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Role of zinc interstitials and oxygen vacancies of ZnO in photocatalysis: a bottom-up approach to control defect density† Fatma Kayaci,ab Sesha Vempati,*a Inci Donmez,ab Necmi Biyikliab and Tamer Uyar*ab Oxygen vacancies (VOs) in ZnO are well-known to enhance photocatalytic activity (PCA) despite various other intrinsic crystal defects. In this study, we aim to elucidate the effect of zinc interstitials (Zni) and VOs on PCA, which has applied as well as fundamental interest. To achieve this, the major hurdle of fabricating ZnO with controlled defect density requires to be overcome, where it is acknowledged that defect level control in ZnO is significantly difficult. In the present context, we fabricated nanostructures and thoroughly characterized their morphological (SEM, TEM), structural (XRD, TEM), chemical (XPS) and optical (photoluminescence, PL) properties. To fabricate the nanostructures, we adopted atomic layer deposition (ALD), which is a powerful bottom-up approach. However, to control defects, we chose polysulfone electrospun nanofibers as a substrate on which the non-uniform adsorption of ALD precursors is inevitable because of the differences in the hydrophilic nature of the functional groups. For the first 100 cycles, Znis were predominant in ZnO quantum dots (QDs), while the presence of VOs was negligible. As the ALD cycle number increased, VOs were introduced, whereas the density of Zni remained unchanged. We employed PL spectra to identify and quantify the density of each defect for all the samples. PCA was performed on all the samples, and the percent change in the decay constant for
Received 8th April 2014 Accepted 6th June 2014 DOI: 10.1039/c4nr01887g www.rsc.org/nanoscale
each sample was juxtaposed with the relative densities of Znis and VOs. A logical comparison of the relative defect densities of Znis and VOs suggested that the former are less efficient than the latter because of the differences in the intrinsic nature and the physical accessibility of the defects. Other reasons for the efficiency differences were elaborated.
Introduction Photocatalysis in the presence of wide band gap semiconductors is a widely researched energy efficient approach for a cleaner environment, in which organic pollutants can be disintegrated.1–3 However, to increase the efficiency of a semiconductor catalyst one needs to isolate4 or at least delay the recombination of photogenerated electrons and holes.1,5–13 To achieve this, lattice defects are induced in the semiconductor1,6,8–13 in addition to the fabrication of heterojunctions (either with noble metals5–7 or another semiconductors4,14). In connection to lattice defects, depending on the type and energetic location of the defect, their character is determined to capture either an electron or a hole, thereby delaying the recombination process.1–4,6,8–13 As a consequence, the captured
a
UNAM-National Nanotechnology Research Centre, Bilkent University, Ankara, 06800, Turkey. E-mail: [emailprotected]; [emailprotected]
b
Institute of Materials Science & Nanotechnology, Bilkent University, Ankara, 06800, Turkey
† Electronic supplementary information (ESI) available: Distribution of particle sizes, NC thickness comparison, XRD, uorescence spectra, TGA and digital photographs. See DOI: 10.1039/c4nr01887g
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or free charge carrier participates in photocatalytic activity (PCA), in which the catalysis occurs at the conduction band (CB), valance band (VB) and the defect site, if available on the surface.4,8 Note that the defects form intermediate bands within the band gap, which allow visible light harvest especially in the case of wide band gap semiconductors.1,4,8,15–17 The lattice defects connected to PCA are basically intrinsic (vacancies, interstitials, anti-site) or extrinsic (induced by impurities) in nature.1,4,6,8–13,15,18 However, it is rather hard, though intriguing, to determine which defect is more benecial for PCA, especially in the context of ZnO because controlling the defect density and maintaining a balance among various defect types is quite challenging.15 On the other hand, dominant defect density can compromise optical quality; nevertheless, it increases PCA, as recently reported by us.8 Among various catalysts, the suitability of ZnO enables its major involvement in photocatalysis,4,6,8,13,19–23 because of easy fabrication of nanostructures21,22,24 ability to harvest UV through visible light of the solar spectrum, if appropriately doped. The defects associated with ZnO within the band gap are either þ * donor type zinc interstitials (Znþþ i ; Zni ; Zni ) and oxygen vacancies (VOþþ ; VOþ and VO ) or acceptor type zinc vacancies 00 0 (VZn and VZn ).25,26 Note that the controllability of the defect
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density depends on the type of defect and the semiconductor itself.1,6,8–13 In the context of ZnO, numerous studies deal with VOs,4,8,10,11,13,21 which is not the case with Znis. It is also notable that the balance between the relative densities of Znis and VOs is still challenging.15 The presence of Znis and VOs in ZnO can be easily identied by simple photoluminescence (PL) experiments,15–17,27 indicated by blue and green emissions, respectively. Note that studies based on Znis are less frequent than VOs because the former defects are difficult to achieve and control in the as-prepared samples.16,17,28–32 In contrast, typically synthesized ZnO is predominant with VOs.4,8,16,17,27,31,32 Consequently, photocatalysis studies involving VOs dominate the literature.1,4,6,8–10,13,21 In this context, it is very useful and important to study the effect of Znis in PCA and their efficiency against VOs. In addition to the fundamental interest, the results of such a study enable an efficient and cost effective design of novel photocatalysts. As shown earlier,33 Znis are very unstable when compared to VOs, which are found to be stable even at 400 C. Aer nearly two years, theoretical support for this instability was provided by Janotti et al.,34 where the authors suggest that Znis diffuse faster through a migration barrier as low as 0.57 eV, cf. for VOs it is from 1.7 to 2.4 eV. Because the ionization energies are in the range of 0.05 to 2.8 eV,33 the formation temperature is detrimental for the relative concentrations of various defects. One needs to choose non-equilibrium processes to induce Znis,35,36 in which the samples are subjected to thermal annealing for controlling the density of defects. Under oxygen decient conditions at high temperatures VOs are typical,37 while Zn vapor-rich environments produce Znis.38,39 For example, when ZnO crystals are annealed in zinc vapor at 1100 C, Znis are introduced into the lattice.40,41 More recently, Zeng et al.15 followed an earlier reported method,35,36 in which a zinc target is laser ablated in aqueous solution. The resultant colloids were subjected to thermal annealing in various atmospheres.15 In addition to the above discussed methods,15,40,41 we will show that atomic layer deposition (ALD) can be employed to produce ZnO on polymer bers consisting of Znis with or without VOs. The good control and precision of ALD19,42,43 along with electrospun bers yielded structures of ZnO with a clear transformation from quantum dots (QDs) to nanocoating (NC). The transformation is achieved simply by increasing the ALD cycles, during which the predominance transfers from Znis to VOs, while the density of the former is sustained in all the cases. Note that the earlier methods15,35,36 required post thermal treatments to control the defect density; in contrast we have noticed predominantly Znis or Znis and VOs in the as-fabricated samples depending on the cycle numbers. In this work, we have employed polysulfone (PSU) electrospun bers as a substrate to deposit ZnO via ALD with varying deposition cycle numbers (viz 100, 200, 300 and 400). This variation has yielded ZnO quantum dots (100 cycles) dominating Zni related defects, while the probable origin of such defects is discussed. Further increase in the cycle numbers ($300) introduced VOs, in which case the density of Znis remains almost constant. The morphological transformation has shown signicant variance in the crystal and optical properties of the samples. Aer thorough characterization
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(structural, optical and surface), the PCA is tested and juxtaposed against defect density. We believe that this study deepens the understanding of the least studied crystal defects of ZnO and Znis in connection to their PCA.
