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GSH-assisted hydrothermal synthesis of MnxCd1−xS solid solution hollow spheres and their application in photocatalytic degradation
Materials Science in Semiconductor Processing 52 (2016) 82–90
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GSH-assisted hydrothermal synthesis of MnxCd1 xS solid solution hollow spheres and their application in photocatalytic degradation Jiasheng Lai b, YiMing Qin a, Lan Yu a, Chunyan Zhang a,n a b
School of Food and Biochemical Engineering, Guangxi Science & Technology Normal University, Laibin 546199, PR China School of Mechanical and electrical engineering, Guangxi Science & Technology Normal University, Laibin 546199, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 December 2015 Received in revised form 9 April 2016 Accepted 5 May 2016
A series of MnxCd1 xS (MCS) solid solutions hollow spheres (x¼0.0, 0.20, 0.33, 0.50 and 0.67) have been synthesized with the assistance of L-Glutathione (GSH) by a simple hydrothermal route for the first time. Different characterization techniques, including X-Ray diffraction (XRD), atomic absorption spectroscopy (AAS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-EmmettTeller surface area (BET), UV–vis and diffuse reflectance spectra (DRS) are performed to investigate the structural and optical properties of the as-prepared samples. The experiment of rhodamine B (RhB) decoloration indicates that all the MCS samples show higher photo-catalytic activities than commercial CdS under visible light irradiation. Among them, the Mn0.33Cd0.67S exhibits the highest photo-catalytic activity. The superior photo-catalytic activity of MCS samples may be attributed predominantly to the synergistic effect of the appropriate band-gap structure as well as the special hollow spherical morphology which makes MCS samples have the ability to harvest exciting visible-light due to multiple scattering within the interior space. Furthermore, the Mn0.33Cd0.67S shows well stability, the photocatalytic activities do not decrease significantly after six recycles. The work may open a novel strategy to fabricate multi-component chalcogenide solid solutions hollow spheres. & 2016 Elsevier Ltd. All rights reserved.
Keywords: MnxCd1 xS solid solutions Hollow spheres GSH Photo-catalysis
1. Introduction Energy crisis and environmental pollution caused by excessive utilization of fossil fuel are the most serious problems the world facing today, which greatly prompt the exploration of new energy sources. Solar energy with distinct advantages of inexhaustible, clean and accessible [1], is becoming a promising energy resource to take the place of the conventional ones. Photo-catalysis which can bring several benefits from energy and environmental viewpoints is one of the most effective approaches to realize the utilization of solar energy [2]. Moreover, micro/nanostructure photocatalysis with different morphologies exhibit diverse physicochemical properties and lead to very different catalytic activity. Therefore, zero dimensional (0-D) photo-catalysis such as quantum dots [3,4] (QDs), 1-D nanorods [5] and nanotubes, 2-D nanosheets, plates, disks and films [6], and 3D nanospheres are synthesized. Among them, 3D hollow nanospheres are worth to be mentioned due to their high light-harvesting efficiency, fast mobility of charge carriers, higher specific surface area, and better permeability compared to their solid counterparts [7,8]. Up to n
Corresponding author. E-mail address: [email protected] (C. Zhang).
http://dx.doi.org/10.1016/j.mssp.2016.05.001 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
now, various hollow spheres including metals, polymers, carbon materials have been investigated widely. Traditional template-directed methods are tedious [9,10]. In contrast, the in situ gasbubble template synthesis strategy [11,12], brings us a new way to synthesize hollow spheres and neglects the template removal step. In recent years, metal sulfides have been widely researched because of having response in the visible light range [13]. Especially, the Cadmium sulfide (CdS) has been proven to be an efficient photo-catalyst materials with a narrow band gap (2.4 eV) and a suitable band structure [14], which matches well with the visible spectral range of solar [15,16]. However, the CdS is easily corroded [17] and its separation efficiency of photo-generated holes and electrons is low, which restrict the applications in solar conversion and environmental remediation. In general, an ideal photo-catalyst not only needs suitable band gap position, but also shows effective separation efficiencies of the photo-generated carriers. All these are strongly dependent on the electronic properties and structural of the photo-catalysts [18]. Many effective methods have been reported to adjust the band gap and inhibit the photo-corrosion process, such as doping metallic elements (Mn and Ni et al.) into CdS [19,20], coating CdS with a thin layer of amorphous carbon [21], pairing CdS with Graphene [22], embedding CdS particles in a polymer matrix [23] and so on. Considering that the solid solutions can be obtained by combining a narrow-
J. Lai et al. / Materials Science in Semiconductor Processing 52 (2016) 82–90
band-gap material with a wide-band-gap material, or even two wide-band-gap materials, thus the solid solutions with a proper band structure may be formed by choosing and adjusting the right components. To present, a huge number of multi-component solid solutions formed by CdS were reported successively, such as ZnxCd1 xS [14,24], Mn1 xCdxS [25], (CuIn)xCd2(1 x)S2 [26], Zn1 xCdxIn2S4 [27,28], CdxCuyZn1 x-yS [29], (ZnxCd1 x)(SexS1 x) [30], Cd0.1SnxZn0.9-2xS [31], Cu-doped Cd0.1Zn0.9S [32] and ZnxCd1 xS/CNTs [33], et al. Among them, the ternary Mn-Cd-S alloyed system formed by combining MnS (Eg ¼3.7 eV) with CdS (Eg ¼2.4 eV) has attracted considerable attention [34–36]. MCS solid solution, due to the tunability of its band gap by regulating the contents of Cd and Mn, is a promising candidate for visible light catalysis. Meiying Liu et al. have reported that Mn1 xCdxS solid solution will be rods, nanorods, nanoparticles while x ¼0.05, 0.16 and Z0.5 separately [25]. In this paper, we report a novel hydrothermal method with template-free to synthesize MCS solid solutions hollow spheres. During the process, GSH as a direct S source and bubble source to avoid traditional messy and complex template removal procedures. The obtained MCS solid solutions hollow spheres feature a uniform size and disperse well, which show much high photo-catalytic activities for organic pollutants removal. This unique structure may favor the harvesting of exciting light and scattering within the interior space. Mn0.33Cd0.67S exhibits the highest photo-catalytic activity due to the appropriate band gap. This one-pot template-free method may provide a costeffective approach to fabricate multi-component chalcogenide solid solutions hollow spheres.
