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AgBr: Experimental and theoretical approaches
Journal of Physics and Chemistry of Solids 135 (2019) 109118
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Mechanism underlying visible-light photocatalytic activity of Ag/AgBr: Experimental and theoretical approaches
T
Drashti K. Bhatta, Upendra D. Patelb,* a b
Civil Engineering Department, C. S. Patel Institute of Technology, Charotar University of Science & Technology, Gujarat, India Civil Engineering Department, Faculty of Technology & Engineering, The M. S. University of Baroda, Gujarat, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Ag/AgBr Photocatalysis Reactive oxidation species Surface plasmon resonance Bacterial cellulose
Silver/silver halides (Ag/AgX) are potential visible-light (VL) photocatalysts due to synergistic effects of surface plasmon resonance (SPR) and semiconductor photocatalysis. However, many contradictory opinions are found in published literature about the mechanism underlying photocatalytic oxidation (PCO) by Ag/AgX. In this study, Ag/AgBr particles were prepared by a simple one-pot synthesis method and used for degradation of Reactive Black 5 (RB5) as a model pollutant aiming to understand the effect of concentration of excess silver ions on SPR phenomenon, and to employ experimental and theoretical approaches to pinpoint the reactive oxidative species (ROS) responsible for RB5 degradation. 16.2% of initial 50 mg/L RB5 was degraded using 0.5 g/L AgBr (Ag+:AgBr = 0, i.e. no excess Ag+) under 60 min VL irradiation. Under identical experimental conditions, RB5 degradation increased to 86.5% using Ag/AgBr synthesized as Ag+:AgBr = 0.4, due to improved SPR effect. Experimental evidence supported by theoretical calculations revealed that superoxide radicals (•O2) played a prime role in PCO followed by photo-generated holes, whereas the contribution of •OH was negligible. The presence of Ag0 on AgBr greatly influenced band energies and regulated the formation of ROS. With an increase in ratio Ag+:AgBr, the conductance and valence bands increasingly became more electro-negative and electropositive, respectively, as compared to that at Ag+:AgBr = 0. The presence of Cl ̅ and SO42− ions adversely affected RB5 degradation, whereas CO32− did not cause any adverse impact. Ag/AgBr could be successfully immobilized over bacterial cellulose which, under identical conditions, provided 79.1% RB5 degradation in 60 min. The Ag/AgBr-BC could be reused five times.
1. Introduction Photocatalytic oxidation (PCO) has evolved as a successful method of environmental pollution remediation and drinking water treatment. When semiconductors are irradiated under a light source, the electrons (ē) from the valance band (VB) get excited and rise to conduction band (CB). The voids left in the VB are the photo-generated holes (h+) (see Eq (1)). TiO2 and ZnO are the two most widely reported photocatalysts. Due to a large band gap of ~3.2 eV, TiO2 and ZnO cannot perform effectively under VL and therefore, cannot exploit abundant and freely available solar energy. Noble metals (Ag, Au, Pt) can assist semiconductor-based photocatalysis by surface plasmon resonance (SPR) phenomenon. When noble metal nanostructures absorb light of wavelength greater than the size of metal particles, the photon frequency matches to the natural frequency of the oscillations of the electrons in the conduction band (CB) and generates local SPR [1]. The SPR enhances light absorption capacity of a noble metal-semiconductor system
*
and improves photocatalysis efficiency. Ag/AgBr photocatalyst is a metal-semiconductor composite photocatalyst. Initially, the excited ē of the AgBr will reduce the Ag+ on its surface and subsequently, along with the electron produced on Ag0 (due to SPR), these electrons reduce O2 to •O2, which is a potent ROS [2–6] (see Eqs (2) and (3)). It has been reported that the reduction of O2 may also produce H2O2 [7,8]. On the other hand, the holes left by the excited electrons on the VB of AgBr and Ag metal (due to SPR), possess significant oxidative potential. These photo-generated holes destroy the pollutants either by oxidizing the pollutant directly or it can oxidize hydroxyl ions to hydroxyl radicals (•OH) which is a non-selective, strong oxidizing species [9–11] (see Eqs (4) and (5)). Semiconductor + hʋ → ē (@CB) + h+ (@ VB)
(1)
At CB, ē + Ag+ → Ag0
Corresponding author. E-mail addresses: [email protected], [email protected] (U.D. Patel).
https://doi.org/10.1016/j.jpcs.2019.109118 Received 31 March 2019; Received in revised form 19 July 2019; Accepted 22 July 2019 Available online 23 July 2019 0022-3697/ © 2019 Elsevier Ltd. All rights reserved.
(2)
Journal of Physics and Chemistry of Solids 135 (2019) 109118
D.K. Bhatt and U.D. Patel
ē + O2 → •O2; ē + H+ 0.5O2 → H2O2; H2O2/•O2 + pollutant → degradation products (3)
2.2. One-pot synthesis of in-situ Ag/AgBr photocatalyst Fifteen milliliters of 55 mM AgNO3 solution were added drop-wise to 15 mL of 45 mM HBr solution with continuous sonication (Electroquip, India). The resulting AgBr suspension was placed in the sonicator for additional 15 min to ensure complete ion exchange. This yellow coloured AgBr suspension was then placed under artificial visible light (150 W halogen lamp, Osram make, India) in a wooden dark box with continuous stirring for 20 min for photo-reduction and deposition of Ag0 on AgBr. The resultant Ag/AgBr suspension (called insitu synthesized, Ag:Br molar ratio 1.25, (designated as Ag/AgBr1.25i) turned greyish in colour indicating the formation of Ag0 over the surface of AgBr particles. Ag/AgBr particle morphology was studied using SEM and EDAX (Nova Nano FEG-SEM 450 with EDAX, Model: TEAM EDS, Make: EDAX Inc, USA). Ag/AgBr1.25i were analyzed before (i.e. immediately after formation of AgBr) and after irradiation by SEM and EDAX. The particles were separated from suspension by centrifugation and then dried prior to the SEM and EDAX analyses. Results of characterization are provided in the Supplementary Information (Section S2).
At VB, h+ + OH ̅ → •OH; •OH + pollutant → degradation products h
+
+ pollutant → oxidized products
(4) (5)
̅
̅
Silver halide semiconductors (AgX, X = Cl, Br) exhibit excellent VL absorption and photocatalytic activity. For instance, Choi et al. [12] studied Methyl Orange (MO) decolourization using Ag/AgCl nanoparticles and reported 15 times faster removal of MO under identical experimental conditions, as compared to N-doped TiO2. Kuai et al. [13] reported complete degradation of 10 mg/L MO in 5min using Ag/AgBr nanoparticles under direct sunlight. In our earlier study, we noted almost complete decolourization ofRB5 in less than 30 min using Ag/ AgBr at pH 7 under artificial VL [14]. Although, several reports deliberating photocatalytic activity of Ag/AgX are published, some teething questions especially concerning mechanistic aspects, remain unanswered. Answering such questions can improve current understanding about Ag/AgX photocatalysts and increase their industrial application and useful life. In the present study, decolourization of RB5 was studied as a model pollutant using Ag/AgBr aiming to answer the following questions.
