nanomaterials Article

Synthesis of Pt@TiO2@CNTs Hierarchical Structure Catalyst by Atomic Layer Deposition and Their Photocatalytic and Photoelectrochemical Activity Shih-Yun Liao, Ya-Chu Yang, Sheng-Hsin Huang and Jon-Yiew Gan * Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan; [email protected] (S.-Y.L.); [email protected] (Y.-C.Y.); [email protected] (S.-H.H.) * Correspondence: [email protected]; Tel.: +886-3-5715131 (ext. 33851) Academic Editor: Thomas Nann Received: 1 March 2017; Accepted: 26 April 2017; Published: 29 April 2017

Abstract: Pt@TiO2 @CNTs hierarchical structures were prepared by first functionalizing carbon nanotubes (CNTs) with nitric acid at 140 ◦ C. Coating of TiO2 particles on the CNTs at 300 ◦ C was then conducted by atomic layer deposition (ALD). After the TiO2 @CNTs structure was fabricated, Pt particles were deposited on the TiO2 surface as co-catalyst by plasma-enhanced ALD. The saturated deposition rates of TiO2 on a-CNTs were 1.5 Å/cycle and 0.4 Å/cycle for substrate-enhanced process and linear process, respectively. The saturated deposition rate of Pt on TiO2 was 0.39 Å/cycle. The photocatalytic activities of Pt@TiO2 @CNTs hierarchical structures were higher than those without Pt co-catalyst. The particle size of Pt on TiO2 @CNTs was a key factor to determine the efficiency of methylene blue (MB) degradation. The Pt@TiO2 @CNTs of 2.41 ± 0.27 nm exhibited the best efficiency of MB degradation. Keywords: CNT; ALD; co-catalyst; TiO2 ; photodegradation

1. Introduction TiO2 has been extensively studied in contaminants degradation and hydrogen generation because of its chemical stability, non-toxicity, and low cost since TiO2 electrode was discovered that photodecomposition of water could be achieved by illumination of UV light in 1972 [1], However, TiO2 has not been widely applied to the environmental industry because the photoexcited electrons and holes inefficiently diffuse to the surface for redox reaction due to high recombination rate of electrons and holes [2–4]. In addition, bulk TiO2 suffers from the property of low surface area and then limits its photocatalytic reactivity [5,6]. Although the nanosize of TiO2 was fabricated by wet chemical process to increase the surface area of TiO2 [4], the TiO2 particles aggregated during phase transformation [7]. In order to improve the efficiencies of semiconductor photocatalyst, CNTs as a template to coat with metal oxides (MOs) have attracted great attention in environmental and energy applications because of several reasons. First, CNTs facilitate high specific surface area for MOs deposition and prevent MOs from agglomeration during annealing due to its 1-D structure. Second, CNTs as sensitizer can enhance the photodegradation reaction. Third, heterojunction of MOs@CNTs provides the interface to lower the recombination rate of photo-induced electron-hole pair [8–10]. Therefore, TiO2 -based structure such as TiO2 /CNTs is a good complementary approach to achieve higher quantum efficiency than that of pure TiO2 [11]. Although the TiO2 /CNTs heterojunction is considered as a good candidate for increasing the quantum efficiencies [11], the effect of MO@CNTs hierarchical catalysts for photodegradation is still limited. For improving the performance of hierarchical catalysts, additional loading with noble metal such as Pt is a good approach because Pt provides more reactive sites in photoreaction as Nanomaterials 2017, 7, 97; doi:10.3390/nano7050097

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a co-catalyst [12]. However, uniform and well-dispersed Pt particles with precisely controllable size on TiO2 are hard to achieve because the deposition usually involves chemical wet processes, such as photo-deposition [13] and impregnation [14], which have some limitations. For instance, for Pt@TiO2 @carbon composites, the loading of Pt is random through photo-deposition because it is difficult to obtain uniformly reacted surface of TiO2 with Pt precursors due to the limitation of irradiation [15]. The agglomeration of Pt particles fabricated by impregnation method takes place when the reduction of metal ions is conducted by chemical reagents or reductive atmosphere such as H2 and NH3 at high temperatures [16,17]. In comparison to other techniques, atomic layer deposition (ALD) provides excellent thickness control and nearly perfect conformal coverage of complex nanostructures as a result of the surface-controlled growth [18]. Besides, ALD has been confirmed not only to prepare thin films with high aspect ratio but also to fabricate particle catalyst in nanoscale [19]. For example, Pt nanoparticles as a catalyst are well distributed on CNTs with controllable loading and size for proton exchange membrane fuel cell (PEMFC) [20,21]. The effectively decreasing the loading of Pt by ALD to meet the commercial requirement of PEMFC was achieved by the evidence of highly specific power density. In this study, pristine CNTs were firstly functionalized by temperature-assisted acid-treatment, and then TiO2 @CNTs structures were prepared by ALD. Pt nanoparticles were subsequently deposited on the TiO2 @CNTs structures by plasma-enhanced ALD (PEALD) to form Pt@TiO2 @CNTs composites. Characterization and photocatalytic degradation of methylene blue (MB) by the nanostructures were also investigated. 2. Experimental 2.1. Acid Treatment of CNTs Multiwalled CNTs (3–12 µm in length, 20–40 nm in outer diameter, 90% purity) were acquired from Powertip Technology Corporation. The pristine CNTs (p-CNTs) were purified and functionalized by soaked in HNO3 (65% purity) and then refluxed at 140 ◦ C for 6 h to obtain acid-treated CNTs (a-CNTs). The a-CNTs were washed with deionized water and alcohol until pH value was 7, collected by centrifuge, and then dried at 80 ◦ C for 24 h. The detail of sample preparation has been reported in the previous work [22]. 2.2. Fabrication of TiO2 @CNTs Structure by ALD The sample of a-CNTs was used as a support for coating with TiO2 by ALD. Firstly, 5 mg of CNTs were sonicated in 15 mL N-Methyl-2-pyrrolidone for 60 min to get good dispersion on a Si wafer. The dispersed CNTs were then deposited with TiO2 at 300 ◦ C by ALD using TiCl4 and H2 O as the precursors. The temperature of precursors was maintained in room temperature. Each cycle consisted of a precursor pulse for 0.08 s and a purge with N2 for 7 s. The number of precursor/purge cycles was from 25 to 200 cycles of ALD to achieve precise control of the particles size of TiO2 . 2.3. Fabrication of Pt@TiO2 @CNTs Composites by PEALD Pt particles were then deposited on the as-prepared TiO2 @CNTs composites by PEALD using (methylcyclopentadienyl) trimethylplatinum (MeCpPtMe3 ) and oxygen plasma as precursors [23]. The sequence of Pt-N2 -O2 plasma-N2 was 0.5-5-3-5 s, respectively. The power of induced couple plasma was 400 W and substrate temperature was 300 ◦ C. The temperature of Pt precursor is heated to 80 ◦ C for enough vapor pressure. The cycles of PEALD were from 10 to 100 to meet precisely controllable size and loading of Pt particles. 2.4. Photocatalytic Activity and Characterization The surface chemistry was analyzed by Fourier-Transform Infrared Spectrometer (FTIR, Bruker, Vertex 80v and Tensor 27, Billerica, Massachusetts, USA). The TiO2 @CNTs and Pt@TiO2 @CNTs

