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Decoration of ZnO microstructures with Ag nanoparticles enhanced the catalytic photodegradation of methylene blue dye
Accepted Manuscript Title: Decoration of ZnO microstructures with Ag nanoparticles enhanced the catalytic photodegradation of methylene blue dye Authors: H.A. Rafaie, R.M. Nor, M.S. Azmina, N.I.T. Ramli, R. Mohamed PII: DOI: Reference:
S2213-3437(17)30374-3 http://dx.doi.org/doi:10.1016/j.jece.2017.07.070 JECE 1785
To appear in: Received date: Revised date: Accepted date:
28-4-2017 22-7-2017 27-7-2017
Please cite this article as: H.A.Rafaie, R.M.Nor, M.S.Azmina, N.I.T.Ramli, R.Mohamed, Decoration of ZnO microstructures with Ag nanoparticles enhanced the catalytic photodegradation of methylene blue dye, Journal of Environmental Chemical Engineeringhttp://dx.doi.org/10.1016/j.jece.2017.07.070 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Decoration of ZnO Microstructures with Ag Nanoparticles enhanced the catalytic photodegradation of Methylene Blue Dye H. A. Rafaiea,*, R. M. Norb, M. S. Azminab, N. I. T. Ramlia, R. Mohameda a Faculty of Applied Sciences, Universiti Teknologi MARA Pahang, Bandar Tun Abdul Razak Jengka, Pahang,Malaysia *[email protected]; [email protected]; [email protected] b Department of Physics, Faculty of Sciences, Universiti of Malaya, 50603, Kuala Lumpur, Malaysia [email protected]; [email protected] * Corresponding author Abstract: ZnO microstructures decorated with Ag nanoparticles photocatalysts synthesized using a simple sol-gel method at different Ag content. The prepared photocatalysts were then characterized by XRD, FESEM, EDX and TEM shows that the ZnO microstructures successfully decorated with Ag nanoparticles and influenced the catalytic photodegradation performance of methelyne blue dye under UV light irradiation. The effect of different Ag content on the structural, surface morphology and catalytic photodegradation of Ag nanoparticles decorated microstructures ZnO were analyzed. The results shows that there is a trend of increasing photocatalytic activity with Ag content of between 2.33 to 63.31 at. % where the highest photodegradation efficiency 87.74 % and photodegradation rate constant, k of 0.032 min-1 for degradation of methelyne blue with 60 min of irradiation. The enhancement of the photocatalysis activity with increasing Ag content can be explained by the increase in the electron-hole pair lifetimes due to electrons promoted to the conduction band are trapped in the Ag nanoparticles which acted as electron sinks. Keywords: Ag nanoparticles; ZnO microstructures; sol-gel; catalytic photodegradation; photocatalyst 1. Introduction Industrial wastewater may contains various organic and inorganic pollutants which can have adverse effect on aquatic life when discharged to environment. Effluents containing dyes is known to create severe environmental pollutions [1]. Dye in wastewater contaminates surface and ground water and can harm ecological resources including water quality, soils, plants, animal and also human health. Textile industry is one of an example that produces a large quantity of highly coloured effluents [2]. Much attention has been devoted to develop effective technologies for the removal of organic pollutants from aqueous media. Recently, technologies including biodegradation, adsorption, and oxidation processes have been developed [3]. Photocatalysis degradation is a promising technique for waste water treatment which can provide good applicability that can rival the methods mentioned earlier [4]. Photocatalysis is a catalytic reaction involving light absorption by a catalyst or a substrate [5]. A photocatalyst is a substance which is activated by adsorbing a photon and is capable of accelerating a reaction without being consumed. To meet the need for practical applications, photocatalyst should have high photocatalytic activity, non-toxic, cost effective and high yield. Oxide-based materials such as TiO2 [6], NiO [7] and ZnO [8] have attracted much interest due to their high photocatalytic activity using uv light to match the values of the band gaps of these materials. Recently, ZnO and TiO2 have received much attention as photocatalyst due to their unique properties and potential applications [9]. The band gap of ZnO (3.25 eV) and TiO2 (3.05 -3.2
eV) are almost similar, so it is expected that ZnO to have the same photocatalytic ability as TiO2 [10,11]. As a photocatalyst, ZnO also exhibit large initial rates of activity due to the presence of active sites with higher surface reactivity [12]. Nevertheless, one of major drawback of ZnO photocatalyst is the low quantum efficiency of ZnO due to the fast recombination of photogenerated electron-hole pairs which significantly decreased the photocatalytic activity of ZnO [13,14]. Thus, the low photocatalytic efficiency of the ZnO photocatalyst still remained a challenge and is insufficient for industrial scaled applications. There have been many reports on studies done to modify the ZnO nanostructures in order to improve the photocatalytic efficiency. The modifications of the photocatalysts are significant in increasing the lifetime of the electron–hole pairs, broadening the absorption spectrum, and to facilitate some specific reactions on the surface of catalysts [15]. The most efficient way to accelerate the charge carrier separation and improve the catalytic activity is by modifying the surface structure of ZnO by decoration with transition metal such as Pt [16,17], Ag [18], Fe [19] and Mg [20]. However, the underlying photocatalytic mechanism is complex and remained controversial. It is widely accepted that Ag nanoparticles on ZnO surface acted as electron trapping and thus increased the lifetime of electron-hole resulting on the enhancement of the photocatalytic activity [13,21]. In this paper, we report a sol-gel method to prepare Ag nanoparticles (AgNPs) decorated microstructures ZnO at various Ag contents. The effect of different amount of Ag contents on the photodegradation efficiency of methylene blue (MB) under UV light irradiation are investigated. Here, the choice of ZnO microstructures was based on our earlier study which showed that the application of nanosized ZnO were limited due to the light shielding effect since the ZnO and Ag NPs were of silimiar size [22]. 2.