2.3. Photocatalytic oxidation of RB5 The experimental setup consisted of a closed wooden black box equipped with a magnetic stirrer (Remi, India) and an artificial VL source including a UV filter (150 W halogen light, Osram, India) was fitted on the top. The Ag/AgBr1.25i @ 500 mg/L was added immediately after synthesis to 250 mL of 50 mg/L RB5 solution and mixture pH was adjusted to 7.0 ± 0.1. The mixture was stirred in dark for 20 min to attain adsorption/desorption equilibrium followed by VL irradiation. All the experiments were performed using this procedure except experiments described under section 2.4 and 2.6. The aliquots were withdrawn at regular time intervals, centrifuged, and the supernatant was analyzed for absorbance at λmax = 595 nm for RB5decolourizationusing a UV–visible spectrophotometer (Cary 60, Agilent, USA). The reaction temperature was maintained at 25 °C ± 1 °C using a water bath in all the experiments unless specified otherwise. The percent
1. The Ag0 nanostructures produced on AgBr by photo-reduction of free Ag+ ions, enhance overall photocatalytic activity due to SPR. What should be the concentration of free Ag+ ions to obtain optimum SPR activity? 2. There is a lack of unanimity among the researchers about which ROS is/are mainly responsible for PCO. For instance, Wang et al. [15] attributed dye decolourization by Ag/AgBr to •O2, •OH, and •Br. On the other hand, Jiang et al. [11], demonstrated that Ag/AgBr could degrade pentachlorophenol mainly by photo-generated holes (h+) whereas the superoxide radicals did not play any significant role. Table S2 (Supplementary information) summarizes similar contrasting arguments reported in published literature. We used theoretical calculations and experimental procedures (using various ROS scavengers) to identify ROS responsible for decolourization and the factors influencing their selection. 3. Furthermore, industrial effluents containing dyes usually contain anions such as chloride, sulfate, and carbonate. The behaviour of Ag/AgBr in the presence of such anions can help understand its suitability for industrial applications. 4. Separation of nano or microparticles of Ag/AgBr following the reaction is a tedious process. An effort is made to immobilize Ag/AgBr on bacterial cellulose and study its photocatalytic activity.
(
C −C
)
⎤ x 100 ; decolourization was calculated as Colour removal, %= ⎡ 0C 0 ⎣ ⎦ where, C0 is the absorbance at time t0, C is the absorbance at time t. All experiments were performed in triplicates and average results were plotted with error bars representing ± 1 standard deviation. Effect of various VL sources was evaluated by conducting experiments indoor (avoiding direct exposure to sunlight at 10:30 a.m., 1:30 p.m., and 4:30 p.m. in May) or under direct sunlight (10 a.m.–12 Noon in May). Effect of the presence of carbonate, chloride, and sulphate ions was evaluated by adding 0.1 M of sodium carbonate (Na2CO3), sodium chloride (NaCl), or sodium sulphate (Na2SO4), respectively.
2. Materials and methods 2.4. Effect of excess Ag+ on photocatalytic activity of Ag/AgBr 2.1. Materials AgBr particles (molar ratio 1:1) were synthesized using 15 mL solutions each of 45 mM AgNO3 and HBr, following the procedure mentioned in section 2.2. The resulting suspension was immediately centrifuged to separate AgBr particles which were then dried in an oven at 105 °C. The dried AgBr, 0.125 g, were resuspended in 30 mL distilled water and varying volumes of AgNO3 stock solution (0.294 M) were added to obtain Ag+:AgBr molar ratios: 0, 0.1, 0.2, 0.3, 0.4, or 0.5. The suspension containing excess Ag+ was irradiated for 1 h under artificial VL to form Ag/AgBr. The resulting Ag/AgBr particles were then separated and dried in an oven at 105 °C. 125 mg of resulting Ag/AgBr particles were added to 250 mL of 50 mg/L RB5 and pH of the mixture was adjusted to 7.0 ± 0.1. The mixture was stirred under dark for 20 min to attain adsorption/desorption equilibrium, followed by VL irradiation to study decolourization.
Silver nitrate (AgNO3), Hydrobromic acid (HBr), Sodium Carbonate (Na2CO3), Sodium Chloride (NaCl), Oxalic acid (OA), and Sodium sulphate (Na2SO4) were supplied by S.D. FineChem Ltd. Sulphuric Acid (H2SO4), Sodium Hydroxide (NaOH) and Dimethyl Sulphoxide (DMSO) were supplied by Finar Chemicals, India. Sodium Acetate was supplied by Samir Tech Chem Pvt Ltd, Vadodara, and Benzoquinone (BQ) was supplied by Loba Chemie Pvt Ltd. Reactive black 5 (RB5) dye was supplied by Astron Chemicals, India. Chemical structure and related information of RB5 are provided in the Supplementary Information (Table S1). Nata-de-coco (bacterial cellulose, marketed as coconut gel) was supplied by Kapasu Company, Philippines. All the chemicals were of analytical grade and used as received without any pre-treatment unless stated otherwise. 2
Journal of Physics and Chemistry of Solids 135 (2019) 109118
D.K. Bhatt and U.D. Patel
the excess Ag concentration in terms of Ag+:AgBr ratio in the range 0.2–0.4 was essential to attaining efficient PCO of RB5 for the applied Ag/AgBr dose of 500 mg/L. Zhang et al. [17] and Xiao et al. [18] reported rapid and complete removal of MO dye using AgBr (no excess Ag); however, they used much smaller dye concentration (10 mg/L) and higher AgBr dose (1 g/L) as compared to those used in the present study. The presence of excess Ag+ in the solution leads to the greater formation of metallic silver over AgBr leading to improved SPR effect and therefore, PCO. It may be noted from Fig. 1(b) that the 1st-order RB5 decolourization rate constant at Ag+:AgBr = 0.5, decreased to 0.027 min−1 (decreases by ~33%) as compared to 0.041 min−1 at Ag+:AgBr = 0.4. Excessive deposition of Ag0 over AgBr surface may cause scattering of photons before they reach the semiconductor. This retards the generation of ē-h+ pair and subsequent redox reactions occurring on the CB and VB of AgBr [6,17]. Moreover, excessive coverage of AgBr may reduce SPR effect due to an increased size of Ag0 particles, and limit the access of RB5 to the catalyst surface resulting in lower decolourization.
2.5. Role of reactive oxidation species (ROS) in the photocatalytic activity of Ag/AgBr The experiments were performed using 500 mg/L Ag/AgBr1.25i on 50 mg/L RB5 solution at pH 7.0 ± 0.1. Role of ROS: superoxide radicals, hydroxyl radicals, and photo-generated holes was evaluated using their respective scavengers: BQ (50 μM), DMSO (100 mM), and oxalic acid (100 mM), respectively. 2.6. Immobilization of Ag/AgBr over bacterial cellulose (BC) Bacterial Cellulose cubes (12–18 mm) were pretreated as described by Patel and Suresh [16]. Firstly, pretreated cubes of BC were cut into smaller cubes of size 4–5 mm. Total 128–130 numbers of these cubes (equivalent wet mass ≅ 12 g) were kept in 30 mL of 45 mM HBr solution overnight with gentle stirring. To this HBr-BC suspension, 2.8 mL of 0.294 M AgNO3 solution was added. Yellow precipitates were formed over the BC surface (Ag:Br = 1.25). Stirring was continued for 1 h in dark followed by artificial VL irradiation for 30 min. These pieces were named as Ag/AgBr-BC. Pieces were removed from suspension, washed, and added to 250 mL of 50 mg/L RB5 solution for photocatalytic reaction. Before the exposure to light, the adsorption-desorption was allowed for 20 min in dark. One lot of 128 numbers cubes contained approximately 0.11–0.12 g Ag/AgBr. Dark adsorption of RB5 on Ag/ AgBr-BC was negligible.
3.2. Performance of in-situ Ag/AgBr photocatalyst in indoor daylight and sunlight It is important to evaluate alternate VL sources as promoters of photocatalytic activity. In a field-scale facility using VL photocatalyst, direct sunlight is always preferred. Thus, the performance of photocatalyst must be evaluated under direct sunlight, under shade, and at different times of the day as the fraction of UV light and intensity of sunlight will vary with time. Fig. 2 shows time course profiles of RB5 decolourization and corresponding 1st-order kinetics using in-situ Ag/ AgBr (Ag:Br = 1.25) exposed to various VL sources. As may be noted from Fig. 2, under indoor light, RB5 decolourization was > 95% in morning time (10:30 a.m.), and decreased to 76.5% in evening time (4:30 p.m.) due to reduction in sunlight intensity from 31000 lux in morning to ~900 lux in the evening. When exposed to direct VL, the extent of RB5 decolourization was > 98% under the artificial light source and sunlight. The radiation intensities under sunlight and the artificial VL were closely matching (~1 × 105 lux). However, it is interesting to note that the 1st-order reaction rate constant in the presence of sunlight was almost 100% higher than that in the presence of the artificial VL source. It seems that UV fraction of sunlight may have contributed to faster RB5 decolourization. The in-situ Ag/AgBr synthesized in this study performed efficiently over widely varying VL intensities and sources, which bolsters its applicability in a real field situation.
