Synthesis, characterization, optical properties investigation and reusability photocatalyst capacity of AgCl-xGO composite

In this work, we report the photocatalytic reuse study of AgCl-xGO composite with different GO concentration (x = 1, 2, 4 and 8 mass %) to degrade cationic dyes. X-ray diffraction and scanning electron microscopy analysis were performed to characterize the formation and morphological features of the synthesized AgCl-xGO composite. Raman spectroscopy and UV–Vis studies were used to characterize the AgCl optical properties. It was observed an Increasing in the photocatalytic efficiency due to interface created by AgCl-GO composite associated to the impeding recombination of the electron–hole pair. Additionally, we notice that after 3 min of exposure to UV radiation, the AgCl-8%GO composite completely degraded the methylene blue dye while pure AgCl sample degraded 75.3%. The AgCl-8%GO composite showed stable photocatalytic activity even after the fourth cycle, indicating a high potential for application in consecutive photocatalytic cycles.

Electronic supplementary material The online version of this article (10.1007/s10854-019-01894-w) contains supplementary material, which is available to authorized users.

Human life quality is the main concern when studying soil, water and air quality due to the discard of industries effluents in the environment [[1]]. The treatment of these effluents are difficult since most of the chemical composition of the effluents belongs to the azo corants family, which are characterized by the presence of one or more groups of -N = N- attached to aromatic rings [[3]]. Hence, the treatment of these compounds by traditional means such as coagulation, flotation and sedimentation are ineffective since they do not act on the -N = N- group [[4]]. Therewith, several methodologies including biodegradation, direct and indirect oxidation and heterogeneous photocatalysis have been widely explored to overcome this issue [[4]]. Among the aforementioned methods, heterogeneous photocatalysis is characterized by the use of a semiconductor to generate an electron–hole (e−/h+) pair that mineralize the organic molecules of azo dyes [[6]]. For instance, zinc, copper, molybdenum and silver are among the many materials that can be used in heterogeneous photocatalysis [[7]].

Silver compounds have gained considerable attention in recent decades due to their high potential application in several areas: catalysis [[9]], fuel cells [[10]], lithium-ion batteries [[11]], etc. Among them, silver-chlorite (AgCl) compound is the most studied since it presents a high efficiency in the degradation of organic materials [[12]–[14]]. Additionally, by combining AgCl with other material (e.g. TiO2 [[15]], iron [[16]] and Bromine [[17]]) its properties can be considerably optimized. Recent reports have shown that silver-graphene composites present high stability and photoactivity [[18]], thus being a potential composite to heterogeneous photocatalytic applications.

In addition, graphene oxide (GO), a carbon-based compound with a lamellar structure, has a high surface area and potential in several technological applications [[20]]. The GO's high surface area makes it a material with excellent adsorbent properties [[21]]. For instance, ZnO-GO composite exhibits better photocatalytic activity in comparison to the ZnO nanoparticles since the dye is adsorbed on the surface of the GO [[22]], where the GO acts as a charge separator occasioned by its two-dimensional π conjugation structure.

In this work, silver chloride (AgCl) and GO powders were used to form the AgCl-xGO composite (x = 0, 1, 2, 4 and 8 mass %). X-ray diffraction (XRD), field emission scanning electron microscopy (SEM), Raman spectroscopy, absorbance spectroscopy in the visible ultraviolet region, diffuse reflectance spectroscopy is used to characterize the crystalline structure and optical properties of the synthetized AgCl-xGO. Further, methylene blue was used to investigate the efficiency of the AgCl-xGO composites as a reusable catalyst.

The composites were synthesized using flake graphite (Vonder, comercial), sodium nitrite (Synth, 99%), sulfuric acid (Neon, 98%), hydrochloric acid (Synth, 38%), potassium permanganate (Neon, 100%), hydrogen peroxide (Synth, 29%), silver nitrate (Synth, 99%), sodium chloride (Dinâmica LTDA, 99,0%), PVP (Alfa Aesar, 99,0%) and deionized water, as received.

