Formation of TiO2 Nanoparticles by Reactive-Layer-Assisted

Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Minnesota, 1978. ..... A...
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NANO LETTERS

Formation of TiO2 Nanoparticles by Reactive-Layer-Assisted Deposition and Characterization by XPS and STM

2005 Vol. 5, No. 7 1327-1332

Zhen Song,† Jan Hrbek,‡ and Richard Osgood*,† Department of Applied Physics and Applied Mathematics, Columbia UniVersity, New York, New York 10027, and Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973 Received March 23, 2005; Revised Manuscript Received May 16, 2005

ABSTRACT Stoichiometric TiO2 nanoparticles (1−5 nm) were prepared by reactive-layer-assisted deposition (RLAD), in which Ti was initially deposited on a multilayer of H2O (or NO2) on a Au(111) substrate at ∼90 K. The composition and atom-resolved structure of the nanoparticles were studied by XPS and STM. The ∼5 nm TiO2 particles had either a rutile or anatase phase with various crystal facets. STS of the nanoparticles suggests size-dependent electronic structure. These well-defined nanoparticles can be used in molecular-level studies of the reactions and mechanisms of photocatalytic processes on TiO2 nanoparticle surfaces.

TiO2 is an important oxide material for a broad range of photocatalytic applications. Because of this interest, its surface chemistry, charge transport, and materials properties have been the subjects of a series of extensive investigations. During optical irradiation, charge carriers are formed by optical absorption across the band gap of the TiO2; these carriers can directly participate in redox processes on TiO2 surfaces. In fact, it has been found that TiO2 particle surfaces (or interfaces) play an important role in photon absorption, charge-carrier trapping and transport.1,2 Thus a decrease in the size of the TiO2 particles, which increases their specific surface area, will enhance the total photoreactivity of TiO2. In addition, the size of the TiO2 particles is known to alter the width of the band gap and the band bending at the interfaces, and thus influence their photochemical properties. Synthesis of well-defined particles is essential for studies of the relationship between reactivity and structure. Many methods have been developed for synthesis of nanometersized TiO2 particles, such as sol-gel, metallorganic chemical vapor deposition,3 ionized cluster beam deposition,4 pulsedlaser ablation,5 gas condensation deposition,6 etc. In our study, we focus on growing TiO2 nanoparticles in situ in an ultrahigh vacuum chamber (UHV). This in situ approach is important for an atomic-level study of the structure of TiO2 nanoparticles and their structural change with various treatments or reactions, as well as the study of the reactions and * Corresponding author. Telephone: (212) 854 4462. Fax: (212) 854 1909. E-mail: [email protected]. † Columbia University. ‡ Brookhaven National Laboratory. 10.1021/nl0505703 CCC: $30.25 Published on Web 06/03/2005

© 2005 American Chemical Society

mechanisms at the molecular-level using state-of-the-art surface science instruments. There are several ways to prepare supported compound nanoparticles in UHV. One is to co-deposit two reactants at elevated substrate temperatures. For example, Helveg et al.7 have grown MoS2 nanoparticles by co-depositing Mo and H2S on Au at a substrate temperature of ∼400 K. However, under certain conditions, this method may result in a wellordered thin film instead of nanoparticles. For example, Guo el al.8 and Ma¨nnig et al.9 reported growing well-ordered TiO2 thin films by co-depositing Ti and O2 on Mo(110) and Ru(0001), respectively, at a temperature of 600 K. A second preparation method first forms a chemisorbed layer of one reactant and then deposits a second reactant on the chemisorbed layer so as to reactively form compound nanoparticles or islands. Cai et al.10 and Biener et al.11 have successfully used this method to prepare RuS2 and TiS2 nanoparticles, respectively, on Au substrates. However, this method may also produce instead large 2-dimensional islands on the substrate. Recently Horn et al.12 and Kim et al.13 have described a novel method for preparation of compound nanoparticles called reactive-layer assisted deposition (RLAD). In this method, a multilayer of one reactant is first grown on a support and the second reactant (metal atoms) is then physical-vapor deposited onto this layer. After the metal atoms react with the molecular multilayers, raising the substrate temperature causes any unreacted adsorbed molecules and volatile reaction products to desorb from the substrate surface; the final product compound is then left on the surface in the form of an ensemble of nanoparticles.

