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ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 8 ) 1 e1 0
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ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production lito a, M.R. Alfaro Cruz a, O. Ceballos-Sanchez b, E. Luevano-Hipo L.M. Torres-Martı´nez c,* a CONACYT - Universidad Autonoma de Nuevo Leon, Facultad de Ingenierı´a Civil-Departamento de Ecomateriales y s de los Garza, NL, Mexico Energı´a, Cd. Universitaria, C.P. 66455 San Nicola b Departamento de Ingenierı´a de Proyectos, CUCEI, Universidad de Guadalajara, Zapopan, Jalisco 45100, Mexico c Universidad Aut onoma de Nuevo Leon, Facultad de Ingenierı´a Civil-Departamento de Ecomateriales y Energı´a, Cd. s de los Garza, NL, Mexico Universitaria, C.P. 66455 San Nicola
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abstract
Article history:
This work studied the effect of different annealing conditions of ZnO thin films grown by
Received 23 January 2018
RF magnetron sputtering and their application as photocatalysts for hydrogen production
Received in revised form
without any sacrificial agent or co-catalyst. ZnO films were annealed in air, nitrogen, and
19 March 2018
argon atmospheres to study the effect of their physical properties in the photocatalytic
Accepted 7 April 2018
activity. ZnO films showed high crystallinity and optical transparence of around 75e90%
Available online xxx
after annealing. Changes in composition and optical properties of the ZnO films were studied by x-ray photoelectron spectroscopy (XPS) and ellipsometry spectroscopy (SE), and
Keywords:
results were correlated with the photocatalytic performance in hydrogen production. The
ZnO thin films
highest photocatalytic hydrogen production was obtained with the ZnO thin film annealed
RF sputtering
in an air atmosphere with a result of 76 mmol.
Hydrogen production
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Photocatalysis
Introduction Currently, the generation of clean and renewable energy from sustainable energy sources such as sunlight is a fundamental topic to avoid the emission of greenhouse gases to the atmosphere. The hydrogen generation from water splitting by photocatalysis processes has become very important because it is an alternative energy vector to replace fossil fuels. In order to produce this energy vector, several semiconductors
materials have been proposed. In particular, TiO2 powder is the most used material as a photocatalyst, mainly in dye degradation. However, there is also literature where TiO2 is used for hydrogen production in powder form or as photoanode in photo-electrochemical water splitting [1e4]. Additionally, the use of tantalates, titanates, and niobates as highly active photocatalysts have been reported; even though their band gaps are too large to absorb visible light, their crystal structure influences positively on the photocatalytic activity in a water-splitting activity reaction [5e8]. Alternative
* Corresponding author. vano-Hipo lito). E-mail addresses: [email protected], [email protected] (E. Lue https://doi.org/10.1016/j.ijhydene.2018.04.054 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054
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photocatalyst based in heterostructures containing TiO2, ZnO, CuO, and CdS have also been used to enhance their photocatalytic efficiency in hydrogen evolution reaction [9e11]. In general, the H2 generation via photocatalysis involves the use of a powder semiconductor to carry out the oxidationreduction process of the water molecule. However, one of the main disadvantages of using semiconductor particles in suspension is their separation and recovery of the liquid phase. In addition, suspensions are associated with a low quantum yield as a consequence of a high recombination speed of the hole-electron pair [12]. However, the use of thin film as a photocatalyst has numerous advantages such as rapid electron transport and homogeneous exposure of the photocatalyst, and it is possible to deposit the photocatalyst over different substrates, efficient absorption of solar energy, and high surface area. ZnO thin films modified by nonmetal and metal elements have been widely studied primarily for dyes degradation [13e18] and for H2 generation. For instance, ZnO thin films deposited by chemical vapor deposition doped with Ag can produce until 0.5 mmol cm2 h1, while the use of Na2S and Na2SO3 as sacrificial agents in ZnO films doped with CeO2 helps to improve the H2 production of the films. The photoelectrochemical H2 production has been reported as well for ZnO thin films impregnated with CueIneZneS with a yield of 0.7 mmol cm2 h1 [19e21]. Some authors have reported the use of different atmospheres to perform heat treatment to ZnO thin films after deposition, with the purpose of modifying his optical or electrical properties, modifying the surface, and increasing the conductivity of ZnO thin films [22e24]. ZnO crystal structure gives rise to polar symmetry along the (002) plane in the hexagonal axis which is a key factor in crystal growth and defect generation, which include oxygen vacancies and zinc interstitial [25]. So, one way to improve ZnO properties in a thin film is to employ different thermal treatments under specific atmospheres, for example, when the thermal treatment is carried out in oxygen poor atmospheres, such as Ar, Nitrogen, etc., which helps to avoid the defects as oxygen vacancies and Zn interstitial [26]. The novelty of this paper is to determinate how concentration of defects, as oxygen vacancies and interstitial zinc, affect the photocatalytic efficiency in ZnO, especially when ZnO is deposited as a thin film. In this work, we propose the use of the ZnO thin films obtained by RF magnetron sputtering as a photocatalyst in H2 generation from water splitting. Where, the absence of oxygen inside the sputtering chamber during deposition, contributes to the presence of defects, as oxygen deficiencies and interstitial zinc. So, the ZnO films were annealed in different atmospheres (air, nitrogen, argon) in order to assessing the structural, optical, and surface properties with respect to its photocatalytic activity in the hydrogen production without using sacrifice agents or co-catalysts.
Experimental Growth of ZnO thin films by RF-Sputtering ZnO thin films were deposited by RF magnetron sputtering over glass substrates using a ZnO target (99.999% Pure, 2.0000
diameter 0.25000 thick; Kurt J. Lesker). Glass substrates were ultrasonically cleaned in acetone, isopropyl alcohol, and deionized water and then dried with air. The vacuum pressure of the chamber was lowered to 8.6 106 Torr using a turbomolecular pump before introducing argon gas. Working pressure was fixed at 1.6 102 mTorr with an argon flow rate of 15 sccm. Substrates were heated at 300 C and deposited at a constant power of 80 W for 2 h; distance between the target and substrate was 8.5 cm. Before deposition, the target was pre-sputtered for 10 min. After the deposit process, ZnO thin films were annealed in argon, nitrogen, and air atmospheres at 400 C for 1 h [27,28], in order to evaluate the photocatalytic activity for hydrogen production, usually the choice of this temperature depends on the type of substrate and their melting fusion [29,30].
Characterization Structural properties were determinate using an X-ray diffractometer PANalytical with a Cu Ka radiation of 1.54 Å in grazing angle. Transmittance measurements were performed using a UVeVis NIR spectrophotometer (Cary 5000). Spectroscopic ellipsometry measurements were collected using a Horiba, Jobin Yvon UVISEL HR 320 ellipsometer at an incident angle of 70 . The morphology of the samples was analyzed by scanning electron microscopy (SEM), using a FEI Nova NanoSEM 200 microscope with an accelerating voltage of 30 kV. The chemical states and elemental composition were determined by X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific, Escalab 250Xi, equipped with an Al Ka monochromatic x-ray source (hv ¼ 1486.7 eV) with a line width of 0.20 eV in an analysis chamber at a bass pressure of <4.3 1010 mbar.
