SnO₂ nanorods/graphene nanoplatelets nanocomposites: towards fast removal of malachite green and pathogen control

Graphene nanoplatelets (Knano, 99.9%), tin (IV) chloride pentahydrate (sigma-Aldrich, 98%), urea (sigma-Aldrich, 99%), hydrochloric acid (HCl) and distilled water were used as received. Solvothermal method was used to prepare SnO₂ nanorods. First, 1.2 g metal precursor and 12 g urea was dissolved in distilled water. Few drops of HCl were added to 400 ml precursor solution to obtain pH 4, under constant stirring for 15 min. This solution was transferred to a Teflon lined autoclave, which was then placed in an electric oven for heat treatment at 90 °C for 24 h. Two nanocomposites were prepared by selecting two different quantities of GNPs (12 wt% and 25 wt%). Two graphene solutions were prepared in mixed solvents. Their sonicated dispersions were introduced to SnO₂ solution. The process was carried out under continuous stirring. This solution was transferred to autoclave and was given a heat treatment at 90 °C for 24 h. The precipitates were obtained via washing, several times in ethanol and water, followed by overnight drying. A schematic presentation of the process is depicted in ESI figure 4(a) (available online at stacks.iop.org/NANO/33/115101/mmedia).

A photocatalytic chamber equipped with UV light source (90 W, Type C and emission peak = 254 nm) was used to investigate the effect of UV light irradiation. Malachite green (C₂₃H₂₅N₂) dye was used to investigate the photocatalytic activity of prepared nanomaterials. A 20 ppm solution of malachite green was prepared from 500 ppm stock solution. 0.028 g sample was added to the solution, with a drop of HCl. The beaker was well covered with aluminum foil and was stirred for 45 min in the dark to achieve adsorption-desorption equilibrium, and 4 ml of solution was collected at t = 0 min. This solution was again stirred in the UV light equipped photocatalytic chamber. The test-solutions were collected in a regular manner. Absorbance profiles were then obtained to monitor the changes. To study the impact of visible light, a same process was repeated using same experimental conditions except a visible light source (200 W) was used for irradiation.

P. aeruginosa and S. aureus were used as model bacteria to evaluate the antibacterial activity of prepared samples. The protocol is briefly described here. The isotonic solution was prepared without sample which was considered as control. Under incubator conditions, SnO₂ and SnO₂/GNPs nanocomposites (GS-I and GS-II) were used in the same concentration (10 mg ml⁻¹) for 24 h, to test the antibacterial activity. 200 μl of prepared samples was added to the 5 ml of Luria Bertani (LB). The 100 μl bacterium culture with medium and samples was incubated for 24 h at 37 °C. The optical density at 600 nm was recorded for 24 h. The protocol is described in detail in one of our previous work [17].

The crystal behavior of the prepared samples is shown in figure 1. The data is obtained from 20° to 90°. The SnO₂ shows peaks located at 26.6° (110), 32.6° (020), 38.8° (101), 51.5° (211), 58.2° (002) and 64.7° (112). The result agrees well with a previous work on SnO₂ [18], and also matches with the standard diffraction peaks data (JCPDS No. 00-001-0625). The findings show that SnO₂ possesses tetragonal crystalline structure with lattice parameters, a = b = 4.7 Å and c = 3.17 Å. The diffraction peaks of SnO₂/GNPs nanocomposites found at 26.6°, 32.6°, 38.8°, 51.5°, 54.6° 58.2°, 61.8°, 64.7° and 73.1° are assigned to (110), (020), (101), (211), (220), (002), (310), (112) and (202) planes, respectively [19, 20]. Carbon registers its presence by showing a peak at 26.6° (002), which confirms the presence of graphene in nanocomposites. The crystallite size of SnO₂ in GS-I and GS-II nanocomposites is 3.98 nm and 5.78 nm respectively (obtained via Scherrer's formula). It is observed that the presence of GNPs has affected the growth of SnO₂ in certain crystallographic directions (figure 1) in GS-I and GS-II nanocomposites, e.g. growth of (101) plane has been supressed in presence of greater content of GNPs in GS(II) nanocomposite.

