TiO2 junction

Materials Research Bulletin 51 (2014) 141–144

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Band alignment of ultra-thin hetero-structure ZnO/TiO2 junction Kai Shen a, Kunjie Wu a, Deliang Wang a,b,* a

Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China b CAS Key Laboratory of Energy Conversion Materials, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 13 April 2013 Received in revised form 17 October 2013 Accepted 9 December 2013 Available online 15 December 2013

The band alignment at the ZnO/TiO2 hetero-structure interface was measured by high resolution X-ray photoelectron spectroscopy. The valence band offset ðEZnO  ETiO2 ÞValence was linearly changed from 0.27 to 0.01 eV at the interface with increased ZnO coating thickness from 0.7 to 7 nm. The interface dipole presented at the ZnO/TiO2 interface was responsible for the decreased band offset. The band alignment of the ZnO/TiO2 heterojunction is a type II alignment. ß 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Interfaces A. Oxides B. Sputtering C. Photoelectron spectroscopy

1. Introduction TiO2 and ZnO are wide band gap oxide semiconductors. In the last 20 years, TiO2 and ZnO have been intensively studied both as an electronic electrode and as an efficient photocatalyst [1–3]. Hetero-structure of ZnO/TiO2 can be used as an active part in photo-electric and photo-catalyst devices, such as the dye sensitized solar cell (DSSC). In a DSSC device, the electron recombination, which occurs between the electrons in the TiO2 conduction band and the ions in the liquid electrolyte and oxidized dyes, is considered to be a relatively severe process that limits the energy conversion efficiency [4]. One of the strategies to mitigate electron recombination in a DSSC is to coat TiO2 nanocrystalline with a wider energy band gap oxide, such as ZnO, to form an energy barrier at the TiO2/oxide interface to blockade electron back transfer, and consequently the performance of the device was reported to be sensitive to the thickness of a ZnO coating layer [5,6]. A heterostructure formed by TiO2 and ZnO can combine the wanted material properties from both oxides. In order to understand the electron transport processes in any electronic devices, the information of the band alignment at the heterostructural interface must be known as accurate as possible. Any

* Corresponding author at: Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China. Tel.: +86 551 63600450; fax: +86 551 63606266. E-mail address: [email protected] (D. Wang). 0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.materresbull.2013.12.013

energy barrier or band offset presented at an interface would have a large consequence for electron transfer. Experimental results regarding to the band alignment of the ZnO/TiO2 hetero-structure are much needed for future device design and analysis of physical processes in the relevant devices. In this study, the band alignment at the ZnO/TiO2 hetero-structural interface with different ZnO coating thickness was characterized by high resolution X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS) and Kelvin probe force microscopy (KPFM). It was found that the valence band offset was decreased with increased ZnO coating layer thickness. The presence of an interface dipole was suggested to be responsible for the decreased band offset. 2. Experimental Anatase nanocrystalline TiO2 thin films were prepared by sol– gel and spin-coating method. Tetrabutyl titanate (Ti(OC4H9)4), acetic acid, DI water (18.2 MV cm), and absolute ethanol were used as the reaction precursors. The film was annealed at 500 8C for 30 min in air. The film thickness was about 300 nm and the grain size was about 20 nm [7]. The ZnO layers were prepared by using the RF magnetron-sputtering technique. A 99.99% ZnO target was sputtered in a reactive gas mixture of O2 and Ar with a pressure ratio O2/Ar of 1/9. The working pressure was 0.4 Pa and the RF power was 60 W. The ZnO coating layer thickness was controlled by varying the sputtering time. After the ZnO deposition, the sample was transported into an XPS system through the air. XPS

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Fig. 1. (a) High resolution XPS spectra of Zn 2p3/2 and Ti 2p3/2 of ZnO/TiO2 heterostructures with different ZnO coating thicknesses; (b) DECL and the valence band offset (VBO) versus the ZnO coating layer thickness.