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composites prepared by using a-CNTs as the template were examined by transmission electron microscopy (TEM, JEOL 2100F, Akishima, Tokyo, Japan). The structural analysis was performed by X-ray diffraction (XRD, Shimadzu 6000, Nakagyo-ku, Kyoto, Japan) with the Cu Kα radiation. The optical properties were obtained by photoluminescence spectroscopy (PL, Horiba Jobin Yvon, Labram HR 800, Minami-Ku, Kyoto, Japan) with excitation wavelength of 325 nm (He-Cd laser, Kimon IK3301R-G Itabashi-Ku, Kyoto, Japan), laser power of 30 mW, and a spot size of 0.79 µm2 . The photocatalytic activities of all the samples were examined by studying the MB degradation with 200 W Hg lamp as irradiation source. The initial concentration of MB is 6.26 × 10−5 M. The distance between photoreactor and lamp was 10 cm. 2 mg of prepared samples were added to the MB solution and P25 was tested as control group. The absorption of MB solution at 664 nm was measured by a UV–Visible spectrometer (Hitachi U-3010, Chiyoda-ku, Tokyo, Japan) for every 10 min illumination until 60 min. The total organic carbon (TOC) concentration was determined with a TOC analyzer (Shimadzu TOC-5000, Nakagyo-ku, Kyoto, Japan). Photoelectrochemical cell (PEC) measurement with 1 M KOH as electrolyte was performed in a three electrode electrochemical cell. Ag/AgCl electrode and Pt foil were used as the reference electrode and counter electrode, respectively. All the samples prepared for PEC measurement were dispersed in polyethylene glycol to form slurry and subsequently printed onto fluorine-doped tin oxide (FTO) by doctor-blade technique as working electrodes [24]. The procedure was repeated to obtain the film thickness of 3 µm that was determined by alpha-step profilometer (Veeco Tektak 150, Plainview, New York, NY, USA). The photocurrent was measured by a potentiostat (Solartron 1286, Bognor Regis, West Sussex, UK) by illumination with a 200 W Hg lamp at a sweeping rate of 10 mV·s−1 from −1.0 to 0.4 V. The active area was 1 cm × 1 cm. 3. Results and Discussion 3.1. Modification of Multiwall CNTs Figure 1 shows the FTIR spectra of p-CNTs and a-CNTs. The FTIR peaks are identified for different types of organic functional groups. The small peak at 1040 cm−1 can be related to the C–O stretching vibration of either alcohol or carboxyl groups [25]. The peak at 1280 cm−1 is attributed to the nitrates NO2 symmetric stretching bands [26]. The peak at around 1400 cm−1 is possibly associated with O–H bending deformation of carboxyl groups and the C=C stretching bands placed by the functional groups. The absorption peaks at around 1700 cm−1 is ascribed to the C=O stretching vibration of carboxyl groups [26]. A clear broad peak at around 3400 cm−1 can be referred to O–H stretching. The outcome of FTIR analysis suggests that oxidation has conducted by the acid treatment and demonstrates that there is significant difference between p-CNTs and a-CNTs. Figure 2 shows the morphology of TiO2 deposited both p-CNTs and a-CNTs by 50 ALD cycles at 300 ◦ C. There was more TiO2 particles deposited on the surface of a-CNTs than that on p-CNTs. The coverage of TiO2 on a-CNTs was more complete because of higher oxygen-containing functional on the surface of a-CNTs. This suggests that the functional groups played an important role in the density of TiO2 particles deposited on the CNTs, resulting in that the TiO2 particles coated on a-CNTs were more uniform and compact than that on p-CNTs. This implies the nucleation sites of TiO2 on a-CNTs were higher than that on p-CNTs after acid treatment. However, the particle size of TiO2 deposited on both p-CNTs and a-CNTs was not obviously different to each other. In order to increasing the amount of TiO2 particles loaded on the CNTs for higher efficiency of MB photodegradation, the acid-treated CNTs for 6 h was choose to be the support as following samples prepared by ALD to form TiO2 @CNTs composites.

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Figure The abbreviations of a-CNTs Figure 1. 1. FTIR FTIR spectra spectra of of pristine pristine and and acid-treated acid-treated CNTs. CNTs. The The abbreviations abbreviations of of a-CNTs and and p-CNTs p-CNTs Figure 1. correspond to acid treatment of CNTs for 6 h by nitric acid and pristine CNTs, respectively. correspond to acid treatment of CNTs for 6 h by nitric acid and pristine CNTs, respectively. correspond to acid treatment of CNTs for 6 h by nitric acid and pristine CNTs, respectively.

Figure 2. 2. TEM TEM morphologies morphologies of of TiO TiO22 deposited deposited on on (a) (a) a-CNTs a-CNTs and and (b) (b) p-CNTs p-CNTs by by 50 50 cycles cycles of of ALD ALD at at Figure 2. Figure TEM morphologies of TiO 2 deposited on (a) a-CNTs and (b) p-CNTs by 50 cycles of ALD at ◦°C. 300 °C. 300 300 C.

3.2. Fabrication TiO22@CNTs Structures 3.2. Fabrication and and Characterization Characterization of of TiO 2 @CNTs Structures The difference of of TiO TiO22 particle size grown by various ALD cycles at 300 ◦°C was observed by The difference 2 particle size grown by various ALD cycles at 300 C was observed by TEM, as shown in Figure 3. It is worthy to note note that that the the particle particle size size distribution distribution of of TiO TiO22 fabricated fabricated TEM, as shown in Figure 3. It is worthy to 2 by variation of ALD cycles was uniform and close to Gaussian distribution. The deviation of average by variation of ALD cycles was uniform and close to Gaussian distribution. The deviation of average size of 8%. This shows thatthat the TiO particles could be precisely and uniformly coated size of TiO TiO222isisless lessthan than 8%. This shows the22TiO 2 particles could be precisely and uniformly on the surface of a-CNTs by ALD to synthesize complete TiO heterojunction structures. By coated on the surface of a-CNTs by ALD to synthesize complete22@CNTs TiO2 @CNTs heterojunction structures. counting more than one hundred particles from TiO22 By counting more than one hundred particles fromthe theTEM TEMimages, images,the the average average diameters diameters of of TiO 2 particles are from 3 to 13 nm and plotted in Figure 4. The particle size increases as the cycle number particles are from 3 to 13 nm and plotted in Figure 4. The particle size increases as the cycle number increases, and and the the relation relation can can be be divided divided into into two two regions regions from from Figure Figure 4. 4. Based Based on on the the generally generally increases, accepted mechanism of of ALD ALD using using TiCl TiCl44 and and H H22O as precursors [27], the process can be separated to accepted mechanism 4 2 O as precursors [27], the process can be separated to two half reactions for a typical self-terminating process in ALD. ALD. In In the the very very beginning, beginning, TiCl TiCl44 reacts reacts two half reactions for a typical self-terminating process in 4 with the surface OH groups and then releases HCl in the first-half reaction: with the surface OH groups and then releases HCl in the first-half reaction: (s) + TiCl4(g) 4(g) → (-O-)nTiCl4-n(s) 4-n(s) + nHCl(g) (g) n(-OH)(s) n(-OH) (s) + TiCl4(g) → (-O-)nTiCl4-n(s) + nHCl(g)

(1) (1) In which (s) presents the surface. The nonstoichiometric titanium chloride (TiClxx) species adsorbed on the surface may react with water and then release HCl in the second-half reaction, which results in the recovery of the surface OH groups again for the next cycle:

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(-O-)nTiCl4-n(s) + (4−n)H2O(g) → (-O-)nTi(OH)4−n(s) + (4−n)HCl(g)

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(2)

In which (s) presents the surface. The nonstoichiometric titanium chloride (TiClx ) species adsorbed on the surface may react with water and then release HCl in the second-half reaction, which results in the recovery of the surface OH groups again for the next cycle: Nanomaterials 2017, 7, 97 5 of 16 (-O-)nTiCl4-n(s) + (4−n)H2 O(g) → (-O-)nTi(OH)−4 − n(s) + (4−n)HCl(g) (-O-)nTiCl4-n(s) + (4−n)H2O(g) → (-O-)nTi(OH)4 n(s) + (4−n)HCl(g)

(2)

(2)

Figure 3. TEM images of TiO2 deposited on a-CNTs with (a) 25; (b) 50; (c) 100; and (d) 200 cycles of ALD at 300 °C. The corresponding figures (e–h)with on (a) the right hand side are Figure 3. TEM images of TiO 2 deposited a-CNTs with (a)25; 25; (b)50; 50; (c)100; 100; and (d)the 200distribution cycles of Figure 3. TEM images of TiO onon a-CNTs (b) (c) and (d) 200 cycles of ALDof TiO2 2 deposited ◦ C. at 2 ALD 300corresponding °C. The corresponding figures on thehand rightside handare side the distribution of TiO particles. at 300 The figures (e–h) on(e–h) the right theare distribution of TiO particles. 2

1616

Particle size of TiO2(nm)

Particle size of TiO2(nm)

particles.