Experimental Materials Polysulfone (PSU, Mw 60 000) was purchased from Sp2, Scientic Polymer Products, Inc. N,N-dimethylacetamide (DMAc, Sigma Aldrich, 99%) was used as solvent. Methylene blue (MB, Sigma Aldrich, certied by the Biological Stain Commission) was employed in the catalysis study. In the ALD process, diethylzinc (DEZn, Sigma Aldrich) and HPLC grade water were used as zinc precursor and oxidant, respectively. All the materials were used as received. Electrospinning of PSU bers PSU (35 wt%) in DMAc was rigorously stirred for 4 h at 60 C to yield a hom*ogeneous, clear yellowish solution. This solution upon reaching room temperature (RT) was taken into a syringe tted with a metallic needle of 0.8 mm inner diameter. This syringe was xed horizontally on a syringe pump (SP 101IZ, WPI), which was set to a feed rate of 0.75 mL h1 15 kV bias was applied (Matsusada, AU Series) across the metal needle and the collector, which were placed about 15 cm apart. The electrospun PSU bers were collected onto an aluminum foil, which was wrapped around a grounded metal electrode. The electrospinning process was performed in an enclosed chamber at 26 C and 29% relative humidity. ALD of ZnO on electrospun bers PSU bers were subjected to an ALD process (Savannah S100 ALD reactor, Cambridge Nanotech Inc.) at 200 C. Exposure mode (a trademark of Cambridge Nanotech Inc.) was employed to deposit ZnO, in which the pump valve was closed during the pulse steps and opened during the purge steps of the ALD cycle. Pulse times of the Zn and oxygen sources were both 0.015 s, and held in the chamber for 10 s. Then, pump valve was opened for purging using N2 at a ow rate of 20 sccm for 10 s. Using this procedure 100, 200, 300 and 400 ALD cycles were deposited. For thickness estimation, the same recipe and cycles were applied to a clean Si/SiOx wafer. Characterization Scanning electron microscopy (SEM, FEI Quanta 200 FEG) was performed on pristine PSU and ZnO coated PSU (PSU–ZnO) bers aer a 5 nm thick Au–Pd sputter coating. Average ber diameter (AFD) was calculated by measuring nearly 100 bers from the SEM images. Spectroscopic ellipsometry (SE, Variable Angle Spectroscopic Ellipsometer, VASE© J.A. Woollam Co.) was employed to determine the thicknesses of the ZnO lms on Si/ SiOx wafers. SE spectra from ZnO lms on Si/SiOx substrates were recorded at three angles of incidence (65 , 70 and 75 ) within the wavelength range of 450–1200 nm. Cauchy dispersion function was employed to estimate the thickness of the
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ZnO lm via Si/SiOx/ZnO models, while the native oxide thicknesses of the Si substrates were measured through SE spectra. XRD patterns from pristine PSU and PSU–ZnO samples were obtained within the range of 2q ¼ 10 –90 by using a PANalytical X'Pert Multi Purpose X-ray Diffractometer with Cu Ka radiation. Transmission electron microscopy (TEM, FEI Tecnai G2F30) was also performed on the PSU–ZnO bers, for which the bers were dispersed in ethanol and a tiny droplet was dried on a holey carbon coated TEM grid. Selected area electron diffraction (SAED) patterns were also recorded. The surface composition of pristine PSU and PSU–ZnO bers were determined via X-ray photoelectron spectroscope (XPS, Thermoscientic, K-alpha) equipped with a monochromated Al Ka X-ray source (hn ¼ 1486.6 eV) and ood gun charge neutralizer. Avantage soware was employed for the deconvolution of various peaks. Horiba Scientic FL-1057 TCSPC was used for the PL measurements performed at excitation wavelengths of 320, 330 or 350 nm. The interband transition was taken as a reference, and each of the spectra was normalized individually. Then, these spectra were subjected to deconvolution using Origin 6.1. The thermal properties of the PSU and PSU–ZnO bers were investigated via thermogravimetric analyzer (TGA, Q500, TA Instruments) from RT to 700 C at a heating rate of 20 C min1 under N2 ow.
