Chinese Journal of Chemical Physics  2017, Vol. 30 Issue (6): 626-630

The article information

Qun-qing Hao, Zhi-qiang Wang, Dong-xu Dai, Chuan-yao Zhou, Xue-ming Yang
郝群庆, 王志强, 戴东旭, 周传耀, 杨学明
Photoelectron Spectroscopic Study of Methanol Adsorbed Rutile TiO2(110) Surface
CH3OH/TiO2(110)界面的光电子能谱研究
Chinese Journal of Chemical Physics, 2017, 30(6): 626-630
化学物理学报, 2017, 30(6): 626-630
http://dx.doi.org/10.1063/1674-0068/30/cjcp1711219

Article history

Received on: November 15, 2017
Accepted on: December 20, 2017
Photoelectron Spectroscopic Study of Methanol Adsorbed Rutile TiO2(110) Surface
Qun-qing Haoa, Zhi-qiang Wanga,b, Dong-xu Daia, Chuan-yao Zhoua, Xue-ming Yanga     
Dated: Received on November 15, 2017; Accepted on December 20, 2017
a. State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian 116023, China;
b. School of Physics and Optoelectronic Engineering, Xidian University, Xi'an 710071 China
*Author to whom correspondence should be addressed. Chuan-yao Zhou, Email:chuanyaozhou@dicp.ac.cn; Xue-ming Yang, Email:xmyang@dicp.ac.cn, Tel:+86-411-84379701, FAX:+86-411-84675584
Part of the special issue for "the Chinese Chemical Society's 15th National Chemical Dynamics Symposium"
Abstract: Methanol/TiO2(110) is a model system in the surface science study of photocatalysis where methanol is taken as a hole capture. However, the highest occupied molecular orbital of adsorbed methanol lies below the valence band maximum of TiO2, preventing the hole transfer. To study the level alignment of this system, electronic structure of methanol covered TiO2(110) surface has been measured by ultraviolet photoelectron spectroscopy and the molecular orbitals of adsorbed methanol have been clearly identified. The results indicate the weak interaction between methanol and TiO2 substrate. The static electronic structure also suggests the mismatch of the energy levels. These static experiments have been performed without band gap excitation which is the prerequisite of a photocatalytic process. Future study of the transient electronic structure using time-resolved UPS has also been discussed.
Key words: Methanol    TiO2    Ultraviolet photoelectron spectroscopy    Level alignment    
Ⅰ. INTRODUCTION

Photocatalysis holds the hope to solve the energy and environment problems we encountered today. Among the countless developed photocatalysts, titanium dioxide has become a prototype due to the excellent characters such as abundance, stability, nontoxicity and relative high photocatalytic efficiency since the seminal work by Honda and Fujishima in 1972 [1]. Adding methanol has been discovered to considerably improve the efficiency of photocatalytic water splitting over TiO$_2$ based photocatalysts [2] and methanol is regarded to act as a hole scavenger [3-5]. As a result, the chemical properties of methanol on TiO$_2$ surface have been extensively investigated [6-9].

Surface science studies combining ultraviolet vacuum (UHV), well ordered single crystals and advanced surface analysis techniques have provide microscopic insights such as reaction sites, elementary reaction steps and reaction kinetics into the photochemistry of methanol on TiO$_2$(110). Methanol prefers molecular adsorption on the five-coordinated Ti sites (Ti$_{5\rm{c}}$) on TiO$_2$(110) and dissociates spontaneously at the bridging bonded oxygen vacancies (O$_\rm{v}$) [10, 11]. Upon band gap excitation, methanol on Ti$_{5\rm{c}}$ sites become dissociated, producing formaldehyde and bridging hydroxyls [12-16].

