Home
Search
Collections
Journals
About
Contact us
My IOPscience
Deposition of uniform Pt nanoparticles with controllable size on TiO2-based nanowires by atomic layer deposition and their photocatalytic properties
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 254002 (http://iopscience.iop.org/0957-4484/26/25/254002) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 132.239.1.230 This content was downloaded on 10/06/2015 at 04:00
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 254002 (9pp)
doi:10.1088/0957-4484/26/25/254002
Deposition of uniform Pt nanoparticles with controllable size on TiO2-based nanowires by atomic layer deposition and their photocatalytic properties Chih-Chieh Wang1, Yang-Chih Hsueh2, Chung-Yi Su2, Chi-Chung Kei3 and Tsong-Pyng Perng2 1
Department of Materials Science and Engineering, Feng Chia University, Taichung 407, Taiwan Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan 3 Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 300, Taiwan 2
E-mail: [email protected] Received 24 February 2015, revised 26 April 2015 Accepted for publication 8 May 2015 Published 4 June 2015 Abstract.
TiO2-based nanowires were prepared by a hydrothermal method, and Pt nanoparticles were deposited on the nanowires by atomic layer deposition (ALD). The size and loading of Pt nanoparticles with very good uniformity could be controlled by the ALD cycle number, along with acid or water treatment of the nanowires. The lower growth rate and the loading of Pt nanoparticles were obtained with water treatment. With acid treatment, the growth rate and loading were increased, and this increase was attributed to the formation of higher density of hydroxide groups on the nanowires. The photocatalytic activities of Pt-deposited nanowires were investigated. The deposition of Pt on the nanowires resulted in both enhanced light absorption in the visible region and electron–hole separation efficiency. The water-treated nanowires exhibited the highest degradation rate of rhodamine B and the highest hydrogen evolution rate. A maximum amount of hydrogen evolution, 13.8 mmol g−1 in 6 h, was achieved. Keywords: atomic layer deposition, Pt nanoparticles, photocatalysis (Some figures may appear in colour only in the online journal) 1. Introduction
reaction process [7–9]. Because of their cation-exchange reactivity on the surface [10, 11], they can also be used as a catalyst support to deposit other active catalysts with uniform distribution and high dispersion. The presence of platinum nanoparticles on the TiO2 surface effectively reduces the electron–hole recombination rate [12–14], which is caused by the formation of the Schottky barrier at the Pt–TiO2 interface. The interfacial charge transfer is facilitated, resulting in improved separation of electrons and holes. Basically, effective electron–hole separation is related to the size, loading, and uniform distribution of Pt particles on TiO2 [12–14]. For instance, Pt particles of several nanometers in diameter can have enhanced efficiency on H2 evolution [12]. On the other hand, larger Pt particles would provide more recombination sites for
Since 1972, when Fujishima and Honda discovered that photodecomposition of water could be achieved by illuminating TiO2 electrodes with ultraviolet (UV) light [1], TiO2 has attracted great attention in wastewater treatment and hydrogen generation because of its low cost, nontoxicity, and chemical stability. However, TiO2 has not been widely applied to the environmental industry because it suffers from a high electron–hole recombination rate [2–4]. Also, the low surface area of bulk TiO2 limits its photocatalytic reactivity [5, 6]. TiO2-based one-dimensional (1D) nanostructures fabricated by the hydrothermal method are of great interest for catalysis due to their high specific surface area, which can facilitate the transport of reagents to the active sites during the 0957-4484/15/254002+09$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
Nanotechnology 26 (2015) 254002
C-C Wang et al
substrate at 50 °C to evaporate the ethanol. A ∼3 μm-thick film was formed. Pt nanoparticles were then deposited on the nanowires by homemade ALD. The deposition of Pt nanoparticles by ALD has been described previously [22–24]. Trimethyl(methylcyclopentadienyl) platinum (IV) (MeCpPtMe3) and oxygen were used as precursors. In a cycle of ALD, a 1 s pulse of MeCpPtMe3 and a 5 s pulse of oxygen were each separated by a 20 s purge of nitrogen. The growth temperature for all samples was 250 °C. The Pt loading was calculated by the ratio of the amount of Pt to Pt + TiO2. After dissolving the sample in a solution composed of HNO3 (3 ml, 65%), HCl (3 ml, 37%), H2SO4 (2 ml, 98%), and HF (2 ml, 48%) that was further diluted to 50 ml, the amount of Pt was then analyzed by inductively coupled plasma mass spectrometry (Perkin Elmer SCIEX ELAN 5000).
photogenerated electrons and holes [12, 13]. In addition, too much Pt loading on TiO2 reduces not only the surface reactive site, but also the light absorption of TiO2 [14]. Therefore, optimum size, loading, and uniform distribution of Pt particles on TiO2 are necessary to obtain effective electron-hole separation and to achieve high photocatalytic activity. Deposition of Pt nanoparticles on TiO2-based 1D nanostructures seems to be a straightforward way to enhance photocatalytic activity. However, deposition of well-dispersed Pt nanoparticles with controllable size and loading on TiO2 is difficult because the deposition usually involves chemical wet processes, such as impregnation [15, 16] and photodeposition [17, 18], which have significant limitations. For instance, in the impregnation method, particle agglomeration occurs during the reduction of metal ions in a hydrogen atmosphere at high temperatures. The loading of Pt is unpredictable in the photodeposition method since it is difficult to attain uniform surface reactivity of TiO2 with a Pt ion precursor. Because of the limitation of these processes, atomic layer depostition (ALD) is used in our fabrication process. ALD is a selflimiting process that enables fabrication of thin films with excellent conformal and uniform coating and precise thickness control on complex nanostructures [19–21]. In addition to the preparation of thin films, nanoparticles can also be fabricated by ALD for a variety of applications [22–25]. For instance, Pt nanoparticles with controllable size and loading as a catalyst are well distributed on carbon nanotubes for proton exchange membrane fuel cells (PEMFCs) [22–24]. The performance of PEMFCs reveals that ALD could effectively reduce the Pt loading to meet the commercial requirements of PEMFCs. However, the substrate requires hydroxide groups as a nucleation site when applying ALD. In this study, TiO2-based nanowires prepared by the hydrothermal method were modified by water and acid treatment to provide more hydroxide groups. Pt nanoparticles were subsequently deposited on the nanowires by ALD. UVvisible light absorption, photocatalytic degradation of rhodamine B, and hydrogen evolution by the nanostructures were also investigated.
2.3. Characterization
X-ray diffraction (XRD) with Cu Kα radiation (Shimadzu 6000) was performed to examine the structures of the nanowires. The morphologies of the nanostructures and the particle sizes of Pt were examined by transmission electron microscopy (TEM, JEOL 3010F). The surface functional groups were analyzed by Fourier transform infrared spectrometry (FTIR). The optical absorption spectra were obtained with a JASCO V-570 spectrophotometer.
2.4. Photocatalytic activity and photoluminescence (PL) measurement
We measured the photocatalytic activity for the degradation of rhodamine B solution (20 ml, 5 × 10−2 M) under illumination from a 200 W Hg lamp (wavelength 240 to 600 nm) by the nanowires. The distance between the Hg lamp and the rhodamine B solution was 25 cm. 0.1 g nanowires were added to the rhodamine B solution. Before the illumination, the solution was stirred for 10 min in the dark for the dye molecules to be adsorbed on the nanowire surface. For 60 min, UV-visible absorption spectra of the samples were then collected before and after every 10 min of irradiation. PL spectra were obtained using microzone Raman spectroscopy (Horiba Jobin Yvon, Labram HR 800). The excitation wavelength was 325 nm (He-Cd laser, Kimon IK3301R-G) with a laser power of 30 mW and a spot size of 0.79 μm2.
2. Experimental 2.1. Preparation of TiO2-based nanowires
TiO2-based nanowires were prepared by a hydrothermal method [25]. 0.5 g anatase TiO2 was reacted with 10 M NaOH at 200 °C for 24 h. After the reaction, the white powder product was washed with water and dried. We called these the pristine nanowires. For modification, 0.1 g pristine nanowires were immersed in water, 1 M hydrochloric acid, or 1 M nitric acid for 1 h at 25 °C. They were then rinsed with water and dried at 50 °C.