Experimental
All the chemical and reagents: zinc oxide (ZnO) powder, silver nitrate (AgNO3) and hexamethylenetetramine (HMTA) were analytically pure and were used without further purification and purchased from Sigma-Aldrich. Deionized (DI) water was also used throughout the experiment. 2.1 Synthesis of the photocatalyst AgNPs decorated microstructures ZnO at different Ag content were synthesized as follows: 5 g ZnO powder was mixed with 0.084, 0.169, 0.849, 1.189, 1.528 or 1.690 g AgNO3 which corresponded to 0.005, 0.01, 0.05, 0.07, 0.09 and 0.01 M Ag with respect to zinc and the mixture were dissolved in 100 mL DI water. The HMTA was added to the solution at a concentration similar to AgNO3. The solutions were heated under stirring at 95 °C for 6 h. After completion, the obtained white-greyish precipitates were filtered and washed repeatedly with DI water and finally dried at 300 °C for 2 h. The samples of AgNPs decorated microstructures ZnO at different Ag content were characterized using a field emission scanning electron microscope (FESEM, JOEL JSM 7600-F) and high resolution transmission electron microscopy (HRTEM, JOEL TEM JEM-2100F) with an acceleration voltage of 200 kV. The chemical composition analysis was performed using electron dispersive X-ray spectroscopy (EDS) in the quantified spot analysis and the chemical mapping mode. The crystallinity of the samples was characterized using XRD diffractometer (PANalytical) with the CuKα radiation probe beam of 1.54056 À wavelength between range 20 – 60 º. 2.2 The catalytic photodegradation measurement The catalytic photodegradation efficiency of the samples was measured based on the decomposition of MB in aqueous solution at room temperature. In a typical process for degradation, 30 mg of pure and AgNPs decorated microstructures ZnO photocatalysts were suspended in a beaker containing 50 mL of 10 mg/L MB dye solution. Prior to irradiation, the
suspensions were stirred in the dark for 30 min to achieve the maximum adsorption of the dye on the surface of the catalysts. The suspension was vertically irradiated from the UV lamp (UV source, λ = 254 nm, 6 watt) under constant stirring. At 10 min intervals and up to 60 min, 5 mL of the dye solution was extracted for concentration measurement. The MB concentration was evaluated using UV-vis spectroscopy where the characteristic of MB absorption peak at 664 nm was used as a measure of MB concentration in the solution. The photodegradation efficiency (%) defined as the maximum photodegradation which was obtained after 60 min, and the photodegaradation rate constant, k for all samples were calculated based on the measurements of the time evolution of the MB concentrations during the photodegradation process. 3.0 Results and Discussion 3.1 XRD analysis The normalized XRD pattern in the range of 20 ° - 60 ° for pure and AgNPs decorated microstructures ZnO at various Ag contents are shown in Fig. 1. The sharp peaks in the XRD pattern for pure and AgNPs decorated microstructures ZnO samples indicate a good crystallinity in the synthesized decorated samples. The diffraction peaks of pure ZnO (Fig. 3a) corresponding to (100), (002), (101) and (110) planes were in agreement with a wurtzite hexagonal ZnO crystal structure (JCPDS No. 36-1451). Meanwhile, for AgNPs decorated microstructures ZnO at concentration of 0.005 M, 0.01 M, 0.05 M, 0.07 M, 0.09 M and 0.1 M (Fig.1 (b)-(g)), peaks marked with “*”were correspond to the (111) and (200) crystal planes of the fcc structure of Ag which are in agreement with the standard Ag pattern (JCPDS No. 040783), respectively [23,24]. There are reports on the doping of ZnO with Ag, where the Ag atoms were incorporated in ZnO lattice as a substituent for Zn2+ or as an interstitial atom [25,26]. If doping occured, peak shifts are expected in the XRD spectra. However, in this work no other peaks and notable peak shifted was observed in the diffraction peaks, indicating that Ag did not incorporate into the lattice of ZnO, but only formed on the surface of ZnO [27–29]. 3.2 Surface Morphology FESEM images of pure and AgNPs decorated microstructures ZnO at various Ag contents are shown in Fig. 2. It is clear from the images that similar structured particles with average size about 100 - 500 nm were observed for pure and AgNPs decorated ZnO microstructures. The incorporation of Ag to the ZnO samples was not visible in the FESEM images. The images revealed that all samples composed of rod-like and cubic-like structure. The morphology and size of the samples do not show any significant changes when Ag was introduced to ZnO surface.
1 μm 1 μm 1 μm 1 μm 1 μm
In agreement to the XRD results, the EDS analysis (Fig. 3) confirmed that Ag element was detected in the sample of AgNPs decorated microstructures ZnO. The EDS analysis shows the presence of Zn, O and Ag elements with no evidence of impurities, confirming the purity of the samples. Moreover, based on EDS analysis, the amount of Ag content was increased as higher molar concentration of Ag was introduced during the sysnthesis process with value of [Ag]/[Zn] 2.33, 4.60, 11.48, 19.90, 40.40 and 61.31 at. %, respectively. The concentrations of the elements together with the percentage ratio of Ag content ([Ag]/[Zn]) for pure and AgNPs decorated microstructures ZnO are presented in Table 1.
Further characterization on the surface morphology has been conducted using the EDS chemical mapping analysis for sample 0.1 M AgNPs decorated microstructures ZnO as shown in Fig. 4 (a) – (f). From the images, elements were represented by blue, red and green for O, Zn and Ag atoms respectively. Fig. 4 (b) shows the mapping for all elements where greenish tint representing Ag appeared on the surface of ZnO microparticles. As expected, the mapping of O and Zn represented in Fig. 4 (c) and (d) showed distribution covering the whole frame. Similarly, the mapping for Ag in Fig. 4 (e) showed complete coverage of the image frame. In general, the elemental mapping indicated that Ag was attached to the ZnO microparticles surfaces. The structural properties of pure and AgNPs decorated microstructures ZnO were further analyzed using TEM as shown in Fig. 5. Fig. 5 (a) and (b) show the low and high magnified TEM image for pure ZnO microstructures. As expected, the structure of the pure ZnO microstructures contains particles structures with size consistent with that observed in the FESEM analysis. High resolution TEM image as shown in Fig. 5 (b) indicates that the particles of ZnO are single-crystal structures. The interplanar spacing of pure ZnO is 0.260 nm which corresponds to the ZnO (002) direction [10]. As shown in Fig. 5 (c), a low magnification image of a cluster of ZnO microparticles was discover where the existence of AgNPs was not apparent. At a higher magnification as illustrate in Fig. 5 (d) shows the ZnO microparticle structure and an expected spherical AgNPs were attached on the ZnO surface. From a close up view as shown in Fig. 5 (e), the spherical particles yielded lattice fringe with crystalline plane spacing of 0.230 nm was assigned to the (111) plane of Ag with fcc structure [30,31]. The crystalline plane spacing of 0.250 nm corresponding to the (002) plane of ZnO also shown in the image. This TEM results proof and agreed with the XRD result that has been discussed earlier showing that AgNPs only formed on the ZnO microparticles surface and did not incorporated in the ZnO crystal structure. 3.3 Evaluation of catalytic photodegradation The catalytic photodegradation activities of the pure and AgNPs decorated microstructures ZnO with various Ag contents are evaluated by measuring the degradation of MB in aqueous solution under UV light irradiation. Fig. 6 shows the time evolution of the absorbance spectra at between 500 and 750 nm, of MB aqueous solution during the photodegradation process up to 60 min using pure and Ag NP decorated photocatalysts and uv light irradiation. A comparison of absorption spectra in the absence of catalyst are also represented as shown in Fig. 6 (a) where it can be seen that, no degradation process occur as no changes on the absorption spectra. The UV–visible absorbance spectra of MB aqueous solutions for pure and AgNPs decorated microstructures ZnO at various Ag content of 2.33, 4.60, 11.48, 19.90, 40.40
and 61.31 at. % was shown in Fig 6 (b) –(h), respectively. It can be seen that the characteristic absorption peak at 664 nm gradually decreased with time in the presence of pure and AgNPs decorated microstructures ZnO indicating the degradation of the dye molecules. The photodegradation efficiency of pure and AgNPs decorated microstructures ZnO at various Ag contents were further evaluated in order to investigate the effect of different Ag content to the degradation percentage of MB dye. The photodegradation efficiency (%) of the photocatalyst could be defined as in equation (1): Photodegradation efficiency (%)= (