As we know, a photocatalytic process include band gap excitation, charge separation, charge transportation, interfacial charge transfer, and finally surface redox reactions. These steps are greatly related to the electronic structure of the system under investigation. Therefore, studies on the occupied and unoccupied electronic structure near the Fermi level ($E_\rm{F}$) are of vital importance in understanding the level alignment which is one of the key issues in photocatalysis. The valence band structure of methanol adsorbed TiO$_2$(110) has been measured by ultraviolet photoelectron spectroscopy (UPS) [17]. Though the molecular orbitals of methanol are not well resolved, one can clearly judge that the highest occupied molecular orbitals (HOMO) of adsorbates is right below the valence band maximum (VBM). The excited states on this interface have also been studied by two-photon photoemission spectroscopy (2PPE). Li et al. detected an excited state at $\sim$2.4 eV above $E_\rm{F}$ and attributed it to the lowest unoccupied molecular orbitals (LUMO) of the adsorbates [18]. However, a later study suggests this excited state was associated with the photodissociation of methanol on this surface and the LUMO of adsorbates lie much higher than this state [12]. A most recent investigation reveals the excited state at $\sim$2.4 eV above $E_\rm{F}$ is an intrinsic character of the TiO$_2$ substrate and it arises from the John-Teller induced splitting of the 3d orbitals of reduced Ti ions, i.e., Ti$^{3+}$ [19]. Theoretical calculations are also used to simulate the levels at methanol/TiO$_2$ interface [20-23]. Accurate experimental data of electronic structure are needed to verify the validity of the calculations.

FIG. 1 Schematic overview of the experimental arrangement of the UV source and the hemispherical electron energy analyzer (HSA). The angle between the axes of the UV source and the HSA is fixed to 50$^{\circ}$.

As mentioned above, methanol is regarded as a hole capture in TiO$_2$ based photocatalysis. However, electronic structure measurement suggest the mismatch of the energy levels. Motivated by such contradiction and the lack of accurate molecular orbital data of adsorbed methanol on TiO$_2$(110), we re-investigate the valence band structure of the interface by UPS. By comparing with gas phase data, molecular orbitals of adsorbed methanol have been clearly identified. The results suggest relative weak interaction between methanol and TiO$_2$ substrate. The mismatch between the methanol HOMO and the TiO$_2$ VBM still exists in this static electronic structure measurement. Future time-resolved UPS studies on the transient electronic structure and ultrafast electron dynamics have been proposed.

Ⅱ. EXPERIMENTS

All the experiments were carried out in the UHV (base pressure$ < $1$\times$10$^{-10}$ mbar) system [24] which contains two chambers. One is for sample preparation and characterization and the other is for photoelectron spectroscopy measurement. Argon ion source (IQE 11/35, SPECS), home-made resistive heating, X-ray photoelectron spectroscopy (XPS; XR 50, SPECS) and low energy electron diffraction (LEED) are equipped for preparation and characterization of the sample. $\text{ }\!\!\mu\!\!\text{ }$-metal is installed in the whole chamber to shield the earth magnetism. A hemispherical electron energy analyzer (HSA; PHOIBOS 100, SPECS) is used to collect and analyze the photoelectrons which are amplified by two micro-channel plates (MCP) and then imaged by a two-dimension charge coupled device (2D-CCD) camera (Sensicam, PCO). This imaging method facilitates the simultaneous recording of the energy and angular distribution of the photoelectrons of interest. The ultimate resolution of the HSA is less than 5 meV.

The TiO$_2$(110) sample (Princeton Scientific Corp, 10 mm$\times$10 mm$\times$1 mm) used in the present study was prepared by circles of Ar$^+$ sputtering (10 min) and UHV annealing at 850 K (30 min). The cleanness and the long range order were confirmed by XPS and LEED. After purification, HPLC grade CH$_3$OH (Sigma-Aldrich) was introduced onto the TiO$_2$(110) via a calibrated effusive beam doser at 120 K. The amount of CH$_3$OH was determined by temperature-programmed desorption spectroscopy (TPD) which was recorded by a modified quadrupole mass spectrometer (RGA 200, Stanford Research System). The coverage was calculated relative to the density of Ti ions (1 monolayer (ML)=5.2$\times$10$^{14}$ molecules/cm$^2$) on the (110) surface of TiO$_2$. The valence band structure of bare TiO$_2$(110) and CH$_3$OH/TiO$_2$(110) were measured by ultraviolet photoelectron spectroscopy (UPS) at about 100 K. Extremely ultraviolet (EUV) source He I (21.2 eV) was generated via discharging of He (UVS 10/35, SPECS). The linewidth of the He I light source is less than 2 meV. Two-photon photoemission spectroscopy (2PPE), which was excited by the second harmonic generation (405 nm) of an 80 MHz oscillator (MaiTai eHP, Spectra-Physics), was chosen to measure the work function of the TiO$_2$ interfaces.