1212 8

8

4

4 0

0

0

0

40

80 120 160 Number of ALD cycles

40

200

80 120 160 Number of ALD cycles

200

Figure 4. Variation of TiO2 particle size on a-CNTs with ALD cycle number.

For TiO2Figure @CNTs4.composites, once the TiClx size species has formed the cycle surface of CNTs, the H2O Variation ofof TiO on on a-CNTs withon ALD number. 2 particle Figure 4. Variation TiO 2 particle size a-CNTs with ALD cycle number. molecules then reacted with TiClx to form TiO2 and subsequently left the OH functional groups again

For TiO2@CNTs composites, once the TiClx species has formed on the surface of CNTs, the H2O molecules then reacted with TiClx to form TiO2 and subsequently left the OH functional groups again

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in the surface. The surface functional groups on a-CNTs would affect the growth rate of TiO2 particles For the TiOreaction composites, once has formed on the surface of 4CNTs, the Hand x speciesamong 2 @CNTswas 2O because determined bythe theTiCl interaction surfaces of CNTs, TiCl molecules molecules then reacted with TiCl to form TiO and subsequently left the OH functional groups again x 2 growth process with a higher growth rate when the H2O molecules. There was a substrate-enhanced in the surface. The surface functional groups on a-CNTs would a-CNTs affect thewith growth of TiO2 particles ALD cycles were less than 50 [28]. In this region, the surface fullyrate oxygen-containing because the reaction was determined by the interaction among surfaces of CNTs, TiCl 4 molecules functional groups enhanced the TiO2 particles to grow. However, that particles were probablyand too H O molecules. There was a substrate-enhanced growth process with a higher growth rate when the 2 tiny to be observed when the cycle number was less than 25. After 50 cycles of ALD, TiO2 was ALD cyclesonwere less thanparticles, 50 [28]. In this resulted region, the with fully deposited the existing which in asurface linearlya-CNTs lower growth rateoxygen-containing in the subsequent functional groups enhanced the TiO particles to grow. However, that particles were toototiny cycles due to consuming out of the 2initial functional groups. The growth rates wereprobably calculated be to be observed when the cycle number was less than 25. After 50 cycles of ALD, TiO was deposited 2 1.5 Å/cycle and 0.4 Å/cycle for substrate-enhanced growth process and linear growth process, on the existing particles, which resulted in a linearly lower growth rate in the subsequent cycles due to respectively. consuming out of the initial functional groups. The growth rates were calculated to be 1.5 Å/cycle and The XRD in Figure 5 growth show the phaseand of TiO 2 deposited on a-CNTs by variable ALD 0.4 Å/cycle for patterns substrate-enhanced process linear growth process, respectively. cycles. TiO 2 particles with 5different on a-CNTs exhibited aALD crystalline TheThe XRD patterns in Figure show theALD phasecycles of TiOdeposited deposited on a-CNTs by variable cycles. 2 anatase Thewith intensity of anatase phase deposited increases as ALD exhibited cycles increase from 25anatase to 200. The TiO2phase. particles different ALD cycles onthe a-CNTs a crystalline Rutile The phase was not observed in the TiO2 particles coated a-CNTfrom at 300 the phase. intensity of anatase phase increases as the ALD cycleson increase 25 to°C. 200.However, Rutile phase ◦ C. crystallinity of TiO hardly observed when the number of cycle was less than 25. According to2 was not observed in2 was the TiO particles coated on a-CNT at 300 However, the crystallinity of TiO 2 Zhang’s report [29], the nucleation of TiO 2 ALD was about 20 cycles. The size of TiO 2 nuclei was less was hardly observed when the number of cycle was less than 25. According to Zhang’s report [29], thannucleation 2 nm when 12020°C and 200 Inofthe meanwhile, theless crystalline particles the of temperatures TiO2 ALD waswere about cycles. The°C. size TiO than 2 nm when 2 nuclei was were also consisted of◦ C amorphous and crystallinethe core. The ratio of amorphous and temperatures were 120 and 200 ◦ C.shell In the meanwhile, crystalline particles were also shell consisted crystalline coreshell decreased as ALD cycles decreased The TiOshell 2 particles less than core 25 ALD cycles of amorphous and crystalline core. The ratio of [29]. amorphous and crystalline decreased were probably too weak to be The observed in XRD patterns to thecycles initialwere nucleation of too TiOweak 2. It was as ALD cycles decreased [29]. TiO2 particles less thandue 25 ALD probably to in agreement with the TEM analysis, as shown in Figure 3a. Although TiO 2 deposited on a-CNTs at be observed in XRD patterns due to the initial nucleation of TiO2 . It was in agreement with the TEM ◦ 300 °C was uniform from 50 200 cycles, TiO2 nanoparticles around nmuniform with higher analysis, as shown in Figure 3a.toAlthough TiOthe on a-CNTs at 300 C10was from 2 deposited photocatalytic been demonstrated Based on that, the TiO2 particles fabricated by 50 to 200 cycles,property the TiO2have nanoparticles around 10 [30]. nm with higher photocatalytic property have been 100 ALD cycles with 9.56 nm were chosen for the following characterization and MB demonstrated [30]. Based on that, the TiO2 particles fabricated by 100 ALD cycles with 9.56 nm were photodegradation. chosen for the following characterization and MB photodegradation.

◦ C. Figure composites with Figure 5. 5. XRD XRD patterns patterns of of TiO TiO22@CNT @CNT composites with various various ALD ALD cycles cycles at at 300 300 °C.

3.3. @CNTs Structures Structures 3.3. Fabrication Fabrication and and Characterization Characterization of of Pt@TiO Pt@TiO2@CNTs After asas co-catalyst were then deposited on After the the TiO TiO22@CNTs @CNTsstructure structurewas wasfabricated, fabricated,PtPtparticles particles co-catalyst were then deposited ◦ C with 25, 50, and 100 cycles by PEALD using MeCpPtMe and oxygen plasma the TiO surface at 300 2 3 on the TiO2 surface at 300 °C with 25, 50, and 100 cycles by PEALD using MeCpPtMe3 and oxygen as precursors. When the lengths of pulseoftime fortime the sequence MeCpPtMe plasma-N 3 -N2 -O32-N 2 were2 plasma as precursors. When the lengths pulse for the sequence MeCpPtMe 2-O2 plasma-N 0.5-5-3-5 s, the saturated deposition rate of Pt rate on TiO @CNTs was 0.39 Å/cycle. TheÅ/cycle. size distributions were 0.5-5-3-5 s, the saturated deposition of 2Pt on TiO 2@CNTs was 0.39 The size and Pt particles by calculating more than one hundred particles from the TEM images are showed in distributions and Pt particles by calculating more than one hundred particles from the TEM images Figure 6. The sizes of Pt particle with 25, 50, and 100 PEALD cycles are 1.47 ± 0.25 nm, 2.41 ± 0.27 nm, are showed in Figure 6. The sizes of Pt particle with 25, 50, and 100 PEALD cycles are 1.47 ± 0.25 nm, and 0.37and nm,3.90 respectively. particle size Pt related cycleofwas shown 2.41 3.90 ± 0.27±nm, ± 0.37 nm, The respectively. Theof particle size to of number Pt relatedoftoALD number ALD cycle