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The size, morphology, and microstructure of SnO₂ nanorods and SnO₂/GNPs nanocomposites have been investigated using TEM. The results are shown in figures 2(a)–(c). Figure 2(a) illustrates that SnO₂ possesses nanorods like morphology. The length and diameter of nanorods are ca. 25 ± 6 nm and 4 ± 2 nm, respectively. Particle size tunes the electronic and optical properties. Particle size also determines the surface area which strongly influences photocatalytic properties. Figure 2(b) demonstrates that SnO₂ nanorods are attached on graphene sheets. Figure 2(c) depicts the detailed view of GS-II nanocomposite. The TEM images show that graphene sheets have well dispersed nanorods. This combined assembly offers high surface area and interfacial contacts, which could possibly play a significant role in photocatalysis and ROS generation.

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The HRTEM and SAED images are shown in figure 3. These images verify the XRD findings, i.e. SnO₂ and SnO₂/GNPs nanocomposites have crystalline nature. Figure 3(a) indicates the spacing between the atomic planes of SnO₂ nanorods. It is 0.288 nm associated with (101) plane of SnO₂. Moreover, figure 3(b) illustrates the spacing between the lattice plane of SnO₂/GNPs nanocomposites. It is found that (101), (110) planes of SnO₂ and (002) plane of carbon are observed with lattice spacing 0.269 nm, 0.326 nm and 0.346, respectively. We performed the selected area electron diffraction (SAED) experiments to further probe the crystallinity of constituent materials and hybrid nature of SnO₂/GNPs (GS-II) nanocomposite. SAED pattern of SnO₂ is presented in figure 3(c). The figure 3(c) shows that the SnO₂ shows (110), (101), (211) and (002) planes which verify XRD results given in figure 1.

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The figure 3(d) also confirms the presence of SnO₂ and carbon in GS-II sample. The presence of carbon (002) plane establishes the inclusion of graphene in the nanocomposite. These findings confirm the biphasic nature of SnO₂/GNPs nanocomposites.

The prepared samples were further analyzed using FTIR spectroscopy to investigate the chemical bonds formation. The successful formation of SnO₂ is presented by a band at ca. 667 cm⁻¹, which is assigned to vibrations associated with O–Sn–O bond [20, 21]. The bands around 1500 and 1620 cm⁻¹ indicate the presence of adsorbed water molecules in SnO₂ and SnO₂/GNPs nanocomposites. The atmospheric CO₂ adsorbed on the sample's surface show a band around 2375 cm⁻¹. Another band centered around 3400 cm⁻¹ is due to –OH bond. No signatures of C–N and H–N bonds are observed in all the samples, which eliminate the possibility of carbon nitride (gC₃N₄) formation [22].

The Raman spectra shown in figure 4(b) indicates the D-band and G-band of neat GNPs that appear at 1350 cm⁻¹ and 1587 cm⁻¹ respectively. These two bands are fingerprint signatures of graphene-based systems. The D-band shows increased intensity for SnO₂/GNPs nanocomposites as compared to neat GNPs, which is due to incorporation of SnO₂ on the surface of GNPs. The G-band shows a shift to higher values i.e., 1601 cm⁻¹ and 1607 cm⁻¹ in GS-I and GS-II nanocomposites, respectively which is due to charge transfer between SnO₂ and GNPs. It indicates the successful formation of nanocomposites.

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The optical properties of SnO₂ and SnO₂/GNPs nanocomposites are investigated to find their suitability for photocatalytic applications. In figure 5(a), the absorbance of prepared nanomaterial has been obtained for wavelengths from 300 to 800 nm.

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The band gap energies are found using Tauc's relation, which is, ${(\alpha h\nu )}^{n}=A(h\nu -{E}_{g}).$ Here, hν, A and α represent the photon energy, constant and absorbance coefficient, respectively. The value of n is determined by indirect bandgap transition and (or) direct bandgap transition.

The optical bandgap energies, calculated using Tauc's equation, are shown in figure 5(b). The bandgap energies change from 3.14 to 2.80 eV in SnO₂ and SnO₂/GNPs nanocomposite (GS-I and GS-II). The band gap energies are reduced with inclusion of graphene. Since, the defects are present in the localized regions and new energy levels may be created between the valance band and conduction band due to the chemical reaction of SnO₂ and GNPs in nanocomposites samples, therefore, a reduction in bandgap energy is observed. This reduction in bandgap energy indicates the potential of the prepared materials towards photocatalysis [23].

