Atomic Layer Deposited (ALD) TiO2 on Fibrous Nano-Silica (KCC-1

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Atomic Layer Deposited (ALD) TiO2 on Fibrous Nano-Silica (KCC-1) for Photocatalysis: Nanoparticle Formation and Size Quantization Effect Rustam Singh,† Rudheer Bapat,† Lijun Qin,‡ Hao Feng,*,‡ and Vivek Polshettiwar*,† †

Nanocatalysis Laboratories (NanoCat), Department of Chemical Sciences, Tata Institute of Fundamental Research (TIFR), Homi Bhabha Road, Colaba, Mumbai 400005, India ‡ Laboratory of Material Surface Engineering and Nanofabrication, Xi’an Modern Chemistry Research Institute, Xi’an 710065, People’s Republic of China S Supporting Information *

ABSTRACT: In this work we report the design and synthesis of high-surface-area photocatalysts by coating TiO2 on fibrous nanosilica (KCC-1) using atomic layer deposition (ALD). Our developed catalyst showed enhanced photocatalytic activity, better than that of the well-known MCM-41- and SBA15-supported TiO2 catalysts using ALD as well as that of other silica-supported TiO2 catalysts reported in the literature to date. This work shows how one can tune the photocatalytic activity of supported TiO2 catalysts by simply tuning the morphology of the support. In addition to extensive characterization of materials using various techniques, comprehensive mechanistic insight into ALD TiO2 coating on KCC-1 fibers was gained using solid-state NMR and UV-DRS. For the first time, we also observed the formation of small and monodispersed TiO2 nanoparticles after heat treatment of these ALD-coated samples of KCC-1. Notably, we observed size quantization effects in these TiO2 nanoparticles, which was confirmed by band gap shift measurements and the Brus effective mass approximation method. We believe that the combination of the unique textural properties and morphology of KCC-1 and TiO2 nanoparticle formation and their size quantization is the reason behind the enhanced photocatalytic activity of KCC-1/ TiO2 catalysts. KEYWORDS: fibrous nanosilica (KCC-1), atomic layer deposition, nanoparticle formation, size quantization effect, SBA-15, MCM-41, TiO2, photocatalysis, nanocatalysis, supported catalysts dispersion,20 preparation of TiO2 thin films, etc.21 Although all of these supported catalysts allowed the isolation and reuse of catalyst more efficiently in comparison to a powder nano-TiO2 suspension, their catalytic activity remained low due to the poor accessibility of active sites (TiO2). In addition, the particle size distribution was broad and difficult to control and particles were nonuniformly distributed on the catalyst surface. This implies that the support plays a very important role in the design of photocatalysts and we need a support which allows more loading of TiO2 with high substrate adsorption capacity and, more importantly, accessible active sites. In addition to TiO2 loading and its accessibility, other main factors which dictate the catalytic efficiency of the photocatalyst are (i) the formation of charge carriers on the surface of the catalyst, (ii) the light-harvesting properties of the catalyst, and (iii) the adsorption−desorption of molecules (e.g., dye) on the catalyst surface. It is well-known that electron−hole recombination occurs on a nanosecond time scale, and hence it is very

1. INTRODUCTION Energy and environment are two of our critical societal challenges. The use of solar energy for efficient photocatalytic water splitting to produce hydrogen1 and dye/perovskitesensitized solar cells to produce electricity2,3 are the most promising potential routes for the development of alternate energy sources. Whereas, photocatalytic decomposition of contaminants such as organic dyes is key for a clean environment.4 TiO2 is one of the favored material used as a photocatalyst for these processes.5 However, the use of unsupported TiO2 nanopowder possesses several drawbacks, such as small surface area, poor substrate adsorption ability, weak lightharvesting properties, and difficulty in isolation and reuse of the catalysts. The issue of isolation and reusability was attempted to be resolved by immobilization of TiO2 on a silica support.4,6 There are several different methods described in the literature to deposit TiO2 on silica supports, which include the acid-catalyzed sol−gel method,7−14 chemical solution deposition,15 supercritical CO2 deposition aided by a liquid-crystal template,16 the impregnation method,17 incipient wet impregnation,18 the hydrothermal method,19 precipitation deposition,6 solid-state © XXXX American Chemical Society