Photocatalytic activity Photocatalytic activity of ZnO thin films (15 cm2) were evaluated using a cylindrical Pyrex batch reactor of 200 mL at room temperature. Thin films were fixed inside the reactor and then filled with nitrogen to remove the oxygen in the medium. Once the oxygen was removed from the reactor, thin films were irradiated using an UV Pen Ray Lamp of 254 nm of irradiance of 4400 mW cm2. Photocatalytic hydrogen production was measured every 30 min using a gas chromatograph Thermo Scientific trace GC Ultra with a thermal conductivity detector (TCD).
Results Structural properties XRD patterns of ZnO thin films deposited, annealed at 400 C in air, nitrogen, and argon atmospheres, are shown in Fig. 1. All thin films are polycrystalline and peaks correspond to ZnO Wurtzite phase according to the JCPDS card 01-089-1397. ZnO thin films deposited have poor crystallinity compared to those heat treated. When thin films were annealed in different atmospheres the crystallinity improved, with the thin film annealed in argon atmosphere being the one that exhibited
Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054
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Fig. 1 e a) The XRD pattern of ZnO thin films deposited and annealed in different atmospheres, b) crystallinity percentage of each film.
the highest crystallinity. All thin films had a preferential orientation over the (002) plane, corresponding to c-axis in the hexagonal structure of the Wurtzite phase. Grain size of the thin films was calculated using the Scherrer equation [17]: D¼
0:9l ; bcosq
(1)
where l is the X-ray wavelength, q is the Bragg's diffraction angle and b is the full width at half maximum (FWHM) of the peak corresponding to the (002) plane. Thin films deposited had a grain size value of 9.54 nm. After annealing in different atmospheres, grain size values were similar for air, nitrogen, and argond20.8, 21.1, and 22.9 nm, respectively (Table 1). It is observed that the grain size increase after annealing, because heat treatments causes the recrystallization of distorted lattices in the bulk [31], so that, the grains are distributed by supplying enough thermal energy, and the small grains are joined to larger ones [22], reducing grain boundaries [32]. The preferred orientation, or textured, is defined as a condition in which the distribution of crystal orientations is nonrandom, when the films are annealed a deformation texture occurs, due to the recrystallization of the material [33]. To study the preferred orientation and growth of the (h k l) planes in each thin film, Eq. (2) was used, defined by Barret and Massaslki [34] for texture coefficient: Tcðh
Iðh ICðh
k lÞ
k lÞ
¼ h P I ðh 1 n
k lÞ
ICðh
k lÞ
i
diffraction peaks considered. A value of texture coefficient greater than the unit indicates that there is a great amount of grains with preference in direction [h k l]; a texture coefficient value equal to the unit means that the grains are randomly oriented as the reference card [35]. Table 1 demonstrates texture coefficient values for each thin film. Values exceeding the unit correspond to [0 0 2] direction in all thin films, confirming a growth along c-axis, perpendicular to the substrate. It was observed that the thin film annealed in the argon atmosphere had the largest value of texture coefficient, as well as the highest crystallinity. To know how much the lattice constant has been modified after heat-treated atmospheres, the lattice constant c [36] was calculated: c¼
l sinq
(3)
where q is the diffraction angle corresponding to the (002) plane. Thin films deposited and annealed in air have similar values of the c parameter (5.1923 and 5.1929 Å, respectively). This value decreased when the films were annealed in reducing atmospheres of nitrogen and argon to 5.1873 and 5.1855 Å, respectively (see Table 1). Generally, thin film lattice constant changes are attributed to different types of strain, like dislocations, stacking faults, grain boundary, etc. [37]. Mostly, thin films deposited by the sputtering process have compressive stresses due to the presence of interstitials or implanted argon in the growing film [38].
(2)
Scanning electronic microscopy (SEM)
k lÞ
where I (h k l) are XRD intensities obtained for thin films deposited by sputtering, I C (h k l) are the XRD intensities for the standard pattern (01-089-1397 card) and n is the number of
Fig. 2 shows the SEM images of ZnO thin films deposited and films annealed in air, nitrogen, and argon atmospheres. The
Table 1 e Texture coefficient Tc of [h k l] planes for each thin film. Textured coefficient TC of (h k l) planes
ZnO (01-089-1397) Deposited Air Nitrogen Argon
Grain size (nm)
c (Ǻ)
TC(002)
TC(101)
TC(102)
TC(100)
TC(103)
TC(112)
9.54 20.88 21.11 22.91
5.233 5.1923 5.1929 5.1873 5.1855
3.11 3.52 3.36 4.18
0.36 0.76 0.80 0.57
0.83 0.76 0.80 0.57
0.33 0.22 0.15 0.05
0.76 0.74 0.84 0.82
0.57 0.48 0.63 0.27
Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054
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Fig. 2 e SEM image for the surface of ZnO thin film (a) deposited and annealed in (b) air, (c) nitrogen, and (d) argon atmosphere.
SEM images revealed for each film that the surface is totally covered with a morphology of irregular grains lower than 500 nm where, for deposited thin film, it is observed that the grains are smaller than the grains in the other films. Thin films annealed in air and nitrogen atmospheres (Fig. 2 (b) and (c)) have a similar surface, with grains larger than those observed in the deposited film. On the other hand, film annealed in an argon atmosphere had a dense surface with more compact and larger grains (Fig. 2(d)). The increase of the grain size imply a better distribution and grain orientation after annealing [22]. In contrast, the deposited thin film has irregular grains which, according to the XRD results, has a grain size smaller than the grain sizes of the other films (Table 1).
XPS Fig. 3 shows the XPS spectra for Zn 2p3/2, O 1s, and C 1s core levels for the deposited and thin films annealed in different atmospheres. The XPS analysis was performed using the Voigt function enclosed in the AAnalyzer Software [39]. A Shirley background was used to fit the Zn 2p and O 1s spectra. For the deposited thin film, the Zn 2p3/2 region was fitted using two singlets peaks centered at 1021.15 eV and 1022.0 eV, respectively. The first peak was associated to zinc in stoichiometric ZnO and the second one was associated to Zn2þ ions in oxygen-deficient regions. For the O 1s region, three singlets peaks were used to fit this region, which are centered at 529.8 eV, 530.48 eV and 531.2 eV, respectively. The first peak
positioned at lower binding energy was associated to O2 ions on the wurtzite structure, while the second one positioned at higher binding energy was attributed to a contribution of O2 ions that are in oxygen-deficient regions within the ZnO film. Finally, the peak at about 531.72 eV, was associated to chemisorbed or absorbed species on the sample surface, such as CeO bond, hydroxyl groups (eOH) or even adsorbed O2. It has been reported that intrinsic defects, as zinc interstitial ) and oxygen vacancies (VOþþ ), are electrically atoms (Znþþ i active and these species can induce localized states close to the conduction band. Both species can be act as donor. For the case of C 1s spectra, typical chemical components associated to adventitious carbon were observed (supporting information), which were attributed to CeC (284.8 eV), CeO (285.9 eV), and CO3 (288.4 eV) [40]. It is noted that all the spectra were aligned to C 1s centered at 284.8 eV. This allows for subtle changes in the binding energies of the measured spectra to be observed. When ZnO films were annealed in different atmospheres, it was possible to observe slight shifts on the binding energy of the Zn 2p3/2 and O 1s regions. This phenomenon was more evident after annealing in nitrogen and argon atmospheres, where a downward energy shift on the binding energy of around 0.6 eV was observed on the Zn 2p3/2 spectrum of both samples. The O 1s spectrum of the film annealed in the argon atmosphere showed an energy shift to a lower binding energy of 0.24 eV. For the film annealed in nitrogen, the O 1s spectrum shifted upward 0.14 eV to a higher binding energy. This energy shift could be related with changes in the
Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054
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Fig. 3 e XPS spectra of the Zn 2p3/2 and O 1s core levels for the ZnO films deposited and annealed in air, argon, and nitrogen atmospheres.
