R., J.D., J.C., D.A., M.M., I.F., A.D., D.L.; supervision, I.F., A.D., D.L.; project administration, M.M., D.A., D.L.; funding acquisition, D.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the "Fond National de la Recherche Luxembourgeoise" (FNR) on the NaneauII project (2015, project number C10/SR/799842).

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

## *Communication* **Core/Shell Ag/SnO<sup>2</sup> Nanowires for Visible Light Photocatalysis**

**Anna Baranowska-Korczyc 1,2, \* , Ewelina Mackiewicz 1 , Katarzyna Ranoszek-Soliwoda 1 , Jaroslaw Grobelny <sup>1</sup> and Grzegorz Celichowski 1, \***


**Abstract:** This study presents core/shell Ag/SnO<sup>2</sup> nanowires (Ag/SnO2NWs) as a new photocatalyst for the rapid degradation of organic compounds by the light from the visible range. AgNWs after coating with a SnO<sup>2</sup> shell change optical properties and, due to red shift of the absorbance maxima of the longitudinal and transverse surface plasmon resonance (SPR), modes can be excited by the light from the visible light region. Rhodamine B and malachite green were respectively selected as a model organic dye and toxic one that are present in the environment to study the photodegradation process with a novel one-dimensional metal/semiconductor Ag/SnO2NWs photocatalyst. The degradation was investigated by studying time-dependent UV/Vis absorption of the dye solution, which showed a fast degradation process due to the presence of Ag/SnO2NWs photocatalyst. The rhodamine B and malachite green degraded after 90 and 40 min, respectively, under irradiation at the wavelength of 450 nm. The efficient photocatalytic process is attributed to two phenomenon surface plasmon resonance effects of AgNWs, which allowed light absorption from the visible range, and charge separations on the Ag core and SnO<sup>2</sup> shell interface of the nanowires which prevents recombination of photogenerated electron-hole pairs. The presented properties of Ag/SnO2NWs can be used for designing efficient and fast photodegradation systems to remove organic pollutants under solar light without applying any external sources of irradiation.

**Keywords:** AgNWs; SnO<sup>2</sup> ; silver nanowires; core-shell nanostructures; photocatalytic activity; visible-light photocatalysis

### **1. Introduction**

In recent years, due to high environmental pollution and fast industrial development, considerable interest has been paid to designing efficient, rapid, and widely applicable photocatalytic systems that are based on semiconductor nanostructures. The unique physical and chemical properties of semiconductors such as TiO2, ZnO, ZnS, and CdSe make them extensively studied materials as photocatalysts [1]. One of the most promising semiconducting metal oxides is SnO<sup>2</sup> because of its high chemical, thermal, and mechanical stability as well as its high performance of organic pollutant degradation. However, a key issue in applying semiconducting nanostructures for practical purposes is the impossibility of visible light utilization. The efficient application of SnO<sup>2</sup> is limited by a large band gap of 3.6–4.1 eV and a very quick recombination of photogenerated electrons and holes [2,3]. To overcome these limitations, various metal nanostructures have been applied to form a heterojunction with semiconductors to induce efficient carrier separation under visible light irradiation. Ag-SnO<sup>2</sup> nanocomposite that is synthesized using an electrochemically active biofilm was proposed as a visible light-driven photocatalyst for the degradation of methyl orange, methylene blue, 4-nitrophenol, and 2-chlorophenol [4]. TiO2/Ag/SnO<sup>2</sup> ternary

**Citation:** Baranowska-Korczyc, A.; Mackiewicz, E.; Ranoszek-Soliwoda, K.; Grobelny, J.; Celichowski, G. Core/Shell Ag/SnO<sup>2</sup> Nanowires for Visible Light Photocatalysis. *Catalysts* **2022**, *12*, 30. https://doi.org/ 10.3390/catal12010030