3. Results and discussion 3.1. 1Effect of the presence of excess Ag+ ions on the photocatalytic activity of Ag/AgBr Fig. 1(a) shows time-course profiles of RB5 decolourization using Ag/AgBr prepared with varying Ag+:AgBr molar ratios. In all experiments concerning Ag/AgBr, dark adsorption of RB5 was negligible and hence all plots of RB5 decolourization begin at 50 mg/L as initial RB5 concentration. Fig. 1(b) illustrates the effect of varying Ag+:AgBr molar ratios on 1st order RB5 decolourization rate constant. Adsorption of RB5 on Ag/AgBr was negligible in all the experiments and hence in all subsequent results, RB5 time-course profiles are shown to commence at 50 mg/L. Results shown in Fig. 1 reveals that extent and rate of RB5 decolourization increase with an increase in excess Ag+ concentration up to Ag+:AgBr molar ratio 0.4, and then decrease at Ag+:AgBr molar ratio 0.5. When no excess Ag was supplied (Ag+:AgBr = 0), RB5 decolourization was limited to 16.2% at the end of 60 min. Fig. 1(b) reveals that
Fig. 1. Influence of varying concentrations of excess Ag+ ions on RB5 decolourization. (Reaction conditions: pH = 7.0, AgBr dose = 500 mg/L, reaction volume = 250 mL, RB5 concentration = 50 mg/L, light source: artificial VL). 3
Journal of Physics and Chemistry of Solids 135 (2019) 109118
D.K. Bhatt and U.D. Patel
Fig. 2. RB5 decolourization using Ag/AgBr particles in indoor natural illumination at different times of the day.(a) Time-course profiles,(b) 1storder kinetics plots. (Reaction conditions: pH = 7.0, Ag/AgBr1.25i dose = 500 mg/L, reaction volume = 250 mL, RB5 concentration = 50 mg/L, light source: various VL sources).
respectively. Here too, the addition of DMSO did not show a significant effect on dye decolourization. Thus, the main ROS responsible for RB5 decolourization were •O2 radicals and photo-generated holes, irrespective of the irradiation source. As mentioned earlier, there are several contradictory opinions in published literature about the ROS responsible for PCO using Ag/AgX photocatalysts. Our experimental results suggested •O2 radicals and photo-generated holes to be the main ROSs, and we further confirmed these observations by some theoretical calculations as discussed under. The relationship among the potentials of conduction and valence bands, band gap, and the energy of free electrons for a semiconductor can be given by the following equations [22].
3.3. Role of reactive oxidation species (ROS) in the photocatalytic activity of Ag/AgBr When a photocatalyst is exposed to light, the electron-hole pair so generated produces reactive species evolving oxidation and reduction reactions. The type, life span, and generation rate of ROS may vary with the type of photocatalyst [19,20]. Fig. 3 reveals the effect of the presence of BQ, DMSO, and OA, respectively as scavengers of •O2, •OH, and h+, on the rate and extent of RB5 decolourization. It may be noted that the rate and extent of RB5 decolourization drastically decreased in the presence of BQ and OA. The effect of the presence of DMSO was negligible indicating a negligible contribution of •OH in PCO of RB5. The RB5 decolourization rate constants were found to follow the sequence, Kno.scavenger > KDMSO > KOA > KBQ (see Fig. 3(b)). Thus, it appears that photo-generated holes and superoxide radicals in increasing order contribute to RB5 decolourization. These results are consistent with those reported by Yan et al. [21], Shu et al. [10], and Zhang et al. [17]. It may be possible that the type of radiation (visible vs. UVC) may also affect the selectivity of ROS responsible for the oxidation of dye. We performed PCO experiments under UVC irradiation which revealed that BQ and OA suppressed RB5 decolourization by 83.62% and 21.82%,
ECB = X− Ee − 1 2 Eg
(6)
EVB = ECB + Eg
(7)
where, ECB and EVB are potential energy (eV) of the conduction and valence bands, respectively; X is the electro-negativity of the semiconductor expressed as the geometric mean of the electro-negativity of the constituent atoms; and Eg is the band gap energy of the semiconductor. Band gap energies (Eg) of AgBr and Ag/AgBr are widely reported in the range of 2.5–2.6 eV [11,23]. For AgBr, the electro-
Fig. 3. Effect of the presence of various ROS scavengers on RB5 decolourization using Ag/AgBr: (a) Time Course Profiles, (b) 1st-order kinetics plots. (Reaction conditions: pH = 7.0, Ag/AgBr1.25i dose = 500 mg/L, reaction volume = 250 mL, RB5 concentration = 50 mg/L, light source: artificial VL, use of different scavengers for ROS). 4
Journal of Physics and Chemistry of Solids 135 (2019) 109118
D.K. Bhatt and U.D. Patel
potentials of the CB and VB, and the corresponding formation of predominant ROS at Ag:Br molar ratios 1 and 1.25. To further strengthen our hypothesis, we carried out experiments using in-situ AgBr (Ag:Br = 1) in the presence of various scavengers. We observed that %RB5 decolourization in the absence of any ROS scavenger was 77% at the end of 60 min, which dropped to 17% and 9%, respectively in the presence of DMSO (•OH scavenger) and OA (hole-scavenger). It may be recalled that theoretical calculations predicted a highly electro-positive EVB of 2.605 eV at Ag:Br = 1, indicating the formation of •OH as the main ROS. Thus, it could be concluded from experimental results and theoretical calculations that the potentials of CB and VB vary according to the composition of Ag/AgBr (i.e. molar ratio of Ag:Br), which subsequently influence the selection of predominant ROS responsible for degradation. We carefully studied methods employed to synthesize Ag/AgX by authors in some published reports. For instance, Jiang et al. [11] synthesized Ag/AgBr particles with Ag:Br molar ratio = 1 and confirmed photo-generated holes and •OH as governing ROS. Hai Li et al. [25] synthesized Ag/AgBr with Ag:Br molar ratio = 1.36 and demonstrated that •O2 played a predominant role in dye decolourization. Thus, although contradictory, the observations made by the above authors can now easily be explained based on the experimental results and the theoretical calculations made by us.