electronic density of the ZnO as consequence of the alteration on the concentration of intrinsic defects or superficial charge. A summary of XPS analysis is shown in Table 2. The semiquantitative analysis shows that there are oxygen deficiencies in the ZnO structure for all films. It can be observed that the quantity of oxygen deficiencies of ZnO thin film annealed in an air atmosphere is higher compared to other films. When ZnO is annealed using an oxygen-rich atmosphere, the
Table 2 e Atomic concentration of oxygen and zinc calculated from XPS spectra for as deposited and annealed at different atmospheres films. Sample Deposited Air Nitrogen Argon
O1 (%)
O2 (%)
Z1 (%)
Z2 (%)
O1/Z1
25.25 25.66 25.91 17.10
16.78 15.24 16.48 18.14
27.73 27.60 29.00 16.62
4.57 9.95 5.66 4.18
0.91 0.93 0.89 1.00
Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054
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as a consequence of its low formation energy in n-type ZnO [42,43]. However, the migration of Zn through the ZnO is more likely, since the energy barrier for the Zn migration in ZnO (0.57 eV) is lower than the energy barrier for the oxygen migration in ZnO (1.7 eV) [42]. This results are consistent with those observed from the XRD and XPS results (see Fig. 1). ZnO thin film annealed in Ar atmosphere has a poor photocatalytic efficiency, because this film has a less concentration of intrinsic defects.
Optical properties
Fig. 4 e Optical transmittance spectra of deposited and annealed in air, nitrogen, and argon atmospheres films.
Fig. 5 e SE spectra representative of the ZnO thin film annealed in air atmosphere.
formation energy de oxygen vacancies and zinc interstitial increases, which limits the formation of a ZnO crystal structure free of intrinsic defects [41] resulting in a low crystallinity. When thin films are annealed in an oxygen-poor atmosphere it promotes the mobility of the oxygen ions and interstitial Zn,
Optical transmittance spectra of ZnO thin films were measured at a wavelength range of 360e1000 nm, as shown in Fig. 4. In general, optical transmittance of ZnO thin films is around 75e90% in the UVeVisible spectrum. The absence of optical interference fringes in the spectrum indicates that the ZnO films are thin. To determine the energy band gap (Eg), optical constants (refractive index (n) and extinction coefficient (k)), superficial roughness, and the ZnO thin film thickness, spectroscopy ellipsometry (SE) were used. These measurements were performed using ellipsometry equipment with a fixed incident angle of the beam of 70 . The dependence of the ellipsometric angles (j and D) against the beam energy was collected in the energy range of 1.5e4.7 eV (or ~264e827 nm), with energy steps of 0.05 eV. These ellipsometric angles determine the change in the light polarization when it interacts with the material. Tauc-Lorentz dispersion model [44] with two oscillators and Bruggeman's effective medium approximation (EMA) [45] were employed to describe the optical response of the three-phase model (air/ZnO/glass substrate). Fig. 5 shows typical spectra for J and D as a function of the wavelength for thin film annealed in air atmosphere (dots). Additionally, the best fit (solid lines) obtained from the modeling of the air/ZnO/ glass structure is presented. It is possible to observe how the theoretical model proposed closely reproduces the optical response of the ZnO thin film over a glass substrate. The rest of the ZnO thin films display a similar behavior to this thin film. Fig. 6 (aeb) show the behavior of refractive index (n) and extinction coefficient (k) for ZnO thin films. The SE results show slight differences in optical properties of the films. Both
Fig. 6 e (a) Refractive index (n) and (b) extinction coefficient (k) of the ZnO thin film annealed in air atmosphere. Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054
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Table 3 e Summary of parameters obtained from SE analysis for the ZnO films. Sample Deposited Air Nitrogen Argon
Thickness (nm)
Roughness (nm)
SE-Eg (eV)
91.3 118.6 124.3 129.8
10.0 26.6 42.8 32.9
3.13 3.10 3.08 3.06
7
Variations on the optical constants in ZnO thin films are related to changes in the grain size, as it was observed in XRD results (see Table 1). When crystallinity of the ZnO film change, after being subjected to a heat treatment, the absorption peaks associated to electronic transitions tending to be less broad as the crystallinity increases, producing changes in optical properties [48]. So, in this work, thin film annealed in Ar atmosphere showed the highest crystallinity, as well as a higher intensity on the extinction coefficient (Fig. 6-b). Since ZnO is transparent, it is possible to observe that the extinction coefficient tends to zero for wavelengths greater that 400 nm (the related optical transmittance spectra is shown in Fig. 4). A summary of SE results is presented in Table 3. From these results, it is possible to identify that ZnO thin film annealed in a nitrogen atmosphere presented a higher superficial roughness compared to the rest of the films. . It should be noted that energy band gap obtained by sputtering is slight smaller than ZnO-bulk (Eg ¼ 3.36 eV). This can be attributed to the presence of intrinsic defects in the ZnO thin film.
Photocatalytic hydrogen production
Fig. 7 e Photocatalytic H2 production of ZnO thin films considering exposed active surface.
n and k increase as wavelength increases from 263 to 375 nm, while for wavelengths greater than 375 nm these parameters tend to decrease. These changes in the optical properties can be associated with variations in the crystal structure and superficial morphology of the ZnO films [46,47].
Photocatalytic H2 production was investigated in a Batch reactor containing 200 mL of deionized water under a nitrogen atmosphere at 25 C. After 3 h of exposure to ultraviolet light, thin film deposited showed a hydrogen production of 46 mmol of H2, while for thin films annealed at 400 C in air, nitrogen, and argon atmospheres, photocatalytic hydrogen productions were 76, 71, and 29 mmol of H2, respectively (Fig. 7). The ZnO thin films annealed in air showed the greatest hydrogen production, which can be associated with the highest oxygen deficiencies percentage, according to the XPS results (Table 2).