C0 − C 𝐴𝑜 − 𝐴 ) x 100 = ( ) × 100 C0 𝐴𝑜
(1)
Where Co is the initial dye concentration, while C is the dye concentration at certain reaction time, t (min), Ao represents the initial absorbance, and A represent the changed of absorbanced of the MB at the characteristic absorption wavelength 664 nm [29,32,33]. As shown in Fig. 7, the photodegradation efficiency of MB is about 57.98 %, 66.60 %, 74.38 %, 80.48 %, 85.25 %, 85.35 % and 87.74 % for pure ZnO, 2.33, 4.60, 11.48, 19.90, 40.40, and 61.31 at. % of AgNPs decorated microstructures ZnO, respectively. It is clear that the degradation of MB after 60 min increased with increasing of Ag contents. Pure ZnO microstructures exhibits the lowest efficiency among all photocatalysts; while 61.31 at. % AgNPs decorated microstructures ZnO shows the highest photodegradation efficiency. The photodegradation rate constant, k of pure and AgNPs decorated microstructures ZnO at different Ag contents also evaluated and quantified by plotting a first order decay plot of the characteristic MB absorption peak 664 nm. The photodegradation rate constant, k, can be obtained from the pseudo-first kinetics model of photocatalysis according to the following equation (2): ln(
C ) kt C0
(2)
Where k is the photodegradation rate constant, Co and C are the MB initial concentration and the concentration of solution after the degradation time of t, respectively [34]. The graph of ln (C/Co) vs irradiation time, t for pure and AgNPs decorated microstructures ZnO at different Ag contents are shown in Fig. 8 below. This linear ln (C/Co) vs t plot agreed that the photodegradation of MB follows pseudofirst-order reaction kinetics. The calculated photodegradation rate constant, k of different photocatalyst inferred from the gradients of the straight lines were 0.014, 0.017, 0.022, 0.026, 0.030, 0.031 and 0.032 min-1 for pure ZnO microsructures, 2.33, 4.60, 11.48, 19.90, 40.40, and 61.31 at. % AgNPs decorated microstructures ZnO, respectively. The value of k for 61.31 at.% of AgNPs decorated microstructures ZnO was greater than twice of pure ZnO microstructures. From the obtained result, AgNPs decorated microstructures ZnO photocatalyst showed a higher k value compared to pure ZnO microstructures and a significant increment on the photodegradation rate constant can be observed as Ag contents was increased. The present results are comparable to previously reported works. Even though conditions were not similar, comparisons can still be made. A study done by Fazhe Sun et al., [35] reported the fabrication of Ag/ZnO heterostructures nanocrystal by a solution method using glucose as a reducing agent and investigated the photocatalytic performance using 0.05 g catalyst into 50 mL of 20 mg/L MB solution under high-pressure Hg-lamp. They found that the photocatalytic activity of the Ag/ZnO heterostructures nanocrystal increases with increasing Ag content from 0.5 to 1.5 wt %. Yang et al., [36] prepared a pompon-like ZnOAg nanocomposites at different Ag contents by hydrothermal method. They studied the
photocatalytic activity of pure ZnO and ZnO-Ag (at different concentration of AgNO3) in the range of 0.05 – 0.20 g/L. They observed that 0.10 g/L ZnO-Ag shows the best performance and the activity of ZnO-Ag decreased when the Ag content exceed 4.82 wt %. The photodegradation rate constant for all Ag/ZnO was higher than pure ZnO also in agreement with Wu et al., [37]. The results suggested that 10.03 wt % Ag/ZnO samples achieves the optimum photocatalyic performance and the photodegradation rate constant, k value is two times higher than that of pure ZnO microsphere. The present findings seem to be consistent with other research which found that Ag content was significantly influenced the photocatalytic activities. Results from this work are tabulated in Table 2, together with previously reported results for comparison. Besides, comparable photodegradation rate constant in other metal doped ZnO reported by Zhang et al., [16] and Fernandez et al., [17] shows that Ag/ZnO is one of the promising photocatalyst. According to Fig. 8, the photodegradation rate constant, k marked increased for sample 19.90 at. % AgNPs decorated microstructures ZnO and showed saturation behaviour for sample 40.40 and 61.31 at. % AgNPs decorated microstructures ZnO, respectively. This situation might be due to an excessive Ag content which resulting in agglomeration of particles hence the photodegradation rate will saturate. A number of studies have reported that the photocatalytic performance was reduced when the more Ag is loaded [38–40]. Excessive amount of Ag is expected will increased the size of Ag particles hence significantly inhibited the recombination of the electron-hole pairs [41]. This argument was also supported by Saravanan and Cheng which conclude that there exist an optimum amount of Ag for enhancing the catalytic degradation process [36,42]. The enhanced photocatalytic activity afforded by AgNPs decorated microstructures ZnO can be explained as follows. When ZnO absorb photons of energy greater than or equal to its band gap, electrons are excited from the valence band (VB) to conduction band (CB), creating an equal number of holes in the VB. Since the energy level of CB of ZnO is higher than Fermi level, Ef of AgNPs decorated microstructures ZnO, electrons will flow from ZnO to Ag nanoparticles. The AgNPs accepts the electrons in the conduction band of ZnO to form Ag+ which act as electron sinks for the photogenerated electrons, preventing the recombination electron holes pair. These situation will prolong the lifetime of the electron hole pairs, hence improve the photocatalytic activity. Afterward, the electrons can be captured by the soluble O2, and the holes can be trapped by the surface hydroxyl, both resulting in the formation of free radicals, which can break organic bonds and oxidize most of the pollutants [27,38]. A schematic band diagram depicting the charge separation processes in AgNPs decorated microstructures ZnO under UV irradiation is schematically illustrated in Fig. 9. Our results showed that, photocatalytic activity improved continuously with Ag content for AgNPs decorated microstructures ZnO ranging from 2.33 to 61.31%. Conclusions In summary, AgNPs decorated microstructures ZnO photocatalyst with Ag loading of 2.33 to 63.31 at. % were synthesized by using sol- gel method. The samples were used to in the photocatalysis degradation of MB in aqueous solution to study the effect of Ag NPs decoration of ZnO photocatalysts. Our results revealed a general trend of increasing photocatalysis activity with increasing Ag loading. The photodegradation rate constant, k values are 0.014, 0.017, 0.022, 0.026, 0.030, 0.031 and 0.032 min-1 for pure ZnO
microstructures, 2.33, 4.60, 11.48, 19.90, 40.40, and 61.31 at. % of AgNPs decorated microstructures ZnO, respectively. The maximum degradation after 60 mins increased to 87.74% using samples with 61.31 at. % Ag, compared to 57.98 % using pure ZnO and 66.60% using the sample with 2.33 at. % Ag. The enhancement can be explained in terms of the increase in the lifetime of the electron-hole pairs due to the trapping of electrons promoted to the conduction band in the Ag nanoparticles. The decoration of Ag NPs on the microstructured ZnO resulted in a Schottky barrier at the interface of the two materials. An electric field is induced at the barrier which drives the electrons promoted to the conduction band to the Ag nanoparticles. This effect of an electron sink afforded by the Ag NPs, effectively extends the lifetime of the electron-hole pairs, and thus allows for longer reaction time which resulted in the enhanced photocatalysis activity. Acknowledgements This work is funded by the University of Malaya under the UM Research Grant No. RG24712AFR and PPP Grant No. PG068-2013A. References [1]
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Fig. 1. Normalized XRD patterns between range 20 – 60 º of (a) pure ZnO, (b) 0.005 M, (c) 0.01 M, (d) 0.05 M, (e) 0.07 M, (f) 0.09 M, (g) 0.1 M AgNPs decorated microstructures ZnO prepared by sol-gel method at a temperature of 95 oC for 6 hours. 1 μm 1 μm
(e)
(a)
(b)
(c)
(d)
(f)
(g)
Fig. 2. FESEM images of (a) pure ZnO, (b-g) 0.005 M, 0.01 M, 0.05 M, 0.07 M, 0.09 M and 0.1 M AgNPs decorated microstructures ZnO (20,000 x magnification) prepared by sol-gel method at a temperature of 95 oC for 6 hours..