Graphene oxide was obtained by a modification of the Hummer's method [[23]]. In this method, 1 g of flake graphite and 0.5 g of sodium nitrate were kept under stirring in 23 mL of sulfuric acid for 1 h. After this time, 3 g of potassium permanganate was gradually added and by always keeping the temperature below 20 °C, then the solution was immersed in an ice bath. The solution was then kept under stirring at a temperature of 35 °C for 14 h. 500 mL of deionized water was added, and the solution was kept under vigorous stirring for 1 h. After that, the solution was treated with 5 mL of hydrogen peroxide (30%) for 30 min to complete the reaction with potassium permanganate. The resulting mixture was washed and filtered with 250 mL of hydrochloric acid and 250 mL of deionized water.

To obtain the powders of silver chloride, silver nitrate, sodium chloride and PVP were added in 100 mL of deionized water and kept under stirring for 15 min, after which the solution was taken to the Branson 102C tip. The AgCl-xGO composite (with x = 1, 2, 4 and 8 mass %) was prepared by adding the AgCl and GO powders in 50 mL of deionized water, kept under magnetic stirring for 3 h. The precipitate was centrifuged and dried at 60 °C.

The compositions of the phases were characterized by the Shimadzu diffractometer (XRD-6000) using CuKα radiation (1.5418Å), step of 0.02° and scanning speed of 1°/min, with 2θ ranging from 5 to 80°.

Field scanning electron microscopy (SEM) was performed in the Hitachi tabletop microscope (model TM-3000) to observe the morphology and interface between the AgCl-GO phases.

Raman spectra were acquired under ambient conditions in a micro-Raman spectrometer (WITec Access) excited by a 632.8 nm (1.96 eV) laser line. A grating of 300 lines/mm was used in the backscattering geometry, and a 100 × objective lens was used to focus a laser spot size of ~ 1 µm onto the sample.

Ultraviolet–visible (UV–Vis) spectroscopy was performed on Shimadzu equipment (UV-2550), with a wavelength range of 200–900 nm and programmed for the diffuse reflectance mode, where from these results it was estimated the band gap energy (Egap) of these materials using the Wood and Tauc Equation [[24]].

The photocatalytic properties of the powders were tested against the methylene blue (MB) dye of molecular formula [C16H18ClN3S] (Mallinckrodt, with 99.5% purity), at pH 5, under UV–Vis radiation. For this, 0.05 g of the composite was placed in a beaker containing 50 mL of MB dye (1.10−5 mol.L−1) and kept under constant stirring, illuminated by six UVC lamps (TUV Philips, 15 W, with maximum intensity of 254 nm = 4.9 eV). At intervals of 30 s, A 2 mL aliquot is collected to analyze the absorbance spectrum variation using the Shimadzu spectrometer (model UV-2600). By these values, the variation of the concentration of the dye was determined by the test time. The reuse capacity of the material was tested, where after each cycle the powder was separated and dried, always maintaining the same ratio of catalyst/dye. The materials were submitted to four cycles.

To identify the reactive species (•O2− and •OH) or the surface charges (h+ and e−) acting on the photocatalytic process were performed in a similar way to the photocatalytic test, however, Silver nitrate (Strem-Chemicals, 99.9%), Ethylenediaminetetraacetic acid disodium salt dihydrate (Sigma-Aldrich, 99.0%), p-Benzoquinone (Sigma-Aldrich, 98%) and isopropyl alcohol (Quimidrol, 99.0%) were added to isolate e−, h+, •O2− and •OH, respectively.

Figure 1 shows the XRD patterns obtained for graphite, GO, AgCl and AgCl-xGO (x = 0, 1, 2, 4 and 8 mass %) samples. One can observe from the patterns the disappearance of the graphite's characteristic refraction peak at around 2θ = 26° and the appearance of its characteristic diffraction peak of the around 2θ = 10° from graphite to the GO formation, referring to the plane (002), being in agreement with previous reports in the literature [[25]–[27]]. The displacement of the characteristic peak from 2θ = 26° to 10° implies in an increment in the interplanar spacing of the material [[28]]. The low intensity refraction peak that appears around 2θ = 43° refers to the (100) plane indicating that the material exhibits a long-range organization [[25]].