2.5. Hydrogen generation measurement
The hydrogen generation measurement was performed in a photoreactor made of quartz and stainless steel containing 0.5 g of catalyst in a 100 ml solution of water and methanol at a 4:1 volume ratio under illumination from a 200 W Hg lamp. Before the measurement, the suspension in the reactor was stirred and purged with argon for 1 h to remove the residual air. For 6 h, a measurement was then conducted for each
2.2. Deposition of Pt nanoparticles on the nanowires by ALD
0.05 g nanowires were dispersed in 5 ml ethanol by ultrasonic vibration. A drop of solution was placed on a 1 × 1 cm silicon 2
Nanotechnology 26 (2015) 254002
C-C Wang et al
Table 1. EDX analysis of the chemical compositions of the
nanowires with acid or water treatment. Element (at%) Treatment H2O HCl HNO3
O
Ti
Na
61.43 66.60 83.44
29.85 33.40 16.26
8.72 0.00 0.30
3. Results and discussion 3.1. Characterization of TiO2-based nanowires with various treatments
The diameter and length of the nanowires were 50 ± 25 nm and 3.00 ± 1.58 μm, respectively. Figure 1(a) shows the TEM image of the nanowires with 45 ± 15 nm diameter and 2.00 ± 0.55 μm length. The morphology was maintained even after hydrochloric or nitric acid treatment. According to the energy dispersive x-ray (EDX) analysis, a many sodium ions were present in the as-prepared nanowires. The concentration of sodium was reduced after acid treatment, which implies that the compositions of the nanowires might be different (table 1). The structures of the water-treated and acid-treated nanowires were examined by XRD patterns, as shown in figure 1(b). Both types of nanowires contain anatase TiO2. The diffraction peaks at approximately 24°, 28°, and 48° are observed for the nanowires with water treatment, corresponding to (110), (211), and (020), respectively, of a trititanate structure (A2Ti3O7, A = H or Na) [26–28]. Note that the intensities of the peaks (211) and (020) are considerably reduced after acid treatment. This may be attributed to the fact that sodium is exchanged by hydrogen during the treatment process [10, 11]. It is also in line with the decrease of sodium content observed in the EDX analysis. An attempt was made to investigate the surface of the nanowires with water and acid treatment by FTIR spectroscopy, as shown in figure 1(c). The absorptions at 3400 and 1630 cm−1 could be assigned to the hydroxide group and the vibration of H-O-H bonds of water, respectively. Both peaks become stronger after acid treatment, which implies that more hydroxyl groups are present in the acid-treated samples. There is an additional nitro group at 1380 cm−1 in the nitric acid treated sample. To check whether the nitro group was caused by the residual nitric acid, the sample was washed five times, first with a 0.1 M NaCl aqueous solution and then by water. The peak was still present, implying that a nitro group is formed on the surface of the nanowires.
Figure 1. (a) TEM image of TiO2-based nanowires. (b) XRD patterns of TiO2-based nanowires with water and HCl treatment. (c) FTIR spectra of TiO2-based nanowires treated with water, nitric acid, and hydrochloric acid.
3.2. Pt-deposited TiO2 nanowires
Pt nanoparticles were deposited uniformly on the TiO2-based nanowires with acid and water treatment, and the size could be well controlled by the cycle number. For the nanowires with nitric acid treatment with 10, 30, and 60 cycles of ALD,
illumination of 1 h. The evolution of hydrogen was analyzed by gas chromatography (Shimazu GC-2014) with a thermal conductive detector. 3
Nanotechnology 26 (2015) 254002
C-C Wang et al
Figure 2. TEM images of nitric acid-treated TiO2-based nanowires deposited with Pt nanoparticles by ALD with (a) 10 cycles, (b) 30 cycles, and (c) 60 cycles. (d) Dependence of Pt particle size on the ALD cycle number for nanowires with various treatments.
the sizes of Pt nanoparticles were ∼0.50 ± 0.03, 1.30 ± 0.08, and 2.60 ± 0.25 nm, respectively, as shown in figures 2(a)–(c). The particle size increases linearly with increases in the cycle number. The growth rate is calculated to be ∼0.046 nm/cycle, which is comparable to that of the nanowires with hydrochloric acid treatment, as shown in figure 2(d). Interestingly, Pt nanoparticles on the water-treated nanowires exhibit lower growth rates that can be divided into two steps. Below 30 cycles, a lower growth rate of 0.033 nm/cycle is observed. At 30 or more cycles of ALD, a higher growth rate of 0.044 nm/ cycle is reached, which is comparable to that of the acidtreated samples. The lower growth rate in the initial stage may be correlated to difficult nucleation. The reaction mechanism of ALD of Pt has been proposed [29]. Surface oxygen would react with the precursor of Pt to form Pt-Ox to initialize the ALD reactions. Pure Pt nanoparticles without any impurity phases are then obtained, even though oxygen is involved in the process [23, 30]. As observed in the FTIR spectrum (figure 1(c)), a lower density of hydroxyl groups is observed in the water-treated nanowires, resulting in the lowest growth rate. Interestingly, in the nitric acid-treated nanowires, although the concentration of hydroxide groups is less than that of the hydrochloric acid-treated nanowires, their growth rates are comparable. Note that an additional nitro group was produced in the nitric acid-treated nanowires. Farmer and
Gordon suggested that NO2 groups on the surface could also assist ALD growth [31, 32] because the nitrogen end is attached to the substrate, leaving the oxygen end to react with the incoming precursor to assist the nucleation of the nanoparticles. Furthermore, the difficult nucleation also has an influence on Pt loading. It is expected that one would observe fewer and smaller Pt nanoparticles on water-treated nanowires, as shown in figure 3(a). With acid treatment, more and larger Pt nanoparticles are decorated on the nanowires (figure 3(b)). As shown in figure 3(c), the Pt loading increases with increases in the cycle number, and the Pt loadings of the water-treated nanowires are consistently 35% lower than those of the other two samples. In addition, the amount of sample on the silicon substrate can affect the deposition of Pt nanoparticles. Hsueh et al observed that the Pt loading increased with an increase in the number of carbon nanotubes (CNTs) on the silicon substrate while the cycle number was kept at a constant [23, 24]. That could also provide an alternative way to control the Pt loading on TiO2 nanowires. The atomic contrast of high-angle annular dark field (HAADF) Z-contrast imaging and EDX mapping of the water-treated nanowires deposited with Pt in 120 cycles of ALD were conducted, as shown in figure 4. From the HAADF Z-contrast image (figure 4(a)), note that the 4
Nanotechnology 26 (2015) 254002
C-C Wang et al
spectrum of EDX reveals that only Pt, Ti, and O signals are present, as shown in figure 4(d). 3.3. UV-visible absorption of Pt-deposited TiO2 nanowires
Figure 5 displays the UV-visible absorption spectra of pristine and Pt-deposited TiO2-based nanowires. The pristine nanowires exhibit an absorption edge at ∼340 nm, which may be attributed to the excitation of electrons from the valence to the conduction band. With the deposition of platinum, the absorption is redshifted. The absorption intensity is also enhanced in the longer wavelength range (>400 nm). It is believed that both the redshift and the enhancement of absorption in the longer wavelength range and are correlated to the presence of new electronic states produced by the deposition of Pt nanoparticles [33, 34]. The Pt-derived electronic states are generated within the band gap. The electrons in the valence band of TiO2 are excited to the Pt-derived electronic states, resulting in the absorption in the longer wavelength range. 3.4. Photocatalytic activity of Pt-deposited TiO2-based nanowires
The photocatalytic activities of the nanowires were investigated using rhodamine B as a reaction reagent, as shown in figure 6. The nanowires with different treatments exhibit essentially the same photocatalytic activities, as shown in figure 6(a). This implies that the surface functional groups, residual sodium on the nanowires, and the A2Ti3O7 phase do not affect the activity. However, the activities of the nanowires are improved as Pt nanoparticles are deposited. This is attributed to the formation of a Schottky barrier between TiO2 and Pt, which facilitates the injection of photogenerated electrons to Pt, resulting in effective separation of electrons and holes [12–14]. The degradation of rhodamine B can be quantified by the value of the reaction constant. The reaction constants of as-prepared nanowires and nitric acid-, hydrochloric acid-, and water-treated nanowires deposited with Pt are 2.00 × 10−3, 2.15 × 10−3, 2.54 × 10−3, and 4.24 × 10−3, respectively. Interestingly, the enhancement of the photocatalytic activity is more pronounced for the water-treated sample than for nitric acid- and hydrochloric acid-treated samples. An attempt is then made to investigate the effects of the particle size and loading of Pt. The sizes of Pt with 60 cycles of ALD on acid-treated and water-treated nanowires are 2.65 ± 0.25 nm and 2.10 ± 0.20 nm, respectively. Wang et al reported that a 0.2 nm difference in Pt particle size could substantially improve the photoreduction of CO2 to CH4 [35]. The production yield increases from 557 to . This implies that a 0.55 nm difference in size in the present case may make a big difference in the photocatalytic activity. Due to the quantum confinement effect, Pt nanoparticles with decreasing size have larger energy band separation [13, 35], thus increasing the transfer of electrons from TiO2 to Pt. As shown in figure 7, Pt nanoparticles larger than ∼1 nm capture electrons efficiently only when the energy levels are below −4.4 eV [13, 35]. If the size becomes larger, the energy level
Figure 3. TEM images of Pt nanoparticles deposited by 60 cycles of
ALD on TiO2-based nanowires treated with (a) water and (b) hydrochloric acid. (c) Pt loadings on the nanowires with various treatments.
