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Enhancement Effect of Gold Nanoparticles on the UV-Light Photochromism of Molybdenum Trioxide Thin Films Tao He,† Ying Ma,† Yaan Cao,† Peng Jiang,‡ Xintong Zhang,† Wensheng Yang,† and Jiannian Yao*,† Center for Molecular Sciences & Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China, and Beijing Laboratory of Vacuum Physics, Center for Condensed Matter Physics & Institute of Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, People’s Republic of China Received May 5, 2001. In Final Form: September 6, 2001 A new kind of photochromic composite film is prepared by depositing gold nanoparticles onto the surface of MoO3 thin film. It is found that gold nanoparticles can improve the UV-light coloration performance of the MoO3 thin film. X-ray photoelectron spectroscopy and surface photovoltage spectroscopy measurements indicate that more water is adsorbed on the composite film surface after the deposition of gold nanoparticles. The increase in the adsorbed water on the film surface allows an efficient utilization of the photogenerated holes during the coloration process. Specifically, the Schottky barrier formed at the MoO3/Au interface facilitates the separation of photogenerated carriers. As a result, the photochromic performance of the MoO3 film is enhanced greatly by the gold nanoparticles.
Introduction Molybdenum trioxide exhibits new optical absorption bands (i.e., shows a new color) after it is irradiated with UV light.1 This effect has resulted in numerous studies1-7 due to its possible technological applications in large-area displays, high-density memory devices, smart windows, and so forth. For photochromic materials, it is of great importance to improve their coloration performance for practical applications. Generally, the coloration performance can be improved by suppressing the recombination process of the photogenerated carriers, which is one of the main factors that affect the photochromic behavior. The Schottky barrier, which may be formed at the interface by depositing noble metals with high work functions onto the surface of a semiconductor, could facilitate the separation of photogenerated electrons and holes. Thus, it is possible to improve the coloration performance of MoO3 by fabricating a Schottky barrier at the metal/ semiconductor interface. The modification of the semiconductor surface with noble metals can enhance the photocatalytic activities of the semiconductor by mediating its surface properties.8,9 However, only a few papers have reported the effect of metal modification on the coloration of materials.10-13 Most of them have been mainly focused * Corresponding author. E-mail:
[email protected]. † Center for Molecular Sciences & Institute of Chemistry. ‡ Center for Condensed Matter Physics & Institute of Physics. (1) Colton, R. J.; Guzman, A. M.; Rabalais, J. W. Acc. Chem. Res. 1978, 11, 170. (2) Deb, S. K. Philos. Mag. 1973, 27, 801. (3) Yao, J. N.; Loo, B. H.; Fujishima, A. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 13. (4) Fleisch, T. H.; Mains, G. J. J. Chem. Phys. 1982, 76, 780. (5) Yao, J. N.; Hashimoto, K.; Fujishima, A. Nature 1992, 355, 624. (6) Pichat, P.; Mozzanega, M.; Hoang-Van, C. J. Phys. Chem. 1988, 92, 467. (7) Yao, J. N.; Loo, B. H.; Hashimoto, K.; Fujishima, A. Ber. BunsenGes. Phys. Chem. 1991, 95, 554. (8) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. (9) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735. (10) Sichel, E. K.; Gittleman, J. I. Appl. Phys. Lett. 1978, 33, 564.
on WO3 and its electrochromism, and in addition, vacuum evaporation or sputtering is usually the metal deposition or doping technique, which is a relatively slow and costly process.14 In this work, gold nanoparticles are used to modify the surface of MoO3 thin film by the spin-on coating technique due to their excellent air stability and isolability.15,16 It is found that the UV-light coloration performance of MoO3 thin film is enhanced greatly by the modification of the MoO3 surface with gold nanoparticles. Experimental Section Chemicals. MoO3 powder (99.99%) was bought from KOSO Chemicals. Sodium borohydride (NaBH4), tetraoctylammonium bromide ([CH3(CH2)7]4N‚Br), hydrogen tetrachloroaurate (HAuCl4‚ 3H2O), and 1-nonanethiol (C9H19SH) were purchased from ACROS Chemicals. The water used in all experiments was highpurity water (18.2 MΩ cm). All other reagents were analytic reagent grade and used as received. Synthesis of Gold Nanoparticles. Gold nanoparticles were prepared according to the reference.17 The mole ratio of C9H19SH and HAuCl4‚3H2O was 1:4. After C9H19SH was added into the organic phase, NaBH4 was used to reduce the HAuCl4 and gold nanoparticles coated with an alkanethiolate monolayer were obtained. The gold colloidal solution (0.5 g/mL) was formed after gold nanoparticles were suspended in toluene. According to the images of transmission electron microscopy (TEM, JEOL 2000EX), (11) Ashrit, P. V.; Bader, G.; Girouard, F. E.; Truong, V.-V.; Yamaguchi, T. Phys. A 1989, 157, 333. (12) Yao, J. N.; Yang, Y. A.; Loo, B. H. J. Phys. Chem. B 1998, 102, 1856. (13) Georg, A.; Graf, W.; Neumann, R.; Wittwer, V. Solid State Ionics 2000, 127, 319. (14) West, A. R. Solid State Chemistry and its Application; Wiley: Chichester, 1984; p 34. (15) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (16) Hostetler, M. J.; Wingate, J. E.; Zhong, C. Z.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (17) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801.