Academic Editors: Sophie Hermans and Julien Mahy

Received: 9 November 2021 Accepted: 24 December 2021 Published: 28 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

heterostructures that were obtained by a one-step reduction approach demonstrated a visible-light photocatalytic effect with high stability and reusability due to the Ag nanoparticle surface plasmon resonance (SPR) influence [5]. An Ag/SnO<sup>2</sup> composite was also fabricated by the one-pot hydrothermal method and revealed high efficiency towards the photodegradation of phenol under visible light irradiation [6]. At the same photocatalytic conditions, Ag/Ag2O/SnO<sup>2</sup> nanoparticles removed malachite green [7]. Ag-doped SnO<sup>2</sup> nanoparticles that were modified with curcumin were found to be an efficient photocatalyst for the degradation of rhodamine B under visible light [8]. The composition of Ag and SnO<sup>2</sup> nanoparticles was applied for the photocatalytic removal of nitrogen oxide under solar light [9]. In addition, Ag-SnO<sup>2</sup> nanocomposites were presented not only as a photocatalytic agent but also their antibacterial and antioxidant properties were investigated [10].

Ag nanostructures of various dimensionalities were used to form Ag/SnO<sup>2</sup> composites for photocatalytic applications, but Ag nanowires (AgNWs), despite their many advantages, have not been applied previously for these purposes. AgNWs reveal high transmittance, excellent plasmonic properties, high electrical performance, mechanical flexibility, nanometric size, and one dimensional (1D) geometry [11]. Moreover, the facile separation process of the AgNWs by filtration or sedimentation allows simple processing and improves their applicability in comparison to other silver nanostructures. In the core/shell Ag/SnO2NWs heterojunction, the advantageous properties of both the nanomaterials can be combined.

This study presents a new photocatalytic system that is based on AgNWs that are coated with an SnO<sup>2</sup> shell. The efficiency of the novel core/shell Ag/SnO2NWs photocatalyst was studied based on the absorbance intensity decrease during the decomposition process of rhodamine B, used as a model dye and under light irradiation at the wavelength from the visible range. Rhodamine B decomposes within 50 and 90 min under 395 nm and 450 nm illumination, respectively. Additionally, malachite green decomposes completely within 40 min with the presence of a new Ag/SnO2NWs catalyst whereas typical TiO<sup>2</sup> photocatalyst only slightly affects the dye under 450 nm irradiation due to the low absorption in the visible region. The proposed Ag/SnO2NWs system combines 1D morphology and the excellent physico-chemical properties of silver nanowires such as photoabsorption from the visible light region as well as the photocatalytic ability of tin oxide. Moreover, the metal/metal oxide arrangement significantly enhances the semiconductor photocatalytic properties due to fast carrier separation and preventing the recombination of photogenerated electron-hole pairs.

### **2. Results and Discussion**

Ag/SnO2NWs that were applied to the study were prepared in two processes, the first one was polyol synthesis to obtain AgNWs and then hydrolysis of sodium stannate in the presence of the nanowires to form a SnO<sup>2</sup> shell on the nanowire surface (Figure 1a). The core/shell Ag/SnO<sup>2</sup> nanowires were composed of 14 nm (±2 nm) thick SnO<sup>2</sup> shell consisting of 7 nm (±2 nm) rutile-type crystals surrounding the metallic core.

Our previous report describes, in detail, both synthesis stages, stability studies, morphological, and structural analysis of the core/shell Ag/SnO2NWs system [12]. Despite the fact that AgNWs are characterized by fast atmospheric corrosion, nine weeks are enough to decompose completely; the Ag/SnO2NWs are stable for over four months at ambient conditions. The core/shell Ag/SnO2NWs show significant stability in the highly complexing environment of KCN solution. They are resistant to harsh CN<sup>−</sup> ions at the concentration range of 0.01 to 0.0001 wt.%. The high stability allows further applying them as a catalyst into various environments for the different pollutant decompositions [12]. The core/shell arrangement allows the formation of the metal-semiconductor junction to prevent the recombination of the photogenerated electron-hole pairs and to enhance photocatalytic efficiency by photoabsorption from the visible region [13]. Moreover, the SnO<sup>2</sup> shell acts as a protective coating on silver nanostructures against the influence of different environmental conditions [12], which provides high stability of the system. The absorbance spectra of AgNWs shows two major bands that are centered around 349 nm