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in Figure 7. From Figure 7, the particle size increases with the cycle number, and the relation only has one slope. In comparison to TiO2 fabricated by ALD using TiCl4 and H2 O as precursors, there is no substrate-enhanced growth process with a higher growth rate before having consumed the initial functional groups [28]. Therefore, the growth rate of Pt particles on the TiO2 surface is the same as that on Pt particles deposited earlier, which results the slope derived from particle sizes of Pt divided to cycle number of ALD is linear. This implies the growth behavior of Pt in PEALD between the interface of TiO2 and Pt precursors is more inert than that of TiO2 in ALD between the interface of a-CNTs and TiO2 precursors. Generally speaking, the precursors of ALD reacting to surface of substrate can be divided to two steps: one is physisorption and the other is chemsorption. The adsorption can be considered reversible in physisorption, as illustrated in reactions (3) and (4), r

a k ∗ + M( g) →k ∗ M( g)

r

d k ∗ M( g) →k ∗ + M( g)

(3) (4)

where k ∗ is the active surface of substrate for adsorption of precursor molecules. Adsorption rate, ra , refers to the amount of molecules M attached to the surface per unit time. Desorption rate, rd , denotes to the amount of molecules M detached from the surface per unit time. For physisorption, the partial pressure of the reactant is a key parameter to describe the adsorption behavior with three assumptions [28], the monolayer is achieved by the maximum amount of adsorbed molecules, all adsorption sites on the surface are assumed equal, and adjacent molecules adsorbed on the surface are assumed not to interact with each other. Once the physisorption is achieved, the precursor would react with functional groups, dangling bond, and defect on the surface of substrate as chemical bonding so called chemsorption. Therefore, the coverage of adsorbed species denoted as Q describing the adsorption process, as following: dQ = r a − r d = k a p (1 − Q ) − k d Q dt

(5)

After saturation in the ALD process, the coverage is constant (dQ/dt = 0), chemsorption coverage Qeq is integrated by the condition of Langmuir isotherm to show the relationship between equilibrium constant of adsorption, K, as following: Qeq =

ka p 1 = k a p + kd 1 + (Kp)−1

(6)

Considering time effect into Equation (5), the chemisorption coverage Q is calculated as a function of time,   Q = Qeq 1 − e−(k a p−kd ) t (7) However, the three assumptions to derive the Equation (5) are not available in real process. For Pt in PEALD, the molecule of MeCpPtMe3 is larger than TiCl4 . Steric hindrance of the MeCpPtMe3 molecules is taken into account. The initially accessible ligands of the chemisorbed species on the surface block up the space for the neighbor reactants [31]. Therefore, the initial ka of MeCpPtMe3 in PEALD is lower than that of TiCl4 in ALD. That presumably explains the initial growth rate of Pt is the same as the following PEALD cycles. By contrast, the molecule of TiCl4 is smaller and easier to react to surface of a-CNTs without steric hindrance, which causes the initially higher growth rate due to higher adsorption rate. However, once the consumption of functional group on the surface of a-CNTs has been achieved, the TiCl4 molecule would deposit on the residual TiO2 particle with lower growth rate because molecules of TiCl4 reacted to the existed TiO2 particles rather than surface of a-CNTs.

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Figure 6. TEM micrographs of Pt particles prepared with (a) 25 cycles; (b) 50 cycles; and (c) 100 cycles Figure 6. TEM micrographs of Pt Pt particles particles prepared with (a)right 25 cycles; cycles; (b) 50 cycles; cycles; and (c) 100 100 cycles cycles of PEALD. corresponding figures prepared of (d–f) on the hand(b) side are theand distribution of Pt Figure 6. TEMThe micrographs of with (a) 25 50 (c) of PEALD. The corresponding figures of (d–f) on the right hand side are the distribution of Pt of particles. PEALD. The corresponding figures of (d–f) on the right hand side are the distribution of Pt particles. particles.

6

Particle size of Pt (nm) Particle size of Pt (nm)

6 4

4 2

2 0

0

0 0

50 100 50 100 Number of PEALD cycle

Number of PEALD cycle Figure 7. 7. Variation of of PtPt particle size onon TiO with PEALD cycle number. 2 @CNTs Figure Variation particle size TiO 2@CNTs with PEALD cycle number. Figure 7. Variation of Pt particle size on TiO2@CNTs with PEALD cycle number.

3.4. Photocatalytic Efficiency of of Pt@TiO Structure 2 @CNTs 3.4. Photocatalytic Efficiency Pt@TiO 2@CNTs Structure 3.4. Photocatalytic Efficiency of Pt@TiO2@CNTs Structure The adsorption-desorption equilibrium of MBofbyMB the samples been reached before irradiation. The adsorption-desorption equilibrium by the has samples has been reached before The adsorption capacities of dye molecules for all TiO @CNTs, Pt@TiO @CNTs samples are almost The adsorption-desorption equilibrium of MB by the samples has been reached before 2 2 irradiation. The adsorption capacities of dye molecules for all TiO2@CNTs, Pt@TiO2@CNTs samples the same. This that capacities the adsorption was mainly affected byPt@TiO the CNTs substrate. irradiation. The adsorption of dye molecules for all TiOwas 2@CNTs, 2@CNTs samples are almost theimplies same. This implies that thecapacity adsorption capacity mainly affected byasthe CNTs as Figure 8 displays the photocatalytic activities of the TiO @CNTs of 9.56 nm with 100 ALD cycles are almost the same. This implies that the adsorption capacity was mainly affected by the CNTs as substrate. Figure 8 displays the photocatalytic activities 2of the TiO2@CNTs of 9.56 nm with 100 ALD ◦ C (Ti@C) and Pt@TiO @CNTs with the particle size of 1.47 ± 0.25 nm, 2.41 ± 0.27 nm, at 300 substrate. displays thePt@TiO activities of the TiO 2@CNTs ALD 2 photocatalytic cycles atFigure 300 °C8(Ti@C) and 2@CNTs with the particle size of 1.47of± 9.56 0.25 nm nm,with 2.41 100 ± 0.27 nm, ◦ C, respectively (Pt25@Ti@C, and 3.90 ± 0.37 nm fabricated by 25, 50, and 100 PEALD cycles at 300 cycles at 300 °C (Ti@C) and Pt@TiO 2 @CNTs with the particle size of 1.47 ± 0.25 nm, 2.41 ± 0.27 nm, and 3.90 ± 0.37 nm fabricated by 25, 50, and 100 PEALD cycles at 300 °C, respectively (Pt25@Ti@C, and 3.90 ± 0.37 nm fabricated by 25, 50, and 100 PEALD cycles at 300 °C, respectively (Pt25@Ti@C,

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Pt50@Ti@C, Pt100@Ti@C). Pt-loaded with 50 PEALD cycles (Pt50@P25) were Pt50@Ti@C, andand Pt100@Ti@C). P25P25 andand Pt-loaded P25P25 with 50 PEALD cycles (Pt50@P25) were alsoalso tested for control specimens. The efficiency of MB degradation can be fitted by a pseudo first-order tested for control specimens. The efficiency of MB degradation can be fitted by a pseudo first-order kinetics with different particle Pt [32], as follows: kinetics with different particle sizesize of Ptof[32], as follows:

=oCexp C =C C (−(kt− ) kt) o exp

(8)(8)

the initial concentrationofofMB, MB,and andk k is where is the concentration of MB after reaction time t, Ciso is where C isCthe concentration of MB after reaction time t, C the initial concentration o the rate constant. According to the photodecomposition result, the k values for each TiO 2@CNTs and is the rate constant. According to the photodecomposition result, the k values for each TiO2 @CNTs 2@CNTs samples were calculated from the slopes, as shown in Figure 8. A blank test reveals andPt@TiO Pt@TiO 2 @CNTs samples were calculated from the slopes, as shown in Figure 8. A blank test that the is slowly degraded by by thethe irradiation after 6060 min. reveals that MB the MB is slowly degraded irradiation after min.The Thekinetic kineticrate rateconstant constantkkfor for the −1. The TiO2@CNTs composite and P25 have almost the same efficiency for MB blank test is 0.0016 min the blank test is 0.0016 min−1 . The TiO2 @CNTs composite and P25 have almost the same efficiency withwith the the k values 0.0060 min min respectively. The Thek value k value −−1 1 ,, respectively. for degradation MB degradation k valuesbeing being0.0063 0.0063 and and 0.0060 of of −1. As the particle size of Pt@TiO 2@CNTs with Pt particles of 1.47 ± 0.25 nm by 25 cycles is 0.0087 min − 1 Pt@TiO2 @CNTs with Pt particles of 1.47 ± 0.25 nm by 25 cycles is 0.0087 min . As the particle size of Pt increases to 2.41 ± 0.27 by cycles, 50 cycles, photodegradation efficiency substantially increases Pt increases to 2.41 ± 0.27 nmnm by 50 the the photodegradation efficiency substantially increases −1. That would presumably is due to a large amount of Pt for with rate constant raising to 0.0158 min − 1 with rate constant raising to 0.0158 min . That would presumably is due to a large amount of Pt for MB MB decomposition. decomposition. However, However, as as the the particle particle size size of of Pt Pt increases increases to to 3.90 3.90 ±±0.37 0.37nm nmby by100 100cycles, cycles,the −1 0.010 min min−1. .To the samples samples of of Pt@TiO Pt@TiO2@CNTs @CNTs with with different different Pt the rate rate constant constant reduces reduces to to 0.010 To sum sum up, up, the 2 loading have a higher decomposition rate of MB than those without Pt loading as co-catalyst. Pt loading have a higher decomposition rate of MB than those without Pt loading as co-catalyst. Pt Pt particles provide more effective reaction reductive reaction cause formation of Schottky particles provide more effective reaction sitessites for for reductive reaction andand cause formation of Schottky barrier between interface andTiO TiO. 2.Once Oncethe the interfacial interfacial charge charge transfer barrier between thethe interface of of PtPt and transfer was was facilitated, facilitated,the 2 recombination rate of electrons and holes would become lower and then result in improving the recombination rate of electrons and holes would become lower and then result in improving separation of electrons holes Pt@TiO 2@CNTs with Pt deposited by 50 PEALD cycles separation of electrons andand holes [33].[33]. TheThe Pt@TiO 2 @CNTs with Pt deposited by 50 PEALD cycles shows the higher k value than that of P25 with the same PEALD cycles of Pt. synergistic effect shows the higher k value than that of P25 with the same PEALD cycles of Pt. TheThe synergistic effect of of Pt loaded on TiO 2@CNTs is more obvious than that of P25. Pt loaded on TiO2 @CNTs is more obvious than that of P25. 0.0

ln C/Co

-0.3 MB blank, k= -0.0016 min-1 P25, k= 0.0060 min-1 Pt50@P25, k= 0.0118 min-1 Ti@C, k= 0.0063 min-1 Pt25@Ti@C, k= 0.0087 min-1 Pt50@Ti@C, k= 0.0158 min-1 Pt100@Ti@C, k= 0.0100 min-1

-0.6

-0.9 0

10

20

30

40

50

60

Time (min) Figure 8. Photodecomposition of MB for P25, Pt50@P25, TiO2 @CNTs (Ti@C), and Pt@TiO2 @CNTs with Figure 8. Photodecomposition of MB for P25, Pt50@P25, TiO2@CNTs (Ti@C), and Pt@TiO2@CNTs with different particle size of Pt (Pt25@Ti@C, Pt50@Ti@C, and Pt100@Ti@C). different particle size of Pt (Pt25@Ti@C, Pt50@Ti@C, and Pt100@Ti@C).

Figure 9 shows the the change of TOC for for the the photocatalytic degradation of MB during UVUV Figure 9 shows change of TOC photocatalytic degradation of MB during irradiation withwith all samples as photocatalysts, which is consistent withwith the tendency shown in Figure 8. irradiation all samples as photocatalysts, which is consistent the tendency shown in Figure The8.gradual decrease of TOC represented the gradual disappearance of organic carbon when the MB The gradual decrease of TOC represented the gradual disappearance of organic carbon when the solution which contained photocatalysts was exposed UV irradiation. k values The calculated MB solution which contained photocatalysts wasunder exposed under UV The irradiation. k values − 1 from TOC were 0.0060, 0.0118, 0.0063, 0.0087, 0.0158, or 0.0100 min with P25, Pt50@P25, Pt25@Ti@C, −1 calculated from TOC were 0.0060, 0.0118, 0.0063, 0.0087, 0.0158, or 0.0100 min with P25, Pt50@P25, Pt50@Ti@C, or Pt100@Ti@C after UV irradiation min. Pt25@Ti@C, Pt50@Ti@C, or Pt100@Ti@C after for UV60 irradiation for 60 min.

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ln TOC/TOCo

0.0

-0.3 MB blank, k= -0.0015 min-1 P25, k= 0.0057 min-1 Pt50@P25, k= 0.0119 min-1 Ti@C, k= 0.0066 min-1 Pt25@Ti@C, k= 0.0091 min-1 Pt50@Ti@C, k= 0.0162 min-1 Pt100@Ti@C, k= 0.0102 min-1

-0.6

-0.9 0

10

20

30

40

50

60

Time (min) Figure 9. thethe TOC during the photocatalytic degradation of MB for P25,for Pt50@P25, Figure 9. Disappearance Disappearanceofof TOC during the photocatalytic degradation of MB P25, Pt50@P25, TiO2 @CNTs (Ti@C), and Pt@TiO2 @CNTs with different particle size of Pt (Pt25@Ti@C, Pt50@Ti@C, TiO2@CNTs (Ti@C), and Pt@TiO2@CNTs with different particle size of Pt (Pt25@Ti@C, Pt50@Ti@C, and and Pt100@Ti@C). Pt100@Ti@C).

Interestingly, the reason why the efficiency of Pt@TiO2 @CNTs with Pt particles of 2.41 ± 0.27 nm