Fig. 3. EDS spectra of (a) pure ZnO, (b) 0.005 M, (c) 0.01 M, (d) 0.05 M, (e) 0.07 M, (f) 0.09 M, (g) 0.1 M AgNPs decorated microstructures ZnO prepared by sol-gel method at a temperature of 95 oC for 6 hours.
Fig. 4. (a) Selected image of 0.1 M AgNPs decorated microstructures ZnO prepared by sol-gel method at a temperature of 95 oC for 6 hours from EDS mapping analysis. The corresponding elemental mapping images for Zn, O and Ag are presented in (b), (c), (d) and (e), respectively and (f) the elemental spectra and composition of the sample.
Fig. 5. The low-magnified and high-resolution TEM image of (a-b) pure ZnO, (c-e) 0.005 M AgNPs decorated microstructures ZnO prepared by sol-gel method at a temperature of 95 oC for 6 hours.
Fig. 6. The time-dependent absorption spectra of MB solution (a) without catalyst and in the presence of catalyst of (b) pure ZnO, AgNPs decorated microstructures ZnO at Ag content of (c) 2.33 at. %, (d) 4.60 at. %, (e) 11.48 at. % (f) 19.90 at. % (g) 40.40 at. %, and (h) 61.31 at. % (initial dye concentration : 10 mg/L, 10 mg catalyst, 60 min irradiation time, under UV light irradiation (6 watt, λ : 254 nm)).
Fig. 7. The photodegradation efficiency curve of MB under UV-light in the presence of pure and AgNPs decorated microstructures ZnO at various Ag contents (initial dye concentration : 10 mg/L, 10 mg catalyst, 60 min irradiation time, under UV light irradiation (6 watt, λ : 254 nm)).
Fig. 8. The curves of ln(C/C0) versus time for photodegradation of MB using pure and AgNPs decorated microstructures ZnO at various Ag contents (initial dye concentration : 10 mg/L, 10 mg catalyst, 60 min irradiation time, under UV light irradiation (6 watt, λ : 254 nm)).
Fig. 9 Schematic band diagram showing the separation charge processes of AgNPs decorated microstructures ZnO under UV irradiation.
Table 1. Atomic % from EDS analysis and ratio of [Ag]/[ZnO] for pure and AgNPs decorated microstructures ZnO at molar concentration of 0.0 – 0.1 M prepared by sol-gel method at a temperature of 95 oC for 6 hours. Amount of AgNO3 relative to ZnO used during the synthesis of AgNPs decorated microstructured ZnO 0 M (Pure ZnO) 0.005 M 0.01 M 0.05 M 0.07 M 0.09 M 0.1 M
Atomic % from EDS analysis
[Ag]/[Zn] (%)
Zn
O
Ag
41.20
58.80
-
-
46.60 39.95 39.20 38.40 32.70 30.00
52.85 58.21 56.30 53.96 54.05 51.58
1.073 1.84 4.50 7.64 13.25 18.42
2.33 4.60 11.48 19.90 40.40 61.31
Table 2. Comparison of photocatalytic activities from previous work with AgNPs decorated microstructures ZnO at different Ag contents (initial dye concentration : 10 mg/L, 10 mg catalyst, 60 min irradiation time, under UV light irradiation (6 watt, λ : 254 nm)). ZnO/Ag samples
Percentage degradation ( %)
R2
Ref.