Photocatalytic activity of PSU–ZnO bers MB aqueous solution (18.8 mM) was used to investigate the PCA of the PSU–ZnO bers, and then compared it with the pristine and no catalyst cases. The samples (11 mg) were immersed into MB solution in quartz cuvettes and exposed to UV irradiation (8 W, UVLMS-38 EL, 365 nm) from a distance of 10 cm. The PSU and PSU–ZnO bers were at the bottom of the cuvette during the experiment, and therefore did not interfere with the measurement. The changes in the absorption peak of MB at 665 nm as a function of UV-irradiation time (t) were analyzed via UV-Vis-NIR spectrophotometer (Varian Cary 5000). The concentrations of MB before and aer UV irradiation are dened as C0 and C, respectively. The rate of dye degradation (C/C0) is quantied with the rst order exponential t (y ¼ y0 + aex/s) for each sample with an automated routine in Origin 6.1, where a ¼ pre exponential factor, x ¼ time axis, and y ¼ C/C0 at different ‘t’ and s-decay constants. All the parameters are set as free (unless otherwise stated) until convergence.
Results and discussion PSU–ZnO bers were fabricated via a two-step process in which electrospinning was followed by ALD with varying cycle numbers (Fig. 1). In the rst step (Fig. 1a), bead free PSU bers were produced and then transferred to an ALD chamber (Fig. 1b) to deposit ZnO for 100, 200, 300 or 400 ALD cycles. On varying the number of cycles, we achieved QDs or NC on PSU bers (Fig. 1c). The PSU–ZnO samples obtained using various cycle numbers viz 100, 200, 300 and 400 ALD cycles, are referred to as PZ1 QD, PZ2 NC, PZ3 NC and PZ4 NC respectively. We will see that this nomenclature is quite appropriate for each sample.
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Fig. 1 Schematic of (a) electrospinning, (b) atomic layer deposition (ALD) and (c) resultant morphology of the samples obtained on applying different ALD cycle numbers.
Polymeric structure can be easily deformed or degraded under the deposition of other materials via the conventional chemical vapor deposition,44 which requires elevated temperatures. In contrast, ALD uses relatively lower temperatures in which the substrate is exposed to two or more precursors separated by purging/evacuation periods (Fig. 1b). Note that ALD is a powerful tool to deposit ZnO and/or other inorganic layers on polymeric lms45 and ber mats.45–47 In this process, the substrate is exposed to gaseous precursor molecules, which do not react with themselves, and new surface sites are formed for the following precursor pulse.48 The morphology of the pristine PSU and PSU–ZnO bers was investigated by SEM. The optimized electrospinning parameters yielded bead-free bers; although, the surface of the ber was noticeably rough (Fig. 2a). Aer the ALD processing we recorded SEM images for all cycle numbers, which are shown in Fig. 2b–e. The bers retained their structure aer ALD in all the cases, where it is notable that compatibility between the precursor and the substrate is vital.45,49 The initial stage of transformation of QDs into NC can be seen from the inset of Fig. 2c, where the grains of ZnO are explicit. The AFDs of the PSU and PSU–ZnO bers are summarized in Fig. 2f, and an obvious increase in ber diameter is observed for higher ALD cycles, as expected. However, given the standard deviation in the diameter measurement, we estimated
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SEM images of electrospun fibers of (a) pristine PSU, (b) PZ1 QD, (c) PZ2 NC, (d) PZ3 NC, (e) PZ4 NC and (f) average fiber diameters.
Fig. 2
the thickness of the ZnO coating by other methods, which will be discussed later. The morphologies of the PSU–ZnO bers were further investigated by TEM (Fig. 3). Fig. 3a shows the QD formation on the surface of PSU bers for 100 ALD cycles. In this sample, highly-dense ZnO QDs with an average diameter of 10 nm were hom*ogenously distributed on the ber surface (Fig. S1 of ESI†). When the number of cycles increased, the process yielded a continuous ZnO coating on the PSU bers with uniform thickness, viz for 200, 300 and 400 cycles, as shown in Fig. 3b, c and d, respectively. Note that the dark region at the edge of the ber indicates that the ZnO coating gets thicker with increasing ALD cycles. Therefore, the ALD process with sufficient cycle numbers yielded a conformal and layer-by-layer deposition of ZnO on PSU bers.4,19,20,49 For cycle numbers $200, a continuous lm is formed because of the coalescence of grains/ islands. Fig. 3e shows a high resolution TEM image of PZ4 NC, where we can see the lattice fringes as well as grains grown in different lattice directions (see double arrows on gure). This grainy structure is consistent with the earlier observation for fewer ALD cycles. Le inset of Fig. 3e shows the lattice resolved image with a lattice spacing of 2.23 nm, which represents the c-axis of ZnO. Furthermore, the SAED pattern from PZ4 NC is shown in the right inset of Fig. 3e, which indicates a polycrystalline sample. The diffraction planes are annotated on the image and found to be consistent with the literature.16,32 Furthermore, we measured the thickness of ZnO coatings from TEM images and determined them to be 43, 56 and 75 nm for PZ2 NC, 3 NC and 4 NC, respectively. The ZnO shell thicknesses as a function of ALD cycle numbers are shown in Fig. S2 of the ESI† for two contexts, namely, PSU–ZnO bers and ZnO coatings on Si/SiOx wafers. The former is from TEM image analysis, while the latter is from SE. We can see the consistency in both the methods, implying the conformal coating of ZnO on the surface of the polymer bers. Note that the thickness of the ZnO shell on the bers for 100 ALD cycles is not shown because it forms island-like structures rather than a continuous lm. A linear t for the data points from TEM analysis yielded a growth rate of 0.20 nm per cycle, where ALD should be acknowledged for the excellent control on the lm thickness against deposition cycles.19,42,43 On the other hand, SE from Si yielded a growth
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TEM images of (a) PZ1 QD, (b) PZ2 NC, (c) PZ3 NC and (d) PZ4 NC; (e) high resolution images and electron diffraction pattern from PZ4 NC.