Fig. 8 e Mechanism of ZnO photoactivation to exemplify how intrinsic defects improve the photocatalytic activity. Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054
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From XPS spectra, it was possible to confirm the presence of defects related to oxygen vacancies (VOþþ ) and zinc interstitial (Znþþ i ), which have been reported that are electrically active and can induce localized state within the energy band gap [41]. Thus, based on the photocatalytic activity results and the physical properties obtained, it is proposed the reaction mechanism shown in Fig. 8. The mechanism of conventional photoactivation of ZnO, involves its excitation with an irradiation energy higher than its band gap value (hv Eg) to promote the transfer of the electrons (e) in valence band (VB) to the conduction band (CB) (Eqs. (5)e(7)). This process is accompanied by a recombination step, which limit the photocatalytic activity of ZnO. Different authors reports some strategies to delays this process, one of those strategies is related to the generation of intrinsic defects such as VOþþ and in the wurtzite structure. As mentioned earlier, the Znþþ i presence of these intrinsic defects has been related to the formation of inter-band gap states that depending of its relative position and can act as electron and hole donors, which eventually delays the recombination of the electron and hole pair. In particular, since the position of VOþþ is 1.16 eV above the CB of ZnO, this defect can capture holes to promote a higher number of electrons available in the medium to react with protons and produce H2 (Eq. (8)) [49]. The holes that have been donated to the medium, can oxidize H2O to produce O2 and Hþ (which react with electrons to form H2). Additionally, the zinc interstitial can act as electron acceptor due to its favorable potential just 0.22 eV above the CB of ZnO (Eq. (9)). Thus, the presence of these both type of intrinsic defects promote a higher efficiency of this application through the simultaneous oxidation and reduction of H2O to produce H2 at neutral pH, without the use of sacrificial agents or co-catalyst. hv þ ZnO ! ZnO e CB þ ZnO hVB
(5)
1 þ H2 OðadsÞ þ 2hþ VB /2H þ O2 [ 2
(6)
2Hþ þ 2e CB /H2 [
(7)
V0þþ 4V0þ þ hþ VB
(8)
þ þ Znþþ i 4Zni þ eCB
(9)
Additionally, we tested the reusability of the ZnO thin films by means of their second evaluation (Fig. 9), where photocatalytic H2 production decreased in all of the samples (more than a half of its initial value) and where thin film annealed in an air atmosphere had a lower reduction in their photocatalytic activity. This reduction can be related to a photocorrosion of ZnO, a phenomenon that has been reported in other works [50e52]. Yield hydrogen in mmol cm2 h1 for each film is attached in supporting information.
Conclusions ZnO thin films deposited by RF sputtering and annealed in different atmospheres were assessed for photocatalytic hydrogen production without using a sacrificial agent or cocatalyst. Thin films annealed in air and nitrogen atmospheres presented the best activities which were associated with similar crystallinity and high oxygen deficiency. It was observed that photocatalytic activity decreased 3.5 times when the film thickness was higher than 125 nm. A direct correlation among the oxygen deficiency percentage and the photocatalytic hydrogen production was also observed, since oxygen deficiencies act as active sites favoring the yield of this reaction. In addition, it was discovered that a deactivation of the samples after a second evaluation was probably due to photocorrosion phenomena, a typical problem when ZnO is used for photocatalytic applications.
Acknowledgments The authors are grateful to CONACYT for their financial sup tedras-CONACYT 363 port through the following projects: Ca and 1060; INFRA-2015-2753; CB-2014-237049; PDCPN-2015-487; and UANL (PAYCIT 2017). We appreciate the support given by Dr. Eduardo Martı´nez Guerra, Dr. Francisco Enrique Longoria Rodriguez, and M. C. Luis Gerardo Silva Vidaurri from CIMAVMonterrey, and Dr. Marco Antonio Garza Navarro from FIME, UANL.
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.04.054.
references
Fig. 9 e Study of the reusability of the ZnO thin films as photocatalyst for H2 production.
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[3] Ferreira de Britoa Juliana, Tavella Francesco, Genovese Chiara, Ampelli Claudio, Zanoni Maria Valnice Boldrin, Centi Gabriele, et al. Role of CuO in the modification of the photocatalytic water splitting behavior of TiO2 nanotube thin films. Appl Catal B Environ 2018;224:136e45. [4] Huang Chao-Wei, Liao Chi-Hung, Wu Jeffrey CS, Liu YuChang, Chang Chun-Ling, Wu Chih-Hung, et al. Hydrogen generation from photocatalytic water splitting over TiO2 thin film prepared by electron beam-induced deposition. Int J Hydrogen Energy 2010;35:12005e10. [5] Kato H, Kudo A. Water splitting into H2 and O2 on alkali tantalate photocatalysts ATaO3 (a ¼ Li, Na, and K). J Phys Chem B 2001;105:4285. [6] Machida M, Murakami S, Kijima T. Photocatalytic property and electronic structure of lanthanide tantalates, LnTaO4 (Ln) La, Ce, Pr, Nd, and Sm). J Phys Chem B 2001;105:3289. [7] Hisatomi Takashi, Kubota Jun, Domen Kazunari. Recent advances in semiconductors for photocatalytic and photoelectrochemical water splitting. Chem Soc Rev 2014;43:7520. [8] Maeda Kazuhiko. Photocatalytic water splitting using semiconductor particles: history and recent developments. J Photochem Photobiol C 2011;12:237e68. [9] Li Zhonghua, Liu Jiawen, Wang Dongjun, Gao Yuan, Shen Jun. Cu2O/Cu/TiO2 nanotube Ohmic heterojunction arrays with enhanced photocatalytic hydrogen production activity. Int J Hydrog Energy 2012;37:6431e7. [10] Hussein AM, Shende RV. Enhanced hydrogen generation using ZrO2-modified coupled ZnO/TiO2 nanocomposites in the absence of noble metal co-catalyst. Int J Hydrogen Energy 2014;39:5557e68. [11] Wang Z, Zhang H, Cao H, Wang L, Wan Z, Hao Y, et al. Facile preparation of ZnS/CdS core/shell nanotubes and their enhanced photocatalytic performance. Int J Hydrog Energy 2017;42:6431e7. [12] Ding Z, Lu GQ, Greenfield PF. A kinetic study on photocatalytic oxidation of phenol in water by silica-dispersed titania nanoparticles. J Colloid Interface Sci 2000;232:1e9. [13] Siuleiman Shahin, Kaneva Nina, Bojinova Assya, Papazova Karolina, Apostolov Anton, Dimitrov Dimitre. Photodegradation of orange II by ZnO and TiO2 powders and nanowire ZnO and