and 376 nm, respectively, that are responsible for the longitudinal and transverse modes in the SPR of the nanowires (Figure 2a). After SnO<sup>2</sup> shell synthesis the above-described peaks were red-shifted to 360 nm and 420 nm for the longitudinal and transverse SPR modes of AgNWs, respectively (Figures 2a and S1). It was shown previously that the SPR of silver nanowires is sensitive to the applied different coating as a result of the surrounding media dielectric constant changes [14]. In this study, the redshift allowed tuning of the absorbance properties and shifting the maxima of main absorbance bands to the visible light region. The absorbance spectra, typical for SnO2, showed a peak at about 193 nm [15]. To study, in detail, the optical properties of the system, the hydrolysis process of sodium stannate in the aqueous solution without the silver nanowires presence was carried out. In this reaction, the pure SnO<sup>2</sup> nanoparticles (SnO2NPs) were prepared that were similar in morphology and structure to the SnO<sup>2</sup> shell (Figure 1b). The nanoparticles revealed a mean diameter of about 25 nm (STEM analysis) and their properties were described in our previous report [16]. The absorbance spectra of SnO2NPs shows the band with the maximum at 195 nm (Figure S2) that was suitable for photocatalysis induced by UV light. The value of the band gap for semiconducting nanostructures that was obtained by our method was calculated based on the Tauc plot (Figure S2) to be about 4.17 eV. This value is in good agreement with the reports for tin oxide nanoparticles [2].

**Figure 1.** STEM images of (**a**) core/shell Ag/SnO2NWs and (**b**) SnO2NPs.

The core/shell Ag/SnO2NWs were used for the degradation of rhodamine B as a model organic dye that is characterized by an absorbance band with a maximum of 553 nm. Rhodamine B is widely applied for the photocatalytic model reaction study because it is broadly representative of organic compounds in its class and, since it strongly absorbs light, this allows facile monitoring of its degradation by UV/Vis spectroscopy [17]. The intensity of the band was studied under the irradiation of precisely defined sources, 395 nm and 450 nm LED lamps, corresponding electromagnetic radiation from the wavelength range of visible light. This approach can optimize the future photodegradation system for removing pollutants using solar irradiation instead of applying sophisticated and expensive UV-light sources (Figure S2). After coating AgNWs with an SnO<sup>2</sup> shell, the nanowires revealed effective photoabsorption at the visible light region due to the absorption band at 420 nm and partially by longitudinal mode at 360 nm (Figure 2a). The core/shell Ag/SnO2NWs were dispersed homogeneously in an aqueous solution of rhodamine B. The shell formation prevented the nanowires from the aggregation process because SnO<sup>2</sup> is an efficient inorganic stabilizer of silver colloidal suspensions [16]. This phenomenon allows AgNWs introduction to the hydrophilic environment without aggregation and acting as a part of efficient photocatalyst as well as can also facilitate the redispersion process of Ag/SnO2NWs after sedimentation. Figure S3a shows the separation process by sedimentation of the nanowires after 48 h. The nanowires were collected as sediment on the bottom of the vessel. This simple separation process of nanomaterials from the solution can be applied as a method of removing solvent with the decomposed pollutants after the photocatalysis process. A gentle mixing of the solution after sedimentation resulted in obtaining a homogenous mixture due to the SnO<sup>2</sup> coating on AgNWs, preventing the aggregation process (Figure S3b).