The photocatalytic activity of SnO₂ and SnO₂/GNPs nanocomposites has been investigated by degradation of malachite green under UV light and visible light. The variant concentration of malachite green under UV light and visible light has been monitored by obtaining absorbance spectra, with maximum absorption observed at 618 nm. The absorbance profiles of aqueous malachite green solution in presence of SnO₂ and SnO₂/GNPs nanocomposites, under UV light irradiation, is depicted in ESI figures 2(a)–(c). Only a slight fraction of dye is decomposed by SnO₂ nanorods, after a 12 min exposure to UV-light. The absorbance profiles in ESI figures 2(b), and (c) show that GS-I degrades the dye to some extent, in same time, whereas, it has been removed completely, using GS-II nanocomposite. The reaction kinetics of the process are explained in figures 6(a), and (b). Langmuir-Hinshelwood model is used to obtain pseudo-first order rate kinetics. It has been obtained that GS-II has the highest rate constant, k = 0.385 min⁻¹, which is an order higher than that of neat SnO₂ nanorods, i.e. k = 0.038 min⁻¹. GS-I possesses an intermediate rate constant k = 0.117 min⁻¹. The photodegradation efficiency of malachite green has been improved from 37.2% (for SnO₂) to 99.11% (for GS-II), whereas GS-I shows only 78.26% photodegradation of malachite green. This is attributed to optimized GNPs content in the nanocomposite.

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To monitor the photodegradation of SnO₂, GS-I and GS-II nanocomposites under visible light irradiation, a similar process was followed. The absorbance profiles are shown in ESI figures 3(a)–(c). The rate kinetics of the photocatalytic reaction taking place under visible light are presented in the figures 6(c), and (d). The reaction rate kinetics have been studied using Langmuir-Hinshelwood model. The highest rate constant has been observed for GS-II nanocomposite i.e. k = 0.332 min⁻¹, whereas k = 0.116 and 0.019 min⁻¹ is obtained for GS-I and SnO₂ nanorods, respectively. Malachite green has been degraded upto 24.7%, 85.52% and 99.1% in 15 min by SnO₂, GS-I and GS-II, respectively. The photocatalysis results are further enriched by photoluminescence results. Figure 6(e) shows that SnO₂ shows a broad band from 625 to 675 nm, however the intensity of the same band is greatly quenched after the inclusion of graphene in GS-II nanocomposite. This can be attributed to the fact that the electronic properties of semiconducting SnO₂ nanorods are altered by graphene. The photoinduced charge carriers behave differently in SnO₂ and GS-II nanocomposite. The PL results agree well with the photocatalytic results. The photoexcited electrons get trapped by the GNPs that are adjacent to SnO₂ nanorods. GNPs being electron acceptor provide conducting channels for the interfacial charge transfer, which delays the recombination of photoinduced charge carriers. Therefore, the PL intensity is greatly reduced in GS-II nanocomposite. This also explains the excellent photocatalytic ability of GS-II nanocomposite. The reduced recombination of charge carriers in GS-II promotes reactive oxygen species to interact with organic dye for longer time, which in turn results in higher photodegradation of malachite green. The PL supported catalytic activity has also been explained previously [24–26]. Surface area is another factor that contributes towards the efficient adsorption of contaminant on the surface of photocatalyst. Smaller the particle size, greater will be the available surface area for the adsorption of MG on the nanocomposite. SnO₂ nanorods have 25 ± 6 nm length and 4 ± 2 nm diameter. These small sized nanorods greatly enhance the surface area, which promotes efficient photocatalytic activity.

A schematic process explaining the reaction mechanism is shown in figure 7. SnO₂ nanorods show a very low photocatalytic activity. The higher band gap and comparatively low surface area is responsible for low photocatalytic activity. When SnO₂ nanorods get exposed to the UV light in malachite green solution, the VB electrons excite to the conduction band (CB), but these excited electrons de-excite from CB to VB quickly. A few conduction electrons of SnO₂ nanorods can be trapped by dissolved oxygen. Therefore, the reactive oxygen species, so generated, are responsible for slight degradation of dye. SnO₂/GNPs nanocomposites exhibit greatly enhanced photocatalytic activity, due to inclusion of graphene. Its good electrical conductivity is responsible for provision of effective charge separation. The low band gap energy assists in the fast photoexcitation of electrons. These electrons are readily accepted by adjacent GNPs, thus enhancing the separation lifetime of the charge carriers. The photogenerated holes and electrons are then used for the generation of ROS. The ROS attack the organic structure of malachite green, and completely decompose it to CO₂ and H₂O [17, 23]. In addition, the increased surface area of nanocomposites also serves for better adsorption of dye on the catalyst surface thus enhancing the photocatalytic activity. The lower bandgap, increased surface area and GNPs availability, synergistically promote higher photodegradation of dye. It is also observed that GS-II degrades the dye with almost same efficiency in 15 min.