Table 1 Energy Potentials (eV vs. NHE) of CB and VB at varying molar ratio of AgBr. Ag:Br molar ratio
X, eV
ECB, eV
EVB, eV
1 1.1 1.2 1.25 1.3 1.4 1.5
5.805 5.339 4.947 4.775 4.615 4.330 4.084
0.005 −0.461 −0.852 −1.025 −1.185 −1.470 −1.716
2.605 2.138 1.747 1.574 1.415 1.130 0.884
negativity X was calculated to be 5.8 eV (see section S4 in Supplementary information for detailed calculations). Using this data, ECB and EVB obtained were 0.005 eV and 2.605 eV, respectively. Surprisingly, the calculated EVB is inadequate (0.005 eV) to reduce O2 to •O2 at VB (O2/•O2 = −0.33 eV vs. NHE) [11,17,24]. On the other hand, theoretical ECB of AgBr (2.605 eV) is sufficiently higher than that required for formation of •OH at CB (•OH/OH− 1.99 eV vs NHE) [11,25]. This strongly suggests the possibility of formation of •OH as an important ROS and almost negligible formation of •O2. Thus, theoretically calculated ECB and EVB values indicate •OH to be the main ROS; a sheer contradiction to the experimental results. It is important to understand that behaviour of Ag/AgBr composite under irradiation is significantly different from that of AgBr semiconductor. The dual excitation of the semiconductor and the noble metal under irradiation changes energy potentials of CB and VB by shifting the Fermi level. The change in ECB and EVB due to the presence of Ag0 on Ag/AgBr subsequently influences the charge separation and the redox reactions at the band levels [26,27]. Table 1 lists theoretically calculated ECB and EVB values of Ag/AgBr photocatalyst at varying Ag:Br molar ratios. It may be noted from Table 1 that with an increase in Ag:Br molar ratio from 1 to 1.5, the corresponding ECB shifts from near zero (0.005 eV) to −1.716 eV, i.e. much more electro-negative than that required to reduce O2 to •O2 (i.e. −0.33 eV). Similarly, EVB values shifted from 2.605 eV to 0.884eV, i.e. much less electro-positive as compared to that required to oxidize OH− to •OH (•OH/OH− 1.99 eV vs NHE). Thus, theoretically, at Ag:Br molar ratio > 1.1, the ECB of Ag/ AgBr becomes more electro-negative enabling the formation of •O2 and disabling the formation of •OH. This observation perfectly coincides with the experimental results obtained using Ag/AgBr (Ag:Br = 1.25) in the presence of scavengers. Based on experimental results and theoretical calculations, Fig. 4 schematically shows the shift in the energy
̅
3.4. Effect of the presence of Cl, CO32−, SO42− ions on RB5 decolourization The Textile and the Dye manufacturing wastewaters contain sig̅ nificant concentrations of anions such as Cl, CO32−, SO42− etc. These anions may interact with the ROSs generated during irradiation [28], and interfere with the access of pollutant to the photocatalyst surface. Thus, it is necessary to evaluate the effect of the presence of these anions on Ag/AgBr photocatalytic activity. As shown in Fig. 5, RB5 decolourization rate constant decreased by ca. 30% in the presence of both, chloride and sulfate ions. Gibb's free energy calculations revealed that AgCl, Ag2CO3, and Ag2SO4 are spontaneously formed by the re̅ action between excess silver ions and Cl, CO32−, SO42− ions, respectively. In other words, the presence of these anions will scavenge Ag+ ions leading to decreased Ag0 formation on AgBr. This will subsequently decrease the SPR activity of Ag/AgBr and thereby, PCO. The presence of sulphate ions also caused a slight decline in RB5 degradation. It has been shown that at higher concentrations, SO42− ions may
Fig. 4. Schematic diagram of energy potential values of CB and VB for Ag:Br molar ratios 1 and 1.25. 5
Journal of Physics and Chemistry of Solids 135 (2019) 109118
D.K. Bhatt and U.D. Patel
Fig. 5. Effect of the presence of sulfate, chloride, and carbonate ions on RB5 decolourization using Ag/AgBr: (a) RB5 time course profiles, (b) 1storder kinetics plots. (Reaction conditions: pH = 7.0, Ag/AgBr1.25i dose = 500 mg/L, reaction volume = 250 mL, RB5 concentration = 50 mg/L, light source: artificial VL, copollutants = Na2CO3, NaCl and Na2SO4).
act as scavenger of photo-generated holes and/or •OH [28]. Besides this, the absence of free Ag+ ions will preclude formation of Ag/ Ag2SO4. Wei et al. [29] reported that pure Ag2SO4 was much slower than Ag/Ag2SO4 for the PCO of Methyl orange and Rhodamine B dyes. It was interesting to note (see Fig. 5) that the presence of carbonate ions did not have any adverse effect on the reaction rate constant and extent of removal. Cheng et al. [28] and Subramonian and Wu [30], revealed that CO32− ions are oxidized by photo-generated holes at VB, forming carbonate radicals (CO32−/•CO3− 1.57 eV) which are potent oxidation species assisting RB5 oxidation. Ag2CO3 semiconductor itself has been reported to be an efficient visible light photocatalyst [31]. It is also reported that Ag2CO3 undergoes significant photo-corrosion to produce Ag+ ions [32] which may be then reduced to Ag0 on the surface of AgBr, improving the SPR activity.
use. Immobilization of the photocatalyst over a substrate will reduce the loss of catalyst and increase reusability. Silver halide particles have been immobilized over various materials like glass, cotton, silk, Fe particles, silica, etc. [19,33–35]. Immobilization of Ag/AgBr over Bacterial cellulose (BC) is not reported so far to the best of our knowledge. Ag/AgBr-BC can be considered as a potential method for immobilization of Ag/AgX photocatalysts. 4. Conclusions Following important conclusions were drawn from the present study. 1. High concentration RB5 solution could be rapidly decolourized using a relatively smaller dose of Ag/AgBr under VL. At the doses used in this study, AgBr alone in the absence of excess Ag+ was much less efficient for RB5 decolourization. Extent and rate of decolourization increased with an increase in the concentration of excess Ag+ due to improved SPR effect. 2. Ag/AgBr performed better under sunlight than the artificial VL. In addition to this, Ag/AgBr demonstrated its versatility by efficient RB5 decolourization under natural indoor luminance at different times of the day. 3. Experimental results and theoretical calculations revealed that superoxide radicals and photo-generated holes are predominant ROSs responsible for RB5 decolourization. It was concluded that the presence of excess Ag+ (i.e. Ag:Br molar ratio > 1) greatly influenced Fermi level of Ag/AgBr, which in turn affected the band energies, and regulated the formation and the selection of ROS. The present study clarifies the paradoxical interpretations related to the
3.5. Immobilization of Ag/AgBr over bacterial cellulose Fig. 6 shows BC cubes at various stages of treatment. Pretreated BC cubes which appear semi-transparent (Fig. 6(a)), become yellowish typically due to the formation of AgBr (Fig. 6(b)). Change in colour from yellow to brownish typically indicates the formation of Ag0. Fig. 7(a)–(b) show SEM images of BC and Ag/AgBr-BC, respectively. Plain BC is characterized by thin fibrils of cellulose parallel to each other (Fig. 7(a)). After immobilization and irradiation, these fibrils can be seen studded with particles of Ag/AgBr (Fig. 7(b)). Fig. 8 shows that 78% of RB5 was decolourized at the end of 60 min using Ag/AgBr-BC. Moreover, there was no adsorption of RB5 on to plain BC cubes or Ag/AgBr-BC. Reusability of Ag/AgBr-BC was studied by spiking RB5 stock solution at regular time intervals. Ag/AgBr-BC could maintain catalytic activity until five reuses with %RB5 decolourization decreasing from 78% at the first use to ~50% at the fifth
Fig. 6. Image of Bacterial Cellulose: (a) BC cubes, (b) small BC pieces immobilized with AgBr over its surface, (c) Irradiated Ag/AgBr-BC pieces. 6
Journal of Physics and Chemistry of Solids 135 (2019) 109118
D.K. Bhatt and U.D. Patel
Fig. 7. SEM images of plain BC (a), and Ag/AgBr-BC (b).
[7]
[8]
[9] [10]
[11]
[12] [13]
Fig. 8. RB5 Decolourization using Ag/AgBr-BC. (Reaction conditions: pH = 7.0, reaction volume = 250 mL, RB5 concentration = 50 mg/L, Ag/AgBrBC = 128 cubes of 4–5 mm containing ~0.11–0.12 g Ag/AgBr, light source: artificial VL).
[14]
[15]
mechanism of photocatalytic oxidation by Ag/AgBr reported in published literature. 4. Ag/AgBr was stable in the presence of common anions like chloride, sulphate, and carbonate, showing its versatility for actual industrial wastewater treatment. 5. The immobilization of photocatalyst over the bacterial cellulose and its reusability can possibly reduce the cost of treatment and improve the useful life of photocatalyst.
[16]
[17]
[18]
[19] [20]
Appendix A. Supplementary data
[21]
Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpcs.2019.109118.