The intensity absorbance of rhodamine B was measured every 10 min after illumination by the selected light source. The samples were centrifuged for photocatalyst separation and further study of the optical properties of the supernatant with the dye. Figure 3a shows absorbance spectra of the supernatant with rhodamine B at the selected time points after 395 nm LED light irradiation. The absorbance intensity significantly decreased after 10 min of illumination and the dye was completely degraded in less than 1 h. It indicates the high efficiency of Ag/SnO2NWs as a photocatalyst under the irradiation from the visible light region. To study the process in detail and the influence of different factors on the photodegradation process, rhodamine B degradation was studied under various conditions (Figure 3b). It is essential to study all the conditions and system components to avoid the possibility of spectral interferences by transformation intermediates which may absorb radiation at the wavelength of the dye's absorption maximum [18]. The ability of the degraded substance to inject electrons into the conduction band of a semiconductor should be primarily tested. Rhodamine B with the Ag/SnO2NWs photocatalyst presence but not light-irradiated (dark experiment) did not show any degradation process (Figure S4). The dye sample without the Ag/SnO2NWs but illuminated by 395 nm LED lamp revealed a slight decrease in the absorbance intensity (Figure S5) and proved that the photodegradation effect appears only in the presence of Ag/SnO2NWs. Moreover, the contribution of SnO<sup>2</sup> in the photodegradation process in this range of illumination was verified. For these purposes, SnO<sup>2</sup> was synthesized using stannate precursor as in the case of the shell but without AgNWs presence. The process resulted in SnO<sup>2</sup> nanoparticles (SnO2NPs) formation, described in our previous report [16]. To compare the influence of SnO<sup>2</sup> on the photocatalytic activity the Ag/SnO2NWs, SnO2NPs were added to the dye solution at the concentration of the whole core/shell complex (2 mg/mL) and at the concentration of the tin oxide in the shell (0.66 mg/mL) (Figure 3b). The weight percentage of the SnO<sup>2</sup> shell in Ag/SnO2NWs complex was determined on the EDS studies and was about 33 wt.% (Figure S6). SnO<sup>2</sup> is known as an efficient photocatalyst under UV irradiation [19–21]. In our system, after irradiation by light from the visible region, both concentrations of SnO2NPs showed a slight degradation rate of rhodamine B as a result of the negligible influence of this radiation on tin oxide nanostructures (Figures 3b and S7).

■ ▲ ▼ ◆ ● **Figure 3.** (**a**) The absorbance spectra of rhodamine B with Ag/SnO2NWs photocatalyst presence under 395 nm light irradiation. (**b**) The degradation of rhodamine B with 2 mg/mL of Ag/SnO2NWs catalyst (), without photocatalyst (N), with 0.66 mg/mL (H), and 2 mg/mL (◆) of SnO2NPs under 395 nm light irradiation and Ag/SnO2NWs without irradiation (dark experiment, •).

− The rapid photodegradation of the dye under 395 nm illumination for about 60 min using Ag/SnO2NWS is due to the formation of a metal/semiconductor heterostructure which prevents the fast recombination of photogenerated electron-hole pairs. The irradiation at the wavelength of 395 nm can excite only SPR of AgNWs in the core/shell composite and cause electron transfer from AgNWs to the conduction band (CB) of SnO<sup>2</sup> (Figure 2b). The carrier migration is a result of combining two materials with different work functions; Ag is characterized by a work function of 4.26 eV and SnO<sup>2</sup> with a work function of about 4.84 eV [22,23]. The photoinduced electrons can get sufficient energy to surmount the Schottky barrier on the Ag/SnO<sup>2</sup> interface despite the uniform energy levels of both components. The electrons are transferred to the semiconductor material to equilibrate the metal-metal oxide alignment and form a new Fermi energy level (Figure 2b). The transferred free electrons are trapped by dissolved oxygen molecules in the water and form a high oxidative species, such as superoxide radical anions (O<sup>2</sup> −•) and hydroxyl radicals (HO• ) [4]. In the core/shell heterostructure arrangement, the recombination of the photoinduced hole-electron pairs is inhibited mainly by forming a complex of free electrons from the CB with oxygen molecules. The trapped electrons can facilitate the formation of O<sup>2</sup> -• and HO• reactive radicals and significantly enhance the rate of their formation, increase photocatalytic activity, and reduce organic substance degradation time. The absorbance intensity of rhodamine B decreased by half after 10 min of irradiation. Moreover, the strong confinement and anisotropic effect in the 1D core/shell metal/semiconductor structures can facilitate carrier separation and increase photocatalytic efficiency.