Received: February 10, 2016 Revised: March 15, 2016

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DOI: 10.1021/acscatal.6b00418 ACS Catal. 2016, 6, 2770−2784

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and various other catalysts.58−65 To compare the KCC-1/TiO2 materials with conventional materials, SBA-15/TiO2 and MCM-41/TiO2 were also prepared using exactly the same ALD and heat treatment procedure. In addition, a detailed insight was gained into the ALD process on KCC-1 fibers using MAS NMR, nanoparticle formation using STEM analysis, band gap shift by UV-DRS, and size quantization effects by the Brus effective mass approximation method.

critical that these charge carriers should travel to the TiO2 surface on a time scale faster than this. One way to overcome this problem is by reducing the TiO2 nanoparticle size. Although a reduction in particle size may solve this issue, the stability/ lifetime of the catalysts gets reduced due to agglomeration and sintering of these small nanoparticles. Moreover, small nanoparticles (∼5−20 nm) have poor light-harvesting ability,22 which then affects the overall efficiency of the catalysts. Therefore, TiO2 having high surface area (without reducing its size) and having better light-harvesting ability is needed. A high surface area with a larger particle size seems impossible fundamentally, but after recent developments in nanomaterials synthesis, this was made possible by synthesizing nanostructured titania23−28 or titania supported on high-surface-area supports.29−34 Another important parameter is the proximity of dye adsorbed on the catalyst surface and the oxidizing species.35−38 To achieve this, the photocatalyst should have very high surface area, with a large surface for adsorption of dye molecules as well as large numbers of active sites on the surface for the interaction of light and generation of more oxidizing species on the surface. We believe that our recently developed fibrous nanosilica (KCC-1) could be the best candidate for a support to design efficient photocatalysts that can satisfy the aforementioned requirements. Fibrous nanosilica (KCC-1)39 has a high surface area that is due to its fibrous morphology and not to pores. Therefore, most of its surface area is accessible, yielding unusually high activity in catalysis40−43 and CO2 capture.44 KCC-1/TiO2 catalysts will have several advantages over conventional silica (MCM-41 and SBA-15)-based TiO2 catalysts (Figure 1), such as (i) high TiO2 loading, minimum reduction in

2. EXPERIMENTAL SECTION Transmission electron microscope (TEM) analysis was performed on FEI TITAN operated at an accelerating voltage of 300 kV. For sample preparation, powders were dispersed in ethanol and a drop of solution was dropped on a 200 mesh carbon-coated TEM grid. X-ray diffraction patterns were recorded using a Panalytical X’Pert Pro powder X-ray diffractometer with Cu Kα radiation. The surface area was calculated by applying Brunauer−Emmett−Teller (BET) theory to N2 physisorption data, recorded using a Micromeritics Flex3 analyzer. About 100 mg of each sample was degassed at 120 °C for 12 h prior to N2 sorption analysis. UV-DRS measurements were carried out using a V-770ST UV/vis/NIR spectrophotometer. 2.1. TiO2 ALD on Silica. KCC-1, MCM-41, and SBA-15 were synthesized as reported earlier.44 ALD experiments were carried out with a homemade viscous flow reactor based on the design of Elam et al.65 The ALD reactor was a 2 in. o.d. stainless steel tube heated on the wall by electronic heating tapes. The reaction temperature was maintained at 150 °C for TiO2 deposition. The powder samples were held in copper sample containers. The cover of the container was made from a fine stainless-steel mesh which could allow precursor molecules to diffuse through. The powder sample (∼150 mg) was evenly spread in the container to form a shallow powder bed less than 1 mm deep. In each ALD experiment three or four sample containers were placed in the reactor. Ultrapure nitrogen was continuously purged through the reactor at a rate of 100 sccm/min. A mechanical pump was used to push the gases through the reactor and to maintain the system base pressure at nearly 1.0 Torr. The reactants (precursors) used for TiO2 growth were Ti(OCH(CH3)2)4 and H2O2 (30% in aqueous solution). Ti(OCH(CH3)2)4 was stored in a stainless steel bubbler. During the TiO2 growth high-purity N2 was purged through the bubbler at a rate of 60 sccm/min to carry out the reactant vapor. The bubbler was maintained at 60 °C to obtain a vapor pressure of 0.05 Torr. The pressure of H2O2 dosed to the reactor was controlled at 0.10−0.15 Torr by adjusting the regulating valve upstream to the ALD reactor. In ALD experiments each deposition cycle includes four consecutive steps: metal precursor dose, N2 purge, oxidant dose, and N2 purge again. These four steps were carried out in a pulse sequence denoted as t1−t2−t3−t4, where “tx’’ (x = 1−4) represents the duration of each step. For TiO2 growth on KCC-1 samples, the pulse sequence used was an optimized dosing time of 60−60−60−60 s. For TiO2 growth on SBA-15 and MCM-41, the pulse sequence was 180−180−180−180 s. The number of ALD cycles applied was 1, 2, 3, 14, 30, and 60, respectively. As-synthesized materials were then fully characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), EDS mapping, powder X-ray diffraction (PXRD), N2 sorption studies, and UV−vis diffuse reflectance spectroscopy (DRS).