Fig. 3
rate 0.18 nm per cycle, which is closely comparable with the results from TEM. Fig. 4 shows the XRD patterns from PSU–ZnO bers of varying cycle numbers. Broadly, the diffraction peaks of ZnO were found to be wurtzite structures for all the cases. Reections corresponding to ZnO, viz (100), (002) and (101) planes (30–38 ) are annotated as (i), (ii) and (iii), respectively, as shown in Fig. 4a. We observed that as the cycle number increased the peaks became sharper (see full width at half maximum, fwhm in Table S1 of ESI†) and more well-dened. As we can see from Fig. 4a (also see Table S1 of ESI†), a signicant shi (d ) is observed for all the three peaks (ICDD 01-074-0040).16,32 In an earlier report,15 the authors observed a similar shi for (100)
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XRD patterns from PZ1 QD, PZ2 NC, PZ3 NC, and PZ4 NC within (a) 30–38 and (b) 45–90 regions.
Fig. 4
peak from the normal, where the shi is attributed to lattice defects. As outlined in the introduction, we are interested in creating defects related to Znis; on the other hand, we note that the lattice has gone through a ‘strain’ induced by defects. Hence, it is important to establish if we can expect the defects of Zni origin in the present samples. In the earlier study,15 the defects were induced by non equilibrium processes, which are sensitive to temperature and can only withstand low temperature annealing.50 Vlasenko et al. suggested that Znis are not stable unlike VOs which are stable up to 400 C.33 In line with this, results from a simulation study also suggested that Znis diffuse faster than VOs, where the migration barriers are 0.57 eV and 1.7 to 2.4 eV, respectively.34 While keeping this in the background, we can see that other reections, (002) and (101) have also shown some nonlinear deviation. These features indicate that the as-prepared samples (PSU–ZnO) are rich in some kinds of defects induced during the ALD process. Since the present deposition temperature is about 200 C, the defects such as Znis are stable, as shown by other researchers.33,34 We will see that these defects are actually interstitial zinc. Various other diffraction planes within 45–90 are annotated on Fig. 4b, and found to be consistent with the literature.16,31,32 As the cycle number increases (300 and 400), we can see that the peaks (200), (112) and (201) have been evolved to be clearer and well-dened when compared to PZ1 QD and PZ2 NC samples. Furthermore, we also observed a single peak for pristine PSU at 17.9 in the XRD patterns because of the semi-crystalline nature of the PSU bers (Fig. S3 of ESI†). As the cycle number increases, this peak is relatively extinguished (not shown here) because it is screened by an inorganic (ZnO) coating. However, it is notable that ALD did not signicantly inuence the crystalline nature of PSU, where we note that the deposition occurred at 200 C. The core-level XPS spectra of O 1s and Zn 2s are shown in Fig. 5 and 6, respectively. O 1s spectra have been deconvoluted for all the cases depending on the chemistry of the surface. The spectral location of the deconvoluted peaks match with the
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Fig. 5 Core-level XPS analysis for O 1s from (a) PSU, (b) PZ1 QD, (c) PZ2 NC, (d) PZ3 NC and (e) PZ4 NC samples. The area under the shaded peak corresponding to chemisorbed oxygen (OCh) is annotated on the image in the units of eV cps.
Fig. 6 Core-level XPS analysis for Zn 2p from (a) PZ1 QD, (b) PZ2 NC, (c) PZ3 NC and (d) PZ4 NC samples.
literature.51 Pristine PSU has shown two principal peaks corresponding to two oxygen molecules in different chemical environments (O/S and O/C), where O/S and O/C represent oxygen bonded to sulfur or carbon, respectively (Fig. 5a).51 In the PZ1
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QD sample we can observe oxygen from ZnO (O/Zn) and PSU due to the absence of continuous ZnO layers on the PSU bers, as shown by TEM images. All the deconvoluted spectra suggested the presence of chemisorbed oxygen (OCh, shaded area on the gure). This OCh can be from any of the species such as –OH, –CO, adsorbed H2O and/or O2 (ref. 4, 8, 52 and 53) or O and O2 ions.4,8 The area under the peak corresponding to OCh is annotated on the image. If we keep aside the OCh of PSU, the origin of such species for the PZ1 QD is at a higher binding energy than the other PSU–ZnO cases. This is not surprising because ZnO has not completely covered the surface of PSU, which implies that the peak in the spectrum is the sum of two contributions. OCh from PZ1 QD has shown an area of 0.28 eV cps, while the other PSU–ZnO samples have shown an average value of 0.42 eV cps. In the case of ZnO, the quantity of OCh is a measure of VOs (ref. 4, 8 and 32) because the former occupies the place of the latter in the lattice. Hence, the density of VOs increased with the thickness of ZnO or with ALD cycle numbers ($200). We will refer to the density of VOs and their effect on emission properties in the context of PL. The Zn 2p core level spectrum is important to study, especially when we suspect that interstitial defects are related to zinc. The spectra in Fig. 6 are broadly similar across all samples and the spectral position of each peak is consistent with literature, conrming the formation of ZnO.51 However, a closer inspection of the spectrum from the PZ1 QD sample has shown a shoulder-like feature at the higher energy side of the Zn 2p3/2 and Zn 2p1/2 peaks (see the shaded area in Fig. 6a). The energetic location of these shoulders (1024.19 and 1047.11 eV) suggests that zinc is in its more oxidized form than ZnO. Note that this does not correspond to Zn(OH)2, in which case the 2p3/2 peak should occur in between 1022.70–1021.80 eV.51 This is an important observation, where the Zn atom has occupied an interstitial site and is surrounded by more than one oxygen atom at the same time. This can be treated as direct evidence for the presence of Znis and on the other hand, we may also expect some VZn defects, which we will discuss in the context of PL in detail.15 We did not see any shoulder like features in the spectra of Zn 2p from the PZ2 through PZ4 NC samples (Fig. 6b–d). We believe that the surface adsorption during the ALD process has taken its toll and shown a signicant effect. Initially, DEZn is adsorbed on the surface of PSU and then H2O vapor reacts with it. However, the adsorption of H2O on the surface of PSU is not uniform because it has two functional groups, one of which is more hydrophilic (O]S]O) than the other (C–O–C). Hence, the molecular adsorption of H2O varies from place to place on the surface. As the cycle number increases, the surface is coated with ZnO, which leads to a change in the surface chemistry, and we can expect more uniform adsorption of precursor molecules. In the initial cycles, the uneven distribution of molecules on the surface of PSU would have caused the growth of ZnO with dominating Zni related defects. Nevertheless, it does not necessarily imply that Znis are not present for the higher cycle numbers, as we will see in the context of PL. However the lack of signal from Znis might be because of increased Zn signal contribution from ZnO.