Graph: Fig. 1XRD patterns for a graphite, GO and AgCl and b AgCl-xGO composites (x = 1, 2, 4 and 8 mass %)

The diffractogram of the AgCl indicate that there was no formation of secondary phases. The characteristic refractive peaks at 2θ = 27.82°, 32.23° and 46.23° refer to the (111), (002) and (002) planes, respectively. AgCl was characterized by the ICSD card 64734, with cubic structure (a = b = c = 5.549 Å) and space group Fm-3 m. Figure 1b shows the diffraction patterns of the AgCl composites with different amount of GO: 1%, 2%, 4% and 8%, respectively. The absence of the GO's peak around 2θ = 10° indicates the reduction of GO [[28]].

Scanning electron microscopy experiment was performed to observe the AgCl morphology, as well as the arrangement of the AgCl particles in relation to the GO concentration. Figure 2 shows the SEM images for the AgCl (a), GO (b) and for the AgCl-xGO (c–j) composites. From Fig. 2a, was noted that the AgCl synthesis by sonochemical method promotes the formation of nanoparticles on its surface. Figures 3b and S1 shows that the nanoparticles are in fact metallic silver. According to the diffractogram shown in Figure S1, it is also possible to observe that in addition to the metallic silver peaks, the formation of AgO occurs. Wang et al. [[30]] synthesized AgCl particles using the precipitation method and also observed the formation of metallic silver nanoparticles on its surface. The presence of noble metal nanoparticles on the surface of the particles promotes the increase of their electronic properties due to the generation of the plasmonic effect [[31]]. Figure 2b shows the surface of the GO, with lamellar structure and irregular surface, generates a larger surface area and, consequently, an enhancement in the surface related properties [[33]]. The SEM micrographs shown in Fig. 2c–i indicates that the mechanical agitation is effective in obtaining the AgCl-xGO composites. This result demonstrates that the AgCl particles decorate the GO layers.

Graph: Fig. 2SEM micrographs for a AgCl, b GO, c–d AgCl-1%GO, e–f AgCl-2%GO, g–h AgCl-4%GO and i–j AgCl-8%GO, respectively

Graph: Fig. 3a EDX area spectra for the AgCl-8%GO composite, b punctual EDX spectra on the surface of the AgCl particle and c mapping of the elements present in the AgCl-8%GO composite

Figure 3 shows the energy-dispersive X-ray (EDX) spectra performed at the area and chemical mapping of the AgCl-8% GO composite (Fig. 3a and c, respectively). In addition, the spot EDX performed on the nanoparticle on the surface of the AgCl particle is also shown in Fig. 3. The mapping shown in Fig. 3c indicates a good uniformity of the dispersed AgCl particles on the GO layers. Figure 2d–j show the interface between AgCl particles and GO for composites for 1, 2, 4 and 8% GO, respectively. Reports have shown that the interface formation between the GO layers with nanoparticles materials promotes an enhancement in the optical and mechanical properties of the composites due to the transfer of charge and for acting as a separation center e−/h+ [[35]]. Zhang et al. [[37]] have observed that the Al-GO composite presents physical and mechanical properties superior to pure Al. Moreover, by further analyzing the micrographs that illustrate the junction of the AgCl particles with the GO (Fig. 2d–j), was noticed that this interface was formed not only physically, but also chemically. Previous works have shown that GO has hydroxyl groups in its basal plane and, carboxylic and ketone groups in its borders, acting on the chemical interaction between AgCl and GO [[38]–[40]]. Hence, the appearance of oxygen-containing functional groups act to facilitate the formation of this chemical interface during mechanical agitation [[35]]. Furthermore, Makheta et al. [[41]] observed that the oxygen-containing functional groups of GO have a strong interaction with metallic agglomerates causing, for example, Cu (tpa) microcubes to deposited on its surface.