In Horn’s and Kim’s experiments, C2H4 and O2 were used as the reactive multilayers to react with Mo and Mg to produced, respectively, MoC2 and MgO nanoparticles on Au substrates. The RLAD method takes advantage of multilayer control; by tuning the thickness of the mutilayer, one can in principle control the size of the nanoparticles14 and even pattern the deposition of the compound nanoparticles.15,16 In this paper, we describe experiments that use the RLAD method to prepare TiO2 nanoparticles with H2O (or NO2) as the reactive layer. We also investigated the growth, electronic structure, and chemical composition of the TiO2 particles by X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS). Finally, we discuss the RLAD growth mechanism and the possibility for patterned growth. The experiments were performed in two separated UHV chambers, dedicated to the STM and XPS measurements, respectively. The STM chamber is equipped with an Omicron VT-STM and a LEED/Auger system. STM images were acquired at room temperature using a W tip. The base pressure of the STM chamber is ∼10-11 Torr. The XPS chamber is equipped with a Gammadata Scienta SES-100 analyzer and a VSW dual-anode X-ray source. All the spectra were measured using Mg KR X-rays with a pass energy of 100 eV for the analyzer. Peak positions were referenced to the Au 4f7/2 core level, which is taken to be at 83.8 eV.17 In both chambers, the sample could be cooled to ∼90 K by connection of the sample to a liquid-nitrogen reservoir. The Au(111) sample was first cleaned by cycles of Ne+ sputtering (1 keV) and annealing (800 K), and then cooled to ∼90 K. The reactive layer of H2O or NO2 was formed on the Au(111) surface by exposing this surface to 5 × 10-8 Torr (as measured with an uncorrected ion-gauge) of the corresponding gases for 5-10 min. The thicknesses of the reactive layers were estimated to be ∼15-30 ML by then assuming unity sticking coefficients for both H2O and NO2 on Au at 90 K. Titanium was physical-vapor deposited onto the reactive multilayer by e-beam evaporation from a Ti rod. After deposition, the sample was warmed to 300 K or higher. The amount of deposited Ti was measured by XPS and STM. Our study shows that the formation of TiO2 depends on the selection of either H2O or NO2 reactive layers, which results in different nanoparticle composition and morphology. In the following, we first discuss our results with H2O and then those obtained with NO2. XPS measurements were carried out first to study the chemical state of the Ti compounds during the reactive process. For example, Figure 1 shows the XPS data for Ti and O core levels with a sample of Ti (0.1 ML) deposited on a ∼30 ML-thick H2O layer formed on the Au(111) surface. The Ti 2p data in Figure 1 show clearly that the Ti oxidation state changes as the sample is heated to room temperature. Below room temperature, Ti is not completely oxidized to TiO2. Intermediate oxidation states such as TiO, Ti2O3, Ti3O5 are found over the range from 90 to 150 K. The peak positions for these Ti valance states, indicated in Figure 1a, agree with those in refs 18 and 19. With an increase in the temperature, Ti is further oxidized until 1328

Figure 1. Ti 2p and O 1s XPS of a sample prepared by depositing 0.1 ML Ti on a 30 ML H2O layer on Au(111) at 90 K and then annealed to 150, 200, 250, and 300 K. Ti is completely oxidized to TiO2 when the temperature is above 250 K.

oxidation is complete at room temperature; pure TiO2 is then formed on the Au surface. This full oxidation is shown clearly by our XPS measurements, which show (see the 300 K spectrum in Figure 1) core-level spectral values comparable to bulk TiO2, i.e., the Ti 2p3/2 peak is at 458.8 eV, the Ti 2p 3/2 and 1/2 spin-orbital-splitting is 5.7 eV, and the O/Ti atomic ratio is ∼2. Figure 1 also shows the corresponding series of shifts in the O 1s peaks as the substrate temperature is raised. We assign the peaks at 533.5 and 530.4 eV to oxygen from H2O and TiO2, respectively. To fit our data it was necessary to include a third broad feature between the H2O and TiO2 peaks; this peak most likely includes features due to intermediate Ti oxidation states and/or hydroxyl groups. Finally, although not shown in Figure 1, we note that experiments with a much smaller amount of Ti, e.g., 0.01 ML, showed full oxidization to TiO2 even at 90 K. Nano Lett., Vol. 5, No. 7, 2005

Figure 2. STM images of TiO2 nanoparticles supported on Au(111), (a) prepared by depositing 0.1 ML Ti on a 30 ML thick H2O layer on a Au surface at 90 K and then annealed to 300 K. The size of the TiO2 particles is ∼1 nm. The particles preferentially aggregate near the elbow sites of the Au herringbone reconstruction. (b) TiO2/Au sample prepared by depositing 0.1 ML of Ti onto a 30 ML thick NO2 layer on Au(111) at 90 K, and then annealed to 300 K, (c) 500 K, and (d) 700 K. The particle size in (b) is ∼1 nm, in (d) is ∼5 nm. TiO2 particles prepared by NO2 reactive layers are randomly distributed on the surfaces. The main image size in (a) is 400×400 nm2; the insert is 40×40 nm2. The main image size in (b) is 200×200 nm2, the inset is 40×40 nm2. The image sizes for (c) and (d) are 40×40 nm2.