nanowire has a darker contrast than the particles due to the difference in the atomic number. Also, the Pt and Ti signals, as shown in figures 4(b) and (c), respectively, are well in line with the contrast of the HAADF Z-contrast image. The full 5
Nanotechnology 26 (2015) 254002
C-C Wang et al
Figure 4. Water-treated TiO2-based nanowire deposited with Pt nanoparticles by 120 cycles of ALD. (a) HAADF Z-contrast image. (b) EDX mapping of Ti. (c) EDX mapping of Pt. (d) Full spectrum of EDX.
sample, 0.72 ± 0.05 wt%. It is believed that the photocatalytic activity can be correlated to the charge carrier separation distance, which can be controlled by the Pt loading on TiO2 [13]. Lower loading of Pt leads to a longer charge carrier separation distance, resulting in higher photocatalytic activity. On the other hand, too high a density of Pt, as in the case of acid treatment, would retard charge carrier separation so that the photocatalytic activity becomes lower. In addition, the light absorption by TiO2 is blocked and the number of surface reactive sites on TiO2 is decreased [15]. Compared to the effect of particle size, Pt loading is probably a more dominant factor in the degradation of rhodamine B. Taking the above samples as an example, the size difference of Pt on watertreated and nitric acid-treated nanowires is 26%, (2.10 nm versus 2.65 nm), but the loading difference increases by 50%, from 0.48 to 0.72 wt%.
Figure 5. UV-visible absorption spectra of pristine and Pt-deposited
nanowires. The Pt nanoparticles were deposited on water- and acidtreated nanowires by 60 cycles of ALD.
3.5. Hydrogen generation of Pt-deposited TiO2-based nanowires
separation tends to gradually narrow like bulk Pt that acts as a recombination center for both electrons and holes. Therefore, the 2.65 nm size may suffer more electron-hole recombination than the 2.10 nm size, to exhibit a lower photocatalytic activity. In addition, the photocatalytic activity may be affected by the Pt loading on TiO2-based nanowires. The Pt loading with 60 cycles of ALD on the water-treated sample is 0.48 ± 0.03 wt%, which is lower than that of the acid-treated
Figure 8 shows the hydrogen generation of the pristine and Pt-deposited TiO2 nanowires from methanol/water decomposition. Compared with P25 and the pristine nanowires, Ptdeposited nanowires exhibit higher hydrogen-generation rates. With water treatment, the maximum amount of hydrogen, 13.8 mmol g−1 in 6 h, is reached under illumination from a 200 W Hg mercury lamp. When the irradiation time is longer than 6 h, the amount of hydrogen evolution then 6
Nanotechnology 26 (2015) 254002
C-C Wang et al
Figure 8. Hydrogen generation from methanol/water on P25 and on
pristine and Pt-deposited TiO2-based nanowires.
nanostructures. The hydrogen-generation rate from this study is higher than the rates of deposition of Pt by impregnation [15] and photodeposition methods [15] on TiO2 nanoparticles and TiO2 B nanotubes [39], and it is comparable to that of TiO2 B nanofibers [40]. However, it is much lower than that of TiO2 nanoparticles photodeposited with 5 nm Pt nanoparticles [41]. Note that TiO2 nanoparticles have good crystallinity and are mainly composed of anatase, which exhibits higher photocatalytic activity than the other phases [42–44]. Less anatase and lower crystallinity were obtained in our sample. The probable evolutionary mechanism of hydrogen from methanol/water on Pt-deposited TiO2-based nanowires could be suggested as follows [45, 46]:
Figure 6. Photocatalytic degradation of rhodamine B. (a) Nanowires with various treatments. (b) Pristine and Pt-deposited nanowires.
[Catalyst] + hν → e− + h+
(1)
h+ + H 2 O → H + + ⋅ OH
(2)
⋅OH + CH 3OH → H 2 O + ⋅ CH 2 OH +
−
⋅CH 2 OH → HCHO + H + e −
2H 2 O + 2e → H 2 + 2OH
−
(3) (4) (5)
and the overall reaction is: CH 3OH → HCHO + H 2
(6)
Electron-hole pairs are generated as the photocatalyst absorbs light energy that is higher than the band gap. The holes in the valence band react with H2O to produce H+ and ·OH radicals (equation (2)) that subsequently oxidize the methanol to form HCHO (equations (3) and (4)). Due to the absence of oxygen, electrons in the conduction band simultaneously reduce H+ to form H2 gas (equation (5)). However, the reactions compete with the recombination of electron-hole pairs. The Pt nanoparticles with appropriate loading and size can capture electrons more effectively for hydrogen evolution from methanol/water due to the formation of a Schottky barrier on the catalyst’s surface. The electric field caused by the Schottky barrier injects electrons into the platinum and leaves holes in the TiO2, resulting in more efficient separation
Figure 7. Schematic diagram of the energy band of TiO2 and the energy levels of Pt nanoparticles of different sizes.
increases slowly to approach a saturated value. This might be due to the balance of the forward and backward reactions of hydrogen evolution, which are associated with a constant pressure in the closed system [36, 37]. Additionally, the reactive sites may become inactive after a long irradiation time because of the adsorption of byproducts [38]. Table 2 summarizes the hydrogen production rates of various Pt-TiO2 7
Nanotechnology 26 (2015) 254002
C-C Wang et al
Table 2. Hydrogen production rates by various Pt-TiO2 nanostructures.
Catalyst
a
Pt-TiO2 NP Pt-TiO2 NP Pt-TiO2 (B) NT Pt-TiO2 (B) NF Pt-TiO2 NP Pt-TiO2− based NW
TiO2 dimensionb
TiO2 phasec
Size (nm)
D = N. A. D = N. A. D = 50 nm L = 2 μm D = 50 nm L = 2 μm D = 8 nm D = 30 nm L = 3 μm
A+R A+R T A+B A T+A
1.4 2.4 N. A. 12 5–10 2.1
Load (wt%) 1 1 1 1 1 0.48
Methodd IP PD PD IP PD ALD
H2 rate (μmol/g · h)
Reference
720 1805 450 2570 6000 2301
[15] [15] [39] [40] [41] This study
a b c d
NP: nanoparticle, NT: nanotube, NF: nanofiber, NW: nanowire. D: diameter L: length. A: anatase, R: rutile, B: TiO2 (B) phase, T: trititanate. IP: impregnation, PD: photodeposition.
loading of Pt nanoparticles can suppress the recombination process of photogenerated electrons and holes. On the other hand, in the case of Pt-deposited nanowires with nitric or hydrochloric acid treatment, a higher intensity is observed. This confirms that too much loading of Pt nanoparticles results in the enhancement of electron-hole recombination, which is well in line with the lower hydrogen evolution, as shown in figure 8.
