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Preparation, characterization of Sb-doped ZnO nanocrystals and their excellent solar light driven photocatalytic activity
Accepted Manuscript Title: Preparation, characterization of Sb-doped ZnO nanocrystals and their excellent solar light driven photocatalytic activity Author: Ramzi Nasser Walid Ben Haj Othmen Habib Elhouichet Mokhtar F´erid PII: DOI: Reference:
S0169-4332(16)32039-6 http://dx.doi.org/doi:10.1016/j.apsusc.2016.09.158 APSUSC 34087
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-7-2016 28-9-2016 30-9-2016
Please cite this article as: Ramzi Nasser, Walid Ben Haj Othmen, Habib Elhouichet, Mokhtar F´erid, Preparation, characterization of Sb-doped ZnO nanocrystals and their excellent solar light driven photocatalytic activity, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.09.158 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation, characterization of Sb-doped ZnO nanocrystals and their excellent solar light driven photocatalytic activity
Ramzi Nasser1,2, Walid Ben Haj Othmen1,2, Habib Elhouichet1,2,*, Mokhtar Férid1
1
Laboratoire de Physico-Chimie des Matériaux Minéraux et leurs Applications, Centre National de Recherches
en Sciences des Matériaux, B.P. 95 Hammam-Lif, 2050, Tunisia. 2
Département de Physique, Faculté des Sciences de Tunis, University of Tunis El Manar 2092, Tunisia.
*Corresponding author: H. Elhouichet: [email protected]
1
Graphical Abstract
CB
VO
.O-2
e-
RhB
O2
(SbZn-2VZn)
.OH h+
VB
Sb-doped ZnO
CO2+H2O
e -
H2O e -
h+ h+
e -
Co h+ mpl e V + h+ h ex Highlights B + acc h ept e Sb-ZnO was obtained by modified sol-gel method using citric acid as stabilizing agent. or Sb incorporated both in lattice and interstitial sites. leve l Zn-2VZn) acceptor level The formation of (Sb e was revealed by photoluminescence studies. (Sb Optimum Sb content to show higher photocatalytic activity was found to be 3%. Zn2VZ n)
h +
Abstract
In the present study, undoped and antimony (Sb) doped ZnO nanocrystals (NCs) were prepared by a simple and economical sol-gel method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed the purity of the obtained phase and its high crystallinity. Raman analysis confirms the hexagonal Wurtzite ZnO structure. According to the diffuse reflectance results, the band gap was found to decrease up to 3% of Sb doping (ZSb3 sample). The results of X-ray photoelectron spectroscopy (XPS) measurements reveal that Sb ions occupied both Zn and interstitials sites. The successful substitution of antimony in ZnO lattice suggests the formation of the complex h+
(SbZn-2VZn) acceptor level above the valence band. Particularly for ZSb3 sample, the UV photoluminescence (PL) band presents an obvious red-shift attributed to the formation of this complex. Rhodamine B (RhB) was used to evaluate the photocatalytic activity of Sb-doped ZnO NCs under sunlight irradiation. It was found that oxygen vacancies play a major role in the photocatalytic process by trapping the excited electrons and inhibiting the radiative recombination. During the photocatalytic mechanism, the Sb doping, expressed through the apparition of the (SbZn-2VZn) correspondent acceptor level, enhances the sunlight absorption within the ZnO band gap, which stimulates the generation of hydroxyl radicals and promotes the photocatalytics reaction rates. Such important contribution of the hydroxyl radicals was confirmed experimentally when using ethanol as scavenger in the photocatalytic reaction. The photodegradation experiments reveal that ZSb3 sample exhibits the highest photocatalytic activity among all the prepared samples and presents a good 2
cycling stability and reusability. The influence of the initial pH in the photodegradation efficiency was also monitored and discussed.
Keywords: ZnO; Sb doping; Modified sol-gel method; Photoluminescence; Acceptor complex; Photocatalysis.
1. Introduction The particular diversified properties of the II–VI Wurtzite phase offer to the ZnO a special ranking among many oxides [1]. The wide band gap 3.37 eV, large and high binding energy 60 meV, and low fabrication cost [2-4] make it more attractive than other transparent conductor oxides such as TiO2 [5]. Furthermore, ZnO based nanomaterials found many applications especially in systems such as light-emitting diodes (LEDs), and photocatalysts [1,6]. Up to now, among various semiconductors used in the photodegradation of organic pollutants, ZnO nanostructures were widely investigated due to their better environmental friendly feature, non-toxicity, low cost and good stability [7,8]. Also, ZnO absorbs large fractions of solar spectrum compared to other photocatalysts like TiO2 and ZnS [5,9]. To improve the visible light absorption of ZnO nanomaterials, the most effective way is to decrease its band gap energy by introducing intermediate states. By doping ZnO nanocrystals (NCs) by antimony, the particles size, morphology and induced surface defects will have an important effect on their photocatalytic activity [10,11]. In addition, doping is a simple and useful concept to introduce defect levels in the bandgap [12], which may inhibit the radiative recombination of electrons-holes pairs. Many papers have been dedicated for the study of the photocatalytic activity of the Sb-doped ZnO. In fact, A. Phuruangrat et al. [13] prepared Sb-doped ZnO with ultrasonic assisted method and monitored its photocatalytic activity under UV irradiation. On the other hand, A. Omidi et al. [14,15] have fabricated Sb-doped ZnO nanostructures using an ultrasonic method and reveals that the as-prepared nanoparticles presents a high photocatalytic activity under UV light irradiation. Then, to the best of your knowledge, the current study presents the first report treating the photocatalytic properties of Sb-doped ZnO NCs under sunlight irradiation. Actually, the nature of the deep defects following the Sb-doping seems to be efficient for the electrons-holes separation under visible sunlight irradiation. Herein, economical and low cost Sb-doped ZnO NCs were prepared by sol-gel method. Series of Zn1-xSbxO with an antimony concentration ranging from x= 0.00 to x= 0.05 were elaborated and characterized through various experimental methods. The photocatalytic activity of the as prepared NCs was monitored for different Sb ratio using the RhB as a probe dye molecule. Moreover, the mechanism of photocatalytic activity of Sb doped ZnO was discussed based on the 3
separation process of electron-hole pairs and the active species detection. Finally, the stability of Sb doped ZnO was studied through successive four cycles of experiments. 2. Experimental a.
Samples preparation
All chemical products were purchased from Sigma–Aldrich Company, and were used as received without further purification. Zinc acetate [Zn(O2CCH3)2(H2O)2] used as a zinc source, antimony trichloride (SbCl3) as a antimony dopant source and citric acid (C6H8O7) as a stabilizer, were the precursors used to prepare the sol gel ZnOSb NCs powders. 2 g of zinc acetate and 4 g of citric acid are dissolved in 200 ml of bi-distilled water under magnetic stirring. The mole fraction of SbCl3 dopant was added to the above solution with different contents (the molar ratio of Sb to Zn was 0, 1, 3, and 5%). The solutions were stirred magnetically for 3h to obtain a homogeneous mixture. Then, the solvent was evaporated at 120°C for 5 days. The obtained gels were ground to make fine powders. The obtained powders were heated at 300°C for 4 h to remove the organic products. After grinding, they are submitted to calcination at 500°C to obtain a crystalline phase of Sb-doped ZnO. In the following, the prepared undoped ZnO and the 1%, 3% and 5% Sb-doped samples will be indexed by ZnO, ZSb1, ZSb3 and ZSb5, respectively.
b. Samples characterizations: X-ray powder diffractometer with Monochromatized CuK𝛼 radiation ( = 1.5418 Å) scanned the samples in the range of (20°–70°). The morphologies of samples were characterized by transmission electron microscopy (TEM) and high resolved TEM (HRTEM) on an FEI Tecnai F20 microscope with an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were analyzed by ESCALSB 200-2 spectrometer under ultra-high vacuum (P<10−9mbar). The data measurements were performed at normal incidence (the sample plane is perpendicular to the emission angle).The mono-chromatized AlKα source (1486.6 eV) was operated at a power of 420 W (30 mA and 14 kV) and C1s (284.6 eV) was used to calibrate the peak positions of the elements. Micro Raman and Photoluminescence measurements were recorded using a Lab RAM HR spectrometer (Horiba Gr, France) using the 632 nm and 325 nm as excitation wavelength from HeCd laser, respectively. Diffuse reflectance spectroscopy measurements were carried by using UV–VIS Lambda 950 spectrometer Perkin Elmer preciosity. c. Photocatalysis experiments The photocatalytic activity of the Sb-doped ZnO nanocrystals was evaluated through the photodegradation of Rodhamine B RhB (C28H31CIN2O3) purchased from Sigma Aldrich chemical. A quantity of 1.916 mg of photocatalyst was added to 800 mL of 5.10-6 M aqueous solution of RhB. In 4
order to reach the adsorption–desorption equilibrium between pollutant and catalyst, the mixture was stirred in the dark for 2h. The solution was then exposed to sunlight irradiation. The degradation efficiency was calculated using the relation [9,16]: 𝑫𝒚𝒆(%) =
(𝑨𝟎 − 𝑨(𝒕)) (𝑪𝟎 − 𝑪(𝒕)) ∗ 𝟏𝟎𝟎 = ∗ 𝟏𝟎𝟎 𝑨𝟎 𝑪𝟎
Where A0 and C0 are respectively the initial (t=0) absorbance and concentration of RhB. A(t) and C(t) 𝐀
are those at an instant t. The kinetic reaction constant can be calculated using the plots of 𝐥𝐧 ( 𝟎 ) 𝐀
versus irradiation time since they are related by [9,17]: 𝒍𝒏 (
𝑨𝟎 ) = 𝒌𝒕 𝑨
Where t is the irradiation time and k is the first-order rate constant of the reaction.