The morphology of TiO2 particles on the Au surface after annealing to 300 K is shown in the STM images in Figure 2a. As seen in the main image, the TiO2 particles form singledomain arrays on individual terraces of the reconstructed Au(111) surface. A close-up image (see insert in Figure 2a) shows clearly that the particles have ∼1 nm diameters and form multiple-particle aggregates near the elbow sites of the Au herringbone reconstruction; it also shows that the Au surface reconstruction is still preserved. This result indicates that the herringbone reconstruction still exists in the presence of the H2O overlayers. Yan et al.20,21 have recently investigated the deposition of Ag nanoparticles on HfO2 and TiO2 using multilayers of H2O ice. Their STM measurements strongly implied that an intermediate, liquid state of water had been formed as the sample was elevated in temperature. In this regard, earlier experiments by Smith and Kay22 suggest that supercooled liquid water is formed at 150 K after heating amorphous solid ice. In Yan et al., the authors proposed a model in which the Ag particles are formed in the top layers of the ice; as the sample warms, these particles are confined within the liquid droplets formed from the melting of the ice. As the oxide surface de-wets during water evaporation, these silver particles are concentrated in the shrinking droplets. Note, however, that these authors did not find any preferential deposition at defect sites. To explain our observation of Nano Lett., Vol. 5, No. 7, 2005

preferential deposition at elbow sites in case of TiO2 on ice/ Au, we have modified Yan’s model and taken into account an important difference between TiO2 and Ag nanoparticles in a H2O liquid film, viz., TiO2 is hydrophilic while Ag is hydrophobic. Thus, for our case, we suggest that TiO2 particles, after being formed in the top layers of the ice film, may diffuse deeper and a small concentration is deposited on the Au surface as the ice melts. The nonzero mobility of these TiO2 particles allows them to find the most energetically favorable positions, the elbow site. The preference for this site for vapor-deposited small particles has been previously reported for example in PVD7 and CVD23 deposition on Au(111) surfaces. Thus, the presence of TiO2 on Au pins the water droplets at each elbow site. With the evaporation and shrinking of the droplets, all TiO2 particles within the droplets are driven to the elbow sites, as seen in Figure 2a, by capillary force.24 We now consider the similarities and differences in the reaction and the morphology of TiO2 nanoparticle formation, which occur when NO2 is used as the reactive layer instead of H2O. First, our XPS data (Figure 3, Ti 2p panel) shows that when the Ti film reacts with the NO2 reactive layer, Ti (0.1 ML) is fully oxidized to TiO2, even at 90 K. The assignment of TiO2 is determined again by our observation of Ti 2p core-level positions that are typical of bulk TiO2.17 A sequence of XPS spectra after annealing to several temperatures from 90 to 700 K is also shown in Figure 3. Note that for temperatures between 200 and 400 K, our XPS data show a ∼1 eV binding energy shift toward lower binding energy for all TiO2-particle-related spectra, i.e., the Ti 2p, O 1s, and N 1s core levels; see red-line spectra in Figure 3. However, the data show no shift for the Au 4f core level after the reaction of the Ti with NO2. We attribute these TiO2-related shifts to the formation of NO3 radicals on the surfaces of TiO2 particles. Such surface NO3 radicals could change the electronic structure, i.e., work function and band bending of the particles, thus causing our observed binding energy shifts. In particular, earlier Rodriguez et al. had investigated the adsorption of NO2 on single-crystal TiO2(110)25 and found that NO3 formed on the surface at temperatures between 200 and 500 K. However, no N-containing species was detected by XPS at TiO2 temperatures of >600 K. In our case, after heating the sample to >500 K, the NO3 XPS peak disappeared from the nanoparticle surfaces; at the same time, the spectrum shifted back to the binding energies of the Ti 2p and O 1s core levels, which are typical for fully oxidized TiO2. Note that we also observed the formation of NO3 on the TiO2 particle surfaces after exposure to NO2 gas at 300 K. In this case, we observed the same ∼1 eV TiO2 related core-level shifts (data not shown). The morphology of the TiO2/Au sample prepared with a NO2 reactive layer at 90 K and then heated to room temperature is shown in Figure 2b. Compared to the TiO2/ H2O/Au samples, the TiO2 particles prepared with NO2 aggregated randomly on the surface. Also, unlike in the case of the H2O-layer process, our STM images suggest that the Au herringbone structure is eliminated immediately after the 1329

Figure 3. Ti 2p, O 1s, and N 1s XPS of a sample prepared by depositing Ti onto a 30 ML-thick NO2 layer on Au(111) at 90 K, and then annealed to 200, 300, 500, and 700 K. Ti is completely oxidized to TiO2 at 90 K.