Ⅲ. RESULTS AND DISCUSSION

FIG. 2 shows the valence electron spectroscopy of clean and 0.5 ML methanol absorbed rutile TiO$_2$(110) surface measured by UPS. The UPS spectra for the clean surface resemble those reported previously [25-27] and can be divided into three regions: (i) the secondary electron (SE) signal which centers at a binding energy of 12.74 eV. The interception of the linear fitting of the rising edge of SE can be used to measure the work function of the surface. (ii) O 2p signal of TiO$_2$ surface between 10 and 3 eV. The two peaks with binding energies of 5.15 and 7.25 eV are ascribed to the O 2p-Ti 4sp $\pi$ bond and O 2p-Ti 3d $\sigma$ bond [28], respectively. (iii) The band gap states signal below 3 eV. Although rutile TiO$_2$ is a wide band gap (3.05 eV) semiconductor, it can be easily reduced to introduce Ti$^{3+}$, which can be reflected by the presence of a gap state (Ti$^{3+}$ 3d) with a binding energy of $\sim$1.00 eV [29-31].

FIG. 2 UPS spectra ($h\nu$=21.22 eV) collected at 100 K on the clean rutile TiO$_2$(110) surface (red), the TiO$_2$(110) surface after adsorption of 0.5 ML methanol (green) and the difference spectrum by subtracting the spectrum of clean surface from that of the methanol covered TiO$_2$(110) surface (blue). The emission angle is 50$^{\circ}$ and the incidence angle is 0$^{\circ}$. The photoemission spectrum of CH$_3$OH in the gas phase [34] is shown as a inset at the left-top.

Upon 0.5 ML methanol adsorption, significant changes can be observed: (ⅰ) though the peak position remains unchanged, the rising edge of the SE moves towards the higher binding energy direction, suggesting the decrease of the work function which is due to the adsorption of electron donating molecules such as water and methanol [32]. (ⅱ) An obvious shoulder develops by the side of the SE peak at 10.95 eV and the overall intensity of the O 2p signal increases by $\sim$10%. We will show later that these features contain the contribution from the molecular orbitals of methanol. (ⅲ) The VBM (labeled by dashed line in FIG. 2) increases by $\sim$0.05 eV, indicating the downward band bending of the surface band structure, which is consistent with the electron donation of the methanol to the surface [33]. (ⅳ) the signals in the band gap shrink by $\sim$20%, which is likely due to the scattering of the photoelectrons by the adsorbates.

In view of investigating the electronic structure of adsorbed methanol on TiO$_2$(110) surface, UPS of clean TiO$_2$ was subtracted from that of 0.5 ML methanol covered rutile. Although such a simple subtraction can't get the exact electronic structure of the pure adsorbed methanol molecules on TiO$_2$(110) because of the adsorbate-substrate interaction, the difference spectra can still reflect the rough results since the methanol-TiO$_2$ interaction is relatively weak, as can be seen in the following analysis. The difference spectrum is shown as blue line in FIG. 2 and the assignments are summarized in Table Ⅰ. There are four broad peaks with the binding energies of 6.25, 8.25, 10.45, and 14.55 eV respectively. To simplify the description, they are labeled as A, B, C and E. For comparison, the photoelectron spectroscopy of the gas phase methanol [34] is also displayed as an inset in FIG. 2. The five peaks at 10.8, 12.8, 15.0, 16.0, and 17.5 eV are assigned to the molecular orbitals 2a$''$, 7a$'$, 6a$'$, 1a$''$ and 5a$'$ respectively. The two orbitals 6a$'$ and 1a$''$ overlap but one can still see the shoulder. It should be noted the relative energies of the photoemission spectra in gas phase and on surface are different. In gas phase, the vertical ionization energy $\varepsilon_\rm{i}$ is measured relative to the vacuum level ($E_\rm{v}$), while on surfaces the binding energy $E_\rm{b}$ is referred to the Fermi level ($E_\rm{F}$). The energies of these two systems can be linked by the following expression [34]:

$ \begin{eqnarray} E_\rm{b} = \varepsilon_\rm{i}-\phi_{\rm{ads}} \end{eqnarray} $ (1)
Table Ⅰ Assignment of the molecular orbitals of gas phase and adsorbed (on TiO$_2$(110)) methanol. The unit of the energy is eV. $E_\rm{v}$, $E_\rm{F}$ and SE stand for the vacuum level, Fermi level and secondary electrons respectively. The capital letters in the adsorbed methanol column are the labels of the difference spectra in FIG. 2. The gas phase data are adapted from Ref.[34].

Where $\phi_{\rm{ads}}$ is the work function of the surface after adsorption. In order to establish the relation between the methanol photoelectron spectra in gas phase and on rutile TiO$_2$(110), we need to measure the work function of the methanol covered surface first. As mentioned above, the work function in UPS spectra can be obtained by linear fitting the rising edge of the SE peak. However, the regions you choose to carry out the fitting can considerably affect the results, due to the curvature of the rising edge. We therefore measured the work function by 2PPE (FIG. 3) in which the lower and upper cut off are determined by the work function and the two-photon energy (2$h\nu$). Usually, the final state energy at the half height of the SE edge (vertical dash lines) is taken as the work function [32]. According to this creteria, the work function of the clean and 0.5 ML CH$_3$OH adsorbed TiO$_2$(110) surface are measured to be 5.17 and 4.50 eV respectively. Such results are consistent with the decrease of work function after methanol adsorption measured by UPS (FIG. 2).

FIG. 3 The 2PPE spectra ($h\nu$=3.06 eV) of the clean and 0.5 ML methanol covered rutile TiO$_2$(110) surface. The signal in these spectra is integrated from -5$^{\circ}$ to +5$^{\circ}$. All the energies are measured relative to $E_\rm{F}$.

By subtracting the 4.50 eV work function of the 0.5 ML methanol covered TiO$_2$(110) from the gas phase methanol photoelectron spectra, we can now compare the electronic structure of gas phase and adsorbed methanol (Table Ⅰ). Obviously the molecular orbitals of methanol molecules become much broader once they are adsorbed onto the TiO$_2$(110) surface. The peak position of 2a$''$(A) and 7a$' $(B) molecular orbitals of the adsorbed methanol agree well with those in the gas phase. However, the 2a$''$ orbitals becomes significantly asymmetric with a long tail towards the VBM, suggesting the interaction between the methanol molecules and the substrate. The 6a$'$ and 1a$''$ orbitals, which are very close and overlap severely even in the gas phase, finally converge into one broad peak (C/D) at 10.50 eV on TiO$_2$(110) surface. The energies are also consistent with those in the gas phase. The 5a$'$ orbital of the adsorbed methanol is completely buried in the intense SE signal. Peak E is an artifact from the subtraction because of the different cut off in the two UPS spectra.