[22]
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Journal of Physics and Chemistry of Solids journal homepage: www.elsevier.com/locate/jpcs
Mechanism underlying visible-light photocatalytic activity of Ag/AgBr: Experimental and theoretical approaches
T
Drashti K. Bhatta, Upendra D. Patelb,* a b
Civil Engineering Department, C. S. Patel Institute of Technology, Charotar University of Science & Technology, Gujarat, India Civil Engineering Department, Faculty of Technology & Engineering, The M. S. University of Baroda, Gujarat, India
A R T I C LE I N FO
A B S T R A C T
Keywords: Ag/AgBr Photocatalysis Reactive oxidation species Surface plasmon resonance Bacterial cellulose
Silver/silver halides (Ag/AgX) are potential visible-light (VL) photocatalysts due to synergistic effects of surface plasmon resonance (SPR) and semiconductor photocatalysis. However, many contradictory opinions are found in published literature about the mechanism underlying photocatalytic oxidation (PCO) by Ag/AgX. In this study, Ag/AgBr particles were prepared by a simple one-pot synthesis method and used for degradation of Reactive Black 5 (RB5) as a model pollutant aiming to understand the effect of concentration of excess silver ions on SPR phenomenon, and to employ experimental and theoretical approaches to pinpoint the reactive oxidative species (ROS) responsible for RB5 degradation. 16.2% of initial 50 mg/L RB5 was degraded using 0.5 g/L AgBr (Ag+:AgBr = 0, i.e. no excess Ag+) under 60 min VL irradiation. Under identical experimental conditions, RB5 degradation increased to 86.5% using Ag/AgBr synthesized as Ag+:AgBr = 0.4, due to improved SPR effect. Experimental evidence supported by theoretical calculations revealed that superoxide radicals (•O2) played a prime role in PCO followed by photo-generated holes, whereas the contribution of •OH was negligible. The presence of Ag0 on AgBr greatly influenced band energies and regulated the formation of ROS. With an increase in ratio Ag+:AgBr, the conductance and valence bands increasingly became more electro-negative and electropositive, respectively, as compared to that at Ag+:AgBr = 0. The presence of Cl ̅ and SO42− ions adversely affected RB5 degradation, whereas CO32− did not cause any adverse impact. Ag/AgBr could be successfully immobilized over bacterial cellulose which, under identical conditions, provided 79.1% RB5 degradation in 60 min. The Ag/AgBr-BC could be reused five times.
1. Introduction Photocatalytic oxidation (PCO) has evolved as a successful method of environmental pollution remediation and drinking water treatment. When semiconductors are irradiated under a light source, the electrons (ē) from the valance band (VB) get excited and rise to conduction band (CB). The voids left in the VB are the photo-generated holes (h+) (see Eq (1)). TiO2 and ZnO are the two most widely reported photocatalysts. Due to a large band gap of ~3.2 eV, TiO2 and ZnO cannot perform effectively under VL and therefore, cannot exploit abundant and freely available solar energy. Noble metals (Ag, Au, Pt) can assist semiconductor-based photocatalysis by surface plasmon resonance (SPR) phenomenon. When noble metal nanostructures absorb light of wavelength greater than the size of metal particles, the photon frequency matches to the natural frequency of the oscillations of the electrons in the conduction band (CB) and generates local SPR [1]. The SPR enhances light absorption capacity of a noble metal-semiconductor system
*
and improves photocatalysis efficiency. Ag/AgBr photocatalyst is a metal-semiconductor composite photocatalyst. Initially, the excited ē of the AgBr will reduce the Ag+ on its surface and subsequently, along with the electron produced on Ag0 (due to SPR), these electrons reduce O2 to •O2, which is a potent ROS [2–6] (see Eqs (2) and (3)). It has been reported that the reduction of O2 may also produce H2O2 [7,8]. On the other hand, the holes left by the excited electrons on the VB of AgBr and Ag metal (due to SPR), possess significant oxidative potential. These photo-generated holes destroy the pollutants either by oxidizing the pollutant directly or it can oxidize hydroxyl ions to hydroxyl radicals (•OH) which is a non-selective, strong oxidizing species [9–11] (see Eqs (4) and (5)). Semiconductor + hʋ → ē (@CB) + h+ (@ VB)
(1)
At CB, ē + Ag+ → Ag0
Corresponding author. E-mail addresses: [email protected], [email protected] (U.D. Patel).
https://doi.org/10.1016/j.jpcs.2019.109118 Received 31 March 2019; Received in revised form 19 July 2019; Accepted 22 July 2019 Available online 23 July 2019 0022-3697/ © 2019 Elsevier Ltd. All rights reserved.
(2)
Journal of Physics and Chemistry of Solids 135 (2019) 109118
D.K. Bhatt and U.D. Patel
ē + O2 → •O2; ē + H+ 0.5O2 → H2O2; H2O2/•O2 + pollutant → degradation products (3)
2.2. One-pot synthesis of in-situ Ag/AgBr photocatalyst Fifteen milliliters of 55 mM AgNO3 solution were added drop-wise to 15 mL of 45 mM HBr solution with continuous sonication (Electroquip, India). The resulting AgBr suspension was placed in the sonicator for additional 15 min to ensure complete ion exchange. This yellow coloured AgBr suspension was then placed under artificial visible light (150 W halogen lamp, Osram make, India) in a wooden dark box with continuous stirring for 20 min for photo-reduction and deposition of Ag0 on AgBr. The resultant Ag/AgBr suspension (called insitu synthesized, Ag:Br molar ratio 1.25, (designated as Ag/AgBr1.25i) turned greyish in colour indicating the formation of Ag0 over the surface of AgBr particles. Ag/AgBr particle morphology was studied using SEM and EDAX (Nova Nano FEG-SEM 450 with EDAX, Model: TEAM EDS, Make: EDAX Inc, USA). Ag/AgBr1.25i were analyzed before (i.e. immediately after formation of AgBr) and after irradiation by SEM and EDAX. The particles were separated from suspension by centrifugation and then dried prior to the SEM and EDAX analyses. Results of characterization are provided in the Supplementary Information (Section S2).
At VB, h+ + OH ̅ → •OH; •OH + pollutant → degradation products h
+
+ pollutant → oxidized products
(4) (5)
̅
̅
Silver halide semiconductors (AgX, X = Cl, Br) exhibit excellent VL absorption and photocatalytic activity. For instance, Choi et al. [12] studied Methyl Orange (MO) decolourization using Ag/AgCl nanoparticles and reported 15 times faster removal of MO under identical experimental conditions, as compared to N-doped TiO2. Kuai et al. [13] reported complete degradation of 10 mg/L MO in 5min using Ag/AgBr nanoparticles under direct sunlight. In our earlier study, we noted almost complete decolourization ofRB5 in less than 30 min using Ag/ AgBr at pH 7 under artificial VL [14]. Although, several reports deliberating photocatalytic activity of Ag/AgX are published, some teething questions especially concerning mechanistic aspects, remain unanswered. Answering such questions can improve current understanding about Ag/AgX photocatalysts and increase their industrial application and useful life. In the present study, decolourization of RB5 was studied as a model pollutant using Ag/AgBr aiming to answer the following questions.