4. Conclusion In conclusion, highly uniform Pt nanoparticles with controllable size and loading were deposited on TiO2-based nanowires by ALD. The size and loading could be controlled by the ALD cycle number and acid or water treatment of the nanowires. The smaller size and lower loading of Pt nanoparticles were obtained by water treatment, which is attributed to the lower density of hydroxide groups. The deposition of Pt nanoparticles resulted in higher visible light absorption and electron-hole separation efficiency of the nanowires. The Ptdeposited nanowires with water treatment exhibited the highest photocatalytic activities in both degradation of rhodamine B and water splitting. A maximum amount of hydrogen evolution, 13.8 mmol g−1, was obtained in 6 h.
Figure 9. PL spectra of P25 and of pristine and Pt-deposited TiO2based nanowires.
of the electrons and holes. As a consequence, the highest photocatalytic activity is achieved for the water-treated nanowires. On the other hand, too much loading and a larger size of platinum nanoparticles (i.e., on the acid-treated nanowires) could result in a higher electron-hole recombination rate and a lower electron–hole recombination distance, light absorption, and effective reactive area of TiO2, resulting in lower photocatalytic activities. Therefore, Pt-deposited nanowires with water treatment exhibit the highest photocatalytic activity.
Acknowledgments This work was supported by the Ministry of Science and Technology of Taiwan under Contract Nos. MOST 1032218-E-035-012, NSC 103-2120-M-007-005 and MOST 104-3113-E-035-001-CC2. We would also like to thank Mr Vitaly Gurykv for the PL measurement.
3.6. PL spectra of Pt-deposited TiO2-based nanowires
Figure 9 displays the PL spectra of P25 and pristine and Ptdeposited nanowires with various treatments. The emission wavelengths of the samples are all located at ∼505 nm, which can be attributed to the charge transfer from Ti3+ to the oxygen anion in the TiO6 8− complex [47]. However, the intensities of the PL emissions that are related to the electronhole recombination rate in semiconductors are very different [48, 49]. The sample with a lower emission intensity could be suggested to have a lower electron-hole recombination rate that would result in higher photocatalytic activity [48, 49]. Indeed, Pt-deposited nanowires with water treatment exhibit the lowest intensity, which demonstrates that an appropriate
References [1] Fujishima A and Honda K 1972 Nature 238 37 [2] Zhang Z, Wang C C, Zakaria R and Ying Y J 1998 J. Phys. Chem. B 102 10871 [3] Wang C C, Zhang Z and Ying Y J 1997 Nanostruct. Mater. 9 583 [4] Serpone N, Lawless D, Khairlutdinov R and Pelizzetti E 1995 J. Phys. Chem. 99 16655 8
Nanotechnology 26 (2015) 254002
C-C Wang et al
[28] Suetake J, Nosaka A Y, Hodouchi K, Matsubara H and Nosaka Y 2008 J. Phys. Chem. C 112 18474 [29] Aaltonen T, Ritala M, Sajavaara T, Keinonen J and Leskela M 2003 Chem. Mater. 15 1924 [30] Lin Y H, Hsueh Y C, Lee P S, Wang C C, Chen J R, Wu J M, Perng T P and Shih H C 2010 J. Electrochem. Soc. 9 K206 [31] Farmer D B and Gordon R G 2006 Nano Lett. 6 699 [32] Farmer D B and Gordon R G 2005 Electrochem. Solid-State Lett. 8 G89 [33] Driessen M D and Grassian V H 1998 J. Phys. Chem. B 102 1418 [34] Schierbaum K D, Fischer S, Torquemada M C, De Segovia J L, Roman E and Martin-Gago J A 1996 Surf. Sci. 345 261 [35] Wang W N, An W J, Ramalingam B, Mukherjee S, Niedzwiedzki D M, Gangopadhyay S and Biswas P 2012 J. Am. Chem. Soc. 134 11276 [36] Karakitsou K E and Verykios X E 1992 J. Catal. 134 629 [37] Sreethawong T, Puangpetch T, Chavadej S and Yoshikawa S 2012 J. Catal. 294 63 [38] Konstantinou I K and Albanis T A 2004 Appl. Catal. B 49 1 [39] Kuo H L, Kuo C Y, Liu C H, Chao J H and Lin C H 2007 Catal. Lett. 113 1 [40] Wang F C, Liu C H, Liu C W, Chao J H and Lin C H 2009 J. Phys. Chem. C 113 13832 [41] Yi H, Peng T, Ke D, Ke D, Zan L and Yan C 2008 Int. J. Hydrog. Energy 33 672 [42] Kamat P V 1993 Chem. Rev. 93 267 [43] Addamo M, Augugliaro V, Paola D A, Lopez G E, Loddo V, Marci G, Molinari R, Palmisano L and Schiavello M 2004 J. Phys. Chem. B 108 3303 [44] Karakitsou K E and Verykios X E 1993 J. Phys. Chem. 97 1184 [45] Kawai T and Sakata T 1980 J. Chem. Soc. Chem. Commun. 15 694 [46] Chen J, Ollis D F, Rulkens W H and Bruning H 1999 Water Res. 33 669 [47] Yu J C, Yu J, Ho W, Jiang Z and Zhang L 2002 Chem. Mater. 14 3808 [48] Li X Z, Li F B, Yang C L and Ge W K 2001 J. Photochem. Photobiol. A 141 209 [49] Li X Z and Li F B 2001 Environ. Sci. Technol. 35 2381
[5] Parra S, Stanca S E, Guasaquillo I and Thampi K R 2004 Appl. Catal. B 51 107 [6] Zhu H, Gao X, Lan Y, Song D, Xi Y and Zhao J 2004 J. Am. Chem. Soc. 126 8380 [7] Jang J S, Kim D H, Choi S H, Jang J W, Kim H G and Lee J S 2012 Int. J. Hydrog. Energy 37 11081 [8] Li C, Yuan J, Han B, Jiang L and Shangguan W 2010 Int. J. Hydrog. Energy 35 7073 [9] Lai C W and Sreekantan S 2013 Electrochim. Acta 87 294 [10] Khan M A, Akhtar M S, Woo S I and Yang O B 2008 Catal. Commun. 10 1 [11] Sun X and Li Y 2003 Chem. Eur. J. 9 2229 [12] Kiwi J and Gratzel M 1984 J. Phys. Chem. 88 1302 [13] Sadeghi M, Liu W, Zhang T G, Stavropoulos P and Levy B 1996 J. Phys. Chem. 100 19466 [14] Rosseler O, Shankar M V, Du M K L, Schmidlin L, Keller N and Keller V 2010 J. Catal. 269 179 [15] Bamwenda G R, Tsubota S, Nakamura T and Haruta M 1995 J. Photochem. Photobiol. A 89 177 [16] Sakthivel S, Shankar M V, Palanichamy M, Arabindoo B, Bahnemann D W and Murugesan V 2004 Water Res. 38 3001 [17] Li F B and Li X Z 2002 Chemosphere 48 1103 [18] Vorontsov A V, Savinov E N and Zhensheng J 1999 J. Photochem. Photobiol. A 125 113 [19] Wang C C, Kei C C, Yu Y W and Perng T P 2007 Nano Lett. 7 1566 [20] Chang W T, Hsueh Y C, Huang S H, Liu K I, Kei C C and Perng T P 2013 J. Mater. Chem. A 1 1987 [21] Liang Y C, Wang C C, Kei C C, Hsueh Y C, Cho W H and Perng T P 2011 J. Phys. Chem. C 115 9498 [22] Liu C, Wang C C, Kei C C, Hsueh Y C and Perng T P 2009 Small 5 1535 [23] Hsueh Y C, Wang C C, Liu C, Kei C C and Perng T P 2012 Nanotechnology 23 405603 [24] Hsueh Y C, Wang C C, Kei C C, Lin Y H, Liu C and Perng T P 2012 J. Catal. 294 63 [25] Lashdaf M, Hatanpaa T, Krause A O I, Lahtinen J, Lindblad M and Tiitta M 2003 Appl. Catal. A 241 51 [26] Mao Y B and Wong S S 2006 J. Am. Chem. Soc. 128 8217 [27] Chen Q, Du G H and Peng Z L 2002 Acta Crystallogr. Sect. B 58 587
9
Search
Collections
Journals
About
Contact us
My IOPscience
Deposition of uniform Pt nanoparticles with controllable size on TiO2-based nanowires by atomic layer deposition and their photocatalytic properties
This content has been downloaded from IOPscience. Please scroll down to see the full text. 2015 Nanotechnology 26 254002 (http://iopscience.iop.org/0957-4484/26/25/254002) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 132.239.1.230 This content was downloaded on 10/06/2015 at 04:00
Please note that terms and conditions apply.