measurements were performed on a Thermo VG Scientific ESCALAB 250 instrument with Al Ka as the X-ray source. All the XPS spectra were calibrated by using the carbon C1s peak (284.60 eV). A Kelvin probe force microscopy (KPFM) system (Veeco Multimode V) was employed to perform characterization on both the surface potential and the topographic microstructure. The tip used was a highly doped silicon tip coated with Pt/Ir. XPS technique has been proved to be a direct and powerful tool for measuring the valence band offset (VBO) of a heterostructure [8]. For an XPS measurement, due to the relatively small inelastic mean free path l of the photoelectron, the coating layer must be thinner than 3–5 times the value of l. So that the photoelectron from the underlying layer and/or from the hetero-structural interface can escape through the upper coating layer. The inelastic mean free path l is a function of the kinetic energy of the photoelectron. It depends on the energy of the X-ray employed as the light source in a XPS system. In our experiment, the XPS system employed the Al Ka radiation (energy of 1486.60 eV) as the excitation source. The Al Ka X-ray source combined with the oxide materials to be studied allowed us to get relatively high intensity XPS signal from the hetero-structural interface with a ZnO coating layer as thick as 7 nm, as can be seen in Fig. 1.

strong intensity of the Ti 2p XPS peaks for the TiO2 films covered with ZnO overlayers, shown in Fig. 1(a), is due to the following reasons: (1) The inelastic mean free path l for the photoelectron in ZnO material is relatively large. The XPS sampling depth (3l) of the Ti 2p electron in ZnO can be calculated by the TPP-2M method and it has a value of 6.2 nm [9]. This value is large due to the large kinetic energy of the Ti 2p photoelectron (1028 eV). (2) In this study the ZnO layers were prepared by using the RF magnetronsputtering technique, and the ZnO films are of polycrystalline nature and they are composed of TiO2 nanocrystalline grains with size of 20 nm. The nanocrystalline structure may help the photoelectrons from the TiO2 underlayer to escape through the ZnO overlayer without much intensity attenuation. The symmetry and peak widths of both the Zn 2p3/2 and the Ti 2p3/2 peaks kept almost unchanged with increased thickness of the ZnO coatings, indicating that at the ZnO/TiO2 interface, there were almost no detectable chemical reactions and/or state mixing between the two oxides. This is important when the XPS technique can be reliably employed to characterize the band offset at a heterostructure interface. The valence band offset EVBO at the ZnO/TiO2 junction can be calculated by employing XPS data using the following equation:

3. Results and discussion

EVBO ¼ ðEVBM  Zn2p3=2 Þbulk ZnO  ðEVBM  Ti2p3=2 Þbulk TiO  DECL

Fig. 1(a) shows the Zn 2p3/2 and the Ti 2p3/2 XPS spectra of the ZnO/TiO2 heterostructures with a ZnO coating thickness of 0.7, 2.2, 5, and 7 nm, respectively. As expected, the intensity of the Zn 2p3/2 signal was increased with increased thickness of the ZnO coating layer. The intensity of the Ti 2p3/2 signal was decreased slightly when the coating layer thickness was less than 5 nm, indicating a photoelectron escape depth of at least 5 nm in ZnO. The relatively

2

(1)

where (EVBM  Zn 2p3/2)bulk ZnO is the energy difference between the valance band maximum and the Zn 2p3/2 core level measured on a thick ZnO film, shown in Fig. 2(a). The valence band maximum was obtained by the linear fittings to the XPS valence band data. The intersect of the two linear extrapolations, see Fig. 2, corresponds to the valence band maximum. The values of EVBM

Fig. 2. (a) Core-level Zn 2p and valence band XPS spectra of bulk ZnO. The inset shows detailed XPS spectrum near the valence band maximum. (b) Core-level Ti 2p and valence band XPS spectra of bulk TiO2. The inset shows detailed XPS spectrum near the valence band maximum.