2.3. Photocatalytic oxidation of RB5 The experimental setup consisted of a closed wooden black box equipped with a magnetic stirrer (Remi, India) and an artificial VL source including a UV filter (150 W halogen light, Osram, India) was fitted on the top. The Ag/AgBr1.25i @ 500 mg/L was added immediately after synthesis to 250 mL of 50 mg/L RB5 solution and mixture pH was adjusted to 7.0 ± 0.1. The mixture was stirred in dark for 20 min to attain adsorption/desorption equilibrium followed by VL irradiation. All the experiments were performed using this procedure except experiments described under section 2.4 and 2.6. The aliquots were withdrawn at regular time intervals, centrifuged, and the supernatant was analyzed for absorbance at λmax = 595 nm for RB5decolourizationusing a UV–visible spectrophotometer (Cary 60, Agilent, USA). The reaction temperature was maintained at 25 °C ± 1 °C using a water bath in all the experiments unless specified otherwise. The percent
1. The Ag0 nanostructures produced on AgBr by photo-reduction of free Ag+ ions, enhance overall photocatalytic activity due to SPR. What should be the concentration of free Ag+ ions to obtain optimum SPR activity? 2. There is a lack of unanimity among the researchers about which ROS is/are mainly responsible for PCO. For instance, Wang et al. [15] attributed dye decolourization by Ag/AgBr to •O2, •OH, and •Br. On the other hand, Jiang et al. [11], demonstrated that Ag/AgBr could degrade pentachlorophenol mainly by photo-generated holes (h+) whereas the superoxide radicals did not play any significant role. Table S2 (Supplementary information) summarizes similar contrasting arguments reported in published literature. We used theoretical calculations and experimental procedures (using various ROS scavengers) to identify ROS responsible for decolourization and the factors influencing their selection. 3. Furthermore, industrial effluents containing dyes usually contain anions such as chloride, sulfate, and carbonate. The behaviour of Ag/AgBr in the presence of such anions can help understand its suitability for industrial applications. 4. Separation of nano or microparticles of Ag/AgBr following the reaction is a tedious process. An effort is made to immobilize Ag/AgBr on bacterial cellulose and study its photocatalytic activity.
(
C −C
)
⎤ x 100 ; decolourization was calculated as Colour removal, %= ⎡ 0C 0 ⎣ ⎦ where, C0 is the absorbance at time t0, C is the absorbance at time t. All experiments were performed in triplicates and average results were plotted with error bars representing ± 1 standard deviation. Effect of various VL sources was evaluated by conducting experiments indoor (avoiding direct exposure to sunlight at 10:30 a.m., 1:30 p.m., and 4:30 p.m. in May) or under direct sunlight (10 a.m.–12 Noon in May). Effect of the presence of carbonate, chloride, and sulphate ions was evaluated by adding 0.1 M of sodium carbonate (Na2CO3), sodium chloride (NaCl), or sodium sulphate (Na2SO4), respectively.
2. Materials and methods 2.4. Effect of excess Ag+ on photocatalytic activity of Ag/AgBr 2.1. Materials AgBr particles (molar ratio 1:1) were synthesized using 15 mL solutions each of 45 mM AgNO3 and HBr, following the procedure mentioned in section 2.2. The resulting suspension was immediately centrifuged to separate AgBr particles which were then dried in an oven at 105 °C. The dried AgBr, 0.125 g, were resuspended in 30 mL distilled water and varying volumes of AgNO3 stock solution (0.294 M) were added to obtain Ag+:AgBr molar ratios: 0, 0.1, 0.2, 0.3, 0.4, or 0.5. The suspension containing excess Ag+ was irradiated for 1 h under artificial VL to form Ag/AgBr. The resulting Ag/AgBr particles were then separated and dried in an oven at 105 °C. 125 mg of resulting Ag/AgBr particles were added to 250 mL of 50 mg/L RB5 and pH of the mixture was adjusted to 7.0 ± 0.1. The mixture was stirred under dark for 20 min to attain adsorption/desorption equilibrium, followed by VL irradiation to study decolourization.
Silver nitrate (AgNO3), Hydrobromic acid (HBr), Sodium Carbonate (Na2CO3), Sodium Chloride (NaCl), Oxalic acid (OA), and Sodium sulphate (Na2SO4) were supplied by S.D. FineChem Ltd. Sulphuric Acid (H2SO4), Sodium Hydroxide (NaOH) and Dimethyl Sulphoxide (DMSO) were supplied by Finar Chemicals, India. Sodium Acetate was supplied by Samir Tech Chem Pvt Ltd, Vadodara, and Benzoquinone (BQ) was supplied by Loba Chemie Pvt Ltd. Reactive black 5 (RB5) dye was supplied by Astron Chemicals, India. Chemical structure and related information of RB5 are provided in the Supplementary Information (Table S1). Nata-de-coco (bacterial cellulose, marketed as coconut gel) was supplied by Kapasu Company, Philippines. All the chemicals were of analytical grade and used as received without any pre-treatment unless stated otherwise. 2
Journal of Physics and Chemistry of Solids 135 (2019) 109118
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the excess Ag concentration in terms of Ag+:AgBr ratio in the range 0.2–0.4 was essential to attaining efficient PCO of RB5 for the applied Ag/AgBr dose of 500 mg/L. Zhang et al. [17] and Xiao et al. [18] reported rapid and complete removal of MO dye using AgBr (no excess Ag); however, they used much smaller dye concentration (10 mg/L) and higher AgBr dose (1 g/L) as compared to those used in the present study. The presence of excess Ag+ in the solution leads to the greater formation of metallic silver over AgBr leading to improved SPR effect and therefore, PCO. It may be noted from Fig. 1(b) that the 1st-order RB5 decolourization rate constant at Ag+:AgBr = 0.5, decreased to 0.027 min−1 (decreases by ~33%) as compared to 0.041 min−1 at Ag+:AgBr = 0.4. Excessive deposition of Ag0 over AgBr surface may cause scattering of photons before they reach the semiconductor. This retards the generation of ē-h+ pair and subsequent redox reactions occurring on the CB and VB of AgBr [6,17]. Moreover, excessive coverage of AgBr may reduce SPR effect due to an increased size of Ag0 particles, and limit the access of RB5 to the catalyst surface resulting in lower decolourization.
2.5. Role of reactive oxidation species (ROS) in the photocatalytic activity of Ag/AgBr The experiments were performed using 500 mg/L Ag/AgBr1.25i on 50 mg/L RB5 solution at pH 7.0 ± 0.1. Role of ROS: superoxide radicals, hydroxyl radicals, and photo-generated holes was evaluated using their respective scavengers: BQ (50 μM), DMSO (100 mM), and oxalic acid (100 mM), respectively. 2.6. Immobilization of Ag/AgBr over bacterial cellulose (BC) Bacterial Cellulose cubes (12–18 mm) were pretreated as described by Patel and Suresh [16]. Firstly, pretreated cubes of BC were cut into smaller cubes of size 4–5 mm. Total 128–130 numbers of these cubes (equivalent wet mass ≅ 12 g) were kept in 30 mL of 45 mM HBr solution overnight with gentle stirring. To this HBr-BC suspension, 2.8 mL of 0.294 M AgNO3 solution was added. Yellow precipitates were formed over the BC surface (Ag:Br = 1.25). Stirring was continued for 1 h in dark followed by artificial VL irradiation for 30 min. These pieces were named as Ag/AgBr-BC. Pieces were removed from suspension, washed, and added to 250 mL of 50 mg/L RB5 solution for photocatalytic reaction. Before the exposure to light, the adsorption-desorption was allowed for 20 min in dark. One lot of 128 numbers cubes contained approximately 0.11–0.12 g Ag/AgBr. Dark adsorption of RB5 on Ag/ AgBr-BC was negligible.
3.2. Performance of in-situ Ag/AgBr photocatalyst in indoor daylight and sunlight It is important to evaluate alternate VL sources as promoters of photocatalytic activity. In a field-scale facility using VL photocatalyst, direct sunlight is always preferred. Thus, the performance of photocatalyst must be evaluated under direct sunlight, under shade, and at different times of the day as the fraction of UV light and intensity of sunlight will vary with time. Fig. 2 shows time course profiles of RB5 decolourization and corresponding 1st-order kinetics using in-situ Ag/ AgBr (Ag:Br = 1.25) exposed to various VL sources. As may be noted from Fig. 2, under indoor light, RB5 decolourization was > 95% in morning time (10:30 a.m.), and decreased to 76.5% in evening time (4:30 p.m.) due to reduction in sunlight intensity from 31000 lux in morning to ~900 lux in the evening. When exposed to direct VL, the extent of RB5 decolourization was > 98% under the artificial light source and sunlight. The radiation intensities under sunlight and the artificial VL were closely matching (~1 × 105 lux). However, it is interesting to note that the 1st-order reaction rate constant in the presence of sunlight was almost 100% higher than that in the presence of the artificial VL source. It seems that UV fraction of sunlight may have contributed to faster RB5 decolourization. The in-situ Ag/AgBr synthesized in this study performed efficiently over widely varying VL intensities and sources, which bolsters its applicability in a real field situation.