ZnO/TiO2 thin films. Colloid Surface A 2014;460:408e13. mez MA, [14] Ahumada-Lazo R, Torres-Martı´nez LM, Ruı´z-Go Vega-Becerra OE, Figueroa-Torres MZ. Photocatalytic efficiency of reusable ZnO thin films deposited by sputtering technique. Appl Surf Sci 2014;322:35e40. [15] Poongodi G, Anandan P, Mohan Kumar R, Jayavel R. Studies on visible light photocatalytic and antibacterial activities of nanostructured cobalt doped ZnO thin films prepared by solegel spin coating method. Spectrochim Acta A 2015;148:237e43. € nmezoglu Savas. [16] Yildirim Ozlem Altintas, Arslan Hanife, So Facile synthesis of cobalt-doped zinc oxide thin films for highly efficient visible light photocatalysts. Appl Surf Sci 2016;390:111e21. rez R, Torres Delgado G, [17] Torres Martı´nez DY, Castanedo Pe Zelaya Angel O. Structural, morphological, optical and photocatalytic characterization of ZnOeSnO2 thin films prepared by the solegel technique. J Photochem Photobiol, A 2012;235:49e55. [18] Li Xiujuan, Wang Chang, Xia Ning, Jiang Ming, Liu Ruina, Huang Jing, et al. Novel ZnO-TiO2 nanocomposite arrays on Ti fabric for enhanced photocatalytic application. J Mol Struct 2017;1148:347e55. [19] Bekermann Daniela. Advanced perspectives in the vapor phase deposition of multifunctional metal oxides € t Bochum; 2012. nanomaterials. Ruhr Universita
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[20] Yang Kai-Yu, Hsu Han-Wen, Hsieh Hsin-Yi, Chang WeiChieh, Li Meng-Chi, Lin Po-Chang, et al. Facile spray deposition of photocatalytic ZnO/Cuein-Zn-S heterostructured composite thin film. Chem Sel 2016;1:4979e86. [21] Zeng Chen-hui, Xie Shilei, Yu Minghao, Yang Yangyi, Lu Xihoung, Tong Yexiang. Facile synthesis of large-area CeO2/ZnO nanotube arrays for enhanced photocatalytic hydrogen evolution. J Power Sources 2014;247:545e50. [22] Wei XQ, Zhang ZG, Liu M, Chen CS, Sun G, Xue CS, et al. Annealing effect on the microstructure and photoluminescence of ZnO thin films. Mater Chem Phys 2007;101:285e90. [23] Ng Zi-Neng, Chana Kah-Yoong, Sin Yew-Keong, Hoon JianWei, Ng Sha-Shiong. Influence of post-annealing condition on the properties of ZnO films. Ceram Int 2013;39:S263e7. [24] Charpentier C, Prod'homme P, Roca i Cabarrocas P. Microstructural, optical and electrical properties of annealed ZnO: Al thin films. Thin Solid Films 2013:424e9. [25] Gupta Monika, Sharma Vidhika, Shrivastava Jaya, Solanki Anjana, Singh AP, Satsangi VR, et al. Preparation and characterization of nanostructured ZnO thin films for photoelectrochemical splitting of water. Bull Mater Sci 2009;32:23e30. [26] Kuprenaite S, Abrutis A, Kubilius V, Murauskas T, Saltyte Z, Plausinaitiene V. Effects of annealing conditions and film thickness on electrical and optical properties of epitaxial Aldoped ZnO films. Thin Solid Films 2016;599:19e26. [27] Bouhssira N, Abed S, Tomasella E, Cellier J, Mosbah A. Influence of annealing temperature on the properties of ZnO thin films deposited by thermal evaporation. Appl Surf Sci 2006;252:5594e7. [28] Panda SK, Jacob C. Preparation of transparent ZnO thin films and their application in UV sensor devices. Solid State Electron 2012;73:44e50. [29] Soylu M, Yakuphanoglu F. Fabrication and characterization of light-sensing device based on transparent ZnO thin film prepared by sol-gel. Optik 2016;127:8479e86. [30] Pati Sumati. Highly textured ZnO thin films grown using solgel route for gas sensing application. J Alloy Comp 2017;695:3552e8. [31] Oh DC, Park SH, Goto H, Im IH, Jung MN, Chang JH, et al. Influence of heat treatments on electrical properties of ZnO films grown by molecular-beam epitaxy. Appl Phys Lett 2009;95:151908. [32] Park Hyun-woo, Chung Kwun-Bum, Park Jin-Seong. A role of oxygen vacancy on annealed ZnO film in the hydrogen atmosphere. Curr Appl Phys 2012;12:S164e7. [33] Cullity BD. Elements of X-ray diffraction. second ed. Addison-Wesley Publishing Company INC; 1978. [34] Barret C, Massalski TB. Structure of metals. Oxford: Pergamon; 1980. p. 204. [35] Romero R, Leinen D, Dalchiele EA, Ramos-Barrado JR, Martin F. The effects of zinc acetate and zinc chloride precursors on the preferred crystalline orientation of ZnO and Al-doped ZnO thin films obtained by spray pyrolysis. Thin Solid Films 2006;515:1942e9. [36] Malek MF, Mamat MH, Musa MZ, Khusaimi Z, Sahdan MZ, Suriani AB, et al. Thermal annealing-induced formation of ZnO nanoparticles: minimum strain and stress ameliorate preferred c-axis orientation and crystal-growth properties. J Alloy Comp 2014;610:575e88. r Tama s. Characterization of nanocrystalline materials [37] Unga by X-ray line profile analysis. J Mater Sci 2007;42:1584e93. [38] Janssen GCAM, Kamminga JD. Stress in hard metal films. Appl Phys Lett 2004;85:3086e8. [39] The peak-fitting software employed was AAnalyzer, www. rdataa.com/aanalyzer.
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ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production lito a, M.R. Alfaro Cruz a, O. Ceballos-Sanchez b, E. Luevano-Hipo L.M. Torres-Martı´nez c,* a CONACYT - Universidad Autonoma de Nuevo Leon, Facultad de Ingenierı´a Civil-Departamento de Ecomateriales y s de los Garza, NL, Mexico Energı´a, Cd. Universitaria, C.P. 66455 San Nicola b Departamento de Ingenierı´a de Proyectos, CUCEI, Universidad de Guadalajara, Zapopan, Jalisco 45100, Mexico c Universidad Aut onoma de Nuevo Leon, Facultad de Ingenierı´a Civil-Departamento de Ecomateriales y Energı´a, Cd. s de los Garza, NL, Mexico Universitaria, C.P. 66455 San Nicola
article info
abstract
Article history:
This work studied the effect of different annealing conditions of ZnO thin films grown by
Received 23 January 2018
RF magnetron sputtering and their application as photocatalysts for hydrogen production
Received in revised form
without any sacrificial agent or co-catalyst. ZnO films were annealed in air, nitrogen, and
19 March 2018
argon atmospheres to study the effect of their physical properties in the photocatalytic
Accepted 7 April 2018
activity. ZnO films showed high crystallinity and optical transparence of around 75e90%
Available online xxx
after annealing. Changes in composition and optical properties of the ZnO films were studied by x-ray photoelectron spectroscopy (XPS) and ellipsometry spectroscopy (SE), and
Keywords:
results were correlated with the photocatalytic performance in hydrogen production. The
ZnO thin films
highest photocatalytic hydrogen production was obtained with the ZnO thin film annealed
RF sputtering
in an air atmosphere with a result of 76 mmol.