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Fig. 3. The O1s XPS spectra for ZnO/TiO2 heterostructures with different ZnO thicknesses.

and Zn 2p3/2 measured on a 180-nm-thick ZnO are 2.04 and 1020.68 eV, respectively. The corresponding values of EVBM and Ti 2p3/2 measured on a 300-nm-thick TiO2 film, are 2.21 and 458.38 eV, respectively, as shown in Fig. 2(b). DECL = (Ti 2p3/ 2  Zn 2p3/2)interface is the energy difference between the two core levels of the Ti 2p3/2 and the Zn 2p3/2, which was measured at a ZnO/TiO2 hetero-junction interface. The values of DECL are 562.20, 562.30, 562.38, and 562.46 eV for the 0.7, 2.2, 5, and 7 nm thick ZnO on TiO2, respectively. It can be seen that for different thick ZnO coated hetero-junction interfaces the values of DECL showed an almost linear increase with increased ZnO thickness, see Fig. 1(b). This point will be discussed in the following paragraphs. The valence band offsets calculated according to Eq. (1) are 0.27, 0.17, 0.09, and 0.01 eV for the ZnO/TiO2 heterojunctions with a ZnO layer thickness of 0.7, 2.2, 5, and 7 nm, respectively. We can see that the valence band offset ðEZnO  ETiO2 ÞValence was linearly changed from 0.27 to 0.01 eV at the ZnO/TiO2 interface with increased ZnO coating thickness. The relatively small valence band offset observed in this study is in consistent with the common anion rule. For a heterojunction made of two oxides, the valence bands of both the ZnO and the TiO2 are mainly formed by the contribution from the oxygen atomic 2p orbital. Therefore the energies of the valance band maximums are near each other for these two materials [10]. Taken the energy band gap energies as 3.37 and 3.20 eV for ZnO and TiO2 [11,12], we can get the conduction band offsets of 0.44, 0.34, 0.26, and 0.18 eV for the ZnO/ TiO2 heterojunctions with a ZnO layer thickness of 0.7, 2.2, 5, and 7 nm, respectively. It can be seen that the value of the energy barrier for electron transport varies with the ZnO coating thickness. ZnO has been reported to act as an energy barrier for electron recombination when coated onto the TiO2 electrode of a DSSC [5,6]. It can be seen that the energy barrier is relatively small, and as reported in the publications, this energy barrier may be easily modified by defects and dipole formation at the heterostructural interface [13]. This may explain why there have been controversial and inconsistent reports on the effect and mechanism of a ZnO coating layer on the device performance of a DSSC [5,6,14,15]. In the ideal case the value of DECL does not vary with the thickness of the deposited coating film. However, it may change if a strong band bending, strain effect, and structural dipole formation were presented at the interface, and/or the electronic structure of the deposited film evolves with film thickness [16,17]. Regarding the band offset variation with the thickness of the ZnO coating layer, we suggested that presence of an interface dipole, which was formed by electron transfer from the ZnO coating layer to the interface states at the ZnO/TiO2 interface, was responsible for the band offset variation. The energy of the ZnO conduction band edge is higher than that of the TiO2, electron transfer from ZnO to the

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interface states occurred upon deposition of a ZnO layer [10]. The dipole direction is directed from TiO2 to ZnO. The formation of the interface dipole may be assisted by the oxygen vacancy presented in the ZnO layer. The O1s XPS spectra for the ZnO coating layers with different thicknesses are shown in Fig. 3. From the XPS data fittings, we can see that three component peaks were detected. The main peak located at 529.9 eV corresponds to the lattice oxygen of ZnO and TiO2. The shoulder peak located at 531.3 eV was related to the oxygen vacancy state or oxygen deficient region (oxygen surrounded by the oxygen vacancies). Another shoulder peak located at 532.2 eV is commonly attributed to adsorbed H2O or – OH on the film surface [7,18–20]. The presence of an anion vacancy changes the distribution of the negative-charge density. The charge density in the region around an oxygen vacancy reduces the screening of the nearest-neighbor oxygen ions, thus raising the effective nuclear charge of an oxygen ion near the vacancy and consequently the binding energy is increased. It is known that oxygen vacancy is a shallow donor in undoped ZnO film, and it increases carrier concentration in the ZnO film [21,22]. Compared to very thin ZnO coating layers, the thicker ZnO layers had a larger amount of oxygen vacancy which provided more electrons to be transported to the conduction band of the TiO2 and therefore would cause a stronger band bending at the interface. This suggestion is consistent with the decrease of the valence band offset for thicker ZnO coating layers. The band alignment of the ZnO/TiO2 heterostructure with a 0.7-nm-thick ZnO coating layer based on the XPS data is shown in Fig. 4. The band alignment is a type II alignment [23]. The surface potentials of the ZnO coating layers with different thickness were directly characterized/measured by the Kelvin probe force microscope technique. As discussed above, a thicker ZnO layer would support more electrons to be transported to the underlying TiO2 layer, resulting in a stronger band bending and leaving the ZnO coating layer in a higher electric potential compared to a thinner ZnO one. Indeed, as shown in Fig. 5, a surface potential difference of 65 mV was measured between the 2.2-nmand the 5-nm-thick ZnO coating layers. This value is in good agreement with the band offset difference of 80 meV obtained by the XPS data for these two ZnO/TiO2 heterostructures as discussed above. Electrochemical impedance spectroscopy (EIS) measurements were carried out to study the effect of ZnO coating layer thickness on electron back transfer of DSSC devices with composite photoelectrodes ZnO/TiO2. The solar cell dark I–V curves showed that the leakage current was decreased with increased thickness of the ZnO coating layers and consequently the solar cells demonstrated higher open-circuit voltage and fill factor (not shown here). The decreased leakage current indicates that the interfacial recombination, which occurred at the interface of TiO2/ZnO and dye/electrolyte, was suppressed. This conclusion has been