3. Results and discussion 3.1. 1Effect of the presence of excess Ag+ ions on the photocatalytic activity of Ag/AgBr Fig. 1(a) shows time-course profiles of RB5 decolourization using Ag/AgBr prepared with varying Ag+:AgBr molar ratios. In all experiments concerning Ag/AgBr, dark adsorption of RB5 was negligible and hence all plots of RB5 decolourization begin at 50 mg/L as initial RB5 concentration. Fig. 1(b) illustrates the effect of varying Ag+:AgBr molar ratios on 1st order RB5 decolourization rate constant. Adsorption of RB5 on Ag/AgBr was negligible in all the experiments and hence in all subsequent results, RB5 time-course profiles are shown to commence at 50 mg/L. Results shown in Fig. 1 reveals that extent and rate of RB5 decolourization increase with an increase in excess Ag+ concentration up to Ag+:AgBr molar ratio 0.4, and then decrease at Ag+:AgBr molar ratio 0.5. When no excess Ag was supplied (Ag+:AgBr = 0), RB5 decolourization was limited to 16.2% at the end of 60 min. Fig. 1(b) reveals that
Fig. 1. Influence of varying concentrations of excess Ag+ ions on RB5 decolourization. (Reaction conditions: pH = 7.0, AgBr dose = 500 mg/L, reaction volume = 250 mL, RB5 concentration = 50 mg/L, light source: artificial VL). 3
Journal of Physics and Chemistry of Solids 135 (2019) 109118
D.K. Bhatt and U.D. Patel
Fig. 2. RB5 decolourization using Ag/AgBr particles in indoor natural illumination at different times of the day.(a) Time-course profiles,(b) 1storder kinetics plots. (Reaction conditions: pH = 7.0, Ag/AgBr1.25i dose = 500 mg/L, reaction volume = 250 mL, RB5 concentration = 50 mg/L, light source: various VL sources).
respectively. Here too, the addition of DMSO did not show a significant effect on dye decolourization. Thus, the main ROS responsible for RB5 decolourization were •O2 radicals and photo-generated holes, irrespective of the irradiation source. As mentioned earlier, there are several contradictory opinions in published literature about the ROS responsible for PCO using Ag/AgX photocatalysts. Our experimental results suggested •O2 radicals and photo-generated holes to be the main ROSs, and we further confirmed these observations by some theoretical calculations as discussed under. The relationship among the potentials of conduction and valence bands, band gap, and the energy of free electrons for a semiconductor can be given by the following equations [22].
3.3. Role of reactive oxidation species (ROS) in the photocatalytic activity of Ag/AgBr When a photocatalyst is exposed to light, the electron-hole pair so generated produces reactive species evolving oxidation and reduction reactions. The type, life span, and generation rate of ROS may vary with the type of photocatalyst [19,20]. Fig. 3 reveals the effect of the presence of BQ, DMSO, and OA, respectively as scavengers of •O2, •OH, and h+, on the rate and extent of RB5 decolourization. It may be noted that the rate and extent of RB5 decolourization drastically decreased in the presence of BQ and OA. The effect of the presence of DMSO was negligible indicating a negligible contribution of •OH in PCO of RB5. The RB5 decolourization rate constants were found to follow the sequence, Kno.scavenger > KDMSO > KOA > KBQ (see Fig. 3(b)). Thus, it appears that photo-generated holes and superoxide radicals in increasing order contribute to RB5 decolourization. These results are consistent with those reported by Yan et al. [21], Shu et al. [10], and Zhang et al. [17]. It may be possible that the type of radiation (visible vs. UVC) may also affect the selectivity of ROS responsible for the oxidation of dye. We performed PCO experiments under UVC irradiation which revealed that BQ and OA suppressed RB5 decolourization by 83.62% and 21.82%,
ECB = X− Ee − 1 2 Eg
(6)
EVB = ECB + Eg
(7)
where, ECB and EVB are potential energy (eV) of the conduction and valence bands, respectively; X is the electro-negativity of the semiconductor expressed as the geometric mean of the electro-negativity of the constituent atoms; and Eg is the band gap energy of the semiconductor. Band gap energies (Eg) of AgBr and Ag/AgBr are widely reported in the range of 2.5–2.6 eV [11,23]. For AgBr, the electro-
Fig. 3. Effect of the presence of various ROS scavengers on RB5 decolourization using Ag/AgBr: (a) Time Course Profiles, (b) 1st-order kinetics plots. (Reaction conditions: pH = 7.0, Ag/AgBr1.25i dose = 500 mg/L, reaction volume = 250 mL, RB5 concentration = 50 mg/L, light source: artificial VL, use of different scavengers for ROS). 4
Journal of Physics and Chemistry of Solids 135 (2019) 109118
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potentials of the CB and VB, and the corresponding formation of predominant ROS at Ag:Br molar ratios 1 and 1.25. To further strengthen our hypothesis, we carried out experiments using in-situ AgBr (Ag:Br = 1) in the presence of various scavengers. We observed that %RB5 decolourization in the absence of any ROS scavenger was 77% at the end of 60 min, which dropped to 17% and 9%, respectively in the presence of DMSO (•OH scavenger) and OA (hole-scavenger). It may be recalled that theoretical calculations predicted a highly electro-positive EVB of 2.605 eV at Ag:Br = 1, indicating the formation of •OH as the main ROS. Thus, it could be concluded from experimental results and theoretical calculations that the potentials of CB and VB vary according to the composition of Ag/AgBr (i.e. molar ratio of Ag:Br), which subsequently influence the selection of predominant ROS responsible for degradation. We carefully studied methods employed to synthesize Ag/AgX by authors in some published reports. For instance, Jiang et al. [11] synthesized Ag/AgBr particles with Ag:Br molar ratio = 1 and confirmed photo-generated holes and •OH as governing ROS. Hai Li et al. [25] synthesized Ag/AgBr with Ag:Br molar ratio = 1.36 and demonstrated that •O2 played a predominant role in dye decolourization. Thus, although contradictory, the observations made by the above authors can now easily be explained based on the experimental results and the theoretical calculations made by us.