3. Results 3.1 XRD analysis Fig.1 illustrates the XRD patterns of the undoped and Sb-doped ZnO NCs annealed at 500°C. The ZnO diffractograms confirm that all the prepared samples crystallize in hexagonal phase (JCPDS no. 36-1451). By adding Sb, no change in the structure phase was detected and the absence of impurity phases in all the samples suggests that most of Sb ions occupy sites within the ZnO lattice. The diffraction peaks present a slight shift to lower angles with Sb doping. This behavior is attributed to the substitution of Zn by Sb ions, which may suggest the successful doping procedure. However, the full width at half-maximum (FWHM) of the principal diffraction peaks further increases with Sb doping, up to 3 at. %, due to the excessive Sb incorporation. The NCs size of the doped ZnO was estimated by the Debye-Scherrer formula [18]: 𝐃=
𝐤𝛌 𝛃𝐜𝐨𝐬𝛉
Where β is the full width at half maximum of the diffraction peak at the angle 𝛉 and is the wavelength for the Cu K component of the employed copper radiation. The calculated average size of the Sb doped ZnO NCs were illustrated in Table 1. As can be seen, the average size decreases with increasing Sb doping until 3%. The decrease in the particle size may be due successful insertion of Sb3+ (0.076 nm) ions in Zn2+(0.074 nm) ions. Thus, it causes the increase in lattice parameters and may affect the growth of grains. 5
Actually, a relatively high doping rate can induces the multiplication of nucleation centers during the formation of the nanocrystals which induces an increase in the grain size [19]. 3.2 Morphological study The morphology study of the undoped ZnO and ZSb3 NCs were performed using TEM and HRTEM imaging (Fig.2). The TEM images indicated nanospheres like crystals with a uniform size distribution ranged between 40-60 nm, which is in good agreement with XRD results. The selected area electron diffraction (SAED) was carried out and indicates that the well resolved diffraction rings originate from high crystallized ZnO as shown in Fig.2.c. From the HRTEM image of the Sb-doped sample, the distance between adjacent lattice fringes was estimated to be about 0.26 nm, which is very close to the theoretical spacing d002 of the Wurtzite ZnO [20]. This fact shows that the insertion of antimony did not affect the hexagonal structure of ZnO (Fig.2.d). The energy dispersive X-ray analysis (EDXA) spectra (Fig.2.e) reveals that the synthesized ZSb3 powder is composed from oxygen, zinc and antimony without any others impurity elements except the Cu and Al that originated from the sample support, which further evidence the purity of the elaborated samples. EDAX results suggest again the successful Sb doping in ZnO NCs.
3.3 X-ray Photoelectron Spectroscopic analysis The surface composition and chemical states can be determined by means of X-ray photoelectron spectroscopy (XPS) through the determination of the binding energies of the different elements present on the ZnO NCs surface. Figure S.1 shows the survey XPS spectra of undoped ZnO and ZSb3 NCs. The figure indicates the presence of the elements Zn, O and adventitious C for the undoped ZnO in addition to the Sb for the Sb-doped sample. No contamination resulted from the elaboration process were detected from the spectra. Fig.3 (a) shows two highly symmetric peaks centered at 1022.20 eV and 1045.54 eV assigned to Zn 2p3/2 and Zn 2p1/2 states, respectively, which is consistent with that reported for ZnO nanostructures [21]. The narrow Zn 2p3/2 band is attributed to Zn2+ ions in an oxygen-deficient ZnO lattice [21,22]. Thus, it can be noted that the majority of the surface Zn ions exists in the ionic form Zn2+ [23- 25]. Furthermore, two peaks related to antimony, one is located at 530.5 eV and were assigned to the coexistence of O1s and Sb3d5/2 and another at 540.1 eV attributed to Sb3d3/2 [24]. The presence of Sb3d3/2 indicated that antimony is well introduced in the ZnO lattice and evidence the presence of SbO bond [21]. An additional peak with a weak intensity was detected at 534.9 eV and corresponds to the Sb 3d3/2 binding energy of metallic antimony Sb0 [22] probably localized on grain boundaries. Some study reveals that the critical doping concentration of antimony in ZnO before the formation of metallic Sb was about 3% [26]. Then, we expect that the formation of metallic antimony is probable 6
especially that we are working around this Sb amount. It is also expected that this metallic Sb phase is located on the ZnO nanocrystals surface. The oxygen species is of major importance in the photocatalytic performance of ZnO NCs. Figure 3(a) illustrated the high resolution of O1s peak of ZSb3 sample. The asymmetric peaks observed in the O1s were deconvoluted into three peaks. The intense one with low binding energy (530.5 eV) could be attributed to the lattice oxygen in the ZnO host. The medium binding energy (531.09 eV) could correspond to the hydroxyl oxygen on the ZnO surface [27,28]. The peak with high binding energy (532.4 eV) is more probably related to Sb 3d5/2 state in ZnO. The hydroxyl oxygen on the surface was a well-known as an important source for the generation of active hydroxyl species responsible for the photocatalytic activity [29]. The deconvoluted Sb 3d3/2 peak shows the existence of two possible states Sb ions in ZnO NCs [29]. Actually, it’s known that Sb ions can exist in two ionizations states: Sb3+ (0.076 nm) when it substitute Zn2+ ions (0.074 nm) and Sb5+ (0.061 nm) when it occupy an interstitial site [22]. In the substitutional case, the isolated SbZn can be associated spontaneously with two induced zinc vacancies (VZn) to form the complex acceptor (SbZn-2VZn) level [12]. In the interstitial case, Sb5+(0.061 nm) can occupy the interstice site (0.062 nm) in the hexagonal ZnO [23]. These defects may support the stability of acceptor levels with low formation energy [24]. From the fitted curve, the higher band intensity related to the Sb3+ states compared to that of the Sb5+ states suggest that the majority of introduced Sb ions substitute the Zn2+ ones. It is also important to note that the substitution presence of Sb ions in interstitial ZnO hexagonal sites may leads to the increase of the charges carriers mobility [30], which can help the carriers to reach the nanoparticle surface. These features seem to be benefic for a photocatalytic purpose.
3.4 Raman scattering Fig.4 shows the Raman spectra of the Sb-doped ZnO NCs. The intense E2High mode, located at 437 cm−1, is attributed to the lattice vibration of the oxygen atoms [18,31]. This mode is particularly characteristic of the Würtzite structure and the internal stress in ZnO [31]. The 338 cm−1 peak present in pure and doped ZnO samples was assigned to the energy difference between E2high and E2low caused by multi phonon vibrations process[32].The weak peak, at about 570 cm−1, is related to the A1 (LO) mode [33] classified among the first-order optical modes in Wurtzite ZnO. The peak at around 380 cm-1 can be related to A1 (TO) mode [34,35]. The Raman spectrum of ZnO:Sb NCs is dominated by the E2High mode confirming then the good crystallinity even with Sb doping. Moreover, the continuous decrease in the intensity relative to the A1(LO) mode can be attributed to the substitution of Zn by Sb, which is consistent with the XRD and XPS results. For high Sb doping, a new Raman mode, located at 712 cm-1 is seen. The associated peak is relatively of a low intensity and may be attributed to weak local vibration mode (LVM) related to Sb-O-Sb vibration [36]. 7
3.5 UV-vis study The optical band gap of the doped ZnO NCs could be extracted from the diffuse reflectance spectrum. Actually, the absorption is calculated using the Kubelka–Munk function [12]: (𝟏−𝑹)𝟐
F (R) =/S
𝟐𝑹
Where R, and S are the reflectance, the absorption coefficient and the scattering coefficient, respectively. Then, the optical band gap energy of the Sb doped ZnO NCs can be obtained by assuming a direct transition between valence and conduction bands using the following expression [37]: h 𝐀(𝒉 − 𝑬𝒈) Where A is constant, is the absorption coefficient; h is photon energy and Eg is the band gap energy of the semiconductor. Eg was determinate through the extrapolation of the linear region to (h)2 = 0 [9]. It is clearly observed that the obtained band gap values were found to decrease, with Sb content, from 3.35 eV for un-doped ZnO to 3.15 eV for ZSb3. Similar results were reported by many studies that reveals a decrease in the ZnO band gap following the introduction of doping elements [38-40]. In fact, the introduction of antimony can induces deep levels within the ZnO band gap. Particularly, the apparition of the complex accepter level, mentioned previously in the XPS analysis, is strongly supported in Sb-doped ZnO [12]. At ambient temperature, electrons can occupy such deep level. Already occupying this complex related level, electrons can be excited directly to the conduction band, which will be expressed by a decrease in the band gap compared to the undoped sample. Moreover, from Fig.5, it can be seen that the curve of the Sb-doped ZnO has a clear absorption shoulder near 3 eV (413 nm). This absorption band is red shifted with Sb-doping, which means that exciting electrons by sunlight irradiation is easier with the Sb-doped sample. The intensity of absorption shoulder increases with Sb doping, this can be attributed to the increase of the electronhole separation rate on the ZnO NCs surface [9,10].