0.9940 0.9780 0.9800 0.9730 0.9920 0.9970 0.9890
(this work)
57.98 after 1 h 66.60 after 1 h 74.38 after 1 h 80.48 after 1 h 85.25 after 1 h 85.35 after 1 h 87.74 after 1 h
Photodegradation rate constant, k (min-1) 0.014 0.017 0.022 0.026 0.030 0.031 0.032
Pure ZnO ZnO/Ag (0.02M) ZnO/Ag (0.05M) ZnO/Ag (0.1M) ZnO/Ag (0.2M)
36.2 after 1 h 39.6 after 1 h 49.3 after 1 h 47.2 after 1 h 42.6 after 1 h
5.16 x 10−3 5.69 x 10−3 7.63 x 10−3 7.17 x 10−3 6.08 x 10−3
-
[32]
Pure ZnO ZnO-Ag (0.05 g/L) ZnO-Ag (0.10 g/L) ZnO-Ag (0.15 g/L) ZnO-Ag (0.20 g/ L)
74.1 after 2 h 99.1 after 2 h 98.0 after 2 h 97.9 after 2 h 94.6 after 2 h
-
-
[36]
-
0.0088 0.1257 0.1396 0.2261 0.1607
0.9899 0.9921 0.9854 0.9563 0.9992
[37]
Pure ZnO microstructures 2.33 % AgNPs decorated microstructures ZnO 4.60 % AgNPs decorated microstructures ZnO 11.48 % AgNPs decorated microstructures ZnO 19.90 % AgNPs decorated microstructures ZnO 40.40 % AgNPs decorated microstructures ZnO 61.31 % AgNPs decorated microstructures ZnO
ZnO 5.41 wt% Ag/ZnO 7.95 wt% Ag/ZnO 10.03 wt% Ag/ZnO 11.37 wt% Ag/ZnO *R2: The square of correlation coefficient of kinetic energy
S2213-3437(17)30374-3 http://dx.doi.org/doi:10.1016/j.jece.2017.07.070 JECE 1785
To appear in: Received date: Revised date: Accepted date:
28-4-2017 22-7-2017 27-7-2017
Please cite this article as: H.A.Rafaie, R.M.Nor, M.S.Azmina, N.I.T.Ramli, R.Mohamed, Decoration of ZnO microstructures with Ag nanoparticles enhanced the catalytic photodegradation of methylene blue dye, Journal of Environmental Chemical Engineeringhttp://dx.doi.org/10.1016/j.jece.2017.07.070 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Decoration of ZnO Microstructures with Ag Nanoparticles enhanced the catalytic photodegradation of Methylene Blue Dye H. A. Rafaiea,*, R. M. Norb, M. S. Azminab, N. I. T. Ramlia, R. Mohameda a Faculty of Applied Sciences, Universiti Teknologi MARA Pahang, Bandar Tun Abdul Razak Jengka, Pahang,Malaysia *[email protected]; [email protected]; [email protected] b Department of Physics, Faculty of Sciences, Universiti of Malaya, 50603, Kuala Lumpur, Malaysia [email protected]; [email protected] * Corresponding author Abstract: ZnO microstructures decorated with Ag nanoparticles photocatalysts synthesized using a simple sol-gel method at different Ag content. The prepared photocatalysts were then characterized by XRD, FESEM, EDX and TEM shows that the ZnO microstructures successfully decorated with Ag nanoparticles and influenced the catalytic photodegradation performance of methelyne blue dye under UV light irradiation. The effect of different Ag content on the structural, surface morphology and catalytic photodegradation of Ag nanoparticles decorated microstructures ZnO were analyzed. The results shows that there is a trend of increasing photocatalytic activity with Ag content of between 2.33 to 63.31 at. % where the highest photodegradation efficiency 87.74 % and photodegradation rate constant, k of 0.032 min-1 for degradation of methelyne blue with 60 min of irradiation. The enhancement of the photocatalysis activity with increasing Ag content can be explained by the increase in the electron-hole pair lifetimes due to electrons promoted to the conduction band are trapped in the Ag nanoparticles which acted as electron sinks. Keywords: Ag nanoparticles; ZnO microstructures; sol-gel; catalytic photodegradation; photocatalyst 1. Introduction Industrial wastewater may contains various organic and inorganic pollutants which can have adverse effect on aquatic life when discharged to environment. Effluents containing dyes is known to create severe environmental pollutions [1]. Dye in wastewater contaminates surface and ground water and can harm ecological resources including water quality, soils, plants, animal and also human health. Textile industry is one of an example that produces a large quantity of highly coloured effluents [2]. Much attention has been devoted to develop effective technologies for the removal of organic pollutants from aqueous media. Recently, technologies including biodegradation, adsorption, and oxidation processes have been developed [3]. Photocatalysis degradation is a promising technique for waste water treatment which can provide good applicability that can rival the methods mentioned earlier [4]. Photocatalysis is a catalytic reaction involving light absorption by a catalyst or a substrate [5]. A photocatalyst is a substance which is activated by adsorbing a photon and is capable of accelerating a reaction without being consumed. To meet the need for practical applications, photocatalyst should have high photocatalytic activity, non-toxic, cost effective and high yield. Oxide-based materials such as TiO2 [6], NiO [7] and ZnO [8] have attracted much interest due to their high photocatalytic activity using uv light to match the values of the band gaps of these materials. Recently, ZnO and TiO2 have received much attention as photocatalyst due to their unique properties and potential applications [9]. The band gap of ZnO (3.25 eV) and TiO2 (3.05 -3.2
eV) are almost similar, so it is expected that ZnO to have the same photocatalytic ability as TiO2 [10,11]. As a photocatalyst, ZnO also exhibit large initial rates of activity due to the presence of active sites with higher surface reactivity [12]. Nevertheless, one of major drawback of ZnO photocatalyst is the low quantum efficiency of ZnO due to the fast recombination of photogenerated electron-hole pairs which significantly decreased the photocatalytic activity of ZnO [13,14]. Thus, the low photocatalytic efficiency of the ZnO photocatalyst still remained a challenge and is insufficient for industrial scaled applications. There have been many reports on studies done to modify the ZnO nanostructures in order to improve the photocatalytic efficiency. The modifications of the photocatalysts are significant in increasing the lifetime of the electron–hole pairs, broadening the absorption spectrum, and to facilitate some specific reactions on the surface of catalysts [15]. The most efficient way to accelerate the charge carrier separation and improve the catalytic activity is by modifying the surface structure of ZnO by decoration with transition metal such as Pt [16,17], Ag [18], Fe [19] and Mg [20]. However, the underlying photocatalytic mechanism is complex and remained controversial. It is widely accepted that Ag nanoparticles on ZnO surface acted as electron trapping and thus increased the lifetime of electron-hole resulting on the enhancement of the photocatalytic activity [13,21]. In this paper, we report a sol-gel method to prepare Ag nanoparticles (AgNPs) decorated microstructures ZnO at various Ag contents. The effect of different amount of Ag contents on the photodegradation efficiency of methylene blue (MB) under UV light irradiation are investigated. Here, the choice of ZnO microstructures was based on our earlier study which showed that the application of nanosized ZnO were limited due to the light shielding effect since the ZnO and Ag NPs were of silimiar size [22]. 2.