Table 1 Energy Potentials (eV vs. NHE) of CB and VB at varying molar ratio of AgBr. Ag:Br molar ratio
X, eV
ECB, eV
EVB, eV
1 1.1 1.2 1.25 1.3 1.4 1.5
5.805 5.339 4.947 4.775 4.615 4.330 4.084
0.005 −0.461 −0.852 −1.025 −1.185 −1.470 −1.716
2.605 2.138 1.747 1.574 1.415 1.130 0.884
negativity X was calculated to be 5.8 eV (see section S4 in Supplementary information for detailed calculations). Using this data, ECB and EVB obtained were 0.005 eV and 2.605 eV, respectively. Surprisingly, the calculated EVB is inadequate (0.005 eV) to reduce O2 to •O2 at VB (O2/•O2 = −0.33 eV vs. NHE) [11,17,24]. On the other hand, theoretical ECB of AgBr (2.605 eV) is sufficiently higher than that required for formation of •OH at CB (•OH/OH− 1.99 eV vs NHE) [11,25]. This strongly suggests the possibility of formation of •OH as an important ROS and almost negligible formation of •O2. Thus, theoretically calculated ECB and EVB values indicate •OH to be the main ROS; a sheer contradiction to the experimental results. It is important to understand that behaviour of Ag/AgBr composite under irradiation is significantly different from that of AgBr semiconductor. The dual excitation of the semiconductor and the noble metal under irradiation changes energy potentials of CB and VB by shifting the Fermi level. The change in ECB and EVB due to the presence of Ag0 on Ag/AgBr subsequently influences the charge separation and the redox reactions at the band levels [26,27]. Table 1 lists theoretically calculated ECB and EVB values of Ag/AgBr photocatalyst at varying Ag:Br molar ratios. It may be noted from Table 1 that with an increase in Ag:Br molar ratio from 1 to 1.5, the corresponding ECB shifts from near zero (0.005 eV) to −1.716 eV, i.e. much more electro-negative than that required to reduce O2 to •O2 (i.e. −0.33 eV). Similarly, EVB values shifted from 2.605 eV to 0.884eV, i.e. much less electro-positive as compared to that required to oxidize OH− to •OH (•OH/OH− 1.99 eV vs NHE). Thus, theoretically, at Ag:Br molar ratio > 1.1, the ECB of Ag/ AgBr becomes more electro-negative enabling the formation of •O2 and disabling the formation of •OH. This observation perfectly coincides with the experimental results obtained using Ag/AgBr (Ag:Br = 1.25) in the presence of scavengers. Based on experimental results and theoretical calculations, Fig. 4 schematically shows the shift in the energy
̅
3.4. Effect of the presence of Cl, CO32−, SO42− ions on RB5 decolourization The Textile and the Dye manufacturing wastewaters contain sig̅ nificant concentrations of anions such as Cl, CO32−, SO42− etc. These anions may interact with the ROSs generated during irradiation [28], and interfere with the access of pollutant to the photocatalyst surface. Thus, it is necessary to evaluate the effect of the presence of these anions on Ag/AgBr photocatalytic activity. As shown in Fig. 5, RB5 decolourization rate constant decreased by ca. 30% in the presence of both, chloride and sulfate ions. Gibb's free energy calculations revealed that AgCl, Ag2CO3, and Ag2SO4 are spontaneously formed by the re̅ action between excess silver ions and Cl, CO32−, SO42− ions, respectively. In other words, the presence of these anions will scavenge Ag+ ions leading to decreased Ag0 formation on AgBr. This will subsequently decrease the SPR activity of Ag/AgBr and thereby, PCO. The presence of sulphate ions also caused a slight decline in RB5 degradation. It has been shown that at higher concentrations, SO42− ions may
Fig. 4. Schematic diagram of energy potential values of CB and VB for Ag:Br molar ratios 1 and 1.25. 5
Journal of Physics and Chemistry of Solids 135 (2019) 109118
D.K. Bhatt and U.D. Patel
Fig. 5. Effect of the presence of sulfate, chloride, and carbonate ions on RB5 decolourization using Ag/AgBr: (a) RB5 time course profiles, (b) 1storder kinetics plots. (Reaction conditions: pH = 7.0, Ag/AgBr1.25i dose = 500 mg/L, reaction volume = 250 mL, RB5 concentration = 50 mg/L, light source: artificial VL, copollutants = Na2CO3, NaCl and Na2SO4).
act as scavenger of photo-generated holes and/or •OH [28]. Besides this, the absence of free Ag+ ions will preclude formation of Ag/ Ag2SO4. Wei et al. [29] reported that pure Ag2SO4 was much slower than Ag/Ag2SO4 for the PCO of Methyl orange and Rhodamine B dyes. It was interesting to note (see Fig. 5) that the presence of carbonate ions did not have any adverse effect on the reaction rate constant and extent of removal. Cheng et al. [28] and Subramonian and Wu [30], revealed that CO32− ions are oxidized by photo-generated holes at VB, forming carbonate radicals (CO32−/•CO3− 1.57 eV) which are potent oxidation species assisting RB5 oxidation. Ag2CO3 semiconductor itself has been reported to be an efficient visible light photocatalyst [31]. It is also reported that Ag2CO3 undergoes significant photo-corrosion to produce Ag+ ions [32] which may be then reduced to Ag0 on the surface of AgBr, improving the SPR activity.
use. Immobilization of the photocatalyst over a substrate will reduce the loss of catalyst and increase reusability. Silver halide particles have been immobilized over various materials like glass, cotton, silk, Fe particles, silica, etc. [19,33–35]. Immobilization of Ag/AgBr over Bacterial cellulose (BC) is not reported so far to the best of our knowledge. Ag/AgBr-BC can be considered as a potential method for immobilization of Ag/AgX photocatalysts. 4. Conclusions Following important conclusions were drawn from the present study. 1. High concentration RB5 solution could be rapidly decolourized using a relatively smaller dose of Ag/AgBr under VL. At the doses used in this study, AgBr alone in the absence of excess Ag+ was much less efficient for RB5 decolourization. Extent and rate of decolourization increased with an increase in the concentration of excess Ag+ due to improved SPR effect. 2. Ag/AgBr performed better under sunlight than the artificial VL. In addition to this, Ag/AgBr demonstrated its versatility by efficient RB5 decolourization under natural indoor luminance at different times of the day. 3. Experimental results and theoretical calculations revealed that superoxide radicals and photo-generated holes are predominant ROSs responsible for RB5 decolourization. It was concluded that the presence of excess Ag+ (i.e. Ag:Br molar ratio > 1) greatly influenced Fermi level of Ag/AgBr, which in turn affected the band energies, and regulated the formation and the selection of ROS. The present study clarifies the paradoxical interpretations related to the
3.5. Immobilization of Ag/AgBr over bacterial cellulose Fig. 6 shows BC cubes at various stages of treatment. Pretreated BC cubes which appear semi-transparent (Fig. 6(a)), become yellowish typically due to the formation of AgBr (Fig. 6(b)). Change in colour from yellow to brownish typically indicates the formation of Ag0. Fig. 7(a)–(b) show SEM images of BC and Ag/AgBr-BC, respectively. Plain BC is characterized by thin fibrils of cellulose parallel to each other (Fig. 7(a)). After immobilization and irradiation, these fibrils can be seen studded with particles of Ag/AgBr (Fig. 7(b)). Fig. 8 shows that 78% of RB5 was decolourized at the end of 60 min using Ag/AgBr-BC. Moreover, there was no adsorption of RB5 on to plain BC cubes or Ag/AgBr-BC. Reusability of Ag/AgBr-BC was studied by spiking RB5 stock solution at regular time intervals. Ag/AgBr-BC could maintain catalytic activity until five reuses with %RB5 decolourization decreasing from 78% at the first use to ~50% at the fifth
Fig. 6. Image of Bacterial Cellulose: (a) BC cubes, (b) small BC pieces immobilized with AgBr over its surface, (c) Irradiated Ag/AgBr-BC pieces. 6
Journal of Physics and Chemistry of Solids 135 (2019) 109118
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Fig. 7. SEM images of plain BC (a), and Ag/AgBr-BC (b).
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[8]
[9] [10]
[11]
[12] [13]
Fig. 8. RB5 Decolourization using Ag/AgBr-BC. (Reaction conditions: pH = 7.0, reaction volume = 250 mL, RB5 concentration = 50 mg/L, Ag/AgBrBC = 128 cubes of 4–5 mm containing ~0.11–0.12 g Ag/AgBr, light source: artificial VL).
[14]
[15]
mechanism of photocatalytic oxidation by Ag/AgBr reported in published literature. 4. Ag/AgBr was stable in the presence of common anions like chloride, sulphate, and carbonate, showing its versatility for actual industrial wastewater treatment. 5. The immobilization of photocatalyst over the bacterial cellulose and its reusability can possibly reduce the cost of treatment and improve the useful life of photocatalyst.
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[17]
[18]
[19] [20]
Appendix A. Supplementary data
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Supplementary data to this article can be found online at https:// doi.org/10.1016/j.jpcs.2019.109118.
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