3.6 Photoluminescence investigations The PL spectra of pure and Sb doped ZnO samples present an important ultra-violet emission at about 377 nm that can be attributed to the free excitons emission [41, 42] (fig. 6). Another emission observed at around 556 nm can be assigned to defects present in ZnO NCs such as oxygen vacancies (VO) and Zn interstitial (Zni) [43,44]. Contrary to the doped samples, the PL spectrum of the undoped
8
sample is dominated by the UV band. The continuous intensity decay of this band with antimony ratio indicates the increase of the defects density in the ZnO band gap. According to many reports [45-48], the defects related to ZnO crystals can affect mainly its optical properties. Actually, Y. Zhang et al. [45] reported that Ag prevents electron-hole recombination which enhances the excitonic emission in Ag-doped ZnO prepared with sol-gel. On the other hand, H. Hong et al. [46] reported that intrinsically fluorescent ZnO nanowires can be adopted for molecularly targeted imaging of cancer cells. However, K. Qi et al. [47] founded that the blue emission collected from the ZnO prepared with solvothermal method, is related to their oxygen vacancies as well as other impurities and defects. Also, L. Irimpon et al. [48] synthesized colloids ZnO by modified polyol precipitation method. They founded a peculiar feature in the fluorescence spectra strongly correlated to the nanoparticles size distribution. Particularly, the UV band of the ZSb3 PL spectrum presents an obvious red-shift to 385 nm. This observation seems to be of a substantial interest in the PL description. This band seems to be directly correlated to the (SbZn–2VZn) complex acceptor level, apparently well manifested with the ZSb3 sample. In fact, Limpijumnong et al.[12] and Tian et al.[49] reported that this level is located at +0.16 eV above the valence band. Taking into account these considerations, the fact that Eg (ZnO)-0.16=3.19 eV, which is very close to the energy of the observed emission (3.22 eV) with the ZSb3 sample, confirms the attribution of this emission to the transition between the conduction band and this complex level as proposed in Fig.6.b. It is interesting to note that the red-shift undergone by the UV band is accompanied by a blue shift of the visible band, principally assigned to oxygen vacancies. This shift of the visible band to higher energies can be attributed to the multiplication of the Zni, energetically shallower than VO level [50,51], since the formation of such interstitial is facilitated by the presence of the complex (SbZn–2VZn) itself [49]. These two observed UV and visible shifts disappear when reaching 5% of antimony. Actually, this comportment will be more comprehensible when describing the SbZn–2VZn formation process. In fact, the presence of such complex is a result of coulombic binding between two oppositely charged defects [12]. During its occupation for the Zn sites, the Sb ions occurs a highly electronegative character while being surrounded by negatively charged oxygen ions. The particular geometry of the Wutzite structure allows these two negatively charged ions to approach each other when the oxygen is 5/3 length bonded away along the [0001] direction [49]. Then, this configuration must be assisted by the presence of two neighboring zinc vacancies [49]. The coulombic nature and the complexity of the described process make from the formation of (SbZn–2VZn) particularly sensitive to Sb content, this feature limits the antimony concentration range in which this complex is formed. Thus, we estimate that 5% of antimony is a sufficient amount to break down the described proceeding and as result the correspondent PL shifts disappear. 9
The PL intensity relative to the ZSb5 sample decreases drastically. This behavior may due to the excess of antimony probably localized at the ZnO surface. Furthermore, PL intensity is related to the PL emission centers density. Then we expect that this density is affected by this doping through the variation in the oxygen vacancies density in the material. These PL observations, coherently with the previous deductions extracted from XPS and UV-Vis measurements, highlights the adequacy of the Sb-doped ZnO for photocatalytic applications.
4. Evaluation of photocatalytic activity of Sb-doped ZnO with RhB RhB characteristic absorbance band located at 560 nm was used to monitor the photocatalytic activity of the Sb-doped ZnO NCs, by measuring, at different regular time intervals, the absorbance of an aqueous solution of RhB containing Sb-doped ZnO powder catalyst. The same experiment, realized without catalyst, reveals that is difficult to decompose the RhB under sunlight irradiation after 100 min (data was not shown), which indicate that the photoinduced selfsensitized photocatalysis of RhB without ZnO is negligible [9]. An obvious decrease in the intensity of the characteristic RhB absorbance peak was observed when introducing the ZnO catalyst. As indicated in Fig.7, the correspondent band decreases continuously during sunlight irradiation. To investigate the effect of the Sb doping on the photocatalytic efficiency of ZnO under sunlight irradiation, the photodegradation experiment was carried out using separately the ZSb1, ZSb3 and ZSb5 powders as catalysts. Fig.7.b illustrates the variation in the UV–Visible absorption spectra at different time intervals in the presence of ZSb3 sample. After 100 min of irradiation, the correspondent peak disappeared completely, indicating the total degradation of RhB with the presence of ZSb3 sample. The above observations reveal that the introducing of antimony enhances clearly the photocatalytic activity of the ZnO. In fact, during 100 min of sunlight irradiation, 98% of RhB was degradated in the presence of the ZSb3 sample while only 68 % of the dye was decomposed with the undoped ZnO during the same irradiation time. This faster photocatalytic activity, under sunlight irradiation, is attributed to the appearance of defects caused by Sb doping and leading to the enhancement of the visible absorption within the band gap. During 100 min of sunlight exposition, the photodegradation rate reaches 68%, 94%, 98%, and 95% with the undoped ZnO, ZSb1, ZSb3, and ZSb5 samples, respectively. This reveals that the ZSb3 sample presents the higher photocatalytic activity. The particularity of the ZSb3 sample was also observed in the previous experimental results and was attributed principally the formation of the (SbZn-2VZn) complex. Then, we suggest that the enhancement of the photocatalytic activity when introducing Sb can be strongly correlated to the appearance of this complex obviously expressed through the photoluminescence results. In fact, 10
electrons transitions from/to the induced (SbZn-2VZn) level seem to be strongly probable under solar excitation due to the low required energy assuring such transitions. Such attribution of the photocatalytic efficiency enhancement to internal band gap transitions is supported by the review works leaded by Kumar et al. [30], Pan et al. [52], Achouri et al. [53] and W. Yu et al .[54]. It is also important to note that the undoped prepared ZnO present already a photocatalytic activity revealing that it possess its own photocatalytic mechanism. Actually, oxygen vacancies and oxygen interstitials act as electrons and holes traps, respectively, then they play a key role in the photodegradation process by inhibiting the electron-hole recombination [30]. In fact, the large PL band centered at 556 nm (green) is consistent with the transition assisted by oxygen vacancies VO [43]. Once exposed to the solar irradiations, excited electrons are trapped by VO located on the ZnO surface [55] and react with O2- to yield superoxide .O2-. The photogenerated holes in the valence band react with OH- to give .OH radicals [30] (Fig.10). As mentioned previously, the photocatalysis mechanism of the Sb-doped samples will be certainly affected by the presence of the SbZn-2VZn complex level since this latter is especially well expressed with the ZSb3 sample that in turn present the better photocatalytic efficiency. Actually, the acceptor nature of this level and the induced p-type conductivity was proved within manifold reports [12,23]. This property can induce an increase of the carrier holes density in the valence band, which further activate the oxidation of hydroxyl radicals and as a result enhances the photocatalytic activity (Fig.10). The attribution of such important role to the complex (SbZn-2VZn) level in the photocatalysis process can be further evidenced when monitoring the intensity of the UV PL band. In fact, the reduction of the UV PL intensity compared to the visible one suggests a decrease in electron-hole radiative recombination, and then we expect an enhancement of the photocatalytic efficiency with Sb doping. The contrary was observed when passing from the ZSb3 to ZSb5. Actually, despite the drastically decay of the PL intensity with the 5% Sb-doped ZnO, the photocatalytic activity of this sample decreases compared to the ZSb3 sample for which the PL shift, attributed to formation of the (SbZn-2VZn) complex, was observed. This observation further evidences the important role of this complex in the photocatalytic mechanism. On the other hand, we expect that the oxygen vacancies kept its role as an electrons trap even for the Sb-doped sample. In fact, in addition to electrons excited directly from the valence band to this level, electrons already present in (SbZn-2VZn) derived level can also be trapped by VO once they are excited by visible irradiation and the transition required energy, estimated to be 2.24 eV, still in the visible range, as shown in figure 10. Then, the reduction of .O2- is also assured by electrons trapped in the VO level even the Sb-doped samples (Fig.10). In order to further confirm the validity of such proposed mechanism and to further highlight the predominant role of the .OH radicals, we precede to the introduction of scavengers species to the reaction system. In fact, ethanol (10mM) was added to the mixture during the irradiation in order to 11
quench the presence of hydroxyl radicals [56-58] and benzoquinone (BQ) (10mM) as .O2- scavenger [59,60]. As shown in figure 9, the ZSb3 was found to be less able to degrade the RhB with the presence of ethanol, contrarily to the experiment containing the BQ that practically do not show any affection by the presence of this .O2- scavenger. Actually, with ethanol, the photodegradation efficiency was found to decrease from 98.2 % to 60.3 %, whereas it reaches 92.12 % with BQ. These results confirm the validity of such proposed mechanism and the important role of the .OH radicals in the photocatalysis process in the presence of Sb-doped ZnO NCs. Under sunlight irradiation, a series of reactions between different entities present in the mixture is activated and can be described as follows: ZnO + hsunlight)ZnO (e-+ h+) H2O + h + O2 + e .
OH/.O2-+ Dye
H+ + .OH .