Experimental
All the chemical and reagents: zinc oxide (ZnO) powder, silver nitrate (AgNO3) and hexamethylenetetramine (HMTA) were analytically pure and were used without further purification and purchased from Sigma-Aldrich. Deionized (DI) water was also used throughout the experiment. 2.1 Synthesis of the photocatalyst AgNPs decorated microstructures ZnO at different Ag content were synthesized as follows: 5 g ZnO powder was mixed with 0.084, 0.169, 0.849, 1.189, 1.528 or 1.690 g AgNO3 which corresponded to 0.005, 0.01, 0.05, 0.07, 0.09 and 0.01 M Ag with respect to zinc and the mixture were dissolved in 100 mL DI water. The HMTA was added to the solution at a concentration similar to AgNO3. The solutions were heated under stirring at 95 °C for 6 h. After completion, the obtained white-greyish precipitates were filtered and washed repeatedly with DI water and finally dried at 300 °C for 2 h. The samples of AgNPs decorated microstructures ZnO at different Ag content were characterized using a field emission scanning electron microscope (FESEM, JOEL JSM 7600-F) and high resolution transmission electron microscopy (HRTEM, JOEL TEM JEM-2100F) with an acceleration voltage of 200 kV. The chemical composition analysis was performed using electron dispersive X-ray spectroscopy (EDS) in the quantified spot analysis and the chemical mapping mode. The crystallinity of the samples was characterized using XRD diffractometer (PANalytical) with the CuKα radiation probe beam of 1.54056 À wavelength between range 20 – 60 º. 2.2 The catalytic photodegradation measurement The catalytic photodegradation efficiency of the samples was measured based on the decomposition of MB in aqueous solution at room temperature. In a typical process for degradation, 30 mg of pure and AgNPs decorated microstructures ZnO photocatalysts were suspended in a beaker containing 50 mL of 10 mg/L MB dye solution. Prior to irradiation, the
suspensions were stirred in the dark for 30 min to achieve the maximum adsorption of the dye on the surface of the catalysts. The suspension was vertically irradiated from the UV lamp (UV source, λ = 254 nm, 6 watt) under constant stirring. At 10 min intervals and up to 60 min, 5 mL of the dye solution was extracted for concentration measurement. The MB concentration was evaluated using UV-vis spectroscopy where the characteristic of MB absorption peak at 664 nm was used as a measure of MB concentration in the solution. The photodegradation efficiency (%) defined as the maximum photodegradation which was obtained after 60 min, and the photodegaradation rate constant, k for all samples were calculated based on the measurements of the time evolution of the MB concentrations during the photodegradation process. 3.0 Results and Discussion 3.1 XRD analysis The normalized XRD pattern in the range of 20 ° - 60 ° for pure and AgNPs decorated microstructures ZnO at various Ag contents are shown in Fig. 1. The sharp peaks in the XRD pattern for pure and AgNPs decorated microstructures ZnO samples indicate a good crystallinity in the synthesized decorated samples. The diffraction peaks of pure ZnO (Fig. 3a) corresponding to (100), (002), (101) and (110) planes were in agreement with a wurtzite hexagonal ZnO crystal structure (JCPDS No. 36-1451). Meanwhile, for AgNPs decorated microstructures ZnO at concentration of 0.005 M, 0.01 M, 0.05 M, 0.07 M, 0.09 M and 0.1 M (Fig.1 (b)-(g)), peaks marked with “*”were correspond to the (111) and (200) crystal planes of the fcc structure of Ag which are in agreement with the standard Ag pattern (JCPDS No. 040783), respectively [23,24]. There are reports on the doping of ZnO with Ag, where the Ag atoms were incorporated in ZnO lattice as a substituent for Zn2+ or as an interstitial atom [25,26]. If doping occured, peak shifts are expected in the XRD spectra. However, in this work no other peaks and notable peak shifted was observed in the diffraction peaks, indicating that Ag did not incorporate into the lattice of ZnO, but only formed on the surface of ZnO [27–29]. 3.2 Surface Morphology FESEM images of pure and AgNPs decorated microstructures ZnO at various Ag contents are shown in Fig. 2. It is clear from the images that similar structured particles with average size about 100 - 500 nm were observed for pure and AgNPs decorated ZnO microstructures. The incorporation of Ag to the ZnO samples was not visible in the FESEM images. The images revealed that all samples composed of rod-like and cubic-like structure. The morphology and size of the samples do not show any significant changes when Ag was introduced to ZnO surface.
1 μm 1 μm 1 μm 1 μm 1 μm
In agreement to the XRD results, the EDS analysis (Fig. 3) confirmed that Ag element was detected in the sample of AgNPs decorated microstructures ZnO. The EDS analysis shows the presence of Zn, O and Ag elements with no evidence of impurities, confirming the purity of the samples. Moreover, based on EDS analysis, the amount of Ag content was increased as higher molar concentration of Ag was introduced during the sysnthesis process with value of [Ag]/[Zn] 2.33, 4.60, 11.48, 19.90, 40.40 and 61.31 at. %, respectively. The concentrations of the elements together with the percentage ratio of Ag content ([Ag]/[Zn]) for pure and AgNPs decorated microstructures ZnO are presented in Table 1.
Further characterization on the surface morphology has been conducted using the EDS chemical mapping analysis for sample 0.1 M AgNPs decorated microstructures ZnO as shown in Fig. 4 (a) – (f). From the images, elements were represented by blue, red and green for O, Zn and Ag atoms respectively. Fig. 4 (b) shows the mapping for all elements where greenish tint representing Ag appeared on the surface of ZnO microparticles. As expected, the mapping of O and Zn represented in Fig. 4 (c) and (d) showed distribution covering the whole frame. Similarly, the mapping for Ag in Fig. 4 (e) showed complete coverage of the image frame. In general, the elemental mapping indicated that Ag was attached to the ZnO microparticles surfaces. The structural properties of pure and AgNPs decorated microstructures ZnO were further analyzed using TEM as shown in Fig. 5. Fig. 5 (a) and (b) show the low and high magnified TEM image for pure ZnO microstructures. As expected, the structure of the pure ZnO microstructures contains particles structures with size consistent with that observed in the FESEM analysis. High resolution TEM image as shown in Fig. 5 (b) indicates that the particles of ZnO are single-crystal structures. The interplanar spacing of pure ZnO is 0.260 nm which corresponds to the ZnO (002) direction [10]. As shown in Fig. 5 (c), a low magnification image of a cluster of ZnO microparticles was discover where the existence of AgNPs was not apparent. At a higher magnification as illustrate in Fig. 5 (d) shows the ZnO microparticle structure and an expected spherical AgNPs were attached on the ZnO surface. From a close up view as shown in Fig. 5 (e), the spherical particles yielded lattice fringe with crystalline plane spacing of 0.230 nm was assigned to the (111) plane of Ag with fcc structure [30,31]. The crystalline plane spacing of 0.250 nm corresponding to the (002) plane of ZnO also shown in the image. This TEM results proof and agreed with the XRD result that has been discussed earlier showing that AgNPs only formed on the ZnO microparticles surface and did not incorporated in the ZnO crystal structure. 3.3 Evaluation of catalytic photodegradation The catalytic photodegradation activities of the pure and AgNPs decorated microstructures ZnO with various Ag contents are evaluated by measuring the degradation of MB in aqueous solution under UV light irradiation. Fig. 6 shows the time evolution of the absorbance spectra at between 500 and 750 nm, of MB aqueous solution during the photodegradation process up to 60 min using pure and Ag NP decorated photocatalysts and uv light irradiation. A comparison of absorption spectra in the absence of catalyst are also represented as shown in Fig. 6 (a) where it can be seen that, no degradation process occur as no changes on the absorption spectra. The UV–visible absorbance spectra of MB aqueous solutions for pure and AgNPs decorated microstructures ZnO at various Ag content of 2.33, 4.60, 11.48, 19.90, 40.40
and 61.31 at. % was shown in Fig 6 (b) –(h), respectively. It can be seen that the characteristic absorption peak at 664 nm gradually decreased with time in the presence of pure and AgNPs decorated microstructures ZnO indicating the degradation of the dye molecules. The photodegradation efficiency of pure and AgNPs decorated microstructures ZnO at various Ag contents were further evaluated in order to investigate the effect of different Ag content to the degradation percentage of MB dye. The photodegradation efficiency (%) of the photocatalyst could be defined as in equation (1): Photodegradation efficiency (%)= (