Hydrogen production
© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Photocatalysis
Introduction Currently, the generation of clean and renewable energy from sustainable energy sources such as sunlight is a fundamental topic to avoid the emission of greenhouse gases to the atmosphere. The hydrogen generation from water splitting by photocatalysis processes has become very important because it is an alternative energy vector to replace fossil fuels. In order to produce this energy vector, several semiconductors
materials have been proposed. In particular, TiO2 powder is the most used material as a photocatalyst, mainly in dye degradation. However, there is also literature where TiO2 is used for hydrogen production in powder form or as photoanode in photo-electrochemical water splitting [1e4]. Additionally, the use of tantalates, titanates, and niobates as highly active photocatalysts have been reported; even though their band gaps are too large to absorb visible light, their crystal structure influences positively on the photocatalytic activity in a water-splitting activity reaction [5e8]. Alternative
* Corresponding author. vano-Hipo lito). E-mail addresses: [email protected], [email protected] (E. Lue https://doi.org/10.1016/j.ijhydene.2018.04.054 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054
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photocatalyst based in heterostructures containing TiO2, ZnO, CuO, and CdS have also been used to enhance their photocatalytic efficiency in hydrogen evolution reaction [9e11]. In general, the H2 generation via photocatalysis involves the use of a powder semiconductor to carry out the oxidationreduction process of the water molecule. However, one of the main disadvantages of using semiconductor particles in suspension is their separation and recovery of the liquid phase. In addition, suspensions are associated with a low quantum yield as a consequence of a high recombination speed of the hole-electron pair [12]. However, the use of thin film as a photocatalyst has numerous advantages such as rapid electron transport and homogeneous exposure of the photocatalyst, and it is possible to deposit the photocatalyst over different substrates, efficient absorption of solar energy, and high surface area. ZnO thin films modified by nonmetal and metal elements have been widely studied primarily for dyes degradation [13e18] and for H2 generation. For instance, ZnO thin films deposited by chemical vapor deposition doped with Ag can produce until 0.5 mmol cm2 h1, while the use of Na2S and Na2SO3 as sacrificial agents in ZnO films doped with CeO2 helps to improve the H2 production of the films. The photoelectrochemical H2 production has been reported as well for ZnO thin films impregnated with CueIneZneS with a yield of 0.7 mmol cm2 h1 [19e21]. Some authors have reported the use of different atmospheres to perform heat treatment to ZnO thin films after deposition, with the purpose of modifying his optical or electrical properties, modifying the surface, and increasing the conductivity of ZnO thin films [22e24]. ZnO crystal structure gives rise to polar symmetry along the (002) plane in the hexagonal axis which is a key factor in crystal growth and defect generation, which include oxygen vacancies and zinc interstitial [25]. So, one way to improve ZnO properties in a thin film is to employ different thermal treatments under specific atmospheres, for example, when the thermal treatment is carried out in oxygen poor atmospheres, such as Ar, Nitrogen, etc., which helps to avoid the defects as oxygen vacancies and Zn interstitial [26]. The novelty of this paper is to determinate how concentration of defects, as oxygen vacancies and interstitial zinc, affect the photocatalytic efficiency in ZnO, especially when ZnO is deposited as a thin film. In this work, we propose the use of the ZnO thin films obtained by RF magnetron sputtering as a photocatalyst in H2 generation from water splitting. Where, the absence of oxygen inside the sputtering chamber during deposition, contributes to the presence of defects, as oxygen deficiencies and interstitial zinc. So, the ZnO films were annealed in different atmospheres (air, nitrogen, argon) in order to assessing the structural, optical, and surface properties with respect to its photocatalytic activity in the hydrogen production without using sacrifice agents or co-catalysts.
Experimental Growth of ZnO thin films by RF-Sputtering ZnO thin films were deposited by RF magnetron sputtering over glass substrates using a ZnO target (99.999% Pure, 2.0000
diameter 0.25000 thick; Kurt J. Lesker). Glass substrates were ultrasonically cleaned in acetone, isopropyl alcohol, and deionized water and then dried with air. The vacuum pressure of the chamber was lowered to 8.6 106 Torr using a turbomolecular pump before introducing argon gas. Working pressure was fixed at 1.6 102 mTorr with an argon flow rate of 15 sccm. Substrates were heated at 300 C and deposited at a constant power of 80 W for 2 h; distance between the target and substrate was 8.5 cm. Before deposition, the target was pre-sputtered for 10 min. After the deposit process, ZnO thin films were annealed in argon, nitrogen, and air atmospheres at 400 C for 1 h [27,28], in order to evaluate the photocatalytic activity for hydrogen production, usually the choice of this temperature depends on the type of substrate and their melting fusion [29,30].
Characterization Structural properties were determinate using an X-ray diffractometer PANalytical with a Cu Ka radiation of 1.54 Å in grazing angle. Transmittance measurements were performed using a UVeVis NIR spectrophotometer (Cary 5000). Spectroscopic ellipsometry measurements were collected using a Horiba, Jobin Yvon UVISEL HR 320 ellipsometer at an incident angle of 70 . The morphology of the samples was analyzed by scanning electron microscopy (SEM), using a FEI Nova NanoSEM 200 microscope with an accelerating voltage of 30 kV. The chemical states and elemental composition were determined by X-ray photoelectron spectroscopy (XPS) using a Thermo Scientific, Escalab 250Xi, equipped with an Al Ka monochromatic x-ray source (hv ¼ 1486.7 eV) with a line width of 0.20 eV in an analysis chamber at a bass pressure of <4.3 1010 mbar.
Photocatalytic activity Photocatalytic activity of ZnO thin films (15 cm2) were evaluated using a cylindrical Pyrex batch reactor of 200 mL at room temperature. Thin films were fixed inside the reactor and then filled with nitrogen to remove the oxygen in the medium. Once the oxygen was removed from the reactor, thin films were irradiated using an UV Pen Ray Lamp of 254 nm of irradiance of 4400 mW cm2. Photocatalytic hydrogen production was measured every 30 min using a gas chromatograph Thermo Scientific trace GC Ultra with a thermal conductivity detector (TCD).
Results Structural properties XRD patterns of ZnO thin films deposited, annealed at 400 C in air, nitrogen, and argon atmospheres, are shown in Fig. 1. All thin films are polycrystalline and peaks correspond to ZnO Wurtzite phase according to the JCPDS card 01-089-1397. ZnO thin films deposited have poor crystallinity compared to those heat treated. When thin films were annealed in different atmospheres the crystallinity improved, with the thin film annealed in argon atmosphere being the one that exhibited
Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054
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Fig. 1 e a) The XRD pattern of ZnO thin films deposited and annealed in different atmospheres, b) crystallinity percentage of each film.
the highest crystallinity. All thin films had a preferential orientation over the (002) plane, corresponding to c-axis in the hexagonal structure of the Wurtzite phase. Grain size of the thin films was calculated using the Scherrer equation [17]: D¼
0:9l ; bcosq
(1)
where l is the X-ray wavelength, q is the Bragg's diffraction angle and b is the full width at half maximum (FWHM) of the peak corresponding to the (002) plane. Thin films deposited had a grain size value of 9.54 nm. After annealing in different atmospheres, grain size values were similar for air, nitrogen, and argond20.8, 21.1, and 22.9 nm, respectively (Table 1). It is observed that the grain size increase after annealing, because heat treatments causes the recrystallization of distorted lattices in the bulk [31], so that, the grains are distributed by supplying enough thermal energy, and the small grains are joined to larger ones [22], reducing grain boundaries [32]. The preferred orientation, or textured, is defined as a condition in which the distribution of crystal orientations is nonrandom, when the films are annealed a deformation texture occurs, due to the recrystallization of the material [33]. To study the preferred orientation and growth of the (h k l) planes in each thin film, Eq. (2) was used, defined by Barret and Massaslki [34] for texture coefficient: Tcðh
Iðh ICðh
k lÞ
k lÞ
¼ h P I ðh 1 n
k lÞ
ICðh
k lÞ
i
diffraction peaks considered. A value of texture coefficient greater than the unit indicates that there is a great amount of grains with preference in direction [h k l]; a texture coefficient value equal to the unit means that the grains are randomly oriented as the reference card [35]. Table 1 demonstrates texture coefficient values for each thin film. Values exceeding the unit correspond to [0 0 2] direction in all thin films, confirming a growth along c-axis, perpendicular to the substrate. It was observed that the thin film annealed in the argon atmosphere had the largest value of texture coefficient, as well as the highest crystallinity. To know how much the lattice constant has been modified after heat-treated atmospheres, the lattice constant c [36] was calculated: c¼
l sinq
(3)
where q is the diffraction angle corresponding to the (002) plane. Thin films deposited and annealed in air have similar values of the c parameter (5.1923 and 5.1929 Å, respectively). This value decreased when the films were annealed in reducing atmospheres of nitrogen and argon to 5.1873 and 5.1855 Å, respectively (see Table 1). Generally, thin film lattice constant changes are attributed to different types of strain, like dislocations, stacking faults, grain boundary, etc. [37]. Mostly, thin films deposited by the sputtering process have compressive stresses due to the presence of interstitials or implanted argon in the growing film [38].