Figure 4a shows the Raman spectra obtained for the AgCl, GO and AgCl-xGO samples collected using a 632.8 nm laser energy. In Fig. 4a, the Raman spectrum of the AgCl present a high fluorescence background characteristic of silver-based metallic materials. As the GO amount is introduced in the AgCl, the fluorescence background considerably reduces and the two characteristic Raman peaks of GO samples start to appears located at around 1329 cm−1 and 1594 cm−1 associated to the D and G bands, respectively [[42]]. The G band are associated to the in-plane vibration of the E2g phonon at the Brillouin zone center. This peak arises from the stretching of the C–C bond of sp2 hybridized carbon-related materials. The D band is a defect-active band and it is related to breathing mode of the six-atom ring [[44]]. Thus, the D band can be used to probe the oxidation of the crystalline structure of the graphite through reducing the domains in the sp2 plane [[45]].

Graph: Fig. 4a Raman spectra and b absorbance spectra of the AgCl-xGO (x = 0, 1, 2, 4 and 8% mass)

The intensity ratio of the D and G (ID/IG) bands is a mean to determine the size of domains in the sp2 plane, where the enhancement of the intensity ratio indicates an effective reduction of oxide in the structure [[46]–[48]]. Notice that as the amount of GO is increased into the AgCl, the D and G bands are more prominent where the D band presents a higher intensity than the G band with an intensity ratio higher than 1. This result characterizes the success of the oxidation into the sample. The increase in intensity of the characteristic bands of GO occurs due to hybridization with the AgCl, where the AgCl act as donor and the GO as receptor of electrons [[18]]. Thus, such results indicate the formation of the AgCl-xGO composite.

The absorbance capacity of the AgCl-xGO samples were characterized by UV–Vis experiment. Figure 4b shows the absorbance spectra of AgCl and AgCl-xGO composites (x = 1, 2, 4 and 8 mass %). The AgCl spectrum shows a maximum absorption peak at 295 nm and a lower absorbance peak is observed above 400 nm. The band formed around 540 nm is attributed to the surface plasmonic effect of silver nanoparticles that may have been photoreduced on the surface of the AgCl-xGO composite [[49]]. Note that the addition of GO promotes a peak shift of the maximum absorption to 305 nm and increases the absorbance to lengths above 400 nm. In addition, it promotes an absorption peak around 340 nm. As previously stated, Hummer's method was used to synthesize the GO samples. This method induces the formation of functional groups containing oxygen between the GO layers, thus generating a high amount of defects that, in contact with particles of other materials, create intermediate levels in the valence-to-conduction transitions and, therefore, increases the absorbance for larger wavelengths [[25], [50]].

The diffuse reflectance spectroscopy technique in the visible ultraviolet region was used to estimate the bandgap (Egap) values of the AgCl-xGO powders (x = 0, 1, 2, 4 and 8% mass) to track how the addition of GO can affect the AgCl bandgap. Therefore, we have used the Kubelka–Munk function [[52]] to convert the reflectance to absorbance data. After that, the Wood and Tauc method [[24]] was used to estimate the value of the optical bandgap given by:

Graph

Here, $\mathit{hv}$ is the photon energy, F(R) is the absorbance and $k$ indicates the different transitions ( $k$ = 1/2, 2, 3/2 or 3 for permissible direct, permissible indirect, direct prohibited and indirect prohibited, respectively). For the composite AgCl-xGO (x = 0, 1, 2, 4 and 8 mass %) was admitted $k=1/2$ , i.e. direct permissible. Absorbance as a function of the photon energy (eV) of each AgCl-xGO (x = 0, 1, 2, 4 and 8 mass %) composites are shown in Fig. 5a–e. Here, the curve linear portion was extrapolated to zero absorption to estimate the Egap. Figure 5f shows the permissible direct Egap values obtained from the linear fitting (red dashed lines in Fig. 5a–e) of each AgCl-xGO (x = 0, 1, 2, 4 and 8 mass %) composites.