C0 − C 𝐴𝑜 − 𝐴 ) x 100 = ( ) × 100 C0 𝐴𝑜
(1)
Where Co is the initial dye concentration, while C is the dye concentration at certain reaction time, t (min), Ao represents the initial absorbance, and A represent the changed of absorbanced of the MB at the characteristic absorption wavelength 664 nm [29,32,33]. As shown in Fig. 7, the photodegradation efficiency of MB is about 57.98 %, 66.60 %, 74.38 %, 80.48 %, 85.25 %, 85.35 % and 87.74 % for pure ZnO, 2.33, 4.60, 11.48, 19.90, 40.40, and 61.31 at. % of AgNPs decorated microstructures ZnO, respectively. It is clear that the degradation of MB after 60 min increased with increasing of Ag contents. Pure ZnO microstructures exhibits the lowest efficiency among all photocatalysts; while 61.31 at. % AgNPs decorated microstructures ZnO shows the highest photodegradation efficiency. The photodegradation rate constant, k of pure and AgNPs decorated microstructures ZnO at different Ag contents also evaluated and quantified by plotting a first order decay plot of the characteristic MB absorption peak 664 nm. The photodegradation rate constant, k, can be obtained from the pseudo-first kinetics model of photocatalysis according to the following equation (2): ln(
C ) kt C0
(2)
Where k is the photodegradation rate constant, Co and C are the MB initial concentration and the concentration of solution after the degradation time of t, respectively [34]. The graph of ln (C/Co) vs irradiation time, t for pure and AgNPs decorated microstructures ZnO at different Ag contents are shown in Fig. 8 below. This linear ln (C/Co) vs t plot agreed that the photodegradation of MB follows pseudofirst-order reaction kinetics. The calculated photodegradation rate constant, k of different photocatalyst inferred from the gradients of the straight lines were 0.014, 0.017, 0.022, 0.026, 0.030, 0.031 and 0.032 min-1 for pure ZnO microsructures, 2.33, 4.60, 11.48, 19.90, 40.40, and 61.31 at. % AgNPs decorated microstructures ZnO, respectively. The value of k for 61.31 at.% of AgNPs decorated microstructures ZnO was greater than twice of pure ZnO microstructures. From the obtained result, AgNPs decorated microstructures ZnO photocatalyst showed a higher k value compared to pure ZnO microstructures and a significant increment on the photodegradation rate constant can be observed as Ag contents was increased. The present results are comparable to previously reported works. Even though conditions were not similar, comparisons can still be made. A study done by Fazhe Sun et al., [35] reported the fabrication of Ag/ZnO heterostructures nanocrystal by a solution method using glucose as a reducing agent and investigated the photocatalytic performance using 0.05 g catalyst into 50 mL of 20 mg/L MB solution under high-pressure Hg-lamp. They found that the photocatalytic activity of the Ag/ZnO heterostructures nanocrystal increases with increasing Ag content from 0.5 to 1.5 wt %. Yang et al., [36] prepared a pompon-like ZnOAg nanocomposites at different Ag contents by hydrothermal method. They studied the
photocatalytic activity of pure ZnO and ZnO-Ag (at different concentration of AgNO3) in the range of 0.05 – 0.20 g/L. They observed that 0.10 g/L ZnO-Ag shows the best performance and the activity of ZnO-Ag decreased when the Ag content exceed 4.82 wt %. The photodegradation rate constant for all Ag/ZnO was higher than pure ZnO also in agreement with Wu et al., [37]. The results suggested that 10.03 wt % Ag/ZnO samples achieves the optimum photocatalyic performance and the photodegradation rate constant, k value is two times higher than that of pure ZnO microsphere. The present findings seem to be consistent with other research which found that Ag content was significantly influenced the photocatalytic activities. Results from this work are tabulated in Table 2, together with previously reported results for comparison. Besides, comparable photodegradation rate constant in other metal doped ZnO reported by Zhang et al., [16] and Fernandez et al., [17] shows that Ag/ZnO is one of the promising photocatalyst. According to Fig. 8, the photodegradation rate constant, k marked increased for sample 19.90 at. % AgNPs decorated microstructures ZnO and showed saturation behaviour for sample 40.40 and 61.31 at. % AgNPs decorated microstructures ZnO, respectively. This situation might be due to an excessive Ag content which resulting in agglomeration of particles hence the photodegradation rate will saturate. A number of studies have reported that the photocatalytic performance was reduced when the more Ag is loaded [38–40]. Excessive amount of Ag is expected will increased the size of Ag particles hence significantly inhibited the recombination of the electron-hole pairs [41]. This argument was also supported by Saravanan and Cheng which conclude that there exist an optimum amount of Ag for enhancing the catalytic degradation process [36,42]. The enhanced photocatalytic activity afforded by AgNPs decorated microstructures ZnO can be explained as follows. When ZnO absorb photons of energy greater than or equal to its band gap, electrons are excited from the valence band (VB) to conduction band (CB), creating an equal number of holes in the VB. Since the energy level of CB of ZnO is higher than Fermi level, Ef of AgNPs decorated microstructures ZnO, electrons will flow from ZnO to Ag nanoparticles. The AgNPs accepts the electrons in the conduction band of ZnO to form Ag+ which act as electron sinks for the photogenerated electrons, preventing the recombination electron holes pair. These situation will prolong the lifetime of the electron hole pairs, hence improve the photocatalytic activity. Afterward, the electrons can be captured by the soluble O2, and the holes can be trapped by the surface hydroxyl, both resulting in the formation of free radicals, which can break organic bonds and oxidize most of the pollutants [27,38]. A schematic band diagram depicting the charge separation processes in AgNPs decorated microstructures ZnO under UV irradiation is schematically illustrated in Fig. 9. Our results showed that, photocatalytic activity improved continuously with Ag content for AgNPs decorated microstructures ZnO ranging from 2.33 to 61.31%. Conclusions In summary, AgNPs decorated microstructures ZnO photocatalyst with Ag loading of 2.33 to 63.31 at. % were synthesized by using sol- gel method. The samples were used to in the photocatalysis degradation of MB in aqueous solution to study the effect of Ag NPs decoration of ZnO photocatalysts. Our results revealed a general trend of increasing photocatalysis activity with increasing Ag loading. The photodegradation rate constant, k values are 0.014, 0.017, 0.022, 0.026, 0.030, 0.031 and 0.032 min-1 for pure ZnO
microstructures, 2.33, 4.60, 11.48, 19.90, 40.40, and 61.31 at. % of AgNPs decorated microstructures ZnO, respectively. The maximum degradation after 60 mins increased to 87.74% using samples with 61.31 at. % Ag, compared to 57.98 % using pure ZnO and 66.60% using the sample with 2.33 at. % Ag. The enhancement can be explained in terms of the increase in the lifetime of the electron-hole pairs due to the trapping of electrons promoted to the conduction band in the Ag nanoparticles. The decoration of Ag NPs on the microstructured ZnO resulted in a Schottky barrier at the interface of the two materials. An electric field is induced at the barrier which drives the electrons promoted to the conduction band to the Ag nanoparticles. This effect of an electron sink afforded by the Ag NPs, effectively extends the lifetime of the electron-hole pairs, and thus allows for longer reaction time which resulted in the enhanced photocatalysis activity. Acknowledgements This work is funded by the University of Malaya under the UM Research Grant No. RG24712AFR and PPP Grant No. PG068-2013A. References [1]
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Fig. 1. Normalized XRD patterns between range 20 – 60 º of (a) pure ZnO, (b) 0.005 M, (c) 0.01 M, (d) 0.05 M, (e) 0.07 M, (f) 0.09 M, (g) 0.1 M AgNPs decorated microstructures ZnO prepared by sol-gel method at a temperature of 95 oC for 6 hours. 1 μm 1 μm
(e)
(a)
(b)
(c)
(d)
(f)
(g)
Fig. 2. FESEM images of (a) pure ZnO, (b-g) 0.005 M, 0.01 M, 0.05 M, 0.07 M, 0.09 M and 0.1 M AgNPs decorated microstructures ZnO (20,000 x magnification) prepared by sol-gel method at a temperature of 95 oC for 6 hours..