Figure 1. Advantages of use of KCC-1 over MCM-41 or SBA-15 as a support for TiO2. Orange dots represent TiO2 nanoparticles.

surface area, and high accessibility of active sites and hence excellent catalytic performance (conversion, kinetics, and stability), (ii) an increase in light-harvesting properties due to the fibrous structure of KCC-1 (enhanced scattering and internal reflections of incident light), and (iii) large adsorption of dye and water molecules during dye degradation due to the high and accessible surface area of KCC-1. To study our hypotheses, we have designed and synthesized high-surface-area TiO2 coated on the fibers of nanosilica (KCC-1/TiO2) (Figure 1). The synthesis of KCC-1/TiO2 was achieved by deposition of the titanium precursor using atomic layer deposition (ALD) on KCC-1 fibers followed by hydrolysiscondensation reaction to produce a TiO2 coating on the fibers. Coating was carried out by ALD and not by a wet deposition technique, which lacks precise control over the thickness of the deposited material. ALD forms a conformal coating on structures by a surface saturative and self-limiting growth process45−47 and hence is used extensively for designing solar cells,48−53 photocatalysts,54−57 2771

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ACS Catalysis 2.2. Photocatalytic Dye Degradation. While these experiments were carried out, best practices in photocatalytic measurements were followed.66 Various parameters such as light intensity, wavelength, and distance between the light source and reaction mixture were exactly same for all of the experiments (setup S1 in the Supporting Information). The catalyst (25 mg) was suspended in 50 mL of 1 × 10−5 M Rhodamine-B (RhB) aqueous solution in a glass reactor maintained at 22 °C. The suspension was stirred in the dark for 2−4 h to ensure the complete adsorption−desorption equilibrium and then exposed to light. A xenon lamp (UV light 250−385 nm; light intensity 75 W, 9.5 mw/cm2, distance between source and reactor 11.6 cm) was used as a light source. The samples were taken at different time intervals from the reaction mixture, and the absorption spectra (after filtration of the catalysts) were measured with a UV−vis spectrophotometer. The concentration of RhB was measured by monitoring the absorption maxima at 555 nm. After dye degradation, there were no additional peaks in the UV−vis spectrum, indicating that the dye was completely degraded and not just photobleached. The best photocatalysts were also screened for methylene blue (MB) and phenol degradation. For methylene blue degradation, experimental conditions were identical with those for RhB degradation (25 mg of catalyst, 50 mL of 1 × 10−5 M aqueous MB solution) and the concentration was measured by monitoring absorption maxima at 665 nm, whereas for phenol degradation 100 mg of catalyst was dispersed in 50 mL of 5 × 10−4 M aqueous phenol solution with a light intensity of 38 mW/cm2. The concentration of phenol was measured by monitoring the absorption maxima at 271 nm.