O2Oxidation products (CO2 + H2O etc…)
The photodegradation reactions can be classified as pseudo-first-order kinetic reactions [12]. From which reaction constants are determinate to be 3.87×10-2, 6.50×10-2, 7.26×10-2, and 6.68×10-2 min-1 for ZnO, ZSb1, ZSb3, and ZSb5, respectively. These values are higher than those reported for Ndoped ZnO [60] and Mg-doped ZnO [61]. This result suggests that Sb-doped ZnO NCs can be used as a potential photocatalyst under sunlight irradiation. It is important to note that the initial pH is an important factor to take into consideration due to its effect on surface charge properties of the semiconductor. For such reason, the effect of initial pH on the photocatalysis efficiency was studied by taking the same experiments under different initial pH values varying between 3 and 12. As shown in figure 11, the photodegradation activity was effectively affected by the initial pH value. Actually, either with the undoped or the Sb-doped ZnO, an acidic medium result in an obvious decrease of this activity contrarily to the basic medium with which the photocatalytic efficiency is very slightly affected (Fig. 11). In fact, the acidity enhances the solubility of the nanostructure in the concerned solution [62] which lowers the stability of the system and induces a decrease in the catalytic reaction activity [15]. As proposed by A. Omidi et al. [15], the adsorption rate of the dye molecule at the surface of the NCs is directly affected by the pH value and this through the electrostatic attraction or repulsion forces that depend on charge difference between the nanoparticle surface and the dye. Actually, under zero point charge pH, estimated to be about pH=9 for nanostructures [63], surface of the photocatalyst will be positively charged similarly to the RhB molecule that will acquire a positive charge under these conditions due to the reaction of its Cl ions with the H+ protons. This configuration promotes the repulsion between the RhB molecules and 12
the surface of ZnO, which decreases the adsorption rate and then quench the photocatalytic activity under acidic conditions. The recycled photocatalytic experiment was also performed under sunlight irradiation (Fig.12). After every cycle, a micro-volume of a very concentrated aqueous solution of RhB was added to the above solution to turn back to the initial concentration of RhB (5.10-6 M). After the first cycle, there was a slight decrease in the photocatalytic activity that may be related to the loss of a small amount of ZnO during the recovery process, then the activity remains stable during the others runs. The photocatalytic activity of ZSb3 is unchangeable after four runs. To further confirm the stability of the elaborated Sb-doped ZnO, the NCs powder was recovered from the reaction mixture, dried and characterized again with XRD measurements. The XRD diffractograms obtained after 4 successive cycles of photocatalytic reaction presents the same peaks as the patterns in figure S.2. These results confirm the structural stability of the hexagonal Sb-doped ZnO used as catalyst in aqueous solution. All the presented results suggest that the ZSb3 catalyst presents a high stability and reusability during the photodegradation of RhB under sunlight irradiation.
Conclusion Pure crystalline phase of Sb-doped ZnO NCs was synthesized by a facile and economical sol gel method. XRD data revealed the formation of Sb-doped ZnO NCs crystallized in Wurtzite structure without any secondary phases.TEM and HRTEM results showed that the nanocrystals have an average size of about 50 nm with a spherical like morphology. X-ray photoelectron spectroscopic analysis confirmed that Sb3+ states were more preferable in comparison to Sb5+ states, which is favorable for the formation of the (SbZn-2VZn) complex in Sb-doped ZnO NCs. The decrease of the band gap energy with Sb doping can be ascribed to the apparition of energy states caused by antimony doping. The formation of the (SbZn-2VZn) complex was also proved by the PL measurements that revealed the highest expression of the correspondent level for the ZSb3 sample. Accordingly, the photocatalytic experiments revealed that the sample ZSb3 shows the highest photocatalytic activity. The key role of the (SbZn-2VZn) complex in the enhancement of the photocatalytic efficiency was demonstrated through a detailed study of the photodegradation mechanism that highlights the contribution of this complex in the oxidation of hydroxyl radicals. Oxygen vacancies play also a principal role in inhibiting the radiative recombination by trapping the excited electrons. The domination of the hydroxyl radicals in the photocatalytic mechanism was proved experimentally. The reusability of the ZSb3 catalyst was also evaluated and reveals its high stability and their good performances in photocatalysis.
13
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16
Figures
Figure 1: XRD patterns of ZnO:Sb NCs with different doping concentrations. Figure 2: (a) TEM images of undoped ZnO (b) ZSb3 NCs. (c) Selected area electron diffraction pattern of ZSb3 NCs. (d) HRTEM image of the ZSb3 sample. (e) EDAX spectrum of ZSb3. Figure 3: (a) XPS spectra of the Zn-2p, O-1s and Sb-3d core level regions for ZSb3 samples. (b) Fitted XPS spectra of the O-1s and (c) Sb-3d3/2 core level regions. Figure 4: Raman spectra of Sb-doped ZnO NCs excited with HeCd laser (exc = 632 nm). Figure 5: (a) Diffuse reflectance spectra of Sb doped ZnO nanocrystals (b) The plots of (h)2 vs. photon energy of undoped ZnO and ZSb3 NCs. Figure 6: (a) PL spectra of Sb-doped ZnO excited with 325 nm for different Sb concentrations (0, 1, 3 and 5% Sb). (b) Proposed photoluminescence mechanism for Sb-doped ZnO. Figure 7: Evolution of the RhB absorption spectra in the presence of (a) ZnO NCs and (b) ZSb3 with sunlight irradiation time. Figure 8: (a) Degradation rate of the RhB with the presence of different Sb-doped ZnO under sunlight irradiation (b) Kinetic curve of photocatalytic activity in the presence of ZnO and ZnOSb (3%). Figure 9: Effects of scavengers on the photocatalytic efficiency of ZSb3 NCs. Figure 10: Proposed photocatalytic mechanism for Sb-doped ZnO under sunlight irradiation. Figure 11: Photodegradation of RhB on ZnO and ZSb3 nanocrystals at various pH values. Figure 12: Cycling photocatalysis experiment in the presence of ZSb3 NCs under sunlight irradiation.
17
200 112 201
103
110
102
100 002 101
Intensity (a.u.)
ZSb5
ZSb3
ZSb1
ZnO 20
30
40
50
60
70
2 Theta(°)
Fig. 1
18
a
b
c
d 19
d=0.26 nm
e
Fig. 2
20
aZn 2p
1.6
3/2
Zn 2p1/2
1.2
1010
1020
1030
0.9
O1s
0.8
1040
1050 1060 Binding Energy (eV)
Sb 3d5/2
0.7 0.6
Sb metal
Sb 3d3/2
0.5 520
525
0.4
530
535
540
545 550 555 Binding Energy (eV)
b O 1s Sb 3d5/2
Fit
0.3
Intensity (a.u.)
Intensity (a.u.)
0.8
0.2
0.1
0.0 524
526
528
530
532
534
536
Bindig energy (eV)
21
c 0.41
Sb 3d3/2 Fit
0.40 3+
Intensity (a.u.)
Sb 0.39 0.38
5+
Sb
0.37 0.36 0.35 538
539
540
541
542
543
544
Binding Energy (eV)
Fig. 3
22
Raman intensity (a.u.)
E2(high) -E2(low)
E2(high)
LVM(Sb-O-Sb)
5% Sb 3% Sb
A1LO
A1TO
1% Sb
0% Sb
100
200
300
400
500
600
700
wavenumber (cm-1)
Fig. 4
23
800
(a)
Diffuse Reflectance (a. u.)
ZnO ZSb1 ZSb3 ZSb5
A b s o r b n a c e (a .u .)
0 .6 0 .5
ZnO ZSb1 ZSb3 ZSb5
0 .4 0 .3 0 .2 0 .1 0 .0 200
300
400
500
600
700
800
W a v e le n g th (n m )
200
300
400
500
600
700
800
Wavelength (nm)
16
ZnO ZSb3
14
(b)
(h)
2
12 10 8 6 4 2 0 2.0
2.4
2.8
3.2
3.6
h (eV)
Fig. 5
24
1000
(a)
ZnO ZSb1 ZSb3 ZSb5
PL intensity (a.u.)
800
600
400
200
0 300
350
400
450
500
550
600
650
700
750
Wavelength (nm)
(b)
CB VO 3.32 eV
3.19 eV vio
2.24 eV
Green
Complex SbZn-2VZn +0.16 eV
VB Fig. 6
25
(a)
Absorbance (a.u.)
ZnO 0 min 20min 40min 60min 80min 100min
400
450
500
550
600
650
700
Wavelength (nm)
Absorbance (a.u.)
(b)
ZSb3 0 min 20 min 40 min 60 min 80 min 100 min
400
450
500
550
600
650
Wavelength (nm) .
Fig. 7
26
700
100
(a)
Degradation rate (%)
80
60
40
ZnO ZSb1 ZSb3 ZSb5
20
0 0
20
40
60
80
100
Time (min)
ln(A0/A)
(b)
ZnO ZSb3
10
1
0
20
40
60
80
100
Time (min)
Fig. 8
27
120
Degradation rate (%)
100
80
60
40
20
0
No scavenger
Ethanol
BQ
Fig. 9
28
CB O2
e-e- e- e- e- e-
VO
. 2.24 eV
O -2
RhB
Green
Complex acceptor level (SbZn-2VZn) 0.16 eV
CO2 +H2O + hOH
h+ h+ h + h + h + h +
VB
.
+ hH 2O
h+
Fig. 10
h+ h
+
h+
h+
29
Degradation rate (%)
100
pH3 ZSb3 pH5 pH7 pH9 pH12
80
60
40
20
0 0
20
40
60
80
100
80
100
Time (min)
Degradation rate (%)
80
pH3 ZnO pH7 pH12
60
40
20
0
0
20
40
60
Time (min)
Fig. 11
30
1.2
ZSb3
1.0
A/A0
0.8 0.6 0.4 0.2 0.0 0
100
200
300
400
500
Time (min)
Fig. 12
31
Table 1: crystalline size of Sb doped ZnO nanocrystals. Samples ZnO ZSb1 ZSb3 ZSb5
Grain size(nm) 54.3 51.9 47.1 52.7
a(nm) 0.321 0.324 0.327 0.331
32
c(nm) 0.524 0.527 0.528 0.530
Table 2: Band gap energy of Sb doped ZnO samples.