Nanotechnology Nanotechnology 26 (2015) 254002 (9pp)
doi:10.1088/0957-4484/26/25/254002
Deposition of uniform Pt nanoparticles with controllable size on TiO2-based nanowires by atomic layer deposition and their photocatalytic properties Chih-Chieh Wang1, Yang-Chih Hsueh2, Chung-Yi Su2, Chi-Chung Kei3 and Tsong-Pyng Perng2 1
Department of Materials Science and Engineering, Feng Chia University, Taichung 407, Taiwan Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan 3 Instrument Technology Research Center, National Applied Research Laboratories, Hsinchu 300, Taiwan 2
E-mail: [email protected] Received 24 February 2015, revised 26 April 2015 Accepted for publication 8 May 2015 Published 4 June 2015 Abstract.
TiO2-based nanowires were prepared by a hydrothermal method, and Pt nanoparticles were deposited on the nanowires by atomic layer deposition (ALD). The size and loading of Pt nanoparticles with very good uniformity could be controlled by the ALD cycle number, along with acid or water treatment of the nanowires. The lower growth rate and the loading of Pt nanoparticles were obtained with water treatment. With acid treatment, the growth rate and loading were increased, and this increase was attributed to the formation of higher density of hydroxide groups on the nanowires. The photocatalytic activities of Pt-deposited nanowires were investigated. The deposition of Pt on the nanowires resulted in both enhanced light absorption in the visible region and electron–hole separation efficiency. The water-treated nanowires exhibited the highest degradation rate of rhodamine B and the highest hydrogen evolution rate. A maximum amount of hydrogen evolution, 13.8 mmol g−1 in 6 h, was achieved. Keywords: atomic layer deposition, Pt nanoparticles, photocatalysis (Some figures may appear in colour only in the online journal) 1. Introduction
reaction process [7–9]. Because of their cation-exchange reactivity on the surface [10, 11], they can also be used as a catalyst support to deposit other active catalysts with uniform distribution and high dispersion. The presence of platinum nanoparticles on the TiO2 surface effectively reduces the electron–hole recombination rate [12–14], which is caused by the formation of the Schottky barrier at the Pt–TiO2 interface. The interfacial charge transfer is facilitated, resulting in improved separation of electrons and holes. Basically, effective electron–hole separation is related to the size, loading, and uniform distribution of Pt particles on TiO2 [12–14]. For instance, Pt particles of several nanometers in diameter can have enhanced efficiency on H2 evolution [12]. On the other hand, larger Pt particles would provide more recombination sites for
Since 1972, when Fujishima and Honda discovered that photodecomposition of water could be achieved by illuminating TiO2 electrodes with ultraviolet (UV) light [1], TiO2 has attracted great attention in wastewater treatment and hydrogen generation because of its low cost, nontoxicity, and chemical stability. However, TiO2 has not been widely applied to the environmental industry because it suffers from a high electron–hole recombination rate [2–4]. Also, the low surface area of bulk TiO2 limits its photocatalytic reactivity [5, 6]. TiO2-based one-dimensional (1D) nanostructures fabricated by the hydrothermal method are of great interest for catalysis due to their high specific surface area, which can facilitate the transport of reagents to the active sites during the 0957-4484/15/254002+09$33.00
1
© 2015 IOP Publishing Ltd Printed in the UK
Nanotechnology 26 (2015) 254002
C-C Wang et al
substrate at 50 °C to evaporate the ethanol. A ∼3 μm-thick film was formed. Pt nanoparticles were then deposited on the nanowires by homemade ALD. The deposition of Pt nanoparticles by ALD has been described previously [22–24]. Trimethyl(methylcyclopentadienyl) platinum (IV) (MeCpPtMe3) and oxygen were used as precursors. In a cycle of ALD, a 1 s pulse of MeCpPtMe3 and a 5 s pulse of oxygen were each separated by a 20 s purge of nitrogen. The growth temperature for all samples was 250 °C. The Pt loading was calculated by the ratio of the amount of Pt to Pt + TiO2. After dissolving the sample in a solution composed of HNO3 (3 ml, 65%), HCl (3 ml, 37%), H2SO4 (2 ml, 98%), and HF (2 ml, 48%) that was further diluted to 50 ml, the amount of Pt was then analyzed by inductively coupled plasma mass spectrometry (Perkin Elmer SCIEX ELAN 5000).
photogenerated electrons and holes [12, 13]. In addition, too much Pt loading on TiO2 reduces not only the surface reactive site, but also the light absorption of TiO2 [14]. Therefore, optimum size, loading, and uniform distribution of Pt particles on TiO2 are necessary to obtain effective electron-hole separation and to achieve high photocatalytic activity. Deposition of Pt nanoparticles on TiO2-based 1D nanostructures seems to be a straightforward way to enhance photocatalytic activity. However, deposition of well-dispersed Pt nanoparticles with controllable size and loading on TiO2 is difficult because the deposition usually involves chemical wet processes, such as impregnation [15, 16] and photodeposition [17, 18], which have significant limitations. For instance, in the impregnation method, particle agglomeration occurs during the reduction of metal ions in a hydrogen atmosphere at high temperatures. The loading of Pt is unpredictable in the photodeposition method since it is difficult to attain uniform surface reactivity of TiO2 with a Pt ion precursor. Because of the limitation of these processes, atomic layer depostition (ALD) is used in our fabrication process. ALD is a selflimiting process that enables fabrication of thin films with excellent conformal and uniform coating and precise thickness control on complex nanostructures [19–21]. In addition to the preparation of thin films, nanoparticles can also be fabricated by ALD for a variety of applications [22–25]. For instance, Pt nanoparticles with controllable size and loading as a catalyst are well distributed on carbon nanotubes for proton exchange membrane fuel cells (PEMFCs) [22–24]. The performance of PEMFCs reveals that ALD could effectively reduce the Pt loading to meet the commercial requirements of PEMFCs. However, the substrate requires hydroxide groups as a nucleation site when applying ALD. In this study, TiO2-based nanowires prepared by the hydrothermal method were modified by water and acid treatment to provide more hydroxide groups. Pt nanoparticles were subsequently deposited on the nanowires by ALD. UVvisible light absorption, photocatalytic degradation of rhodamine B, and hydrogen evolution by the nanostructures were also investigated.
2.3. Characterization
X-ray diffraction (XRD) with Cu Kα radiation (Shimadzu 6000) was performed to examine the structures of the nanowires. The morphologies of the nanostructures and the particle sizes of Pt were examined by transmission electron microscopy (TEM, JEOL 3010F). The surface functional groups were analyzed by Fourier transform infrared spectrometry (FTIR). The optical absorption spectra were obtained with a JASCO V-570 spectrophotometer.
2.4. Photocatalytic activity and photoluminescence (PL) measurement
We measured the photocatalytic activity for the degradation of rhodamine B solution (20 ml, 5 × 10−2 M) under illumination from a 200 W Hg lamp (wavelength 240 to 600 nm) by the nanowires. The distance between the Hg lamp and the rhodamine B solution was 25 cm. 0.1 g nanowires were added to the rhodamine B solution. Before the illumination, the solution was stirred for 10 min in the dark for the dye molecules to be adsorbed on the nanowire surface. For 60 min, UV-visible absorption spectra of the samples were then collected before and after every 10 min of irradiation. PL spectra were obtained using microzone Raman spectroscopy (Horiba Jobin Yvon, Labram HR 800). The excitation wavelength was 325 nm (He-Cd laser, Kimon IK3301R-G) with a laser power of 30 mW and a spot size of 0.79 μm2.
2. Experimental 2.1. Preparation of TiO2-based nanowires
TiO2-based nanowires were prepared by a hydrothermal method [25]. 0.5 g anatase TiO2 was reacted with 10 M NaOH at 200 °C for 24 h. After the reaction, the white powder product was washed with water and dried. We called these the pristine nanowires. For modification, 0.1 g pristine nanowires were immersed in water, 1 M hydrochloric acid, or 1 M nitric acid for 1 h at 25 °C. They were then rinsed with water and dried at 50 °C.