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irradiation time intervals, 3 mL liquid was taken out and then filtered. The concentration of RhB in filtrates was analyzed using a UV–vis spectrophotometer (UV-2450). In order to test the durability of samples, six cycles of photocatalytic degradation RhB and phenol were assessed by using Mn0.33Cd0.67S as a representative sample. After each run of reactions, the catalyst was collected carefully, washed by anhydrous ethanol and reused in the following recycling experiment. We detected also the XRD spectrum of the final catalyst after the experiment. 2.3. Analysis of reactive species and active species trapping As we know, coumarin can readily react with an readily reaa kind of highly fluorescent product, 7-hydroxycoumarin, so we used coumarin as a probe molecule to detect the formation of frmationt the robe molecul-catalyst by a photoluminescence (PL) method. The experimental procedure was similar to the previous activity test. The PL intensity was measured around 445 nm excited by 332 nm light. To detect what is the primary active species during the photocatalytic reaction, we used tert-butyl alcohol (TBA), p-benzoquinone (PBQ) and triethanol amine (TEOA) as scavengers [37] to prove the presence of hydroxyl radicals ( OH), superoxide radicals ( O2-) and holes (h þ ), respectively. The scavenger was added into the RhB solution prior to the addition of the photo-catalysts using a similar procedure to the previous activity test. 2.4. Characterization
2. Experimental 2.1. Preparation of MnxCd1 xS solid solutions hollow spheres C6H12N2O4S2 (GSH) and cadmium nitrate tetrahydrate (Cd(NO3)2 4H2O) were provided by Aladdin, manganese acetate tetrahydrate (Mn(CH3COO)2 4H2O) was obtained from Sinopharm Chemical Reagent Co, Ltd (Shanghai, China). All chemicals were analytical grade reagents and used without any further purification. In a typical synthesis procedure, 0.60 mmol of GSH was dissolved into 40 mL distilled water as solution A. Meanwhile, the mixture of x mmol Mn(CH3COO)2 4H2O and (0.6-x) mmol Cd(NO3)2 4H2O were dissolved into another 40 mL distilled water as solution B. Subsequently, solution B was slowly added to solution A under vigorous stirring. The mixture was then transferred to a 100 mL Teflon-lined stain-less steel autoclave, heated to 180 °C and maintained for 5 h, and then air-cooled to room temperature. The product was filtrated, washed alternately with deionized water and ethanol several times and dried at 60 °C for overnight. A series of MCS solid solutions hollow spheres with different compositions were synthesized by varying the molar ratio of different precursors. 2.2. Photo-catalytic activities and durability measurements The photo-catalytic activity of MCS was assessed by degrading RhB under visible light irradiation, using a 350 W Xe lamp equipped with a 420 nm cutoff filter. After the pH of the solution were adjusted at desired values using hydrochloric acid and sodium hydroxide solutions, a certain quality of MCS was suspended in 50 mL a certain concentration of RhB aqueous solution, then ultrasonic treated for 10 min and stirred in the dark for 1 h in order to meet the adsorption/desorption equilibrium between the catalyst and the pollutants. In order to ensure the reactions at room temperature and reduce the interference of the infrared radiation, condensate water was passed into the flask. At given
The microstructure and particle sizes of MCS were observed by TEM using a Tecnai G20 (FEI Co., Holland) TEM microscope worked at an accelerating voltage of 200 kV, high-resolution TEM (HRTEM) were collected using an FEI Titan TEM operated at 300 kV. SEM was performed with SU8000 (FESEM, Hitachi, Japan) which worked at an accelerating voltage of 15 KV. SEM was used to investigate the overall morphology of the product. The BET specific surface areas of the samples were evaluated on the basis of nitrogen adsorption isotherms using a Micromeritics ASAP2020 gas adsorption apparatus (USA). A Shimadzu AA6300 atomic absorption spectrum (AAS), equipped with a graphite oven, was used to determine the content of Cd and Mn in the as-prepared samples. The UV–vis diffused reflectance spectra (UV–vis DRS) were obtained using a UV–vis spectrometer (UV-2450, Shimadzu, Japan) from 200 to 800 nm, BaSO4 was used as background.
3. Results and discussion 3.1. Photo-catalytic and optical properties Photo-catalysts are potentially promising materials in the fields of environmental restoration and water splitting, thereby high catalytic activity is essential for large scale applications. Here, we have chosen the photo-catalytic degradation of non-biodegradable pollutant RhB dye under visible light irradiation as a model reaction to evaluate the photo-catalytic activities of the as-prepared MCS solid solutions hollow spheres (Fig. 1). For comparison, commercial CdS is also performed at the same experimental conditions. The photo-catalytic degradation process can be monitored by detecting the intensity changes of the RhB absorption peak about 553 nm. As shown in Fig. 1, all MnxCd1 xS solid solutions hollow spheres exhibit higher photo-catalytic activity than commercial CdS. This can be attributed to the formation of solid solutions as well as unique hollow structure, which enables the catalyst to response in visible light and has excellent performance
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1.0
0.8
g f
0.8
0 min
C t / C0
0.6
Absorption
0.6
e d
0.4
c
a 5
10 15 Time(min)
20
300
25
Fig. 1. Photocatalytic decoloration curves of RhB under visible light (λ4420 nm) irradiation. (a) Mn0.33Cd0.67S, (b) Mn0.2Cd0.8S, (c) Mn0.5Cd0.5S, (d) Mn0.67Cd0.33S, (e) CdS hollow spheres, (f) commercial CdS, (g) without adding catalyst.
for dyes removal. On the one hand, the band gap and position of MCS can be adjusted by altering the ratio of compositions, which enables the catalyst to response in visible light. On the other hand, the unique hollow structure can provide more active sites to contact with RhB closely, and also allows multiple reflections of visible light in the interior cavities, which improves the light harvesting efficiently [38,39]. The Mn0.33Cd0.67S shows the highest photo-catalytic activity among the series of MCS samples, about 99.85% of RhB is degraded under visible light irradiation in 25 min. In order to compare the photo-catalytic activities of these solid solutions, the reaction rate constants (k) are calculated by adopting the pseudo first-order model as expressed by Eq. (1), which is commonly used in photocatalytic decoloration process [40].
ln
15 min 20 min
0.0
0.0 0
0.4
0.2
b
0.2
5 min 10 min
(C0 /Ct ) = kt
(1)
C0 and Ct are the concentrations of RhB aqueous solution at 0 and t min, respectively. As shown in Table 1, the RhB degradation rate of the Mn0.33Cd0.67S solid solutions hollow spheres (k ¼0.2372 min 1) is about 4 times as much as that of CdS hollow sphere (k ¼0.0545 min 1). Therefore, it is believed that the formation of MCS solid solutions contributed to the activity enhancement of CdS hollow sphere. In Fig. 2, with the time go on, the peak of RhB declines promptly and the color of RhB aqueous solution changed from pink to colorless, which indicate that the ethyl groups of RhB are removed during irradiation [41,42]. The RhB is a macromolecular substance with much groups, including some properties unstable groups which easy to react. The unstable groups initiate a chain reaction during irradiation, which may be the cause of RhB decolor quickly. Generally, RhB degradation under visible light irradiation contained a direct photocatalytic process and an indirect dye photosensitization process [43]. Therefore, this superior photocatalytic activity can be attributed to a synergistic effect of photocatalytic and photosensitization. A good
350
400
450 500 550 Wavelength (nm)
600
650
700
Fig. 2. The absorption spectrum of RhB (1.0 10 5 M, 50 mL) in the presence of Mn0.33Cd0.67S (50 mg), the inset is the corresponding decoloration kinetics graph in line with a first order reaction.
linear relationship of ln(Ct/C0) versus reaction time is shown in the inset of Fig. 2, which indicates that the degradation of RhB obeys the first order reaction well. The effects of various factors such as the initial concentration of dye, pH and the dosage of catalyst on the removal efficiency of RhB are studied. Fig. 3 shows the relationship between initial concentration and the decoloration rate of the RhB in the presence of 50.0 mg Mn0.33Cd0.67S photocatalysts. The decoloration rate declines with the increase of initial concentration of RhB, which may be attributed to two reasons [44]: (i) more RhB molecules will be adsorbed on the surface of photocatalysts with the increase of initial concentration and occupy the active sites of photocatalysts; leads to a decrease in the number of active sites and (ii) The RhB molecules can absorb visible light, an increase in initial concentration of RhB will lead to a reduction in the number of photons that reach the catalyst surface. Due to the surface charge properties of MCS change with the changes of pH values, the decolorzation efficiency of RhB strongly depends on the pH of the reaction solution too. It is obvious in Fig. 4, the maximum decolorization rate is obtained in pH 7.5, and then decreased with increasing of pH value. This can be attributed to that weak alkaline solution is conducive to the adsorption of the RhB molecules onto MCS surfaces since RhB is a cationic dye. Meanwhile, however, when the pH value is so high (pH 410), the hydroxyl ions compete with RhB molecules in the adsorption on the surface of photocatalysts [45], and the photodecolorant reaction is inhibited. Photodecolorant reaction of dye takes place both on the active surface of photocatalyst and in bulk solution generated active oxidant species. Therefore the dosage of photocatalyst plays an important role in photo-degradation processes [46]. The effect of MCS concentration on removal of RhB is examined by varying Mn0.33Cd0.67S dose of 25–100 mg while keeping initial dye concentration at 1.0 10 5 mol/L and pH 7.5, As shown in Fig. 5, the
Table 1 Precusor ratio of Mn/Cd, reaction rate constants (k), R-square, energy band gap (Eg), BET specific surface area and the composition values estimated from AAS of MCS solid solutions. MnxCd1 xS
0 0.2 0.33 0.5 0.67
k (min 1)
0.05459 0.09611 0.23724 0.09179 0.05748
R2
0.9678 0.9523 0.9595 0.9704 0.9642
Eg (eV)
2.31 2.37 2.38 2.39 2.41
λg (nm)
536.8 523.2 521 518.8 514.5
BET Surface
245 414 464 675 698
Composition values of MCS During Synthesis
From AAS
0 0.2 0.33 0.5 0.67
0 0.17 0.31 0.48 0.63
J. Lai et al. / Materials Science in Semiconductor Processing 52 (2016) 82–90
1.0
0.6
(d)
0.4
0.5
0.6
Absorption
Ct /C0
0.8
(a) (b) (c)
0.4 0.2
0
5
10 Time (min)
15
1.5 h 2.5 h 3.5 h
0.2
0.0 260
20
Fig. 3. The effect of initial RhB concentration on decolorization efficiency: (a) 0.5 10 5 mol/L, (b) 1.0 10 5 mol/L, (c) 1.5 10 5 mol/L, (d) 2.0 10 5 mol/L.