Samples
ZnO
ZSb1
ZSb3
ZSb5
Eg(eV)
3.35
3.25
3.14
3.18
33
S0169-4332(16)32039-6 http://dx.doi.org/doi:10.1016/j.apsusc.2016.09.158 APSUSC 34087
To appear in:
APSUSC
Received date: Revised date: Accepted date:
15-7-2016 28-9-2016 30-9-2016
Please cite this article as: Ramzi Nasser, Walid Ben Haj Othmen, Habib Elhouichet, Mokhtar F´erid, Preparation, characterization of Sb-doped ZnO nanocrystals and their excellent solar light driven photocatalytic activity, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.09.158 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Preparation, characterization of Sb-doped ZnO nanocrystals and their excellent solar light driven photocatalytic activity
Ramzi Nasser1,2, Walid Ben Haj Othmen1,2, Habib Elhouichet1,2,*, Mokhtar Férid1
1
Laboratoire de Physico-Chimie des Matériaux Minéraux et leurs Applications, Centre National de Recherches
en Sciences des Matériaux, B.P. 95 Hammam-Lif, 2050, Tunisia. 2
Département de Physique, Faculté des Sciences de Tunis, University of Tunis El Manar 2092, Tunisia.
*Corresponding author: H. Elhouichet: [email protected]
1
Graphical Abstract
CB
VO
.O-2
e-
RhB
O2
(SbZn-2VZn)
.OH h+
VB
Sb-doped ZnO
CO2+H2O
e -
H2O e -
h+ h+
e -
Co h+ mpl e V + h+ h ex Highlights B + acc h ept e Sb-ZnO was obtained by modified sol-gel method using citric acid as stabilizing agent. or Sb incorporated both in lattice and interstitial sites. leve l Zn-2VZn) acceptor level The formation of (Sb e was revealed by photoluminescence studies. (Sb Optimum Sb content to show higher photocatalytic activity was found to be 3%. Zn2VZ n)
h +
Abstract
In the present study, undoped and antimony (Sb) doped ZnO nanocrystals (NCs) were prepared by a simple and economical sol-gel method. X-ray diffraction (XRD) and transmission electron microscopy (TEM) revealed the purity of the obtained phase and its high crystallinity. Raman analysis confirms the hexagonal Wurtzite ZnO structure. According to the diffuse reflectance results, the band gap was found to decrease up to 3% of Sb doping (ZSb3 sample). The results of X-ray photoelectron spectroscopy (XPS) measurements reveal that Sb ions occupied both Zn and interstitials sites. The successful substitution of antimony in ZnO lattice suggests the formation of the complex h+
(SbZn-2VZn) acceptor level above the valence band. Particularly for ZSb3 sample, the UV photoluminescence (PL) band presents an obvious red-shift attributed to the formation of this complex. Rhodamine B (RhB) was used to evaluate the photocatalytic activity of Sb-doped ZnO NCs under sunlight irradiation. It was found that oxygen vacancies play a major role in the photocatalytic process by trapping the excited electrons and inhibiting the radiative recombination. During the photocatalytic mechanism, the Sb doping, expressed through the apparition of the (SbZn-2VZn) correspondent acceptor level, enhances the sunlight absorption within the ZnO band gap, which stimulates the generation of hydroxyl radicals and promotes the photocatalytics reaction rates. Such important contribution of the hydroxyl radicals was confirmed experimentally when using ethanol as scavenger in the photocatalytic reaction. The photodegradation experiments reveal that ZSb3 sample exhibits the highest photocatalytic activity among all the prepared samples and presents a good 2
cycling stability and reusability. The influence of the initial pH in the photodegradation efficiency was also monitored and discussed.
Keywords: ZnO; Sb doping; Modified sol-gel method; Photoluminescence; Acceptor complex; Photocatalysis.
1. Introduction The particular diversified properties of the II–VI Wurtzite phase offer to the ZnO a special ranking among many oxides [1]. The wide band gap 3.37 eV, large and high binding energy 60 meV, and low fabrication cost [2-4] make it more attractive than other transparent conductor oxides such as TiO2 [5]. Furthermore, ZnO based nanomaterials found many applications especially in systems such as light-emitting diodes (LEDs), and photocatalysts [1,6]. Up to now, among various semiconductors used in the photodegradation of organic pollutants, ZnO nanostructures were widely investigated due to their better environmental friendly feature, non-toxicity, low cost and good stability [7,8]. Also, ZnO absorbs large fractions of solar spectrum compared to other photocatalysts like TiO2 and ZnS [5,9]. To improve the visible light absorption of ZnO nanomaterials, the most effective way is to decrease its band gap energy by introducing intermediate states. By doping ZnO nanocrystals (NCs) by antimony, the particles size, morphology and induced surface defects will have an important effect on their photocatalytic activity [10,11]. In addition, doping is a simple and useful concept to introduce defect levels in the bandgap [12], which may inhibit the radiative recombination of electrons-holes pairs. Many papers have been dedicated for the study of the photocatalytic activity of the Sb-doped ZnO. In fact, A. Phuruangrat et al. [13] prepared Sb-doped ZnO with ultrasonic assisted method and monitored its photocatalytic activity under UV irradiation. On the other hand, A. Omidi et al. [14,15] have fabricated Sb-doped ZnO nanostructures using an ultrasonic method and reveals that the as-prepared nanoparticles presents a high photocatalytic activity under UV light irradiation. Then, to the best of your knowledge, the current study presents the first report treating the photocatalytic properties of Sb-doped ZnO NCs under sunlight irradiation. Actually, the nature of the deep defects following the Sb-doping seems to be efficient for the electrons-holes separation under visible sunlight irradiation. Herein, economical and low cost Sb-doped ZnO NCs were prepared by sol-gel method. Series of Zn1-xSbxO with an antimony concentration ranging from x= 0.00 to x= 0.05 were elaborated and characterized through various experimental methods. The photocatalytic activity of the as prepared NCs was monitored for different Sb ratio using the RhB as a probe dye molecule. Moreover, the mechanism of photocatalytic activity of Sb doped ZnO was discussed based on the 3
separation process of electron-hole pairs and the active species detection. Finally, the stability of Sb doped ZnO was studied through successive four cycles of experiments. 2. Experimental a.
Samples preparation
All chemical products were purchased from Sigma–Aldrich Company, and were used as received without further purification. Zinc acetate [Zn(O2CCH3)2(H2O)2] used as a zinc source, antimony trichloride (SbCl3) as a antimony dopant source and citric acid (C6H8O7) as a stabilizer, were the precursors used to prepare the sol gel ZnOSb NCs powders. 2 g of zinc acetate and 4 g of citric acid are dissolved in 200 ml of bi-distilled water under magnetic stirring. The mole fraction of SbCl3 dopant was added to the above solution with different contents (the molar ratio of Sb to Zn was 0, 1, 3, and 5%). The solutions were stirred magnetically for 3h to obtain a homogeneous mixture. Then, the solvent was evaporated at 120°C for 5 days. The obtained gels were ground to make fine powders. The obtained powders were heated at 300°C for 4 h to remove the organic products. After grinding, they are submitted to calcination at 500°C to obtain a crystalline phase of Sb-doped ZnO. In the following, the prepared undoped ZnO and the 1%, 3% and 5% Sb-doped samples will be indexed by ZnO, ZSb1, ZSb3 and ZSb5, respectively.
b. Samples characterizations: X-ray powder diffractometer with Monochromatized CuK𝛼 radiation ( = 1.5418 Å) scanned the samples in the range of (20°–70°). The morphologies of samples were characterized by transmission electron microscopy (TEM) and high resolved TEM (HRTEM) on an FEI Tecnai F20 microscope with an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were analyzed by ESCALSB 200-2 spectrometer under ultra-high vacuum (P<10−9mbar). The data measurements were performed at normal incidence (the sample plane is perpendicular to the emission angle).The mono-chromatized AlKα source (1486.6 eV) was operated at a power of 420 W (30 mA and 14 kV) and C1s (284.6 eV) was used to calibrate the peak positions of the elements. Micro Raman and Photoluminescence measurements were recorded using a Lab RAM HR spectrometer (Horiba Gr, France) using the 632 nm and 325 nm as excitation wavelength from HeCd laser, respectively. Diffuse reflectance spectroscopy measurements were carried by using UV–VIS Lambda 950 spectrometer Perkin Elmer preciosity. c. Photocatalysis experiments The photocatalytic activity of the Sb-doped ZnO nanocrystals was evaluated through the photodegradation of Rodhamine B RhB (C28H31CIN2O3) purchased from Sigma Aldrich chemical. A quantity of 1.916 mg of photocatalyst was added to 800 mL of 5.10-6 M aqueous solution of RhB. In 4
order to reach the adsorption–desorption equilibrium between pollutant and catalyst, the mixture was stirred in the dark for 2h. The solution was then exposed to sunlight irradiation. The degradation efficiency was calculated using the relation [9,16]: 𝑫𝒚𝒆(%) =
(𝑨𝟎 − 𝑨(𝒕)) (𝑪𝟎 − 𝑪(𝒕)) ∗ 𝟏𝟎𝟎 = ∗ 𝟏𝟎𝟎 𝑨𝟎 𝑪𝟎
Where A0 and C0 are respectively the initial (t=0) absorbance and concentration of RhB. A(t) and C(t) 𝐀
are those at an instant t. The kinetic reaction constant can be calculated using the plots of 𝐥𝐧 ( 𝟎 ) 𝐀
versus irradiation time since they are related by [9,17]: 𝒍𝒏 (
𝑨𝟎 ) = 𝒌𝒕 𝑨
Where t is the irradiation time and k is the first-order rate constant of the reaction.