NO2 deposition, apparently because of a much stronger interaction of the NO2 with the Au surface. The different TiO2 nanoparticle patterns generated from H2O and NO2 imply that RLAD growth is sensitive to the substrate structures; therefore, one might anticipate that spatially modulating the substrate structure when using RLAD would allow patterned growth of nanoparticles. Patterned growth can also be realized by spatially modulating the initial reactive multilayers, using for example laserinterference desorption. This latter approach is used in bufferlayer-assisted growth, in which metal atoms were subsequently deposited on a buffer layer such as solid Xe at below 50 K on a substrate, see Kerner and Asscher.15,16 The thermal stability of the RLAD-prepared TiO2/Au samples was studied by heating the sample from room temperature to 700 K. STM images (Figure 2c and d) show the morphology change upon the heating. For the sample prepared by using a NO2 reactive layer, heat treatment of the sample results in the re-appearance of the herringbone structure, but with a highly disordered state. In addition, heat treatment also induces coalescence and ripening of the TiO2 particles. This same heat-induced phenomenon is also observed for samples prepared with H2O reactive layers. Note that as the particles are heated to ∼700 K, they gradually assume a flat-crystalline structure. The average particle diameter of the sample shown in Figure 2d is ∼5 nm. Their average apparent height is ∼0.5 nm, as measured at a bias voltage of 2.2 V. At this tip voltage, the electron local densities of states for Au and TiO2 are relatively close, as indicated by STS (see Figure 5b). Atomic-resolution STM images of various structures of TiO2 particles formed after annealing to 700 K are shown in Figure 4. The two particles shown in the lower panels of Figure 4 have a surface structure with an unit cell of 0.46 nm × 0.30 nm, which is consistent with the TiO2 rutile (100) structure. Therefore, we assign these particle facets to rutile (100). The TiO2 nanoparticle structures shown in the upper panels of Figure 4 have short-range periodic arrangements not consistent with any TiO2 structure. However, there are 1330

Figure 4. Close-up STM images of the individual TiO2 particles shown in Figure 2d. The particles display different facets. The lattice constants denoted in the figure are in nm.

regions of the surfaces showing two lattice constants, 0.37 and 0.30 nm, as denoted in the figure. These two lattice constants are typical for anatase (0.37 nm) and rutile (0.30 nm) TiO2, respectively. We therefore assume that these images show an intermediate state of the transformation from anatase to a rutile phase. Thus, repeated heating of the sample to 700 K after exposure to NO2 at 300 K induces the formation of an increasing concentration of rutile-(100)-facet TiO2 particles. In the experiment by Guo et al.,8 TiO2 thin films, grown on Mo(110) by evaporating Ti in an O2 atmosphere at 600 K and then annealing to 700 K, were shown via LEED measurements to have a rutile (100) structure. Finally, we have used scanning tunneling spectroscopy (STS) measurements in order to obtain a preliminary glimpse into any size-dependent nanoparticle electronic structure. Figure 5 shows two sets of STS (|I|-V curves) from the ∼1 Nano Lett., Vol. 5, No. 7, 2005

Figure 5. STS of TiO2 particles supported on Au(111) surfaces. (a) TiO2 prepared by RLAD with H2O as the reactive layer and then annealed to 300 K. The particle size of this sample is ∼1 nm. (b) TiO2 prepared with NO2 as the reactive layer and then annealed to 800 K. The particle size of this sample is ∼5 nm. STS set point: 2.3 V, 0.05 nA.