(2)
Scanning electronic microscopy (SEM)
k lÞ
where I (h k l) are XRD intensities obtained for thin films deposited by sputtering, I C (h k l) are the XRD intensities for the standard pattern (01-089-1397 card) and n is the number of
Fig. 2 shows the SEM images of ZnO thin films deposited and films annealed in air, nitrogen, and argon atmospheres. The
Table 1 e Texture coefficient Tc of [h k l] planes for each thin film. Textured coefficient TC of (h k l) planes
ZnO (01-089-1397) Deposited Air Nitrogen Argon
Grain size (nm)
c (Ǻ)
TC(002)
TC(101)
TC(102)
TC(100)
TC(103)
TC(112)
9.54 20.88 21.11 22.91
5.233 5.1923 5.1929 5.1873 5.1855
3.11 3.52 3.36 4.18
0.36 0.76 0.80 0.57
0.83 0.76 0.80 0.57
0.33 0.22 0.15 0.05
0.76 0.74 0.84 0.82
0.57 0.48 0.63 0.27
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Fig. 2 e SEM image for the surface of ZnO thin film (a) deposited and annealed in (b) air, (c) nitrogen, and (d) argon atmosphere.
SEM images revealed for each film that the surface is totally covered with a morphology of irregular grains lower than 500 nm where, for deposited thin film, it is observed that the grains are smaller than the grains in the other films. Thin films annealed in air and nitrogen atmospheres (Fig. 2 (b) and (c)) have a similar surface, with grains larger than those observed in the deposited film. On the other hand, film annealed in an argon atmosphere had a dense surface with more compact and larger grains (Fig. 2(d)). The increase of the grain size imply a better distribution and grain orientation after annealing [22]. In contrast, the deposited thin film has irregular grains which, according to the XRD results, has a grain size smaller than the grain sizes of the other films (Table 1).
XPS Fig. 3 shows the XPS spectra for Zn 2p3/2, O 1s, and C 1s core levels for the deposited and thin films annealed in different atmospheres. The XPS analysis was performed using the Voigt function enclosed in the AAnalyzer Software [39]. A Shirley background was used to fit the Zn 2p and O 1s spectra. For the deposited thin film, the Zn 2p3/2 region was fitted using two singlets peaks centered at 1021.15 eV and 1022.0 eV, respectively. The first peak was associated to zinc in stoichiometric ZnO and the second one was associated to Zn2þ ions in oxygen-deficient regions. For the O 1s region, three singlets peaks were used to fit this region, which are centered at 529.8 eV, 530.48 eV and 531.2 eV, respectively. The first peak
positioned at lower binding energy was associated to O2 ions on the wurtzite structure, while the second one positioned at higher binding energy was attributed to a contribution of O2 ions that are in oxygen-deficient regions within the ZnO film. Finally, the peak at about 531.72 eV, was associated to chemisorbed or absorbed species on the sample surface, such as CeO bond, hydroxyl groups (eOH) or even adsorbed O2. It has been reported that intrinsic defects, as zinc interstitial ) and oxygen vacancies (VOþþ ), are electrically atoms (Znþþ i active and these species can induce localized states close to the conduction band. Both species can be act as donor. For the case of C 1s spectra, typical chemical components associated to adventitious carbon were observed (supporting information), which were attributed to CeC (284.8 eV), CeO (285.9 eV), and CO3 (288.4 eV) [40]. It is noted that all the spectra were aligned to C 1s centered at 284.8 eV. This allows for subtle changes in the binding energies of the measured spectra to be observed. When ZnO films were annealed in different atmospheres, it was possible to observe slight shifts on the binding energy of the Zn 2p3/2 and O 1s regions. This phenomenon was more evident after annealing in nitrogen and argon atmospheres, where a downward energy shift on the binding energy of around 0.6 eV was observed on the Zn 2p3/2 spectrum of both samples. The O 1s spectrum of the film annealed in the argon atmosphere showed an energy shift to a lower binding energy of 0.24 eV. For the film annealed in nitrogen, the O 1s spectrum shifted upward 0.14 eV to a higher binding energy. This energy shift could be related with changes in the
Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054
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Fig. 3 e XPS spectra of the Zn 2p3/2 and O 1s core levels for the ZnO films deposited and annealed in air, argon, and nitrogen atmospheres.
electronic density of the ZnO as consequence of the alteration on the concentration of intrinsic defects or superficial charge. A summary of XPS analysis is shown in Table 2. The semiquantitative analysis shows that there are oxygen deficiencies in the ZnO structure for all films. It can be observed that the quantity of oxygen deficiencies of ZnO thin film annealed in an air atmosphere is higher compared to other films. When ZnO is annealed using an oxygen-rich atmosphere, the
Table 2 e Atomic concentration of oxygen and zinc calculated from XPS spectra for as deposited and annealed at different atmospheres films. Sample Deposited Air Nitrogen Argon
O1 (%)
O2 (%)
Z1 (%)
Z2 (%)
O1/Z1
25.25 25.66 25.91 17.10
16.78 15.24 16.48 18.14
27.73 27.60 29.00 16.62
4.57 9.95 5.66 4.18
0.91 0.93 0.89 1.00
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as a consequence of its low formation energy in n-type ZnO [42,43]. However, the migration of Zn through the ZnO is more likely, since the energy barrier for the Zn migration in ZnO (0.57 eV) is lower than the energy barrier for the oxygen migration in ZnO (1.7 eV) [42]. This results are consistent with those observed from the XRD and XPS results (see Fig. 1). ZnO thin film annealed in Ar atmosphere has a poor photocatalytic efficiency, because this film has a less concentration of intrinsic defects.
Optical properties
Fig. 4 e Optical transmittance spectra of deposited and annealed in air, nitrogen, and argon atmospheres films.
Fig. 5 e SE spectra representative of the ZnO thin film annealed in air atmosphere.