1.0
pH=3 pH=6.5 pH=7.5 pH=8.5 pH=10
0.8
Ct / C0
0h 0.5 h
0.1
0.0
0.6 0.4 0.2 0.0 0
5
10 15 Time(min)
20
25
Fig. 4. The effect of pH on decolorization efficiency of RhB.
25 mg 50 mg 75 mg 100 mg
1.0 0.8
Ct / C0
0.3
85
0.6 0.4
280
300 320 Wavelength (nm)
340
360
Fig. 6. The absorption spectrum of salicylic acid solution (20 mg L 1, 50 mL) in the presence of Mn0.33Cd0.67S (50 mg) under visible light ( 4420 nm) irradiation. The inset is the corresponding Ct/C0 versus time curves.
and stable pollutant, salicylic acid which has no visible light adsorption, has been also chosen to photo-catalytic degrade under visible light irradiation. As shown in Fig. 6, in 3.5 h irradiation of visible light, the peak of salicylic acid disappears completely, which indicates that the MCS hollow spheres solid solutions have high activity and salicylic acid can be decomposed efficiently in 3.5 h. The result demonstrates again the high activity of the MCS hollow spheres solid solutions. In order to investigate the recyclability of the MCS solid solutions hollow spheres, the Mn0.33Cd0.67S is chosen to be a representative sample for recycling test. As shown in Fig. 7, no matter RhB which easy to adsorption or phenol which difficult to adsorption on surface of catalytic, the Mn0.33Cd0.67S exhibits excellent stability and isn't significant deactivation after 6 recycles. The XRD patterns of the fresh and used Mn0.33Cd0.67S are also compared in Fig. 8. Apart from a slightly weakened intensity in diffraction peaks, no notable peak shift is observed, indicating the high stability of our MCS hollow spheres solid solutions. UV–vis diffuse reflectance spectra (DRS) are measured to determine the optical properties of the solid solutions. The direct band gap values and maximum absorbance values of the CdS and MCS hollow spheres samples have been shown in Fig. 9 and listed in Table 1. The maximum absorption light wavelengths of CdS and MCS hollow spheres are in visible light. To now, we can conclude that the resultant MCS have a nature of visible-light response, which promotes their application in visible-light photo-catalysts
1.0
0.2
0.9
0
5
10 15 Time(min)
20
25
Fig. 5. The effect of Mn0.33Cd0.67S dosage on decolorization efficiency of RhB.
removal efficiency was increased with MCS dose which is attributed to the exposed active surface area increases with the amount of MCS. Meanwhile, however, the decolorization of RhB is inhibited when the dosage of photocatalyst is high, which may be due to this fact that the suspended MCS can scatter visible light, and therefore reduce the formation of electron/hole pairs and active sites [47], leading to lower efficiency in photodegradation. To eliminate the self-decomposition of dye, a typical colorless
(C0-Ct)/C 0
0.0
0.8
0.7 phenol RhB
0.6
1
2
3 4 Number of cycle
5
6
Fig. 7. Recycled photo-decoloration under the visible light (4420 nm) irradiation over Mn0.33Cd0.67S hollow spheres for different reaction time: RhB 20 min, Phenol 3 h.
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250
3000
3 Quantity adsorbed (cm /g )
Intensity / a.u.
No shift
2000 new 1000
used
200
150
100
50
0 10
20
30
40 50 2 Theta / Degree
60
70
0.0
Absorption
(Ahυ)2
0.5 0.4 0.3 0.2 0.1
2.3
2.4
2.5 hυ (eV)
2.6
2.7
2.8
3 Quantity adsorbed (cm /g )
0.6
0.6
0.8
1.0
300
200
100 Adsorption Desorption
0
0 0.33 0.5 0.67 0.2
0.0
500
0.4
400
0.7
0.0 2.2
0.2
500
Fig. 8. XRD patterns of Mn0.33Cd0.67S before and after being used for six times.
450
Adsorption Desorption
0
550 Wavelength (nm)
600
650
Fig. 9. UV–vis diffuse reflectance spectra (DRS) of the MnxCd1 xS hollow spheres.
[48]. It can be seen that the band gap (Eg) value of bulk CdS is estimated to be 2.42 eV. Compared with pure CdS hollow spheres (2.31 eV), this result exhibits a red shift, which is attributed to the size quantization [49]. With the introduction of Mn2 þ into the CdS, the band gap of the semiconductor MCS gradually widened, which exhibits intense absorption bands in the visible light region. Moreover, with the increasing of Mn/Cd atomic ratio in the synthesis solution, band gaps increasing from 2.37 eV to 2.41 eV, which shows a slight blue shift. The strong absorption in the visible region indicates that the absorption of the Mn1 xCdxS samples is attributed to the intrinsic transition from the valence band to the conduction band but not the transition between impurity levels to the conduction band [50].
0.4 0.6 Relative Pressure (P/Po)
0.8
1.0
Fig. 10. BET adsorption/desorption isotherms of (a) blank-CdS hollow spheres and (b) Mn0.33Cd0.67S hollow spheres; the inset are the corresponding pore size distribution.
(insert in the Fig. 10) than CdS hollow structure, demonstrating that the relatively small pore present in the products. Those results suggest that there is obvious difference of adsorptivity between CdS and MnxCd1 xS hollow spheres. In order to compare the adsorption performance of the as-prepared samples, dark experiments have been studied keeping initial dye concentration at 1.0 10 5 mol/L, 50 mg Mn0.33Cd0.67S, and pH7.5. As shown in Fig. 11, all MCS solid solutions hollow spheres exhibit stronger adsorption ability than commercial CdS. Therefore, the enhanced photo-catalytic activity of MnxCd1 xS as compared to pure CdS hollow spheres can be ascribed to the change in surface area and porosity.
1.0
3.2. BET surface areas and adsorption performance
0.8
Ct/C0
The BET specific surface areas of the as-synthesized hollow structures are measured by using nitrogen adsorption and desorption isotherms. Fig. 10(a) and (b) show the representative N2 adsorption and desorption isotherms of CdS and Mn0.33Cd0.67S hollow spheres, respectively. All the two curves can be categorized as type IV isotherms with a typical H3 hysteresis loop in the range of 0.4–0.95 P/P0, indicating the mesoporous structure of the products. We can see clearly that the specific surface areas of MnxCd1 xS hollow spheres are larger than that of CdS hollow sphere (Table 1). High surface area can create abundant active sites in favor of the adsorption of dye. Therefore, surface area plays a key role in their photo-catalytic reactions [51]. All the sample of MnxCd1 xS show narrower pore distribution ranging below 15 nm
0.2
0.6 0.4
CdS Mn0.67Cd0.33S
0.2
Mn0.5Cd0.5S Mn0.33Cd0.67S Mn0.2Cd0.8S
0.0 0
20
40
60 80 Time (min)
100
Fig. 11. The adsorption curve of RhB in dark.
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87
1.0 PBQ
CdS
0.8
Intensity (a.u.)