and ∼5 nm diameter TiO2 particles shown in Figure 2a and d, respectively, along with STS (|I|-V curves) of the corresponding Au substrates. Note that the Au STS from both measurements are identical indicating the same STMtip conditions in the two measurements. In our STS measurements for large band-gap TiO2, we used a set-point bias voltage of +2.3 V, i.e., ∼1 order of magnitude higher than that for typical STS measurements on metals; this set-point plus the low density of Au 6s states causes STS curves to show an artificial “band gap” near 0 V. Despite the fact that our measurements of TiO2 nanoparticles are convoluted with the response of the Au substrate, we can extract a useful estimate of the band gap of our nanoparticles. In particular, it can be shown that the crossing points in the Au and the TiO2 I-V curves at both positive and negative bias give an upper limit of the band gap; this approach is similar to the image contrast technique used for oxide film band gap estimation in STM imaging experiments.26 Thus in our case, for the 5 nm nanoparticle, the crossing of the TiO2 curve with that of Au occurs at +2.3 V and -1.5 V, corresponding to a band gap close to ∼3.8 eV. In the case of the 1 nm particle, the positive crossing point is at +2.5 V; however, the negative crossing point has shifted down to the point that is out of the bias voltage range covered by our measurement. Despite this, it is clear that the band gap is far larger than that for the 5 nm particle. Our XPS data show no evidence of detectable impurities in the TiO2 nanoparticles, i.e., the TiO2 is an “intrinsic” material. Therefore, any contaminant-induced alteration of the particle electronic properties can be ruled out and hence the size effects seen in the data appear to be intrinsic in origin. Our data also suggest other interesting differences. For example, the higher tunneling currents seen for the larger particles would be expected if the density of electronic states were higher for this particle due to, say, annealing-induced defects. Further, as shown in Figure 5a, the Fermi energy for the smaller TiO2 particle lies closer to the conduction band of TiO2; this property would be expected for an n-type semiconductor, such as TiO2, due to a lattice-oxygen deficiency. On the other hand, in the case of the larger particle, the Fermi energy is located approximately at the center of the band gap, implying a shift of the Fermi energy toward the valence band. This Nano Lett., Vol. 5, No. 7, 2005

shift can be explained by several phenomena including different amounts of oxygen vacancies or interfacial charging. In conclusion, we have demonstrated the preparation of TiO2 nanoparticles using the RLAD method. Both H2O and NO2 can be used as the reactive layers for preparing pure TiO2 nanoparticles. This method is UHV compatible and, while only Au(111) is used here, the procedure is independent of the nature of support material. The thickness of the deposited TiO2 is not limited by the method itself. By varying temperature, one can control the size of the particles from ∼1 nm to much larger diameters. After annealing to 700 K, the particle crystalline structure is composed of a mixture of rutile and anatase phases, each having a variety of different exposed facets. The electronic structure of the TiO2 particles as measured by STS is consistent with size dependence; this property could play a role in determining both the reactivity and photochemical properties of the nanoparticles. A study of the chemical properties at the molecular level with these TiO2/Au samples is in progress in our group. Acknowledgment. This research was carried out at the Chemistry Department and the Center for Functional Nanomaterials of the Brookhaven National Laboratory under Contract DE-AC02-98CH10886 with the U.S. Department of Energy (Division of Chemical Sciences). R.O. and Z.S. gratefully acknowledge support from the U.S. Department of Energy, Contract No. DE-FG02-90ER14104. References (1) Nozik, A. J.; Memming, R. J. Phys. Chem. 1996, 100, 13061. (2) De Jongh, P. E.; Vanmaekelbergh, D.; Phys. ReV. Lett. 1996, 77, 3427. (3) Jung, O.-J.; Kim, S.-H.; Cheong, K.-H.; Li, W.; Saha, S. I. Bull. Korean Chem. Soc. 2003, 24, 49. (4) Dohshi, S.; Takeuchi, M. Anpo, M. Catal. Today 2003, 85, 199. (5) Liang, C.; Shimizu, Y.; Sasaki, T.; Koshizaki, N. J. Mater. Res. 2004, 19, 1551. (6) Siegel, R. W.; Ramasamy, S.; Hahn, H.; Li, Z.; Ting. L.; Gronsky, R. J. Mater. Res. 1988, 3, 1367. (7) Helveg, S.; Lauritsen, J. V.; Lægsgaard, E.; Stensgaard, I.; Nørskov, J. K.; Clausen, B. S.; Topsøe, H.; Besenbacher, F. Phys. ReV. Lett. 2000, 84, 951. (8) Guo, Q.; Oh, W. S.; Goodman, D. W. Surf. Sci. 1999, 437, 49. (9) Ma¨nnig, A.; Zhao, Z.; Rosenthal, D.; Christmann, K.; Hoster, H.; Rauscher, H.; Behm, R. J. Surf. Sci. 2005, 576, 29. 1331

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Nano Lett., Vol. 5, No. 7, 2005