10.1021/la010671q CCC: $20.00 © 2001 American Chemical Society Published on Web 12/01/2001
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the average diameter of the monodispersed gold nanoparticles was ca. 5.0 nm. Fabrication of Thin Films. The MoO3 thin film was prepared by vacuum evaporation of high-purity powder onto tin oxide coated glass substrates (NESA). The vacuum of the evaporation chamber was maintained at about 1.5 × 10-3 Pa during the evaporation. Gold nanoparticles were deposited onto the surface of MoO3 film by the spin-on coating technique at 1820 rpm for 15 s, and the composite thin film (MoO3/Au) was obtained after being dried in air away from light. The thickness of the MoO3 thin film was about 1000 nm (Talystep, Rank Taylor Hobson). The gold nanoparticle overlayer was about 10-20 nm in thickness according to section analysis of the images of scanning electron microscopy (SEM, Amray 1910FE). Physical Measurements. Before and after the films were directly irradiated in air for 3 min with the UV output of a 500 W high-pressure mercury lamp, the absorbance measurements of the films were conducted on a double-beam UV-visible spectrophotometer (Shimadzu UV-1601PC). The surface photovoltage spectroscopy (SPS) measurements were performed with a solid junction photovoltaic cell, NESA/sample/NESA sandwich structure, using a light source-monochromator lock-in detection technique.18,19 The spectra were normalized considering the correction arising from the intensity distribution of the light source. The X-ray photoelectron spectroscopy (XPS) measurements were carried out on as-prepared and irradiated thin films by using XSAM800 (KRATOS) with Al ΚR (1486.6 eV) radiation. The instrumental resolution was ca. 0.8 eV measured as full width at half-maximum (fwhm) of Ag 3d5/2 (368.3 eV). The results were calibrated by the value of C 1s at 284.8 eV.
Results and Discussion UV/Visible Spectra and Coloration Mechanism. Photochromic response of a thin film is accompanied by a change in its absorbance. Hence, the magnitude of the absorbance change is used to evaluate the coloration performance. Figure 1 is the absorption spectra for the thin films of MoO3 (A) and MoO3/Au (B). Curves a and b correspond to the as-prepared and colored films, respectively. Both the MoO3 and MoO3/Au thin films are almost transparent before UV-light irradiation. When these samples are irradiated with UV light, they turn blue in color and broad absorption peaks appear accordingly in the spectra. The difference in absorbance at 900 nm after and before coloration is denoted as ∆ABS. According to Figure 1, the ∆ABS for the MoO3/Au thin films (0.443) is about 2.55 times that of the MoO3 thin films (0.174), which indicates that the MoO3 thin films modified with the gold nanoparticle overlayer exhibit enhanced UV-light photochromic response over the plain films. The photochromic mechanism for MoO3 can be explained as follows:3
MoO3 + hν f MoO3* + e- + h+
(1)
2h+ + H2O f 2H+ + O
(2)
VI MoVx O3 MoO3 + xH+ + xe- f HxMo1-x
(3)
hν
V V VI MoVI A + MoB 98 MoA + MoB
(4)
h+ + e- f heat
(5)
Electrons and holes are formed when the films are irradiated with UV light (hν g Eg) in air (eq 1). The photogenerated electrons are injected into the conduction band of MoO3, and the holes react with the adsorbed H2O (18) Wang, B. H.; Wang, D. J.; Cao, Y. W.; Chai, X. D.; Geng, X. H.; Li, T. J. Thin Solid Films 1996, 284/285, 588. (19) Gatos, H. G.; Lagowski, J. J. Vac. Sci. Technol. 1973, 10, 130.