To the best of our knowledge, it is the first report presenting the photocatalytic properties of the 1D core/shell Ag/SnO2NWs nano-system. It can broaden the range of AgNWs applications since SnO<sup>2</sup> revealed a high environmental stability [12]. The silver nanostructures tend to aggregate and dissolve in the aquatic environment, so the protective shells are also applied to increase stability, processability, and range of applications. The ability of the new catalyst to decompose organic compounds under solar irradiation combined with a high resistance even to harsh conditions can allow the designing of photodegradation systems for various environments and without any external irradiation sources. The advantage of the presented Ag/SnO2NWs photocatalyst is its high stability under different conditions in comparison to other silver/wide band semiconductor composites. An example of that hybrid is Ag/ZnO heterostructure, which, although revealed high photocatalytic activity [24], both its components show a low stability. Silver nanostructures are affected by atmospheric corrosion due to the effective interaction of Ag+ ions with

sulphides and the formation of a silver sulphide layer [25]. ZnO is easily degraded at the nanoscale in hydrophilic environments [26]. Another, more stable semiconductor of TiO<sup>2</sup> that was combined with various Ag nanostructures revealed photocatalytic activity that was characterized by a degradation time of 60 min [27] and more than 90 min [28], but under UV-light. Ag nanowires that were modified with an α-Fe2O<sup>3</sup> show similar efficiency and time of the process; methylene blue was degraded for 30 min under visible light illumination, according to the authors, due to the synergetic effect of LSPR and the effective separation of photogenerated carriers between both materials [29]. A ternary TiO2/Ag/SnO<sup>2</sup> system was applied for the photodegradation of methylene blue for more than 140 min for 40 mg of the photocatalyst of 3.12 × 10−<sup>5</sup> methylene blue (100 mL) under visible light irradiation [5]. Ag/Ag2O/SnO<sup>2</sup> nanocomposites removed malachite green (20 mg/L) after 120 min by using 30 mg of the photocatalyst [7]. The photocatalysis process under visible light with the addition of Ag-SnO<sup>2</sup> nanocomposites that were synthesized using electrochemically active biofilm was measured in hours, but was significantly efficient than for pure SnO<sup>2</sup> [4]. The sphere-like plasmonic Ag/SnO<sup>2</sup> photocatalyst revealed a phenol decomposition time of 50 min that was similar to our results but with different dimensionalities allowing the application of them to other purposes [6]. α −

To study the photodegradation process with Ag/SnO2NWs catalyst that was induced by the light from the visible region, an additional light source from this area at the wavelength of 450 nm was chosen. The removal efficiency of rhodamine B was similar for 395 nm and 450 nm light sources and was calculated to be about 87% and 88% for irradiation at 395 nm and 450 nm (Figure S8). However, the degradation time was not comparable; the process took about 50 and 90 min for irradiation at 395 nm and 450 nm, respectively. The efficiency of the degradation was also high, but the time that was needed to decompose the dye increased to about 90 min (Figure 4a). Both the SPR bands that were irradiated at 450 nm were not so effectively excited as for the 395 nm source. The AgNWs absorbed only the irradiation above 450 nm and their absorbance spectrum was only partially excited (Figure 2a). The process was still efficient and significantly higher than for pure SnO<sup>2</sup> irradiated under 450 nm (Figure 4b).

■ ▲ ▼ ◆ ● **Figure 4.** (**a**) The absorbance spectra of rhodamine B with Ag/SnO2NWs photocatalyst presence under 450 nm light irradiation. (**b**) The degradation of rhodamine B with 2 mg/mL of Ag/SnO2NWs catalyst (), with no photocatalyst (N), with 0.66 mg/mL (H), and 2 mg/mL (◆) of SnO2NPs under 450 nm light irradiation and Ag/SnO2NWs without irradiation (dark experiment, •).

Similar to the 395 nm excitation source (Figure 3b), the absorbance intensity of the dye under illumination at the wavelength of 450 nm without the photocatalyst did not change significantly, indicating the minimal influence of the irradiation on the optical properties of rhodamine B (Figures 4b and S9). The experiment with Ag/SnO2NWs photocatalyst, without applying irradiation (dark experiment, Figure S4), showed only a minimal decrease in the absorbance intensity and demonstrated the stability of the dye and the essential influence of light illumination in the decomposition process. The irradiation at the wavelength of 450 nm on SnO2NPs at both concentrations, 2 mg/mL as Ag/SnO2NWs and 0.66 mg/mL corresponding to SnO<sup>2</sup> amount in the core/shell complex, showed a slight degradation rate of rhodamine B (Figure S10). It indicated that the selected irradiation source did not significantly affect the organic substances when using pure semiconductor as a photocatalyst, only the Ag/SnO<sup>2</sup> heterojunction can be considered as a source for the utilization of organic pollutants under visible light.