2.5. Hydrogen generation measurement
The hydrogen generation measurement was performed in a photoreactor made of quartz and stainless steel containing 0.5 g of catalyst in a 100 ml solution of water and methanol at a 4:1 volume ratio under illumination from a 200 W Hg lamp. Before the measurement, the suspension in the reactor was stirred and purged with argon for 1 h to remove the residual air. For 6 h, a measurement was then conducted for each
2.2. Deposition of Pt nanoparticles on the nanowires by ALD
0.05 g nanowires were dispersed in 5 ml ethanol by ultrasonic vibration. A drop of solution was placed on a 1 × 1 cm silicon 2
Nanotechnology 26 (2015) 254002
C-C Wang et al
Table 1. EDX analysis of the chemical compositions of the
nanowires with acid or water treatment. Element (at%) Treatment H2O HCl HNO3
O
Ti
Na
61.43 66.60 83.44
29.85 33.40 16.26
8.72 0.00 0.30
3. Results and discussion 3.1. Characterization of TiO2-based nanowires with various treatments
The diameter and length of the nanowires were 50 ± 25 nm and 3.00 ± 1.58 μm, respectively. Figure 1(a) shows the TEM image of the nanowires with 45 ± 15 nm diameter and 2.00 ± 0.55 μm length. The morphology was maintained even after hydrochloric or nitric acid treatment. According to the energy dispersive x-ray (EDX) analysis, a many sodium ions were present in the as-prepared nanowires. The concentration of sodium was reduced after acid treatment, which implies that the compositions of the nanowires might be different (table 1). The structures of the water-treated and acid-treated nanowires were examined by XRD patterns, as shown in figure 1(b). Both types of nanowires contain anatase TiO2. The diffraction peaks at approximately 24°, 28°, and 48° are observed for the nanowires with water treatment, corresponding to (110), (211), and (020), respectively, of a trititanate structure (A2Ti3O7, A = H or Na) [26–28]. Note that the intensities of the peaks (211) and (020) are considerably reduced after acid treatment. This may be attributed to the fact that sodium is exchanged by hydrogen during the treatment process [10, 11]. It is also in line with the decrease of sodium content observed in the EDX analysis. An attempt was made to investigate the surface of the nanowires with water and acid treatment by FTIR spectroscopy, as shown in figure 1(c). The absorptions at 3400 and 1630 cm−1 could be assigned to the hydroxide group and the vibration of H-O-H bonds of water, respectively. Both peaks become stronger after acid treatment, which implies that more hydroxyl groups are present in the acid-treated samples. There is an additional nitro group at 1380 cm−1 in the nitric acid treated sample. To check whether the nitro group was caused by the residual nitric acid, the sample was washed five times, first with a 0.1 M NaCl aqueous solution and then by water. The peak was still present, implying that a nitro group is formed on the surface of the nanowires.
Figure 1. (a) TEM image of TiO2-based nanowires. (b) XRD patterns of TiO2-based nanowires with water and HCl treatment. (c) FTIR spectra of TiO2-based nanowires treated with water, nitric acid, and hydrochloric acid.
3.2. Pt-deposited TiO2 nanowires
Pt nanoparticles were deposited uniformly on the TiO2-based nanowires with acid and water treatment, and the size could be well controlled by the cycle number. For the nanowires with nitric acid treatment with 10, 30, and 60 cycles of ALD,
illumination of 1 h. The evolution of hydrogen was analyzed by gas chromatography (Shimazu GC-2014) with a thermal conductive detector. 3
Nanotechnology 26 (2015) 254002
C-C Wang et al
Figure 2. TEM images of nitric acid-treated TiO2-based nanowires deposited with Pt nanoparticles by ALD with (a) 10 cycles, (b) 30 cycles, and (c) 60 cycles. (d) Dependence of Pt particle size on the ALD cycle number for nanowires with various treatments.
the sizes of Pt nanoparticles were ∼0.50 ± 0.03, 1.30 ± 0.08, and 2.60 ± 0.25 nm, respectively, as shown in figures 2(a)–(c). The particle size increases linearly with increases in the cycle number. The growth rate is calculated to be ∼0.046 nm/cycle, which is comparable to that of the nanowires with hydrochloric acid treatment, as shown in figure 2(d). Interestingly, Pt nanoparticles on the water-treated nanowires exhibit lower growth rates that can be divided into two steps. Below 30 cycles, a lower growth rate of 0.033 nm/cycle is observed. At 30 or more cycles of ALD, a higher growth rate of 0.044 nm/ cycle is reached, which is comparable to that of the acidtreated samples. The lower growth rate in the initial stage may be correlated to difficult nucleation. The reaction mechanism of ALD of Pt has been proposed [29]. Surface oxygen would react with the precursor of Pt to form Pt-Ox to initialize the ALD reactions. Pure Pt nanoparticles without any impurity phases are then obtained, even though oxygen is involved in the process [23, 30]. As observed in the FTIR spectrum (figure 1(c)), a lower density of hydroxyl groups is observed in the water-treated nanowires, resulting in the lowest growth rate. Interestingly, in the nitric acid-treated nanowires, although the concentration of hydroxide groups is less than that of the hydrochloric acid-treated nanowires, their growth rates are comparable. Note that an additional nitro group was produced in the nitric acid-treated nanowires. Farmer and
Gordon suggested that NO2 groups on the surface could also assist ALD growth [31, 32] because the nitrogen end is attached to the substrate, leaving the oxygen end to react with the incoming precursor to assist the nucleation of the nanoparticles. Furthermore, the difficult nucleation also has an influence on Pt loading. It is expected that one would observe fewer and smaller Pt nanoparticles on water-treated nanowires, as shown in figure 3(a). With acid treatment, more and larger Pt nanoparticles are decorated on the nanowires (figure 3(b)). As shown in figure 3(c), the Pt loading increases with increases in the cycle number, and the Pt loadings of the water-treated nanowires are consistently 35% lower than those of the other two samples. In addition, the amount of sample on the silicon substrate can affect the deposition of Pt nanoparticles. Hsueh et al observed that the Pt loading increased with an increase in the number of carbon nanotubes (CNTs) on the silicon substrate while the cycle number was kept at a constant [23, 24]. That could also provide an alternative way to control the Pt loading on TiO2 nanowires. The atomic contrast of high-angle annular dark field (HAADF) Z-contrast imaging and EDX mapping of the water-treated nanowires deposited with Pt in 120 cycles of ALD were conducted, as shown in figure 4. From the HAADF Z-contrast image (figure 4(a)), note that the 4
Nanotechnology 26 (2015) 254002
C-C Wang et al
spectrum of EDX reveals that only Pt, Ti, and O signals are present, as shown in figure 4(d). 3.3. UV-visible absorption of Pt-deposited TiO2 nanowires
Figure 5 displays the UV-visible absorption spectra of pristine and Pt-deposited TiO2-based nanowires. The pristine nanowires exhibit an absorption edge at ∼340 nm, which may be attributed to the excitation of electrons from the valence to the conduction band. With the deposition of platinum, the absorption is redshifted. The absorption intensity is also enhanced in the longer wavelength range (>400 nm). It is believed that both the redshift and the enhancement of absorption in the longer wavelength range and are correlated to the presence of new electronic states produced by the deposition of Pt nanoparticles [33, 34]. The Pt-derived electronic states are generated within the band gap. The electrons in the valence band of TiO2 are excited to the Pt-derived electronic states, resulting in the absorption in the longer wavelength range. 3.4. Photocatalytic activity of Pt-deposited TiO2-based nanowires
The photocatalytic activities of the nanowires were investigated using rhodamine B as a reaction reagent, as shown in figure 6. The nanowires with different treatments exhibit essentially the same photocatalytic activities, as shown in figure 6(a). This implies that the surface functional groups, residual sodium on the nanowires, and the A2Ti3O7 phase do not affect the activity. However, the activities of the nanowires are improved as Pt nanoparticles are deposited. This is attributed to the formation of a Schottky barrier between TiO2 and Pt, which facilitates the injection of photogenerated electrons to Pt, resulting in effective separation of electrons and holes [12–14]. The degradation of rhodamine B can be quantified by the value of the reaction constant. The reaction constants of as-prepared nanowires and nitric acid-, hydrochloric acid-, and water-treated nanowires deposited with Pt are 2.00 × 10−3, 2.15 × 10−3, 2.54 × 10−3, and 4.24 × 10−3, respectively. Interestingly, the enhancement of the photocatalytic activity is more pronounced for the water-treated sample than for nitric acid- and hydrochloric acid-treated samples. An attempt is then made to investigate the effects of the particle size and loading of Pt. The sizes of Pt with 60 cycles of ALD on acid-treated and water-treated nanowires are 2.65 ± 0.25 nm and 2.10 ± 0.20 nm, respectively. Wang et al reported that a 0.2 nm difference in Pt particle size could substantially improve the photoreduction of CO2 to CH4 [35]. The production yield increases from 557 to . This implies that a 0.55 nm difference in size in the present case may make a big difference in the photocatalytic activity. Due to the quantum confinement effect, Pt nanoparticles with decreasing size have larger energy band separation [13, 35], thus increasing the transfer of electrons from TiO2 to Pt. As shown in figure 7, Pt nanoparticles larger than ∼1 nm capture electrons efficiently only when the energy levels are below −4.4 eV [13, 35]. If the size becomes larger, the energy level
Figure 3. TEM images of Pt nanoparticles deposited by 60 cycles of
ALD on TiO2-based nanowires treated with (a) water and (b) hydrochloric acid. (c) Pt loadings on the nanowires with various treatments.