formation energy de oxygen vacancies and zinc interstitial increases, which limits the formation of a ZnO crystal structure free of intrinsic defects [41] resulting in a low crystallinity. When thin films are annealed in an oxygen-poor atmosphere it promotes the mobility of the oxygen ions and interstitial Zn,
Optical transmittance spectra of ZnO thin films were measured at a wavelength range of 360e1000 nm, as shown in Fig. 4. In general, optical transmittance of ZnO thin films is around 75e90% in the UVeVisible spectrum. The absence of optical interference fringes in the spectrum indicates that the ZnO films are thin. To determine the energy band gap (Eg), optical constants (refractive index (n) and extinction coefficient (k)), superficial roughness, and the ZnO thin film thickness, spectroscopy ellipsometry (SE) were used. These measurements were performed using ellipsometry equipment with a fixed incident angle of the beam of 70 . The dependence of the ellipsometric angles (j and D) against the beam energy was collected in the energy range of 1.5e4.7 eV (or ~264e827 nm), with energy steps of 0.05 eV. These ellipsometric angles determine the change in the light polarization when it interacts with the material. Tauc-Lorentz dispersion model [44] with two oscillators and Bruggeman's effective medium approximation (EMA) [45] were employed to describe the optical response of the three-phase model (air/ZnO/glass substrate). Fig. 5 shows typical spectra for J and D as a function of the wavelength for thin film annealed in air atmosphere (dots). Additionally, the best fit (solid lines) obtained from the modeling of the air/ZnO/ glass structure is presented. It is possible to observe how the theoretical model proposed closely reproduces the optical response of the ZnO thin film over a glass substrate. The rest of the ZnO thin films display a similar behavior to this thin film. Fig. 6 (aeb) show the behavior of refractive index (n) and extinction coefficient (k) for ZnO thin films. The SE results show slight differences in optical properties of the films. Both
Fig. 6 e (a) Refractive index (n) and (b) extinction coefficient (k) of the ZnO thin film annealed in air atmosphere. Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054
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Table 3 e Summary of parameters obtained from SE analysis for the ZnO films. Sample Deposited Air Nitrogen Argon
Thickness (nm)
Roughness (nm)
SE-Eg (eV)
91.3 118.6 124.3 129.8
10.0 26.6 42.8 32.9
3.13 3.10 3.08 3.06
7
Variations on the optical constants in ZnO thin films are related to changes in the grain size, as it was observed in XRD results (see Table 1). When crystallinity of the ZnO film change, after being subjected to a heat treatment, the absorption peaks associated to electronic transitions tending to be less broad as the crystallinity increases, producing changes in optical properties [48]. So, in this work, thin film annealed in Ar atmosphere showed the highest crystallinity, as well as a higher intensity on the extinction coefficient (Fig. 6-b). Since ZnO is transparent, it is possible to observe that the extinction coefficient tends to zero for wavelengths greater that 400 nm (the related optical transmittance spectra is shown in Fig. 4). A summary of SE results is presented in Table 3. From these results, it is possible to identify that ZnO thin film annealed in a nitrogen atmosphere presented a higher superficial roughness compared to the rest of the films. . It should be noted that energy band gap obtained by sputtering is slight smaller than ZnO-bulk (Eg ¼ 3.36 eV). This can be attributed to the presence of intrinsic defects in the ZnO thin film.
Photocatalytic hydrogen production
Fig. 7 e Photocatalytic H2 production of ZnO thin films considering exposed active surface.
n and k increase as wavelength increases from 263 to 375 nm, while for wavelengths greater than 375 nm these parameters tend to decrease. These changes in the optical properties can be associated with variations in the crystal structure and superficial morphology of the ZnO films [46,47].
Photocatalytic H2 production was investigated in a Batch reactor containing 200 mL of deionized water under a nitrogen atmosphere at 25 C. After 3 h of exposure to ultraviolet light, thin film deposited showed a hydrogen production of 46 mmol of H2, while for thin films annealed at 400 C in air, nitrogen, and argon atmospheres, photocatalytic hydrogen productions were 76, 71, and 29 mmol of H2, respectively (Fig. 7). The ZnO thin films annealed in air showed the greatest hydrogen production, which can be associated with the highest oxygen deficiencies percentage, according to the XPS results (Table 2).
Fig. 8 e Mechanism of ZnO photoactivation to exemplify how intrinsic defects improve the photocatalytic activity. Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054
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From XPS spectra, it was possible to confirm the presence of defects related to oxygen vacancies (VOþþ ) and zinc interstitial (Znþþ i ), which have been reported that are electrically active and can induce localized state within the energy band gap [41]. Thus, based on the photocatalytic activity results and the physical properties obtained, it is proposed the reaction mechanism shown in Fig. 8. The mechanism of conventional photoactivation of ZnO, involves its excitation with an irradiation energy higher than its band gap value (hv Eg) to promote the transfer of the electrons (e) in valence band (VB) to the conduction band (CB) (Eqs. (5)e(7)). This process is accompanied by a recombination step, which limit the photocatalytic activity of ZnO. Different authors reports some strategies to delays this process, one of those strategies is related to the generation of intrinsic defects such as VOþþ and in the wurtzite structure. As mentioned earlier, the Znþþ i presence of these intrinsic defects has been related to the formation of inter-band gap states that depending of its relative position and can act as electron and hole donors, which eventually delays the recombination of the electron and hole pair. In particular, since the position of VOþþ is 1.16 eV above the CB of ZnO, this defect can capture holes to promote a higher number of electrons available in the medium to react with protons and produce H2 (Eq. (8)) [49]. The holes that have been donated to the medium, can oxidize H2O to produce O2 and Hþ (which react with electrons to form H2). Additionally, the zinc interstitial can act as electron acceptor due to its favorable potential just 0.22 eV above the CB of ZnO (Eq. (9)). Thus, the presence of these both type of intrinsic defects promote a higher efficiency of this application through the simultaneous oxidation and reduction of H2O to produce H2 at neutral pH, without the use of sacrificial agents or co-catalyst. hv þ ZnO ! ZnO e CB þ ZnO hVB
(5)
1 þ H2 OðadsÞ þ 2hþ VB /2H þ O2 [ 2
(6)
2Hþ þ 2e CB /H2 [
(7)
V0þþ 4V0þ þ hþ VB
(8)
þ þ Znþþ i 4Zni þ eCB
(9)
Additionally, we tested the reusability of the ZnO thin films by means of their second evaluation (Fig. 9), where photocatalytic H2 production decreased in all of the samples (more than a half of its initial value) and where thin film annealed in an air atmosphere had a lower reduction in their photocatalytic activity. This reduction can be related to a photocorrosion of ZnO, a phenomenon that has been reported in other works [50e52]. Yield hydrogen in mmol cm2 h1 for each film is attached in supporting information.
Conclusions ZnO thin films deposited by RF sputtering and annealed in different atmospheres were assessed for photocatalytic hydrogen production without using a sacrificial agent or cocatalyst. Thin films annealed in air and nitrogen atmospheres presented the best activities which were associated with similar crystallinity and high oxygen deficiency. It was observed that photocatalytic activity decreased 3.5 times when the film thickness was higher than 125 nm. A direct correlation among the oxygen deficiency percentage and the photocatalytic hydrogen production was also observed, since oxygen deficiencies act as active sites favoring the yield of this reaction. In addition, it was discovered that a deactivation of the samples after a second evaluation was probably due to photocorrosion phenomena, a typical problem when ZnO is used for photocatalytic applications.
Acknowledgments The authors are grateful to CONACYT for their financial sup tedras-CONACYT 363 port through the following projects: Ca and 1060; INFRA-2015-2753; CB-2014-237049; PDCPN-2015-487; and UANL (PAYCIT 2017). We appreciate the support given by Dr. Eduardo Martı´nez Guerra, Dr. Francisco Enrique Longoria Rodriguez, and M. C. Luis Gerardo Silva Vidaurri from CIMAVMonterrey, and Dr. Marco Antonio Garza Navarro from FIME, UANL.
Appendix A. Supplementary data Supplementary data related to this article can be found at https://doi.org/10.1016/j.ijhydene.2018.04.054.
references
Fig. 9 e Study of the reusability of the ZnO thin films as photocatalyst for H2 production.
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Please cite this article in press as: Alfaro Cruz MR, et al., ZnO thin films deposited by RF magnetron sputtering: Effects of the annealing and atmosphere conditions on the photocatalytic hydrogen production, International Journal of Hydrogen Energy (2018), https:// doi.org/10.1016/j.ijhydene.2018.04.054