3. Results 3.1 XRD analysis Fig.1 illustrates the XRD patterns of the undoped and Sb-doped ZnO NCs annealed at 500°C. The ZnO diffractograms confirm that all the prepared samples crystallize in hexagonal phase (JCPDS no. 36-1451). By adding Sb, no change in the structure phase was detected and the absence of impurity phases in all the samples suggests that most of Sb ions occupy sites within the ZnO lattice. The diffraction peaks present a slight shift to lower angles with Sb doping. This behavior is attributed to the substitution of Zn by Sb ions, which may suggest the successful doping procedure. However, the full width at half-maximum (FWHM) of the principal diffraction peaks further increases with Sb doping, up to 3 at. %, due to the excessive Sb incorporation. The NCs size of the doped ZnO was estimated by the Debye-Scherrer formula [18]: 𝐃=
𝐤𝛌 𝛃𝐜𝐨𝐬𝛉
Where β is the full width at half maximum of the diffraction peak at the angle 𝛉 and is the wavelength for the Cu K component of the employed copper radiation. The calculated average size of the Sb doped ZnO NCs were illustrated in Table 1. As can be seen, the average size decreases with increasing Sb doping until 3%. The decrease in the particle size may be due successful insertion of Sb3+ (0.076 nm) ions in Zn2+(0.074 nm) ions. Thus, it causes the increase in lattice parameters and may affect the growth of grains. 5
Actually, a relatively high doping rate can induces the multiplication of nucleation centers during the formation of the nanocrystals which induces an increase in the grain size [19]. 3.2 Morphological study The morphology study of the undoped ZnO and ZSb3 NCs were performed using TEM and HRTEM imaging (Fig.2). The TEM images indicated nanospheres like crystals with a uniform size distribution ranged between 40-60 nm, which is in good agreement with XRD results. The selected area electron diffraction (SAED) was carried out and indicates that the well resolved diffraction rings originate from high crystallized ZnO as shown in Fig.2.c. From the HRTEM image of the Sb-doped sample, the distance between adjacent lattice fringes was estimated to be about 0.26 nm, which is very close to the theoretical spacing d002 of the Wurtzite ZnO [20]. This fact shows that the insertion of antimony did not affect the hexagonal structure of ZnO (Fig.2.d). The energy dispersive X-ray analysis (EDXA) spectra (Fig.2.e) reveals that the synthesized ZSb3 powder is composed from oxygen, zinc and antimony without any others impurity elements except the Cu and Al that originated from the sample support, which further evidence the purity of the elaborated samples. EDAX results suggest again the successful Sb doping in ZnO NCs.
3.3 X-ray Photoelectron Spectroscopic analysis The surface composition and chemical states can be determined by means of X-ray photoelectron spectroscopy (XPS) through the determination of the binding energies of the different elements present on the ZnO NCs surface. Figure S.1 shows the survey XPS spectra of undoped ZnO and ZSb3 NCs. The figure indicates the presence of the elements Zn, O and adventitious C for the undoped ZnO in addition to the Sb for the Sb-doped sample. No contamination resulted from the elaboration process were detected from the spectra. Fig.3 (a) shows two highly symmetric peaks centered at 1022.20 eV and 1045.54 eV assigned to Zn 2p3/2 and Zn 2p1/2 states, respectively, which is consistent with that reported for ZnO nanostructures [21]. The narrow Zn 2p3/2 band is attributed to Zn2+ ions in an oxygen-deficient ZnO lattice [21,22]. Thus, it can be noted that the majority of the surface Zn ions exists in the ionic form Zn2+ [23- 25]. Furthermore, two peaks related to antimony, one is located at 530.5 eV and were assigned to the coexistence of O1s and Sb3d5/2 and another at 540.1 eV attributed to Sb3d3/2 [24]. The presence of Sb3d3/2 indicated that antimony is well introduced in the ZnO lattice and evidence the presence of SbO bond [21]. An additional peak with a weak intensity was detected at 534.9 eV and corresponds to the Sb 3d3/2 binding energy of metallic antimony Sb0 [22] probably localized on grain boundaries. Some study reveals that the critical doping concentration of antimony in ZnO before the formation of metallic Sb was about 3% [26]. Then, we expect that the formation of metallic antimony is probable 6
especially that we are working around this Sb amount. It is also expected that this metallic Sb phase is located on the ZnO nanocrystals surface. The oxygen species is of major importance in the photocatalytic performance of ZnO NCs. Figure 3(a) illustrated the high resolution of O1s peak of ZSb3 sample. The asymmetric peaks observed in the O1s were deconvoluted into three peaks. The intense one with low binding energy (530.5 eV) could be attributed to the lattice oxygen in the ZnO host. The medium binding energy (531.09 eV) could correspond to the hydroxyl oxygen on the ZnO surface [27,28]. The peak with high binding energy (532.4 eV) is more probably related to Sb 3d5/2 state in ZnO. The hydroxyl oxygen on the surface was a well-known as an important source for the generation of active hydroxyl species responsible for the photocatalytic activity [29]. The deconvoluted Sb 3d3/2 peak shows the existence of two possible states Sb ions in ZnO NCs [29]. Actually, it’s known that Sb ions can exist in two ionizations states: Sb3+ (0.076 nm) when it substitute Zn2+ ions (0.074 nm) and Sb5+ (0.061 nm) when it occupy an interstitial site [22]. In the substitutional case, the isolated SbZn can be associated spontaneously with two induced zinc vacancies (VZn) to form the complex acceptor (SbZn-2VZn) level [12]. In the interstitial case, Sb5+(0.061 nm) can occupy the interstice site (0.062 nm) in the hexagonal ZnO [23]. These defects may support the stability of acceptor levels with low formation energy [24]. From the fitted curve, the higher band intensity related to the Sb3+ states compared to that of the Sb5+ states suggest that the majority of introduced Sb ions substitute the Zn2+ ones. It is also important to note that the substitution presence of Sb ions in interstitial ZnO hexagonal sites may leads to the increase of the charges carriers mobility [30], which can help the carriers to reach the nanoparticle surface. These features seem to be benefic for a photocatalytic purpose.
3.4 Raman scattering Fig.4 shows the Raman spectra of the Sb-doped ZnO NCs. The intense E2High mode, located at 437 cm−1, is attributed to the lattice vibration of the oxygen atoms [18,31]. This mode is particularly characteristic of the Würtzite structure and the internal stress in ZnO [31]. The 338 cm−1 peak present in pure and doped ZnO samples was assigned to the energy difference between E2high and E2low caused by multi phonon vibrations process[32].The weak peak, at about 570 cm−1, is related to the A1 (LO) mode [33] classified among the first-order optical modes in Wurtzite ZnO. The peak at around 380 cm-1 can be related to A1 (TO) mode [34,35]. The Raman spectrum of ZnO:Sb NCs is dominated by the E2High mode confirming then the good crystallinity even with Sb doping. Moreover, the continuous decrease in the intensity relative to the A1(LO) mode can be attributed to the substitution of Zn by Sb, which is consistent with the XRD and XPS results. For high Sb doping, a new Raman mode, located at 712 cm-1 is seen. The associated peak is relatively of a low intensity and may be attributed to weak local vibration mode (LVM) related to Sb-O-Sb vibration [36]. 7
3.5 UV-vis study The optical band gap of the doped ZnO NCs could be extracted from the diffuse reflectance spectrum. Actually, the absorption is calculated using the Kubelka–Munk function [12]: (𝟏−𝑹)𝟐
F (R) =/S
𝟐𝑹
Where R, and S are the reflectance, the absorption coefficient and the scattering coefficient, respectively. Then, the optical band gap energy of the Sb doped ZnO NCs can be obtained by assuming a direct transition between valence and conduction bands using the following expression [37]: h 𝐀(𝒉 − 𝑬𝒈) Where A is constant, is the absorption coefficient; h is photon energy and Eg is the band gap energy of the semiconductor. Eg was determinate through the extrapolation of the linear region to (h)2 = 0 [9]. It is clearly observed that the obtained band gap values were found to decrease, with Sb content, from 3.35 eV for un-doped ZnO to 3.15 eV for ZSb3. Similar results were reported by many studies that reveals a decrease in the ZnO band gap following the introduction of doping elements [38-40]. In fact, the introduction of antimony can induces deep levels within the ZnO band gap. Particularly, the apparition of the complex accepter level, mentioned previously in the XPS analysis, is strongly supported in Sb-doped ZnO [12]. At ambient temperature, electrons can occupy such deep level. Already occupying this complex related level, electrons can be excited directly to the conduction band, which will be expressed by a decrease in the band gap compared to the undoped sample. Moreover, from Fig.5, it can be seen that the curve of the Sb-doped ZnO has a clear absorption shoulder near 3 eV (413 nm). This absorption band is red shifted with Sb-doping, which means that exciting electrons by sunlight irradiation is easier with the Sb-doped sample. The intensity of absorption shoulder increases with Sb doping, this can be attributed to the increase of the electronhole separation rate on the ZnO NCs surface [9,10].