nanowire has a darker contrast than the particles due to the difference in the atomic number. Also, the Pt and Ti signals, as shown in figures 4(b) and (c), respectively, are well in line with the contrast of the HAADF Z-contrast image. The full 5
Nanotechnology 26 (2015) 254002
C-C Wang et al
Figure 4. Water-treated TiO2-based nanowire deposited with Pt nanoparticles by 120 cycles of ALD. (a) HAADF Z-contrast image. (b) EDX mapping of Ti. (c) EDX mapping of Pt. (d) Full spectrum of EDX.
sample, 0.72 ± 0.05 wt%. It is believed that the photocatalytic activity can be correlated to the charge carrier separation distance, which can be controlled by the Pt loading on TiO2 [13]. Lower loading of Pt leads to a longer charge carrier separation distance, resulting in higher photocatalytic activity. On the other hand, too high a density of Pt, as in the case of acid treatment, would retard charge carrier separation so that the photocatalytic activity becomes lower. In addition, the light absorption by TiO2 is blocked and the number of surface reactive sites on TiO2 is decreased [15]. Compared to the effect of particle size, Pt loading is probably a more dominant factor in the degradation of rhodamine B. Taking the above samples as an example, the size difference of Pt on watertreated and nitric acid-treated nanowires is 26%, (2.10 nm versus 2.65 nm), but the loading difference increases by 50%, from 0.48 to 0.72 wt%.
Figure 5. UV-visible absorption spectra of pristine and Pt-deposited
nanowires. The Pt nanoparticles were deposited on water- and acidtreated nanowires by 60 cycles of ALD.
3.5. Hydrogen generation of Pt-deposited TiO2-based nanowires
separation tends to gradually narrow like bulk Pt that acts as a recombination center for both electrons and holes. Therefore, the 2.65 nm size may suffer more electron-hole recombination than the 2.10 nm size, to exhibit a lower photocatalytic activity. In addition, the photocatalytic activity may be affected by the Pt loading on TiO2-based nanowires. The Pt loading with 60 cycles of ALD on the water-treated sample is 0.48 ± 0.03 wt%, which is lower than that of the acid-treated
Figure 8 shows the hydrogen generation of the pristine and Pt-deposited TiO2 nanowires from methanol/water decomposition. Compared with P25 and the pristine nanowires, Ptdeposited nanowires exhibit higher hydrogen-generation rates. With water treatment, the maximum amount of hydrogen, 13.8 mmol g−1 in 6 h, is reached under illumination from a 200 W Hg mercury lamp. When the irradiation time is longer than 6 h, the amount of hydrogen evolution then 6
Nanotechnology 26 (2015) 254002
C-C Wang et al
Figure 8. Hydrogen generation from methanol/water on P25 and on
pristine and Pt-deposited TiO2-based nanowires.
nanostructures. The hydrogen-generation rate from this study is higher than the rates of deposition of Pt by impregnation [15] and photodeposition methods [15] on TiO2 nanoparticles and TiO2 B nanotubes [39], and it is comparable to that of TiO2 B nanofibers [40]. However, it is much lower than that of TiO2 nanoparticles photodeposited with 5 nm Pt nanoparticles [41]. Note that TiO2 nanoparticles have good crystallinity and are mainly composed of anatase, which exhibits higher photocatalytic activity than the other phases [42–44]. Less anatase and lower crystallinity were obtained in our sample. The probable evolutionary mechanism of hydrogen from methanol/water on Pt-deposited TiO2-based nanowires could be suggested as follows [45, 46]:
Figure 6. Photocatalytic degradation of rhodamine B. (a) Nanowires with various treatments. (b) Pristine and Pt-deposited nanowires.
[Catalyst] + hν → e− + h+
(1)
h+ + H 2 O → H + + ⋅ OH
(2)
⋅OH + CH 3OH → H 2 O + ⋅ CH 2 OH +
−
⋅CH 2 OH → HCHO + H + e −
2H 2 O + 2e → H 2 + 2OH
−
(3) (4) (5)
and the overall reaction is: CH 3OH → HCHO + H 2
(6)
Electron-hole pairs are generated as the photocatalyst absorbs light energy that is higher than the band gap. The holes in the valence band react with H2O to produce H+ and ·OH radicals (equation (2)) that subsequently oxidize the methanol to form HCHO (equations (3) and (4)). Due to the absence of oxygen, electrons in the conduction band simultaneously reduce H+ to form H2 gas (equation (5)). However, the reactions compete with the recombination of electron-hole pairs. The Pt nanoparticles with appropriate loading and size can capture electrons more effectively for hydrogen evolution from methanol/water due to the formation of a Schottky barrier on the catalyst’s surface. The electric field caused by the Schottky barrier injects electrons into the platinum and leaves holes in the TiO2, resulting in more efficient separation
Figure 7. Schematic diagram of the energy band of TiO2 and the energy levels of Pt nanoparticles of different sizes.
increases slowly to approach a saturated value. This might be due to the balance of the forward and backward reactions of hydrogen evolution, which are associated with a constant pressure in the closed system [36, 37]. Additionally, the reactive sites may become inactive after a long irradiation time because of the adsorption of byproducts [38]. Table 2 summarizes the hydrogen production rates of various Pt-TiO2 7
Nanotechnology 26 (2015) 254002
C-C Wang et al
Table 2. Hydrogen production rates by various Pt-TiO2 nanostructures.
Catalyst
a
Pt-TiO2 NP Pt-TiO2 NP Pt-TiO2 (B) NT Pt-TiO2 (B) NF Pt-TiO2 NP Pt-TiO2− based NW
TiO2 dimensionb
TiO2 phasec
Size (nm)
D = N. A. D = N. A. D = 50 nm L = 2 μm D = 50 nm L = 2 μm D = 8 nm D = 30 nm L = 3 μm
A+R A+R T A+B A T+A
1.4 2.4 N. A. 12 5–10 2.1
Load (wt%) 1 1 1 1 1 0.48
Methodd IP PD PD IP PD ALD
H2 rate (μmol/g · h)
Reference
720 1805 450 2570 6000 2301
[15] [15] [39] [40] [41] This study
a b c d
NP: nanoparticle, NT: nanotube, NF: nanofiber, NW: nanowire. D: diameter L: length. A: anatase, R: rutile, B: TiO2 (B) phase, T: trititanate. IP: impregnation, PD: photodeposition.
loading of Pt nanoparticles can suppress the recombination process of photogenerated electrons and holes. On the other hand, in the case of Pt-deposited nanowires with nitric or hydrochloric acid treatment, a higher intensity is observed. This confirms that too much loading of Pt nanoparticles results in the enhancement of electron-hole recombination, which is well in line with the lower hydrogen evolution, as shown in figure 8.