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In the discussion of OCh (or VOs), it is noted that PZ1 QD possesses the least amount of VOs while Zn 2p analysis suggests that it has a higher amount of Zni related defects. As the thickness of ZnO coating increased ($200 ALD cycles), Zni related defects were not detected, and simultaneously the OCh signal was increased. Finally, for the rst 100 ALD cycles Znis are seen with limited VOs; as the thickness of ZnO increases, although the presence of Znis is not explicit, the density of VOs is denitely increased. In other words, VOs and Znis are predominant in thicker and thinner coatings, respectively. The PCA of a semiconductor can be understood by carefully analyzing the optical properties,4,8 where information regarding the radiative recombination centers (other than CB) associated with crystal defects would be revealed. In principle, there are numerous defect states within the band gap of ZnO. The þþ þ þþ * * donor defects are Znþ i ; Zni ; Zni (neutral), VO ; VO ; andVO 00 0 25,26 (neutral), and the acceptor defects are VZn and VZn . Znis and VOs are known to be the predominant ionic defect types,16,31,54–56 where the former are shallow donors, while the latter create deep levels.17,54,56 At higher oxygen partial pressures VZn and Oi may also be thermodynamically stabilized.55 However, which defect (Zni or VO) dominates in native, undoped ZnO is still under debate.15 To emphasize, in the present case, the density of Zni is controlled by simply altering the ALD cycle numbers, for which researchers have used non-equilibrium processes,15,35,36 followed by thermal annealing to control the defect density; in contrast we will see that the as-prepared samples possess defects such as Znis. Znis come from the Frenkel reaction, eqn (1), and further ionization reactions, eqn (2) and (3). * ZnZn 5Zn*i þ VZn
(1)
Zn*i 5Znþ i þe
(2)
þþ þ e Znþ i 5Zni
(3)
Blue emission from ZnO nanoparticles has been explained earlier by H. Zeng et al.,15 in which Zni are at the core of the emission. Furthermore, the extended states of Zni (ex-Znis) can be formed according to the defect ionization reactions, eqn (2) and (3), and can result in defect localization coupled with a disordered lattice. It is notable that blue emissions are quite infrequent15 for the as-prepared samples when compared with the literature.16,17,28–32 Note that Zni is 0.22 eV below the CB,57 VZn is 0.30 eV above the VB58–60 and ex-Znis are 0.54 to 0.635 eV below the CB15 in ZnO for a typical band gap of 3.36 eV.16 As mentioned earlier, the another set of major defects are VOs, which have been extensively addressed in the literature (see the discussion and cross-references in ref. 16). Despite various interpretations, previous studies17,27 are found to be rather useful in which the energetic location of VOs has been conrmed through an indirect method. Based on this the VOþ state captures an electron from CB and forms a neutral state (VO* ), from which a transition to VB occurs by the emission of green light at a 500 nm wavelength, eqn (4).16,17,27 On the other hand, VOþ captures a hole from VB and forms a VOþþ state into Nanoscale, 2014, 6, 10224–10234 | 10229
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which an electron recombines from CB and emits green light at a 565 nm wavelength, eqn (5). VO* 5VOþ þ e
(4)
VOþþ 5VOþ þ hþ
(5)
Note that VO* is 0.86 eV below the CB and VOþþ is 1.16 eV above the VB,27 for a typical band gap of 3.36 eV with an exciton binding energy of 60 meV.16 The processes described in eqn (4) and (5) occur in the bulk grain region (BGR) and depletion region (DR), respectively. Based on the above discussed defects and their energetic location within the band gap, the present PL spectra were deconvoluted, as shown in Fig. 7a, and the possible emission mechanism is schematized in Fig. 7b. Emissions from ZnO, a1–4, b1–4, c1–4, d1–4 and e1–4 in Fig. 7a correspond to the transitions a, b, c, d, and e in Fig. 7b. The spectral location of each peak (in eV and nm) is tabulated in Table 1 along with the emission mechanism. Before we discuss the various emission lines, it would be appropriate to discuss the uorescence peak of PSU from the PZ1 QD sample on which the ZnO coating is not continuous. We employed 350 nm excitation (lEx) for analyzing the emission from ZnO for which PSU has shown a shoulderlike structure; see the normalized emission spectra for three different excitations in Fig. S4 of the ESI.† By considering the shape of the emission, Lorentzian t is employed for PSU, and we can see a perfect retrace of the data points on Fig. 7a (top le panel), which peaked at 363 nm (P0 ) for 320 nm excitation. Keeping this in the background, in the case of PZ1 QD, during
the deconvolution the spectral location of this peak is xed (P00 ) and the other parameters are allowed to vary, where the excitation wavelength is 350 nm. As mentioned earlier, in the case of ZnO, the peak attributions and their wavelengths are still under discussion;15 however, the present assignments were performed based on the available evidence in the literature while using a logical approach for the cases where more than one transition is feasible. Refer to the schematic diagram Fig. 7b for each transition that is described below. The least controversial emission from interband transition occurred in the range of 382–390 nm (a1–4) for all the samples, where an electron from the free exciton (FX) level recombines with a hole in the VB. The blue region of the emission contains two probable transitions related to VZn and Zni, i.e. an electron from CB recombines with the hole at VZn or an electron is captured by Zni then recombines with the hole in the VB. Either or both of these transitions occur by the emission of b1–4.16,57,58 Aer a careful inspection the emission (b1–4) should be ascribed to Zni / VB transition, as we have noted interstitial zinc in the XPS, at least for PZ1 QD. In 2010, a study by Zeng et al.15 introduced probable transitions from Zni or ex-Znis to the VB, while an earlier investigation in 2009 by Ahn et al. suggested transitions form Zni to VZn states.57 Further complicating the attribution, in our case both the arguments have a reasonable overlap within the blue wavelength region, c1–4 (Table 1). In the discussion given by Zeng et al.,15 electrons from CB are relaxed to Zni (in line with Ahn et al.57) and/or ex-Zni via a non-radiative process and then to VB emitting violet and/or blue emissions respectively. On the other hand, Ahn et al.57 suggest two transitions from Zni to VB and/or VZn; nevertheless, in the present