3.6 Photoluminescence investigations The PL spectra of pure and Sb doped ZnO samples present an important ultra-violet emission at about 377 nm that can be attributed to the free excitons emission [41, 42] (fig. 6). Another emission observed at around 556 nm can be assigned to defects present in ZnO NCs such as oxygen vacancies (VO) and Zn interstitial (Zni) [43,44]. Contrary to the doped samples, the PL spectrum of the undoped
8
sample is dominated by the UV band. The continuous intensity decay of this band with antimony ratio indicates the increase of the defects density in the ZnO band gap. According to many reports [45-48], the defects related to ZnO crystals can affect mainly its optical properties. Actually, Y. Zhang et al. [45] reported that Ag prevents electron-hole recombination which enhances the excitonic emission in Ag-doped ZnO prepared with sol-gel. On the other hand, H. Hong et al. [46] reported that intrinsically fluorescent ZnO nanowires can be adopted for molecularly targeted imaging of cancer cells. However, K. Qi et al. [47] founded that the blue emission collected from the ZnO prepared with solvothermal method, is related to their oxygen vacancies as well as other impurities and defects. Also, L. Irimpon et al. [48] synthesized colloids ZnO by modified polyol precipitation method. They founded a peculiar feature in the fluorescence spectra strongly correlated to the nanoparticles size distribution. Particularly, the UV band of the ZSb3 PL spectrum presents an obvious red-shift to 385 nm. This observation seems to be of a substantial interest in the PL description. This band seems to be directly correlated to the (SbZn–2VZn) complex acceptor level, apparently well manifested with the ZSb3 sample. In fact, Limpijumnong et al.[12] and Tian et al.[49] reported that this level is located at +0.16 eV above the valence band. Taking into account these considerations, the fact that Eg (ZnO)-0.16=3.19 eV, which is very close to the energy of the observed emission (3.22 eV) with the ZSb3 sample, confirms the attribution of this emission to the transition between the conduction band and this complex level as proposed in Fig.6.b. It is interesting to note that the red-shift undergone by the UV band is accompanied by a blue shift of the visible band, principally assigned to oxygen vacancies. This shift of the visible band to higher energies can be attributed to the multiplication of the Zni, energetically shallower than VO level [50,51], since the formation of such interstitial is facilitated by the presence of the complex (SbZn–2VZn) itself [49]. These two observed UV and visible shifts disappear when reaching 5% of antimony. Actually, this comportment will be more comprehensible when describing the SbZn–2VZn formation process. In fact, the presence of such complex is a result of coulombic binding between two oppositely charged defects [12]. During its occupation for the Zn sites, the Sb ions occurs a highly electronegative character while being surrounded by negatively charged oxygen ions. The particular geometry of the Wutzite structure allows these two negatively charged ions to approach each other when the oxygen is 5/3 length bonded away along the [0001] direction [49]. Then, this configuration must be assisted by the presence of two neighboring zinc vacancies [49]. The coulombic nature and the complexity of the described process make from the formation of (SbZn–2VZn) particularly sensitive to Sb content, this feature limits the antimony concentration range in which this complex is formed. Thus, we estimate that 5% of antimony is a sufficient amount to break down the described proceeding and as result the correspondent PL shifts disappear. 9
The PL intensity relative to the ZSb5 sample decreases drastically. This behavior may due to the excess of antimony probably localized at the ZnO surface. Furthermore, PL intensity is related to the PL emission centers density. Then we expect that this density is affected by this doping through the variation in the oxygen vacancies density in the material. These PL observations, coherently with the previous deductions extracted from XPS and UV-Vis measurements, highlights the adequacy of the Sb-doped ZnO for photocatalytic applications.
4. Evaluation of photocatalytic activity of Sb-doped ZnO with RhB RhB characteristic absorbance band located at 560 nm was used to monitor the photocatalytic activity of the Sb-doped ZnO NCs, by measuring, at different regular time intervals, the absorbance of an aqueous solution of RhB containing Sb-doped ZnO powder catalyst. The same experiment, realized without catalyst, reveals that is difficult to decompose the RhB under sunlight irradiation after 100 min (data was not shown), which indicate that the photoinduced selfsensitized photocatalysis of RhB without ZnO is negligible [9]. An obvious decrease in the intensity of the characteristic RhB absorbance peak was observed when introducing the ZnO catalyst. As indicated in Fig.7, the correspondent band decreases continuously during sunlight irradiation. To investigate the effect of the Sb doping on the photocatalytic efficiency of ZnO under sunlight irradiation, the photodegradation experiment was carried out using separately the ZSb1, ZSb3 and ZSb5 powders as catalysts. Fig.7.b illustrates the variation in the UV–Visible absorption spectra at different time intervals in the presence of ZSb3 sample. After 100 min of irradiation, the correspondent peak disappeared completely, indicating the total degradation of RhB with the presence of ZSb3 sample. The above observations reveal that the introducing of antimony enhances clearly the photocatalytic activity of the ZnO. In fact, during 100 min of sunlight irradiation, 98% of RhB was degradated in the presence of the ZSb3 sample while only 68 % of the dye was decomposed with the undoped ZnO during the same irradiation time. This faster photocatalytic activity, under sunlight irradiation, is attributed to the appearance of defects caused by Sb doping and leading to the enhancement of the visible absorption within the band gap. During 100 min of sunlight exposition, the photodegradation rate reaches 68%, 94%, 98%, and 95% with the undoped ZnO, ZSb1, ZSb3, and ZSb5 samples, respectively. This reveals that the ZSb3 sample presents the higher photocatalytic activity. The particularity of the ZSb3 sample was also observed in the previous experimental results and was attributed principally the formation of the (SbZn-2VZn) complex. Then, we suggest that the enhancement of the photocatalytic activity when introducing Sb can be strongly correlated to the appearance of this complex obviously expressed through the photoluminescence results. In fact, 10
electrons transitions from/to the induced (SbZn-2VZn) level seem to be strongly probable under solar excitation due to the low required energy assuring such transitions. Such attribution of the photocatalytic efficiency enhancement to internal band gap transitions is supported by the review works leaded by Kumar et al. [30], Pan et al. [52], Achouri et al. [53] and W. Yu et al .[54]. It is also important to note that the undoped prepared ZnO present already a photocatalytic activity revealing that it possess its own photocatalytic mechanism. Actually, oxygen vacancies and oxygen interstitials act as electrons and holes traps, respectively, then they play a key role in the photodegradation process by inhibiting the electron-hole recombination [30]. In fact, the large PL band centered at 556 nm (green) is consistent with the transition assisted by oxygen vacancies VO [43]. Once exposed to the solar irradiations, excited electrons are trapped by VO located on the ZnO surface [55] and react with O2- to yield superoxide .O2-. The photogenerated holes in the valence band react with OH- to give .OH radicals [30] (Fig.10). As mentioned previously, the photocatalysis mechanism of the Sb-doped samples will be certainly affected by the presence of the SbZn-2VZn complex level since this latter is especially well expressed with the ZSb3 sample that in turn present the better photocatalytic efficiency. Actually, the acceptor nature of this level and the induced p-type conductivity was proved within manifold reports [12,23]. This property can induce an increase of the carrier holes density in the valence band, which further activate the oxidation of hydroxyl radicals and as a result enhances the photocatalytic activity (Fig.10). The attribution of such important role to the complex (SbZn-2VZn) level in the photocatalysis process can be further evidenced when monitoring the intensity of the UV PL band. In fact, the reduction of the UV PL intensity compared to the visible one suggests a decrease in electron-hole radiative recombination, and then we expect an enhancement of the photocatalytic efficiency with Sb doping. The contrary was observed when passing from the ZSb3 to ZSb5. Actually, despite the drastically decay of the PL intensity with the 5% Sb-doped ZnO, the photocatalytic activity of this sample decreases compared to the ZSb3 sample for which the PL shift, attributed to formation of the (SbZn-2VZn) complex, was observed. This observation further evidences the important role of this complex in the photocatalytic mechanism. On the other hand, we expect that the oxygen vacancies kept its role as an electrons trap even for the Sb-doped sample. In fact, in addition to electrons excited directly from the valence band to this level, electrons already present in (SbZn-2VZn) derived level can also be trapped by VO once they are excited by visible irradiation and the transition required energy, estimated to be 2.24 eV, still in the visible range, as shown in figure 10. Then, the reduction of .O2- is also assured by electrons trapped in the VO level even the Sb-doped samples (Fig.10). In order to further confirm the validity of such proposed mechanism and to further highlight the predominant role of the .OH radicals, we precede to the introduction of scavengers species to the reaction system. In fact, ethanol (10mM) was added to the mixture during the irradiation in order to 11
quench the presence of hydroxyl radicals [56-58] and benzoquinone (BQ) (10mM) as .O2- scavenger [59,60]. As shown in figure 9, the ZSb3 was found to be less able to degrade the RhB with the presence of ethanol, contrarily to the experiment containing the BQ that practically do not show any affection by the presence of this .O2- scavenger. Actually, with ethanol, the photodegradation efficiency was found to decrease from 98.2 % to 60.3 %, whereas it reaches 92.12 % with BQ. These results confirm the validity of such proposed mechanism and the important role of the .OH radicals in the photocatalysis process in the presence of Sb-doped ZnO NCs. Under sunlight irradiation, a series of reactions between different entities present in the mixture is activated and can be described as follows: ZnO + hsunlight)ZnO (e-+ h+) H2O + h + O2 + e .
OH/.O2-+ Dye
H+ + .OH .