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The recyclability performance of a nanocatalyst is an essential criterion that determines its practical use, particularly in countries with economical constraints. The recyclability test of GS-II nanocomposite was conducted by washing and drying the sample and re-using it four times (figures 8(a), and (b)) both under UV light and visible light irradiation. GS-II shows sustainable photodegradation efficiency for 4 cycles, with only a very slight reduction in efficiency in the 4th cycle, in both cases.

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The GS-II nanocomposite was also tested, after 4 recycles, for microstructure, and crystal structure. The results are presented in figures 8(c), and (d) respectively. Figure 8(c) depicts that SnO₂ nanorods are anchored on graphene nanosheet even after 4 cycles of use as photocatalyst, which shows that the microstructure of the catalyst remains intact. Figure 8(d) presents the crystallinity of GS-II after 4 cycles of photocatalytic activity.

The interplanar spacing in SnO₂ nanorods and graphene is also presented. The result shows that the crystal structure is not deteriorated after photocatalysis.

Graphene and SnO₂ based nanocomposites have rarely been investigated for the removal of malachite green (MG), however, some relevant literature can be found for the removal of other organic contaminants [27, 28]. A systematic comparison of this study with the previous work is cataloged in the table 1. First, a comparison has been established for the degradation of malachite green dye using different materials [29–38], where adsorption and photocatalysis both have been covered. It can be readily inferred from this comparison that SnO₂/GNPs nanocomposite (GS-II) has shown a superior performance for the fast and complete removal of MG (99.11% in 12 min only). In a previous work, a competitive material, Ag₃PO₄@MWCNTs@Cr:SrTiO₃ shows comparable performance of MG removal, but involves expensive material (Ag₃PO₄) and tedious preparation method. Whereas SnO₂/GNPs nanocomposite could be prepared using few and cheap precursors via a simple synthesis method (see ESI figure 3), which makes it superior. In the second part, the table 1 anthologizes the use of graphene-based materials for the elimination of malachite green [39–43], which also establishes the merits of prepared nano-photocatalyst. In addition, the literature shows that graphene-based nanocomposites have been used for decontamination of variety of organic dyes, but few reports are available for the removal of malachite green [44, 45]. In the end, a comparison of SnO₂ based materials, to degrade malachite green dye, has also been presented [46–49]. It not only depicts the outstanding ability of SnO₂/GNPs nanocomposite (GS-II) to remove selected dye (MG) but also shows that SnO₂/GNPs nanocomposite has not been investigated so far, for the removal of malachite green. Therefore, this work adds significantly to the existing knowledge, by making SnO₂/GNPs nanocomposite superior to the contemporary materials in terms of affordability and re-usability.

The testing of novel nanomedicine is inevitable to diffuse the long lasting and harmful effects of drug resistant pathogens. To determine the synergic antibacterial effects of SnO₂ and GNPs, we have tested all the prepared materials against two model Gram-positive and Gram-negative bacterial strains (P. aeruginosa and S. aureus). Figures 9(a) and (c) represent the OD profiles and cell viabilities of the selected bacterial strains in the presence of SnO₂, and SnO₂/GNPs nanocomposites (GS-I and GS-II). The results show that SnO₂ inhibits 26.03% and 13.46% growth of P. aeruginosa and S. aureus, respectively. SnO₂/GNPs nanocomposite (GS-I) inhibits 42.41% and 44% growth of P. aeruginosa and S. aureus, whereas GS-II nanocomposite has resulted in 79.57% and 78.51% inhibition of P. aeruginosa and S. aureus, respectively. Hence, GS-II has exhibited a significant antimicrobial effect.

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The overall performance of GS-I and GS-II is considerably better than that of SnO₂, which is due to an increased amount of graphene. GS-II has significantly controlled the life threatening pathogens from 2 to 24 h [17, 52].