Interestingly, reason the efficiency Pt@TiO2@CNTs with Pt particles of 2.41 ± 0.27 nm is higher than that ofthe 3.90 ± 0.37why nm deserves furtherof investigation. As a whole, effective electron-hole isseparation higher than that of 3.90 ± 0.37 nm deserves further investigation. As a whole, effective is related to the loading, size, and distribution of Pt particles on TiO2 [34,35]. From theelectron-hole view separation is related to the loading, and distribution of Pt particles TiO2 [34,35]. From the of photocatalytic reaction, Pt particles of size, several nanometers can enhance activity ofon photodegradation. However, larger Pt particlesreaction, may provide recombination sites nanometers for photo-generated electrons and view of photocatalytic Pt more particles of several can enhance activity of holes [35,36]. In addition, not only surface reactive site on TiO but also the light absorption of TiO 2 2 photodegradation. However, larger Pt particles may provide more recombination sites are for photoreduced byelectrons the too high Pt holes loading [34]. Therefore, optimum size, loading, uniform of also the generated and [35,36]. In addition, not only surface and reactive sitedistribution on TiO2 but Pt particles on TiO are necessary to obtain effective electron-hole separation and to achieve a higher light absorption of2 TiO 2 are reduced by the too high Pt loading [34]. Therefore, optimum size, loading, photocatalytic activity. Pt particles with 0.2 nm different size could particularly increase the efficiency and uniform distribution of Pt particles on TiO2 are necessary to obtain effective electron-hole of photoreduction of CO2 to CH4 [37]. The conversion rate increases almost twice within only 0.2 nm separation and to achieve a higher photocatalytic activity. Pt particles with 0.2 nm different size could different of Pt particles. This implies that a 1–2 nm different particle size in this study may make a big particularly increase the efficiency of CO 2 to CH4 [37]. The conversion rate difference in the MB degradation. Basedof on photoreduction quantum confinement effect, decreased particle size of Pt increases almost twice within only 0.2 nm different of Pt particles. This implies that a 1–2 nm different has larger energy band gap [37]. Pt particles have higher energy band separation and prevent electron particle study may a big the difference in the MB degradation. Based on quantum transfer size from in TiOthis bandmake to Pt when particles are extremely small. However, electrons 2 conduction confinement effect, decreased particle size energy of Pt has band gap the [37].particle Pt particles have can easily transfer to Pt particles with a suitable levellarger lower energy than −4.4 eV when size of Pt increases asseparation shown in Figure 10. If the electron size becomes too larger, behavior of Pt particles higher energy [37], band and prevent transfer fromthe TiO 2 conduction band toisPt when more like bulk Pt due to energy level narrowing, which increases the recombination rate of electrons the particles are extremely small. However, electrons can easily transfer to Pt particles with a suitable and holes [36]. To sum up,−4.4 the eV Pt particles with 2.41 ±size 0.27 of nmPtreconcile the[37], quantum confinement energy level lower than when the particle increases as shown in Figure 10. If effect with suitabletoo energy band gap and avoid Pt like as recombination of the size becomes larger, the behavior ofthe Pt behavior particlesofisbulk more bulk Pt duecenter to energy level electrons and holes. This presumably explains that the sample with Pt particles of 2.41 ± 0.27 nm narrowing, which increases the recombination rate of electrons and holes [36]. To sum up, the Pt shows the highest efficiency for MB degradation. Table 1 summarizes the particle sizes of Pt for the particles with 2.41 ± 0.27 nm reconcile the quantum confinement effect with suitable energy band gap highest reaction rate of some applications from the literature. The optimum particle size of Pt in this and avoid the bulk Pt asofrecombination center electrons and This presumably study is 2.41 ± behavior 0.27 nm forofdegradation MB, compared to 1.9, 2,of1.75, 2, and 2.2 nmholes. for applications explains that the glycerol sample reforming, with Pt particles of 2.41 ± CO 0.27oxidation nm shows the highest efficiency of C2 H4 oxidation, water splitting, and by using different process for for MB degradation. Table 1 summarizes the an particle sizes of Ptsize forclose thetohighest reaction for rate deposition of Pt, respectively. The all show optimum particle 2 nm. Therefore, theof some applications from the literature. The optimum particle size of Pt in this study is 2.41 ± 0.27 Pt@TiO2 @CNTs structure with the size of 2.41 ± 0.27 nm of Pt for superior efficiency of MB degradation nm for is probably because its high area and 2, appropriate diffusion for electronsofand holes. degradation of MB,of compared to 1.9, 1.75, 2, and 2.2 nmlength for applications C2H 4 oxidation, glycerol Besides, it is well know the activity of photocatalyst can be demonstrated by the PEC analysis, of Pt, reforming, water splitting, and CO oxidation by using different process for deposition as shown in Figure In comparison to pure P25 and the Therefore, open circuitfor potential (Eocp )2@CNTs 2 @CNTs, respectively. The all11.show an optimum particle sizeTiO close to 2 nm. the Pt@TiO of the Pt@TiO @CNTs with different particle sizes of Pt are all − 951 mV (versus Ag/AgCl) structure with2 the size of 2.41 ± 0.27 nm of Pt for superior efficiency of MB degradationwith is probably negatively shifted 130 mV. The lower open circuit potential implies that Pt@TiO2 @CNTs has less because of its high area and appropriate diffusion length for electrons and holes. recombination sites than that of TiO2 @CNTs and P25, which results in higher photocurrent density and

Besides, it is well know the activity of photocatalyst can be demonstrated by the PEC analysis, as shown in Figure 11. In comparison to pure P25 and TiO2@CNTs, the open circuit potential (Eocp) of the Pt@TiO2@CNTs with different particle sizes of Pt are all −951 mV (versus Ag/AgCl) with negatively shifted 130 mV. The lower open circuit potential implies that Pt@TiO2@CNTs has less recombination sites than that of TiO2@CNTs and P25, which results in higher photocurrent density

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higher photodecomposition rate of MB [38]. The photocurrent of the Pt@TiO2 @CNTs with Pt particles of 2.41 ± 0.27 nm has higher photocurrent densities than those other samples. The result is consistent to the MB degradation and shows that the optimized particle size of Pt plays an important role for design of noble metal as co-catalyst to improve efficiency of photocatalyst. Table 1. Summary of particle sizes of Pt for the highest photodegradation rate.

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Size Distribution

Optimized Size (nm)

Preparation Method a

Test Reagent

Platinum Precursor

Ref.

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1.9

Hydrothermal

CO, C2 H4

PtCl4

[42]

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1.2–4.2 2 Impregnation Glycerol H2 PtCl6 ·6H2 O [43] particles of 2.41 ± 0.27 nm has 1.75 higher photocurrent densities those samples. 1.8–4.9 MW-solvothermal COthan H2other PtCl6 [44] The result is 2 2 and shows sputterthedensities EtOH/H Ptsize target [45] an 2O particles ofto2.41 ± 0.27 nm has higher photocurrent than those other samples. The result is consistent the1–15 MB degradation that optimized particle of Pt plays important 2.2–16.7 2.2 H2 reduction CO2 N.A [46] consistent to the degradation and showstoPEALD that the optimized particle size of an important 1.47–3.9 2.41co-catalyst MB MeCpPtMe This study role for design ofMB noble metal as improve efficiency of photocatalyst. 3 Pt plays a : MW: microwave assited List of figures. role for design of noble metal as co-catalyst to improve efficiency of photocatalyst.

Figure 10. Schematic diagram of the energy levels of Pt particles with different sizes and the energy Figure 10. Schematic diagram energy levels levels ofofPtPt particles withwith different sizes and the and energy Figure diagram of of thetheenergy particles different sizes the energy band of10. TiOSchematic 2 after irradiation. band of TiO2 after irradiation. band of TiO2 after irradiation.

Figure 11. Current-voltage characteristics of P25, TiO2 @CNTs (Ti@C), and Pt@TiO2 @CNTs with

Figure 11. Current-voltage characteristics of P25, TiO2@CNTs (Ti@C), and Pt@TiO2@CNTs with different particle size of Pt (Pt25@Ti@C, Pt50@Ti@C, and Pt100@Ti@C). Figure 11. Current-voltage characteristics of P25, and TiOPt100@Ti@C). 2@CNTs (Ti@C), and Pt@TiO2@CNTs with different particle size of Pt (Pt25@Ti@C, Pt50@Ti@C, different particle size of Pt (Pt25@Ti@C, Pt50@Ti@C, and Pt100@Ti@C).

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Furthermore, the PL spectra are useful to study the efficiency of trapping, immigration, and transfer of charge carriers for understanding the recombination of electrons and holes because recombination of free charge carries result in signal of PL emissions [39]. Figure 12 shows the PL spectra of TiO2 @CNTs and Pt@TiO2 @CNTs with various particle size of Pt are all located at around 505 nm. For TiO2 , the emission wavelength at around 505 nm is the charge transfer from Ti3+ to the oxygen anion in the TiO6 8− complex [39], and the recombination rate of electrons and holes is related to the intensity of the PL emission [40,41]. Therefore, the samples with the higher intensity of PL emission could be suggested to the existence of more trapping centers that enhance the recombination rate of electrons and holes. For Pt@TiO2 @CNTs with Pt particles of 2.41 ± 0.27 nm, the lowest intensity Figure 11. Current-voltage characteristics of P25, TiO2@CNTs (Ti@C), and Pt@TiO2@CNTs with proves that the proper particle size of Pt can suppresses the recombination of electrons and holes in different particletosize ofsamples. Pt (Pt25@Ti@C, Pt50@Ti@C, and Pt100@Ti@C). comparison other The result is well in line with the PEC analysis and MB degradation.