TEOA Ct/C0
0.6 0.4 TBA
Mn0.33Cd0.67S
0.2 No scavenger 0.0 0
520
540
560 580 Wavelength (nm)
Fig. 12. Photoluminescence (PL) spectra of CdS and Mn0.33 Cd0.67S hollow spheres.
3.3. Investigation of the photocatalytic mechanism Photoluminescence (PL) emission spectra is utilized to investigate the separation and transfer efficiency of photogenerated charge carriers in semiconductors, since PL emissions may result from the recombination of free carriers. Generally, a decrease in recombination rate leads to a lower PL intensity, thus higher photocatalytic activity. As shown in Fig. 12, the PL intensity obtained over Mn0.33Cd0.67S hollow spheres is much weaker than the pure CdS hollow spheres, which indicates the lifetime of photo generated charge carriers in Mn0.33Cd0.67S is longer [52] and suggests the amount of photo-generation electrons which quenched through radiative recombination becomes fewer, hence reduced recombination rate of excited carriers. This may be nonradiative recombination centers. Generally, the lower recombination rate of electron and hole is attributed to the more crystal defects with introduction of Mn2 þ which can act as radiative or often associated with high photo-catalytic activity [53]. The decreased recombination of electron-hole pairs may be responsible for the enhancement of photo-catalytic performance, which is in good agreement with the results from the photodegradation experiment. The photoluminescence (PL) technique is often employed to detect hydroxyl radicals ( OH) too. As can be seen form Fig. 13, with the irradiation time extending, the PL intensity at about 445 nm increases gradually, indicating the increase in the production of OH radicals at the catalysis-water interface during
20
25
photo-catalytic process [54]. But, the PL intensity of MCS hollow spheres is much weaker than that of CdS [55], which indicates that the OH radicals isn’t the most important reactive species. It is well know that O2- and h þ can also be generated besides OH radicals in the photo-oxidation process [37]. In order to find out which active species dominate the degradation of RhB during photo-catalytic process, PBQ and TEOA are introduced as molecular detectors to quench O2- and h þ , respectively. Although the hydroxyl radicals can react with PBQ and TEOA too, the TBA was chosen as OH radicals scavenger because it reacts with OH radicals at a high rate constant (k ¼6 108) [56]. As we know, the scavengers can react with active species preferentially and hinder the photo-decoloration of RhB. As shown in Fig. 14, the photodecoloration rate of RhB is slightly reduced in the presence of TBA which reveals a few of OH radicals involved in the RhB degradation process and markedly suppressed in the presence of PBQ and TEOA which suggests O2- and h þ are primarily responsible for the degradation of RhB. The active species trapping experiments indicate that O2- and h þ are the most important reactive species for RhB degradation. The adsorption of RhB in the absence and presence of scavenger have been compared. Although the presence of scavenger can reduce the hole-mediated oxidation and dye sensitization process, as shown in Fig. 15, there is a slightly difference of adsorption which might be the small amount of scavengers in our degradation experiment. The conduction band (CB) and valence band (VB) potentials of MCS solid solutions are located between those of MnS and CdS and can be adjusted continuously by varying the composition of the
1.0 25 min
100
0.8
20 min 15 min
Ct/C0
PL Intensity / a.u.
10 15 Time(min)
Fig. 14. The photocatalytic activity of Mn0.33Cd0.67S hollow spheres with different quenchers.
150
10 min 50
0 min
0.6
0.4 No scavenger TBA TEOA PBQ
0.2
0 350
5
600
400
450 500 Wavelength (nm)
550
600
Fig. 13. PL spectra changes observed during visible light illumination( 4 420 nm) of Mn0.33 Cd0.67S hollow spheres in 5 10 4 M coumarin solution (excitation at 332 nm).
0.0 0
10
20
30
40 50 Time(min)
60
70
80
90
Fig. 15. The adsorption of RhB in the absence and presence of scavenger.
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degradation process over MCS is shown in Fig. 16. The photodegradation of RhB is mainly attributed to the photogenerated hole oxidation and O2- photoreduction process. 3.4. Morphology investigation
Fig. 16. Schematic diagram of photocatalytic processes Mn0.33Cd0.67S photocatalyst under the visible light irradiation.
occurring
over
solid solutions [25]. With the increase of Mn content, the positions of the CB and VB are shifted toward more negative potential and more positive potential, which leads to the remarkable increase in the band gap of MCS solid solutions. Generally, more negative potential of the CB of catalytic leads to more facile charge transfer, which are beneficial to the photocatalytic activities. On the other hand, as shown in Table 1, MCS solid solution possesses a much larger surface area than CdS, which increases the active sites and also contributes to the RhB photo-degradation. Aforementioned results can help us to discuss photocatalytic mechanism of degrading RhB over MCS solid solutions. A schematic diagram of the carriers generated and the mechanism of the
The morphology of MCS solid solutions hollow spheres are studied by SEM and TEM with different magnifications and the corresponding HRTEM images. Fig. 17(a) shows these hollow spheres with an average diameter of 300–600 nm, which are composed of loosely packed nano-particles. We can also observe that the surface of sphere is coarse obviously. There is a clear contrast between the pale center and the dark edge in Fig. 17(b), which demonstrates that the obtained spheres have a hollow interior. Fig. 17(c) shows these hollow spheres are constructed from some crystal clear tetragonum pillars with diameter of 10–30 nm. The Fig. 17a-c together indicate that these rough shells can create inherent porosity which produces plentiful nanoscale channels for chemical group travelling between the exterior space and interior cavity. This special structure makes it particularly suitable for further studies in catalytic applications and adsorption [55]. Fig. 17 (d) is the corresponding high resolution TEM image with clear lattice fringes, which allowed for the identification of crystallographic spacing. To make a better understand of the evolution process of hollow spheres, fixing other reaction parameters invariant, the time-dependent experiments are carried out. Fig. 18 show the detailed
d=0.3347nm
d=0.3572nm (002)
(100)
Fig. 17. Images of Mn0.33Cd0.67S hollow spheres, (a) is SEM, (b) is TEM, (c) and (d) are the corresponding HRTEM.
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Fig. 18. Typical TEM images of the Mn0.33Cd0.67S samples prepared at 180 °C for different reaction time: (a) 5, (b) 10, (c) 15, and (d) 20 h.
surface morphology of the Mn0.33Cd0.67S solid solutions hollow spheres obtained at 180 °C at different reaction time. As shown in Fig. 18(a), after the reaction time of 5 h, the Mn0.33Cd0.67S hollow spheres with the shell thickness of about 50 nm. The hollow spheres are aggregate by the crystalline nanoparticles around the gas/liquid interface between bubble and water. The continuous aggregation process occurs when reaction time reach to 10 h, the thickness of the shell is increased from 50 nm to 100 nm (Fig. 18 (b)). Interestingly, when upon increasing reaction time to 15 h, the clear contrast between the pale center and the dark edge is disappear because of the increase for shell thickness, and there is a clear change in the surface morphology. As shown in Fig. 18(c), the surface of the shell is covered with a few of nanorods, and these nanorods are perpendicular to the surface. Large amounts of crystalline and well-arranged nanorods appeared in the surface of hollow spheres (Fig. 18(d)) when the reaction time further prolonged to 20 h or longer.
4. Conclusions In summary, a series of MCS solid solutions hollow spheres with visible light respond have successfully synthesized by varying the molar ratio of different precursors via a simple template-free hydrothermal method. All the samples show the higher photocatalytic activities for the RhB degradation under visible light irradiation than commercial CdS. The synergy of the hollow porous spherical morphology, optimized band gap, and proper mole ratio of Mn/Cd in MnxCd1 xS are proposed to be responsible for its high
activity. Moreover, the Mn0.33Cd0.67S solid solutions hollow sphere shows the highest photo-catalytic activity and also demonstrates to be a stable. This work indicates that MCS solid solutions hollow spheres, as highly efficient photo-catalysts, may be a very promising candidate for scaled up production and environmental applications.
Notes The authors declare no competing financial interest.
Acknowledgements This work was supported by University Scientific Technology Research Program of Guangxi Zhuang Autonomous Region Education Department (KY2015YB362), University Scientific Technology Research Program of Guangxi Zhuang Autonomous Region Education Department (LX2014496).