4. Conclusion In conclusion, highly uniform Pt nanoparticles with controllable size and loading were deposited on TiO2-based nanowires by ALD. The size and loading could be controlled by the ALD cycle number and acid or water treatment of the nanowires. The smaller size and lower loading of Pt nanoparticles were obtained by water treatment, which is attributed to the lower density of hydroxide groups. The deposition of Pt nanoparticles resulted in higher visible light absorption and electron-hole separation efficiency of the nanowires. The Ptdeposited nanowires with water treatment exhibited the highest photocatalytic activities in both degradation of rhodamine B and water splitting. A maximum amount of hydrogen evolution, 13.8 mmol g−1, was obtained in 6 h.
Figure 9. PL spectra of P25 and of pristine and Pt-deposited TiO2based nanowires.
of the electrons and holes. As a consequence, the highest photocatalytic activity is achieved for the water-treated nanowires. On the other hand, too much loading and a larger size of platinum nanoparticles (i.e., on the acid-treated nanowires) could result in a higher electron-hole recombination rate and a lower electron–hole recombination distance, light absorption, and effective reactive area of TiO2, resulting in lower photocatalytic activities. Therefore, Pt-deposited nanowires with water treatment exhibit the highest photocatalytic activity.
Acknowledgments This work was supported by the Ministry of Science and Technology of Taiwan under Contract Nos. MOST 1032218-E-035-012, NSC 103-2120-M-007-005 and MOST 104-3113-E-035-001-CC2. We would also like to thank Mr Vitaly Gurykv for the PL measurement.
3.6. PL spectra of Pt-deposited TiO2-based nanowires
Figure 9 displays the PL spectra of P25 and pristine and Ptdeposited nanowires with various treatments. The emission wavelengths of the samples are all located at ∼505 nm, which can be attributed to the charge transfer from Ti3+ to the oxygen anion in the TiO6 8− complex [47]. However, the intensities of the PL emissions that are related to the electronhole recombination rate in semiconductors are very different [48, 49]. The sample with a lower emission intensity could be suggested to have a lower electron-hole recombination rate that would result in higher photocatalytic activity [48, 49]. Indeed, Pt-deposited nanowires with water treatment exhibit the lowest intensity, which demonstrates that an appropriate
References [1] Fujishima A and Honda K 1972 Nature 238 37 [2] Zhang Z, Wang C C, Zakaria R and Ying Y J 1998 J. Phys. Chem. B 102 10871 [3] Wang C C, Zhang Z and Ying Y J 1997 Nanostruct. Mater. 9 583 [4] Serpone N, Lawless D, Khairlutdinov R and Pelizzetti E 1995 J. Phys. Chem. 99 16655 8
Nanotechnology 26 (2015) 254002
C-C Wang et al
[28] Suetake J, Nosaka A Y, Hodouchi K, Matsubara H and Nosaka Y 2008 J. Phys. Chem. C 112 18474 [29] Aaltonen T, Ritala M, Sajavaara T, Keinonen J and Leskela M 2003 Chem. Mater. 15 1924 [30] Lin Y H, Hsueh Y C, Lee P S, Wang C C, Chen J R, Wu J M, Perng T P and Shih H C 2010 J. Electrochem. Soc. 9 K206 [31] Farmer D B and Gordon R G 2006 Nano Lett. 6 699 [32] Farmer D B and Gordon R G 2005 Electrochem. Solid-State Lett. 8 G89 [33] Driessen M D and Grassian V H 1998 J. Phys. Chem. B 102 1418 [34] Schierbaum K D, Fischer S, Torquemada M C, De Segovia J L, Roman E and Martin-Gago J A 1996 Surf. Sci. 345 261 [35] Wang W N, An W J, Ramalingam B, Mukherjee S, Niedzwiedzki D M, Gangopadhyay S and Biswas P 2012 J. Am. Chem. Soc. 134 11276 [36] Karakitsou K E and Verykios X E 1992 J. Catal. 134 629 [37] Sreethawong T, Puangpetch T, Chavadej S and Yoshikawa S 2012 J. Catal. 294 63 [38] Konstantinou I K and Albanis T A 2004 Appl. Catal. B 49 1 [39] Kuo H L, Kuo C Y, Liu C H, Chao J H and Lin C H 2007 Catal. Lett. 113 1 [40] Wang F C, Liu C H, Liu C W, Chao J H and Lin C H 2009 J. Phys. Chem. C 113 13832 [41] Yi H, Peng T, Ke D, Ke D, Zan L and Yan C 2008 Int. J. Hydrog. Energy 33 672 [42] Kamat P V 1993 Chem. Rev. 93 267 [43] Addamo M, Augugliaro V, Paola D A, Lopez G E, Loddo V, Marci G, Molinari R, Palmisano L and Schiavello M 2004 J. Phys. Chem. B 108 3303 [44] Karakitsou K E and Verykios X E 1993 J. Phys. Chem. 97 1184 [45] Kawai T and Sakata T 1980 J. Chem. Soc. Chem. Commun. 15 694 [46] Chen J, Ollis D F, Rulkens W H and Bruning H 1999 Water Res. 33 669 [47] Yu J C, Yu J, Ho W, Jiang Z and Zhang L 2002 Chem. Mater. 14 3808 [48] Li X Z, Li F B, Yang C L and Ge W K 2001 J. Photochem. Photobiol. A 141 209 [49] Li X Z and Li F B 2001 Environ. Sci. Technol. 35 2381
[5] Parra S, Stanca S E, Guasaquillo I and Thampi K R 2004 Appl. Catal. B 51 107 [6] Zhu H, Gao X, Lan Y, Song D, Xi Y and Zhao J 2004 J. Am. Chem. Soc. 126 8380 [7] Jang J S, Kim D H, Choi S H, Jang J W, Kim H G and Lee J S 2012 Int. J. Hydrog. Energy 37 11081 [8] Li C, Yuan J, Han B, Jiang L and Shangguan W 2010 Int. J. Hydrog. Energy 35 7073 [9] Lai C W and Sreekantan S 2013 Electrochim. Acta 87 294 [10] Khan M A, Akhtar M S, Woo S I and Yang O B 2008 Catal. Commun. 10 1 [11] Sun X and Li Y 2003 Chem. Eur. J. 9 2229 [12] Kiwi J and Gratzel M 1984 J. Phys. Chem. 88 1302 [13] Sadeghi M, Liu W, Zhang T G, Stavropoulos P and Levy B 1996 J. Phys. Chem. 100 19466 [14] Rosseler O, Shankar M V, Du M K L, Schmidlin L, Keller N and Keller V 2010 J. Catal. 269 179 [15] Bamwenda G R, Tsubota S, Nakamura T and Haruta M 1995 J. Photochem. Photobiol. A 89 177 [16] Sakthivel S, Shankar M V, Palanichamy M, Arabindoo B, Bahnemann D W and Murugesan V 2004 Water Res. 38 3001 [17] Li F B and Li X Z 2002 Chemosphere 48 1103 [18] Vorontsov A V, Savinov E N and Zhensheng J 1999 J. Photochem. Photobiol. A 125 113 [19] Wang C C, Kei C C, Yu Y W and Perng T P 2007 Nano Lett. 7 1566 [20] Chang W T, Hsueh Y C, Huang S H, Liu K I, Kei C C and Perng T P 2013 J. Mater. Chem. A 1 1987 [21] Liang Y C, Wang C C, Kei C C, Hsueh Y C, Cho W H and Perng T P 2011 J. Phys. Chem. C 115 9498 [22] Liu C, Wang C C, Kei C C, Hsueh Y C and Perng T P 2009 Small 5 1535 [23] Hsueh Y C, Wang C C, Liu C, Kei C C and Perng T P 2012 Nanotechnology 23 405603 [24] Hsueh Y C, Wang C C, Kei C C, Lin Y H, Liu C and Perng T P 2012 J. Catal. 294 63 [25] Lashdaf M, Hatanpaa T, Krause A O I, Lahtinen J, Lindblad M and Tiitta M 2003 Appl. Catal. A 241 51 [26] Mao Y B and Wong S S 2006 J. Am. Chem. Soc. 128 8217 [27] Chen Q, Du G H and Peng Z L 2002 Acta Crystallogr. Sect. B 58 587
9