Graph: Fig. 5Extrapolation of the linear portion of the absorbance curve as a function of the photon energy for a AgCl, b AgCl-1%GO, c AgCl-2%GO, d AgCl-4%GO, e AgCl-8%GO and f E gap variation of the GO amount (Color figure online)

The obtained Egap value for AgCl (2.82 eV) agrees with previous literature reports [[12], [53]–[55]]. From Fig. 5f, we notice that as the amount of GO increases the Egap value decreases with a bandgap variation of around 0.63 eV from the GO concentration goes from 0 to 8%. Such reduction of the bandgap value is expected since from the absorbance curves shown in Fig. 4b indicates that the addition of GO increases the absorbance in the visible region, in relation to the pure AgCl. The use of GO in the production of composites provides an improvement of the composite's optical properties which is related to the synergy effect from the interfacial interaction between the GO-composite phases [[56]].

The photocatalytic activity of AgCl and AgCl-xGO composites was monitored by varying the concentration of methylene blue dye. The MB solution was investigated under UV–Vis radiation in time interval of 4 min, with a withdrawn aliquot every 30 s. As can be seen in Fig. 6a, when AgCl-xGO are employed as a photocatalyst, an appreciable reduction of the MB concentration is observed. Degradation of the MB is indicated by the decay of the absorbance signal of samples near 660 nm at different time intervals, as shown in Fig. 6a. Comparison of photodegradation of MB dye catalyzed with AgCl-xGO in terms of the change in concentration with respect to the initial concentration is shown in Fig. 6b. For a better visualization of the photocatalytic efficiency (see Eq. 2) of the powders, Fig. 6c shows the percentage degradation of MB to each GO amount after 3 min. The photocatalytic effect of AgCl-xGO can be described by determining the first-order kinetic constant, as shown in Eq. 3 [[58]]:

Graph: Fig. 6a Variation of absorbance for AgCl, b Variation of MB concentration as a function of the test time for AgCl-xGO (x = 0, 1, 2, 4 and 8% mas), c Percentage of degradation without and with catalyst after 3 min and d Evolution of photodegradation kinetic

2 $$\left(1-C/C_0\right).100$$

Graph

3 $$\left(-ln{C}_t/C_0\right)=kT$$

Graph

Here, $C_t$ is the absorbance of methylene blue after irradiation at a selected time interval, $C_0$ is the initial absorbance, $T$ is the irradiation time and $k$ corresponds to kinetic constant. Figure 6d shows the linear fitting of experimental results by Eq. (3) and the extracted for kinetic constant ( $k$ ) and the correlation coefficient (R2) are shown in Table 1.

The apparent first-order rate constant, k of photocatalytic degradation and correlation coefficient, R2

The above results demonstrated that the synthesized powders produced in this work show a high photocatalytic property in the degradation of MB dye. Thus, the obtained composites completely degraded the MB dye after 4 min of time showing that the pure AgCl degrades by 96%. The extracted values of the kinetic constant provide the behavior of the material along the photocatalysis. According to Table 1, the sample with 8% of GO has a constant k to be 1.558 min−1 which is higher as compared to that of pure AgCl sample (0.477 min−1). This result indicates that the composite which has 8% of GO reduces the MB dye in three orders of magnitudes. Sharma et al. [[59]] showed that the addition of 1% of GO into TiO2 doubles the photocatalytic kinetic constant (from 0.003 to 0.006 min−1) compared to the crystal violet dye after 150 min. The photocatalytic activity of semiconducting materials is directly related to the generation and impediment of the electron–hole pair recombination in which acts in the generation of hydroxyl radicals (•OH), which are strong oxidizing agents of organic materials, such dyes [[60]–[62]].

The addition of GO causes the absorption spectrum to be displaced to the visible light length, facilitating the creation of electron–hole pairs [[18]]. The GO reduces the movement of the dye molecules by adsorbing them through strong π–π bonds [[63]]. Furthermore, the AgCl possess a strong electron donor character whereas the GO is one of the best receptors, thus the AgCl-GO interface (shown in Fig. 4a) acts by restricting the recombination of the photogenerated e−/h+ pairs, increasing the number of reactive species of oxygen (ROS) available for dye degradation [[49], [65]].