Ag/SnO2NWs were also applied for the degradation of malachite green, which is present in the environment and its remediation is highly required. Malachite green is a dye that is commonly used in the textile and food industry. It should be removed after industrial processes due to the fact that it is highly toxic, especially for aquatic flora and fauna [30–32]. Figure 5a shows the rapid degradation of malachite green by Ag/SnO2NWs under the light from the visible range (450 nm); malachite green was completely decomposed after 40 min. The absorbance intensity of the dye with Ag/SnO2NWs photocatalyst presence but without any irradiation did not change significantly (Figures 5b and S11). The dark experiment proved the photodegradation mechanism for fast malachite green decomposition. Malachite green irradiation at the wavelength of 450 nm without photocatalyst revealed only a slight decrease in the absorbance intensity (Figures 5b and S12). The irradiation of SnO2NPs at both concentrations, 2 mg/mL as Ag/SnO2NWs and 0.66 mg/mL corresponding to SnO<sup>2</sup> amount in the core/shell complex also showed only a slight degradation rate of malachite green (Figures 5b and S13). Moreover, to compare Ag/SnO2NWs photocatalyst efficiency at the visible range to a well-known commercial photocatalyst, malachite green was treated with TiO<sup>2</sup> under 450 nm irradiation. TiO<sup>2</sup> as a semiconductor that is characterized by absorption in the UV range, decomposed the dye only to a minimal extent (Figures 5b, S14 and S15) [33]. The core/shell Ag/SnO2NWs system shows a high degradation rate of organic pollutants under visible light irradiation and can be used in practical photocatalysis reactions for efficient remediation.

■ ▲ ▼ ◆ ★ ● **Figure 5.** (**a**) The absorbance spectra of malachite green with Ag/SnO2NWs photocatalyst presence under 450 nm light irradiation. (**b**) The degradation of malachite green with 2 mg/mL of Ag/SnO2NWs catalyst (), with no photocatalyst (N), with 0.66 mg/mL of SnO2NPs (H), 2 mg/mL of SnO2NPs (◆), and with 2 mg/mL TiO<sup>2</sup> catalyst (⋆) under 450 nm light irradiation and Ag/SnO2NWs without irradiation (dark experiment, •).

### **3. Experimental Section**

### *3.1. Preparation of Core/Shell Ag/SnO2NWs*

In the first stage, AgNWs were synthesized by a polyol process and then covered by an SnO<sup>2</sup> shell as a result of the hydrolysis process of sodium stannate; both stages were described in detail in our previous studies [12,34]. In brief, 0.408 g of AgNO<sup>3</sup> solution (purity 99.9999%, Sigma-Aldrich, St Louis, MO, USA) in ethylene glycol (EG, POCH) was added (feed rate of 16 mL/h) to the mixture of 40 mL of EG, 2 g of polyvinyl pyrrolidone (PVP, molecular weight of 55 kDa, Sigma Aldrich) and 0.028 g of sodium chloride (NaCl, Chempur, Karlsruhe, Germany) was constantly heated to 170 ◦C, refluxed, and stirred at 570 rpm. Then, the solution was maintained at the same conditions for 1 h and cooled to room temperature. The mixture of AgNWs was diluted by acetone followed by dispersion in 60 mL of ethanol (anhydrous, POCH).

An SnO<sup>2</sup> shell on AgNWs was obtained in a one-step process by adding 5.051 g of an aqueous solution of sodium stannate trihydrate (0.25 wt.%. Na2SnO3·3H2O, Sigma-Aldrich, 95%) to the AgNWs mixture that was heated to 100 ◦C, refluxed, and stirred at 300 rpm. The solution was kept at the above-described conditions for 15 min and then cooled in a cold water-bath. The mixture of AgNWs was previously dispersed in water by adding 2.5 g of AgNWs that was obtained in polyol process (ethanol solution), to 92.7 g deionized water and 1 wt.% aqueous solution of sodium citrate (Na3C6H5O7·2H2O, purity 99.0%, Sigma Aldrich).