The comparison of the molecular orbitals of gas phase and adsorbed methanol tell us the following information: (ⅰ) the peak position of the five molecular orbitals of the adsorbed methanol agree well with those in the gas phase, indicating methanol remains molecular adsorption on TiO$_2$(110) and their interaction with the substrate is relatively weak. In fact in a control experiment to check whether the He I light source can affect the adsorption state of methanol on TiO$_2$(110), all the possible chemical reaction products were measured by TPD when the 0.5 ML methanol/TiO$_2$(110) interface has been exposed to the light source for adequately long time, and no detectable products have been observed. Therefore, in our experiments the effect of the He I light source on methanol can be neglected. This control study also suggests methanol prefers molecular adsorption in the current experimental conditions, which is consistent with previous experimental and theoretical investigations [11, 12]. (ⅱ) Although the peak position of the highest occupied molecular orbital (HOMO) of the adsorbed methanol, i.e., 2a$''$, remains the same as that in the gas phase, it develops a long tail towards the VBM. Feature 2a$''$ is from the ionization of electrons in an out-of-plane (C-O-H plane), non-bonding orbital on oxygen [34]. As we know, the adsorption of methanol on TiO$_2$ is realized by the donation of the lone pair electrons to the empty Ti 3d orbitals of the substrate. Such a configuration leads to the changes of the oxygen related orbitals of adsorbed methanol compared to those in the gas phase. TiO$_2$ is an n-type semiconductor, we simply take the conduction band minimum (CBM) as the Fermi level. The HOMO peak of adsorbed methanol and the VBM are located at 6.25 and 3.05 eV respectively. Even when the interaction of methanol with TiO$_2$ extends the HOMO upper edge to 4.20 eV, there is a big mismatch of the energy level, preventing the hole transferring from VBM to adsorbate HOMO since in most cases the hot photogenerated charge carriers cool to the relative band edges in ultrafast time scale [5].

The above level alignment are measured without band gap excitation which is the prerequisite of photocatalysis. To investigate the effect of band gap excitation on the interfacial electronic structure, we repeated the above experiments and added a continuous ultraviolet (UV) light source (360 nm) simultaneously during the measurements. However, we did not observe any change which is probably due to the low flux of the UV light source. The development of ultrafast laser techniques in the EUV and soft X-ray range, for example, free electron laser (FEL) and high order harmonic generation (HHG), provides unprecedented opportunities for the investigation of the transient electronic structure. An HHG based time-resolved UPS spectrometer is under construction in our lab. Such a method has been used to study the ultrafast electronic structure [35] and chemical reactions [36, 37] on surfaces. We are going to measure the transient level alignment and ultrafast valence electronic structure of the prototypical TiO$_2$ based photocatalytic systems with the newly built spectrometer.

Ⅳ. CONCLUSION

Valence band structure of methanol covered rutile TiO$_2$(110) surface has been studied by UPS. Several molecular orbitals of the adsorbed methanol have been identified by comparing with those in the gas phase. The results indicate methanol interacts weakly with the TiO$_2$ substrate. Static UPS measurements also suggest the mismatch of the adsorbate HOMO and the substrate VBM, contradicting with the generally accepted fact that methanol acts as a hole scavenger in TiO$_2$ based photocatalysis. Transient level alignment of the photocatalytic systems will be studied with time-resolved UPS in the future.

Ⅴ. ACKNOWLEDGMENTS

This work was supported by the National Natural Science Foundation of China (No.21573225 and No.21703164), the Youth Innovation Promotion Association of CAS (No.2017224) and the Natural Science Foundation of Liaoning Province (No.2015020242).

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CH3OH/TiO2(110)界面的光电子能谱研究
郝群庆a, 王志强a,b, 戴东旭a, 周传耀a, 杨学明a     
a. 中国科学院大连化学物理研究所分子反应动力学国家重点实验室, 大连 116023;
b. 西安电子科技大学物理与光电工程学院, 西安 710071
摘要: 为研究CH3OH/TiO2(110)系统的能级结构,本文使用了紫外光电子能谱(UPS)重新对CH3OH/TiO2(110)界面的价带电子结构进行了研究.结果清晰地分辨了吸附态甲醇的分子轨道,相比于气相分子其能量没有变化,表明甲醇与二氧化钛基底的相互作用较弱.体系的静态电子结构表明甲醇的HOMO与基底VBM存在很大的能级失配.上述研究都是在没有带隙激发的前提下研究得出的,但带隙激发恰恰又是光催化反应的先决条件.因此,本文讨论了在未来工作中使用时间分辨的紫外光电子能谱研究瞬态电子结构的相关问题.
关键词: 甲醇    二氧化钛    紫外光电子能谱    能级匹配