The same experiment was conducted in dark as well. The results are shown graphically in figures 10(a), and (b). The OD profiles show that the growth inhibition of both bacterial strains in dark is significantly less than the one carried out in ordinary setup. Figure 10(a) shows that SnO₂, GS-I, and GS-II inhibit 6.36%, 14.05% and 24.39% growth of P. aeruginosa in 24 h. Figure 10(b) shows that SnO₂, GS-I, and GS-II inhibit 7.46%, 14.26% and 26.36% growth of S. aureus in 24 h.

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There have been several explanations about pathogen control by graphene-based nanocomposites. Figure 11 details a schematic presentation of the possible pathways that lead to the bacterial death. Graphene has wrinkled surface (being flexible) and sharp edges (like cutters). It has been proposed that the edges play a role of cutter by directly disrupting bacterial membranes. The damaged bacterial membrane can ultimately lead to the leakage of cytoplasm [53]. This membrane disruption may result immediate cell death. The TEM image (figure 2(b)) shows that the graphene has a sheet like structure with a rough surface due to presence of SnO₂ nanorods. The thin and sharp edges of graphene are also visible. This morphology has been experimentally reported to impair cell membranes in Gram-negative bacteria like E. coli. It has been explained by direct experimental evidence, using TEM images, that edges of graphene act like blade that cut the bacterial membranes. It has been clearly demonstrated that graphene may get inserted into E. coli to damage it completely. The loss of membrane integrity has been found to be associated with the lipid extraction within few nanoseconds, mainly caused by graphene wrapping around the cell and its cutter like activity [54–57]. The membrane disintegration is schematically described in figure 11.

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It has been experimentally proven that the bacterial membranes are electron rich surfaces. The graphene sheets being an outstanding electron acceptor material, are readily available for membrane-electrons, thus creating an electron deficit at the membrane surface. The unbalanced charges appear as another cause to damage the toxic bacterial cells [53].

The antibacterial performance of a nanomaterial depends strongly on its particle size. The dimensions of a nanomaterial determine its penetrability into the bacterial cell. Nanomaterials with small particle sizes can easily penetrate in the bacterial cell membranes by passing through the porous cell membranes. SnO₂ nanorods with 25 ± 6 nm length and 4 ± 2 nm diameter can easily penetrate through the permeable membranes pores. The easy inflow of nanostructures is only possible if their size is smaller than that of membrane pores [58]. When the SnO₂ nanorods enter the cytoplasm, they may interrupt normal function of cell along with many other destructive mechanisms. This also explains the slight antibacterial activity exhibited by neat SnO₂ nanorods.

The formation of ROS has been a common theme for toxicity of nanomaterials. ROS initiated oxidation-reduction reactions cause cell death with onset of DNA destruction and protein denaturation. It is an established fact that the positive metal ions, free electrons could also lead towards ROS initiated oxidative stress, which is primarily responsible to damage the pathogenic bacteria cells [6]. All the above-mentioned activity, in a coherent manner, ultimately results the significant inhibition of bacterial growth. However, it is important to address the effect of light in ROS generation. In normal day light experiment, the generation of ROS is considerably enhanced due to creation of photoinduced charge carriers. These electrons, holes, and ROS are ultimately used to generate oxidative stress in bacterial cells. The basic conclusion derived from the antibacterial assessments is that ROS generation has significant effect in bacterial growth inhibition. The ROS generation varies in ambient light and in dark. In the absence of light, the charge carriers (e−h pairs) can't be generated which reduces the growth inhibition of bacteria. The large cell viabilities have been observed in dark which are mainly due to the absence or a smaller number of ROS. The slight bacterial death that has been found in the dark is largely due to sheet like cutting effect of GNPs in the nanocomposite.

There are other factors that play an important role for the observed antibacterial activity of SnO₂ and SnO₂/GNPs nanocomposites in the dark. First, the electrostatic interaction between SnO₂ nanorods and bacterial cell membrane leads to transport of Sn²⁺ ions into the cell cytoplasm. The interaction between Sn²⁺ and cytoplasmic organelles may cause DNA damage which may lead to bacterial cell death. Also, the membrane malfunctioning due to attached SnO₂ nanorods is one of the reasons of bacterial growth inhibition. These mechanisms have been proposed previously for experiments that are conducted in dark [59, 60].

Table 2 depicts the performance of various graphene/metal oxide nanocomposites. It can be readily concluded from the comparison that SnO₂/GNPs nanocomposite (GS-II) possesses significant control over bacterial growth, both for Gram-positive and Gram-negative bacteria