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
GSH-assisted hydrothermal synthesis of MnxCd1 xS solid solution hollow spheres and their application in photocatalytic degradation Jiasheng Lai b, YiMing Qin a, Lan Yu a, Chunyan Zhang a,n a b
School of Food and Biochemical Engineering, Guangxi Science & Technology Normal University, Laibin 546199, PR China School of Mechanical and electrical engineering, Guangxi Science & Technology Normal University, Laibin 546199, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 3 December 2015 Received in revised form 9 April 2016 Accepted 5 May 2016
A series of MnxCd1 xS (MCS) solid solutions hollow spheres (x¼0.0, 0.20, 0.33, 0.50 and 0.67) have been synthesized with the assistance of L-Glutathione (GSH) by a simple hydrothermal route for the first time. Different characterization techniques, including X-Ray diffraction (XRD), atomic absorption spectroscopy (AAS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Brunauer-EmmettTeller surface area (BET), UV–vis and diffuse reflectance spectra (DRS) are performed to investigate the structural and optical properties of the as-prepared samples. The experiment of rhodamine B (RhB) decoloration indicates that all the MCS samples show higher photo-catalytic activities than commercial CdS under visible light irradiation. Among them, the Mn0.33Cd0.67S exhibits the highest photo-catalytic activity. The superior photo-catalytic activity of MCS samples may be attributed predominantly to the synergistic effect of the appropriate band-gap structure as well as the special hollow spherical morphology which makes MCS samples have the ability to harvest exciting visible-light due to multiple scattering within the interior space. Furthermore, the Mn0.33Cd0.67S shows well stability, the photocatalytic activities do not decrease significantly after six recycles. The work may open a novel strategy to fabricate multi-component chalcogenide solid solutions hollow spheres. & 2016 Elsevier Ltd. All rights reserved.
Keywords: MnxCd1 xS solid solutions Hollow spheres GSH Photo-catalysis
1. Introduction Energy crisis and environmental pollution caused by excessive utilization of fossil fuel are the most serious problems the world facing today, which greatly prompt the exploration of new energy sources. Solar energy with distinct advantages of inexhaustible, clean and accessible [1], is becoming a promising energy resource to take the place of the conventional ones. Photo-catalysis which can bring several benefits from energy and environmental viewpoints is one of the most effective approaches to realize the utilization of solar energy [2]. Moreover, micro/nanostructure photocatalysis with different morphologies exhibit diverse physicochemical properties and lead to very different catalytic activity. Therefore, zero dimensional (0-D) photo-catalysis such as quantum dots [3,4] (QDs), 1-D nanorods [5] and nanotubes, 2-D nanosheets, plates, disks and films [6], and 3D nanospheres are synthesized. Among them, 3D hollow nanospheres are worth to be mentioned due to their high light-harvesting efficiency, fast mobility of charge carriers, higher specific surface area, and better permeability compared to their solid counterparts [7,8]. Up to n
Corresponding author. E-mail address: [email protected] (C. Zhang).
http://dx.doi.org/10.1016/j.mssp.2016.05.001 1369-8001/& 2016 Elsevier Ltd. All rights reserved.
now, various hollow spheres including metals, polymers, carbon materials have been investigated widely. Traditional template-directed methods are tedious [9,10]. In contrast, the in situ gasbubble template synthesis strategy [11,12], brings us a new way to synthesize hollow spheres and neglects the template removal step. In recent years, metal sulfides have been widely researched because of having response in the visible light range [13]. Especially, the Cadmium sulfide (CdS) has been proven to be an efficient photo-catalyst materials with a narrow band gap (2.4 eV) and a suitable band structure [14], which matches well with the visible spectral range of solar [15,16]. However, the CdS is easily corroded [17] and its separation efficiency of photo-generated holes and electrons is low, which restrict the applications in solar conversion and environmental remediation. In general, an ideal photo-catalyst not only needs suitable band gap position, but also shows effective separation efficiencies of the photo-generated carriers. All these are strongly dependent on the electronic properties and structural of the photo-catalysts [18]. Many effective methods have been reported to adjust the band gap and inhibit the photo-corrosion process, such as doping metallic elements (Mn and Ni et al.) into CdS [19,20], coating CdS with a thin layer of amorphous carbon [21], pairing CdS with Graphene [22], embedding CdS particles in a polymer matrix [23] and so on. Considering that the solid solutions can be obtained by combining a narrow-
J. Lai et al. / Materials Science in Semiconductor Processing 52 (2016) 82–90
band-gap material with a wide-band-gap material, or even two wide-band-gap materials, thus the solid solutions with a proper band structure may be formed by choosing and adjusting the right components. To present, a huge number of multi-component solid solutions formed by CdS were reported successively, such as ZnxCd1 xS [14,24], Mn1 xCdxS [25], (CuIn)xCd2(1 x)S2 [26], Zn1 xCdxIn2S4 [27,28], CdxCuyZn1 x-yS [29], (ZnxCd1 x)(SexS1 x) [30], Cd0.1SnxZn0.9-2xS [31], Cu-doped Cd0.1Zn0.9S [32] and ZnxCd1 xS/CNTs [33], et al. Among them, the ternary Mn-Cd-S alloyed system formed by combining MnS (Eg ¼3.7 eV) with CdS (Eg ¼2.4 eV) has attracted considerable attention [34–36]. MCS solid solution, due to the tunability of its band gap by regulating the contents of Cd and Mn, is a promising candidate for visible light catalysis. Meiying Liu et al. have reported that Mn1 xCdxS solid solution will be rods, nanorods, nanoparticles while x ¼0.05, 0.16 and Z0.5 separately [25]. In this paper, we report a novel hydrothermal method with template-free to synthesize MCS solid solutions hollow spheres. During the process, GSH as a direct S source and bubble source to avoid traditional messy and complex template removal procedures. The obtained MCS solid solutions hollow spheres feature a uniform size and disperse well, which show much high photo-catalytic activities for organic pollutants removal. This unique structure may favor the harvesting of exciting light and scattering within the interior space. Mn0.33Cd0.67S exhibits the highest photo-catalytic activity due to the appropriate band gap. This one-pot template-free method may provide a costeffective approach to fabricate multi-component chalcogenide solid solutions hollow spheres.