The core/shell Ag/SnO2NWs were filtered (Merck Millipore (Burlington, MA, USA) type RTTR, Isopore membrane Filter, the pore size of 1.2 µm) to remove any by-products of the reactions and obtain high purity samples. To obtain pure SnO<sup>2</sup> that was relevant to the shell part of the complex for the control experiments, 5.70 g sodium stannate trihydrate (0.25 wt.% aqueous solution) was added to 40 g of boiling water and this sample was heated to 100 ◦C and stirred at 600 rpm for 15 min. As a result, the SnO<sup>2</sup> nanoparticles (SnO2NPs) were synthesized. Their morphology and structure were described in our previous work [16].

### *3.2. Photocatalytic Activity Study*

Photocatalytic activities of the core/shell Ag/SnO2NWs were determined by the decomposition of rhodamine B (≥95%, Sigma Aldrich) as a model system. The aqueous solution of Ag/SnO2NWs (2 mg/mL) and rhodamine B (5 mg/L, 10.4 µM) was irradiated by 395 nm and 450 nm LED lamps (100 W). The removal efficiency of rhodamine B by Ag/SnO2NWs photocatalyst for both irradiation sources was calculated based on the initial (*C*0) and final concentration (*C*) of the dye according to Equation (1).

$$H\left(\%\right) = \frac{\mathcal{C}\_0 - \mathcal{C}}{\mathcal{C}\_0} \times 100\tag{1}$$

As control samples, SnO2NPs were also illuminated by 395 nm and 450 nm as well as 365 nm LED lamps at concentrations of 2 mg/mL (the same as Ag/SnO2NWs system) and 0.66 mg/mL (as shell SnO<sup>2</sup> percentage wt.% in the whole complex). The weight percentage of the shell and core of the Ag/SnO2NWs composite was determined based on EDS (Energy Dispersive X-ray Spectroscopy) studies using an FEI Nova NanoSEM 450 microscope that was equipped with EDAX Roentgen spectrometer (EDS) and an Octane Pro Silicon Drift Detector (SDD). The samples were previously collected on silicon wafers for EDS measurements.

Moreover, malachite green oxalate salt (7 mg/L, 15.1 µM, Sigma Aldrich) was degraded with the presence of Ag/SnO2NWs (2 mg/mL) under irradiation of a 450 nm LED lamp (100 W). To compare the photocatalytic ability of Ag/SnO2NWs to commercial photocatalyst, 2 mg/mL of TiO<sup>2</sup> (titanium (IV) oxide, anatase, nanopowder, <25 nm particle size, 99.7%, Sigma Aldrich) was added to malachite green oxalate salt solution (7 mg/L, 15.1 µM) and irradiated by 450 nm LED lamp (100 W). The degradation reactions of rhodamine B and malachite green were monitored by measuring the UV/Vis absorption spectra (UV5600 spectrophotometer, Biosens) of the sample solution taken out at regular intervals, every 10 min for 395 nm and 450 nm LED lamps or every 30 min for 365 nm LED lamp illumination. The sample solution was constantly stirred (400 rpm) and cooled during illumination. The LED lamps were fixed over the vessel that was filled with a sample solution. The absorbance spectra of each sample supernatant were recorded in the

wavelength range of 200 to 800 nm after the centrifugation process (8000 rpm, 2 min) of a 2 mL sample after selected illumination time.