Figure PL spectra of 2TiO (Ti@C)and and Pt@TiO Pt@TiO2 @CNTs with different particle size of Pt 2 @CNTs Figure 12. PL 12. spectra of TiO @CNTs (Ti@C) 2@CNTs with different particle size of Pt (Pt25@Ti@C, Pt50@Ti@C, and Pt100@Ti@C). (Pt25@Ti@C, Pt50@Ti@C, and Pt100@Ti@C).

4. Conclusions The formation of TiO2 @CNTs and Pt@TiO2 @CNTs structures with various ALD and PEALD cycles was studied in detailed. It demonstrated that the CNTs with nitric acid treatment could generated oxygen-containing functional groups on the surface. The density of TiO2 particles deposited by ALD is related to the amount of oxygen-containing functional groups on the surface of CNTs. The saturated growth rates of TiO2 on a-CNTs were 1.5 Å/cycle and 0.4 Å/cycle, which result from substrate-enhanced process and linear process, respectively. Compared to TiO2 , the saturated growth rate of Pt on TiO2 @CNTs was linearly 0.39 Å/cycle due to the steric hindrance of Pt precursors. With increasing the ALD cycles, the as-deposited TiO2 particles all show anatase phase and increase the intensity of crystallinity at 300 ◦ C. The composite with Pt particles of 2.41 ± 0.27 nm has the highest efficiency MB degradation because it reconciles the quantum confinement effect with suitable energy band gap and avoid the behavior of bulk Pt as recombination center of electrons and holes. In comparison to TiO2 @CNTs, the samples with Pt loading show the relatively negative Eocp and higher photocurrent due to the Pt particles as co-catalyst. The result of PL spectra proved the lower electrons and holes recombination rate with optimum particle size of Pt. For Pt@TiO2 @CNTs of 2.41 ± 0.27 nm, the higher photocurrent and lowest intensity of PL emission all demonstrate lower recombination rate of electrons and holes in agreement with MB degradation. The key achievements of this study are the precise control of particle sizes of TiO2 and Pt and quantification of the photocatalytic efficiencies

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of TiO2 @CNTs coupled with Pt particles, which provide useful information for further design of Pt@TiO2 @CNTs structure for catalytic, sensor, and photochemical applications. Acknowledgments: This work was supported by the Ministry of Science and Technology of Taiwan under the Contract No. MOST 105-2221-E-007-110. Author Contributions: Shih-Yun Liao, Ya-Chu Yang, Sheng-Hsin Huang and Jon-Yiew Gan designed the project. Shih-Yun Liao and Ya-Chu Yang performed the experiments and analyzed the data; Sheng-Hsin Huang and Jon-Yiew Gan assisted in data processing and the manuscript preparation; and all authors contributed to the manuscript. Conflicts of Interest: The authors declare no competing financial interest.

References 1. 2.

3. 4. 5. 6. 7. 8. 9. 10. 11.

12.

13. 14.

15. 16. 17.

18.

Fujishima, A.; Honda, K. Electrochemical photolysis of water at a semiconductor electrode. Nature 1972, 238, 37–38. [CrossRef] [PubMed] Serpone, N.; Lawless, D.; Khairutdinov, R.; Pelizzetti, E. Subnanosecond relaxation dynamics in TiO2 colloidal sols (particle sizes RP = 1.0-13.4 nm). Relevance to heterogeneous photocatalysis. J. Phys. Chem. 1995, 99, 16655–16661. [CrossRef] Wang, C.C.; Zhang, Z.B.; Ying, J.Y. Photocatalytic decomposition of halogenated organics over nanocrystalline titania. Nanostruct. Mater. 1997, 9, 583–586. [CrossRef] Zhang, Z.B.; Wang, C.C.; Zakaria, R.; Ying, J.Y. Role of particle size in nanocrystalline TiO2 -based photocatalysts. J. Phys. Chem. B 1998, 102, 10871–10878. [CrossRef] Parra, S.; Stanca, S.E.; Guasaquillo, I.; Thampi, K.R. Photocatalytic degradation of atrazine using suspended and supported TiO2 . Appl. Catal. B Environ. 2004, 51, 107–116. [CrossRef] Zhu, H.Y.; Gao, X.P.; Lan, Y.; Song, D.Y.; Xi, Y.X.; Zhao, J.C. Photocatalytic degradation of atrazine using suspended and supported TiO2 . J. Am. Chem. Soc. 2004, 126, 8380–8381. [CrossRef] [PubMed] Zhang, H.Z.; Banfield, J.F. Understanding polymorphic phase transformation behavior during growth of nanocrystalline aggregates: Insights from TiO2 . J. Phys. Chem. B 2000, 104, 3481–3487. [CrossRef] Britto, P.J.; Santhanam, K.S.V.; Rubio, A.; Alonso, J.A.; Ajayan, P.M. Improved charge transfer at carbon nanotube electrodes. Adv. Mater. 1999, 11, 154–157. [CrossRef] Che, J.W.; Cagin, T.; Goddard, W.A. Thermal conductivity of carbon nanotubes. Nanotechnology 2000, 11, 65–69. [CrossRef] Collins, P.C.; Arnold, M.S.; Avouris, P. Engineering carbon nanotubes and nanotube circuits using electrical breakdown. Science 2001, 292, 706–709. [CrossRef] [PubMed] Orlanducci, S.; Sessa, V.; Terranova, M.L.; Battiston, G.A.; Battiston, S.; Gerbasi, R. Nanocrystalline TiO2 on single walled carbon nanotube arrays: Towards the assembly of organized C/TiO2 nanosystems. Carbon 2006, 44, 2839–2843. [CrossRef] Li, C.; Yuan, J.; Han, B.; Jiang, L.; Shangguan, W. TiO2 nanotubes incorporated with CdS for photocatalytic hydrogen production from splitting water under visible light irradiation. Int. J. Hydrog. Energy 2010, 35, 7073–7079. [CrossRef] Li, F.B.; Li, X.Z. The enhancement of photodegradation efficiency using Pt-TiO2 catalyst. Chemosphere 2002, 48, 1103–1111. [CrossRef] Bamwenda, G.R.; Tsubota, S.; Nakamura, T.; Haruta, M. Photoassisted hydrogen-production from a water-ethanol solution: A compasison activities of Au-TiO2 and Pt-TiO2 . J. Photochem. Photobiol. A 1995, 89, 177–189. [CrossRef] Gomes, H.T.; Machado, B.F.; Silva, A.M.T.; Draži´c, G.; Faria, J.L. Photodeposition of Pt nanoparticles on TiO2–carbon xerogel composites. Mater. Lett. 2011, 65, 966–969. [CrossRef] Chen, M.-L.; Zhang, F.-J.; Oh, W.-C. Preparation and Catalytic Properties of Pt/CNT/TiO2 Composite. J. Korean Ceram. Soc. 2010, 47, 269–275. [CrossRef] Bedolla-Valdez, Z.I.; Verde-Gómez, Y.; Valenzuela-Muñiz, A.M.; Gochi-Ponce, Y.; Oropeza-Guzmán, M.T.; Berhault, G.; Alonso-Núñez, G. Sonochemical synthesis and characterization of Pt/CNT, Pt/TiO2, and Pt/CNT/TiO2 electrocatalysts for methanol electro-oxidation. Electrochim. Acta 2015, 186, 76–84. [CrossRef] Marichy, C.; Pinna, N. Carbon-nanostructures coated/decorated by atomic layer deposition: Growth and applications. Coordin. Chem. Rev. 2013, 257, 3232–3253. [CrossRef]

Nanomaterials 2017, 7, 97

19.

20.

21. 22.

23.

24. 25. 26. 27. 28. 29.

30. 31. 32.

33.

34.

35. 36.

37.

38. 39. 40.