83
irradiation time intervals, 3 mL liquid was taken out and then filtered. The concentration of RhB in filtrates was analyzed using a UV–vis spectrophotometer (UV-2450). In order to test the durability of samples, six cycles of photocatalytic degradation RhB and phenol were assessed by using Mn0.33Cd0.67S as a representative sample. After each run of reactions, the catalyst was collected carefully, washed by anhydrous ethanol and reused in the following recycling experiment. We detected also the XRD spectrum of the final catalyst after the experiment. 2.3. Analysis of reactive species and active species trapping As we know, coumarin can readily react with an readily reaa kind of highly fluorescent product, 7-hydroxycoumarin, so we used coumarin as a probe molecule to detect the formation of frmationt the robe molecul-catalyst by a photoluminescence (PL) method. The experimental procedure was similar to the previous activity test. The PL intensity was measured around 445 nm excited by 332 nm light. To detect what is the primary active species during the photocatalytic reaction, we used tert-butyl alcohol (TBA), p-benzoquinone (PBQ) and triethanol amine (TEOA) as scavengers [37] to prove the presence of hydroxyl radicals ( OH), superoxide radicals ( O2-) and holes (h þ ), respectively. The scavenger was added into the RhB solution prior to the addition of the photo-catalysts using a similar procedure to the previous activity test. 2.4. Characterization
2. Experimental 2.1. Preparation of MnxCd1 xS solid solutions hollow spheres C6H12N2O4S2 (GSH) and cadmium nitrate tetrahydrate (Cd(NO3)2 4H2O) were provided by Aladdin, manganese acetate tetrahydrate (Mn(CH3COO)2 4H2O) was obtained from Sinopharm Chemical Reagent Co, Ltd (Shanghai, China). All chemicals were analytical grade reagents and used without any further purification. In a typical synthesis procedure, 0.60 mmol of GSH was dissolved into 40 mL distilled water as solution A. Meanwhile, the mixture of x mmol Mn(CH3COO)2 4H2O and (0.6-x) mmol Cd(NO3)2 4H2O were dissolved into another 40 mL distilled water as solution B. Subsequently, solution B was slowly added to solution A under vigorous stirring. The mixture was then transferred to a 100 mL Teflon-lined stain-less steel autoclave, heated to 180 °C and maintained for 5 h, and then air-cooled to room temperature. The product was filtrated, washed alternately with deionized water and ethanol several times and dried at 60 °C for overnight. A series of MCS solid solutions hollow spheres with different compositions were synthesized by varying the molar ratio of different precursors. 2.2. Photo-catalytic activities and durability measurements The photo-catalytic activity of MCS was assessed by degrading RhB under visible light irradiation, using a 350 W Xe lamp equipped with a 420 nm cutoff filter. After the pH of the solution were adjusted at desired values using hydrochloric acid and sodium hydroxide solutions, a certain quality of MCS was suspended in 50 mL a certain concentration of RhB aqueous solution, then ultrasonic treated for 10 min and stirred in the dark for 1 h in order to meet the adsorption/desorption equilibrium between the catalyst and the pollutants. In order to ensure the reactions at room temperature and reduce the interference of the infrared radiation, condensate water was passed into the flask. At given
The microstructure and particle sizes of MCS were observed by TEM using a Tecnai G20 (FEI Co., Holland) TEM microscope worked at an accelerating voltage of 200 kV, high-resolution TEM (HRTEM) were collected using an FEI Titan TEM operated at 300 kV. SEM was performed with SU8000 (FESEM, Hitachi, Japan) which worked at an accelerating voltage of 15 KV. SEM was used to investigate the overall morphology of the product. The BET specific surface areas of the samples were evaluated on the basis of nitrogen adsorption isotherms using a Micromeritics ASAP2020 gas adsorption apparatus (USA). A Shimadzu AA6300 atomic absorption spectrum (AAS), equipped with a graphite oven, was used to determine the content of Cd and Mn in the as-prepared samples. The UV–vis diffused reflectance spectra (UV–vis DRS) were obtained using a UV–vis spectrometer (UV-2450, Shimadzu, Japan) from 200 to 800 nm, BaSO4 was used as background.
3. Results and discussion 3.1. Photo-catalytic and optical properties Photo-catalysts are potentially promising materials in the fields of environmental restoration and water splitting, thereby high catalytic activity is essential for large scale applications. Here, we have chosen the photo-catalytic degradation of non-biodegradable pollutant RhB dye under visible light irradiation as a model reaction to evaluate the photo-catalytic activities of the as-prepared MCS solid solutions hollow spheres (Fig. 1). For comparison, commercial CdS is also performed at the same experimental conditions. The photo-catalytic degradation process can be monitored by detecting the intensity changes of the RhB absorption peak about 553 nm. As shown in Fig. 1, all MnxCd1 xS solid solutions hollow spheres exhibit higher photo-catalytic activity than commercial CdS. This can be attributed to the formation of solid solutions as well as unique hollow structure, which enables the catalyst to response in visible light and has excellent performance
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1.0
0.8
g f
0.8
0 min
C t / C0
0.6
Absorption
0.6
e d
0.4
c
a 5
10 15 Time(min)
20
300
25
Fig. 1. Photocatalytic decoloration curves of RhB under visible light (λ4420 nm) irradiation. (a) Mn0.33Cd0.67S, (b) Mn0.2Cd0.8S, (c) Mn0.5Cd0.5S, (d) Mn0.67Cd0.33S, (e) CdS hollow spheres, (f) commercial CdS, (g) without adding catalyst.
for dyes removal. On the one hand, the band gap and position of MCS can be adjusted by altering the ratio of compositions, which enables the catalyst to response in visible light. On the other hand, the unique hollow structure can provide more active sites to contact with RhB closely, and also allows multiple reflections of visible light in the interior cavities, which improves the light harvesting efficiently [38,39]. The Mn0.33Cd0.67S shows the highest photo-catalytic activity among the series of MCS samples, about 99.85% of RhB is degraded under visible light irradiation in 25 min. In order to compare the photo-catalytic activities of these solid solutions, the reaction rate constants (k) are calculated by adopting the pseudo first-order model as expressed by Eq. (1), which is commonly used in photocatalytic decoloration process [40].
ln
15 min 20 min
0.0
0.0 0
0.4
0.2
b
0.2
5 min 10 min
(C0 /Ct ) = kt
(1)
C0 and Ct are the concentrations of RhB aqueous solution at 0 and t min, respectively. As shown in Table 1, the RhB degradation rate of the Mn0.33Cd0.67S solid solutions hollow spheres (k ¼0.2372 min 1) is about 4 times as much as that of CdS hollow sphere (k ¼0.0545 min 1). Therefore, it is believed that the formation of MCS solid solutions contributed to the activity enhancement of CdS hollow sphere. In Fig. 2, with the time go on, the peak of RhB declines promptly and the color of RhB aqueous solution changed from pink to colorless, which indicate that the ethyl groups of RhB are removed during irradiation [41,42]. The RhB is a macromolecular substance with much groups, including some properties unstable groups which easy to react. The unstable groups initiate a chain reaction during irradiation, which may be the cause of RhB decolor quickly. Generally, RhB degradation under visible light irradiation contained a direct photocatalytic process and an indirect dye photosensitization process [43]. Therefore, this superior photocatalytic activity can be attributed to a synergistic effect of photocatalytic and photosensitization. A good
350
400
450 500 550 Wavelength (nm)
600
650
700
Fig. 2. The absorption spectrum of RhB (1.0 10 5 M, 50 mL) in the presence of Mn0.33Cd0.67S (50 mg), the inset is the corresponding decoloration kinetics graph in line with a first order reaction.
linear relationship of ln(Ct/C0) versus reaction time is shown in the inset of Fig. 2, which indicates that the degradation of RhB obeys the first order reaction well. The effects of various factors such as the initial concentration of dye, pH and the dosage of catalyst on the removal efficiency of RhB are studied. Fig. 3 shows the relationship between initial concentration and the decoloration rate of the RhB in the presence of 50.0 mg Mn0.33Cd0.67S photocatalysts. The decoloration rate declines with the increase of initial concentration of RhB, which may be attributed to two reasons [44]: (i) more RhB molecules will be adsorbed on the surface of photocatalysts with the increase of initial concentration and occupy the active sites of photocatalysts; leads to a decrease in the number of active sites and (ii) The RhB molecules can absorb visible light, an increase in initial concentration of RhB will lead to a reduction in the number of photons that reach the catalyst surface. Due to the surface charge properties of MCS change with the changes of pH values, the decolorzation efficiency of RhB strongly depends on the pH of the reaction solution too. It is obvious in Fig. 4, the maximum decolorization rate is obtained in pH 7.5, and then decreased with increasing of pH value. This can be attributed to that weak alkaline solution is conducive to the adsorption of the RhB molecules onto MCS surfaces since RhB is a cationic dye. Meanwhile, however, when the pH value is so high (pH 410), the hydroxyl ions compete with RhB molecules in the adsorption on the surface of photocatalysts [45], and the photodecolorant reaction is inhibited. Photodecolorant reaction of dye takes place both on the active surface of photocatalyst and in bulk solution generated active oxidant species. Therefore the dosage of photocatalyst plays an important role in photo-degradation processes [46]. The effect of MCS concentration on removal of RhB is examined by varying Mn0.33Cd0.67S dose of 25–100 mg while keeping initial dye concentration at 1.0 10 5 mol/L and pH 7.5, As shown in Fig. 5, the
Table 1 Precusor ratio of Mn/Cd, reaction rate constants (k), R-square, energy band gap (Eg), BET specific surface area and the composition values estimated from AAS of MCS solid solutions. MnxCd1 xS
0 0.2 0.33 0.5 0.67
k (min 1)
0.05459 0.09611 0.23724 0.09179 0.05748
R2
0.9678 0.9523 0.9595 0.9704 0.9642
Eg (eV)
2.31 2.37 2.38 2.39 2.41
λg (nm)
536.8 523.2 521 518.8 514.5
BET Surface
245 414 464 675 698
Composition values of MCS During Synthesis
From AAS
0 0.2 0.33 0.5 0.67
0 0.17 0.31 0.48 0.63
J. Lai et al. / Materials Science in Semiconductor Processing 52 (2016) 82–90
1.0
0.6
(d)
0.4
0.5
0.6
Absorption
Ct /C0
0.8
(a) (b) (c)
0.4 0.2
0
5
10 Time (min)
15
1.5 h 2.5 h 3.5 h
0.2
0.0 260
20
Fig. 3. The effect of initial RhB concentration on decolorization efficiency: (a) 0.5 10 5 mol/L, (b) 1.0 10 5 mol/L, (c) 1.5 10 5 mol/L, (d) 2.0 10 5 mol/L.