### **4. Conclusions**

The study demonstrates Ag nanowires that are covered with SnO<sup>2</sup> shell as a new, efficient photocatalyst under the irradiation from the visible light range. The fast degradation process is the effect of the combination of advantages of both the components forming the core/shell Ag/SnO2NWs hybrid as well as phenomena appearing on the material interfaces. The SPR absorbance spectrum of AgNWs after coating with SnO<sup>2</sup> shifts towards the visible region and facilitates excitation of the electrons in the nanowires by photons at this wavelength range. The photocatalytic activity of SnO<sup>2</sup> is enhanced significantly and achievable without UV irradiation. The excited electrons from the metal core are transferred to the metal oxide shell and captured by oxygen molecules and involved in the formation of reactive radicals that are essential for the degradation of organic compounds. This report utilizes rhodamine B as a model organic dye for studying the activity of novel 1D metal/semiconductor Ag/SnO2NWs photocatalyst. The rhodamine B is degraded after 50 and 90 min under irradiation at the wavelength of 395 nm and 450 nm, respectively. Moreover, malachite green as an environmental organic pollutant is decomposed after 40 min by Ag/SnO2NWs and only slightly degraded by the common catalyst of TiO<sup>2</sup> under 450 nm irradiation. High photocatalytic activity of the Ag/SnO2NWs system is attributed to the core/shell metal/semiconductor arrangement which results in carrier separations and prevents the recombination of photogenerated electron-hole pairs. The facile processing of an Ag/SnO2NWs hybrid by simple separation, such as filtration or sedimentation, is beneficial for photocatalytic applications.

Our findings indicate that the core/shell of Ag/SnO2NWs represents a very promising material that is characterized by high environmental stability for the designing of future photocatalytic systems under solar irradiation for effective remediation processes of various environments.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/ 10.3390/catal12010030/s1, Figure S1: Absorbance spectra of core/shell Ag/SnO2NWs. Figure S2: The absorbance spectra of SnO2NPs at the concentrations of 2 and 0.66 mg/mL and (inset) Tauc plot for SnO<sup>2</sup> energy gap value determination. Figure S3: The images of an aqueous solution of Ag/SnO2NWS (a) left for 48h for sedimentation and (b) then gently mixed to redisperse the core/shell nanowires. Figure S4: The absorbance spectra of rhodamine B without any irradiation after centrifugation of Ag/SnO2NWs photocatalyst (dark experiment). Figure S5: The absorbance spectra of rhodamine B that was irradiated under 359 nm without catalyst. Figure S6: The EDS spectrum of Ag/SnO2NWs, inset: The weight percentage of O, Ag, and Sn in the hybrid, and STEM image of the sample area for EDS analysis. Figure S7: The absorbance spectra of rhodamine B and SnO2NPs at a concentration of (a) 0.66 mg/mL and (b) 2 mg/mL under 395 nm light irradiation. Figure S8: The degradation of rhodamine B with Ag/SnO2NWs photocatalysts presence under 395 nm and 450 nm irradiation. Figure S9: The absorbance spectra of rhodamine B that was irradiated under 450 nm. Figure S10: The absorbance spectra of rhodamine B with the presence of SnO2NPs at a concentration of (a) 0.66 mg/mL and (b) 2 mg/mL under 450 nm light irradiation. Figure S11: The absorbance spectra of malachite green without any irradiation with the presence of Ag/SnO2NWs photocatalyst (dark experiment). Figure S12: The absorbance spectra of malachite green that was irradiated under 450 nm without catalyst. Figure S13: The absorbance spectra of malachite green with the presence of SnO2NPs at a concentration of (a) 0.66 mg/mL and (b) 2 mg/mL under 450 nm light irradiation. Figure S14: The absorbance spectra of malachite green with the presence of 2 mg/mL TiO<sup>2</sup> photocatalyst that was irradiated under 450 nm with. Figure S15: The absorbance spectra of TiO<sup>2</sup> (P25).

**Author Contributions:** Conceptualization, methodology, validation, and formal analysis, A.B.-K. and G.C.; investigation, A.B.-K., E.M. and K.R.-S.; resources, J.G. and G.C.; data curation, G.C.; writing—original draft preparation, A.B.-K.; writing—review and editing, A.B.-K. and G.C.; funding acquisition, G.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** The research was financially supported by a grant from the National Science Centre, Poland (Opus 15 no. 2018/29/B/ST8/02016).

**Data Availability Statement:** The data presented in this study are available on request from the corresponding authors.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**