Figure 1. Absorption spectra of vacuum-evaporated thin films of MoO3 (A) and MoO3/Au (B): (curve a) as-prepared films and (curve b) spectra taken after film a was irradiated with UV light for 3 min in air.
to form protons simultaneously (eq 2). The protons diffuse into the lattice of MoO3 subsequently and hydrogen VI MoVx O3) is formed (eq 3), molybdenum bronze (HxMo1-x whereas the oxygen radicals may occupy the vacancy sites inside the sample or escape as molecular oxygen into the atmosphere.20 As a result, the films turn blue in color due to the intervalence transition (eq 4).21-23 In addition, the photogenerated electrons can react with the holes (eq 5), which is detrimental to the photochromism. Determination of Fermi Energy Level (EF). Molybdenum trioxide is an n-type semiconductor. The optical band gap (Eg) of MoO3 thin films can be derived as 3.15 eV from a plot of (Rhν)1/2 versus hν, where R is the absorption coefficient.2,24 Figure 2 gives the XPS spectra of the valence band for as-prepared thin films of MoO3 (A) and MoO3/Au (B). The bands at about 6.27 eV (Figure 2A) and 6.43 eV (Figure 2B) are assigned to the O 2p level of MoO3.1,4 The band at 3.60 eV in Figure 2B is ascribed to the Au 5d level of the gold nanoparticles. The top of the experimental valence band (EVB) of MoO3 in MoO3 plain film is determined to be 2.7 eV by linear extrapolation of the segment of maximum negative slope to the background level;25,26 that is, EVB lies 2.7 eV below EF. (20) Bechinger, C.; Oefinger, G.; Herminghaus, S.; Leiderer, P. J. Appl. Phys. 1993, 74, 4527. (21) Faughnan, B. W.; Crandall, R. S.; Hyman, P. M. RCA Rev. 1975, 36, 177. (22) Allen, G. C.; Hush, N. S. In Progress in Inorganic Chemistry; Cotton, F. A., Ed.; Interscience: New York, 1967; Vol. 8, p 357. (23) Hush, N. S. In Progress in Inorganic Chemistry; Cotton, F. A., Ed.; Interscience: New York, 1967; Vol. 8, p 391. (24) Granqvist, C. G. Handbook of Inorganic Electrochromic Materials; Elsevier: Amsterdam, 1995; p 141, p 221. (25) Kowalczyk, S. P.; McFeely, F. R.; Ley, L.; Pollak, R. A.; Shirley, D. A. Phys. Rev. B 1974, 9, 3573. (26) Pantelides, S. T. Phys. Rev. B 1975, 11, 2391.
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Figure 2. Deconvoluted XPS valence band spectra for asprepared thin films of MoO3 (A) and MoO3/Au (B).
On the other hand, the conduction band minimum (ECB) of MoO3 can be calculated in the light of the following semiempirical rule:27,28
φ ) 1.794χp + 1.11
(6)
1 ECB ) ψ - Eg 2
(7)
where φ is the “atomic Fermi energy” of elements, χp is the Pauling electronegativity, and ψ is the intrinsic Fermi energy of the solid, which is calculated as the arithmetic mean of atomic values of the constituents.28 The ECB of MoO3 is calculated to be 4.17 eV using the χp of 3.44 for oxygen29 and φ of 4.21 eV for molybdenum.30 Since the Eg of MoO3 is 3.15 eV and EVB lies 2.7 eV below EF, the EF of MoO3 is determined to be 4.62 eV on the vacuum scale. XPS and SPS Measurements. Figure 3 gives the XPS spectra of O 1s for thin films of MoO3 and MoO3/Au before and after the UV-light irradiation. The peak at ca. 532.7 eV is ascribed to the adsorbed water, while the peak at ca. 530.6 eV is assigned to the oxygen in MoO6 octahedra.31 The quantity ratio of hydroxide oxygen atoms to oxygen atoms in MoO6 octahedra is determined32 to be 0.15 and (27) Sanderson, R. T. Chemical Periodicity; Reinhold: New York, 1960. (28) Frese, K. W., Jr. J. Vac. Sci. Technol. 1979, 16, 1042. (29) Pauling, L. The Nature of the Chemical Bond, 3rd ed.; Cornell University Press: New York, 1960. (30) Riviere, J. C. Solid State Surf. Sci. 1969, 1, 179. (31) Wang, J. Q.; Wu, W. H.; Feng, D. M. An Introduction to Electron Spectroscopy: XPS/XAES/UPS; National Defense Industrial Press: Beijing (in Chin.), 1992. (32) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymond, R. H.; Gale, L. H. Surf. Interface Anal. 1981, 3, 211.