1.0
pH=3 pH=6.5 pH=7.5 pH=8.5 pH=10
0.8
Ct / C0
0h 0.5 h
0.1
0.0
0.6 0.4 0.2 0.0 0
5
10 15 Time(min)
20
25
Fig. 4. The effect of pH on decolorization efficiency of RhB.
25 mg 50 mg 75 mg 100 mg
1.0 0.8
Ct / C0
0.3
85
0.6 0.4
280
300 320 Wavelength (nm)
340
360
Fig. 6. The absorption spectrum of salicylic acid solution (20 mg L 1, 50 mL) in the presence of Mn0.33Cd0.67S (50 mg) under visible light ( 4420 nm) irradiation. The inset is the corresponding Ct/C0 versus time curves.
and stable pollutant, salicylic acid which has no visible light adsorption, has been also chosen to photo-catalytic degrade under visible light irradiation. As shown in Fig. 6, in 3.5 h irradiation of visible light, the peak of salicylic acid disappears completely, which indicates that the MCS hollow spheres solid solutions have high activity and salicylic acid can be decomposed efficiently in 3.5 h. The result demonstrates again the high activity of the MCS hollow spheres solid solutions. In order to investigate the recyclability of the MCS solid solutions hollow spheres, the Mn0.33Cd0.67S is chosen to be a representative sample for recycling test. As shown in Fig. 7, no matter RhB which easy to adsorption or phenol which difficult to adsorption on surface of catalytic, the Mn0.33Cd0.67S exhibits excellent stability and isn't significant deactivation after 6 recycles. The XRD patterns of the fresh and used Mn0.33Cd0.67S are also compared in Fig. 8. Apart from a slightly weakened intensity in diffraction peaks, no notable peak shift is observed, indicating the high stability of our MCS hollow spheres solid solutions. UV–vis diffuse reflectance spectra (DRS) are measured to determine the optical properties of the solid solutions. The direct band gap values and maximum absorbance values of the CdS and MCS hollow spheres samples have been shown in Fig. 9 and listed in Table 1. The maximum absorption light wavelengths of CdS and MCS hollow spheres are in visible light. To now, we can conclude that the resultant MCS have a nature of visible-light response, which promotes their application in visible-light photo-catalysts
1.0
0.2
0.9
0
5
10 15 Time(min)
20
25
Fig. 5. The effect of Mn0.33Cd0.67S dosage on decolorization efficiency of RhB.
removal efficiency was increased with MCS dose which is attributed to the exposed active surface area increases with the amount of MCS. Meanwhile, however, the decolorization of RhB is inhibited when the dosage of photocatalyst is high, which may be due to this fact that the suspended MCS can scatter visible light, and therefore reduce the formation of electron/hole pairs and active sites [47], leading to lower efficiency in photodegradation. To eliminate the self-decomposition of dye, a typical colorless
(C0-Ct)/C 0
0.0
0.8
0.7 phenol RhB
0.6
1
2
3 4 Number of cycle
5
6
Fig. 7. Recycled photo-decoloration under the visible light (4420 nm) irradiation over Mn0.33Cd0.67S hollow spheres for different reaction time: RhB 20 min, Phenol 3 h.
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250
3000
3 Quantity adsorbed (cm /g )
Intensity / a.u.
No shift
2000 new 1000
used
200
150
100
50
0 10
20
30
40 50 2 Theta / Degree
60
70
0.0
Absorption
(Ahυ)2
0.5 0.4 0.3 0.2 0.1
2.3
2.4
2.5 hυ (eV)
2.6
2.7
2.8
3 Quantity adsorbed (cm /g )
0.6
0.6
0.8
1.0
300
200
100 Adsorption Desorption
0
0 0.33 0.5 0.67 0.2
0.0
500
0.4
400
0.7
0.0 2.2
0.2
500
Fig. 8. XRD patterns of Mn0.33Cd0.67S before and after being used for six times.
450
Adsorption Desorption
0
550 Wavelength (nm)
600
650
Fig. 9. UV–vis diffuse reflectance spectra (DRS) of the MnxCd1 xS hollow spheres.
[48]. It can be seen that the band gap (Eg) value of bulk CdS is estimated to be 2.42 eV. Compared with pure CdS hollow spheres (2.31 eV), this result exhibits a red shift, which is attributed to the size quantization [49]. With the introduction of Mn2 þ into the CdS, the band gap of the semiconductor MCS gradually widened, which exhibits intense absorption bands in the visible light region. Moreover, with the increasing of Mn/Cd atomic ratio in the synthesis solution, band gaps increasing from 2.37 eV to 2.41 eV, which shows a slight blue shift. The strong absorption in the visible region indicates that the absorption of the Mn1 xCdxS samples is attributed to the intrinsic transition from the valence band to the conduction band but not the transition between impurity levels to the conduction band [50].
0.4 0.6 Relative Pressure (P/Po)
0.8
1.0
Fig. 10. BET adsorption/desorption isotherms of (a) blank-CdS hollow spheres and (b) Mn0.33Cd0.67S hollow spheres; the inset are the corresponding pore size distribution.
(insert in the Fig. 10) than CdS hollow structure, demonstrating that the relatively small pore present in the products. Those results suggest that there is obvious difference of adsorptivity between CdS and MnxCd1 xS hollow spheres. In order to compare the adsorption performance of the as-prepared samples, dark experiments have been studied keeping initial dye concentration at 1.0 10 5 mol/L, 50 mg Mn0.33Cd0.67S, and pH7.5. As shown in Fig. 11, all MCS solid solutions hollow spheres exhibit stronger adsorption ability than commercial CdS. Therefore, the enhanced photo-catalytic activity of MnxCd1 xS as compared to pure CdS hollow spheres can be ascribed to the change in surface area and porosity.
1.0
3.2. BET surface areas and adsorption performance
0.8
Ct/C0
The BET specific surface areas of the as-synthesized hollow structures are measured by using nitrogen adsorption and desorption isotherms. Fig. 10(a) and (b) show the representative N2 adsorption and desorption isotherms of CdS and Mn0.33Cd0.67S hollow spheres, respectively. All the two curves can be categorized as type IV isotherms with a typical H3 hysteresis loop in the range of 0.4–0.95 P/P0, indicating the mesoporous structure of the products. We can see clearly that the specific surface areas of MnxCd1 xS hollow spheres are larger than that of CdS hollow sphere (Table 1). High surface area can create abundant active sites in favor of the adsorption of dye. Therefore, surface area plays a key role in their photo-catalytic reactions [51]. All the sample of MnxCd1 xS show narrower pore distribution ranging below 15 nm
0.2
0.6 0.4
CdS Mn0.67Cd0.33S
0.2
Mn0.5Cd0.5S Mn0.33Cd0.67S Mn0.2Cd0.8S
0.0 0
20
40
60 80 Time (min)
100
Fig. 11. The adsorption curve of RhB in dark.
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87
1.0 PBQ
CdS
0.8
Intensity (a.u.)
TEOA Ct/C0
0.6 0.4 TBA
Mn0.33Cd0.67S
0.2 No scavenger 0.0 0
520
540
560 580 Wavelength (nm)
Fig. 12. Photoluminescence (PL) spectra of CdS and Mn0.33 Cd0.67S hollow spheres.