Fig. 7 (a) Luminescence spectra from electrospun fibers of PSU and PZ1 QD, PZ2 NC, PZ3 NC, and PZ4 NC. Spectrum from PSU is co-plotted with that of PZ1 QD for 320 nm excitation with Lorentzian fit. Spectra from PSU–ZnO samples are decomposed into Gaussian peaks. (b) Schematic diagram depicting the various defects and the ascribed transition, where a1–4, b1–4, c1–4, d1–4 and e1–4 in (a) correspond to the transitions a, b, c, d, and e in (b). FX and Zni are 0.06 eV and 0.22 eV below the CB, respectively,57 while ex-Znis are 0.54 to 0.635 eV below the CB.15 VO* is 0.86 eV below the CB and VOþþ is 1.16 eV above the VB,27 for a typical band gap of 3.36 eV (ref. 16) The CB and VB locations are taken from ref. 61. Numerals (1) through (5) represent PCA that can occur at different places.
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Table 1 Spectral locations of deconvoluted peaks and ascribed emission mechanism. The wavelength of the emission is given in nm, and eV for convenience
Sample
Interband (nm, eV)
VZn or Zni related (nm, eV)
VO related (nm, eV)
PZ1 QD PZ2 NC PZ3 NC PZ4 NC Mechanism
382, 3.25 (a1) 386, 3.21 (a2) 390, 3.18 (a3) 389, 3.19 (a4)
400, 3.10 (b1) 404, 3.06 (b2) 407, 3.04 (b3) 407, 3.04 (b4)
501, 2.47 498, 2.49 499, 2.49 500, 2.48
395 nm
FX ! VB
395 nm
ex-Zni ! VB
405 nm
Zni ! VZn
Zni ! VB CB ! VZn
context, this description might not be feasible because we have provided evidence of Znis by XPS. Hence, the peaks, c1–4, are attributed to the ex-Zni to VB transition because the dominant Znis can form extended states. Finally, the green emission occurs due to VOs constituting two transitions,17,27,32 as shown in Table 1. These transitions are annotated with d1–4 and e1–4 in Table 1, and fall within 498–501 nm and 562–566 nm, respectively. The spectral locations of all of the abovementioned peaks are quite consistent with the literature.4,15–17,31,32 It is clear that the present samples possess defects such as Znis and VOs; however, in the following we will consider the relative density (integrated area of the peak) of each transition that is ascribed in Table 1 while referring to Table S2 of the ESI.† We have previously found this comparison to be very useful in explaining the PCA.4,8 a1–4 are considered to be sharp and intense for defect free ZnO (ref. 16) (see cross-references therein). However, in the present case, the fwhm does not vary signicantly across all samples, while the area under the peak (A) appeared to improve from PZ1 QD to the rest of the samples, indicating slightly improved optical quality.16,31,32 The next region to consider is violet emission, b1–4, where the fwhm is almost the same for all samples. Further, PZ1 QD has shown the lowest A, which nominally increased with ALD cycle numbers. The blue emission, c1–4, has shown interesting features, where the fwhm is similar for all samples, while in contrast A for PZ1 QD is almost twice that of other PSU–ZnO samples. It will be more contextual to discuss the reason for this difference aer addressing the rest of the emissions. We can see that the fwhm of d1–4 increased with the ALD cycle numbers, while A is comparable between the samples PZ1 QD and PZ2 NC, and PZ3 NC and PZ4 NC. In the case of PZ1 QD and PZ2 NC, the thickness of the ZnO is not more than 40 nm, while the transitions d1 and d2 occur in the BGR. Because the grains are not completely formed for such thicknesses, the intensity of the transition is comparatively low. In the context of d3 and d4, there is a signicant increase in the thickness, which has led to the formation of high density BGRs emitting relatively higher intensity green light, and thereby higher A. Peak e occurs from the DR, in which case the argument given for d1–4 is still valid (Fig. 7b). At lower thickness values, the density of VOs is relatively less, especially for PZ1 QD. Therefore, it is clear that the capture of electrons by the VOþ state is almost negligible; hence, a predominant emission is noticed for c1 with an A value of 67 nm counts which is almost double the intensity of c2–4. This journal is © The Royal Society of Chemistry 2014
433, 2.86 (c1) 432, 2.87 (c2) 436, 2.85 (c3) 437, 2.83 (c4) 440455 nm
(d1) (d2) (d3) (d4)
500 nm
VO* ! VB
564, 2.20 (e1) 565, 2.20 (e2) 562, 2.21 (e3) 566, 2.19 (e4) 564 nm
CB ! VOþþ
437 nm