O2Oxidation products (CO2 + H2O etc…)
The photodegradation reactions can be classified as pseudo-first-order kinetic reactions [12]. From which reaction constants are determinate to be 3.87×10-2, 6.50×10-2, 7.26×10-2, and 6.68×10-2 min-1 for ZnO, ZSb1, ZSb3, and ZSb5, respectively. These values are higher than those reported for Ndoped ZnO [60] and Mg-doped ZnO [61]. This result suggests that Sb-doped ZnO NCs can be used as a potential photocatalyst under sunlight irradiation. It is important to note that the initial pH is an important factor to take into consideration due to its effect on surface charge properties of the semiconductor. For such reason, the effect of initial pH on the photocatalysis efficiency was studied by taking the same experiments under different initial pH values varying between 3 and 12. As shown in figure 11, the photodegradation activity was effectively affected by the initial pH value. Actually, either with the undoped or the Sb-doped ZnO, an acidic medium result in an obvious decrease of this activity contrarily to the basic medium with which the photocatalytic efficiency is very slightly affected (Fig. 11). In fact, the acidity enhances the solubility of the nanostructure in the concerned solution [62] which lowers the stability of the system and induces a decrease in the catalytic reaction activity [15]. As proposed by A. Omidi et al. [15], the adsorption rate of the dye molecule at the surface of the NCs is directly affected by the pH value and this through the electrostatic attraction or repulsion forces that depend on charge difference between the nanoparticle surface and the dye. Actually, under zero point charge pH, estimated to be about pH=9 for nanostructures [63], surface of the photocatalyst will be positively charged similarly to the RhB molecule that will acquire a positive charge under these conditions due to the reaction of its Cl ions with the H+ protons. This configuration promotes the repulsion between the RhB molecules and 12
the surface of ZnO, which decreases the adsorption rate and then quench the photocatalytic activity under acidic conditions. The recycled photocatalytic experiment was also performed under sunlight irradiation (Fig.12). After every cycle, a micro-volume of a very concentrated aqueous solution of RhB was added to the above solution to turn back to the initial concentration of RhB (5.10-6 M). After the first cycle, there was a slight decrease in the photocatalytic activity that may be related to the loss of a small amount of ZnO during the recovery process, then the activity remains stable during the others runs. The photocatalytic activity of ZSb3 is unchangeable after four runs. To further confirm the stability of the elaborated Sb-doped ZnO, the NCs powder was recovered from the reaction mixture, dried and characterized again with XRD measurements. The XRD diffractograms obtained after 4 successive cycles of photocatalytic reaction presents the same peaks as the patterns in figure S.2. These results confirm the structural stability of the hexagonal Sb-doped ZnO used as catalyst in aqueous solution. All the presented results suggest that the ZSb3 catalyst presents a high stability and reusability during the photodegradation of RhB under sunlight irradiation.
Conclusion Pure crystalline phase of Sb-doped ZnO NCs was synthesized by a facile and economical sol gel method. XRD data revealed the formation of Sb-doped ZnO NCs crystallized in Wurtzite structure without any secondary phases.TEM and HRTEM results showed that the nanocrystals have an average size of about 50 nm with a spherical like morphology. X-ray photoelectron spectroscopic analysis confirmed that Sb3+ states were more preferable in comparison to Sb5+ states, which is favorable for the formation of the (SbZn-2VZn) complex in Sb-doped ZnO NCs. The decrease of the band gap energy with Sb doping can be ascribed to the apparition of energy states caused by antimony doping. The formation of the (SbZn-2VZn) complex was also proved by the PL measurements that revealed the highest expression of the correspondent level for the ZSb3 sample. Accordingly, the photocatalytic experiments revealed that the sample ZSb3 shows the highest photocatalytic activity. The key role of the (SbZn-2VZn) complex in the enhancement of the photocatalytic efficiency was demonstrated through a detailed study of the photodegradation mechanism that highlights the contribution of this complex in the oxidation of hydroxyl radicals. Oxygen vacancies play also a principal role in inhibiting the radiative recombination by trapping the excited electrons. The domination of the hydroxyl radicals in the photocatalytic mechanism was proved experimentally. The reusability of the ZSb3 catalyst was also evaluated and reveals its high stability and their good performances in photocatalysis.
13
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16
Figures
Figure 1: XRD patterns of ZnO:Sb NCs with different doping concentrations. Figure 2: (a) TEM images of undoped ZnO (b) ZSb3 NCs. (c) Selected area electron diffraction pattern of ZSb3 NCs. (d) HRTEM image of the ZSb3 sample. (e) EDAX spectrum of ZSb3. Figure 3: (a) XPS spectra of the Zn-2p, O-1s and Sb-3d core level regions for ZSb3 samples. (b) Fitted XPS spectra of the O-1s and (c) Sb-3d3/2 core level regions. Figure 4: Raman spectra of Sb-doped ZnO NCs excited with HeCd laser (exc = 632 nm). Figure 5: (a) Diffuse reflectance spectra of Sb doped ZnO nanocrystals (b) The plots of (h)2 vs. photon energy of undoped ZnO and ZSb3 NCs. Figure 6: (a) PL spectra of Sb-doped ZnO excited with 325 nm for different Sb concentrations (0, 1, 3 and 5% Sb). (b) Proposed photoluminescence mechanism for Sb-doped ZnO. Figure 7: Evolution of the RhB absorption spectra in the presence of (a) ZnO NCs and (b) ZSb3 with sunlight irradiation time. Figure 8: (a) Degradation rate of the RhB with the presence of different Sb-doped ZnO under sunlight irradiation (b) Kinetic curve of photocatalytic activity in the presence of ZnO and ZnOSb (3%). Figure 9: Effects of scavengers on the photocatalytic efficiency of ZSb3 NCs. Figure 10: Proposed photocatalytic mechanism for Sb-doped ZnO under sunlight irradiation. Figure 11: Photodegradation of RhB on ZnO and ZSb3 nanocrystals at various pH values. Figure 12: Cycling photocatalysis experiment in the presence of ZSb3 NCs under sunlight irradiation.
17
200 112 201
103
110
102
100 002 101
Intensity (a.u.)
ZSb5
ZSb3
ZSb1
ZnO 20
30
40
50
60
70
2 Theta(°)
Fig. 1
18
a
b
c
d 19
d=0.26 nm
e
Fig. 2
20
aZn 2p
1.6
3/2
Zn 2p1/2
1.2
1010
1020
1030
0.9
O1s
0.8
1040
1050 1060 Binding Energy (eV)
Sb 3d5/2
0.7 0.6
Sb metal
Sb 3d3/2
0.5 520
525
0.4
530
535
540
545 550 555 Binding Energy (eV)
b O 1s Sb 3d5/2
Fit
0.3
Intensity (a.u.)
Intensity (a.u.)
0.8
0.2
0.1
0.0 524
526
528
530
532
534
536
Bindig energy (eV)
21
c 0.41
Sb 3d3/2 Fit
0.40 3+
Intensity (a.u.)
Sb 0.39 0.38
5+
Sb
0.37 0.36 0.35 538
539
540
541
542
543
544
Binding Energy (eV)
Fig. 3
22
Raman intensity (a.u.)
E2(high) -E2(low)
E2(high)
LVM(Sb-O-Sb)
5% Sb 3% Sb
A1LO
A1TO
1% Sb
0% Sb
100
200
300
400
500
600
700
wavenumber (cm-1)
Fig. 4
23
800
(a)
Diffuse Reflectance (a. u.)
ZnO ZSb1 ZSb3 ZSb5
A b s o r b n a c e (a .u .)
0 .6 0 .5
ZnO ZSb1 ZSb3 ZSb5
0 .4 0 .3 0 .2 0 .1 0 .0 200
300
400
500
600
700
800
W a v e le n g th (n m )
200
300
400
500
600
700
800
Wavelength (nm)
16
ZnO ZSb3
14
(b)
(h)
2
12 10 8 6 4 2 0 2.0
2.4
2.8
3.2
3.6
h (eV)
Fig. 5
24
1000
(a)
ZnO ZSb1 ZSb3 ZSb5
PL intensity (a.u.)
800
600
400
200
0 300
350
400
450
500
550
600
650
700
750
Wavelength (nm)
(b)
CB VO 3.32 eV
3.19 eV vio
2.24 eV
Green
Complex SbZn-2VZn +0.16 eV
VB Fig. 6
25
(a)
Absorbance (a.u.)
ZnO 0 min 20min 40min 60min 80min 100min
400
450
500
550
600
650
700
Wavelength (nm)
Absorbance (a.u.)
(b)
ZSb3 0 min 20 min 40 min 60 min 80 min 100 min
400
450
500
550
600
650
Wavelength (nm) .
Fig. 7
26
700
100
(a)
Degradation rate (%)
80
60
40
ZnO ZSb1 ZSb3 ZSb5
20
0 0
20
40
60
80
100
Time (min)
ln(A0/A)
(b)
ZnO ZSb3
10
1
0
20
40
60
80
100
Time (min)
Fig. 8
27
120
Degradation rate (%)
100
80
60
40
20
0
No scavenger
Ethanol
BQ
Fig. 9
28
CB O2
e-e- e- e- e- e-
VO
. 2.24 eV
O -2
RhB
Green
Complex acceptor level (SbZn-2VZn) 0.16 eV
CO2 +H2O + hOH
h+ h+ h + h + h + h +
VB
.
+ hH 2O
h+
Fig. 10
h+ h
+
h+
h+
29
Degradation rate (%)
100
pH3 ZSb3 pH5 pH7 pH9 pH12
80
60
40
20
0 0
20
40
60
80
100
80
100
Time (min)
Degradation rate (%)
80
pH3 ZnO pH7 pH12
60
40
20
0
0
20
40
60
Time (min)
Fig. 11
30
1.2
ZSb3
1.0
A/A0
0.8 0.6 0.4 0.2 0.0 0
100
200
300
400
500
Time (min)
Fig. 12
31
Table 1: crystalline size of Sb doped ZnO nanocrystals. Samples ZnO ZSb1 ZSb3 ZSb5
Grain size(nm) 54.3 51.9 47.1 52.7
a(nm) 0.321 0.324 0.327 0.331
32
c(nm) 0.524 0.527 0.528 0.530
Table 2: Band gap energy of Sb doped ZnO samples.
Samples
ZnO
ZSb1
ZSb3
ZSb5
Eg(eV)
3.35
3.25
3.14
3.18
33