In order to identify the mechanisms acting on the photocatalysis, the AgCl and AgCl-8% GO samples were submitted to photocatalytic tests in the presence of e−, h+, •O2− and •OH, respectively. Thus, silver nitrate, ethylenediaminetetraacetic acid disodium salt dehydrate (EDTA), p-Benzoquinone (BZQ) and isopropyl alcohol were added at a ratio of 0.10 M to MB dye (50 mL, 1.10−5 mol.L−1) [[66]]. Figure 7a, b shows the degradation of MB dye in the presence of different scavengers. Figure 7c shows the percentage degradation of the photocatalytic for both AgCl and AgCl-8%GO for different scavengers after 3 min.

Graph: Fig. 7Photodegradation for the a AgCl and b AgCl-8%GO in the presence of different scavengers. c Comparison of photocatalytic activity with and without scavengers for the AgCl and AgCl-8%GO after three minutes

The presence of isopropyl alcohol and BZQ do not significantly alter the photocatalytic activity of AgCl after 4 min, but it was observed that for shorter times the reduction of MB dye concentration occurs more rapidly. In stark contrast, the addition of EDTA and silver nitrate reduces photocatalytic activity. These results indicate that the main mechanism acting on photocatalysis is related to the surface charges (h+ and e−) [[68]]. As previously mentioned, the interface formed between the AgCl-GO composite provides greater generation of e−/h+ [[35]] and this is checked by the addition of scavengers (i.e. e− and h+) which showed a considerable reduction in photocatalytic activity [[49]]. Also, the observed reduction in photocatalytic activity by adding isopropyl alcohol and BZQ indicate that •OH and •O2− significantly in the photocatalytic activity [[69]]. Thus, the increase of the photocatalytic activity by the formation of the AgCl-GO composite is associated with the use of the radicals •OH and •O2− in the degradation of the MB dye.

Due to degradation capacity of organic contaminants, mainly dyes, which are difficult to degrade by conventional methods, heterogeneous catalysis has been extensively studied [[70]]. The reusability of the photocatalyst is related to the material's ability to remain active even after its use hence avoiding generate another type of tailings. Basically, the evaluation of the catalyst can be based on two criterion [[70]]: (I) the ability of the catalyst retain its activity over time and (II) the facility that the catalyst could be recycled from the solution. The recyclability of AgCl-xGO materials (x = 0, 1, 2, 4 and 8 mass %) was also investigated. The photocatalytic tests were performed using the same powders four times and the results are shown in Fig. 8. Here, both the proportion of catalyst and solution were kept the same during the 4 min cycle. From Fig. 8, it can be observed that both the AgCl and the AgCl-xGO composite (x = 1, 2, 4 and 8 mass %) maintain their photocatalytic efficiency after four cycles.

Graph: Fig. 8Variation of MB concentration by the test time for a AgCl, b AgCl-1%GO, c AgCl-2%GO, d AgCl-4%GO and e AgCl-8%GO samples during four cycles

The diffractograms confirmed the AgCl formation with high crystallinity and no formation of secondary phases. SEM images showed the efficient in obtaining the AgCl-xGO composite through the mechanical agitation method, was has observed the formation of interface between the components. Raman results show the reduction of the high fluorescence background characteristic of AgCl by increasing the amount of the GO. The increase in GO concentration increased the absorbance in the visible region, resulting in the Egap reduction. These optical results indicate that the addition of GO reduces the energy required for electronic excitation of the valence band to conduction band. Finally, photocatalytic studies of the as-synthesized AgCl-xGO composites showed a high efficient in the degradation of methylene blue dye, where the 8% GO sample degraded completely in only 3 min under UV radiation. Our composite showed a high efficient as a reusable material, where there was practically no loss of its efficiency in four photocatalytic cycles. Therefore, it is concluded that GO concentration increase in AgCl-xGO can enhance the photocatalytic activity and thus can better degrade organic pollutants.

This study was partially financed in part by the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior—Brasil (CAPES/PROCAD)—Finance Code 2013/2998/2014. The authors also thank the financial support from the Brazilian research financing institution: CNPq No. 307546/2014. B.R.C acknowledges the financial support from the Brazilian agencies CAPES, CNPq and FINEP.

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