3.3. Investigation of the photocatalytic mechanism Photoluminescence (PL) emission spectra is utilized to investigate the separation and transfer efficiency of photogenerated charge carriers in semiconductors, since PL emissions may result from the recombination of free carriers. Generally, a decrease in recombination rate leads to a lower PL intensity, thus higher photocatalytic activity. As shown in Fig. 12, the PL intensity obtained over Mn0.33Cd0.67S hollow spheres is much weaker than the pure CdS hollow spheres, which indicates the lifetime of photo generated charge carriers in Mn0.33Cd0.67S is longer [52] and suggests the amount of photo-generation electrons which quenched through radiative recombination becomes fewer, hence reduced recombination rate of excited carriers. This may be nonradiative recombination centers. Generally, the lower recombination rate of electron and hole is attributed to the more crystal defects with introduction of Mn2 þ which can act as radiative or often associated with high photo-catalytic activity [53]. The decreased recombination of electron-hole pairs may be responsible for the enhancement of photo-catalytic performance, which is in good agreement with the results from the photodegradation experiment. The photoluminescence (PL) technique is often employed to detect hydroxyl radicals ( OH) too. As can be seen form Fig. 13, with the irradiation time extending, the PL intensity at about 445 nm increases gradually, indicating the increase in the production of OH radicals at the catalysis-water interface during
20
25
photo-catalytic process [54]. But, the PL intensity of MCS hollow spheres is much weaker than that of CdS [55], which indicates that the OH radicals isn’t the most important reactive species. It is well know that O2- and h þ can also be generated besides OH radicals in the photo-oxidation process [37]. In order to find out which active species dominate the degradation of RhB during photo-catalytic process, PBQ and TEOA are introduced as molecular detectors to quench O2- and h þ , respectively. Although the hydroxyl radicals can react with PBQ and TEOA too, the TBA was chosen as OH radicals scavenger because it reacts with OH radicals at a high rate constant (k ¼6 108) [56]. As we know, the scavengers can react with active species preferentially and hinder the photo-decoloration of RhB. As shown in Fig. 14, the photodecoloration rate of RhB is slightly reduced in the presence of TBA which reveals a few of OH radicals involved in the RhB degradation process and markedly suppressed in the presence of PBQ and TEOA which suggests O2- and h þ are primarily responsible for the degradation of RhB. The active species trapping experiments indicate that O2- and h þ are the most important reactive species for RhB degradation. The adsorption of RhB in the absence and presence of scavenger have been compared. Although the presence of scavenger can reduce the hole-mediated oxidation and dye sensitization process, as shown in Fig. 15, there is a slightly difference of adsorption which might be the small amount of scavengers in our degradation experiment. The conduction band (CB) and valence band (VB) potentials of MCS solid solutions are located between those of MnS and CdS and can be adjusted continuously by varying the composition of the
1.0 25 min
100
0.8
20 min 15 min
Ct/C0
PL Intensity / a.u.
10 15 Time(min)
Fig. 14. The photocatalytic activity of Mn0.33Cd0.67S hollow spheres with different quenchers.
150
10 min 50
0 min
0.6
0.4 No scavenger TBA TEOA PBQ
0.2
0 350
5
600
400
450 500 Wavelength (nm)
550
600
Fig. 13. PL spectra changes observed during visible light illumination( 4 420 nm) of Mn0.33 Cd0.67S hollow spheres in 5 10 4 M coumarin solution (excitation at 332 nm).
0.0 0
10
20
30
40 50 Time(min)
60
70
80
90
Fig. 15. The adsorption of RhB in the absence and presence of scavenger.
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degradation process over MCS is shown in Fig. 16. The photodegradation of RhB is mainly attributed to the photogenerated hole oxidation and O2- photoreduction process. 3.4. Morphology investigation
Fig. 16. Schematic diagram of photocatalytic processes Mn0.33Cd0.67S photocatalyst under the visible light irradiation.
occurring
over
solid solutions [25]. With the increase of Mn content, the positions of the CB and VB are shifted toward more negative potential and more positive potential, which leads to the remarkable increase in the band gap of MCS solid solutions. Generally, more negative potential of the CB of catalytic leads to more facile charge transfer, which are beneficial to the photocatalytic activities. On the other hand, as shown in Table 1, MCS solid solution possesses a much larger surface area than CdS, which increases the active sites and also contributes to the RhB photo-degradation. Aforementioned results can help us to discuss photocatalytic mechanism of degrading RhB over MCS solid solutions. A schematic diagram of the carriers generated and the mechanism of the
The morphology of MCS solid solutions hollow spheres are studied by SEM and TEM with different magnifications and the corresponding HRTEM images. Fig. 17(a) shows these hollow spheres with an average diameter of 300–600 nm, which are composed of loosely packed nano-particles. We can also observe that the surface of sphere is coarse obviously. There is a clear contrast between the pale center and the dark edge in Fig. 17(b), which demonstrates that the obtained spheres have a hollow interior. Fig. 17(c) shows these hollow spheres are constructed from some crystal clear tetragonum pillars with diameter of 10–30 nm. The Fig. 17a-c together indicate that these rough shells can create inherent porosity which produces plentiful nanoscale channels for chemical group travelling between the exterior space and interior cavity. This special structure makes it particularly suitable for further studies in catalytic applications and adsorption [55]. Fig. 17 (d) is the corresponding high resolution TEM image with clear lattice fringes, which allowed for the identification of crystallographic spacing. To make a better understand of the evolution process of hollow spheres, fixing other reaction parameters invariant, the time-dependent experiments are carried out. Fig. 18 show the detailed
d=0.3347nm
d=0.3572nm (002)
(100)
Fig. 17. Images of Mn0.33Cd0.67S hollow spheres, (a) is SEM, (b) is TEM, (c) and (d) are the corresponding HRTEM.
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Fig. 18. Typical TEM images of the Mn0.33Cd0.67S samples prepared at 180 °C for different reaction time: (a) 5, (b) 10, (c) 15, and (d) 20 h.
surface morphology of the Mn0.33Cd0.67S solid solutions hollow spheres obtained at 180 °C at different reaction time. As shown in Fig. 18(a), after the reaction time of 5 h, the Mn0.33Cd0.67S hollow spheres with the shell thickness of about 50 nm. The hollow spheres are aggregate by the crystalline nanoparticles around the gas/liquid interface between bubble and water. The continuous aggregation process occurs when reaction time reach to 10 h, the thickness of the shell is increased from 50 nm to 100 nm (Fig. 18 (b)). Interestingly, when upon increasing reaction time to 15 h, the clear contrast between the pale center and the dark edge is disappear because of the increase for shell thickness, and there is a clear change in the surface morphology. As shown in Fig. 18(c), the surface of the shell is covered with a few of nanorods, and these nanorods are perpendicular to the surface. Large amounts of crystalline and well-arranged nanorods appeared in the surface of hollow spheres (Fig. 18(d)) when the reaction time further prolonged to 20 h or longer.
4. Conclusions In summary, a series of MCS solid solutions hollow spheres with visible light respond have successfully synthesized by varying the molar ratio of different precursors via a simple template-free hydrothermal method. All the samples show the higher photocatalytic activities for the RhB degradation under visible light irradiation than commercial CdS. The synergy of the hollow porous spherical morphology, optimized band gap, and proper mole ratio of Mn/Cd in MnxCd1 xS are proposed to be responsible for its high
activity. Moreover, the Mn0.33Cd0.67S solid solutions hollow sphere shows the highest photo-catalytic activity and also demonstrates to be a stable. This work indicates that MCS solid solutions hollow spheres, as highly efficient photo-catalysts, may be a very promising candidate for scaled up production and environmental applications.
Notes The authors declare no competing financial interest.
Acknowledgements This work was supported by University Scientific Technology Research Program of Guangxi Zhuang Autonomous Region Education Department (KY2015YB362), University Scientific Technology Research Program of Guangxi Zhuang Autonomous Region Education Department (LX2014496).
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