Fig. 3. EDS spectra of (a) pure ZnO, (b) 0.005 M, (c) 0.01 M, (d) 0.05 M, (e) 0.07 M, (f) 0.09 M, (g) 0.1 M AgNPs decorated microstructures ZnO prepared by sol-gel method at a temperature of 95 oC for 6 hours.
Fig. 4. (a) Selected image of 0.1 M AgNPs decorated microstructures ZnO prepared by sol-gel method at a temperature of 95 oC for 6 hours from EDS mapping analysis. The corresponding elemental mapping images for Zn, O and Ag are presented in (b), (c), (d) and (e), respectively and (f) the elemental spectra and composition of the sample.
Fig. 5. The low-magnified and high-resolution TEM image of (a-b) pure ZnO, (c-e) 0.005 M AgNPs decorated microstructures ZnO prepared by sol-gel method at a temperature of 95 oC for 6 hours.
Fig. 6. The time-dependent absorption spectra of MB solution (a) without catalyst and in the presence of catalyst of (b) pure ZnO, AgNPs decorated microstructures ZnO at Ag content of (c) 2.33 at. %, (d) 4.60 at. %, (e) 11.48 at. % (f) 19.90 at. % (g) 40.40 at. %, and (h) 61.31 at. % (initial dye concentration : 10 mg/L, 10 mg catalyst, 60 min irradiation time, under UV light irradiation (6 watt, λ : 254 nm)).
Fig. 7. The photodegradation efficiency curve of MB under UV-light in the presence of pure and AgNPs decorated microstructures ZnO at various Ag contents (initial dye concentration : 10 mg/L, 10 mg catalyst, 60 min irradiation time, under UV light irradiation (6 watt, λ : 254 nm)).
Fig. 8. The curves of ln(C/C0) versus time for photodegradation of MB using pure and AgNPs decorated microstructures ZnO at various Ag contents (initial dye concentration : 10 mg/L, 10 mg catalyst, 60 min irradiation time, under UV light irradiation (6 watt, λ : 254 nm)).
Fig. 9 Schematic band diagram showing the separation charge processes of AgNPs decorated microstructures ZnO under UV irradiation.
Table 1. Atomic % from EDS analysis and ratio of [Ag]/[ZnO] for pure and AgNPs decorated microstructures ZnO at molar concentration of 0.0 – 0.1 M prepared by sol-gel method at a temperature of 95 oC for 6 hours. Amount of AgNO3 relative to ZnO used during the synthesis of AgNPs decorated microstructured ZnO 0 M (Pure ZnO) 0.005 M 0.01 M 0.05 M 0.07 M 0.09 M 0.1 M
Atomic % from EDS analysis
[Ag]/[Zn] (%)
Zn
O
Ag
41.20
58.80
-
-
46.60 39.95 39.20 38.40 32.70 30.00
52.85 58.21 56.30 53.96 54.05 51.58
1.073 1.84 4.50 7.64 13.25 18.42
2.33 4.60 11.48 19.90 40.40 61.31
Table 2. Comparison of photocatalytic activities from previous work with AgNPs decorated microstructures ZnO at different Ag contents (initial dye concentration : 10 mg/L, 10 mg catalyst, 60 min irradiation time, under UV light irradiation (6 watt, λ : 254 nm)). ZnO/Ag samples
Percentage degradation ( %)
R2
Ref.
0.9940 0.9780 0.9800 0.9730 0.9920 0.9970 0.9890
(this work)
57.98 after 1 h 66.60 after 1 h 74.38 after 1 h 80.48 after 1 h 85.25 after 1 h 85.35 after 1 h 87.74 after 1 h
Photodegradation rate constant, k (min-1) 0.014 0.017 0.022 0.026 0.030 0.031 0.032
Pure ZnO ZnO/Ag (0.02M) ZnO/Ag (0.05M) ZnO/Ag (0.1M) ZnO/Ag (0.2M)
36.2 after 1 h 39.6 after 1 h 49.3 after 1 h 47.2 after 1 h 42.6 after 1 h
5.16 x 10−3 5.69 x 10−3 7.63 x 10−3 7.17 x 10−3 6.08 x 10−3
-
[32]
Pure ZnO ZnO-Ag (0.05 g/L) ZnO-Ag (0.10 g/L) ZnO-Ag (0.15 g/L) ZnO-Ag (0.20 g/ L)
74.1 after 2 h 99.1 after 2 h 98.0 after 2 h 97.9 after 2 h 94.6 after 2 h
-
-
[36]
-
0.0088 0.1257 0.1396 0.2261 0.1607
0.9899 0.9921 0.9854 0.9563 0.9992
[37]
Pure ZnO microstructures 2.33 % AgNPs decorated microstructures ZnO 4.60 % AgNPs decorated microstructures ZnO 11.48 % AgNPs decorated microstructures ZnO 19.90 % AgNPs decorated microstructures ZnO 40.40 % AgNPs decorated microstructures ZnO 61.31 % AgNPs decorated microstructures ZnO
ZnO 5.41 wt% Ag/ZnO 7.95 wt% Ag/ZnO 10.03 wt% Ag/ZnO 11.37 wt% Ag/ZnO *R2: The square of correlation coefficient of kinetic energy