14 of 15

Liu, C.; Wang, C.C.; Kei, C.C.; Hsueh, Y.C.; Perng, T.P. Atomic layer deposition of platinum nanoparticles on carbon nanotubes for application in proton-exchange membrane fuel cells. Small 2009, 5, 1535–1538. [CrossRef] [PubMed] Hsueh, Y.C.; Wang, C.C.; Kei, C.C.; Lin, Y.H.; Liu, C.; Perng, T.P. Fabrication of catalyst by atomic layer deposition for high specific power density proton exchange membrane fuel cells. J. Catal. 2012, 294, 63–68. [CrossRef] Hsueh, Y.C.; Wang, C.C.; Liu, C.; Kei, C.C.; Perng, T.P. Deposition of platinum on oxygen plasma treated carbon nanotubes by atomic layer deposition. Nanotechnology 2012, 23, 405603. [CrossRef] [PubMed] Huang, S.H.; Wang, C.C.; Liao, S.Y.; Gan, J.Y.; Perng, T.P. CNT/TiO2 core-shell structures prepared by atomic layer deposition and characterization of their photocatalytic properties. Thin Solid Films 2015, 616, 151–159. [CrossRef] Zhang, J.K.; Chen, C.Q.; Chen, S.; Hu, Q.M.; Gao, Z.; Li, Y.Q.; Qin, Y. Highly dispersed Pt nanoparticles supported on carbon nanotubes produced by atomic layer deposition for hydrogen generation from hydrolysis of ammonia borane. Catal. Sci. Technol. 2017, 7, 322–329. [CrossRef] Lin, C.J.; Yu, Y.H.; Liou, Y.H. Free-standing TiO2 nanotube array films sensitized with CdS as highly active solar light-driven photocatalysts. Appl. Catal. B Environ. 2009, 93, 119–125. [CrossRef] El-Hendawy, A.N.A. Influence of HNO3 oxidation on the structure and adsorptive properties of corncob-based activated carbon. Carbon 2003, 41, 713–722. [CrossRef] Pradhan, B.K.; Sandle, N.K. Effect of different oxidizing agent treatments on the surface properties of activated carbons. Carbon 1999, 37, 1323–1332. [CrossRef] Aarik, J.; Aidla, A.; Mandar, H.; Uustare, T. Atomic layer deposition of titanium dioxide from TiCl4 and H2 O: Investigation of growth mechanism. Appl. Surf. Sci. 2001, 172, 148–158. [CrossRef] Puurunen, R.L. Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process. J. Appl. Phys. 2005, 97, 121301. [CrossRef] Zhang, Y.C.; Guerra-Nunez, C.; Utke, I.; Michler, J.; Rossell, M.D.; Erni, R. Morphology and crystallinity control of ultrathin TiO2 layers deposited on carbon nanotubes by temperature-step atomic layer deposition. J. Phys. Chem. C 2015, 119, 3379–3387. [CrossRef] Koˇcí, K.; Obalová, L.; Matˇejová, L.; Plachá, D.; Lacný, Z.; Jirkovský, J.; Šolcová, O. Effect of TiO2 particle size on the photocatalytic reduction of CO2 . Appl. Catal. B Environ. 2009, 89, 494–502. [CrossRef] George, S.M. Atomic layer deposition: An overview. Chem. Rev. 2010, 110, 111–131. [CrossRef] [PubMed] Barka, N.; Qourzal, S.; Assabbane, A.; Nounah, A.; Ait-Ichou, Y. Factors influencing the photocatalytic degradation of Rhodamine B by TiO2 -coated non-woven paper. J. Photochem. Photobiol. A 2008, 195, 346–351. [CrossRef] Wang, C.C.; Hsueh, Y.C.; Su, C.Y.; Kei, C.C.; Perng, T.P. Deposition of uniform Pt nanoparticles with controllable size on TiO2 -based nanowires by atomic layer deposition and their photocatalytic properties. Nanotechnology 2015, 26, 254002. [CrossRef] [PubMed] Rosseler, O.; Shankar, M.V.; Du, M.K.-L.; Schmidlin, L.; Keller, N.; Keller, V. Solar light photocatalytic hydrogen production from water over Pt and Au/TiO2 (anatase/rutile) photocatalysts: Influence of noble metal and porogen promotion. J. Catal. 2010, 269, 179–190. [CrossRef] Kiwi, J.; Gratzel, M. Optimization of conditions for photochemical water cleavage-aqueous Pt/TiO2 (anatase) dispersions under ultraviolet-light. J. Phys. Chem. 1984, 88, 1302–1307. [CrossRef] Sadeghi, M.; Liu, W.; Zhang, T.G.; Stavropoulos, P.; Levy, B. Role of photoinduced charge carrier separation distance in heterogeneous photocatalysis: Oxidative degradation of CH3 OH vapor in contact with Pt/TiO2 and cofumed TiO2 -Fe2 O3 . J. Phys. Chem. 1996, 100, 19466–19474. [CrossRef] Wang, W.N.; An, W.J.; Ramalingam, B.; Mukherjee, S.; Niedzwiedzki, D.M.; Gangopadhyay, S.; Biswas, P. Size and structure matter: enhanced CO2 photoreduction efficiency by size-resolved ultrafine Pt nanoparticles on TiO2 single crystals. J. Am. Chem. Soc. 2012, 134, 11276–11281. [CrossRef] [PubMed] Wang, C.C.; Kei, C.C.; Perng, T.P. Fabrication of TiO2 nanotubes by atomic layer deposition and their photocatalytic and photoelectrochemical applications. Nanotechnology 2011, 22, 365702. [CrossRef] [PubMed] Li, X.Z.; Li, F.B. Study of Au/Au3+ -TiO2 photocatalysts toward visible photooxidation for water and wastewater treatment. Environ. Sci. Technol. 2001, 35, 2381–2387. [CrossRef] [PubMed] Li, X.Z.; Li, F.B.; Yang, C.L.; Ge, W.K. Photocatalytic activity of WOx-TiO2 under visible light irradiation. J. Photochem. Photobiol. A 2001, 141, 209–217. [CrossRef]

Nanomaterials 2017, 7, 97

41. 42.

43.

44.

45.

46.

15 of 15

Yu, J.C.; Yu, J.G.; Ho, W.K.; Jiang, Z.T.; Zhang, L.Z. Effects of F-doping on the photocatalytic activity and microstructures of nanocrystalline TiO2 powders. Chem. Mater. 2002, 14, 3808–3816. [CrossRef] Isaifan, R.J.; Ntais, S.; Couillard, M.; Baranova, E.A. Size-dependent activity of Pt/yttria-stabilized zirconia catalyst for ethylene and carbon monoxide oxidation in oxygen-free gas environment. J. Catal. 2015, 324, 32–40. [CrossRef] Ciftci, A.; Ligthart, D.; Hensen, E.J.M. Influence of Pt particle size and Re addition by catalytic reduction on aqueous phase reforming of glycerol for carbon-supported Pt(Re) catalysts. Appl. Catal. B Environ. 2015, 174, 126–135. [CrossRef] Feng, X.J.; Sloppy, J.D.; LaTemp, T.J.; Paulose, M.; Komarneni, S.; Bao, N.Z.; Grimes, C.A. Synthesis and deposition of ultrafine Pt nanoparticles within high aspect ratio TiO2 nanotube arrays: Application to the photocatalytic reduction of carbon dioxide. J. Mater. Chem. 2011, 21, 13429–13433. [CrossRef] Yoo, J.; Altomare, M.; Mokhtar, M.; Alshehri, A.; Al-Thabaiti, S.A.; Mazare, A.; Schmuki, P. Photocatalytic H2 generation using dewetted Pt-decorated TiO2 nanotubes: Optimized dewetting and oxide crystallization by a multiple annealing process. J. Phy. Chem. C 2016, 120, 15884–15892. [CrossRef] Kim, G.J.; Kwon, D.W.; Hong, S.C. Effect of Pt particle size and valence state on the performance of Pt/TiO2 catalysts for CO oxidation at room temperature. J. Phy. Chem. C 2016, 120, 17996–18004. [CrossRef] © 2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).