Finally, to comment on the overall defect density and their predominance, the emission because of VOs is gradually increasing where the intensity due to Zni is almost stable. This is a very signicant nding15 where it is believed that obtaining a balance between VOs and Znis is a rather hard task, i.e. if the density of Zni increases than that of VOs decreases. To further justify the existence of Znis, their origin and their balance against VOs requires to be considered. The relative concentration of these defects is strongly dependent on formation temperature because their ionization energies lie in the range of 0.05 to 2.8 eV.33 In the case of vapor phase deposition, the partial pressure and the relative pressures of zinc and oxygen are also found to be vital,37,62 and as expected a highly reducing environment together with high temperatures causes VOs to dominate,37 while under zinc rich environments Znis are the predominant defects. Under high temperature (1100 C), the annealing of ZnO crystals depicted Znis.40 Therefore, the chosen parameters of ALD yielded high densities of Znis, where the duration of the precursor pulse, substrate and its temperature are believed to be detrimental. Since we employed different cycle numbers to manipulate the morphology, the amount of ZnO is quantied from TGA thermograms (see Fig. S5 of ESI† and its explanation). The weight percentages of ZnO in the PSU–ZnO bers were calculated by subtracting the char yield (%) of the pristine PSU bers at 700 C yielding 10, 14, 19 and 25% in PZ1 QD, PZ 2 NC, PZ 3 NC and PZ4 NC, respectively. PCA on MB in the presence of PSU–ZnO samples was tested under a UV-illumination of 365 nm wavelength; as a result, electrons are excited to CB rather than any defect sites. The degradation of MB (C/C0) as a function of ‘t’ is plotted in Fig. 8a. We observed exponential decay behavior, which follows pseudo-rst-order kinetics, as explained by the Langmuir–Hinshelwood model. Note that the degradation follows1 Langmuir sorption isotherms,63 in which organic molecules are adsorbed on the surface as a monolayer ˚ These adsorbed molecules are oxidized at a distance of 1 A. through a process mediated by the hydroxyl radical (cOH).2,3 This cOH radical forms either from an electron3 (at CB or defect site) via molecular oxygen or from a hole2 (at VB or defect site) via adsorbed H2O or OH groups on the surface. The mechanisms that govern these processes are given in eqn (6) and (7), according to Izumi et al.3 and Matthews.2 Please refer to our earlier articles4,8 for detailed discussion involving the eqn (6) and (7). Note that oxidation involves the charge carrier from a
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Fig. 8 (a) PCA of various samples compared with pristine PSU and no catalyst cases, (b) densities of various defects and (c) improvement in decay constants are referenced with PZ1 QD, and shown in percentage.
semiconductor, hence one needs to balance the interfacial charge transfer between the semiconductor and dye against carrier recombination to achieve higher efficiencies. 3 eCB þ O2 /cO2 þ 7 cO2 þ H /cHO2 7 7 2cHO2 / H2 O2 þ O2 7 7 cO2 þ cHO2 /cHO2 þ O2 7 (6) þ 7 cHO2 þ H / H2 O2 7 7 H2 O2 þ cO2 /cOH þ OH þ O2 ðorÞ 7 H2 O2 þ eCB /cOH þ OH ðorÞ 5 hn
H2 O2 ! 2cOH hVB þ þ H2 O/cOH þ Hþ hVB þ þ HO /cOH
(7)
In the no catalyst and PSU cases, no degradation is observed within the UV exposure; in contrast, PSU–ZnO samples have shown a nite decay time of varying spans (Fig. 8a). Eqn (6) and (7) govern the PCA on the surface of ZnO, where the defect sites play a crucial role in trapping the charge carriers. Please refer to Fig. 7b for the energetic location of each defect with reference to
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CB and VB of ZnO where the active sites for PCA are indicated with numerals, (1) through (5). Zni, and ex-Zni can capture electrons from CB and can take part in PCA. Although VO* exists in the bulk of the grain, it depicts indirect effect by capturing an electron from CB, and thereby delaying the recombination process. Hence, because of VO* , PCA occurs at VB via a hole. On the other hand VOþþ exists in the DR, which may be available for PCA, showing an enhanced effect when compared to VO* . This is because VOþþ can be on the surface or at grain boundaries where the captured hole and the delayed electron at CB can participate in PCA. As mentioned in the introduction, we aim to elucidate the effect of Znis and VOs on PCA. To achieve this we compared the activities and the percentage increase for each defect density (area under the peaks of a1–4 through e1–4 corresponding to each defect type) is calculated (please refer to the description in the Table S2 legend of the ESI†) with reference to PZ1 QD, which were correspondingly annotated as primed alphabets (Fig. 8b). In addition, note that if an electron is captured by a defect, the associated hole at VB participates in catalysis. Conversely, if a hole is captured by a defect, then an electron in CB can participate in catalysis. The percentage decrease in s is also calculated with reference to that of PZ1 QD ([sPZ1 QD sPZ2,3 or 4 NC] 100/sPZ1 QD), as shown in Fig. 8c. By taking into account of the discussion on relative densities of various defects, in the following we will attribute the difference in s for PSU–ZnO samples referring to Fig. 8b and c. Note that PZ1 QD has shown longer s than the others (Fig. 8a). However, the percentage decrement of s suggests an improvement in the performance of 77% for PZ2 NC. As expected, this increment is cumulative from various defects. PZ1 QD is predominant with ex-Znis, and hence the PCA occurs at (2) and (3) in addition to (1) and (5) (Fig. 7b). In the case of PZ2 NC, Znis are as dense as the PZ1 QD sample while ex-Zni states are subdued. On the other hand a0 and e0 are almost doubled, which enhances the PCA at (1), (5) and (1), (4) respectively. b2 emission is slightly improved while ignoring that of d2. Hence, majorly, 77% improvement is a sum effect of contributions from a0 [(1), (5)], e0 [(1), (4)] and b0 [(2), (5)] in that order. If similarly argued, the following conclusions can be drawn for the rest of the samples. From PZ1 QD to PZ3 NC an improvement of 80% is noticed. This is a sum effect of contributions from e0 [(1), (4)], d0 [(5)], a0 [(1), (5)] and b0 [(2), (5)] in that order. Finally, from PZ1 QD to PZ4 NC an improvement of 88% is observed, which is a sum effect of contributions from e0 [(1), (4)], d0 [(5)], a0 [(1), (5)] and b0 [(2), (5)] in that order. As discussed earlier, the physical location of VOs suggests that for the same defect levels of e0 and d0 , the former shows higher PCA than the latter. Given the overall scenario, the important thing to note is that PCA is not severely affected by defect density above a certain degree. For example, from PZ1 QD to PZ3 NC, the net increase in the defect density is nearly 770%; however, the PCA has shown an improvement from 77 to 80%. On the other hand, from PZ1 QD to PZ2 NC, the net increase in the defect density is 200%, which has shown 77% improvement in PCA. Although the ex-Znis have subdued, the increased optical quality and VOþþ states have shown an improvement of 77% in PCA, which is a signicant nding. Despite such