Fig. 4. The band alignment of the 0.7-nm-thick ZnO on TiO2 heterostructure obtained based on the XPS data.

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Fig. 5. Kelvin probe force microscopy surface potential images of (a) the 5-nm-thick and (b) the 2.2-nm-thick ZnO coating layers on TiO2; (c) potential profiles obtained along the two lines indicated in Fig. 5(a) and (b).

confirmed by the EIS measurements shown in Fig. 6. EIS is a powerful technique to study the kinetics of electrochemical and photo-electrochemical processes occurred in a DSSC [24,25]. The EIS spectra carried out on DSSCs with ZnO thickness of 0.7 and 2.2 nm, measured under a forward bias of 0.75 V, are shown in Fig. 6. The responses were recorded in a frequency region of 1– 105 Hz. It can be seen that the two DSSCs showed a typical EI spectrum containing two semicircles. The small semicircle in the high frequency region, 103–105 Hz, corresponded to the charge transfer processes occurred at the Pt/electrolyte interface. The big semicircle in the low frequency region of 1–103 Hz, can be assigned to the electron back transfer reaction at the interface of TiO2/ZnO and dye/electrolyte [25]. A semicircle with a larger radius in the Nyquist plot means a less possibility of electron recombination occurred at the TiO2/ZnO/dye/electrolyte interface. The EIS spectra demonstrated a clear trend of reduced electron recombination upon coating a thin ZnO layer on TiO2. Once an electron is injected into the TiO2 conduction band through a ZnO coating layer, it will have a much less possibility to recombine with the I3 in the electrolyte and the oxidized dyes, leading to a decreased leakage current. As discussed above, the conduction band offsets was decreased from 0.44, 0.34, 0.26, to 0.18 eV for the ZnO/TiO2 heterojunctions with a ZnO layer thickness of 0.7, 2.2, 5, and 7 nm, respectively. Even though the energy barrier for electron back transfer was decreased for thicker ZnO coating layers, the DSSC device performance showed that the possibility for electron back transfer depended on both the energy barrier and the thickness of the ZnO cover layer. Therefore the overall effect of a ZnO coating layer on the DSSC device performance must consider both the barrier height and barrier thickness, just as it is usual for a tunneling transfer process.

4. Conclusion In summary, the valence band offset ðEZnO  ETiO2 ÞValence was found to be linearly decreased from 0.27 to 0.01 eV at the ZnO/TiO2 interface with increased ZnO coating thickness from 0.7 to 7 nm. The relatively small valence band offset is in consistent with the common anion rule, which states that the valence bands of both the ZnO and the TiO2 are mainly formed by the contribution from the oxygen atomic 2p orbital. The band alignment of the ZnO/TiO2 heterojunction is a type II alignment. Acknowledgement This work was financially supported by the National Natural Science Foundation of China (Nos. 60876047 and 60976054). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]

Fig. 6. Electrochemical impedance spectra of DSSCs based on bare TiO2 and ZnO– TiO2 electrodes with a ZnO thickness of 0.7 and 2.2 nm, respectively.

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