0.11 for MoO3/Au and MoO3 thin films, respectively. This means that more water is adsorbed on the film surface after the modification with gold nanoparticles. It is reasonable since gold nanoparticles exhibit strong surface adsorbability owing to the surface effect of nanoparticles. After the UV-light irradiation, it is found that the amount of adsorbed water on either MoO3 or MoO3/Au films decreases. This is because a part of water is consumed during the photochromic process (see eq 2). Figure 4 shows the SPS for thin films of MoO3 (A) and MoO3/Au (B) before and after the UV-light irradiation. The response at about 340 nm (peaks a and a′) corresponds to the transition from the valence band to the conduction band (TVC) of MoO3. From the band edge of this bandband transition, the Eg is calculated to be 3.15 eV, which is consistent with the value derived from the UV-visible spectra. The peaks at about 430 nm (peaks b and b′) could be assigned to the transition from the valence band of MoO3 to the surface states (TVS) located in the region of Eg. The energy level of TVS is calculated to be about 4.44 eV on the vacuum scale according to the Eg and EVB of MoO3, corresponding approximately to the potential of H2O/H2. Since the presence of adsorbed water has been confirmed by the O 1s XPS spectra, it is suggested that it is the adsorbed water that contributed these surface states, located at about 2.88 eV above the valence band. It can be calculated that before and after the UV-light coloration the peak intensity ratio of TVS to TVC changes from 15:2 to 5:2 for the MoO3/Au film and from 15:5 to 4:5 for the MoO3 film. This also indicates that the amount of adsorbed water decreases after the UV-light coloration. For the virgin thin films, it can be found that the intensity ratio of peak b to peak a of MoO3 (ca. 15:5) is smaller than that of MoO3/Au (ca. 15:2). That is, the SPS signal corresponding to the surface states becomes more extensive after the modification with gold nanoparticles, indicating that more water is adsorbed on the surface of MoO3/Au thin films. This is consistent with the O 1s XPS results. Enhancement Mechanism. The EF of gold nanoparticles is 5.10 eV on the vacuum scale33 as they remain largely metallic in nature.16 So the contact of gold nanoparticles with MoO3 thin film will result in the formation of a Schottky barrier at the MoO3/Au interface due to the different EF between the two species.34,35 Therefore, the energy band structure of MoO3/Au thin film can be illustrated as shown in Figure 5 according to the above discussion. Under the effect of the built-in electric field, photogenerated electrons migrate to the bulk of MoO3 thin film along the conduction band after the UV-light excitation, while the holes move toward the interface via the valence band. So the separation of photogenerated electron/hole for the MoO3/Au thin film is more effective than that for the MoO3 thin film and the recombination of the photogenerated electron/hole is suppressed rather efficiently. As a result, more photogenerated carriers can contribute to the coloration process, resulting in the formation of more molybdenum bronze on the film surface (see eqs 1-3). Owing to the surface effect of gold nanoparticles, there is more adsorbed water presented on the surface of MoO3/ Au thin films than on that of MoO3 thin films. The increase in the adsorbed water on the film surface is favorable to the utilization of the photogenerated holes arising from the TVC and TVS. As a result, more coloration active (33) Thanailakis, A. J. Phys. C: Solid State Phys. 1975, 8, 655. (34) Rhoderick, E. H. J. Phys. D: Appl. Phys. 1970, 3, 1153. (35) Rhoderick, E. H.; Williams, R. H. Metal-Semiconductor Contacts, 2nd ed.; Clarendon Press: Oxford [U.K.], 1988.
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Figure 3. The XPS spectra of O 1s level: (A) as-prepared MoO3 thin film; (B) UV-light colored MoO3 thin film; (C) as-prepared MoO3/Au thin film; (D) UV-light colored MoO3/Au thin film.
Figure 5. The schematic diagram for the Schottky barrier and the charge-transfer process at the MoO3/Au interface.
In summary, a new kind of MoO3/Au composite film is fabricated by depositing gold nanoparticles onto the surface of MoO3 thin film. The UV-light coloration performance of the composite film is greatly improved due to the formation of a Schottky barrier at the metal/ semiconductor interface and the increased adsorbed water on the film surface contributed by the surface effect of gold nanoparticles. This study suggests that the coloration performance of photochromic materials can be mediated by controlling their surface states. Figure 4. SPS for thin films of MoO3 (A) and MoO3/Au (B) before and after the UV-light irradiation.
species (protons) are produced in the MoO3/Au film than in the MoO3 plain film upon excitation (see eq 2). So the coloration reactions (eq 2 and 3) are promoted and the detrimental reaction (eq 5) is inhibited, resulting in the production of more molybdenum bronze.
Acknowledgment. The authors are grateful that this work is sponsored by the National Research Fund for Fundamental Key Projects No. 973 (G19990330), National Science Foundation of China and Chinese Academy of Sciences. LA010671Q