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signicant contribution from VOs it is notable that further increase in their density did not help to enhance the PCA. However, it is convincing that VOs enhance PCA,4,8,10,11,13,21 which can probably dominate the contribution from Znis and/or exZnis. This is because of the differences in the intrinsic nature of the defects. From eqn (1) to (5), it appears that both the Zni and VOs can form ionized levels; however, in the earlier explanation by Zeng et al.15 there is no discussion on charged zinc interstitials forming defect bands within the band gap. In clear contrast to this, VOs form two defect levels17,27,64,65 within the band gap by following the eqn (4) and (5). Among these two bands, although one is not directly accessible for PCA, it shows an indirect effect by capturing an electron from CB. Furthermore, eqn (5) is feasible17,27 in the DR of the semiconductor, where the obvious sites on the surface are the grain boundaries. As the thickness increases, initially grains take shape along with grain boundaries, in which the latter are very favorable places for the dye to adsorb. As shown by XPS, VOs are occupied by oxygen related functional groups, for example, –OH, H2O and/or O2. These species can directly participate and/or mediate the PCA. Although the point of zero charge for ZnO is 9.0 0.3 due to the predominance of VOs the surface of ZnO can be negatively charged in the presence of –OH groups, which are more favorable for cationic dyes such as MB. In the present context, we did not consider the pH of the solution and its effects. Also note (eqn (7)) that the PCA involving H2O is associated with hVB+ forming the most reactive hydroxyl radical (E0 ¼ +3.06 V) where the holes can be captured by VOþ . In our earlier study,4 we have given further explanation why holes are more reactive than electrons. Hence, given the above reasons, it is concluded that Znis are less reactive than VOs.
Conclusions We have fabricated ZnO QDs on the surface of electrospun PSU bers and carefully investigated the changes in the properties when QDs transform into NC. Since polymers are known for their exibility, PSU–ZnO bers were no exception in ease of handling, as shown in Fig. S6 of ESI.† Morphological characterization suggested that the ALD precursors and PSU are well compatible. The growth rate of ZnO lm is determined to be 0.2 nm per cycle. The morphological transformation has shown signicant variance in the crystal and optical properties of the samples. The average size of the ZnO QDs is determined to be around 10 nm from TEM images. SAED patterns were recorded and a representative pattern identied as polycrystalline ZnO. In addition to the conrmation of wurtzite structure, the XRD data suggested that the samples have suffered from severe strain, which might be due to the intrinsic lattice defects. XPS has evidenced Zni related defects quite explicitly for the PZ1 QD sample, which is because of nonuniform molecular adsorption of H2O vapor on the surface of PSU to react with DEZn. The PL spectra are recorded for all the samples, and the probable transitions are recognized based on the available evidence in line with literature. Interestingly, PZ1 QD has shown a nominal signature of VOs from XPS, which is consistent with PL data. Signicantly, the increase in the
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density of VOs occurred for higher ALD cycle numbers viz $300 despite the presence of almost the same density of Znis. The PCA experiment was performed on the samples, which depicted pseudo rst order kinetics. The decay constant was estimated for all the PSU–ZnO samples via rst order exponential t. The tting outputs of PL spectra were employed to estimate the relative densities of Znis and VOs by considering the area under each peak, corresponding to defect related transitions. The percent improvement in PCA was juxtaposed against the defect densities. The results suggest that Znis and related defects are less efficient than VOs in the context of photocatalysis. This is mainly because of the accessibility of these defects for PCA. Both Zni and VO capture charge carriers, thereby delaying the recombination process; however, the intrinsic nature of VOs allow them to exist in the grain boundaries (VOþþ ) where the region is depleted with charge carriers and in the bulk of the grain (VO* ). A hole is trapped at VOþþ ; consequently an electron is delayed at CB and can participate in PCA in addition to a delayed hole at VB due to VO* . This double effect is not observed in the case of Znis, which makes them less efficient when compared to VOs.
Acknowledgements F. K. thanks TUBITAK-BIDEB for a PhD scholarship. S. V. thanks The Scientic & Technological Research Council of Turkey (TUBITAK) (TUBITAK-BIDEB 2221 – Fellowships for Visiting Scientists and Scientists on Sabbatical) for fellowship. N. B. thanks EU FP7-Marie Curie-IRG for funding NEMSmart (PIRG05-GA-2009-249196). T. U. thanks EU FP7-Marie Curie-IRG NANOWEB (PIRG06-GA-2009-256428) and The Turkish Academy of Sciences – Outstanding Young Scientists Award Program (TUBA-GEBIP) for partial funding. Authors thank M. Guler for technical support for TEM analysis.
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