isopropoxide. In the second pulse (water), the supported surface titanium isopropoxide becomes hydrolyzed, which condenses to form a titania (TiO2) atomic layer on the surface of KCC-1. During the water pulse, tilanols (Si−O−TiO2OH) will be generated along with the generation of silanols on the surface. This completes the first ALD cycle (C1). In the second ALD cycle, the precursor pulse will react with surface tilanols (in addition to silanols) for another ligand exchange reaction and, in the water pulse step, they hydrolyze−condense to form a second atomic layer of TiO2 (C2). These steps were repeated until 60 cycles to achieve a good amount of TiO2 loading on KCC-1. To gain insight into the ALD mechanism, these as-prepared (ASP) materials were then characterized by 29Si and 1H solidstate magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy.57,67,68 Figure 2 shows the CP-MAS and single-pulse direct excitation 29Si NMR spectra and 1H spectra. In CP-MAS three main signals at δ −96, −105, and −115 ppm were observed, which can be attributed to Q2 (SiO2(OH)2), Q3 (SiO3OH), and Q4 (SiO4) sites, respectively. Q4 sites could also be due to Si−O−Ti bond formation.67 However, we were not able to resolve the Q4 sites due to Si−O−Si and Si−O−Ti in CP-MAS as well as in single-pulse 29Si NMR experiments. With an increase in ALD cycles from C1 to C30, a decrease in the ratio of Q3 to Q4 was observed (Figure 2), which indicates that more Q4 sites are forming, due to the formation of more and more Si−O−Ti bonds. However, if it were just the (TiOH groups in the) growing TiO2 chain reacting with the Ti precursor, this ratio should not have changed, which indicates the formation of more silanols during the water pulse by the attack of water molecules on siloxanes, which are then reacting with the Ti precursor. 1 H MAS NMR spectra show signals at δ 1.2 and 4.1 ppm for surface silanols and tilanols, respectively (Figure 2).68 The presence of silanols even at 30 cycles indicates that in every ALD cycle, in addition to tilanols, silanols are also produced by the attack of water molecules on siloxane bonds. Various numbers of ALD cycles (1, 2, 3, 14, 30, and 60) of TiO2 on all three supports (KCC-1, MCM-41, and SBA-15) were carried out. These materials are denoted KCC-1/TiO2-Cn, SBA-15/TiO2-Cn, and MCM-41/TiO2-Cn, where n = 1, 2, 3, 14, 30, 60. STEM and EDS mapping indicates that the coating of TiO2 on fibrous nanosilica KCC-1 was homogeneous and uniform (Figure 3). Similar observations were also made for MCM-41 and SBA-15 series (see Figure S1 in the Supporting Information). 3.2. Effect of Surface Area and Morphology of Silicas on TiO2 Loading and Accessibility of Active Sites. TiO2 loading of the as-synthesized materials were calculated by EDS analysis.69 On comparison, we found that the loadings of TiO2 were nearly the same up to three deposition cycles (Figure 4a). However, with an increase in the ALD cycles, the TiO2 loading kept increasing in the case of KCC-1 and SBA-15 but saturated at 30 deposition cycles in the case of MCM-41 (Figure 4a). At 60 cycles, for KCC-1/TiO2-C60, MCM-41/TiO2-C60, and SBA-15/TiO2-C60, good amounts of TiO2 loading of 57, 39, and 63 wt %, respectively, were achieved. One important premise of all ALD processes is the saturated surface reaction. The saturated surface reaction defines the layer by layer growth pattern of ALD films and ensures uniform distribution of the deposited species on the support. Under saturated surface reaction conditions the catalyst loading is only dependent on the number of ALD cycles performed (not on the precursor exposure time). In this work also, all ALD experiments

3. RESULTS AND DISCUSSION To assess and compare the properties and photocatalytic performance of TiO2-loaded KCC-1 materials, we evaluated them on the basis of five critical factors: (1) synthesis and formation mechanism of TiO2-coated KCC-1 by ALD (2) effect of the surface area and morphology of silicas on TiO2 loading and accessibility of active sites (3) heat treatment of TiO2-coated KCC-1 and nanoparticle formation (4) quantum confinement (QC) effects (5) photocatalytic efficiency (conversion, kinetics, stability, and reusability) of KCC-1/TiO2 and comparison with that of SBA-15- and MCM-41-supported TiO2 catalysts 3.1. Synthesis and Formation Mechanism of TiO2Coated KCC-1 by Atomic Layer Deposition (ALD). ALD was employed to coat TiO2 on the fibers of KCC-1 using titanium isopropoxide [Ti(OCH(CH3)2)4] as a Ti precursor. The materials were then characterized to determine the TiO2 crystallographic forms, particle size, band gap, and overall textural properties of the catalysts by scanning transmission electron microscopy (STEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), EDS mapping, powder X-ray diffraction (PXRD), N2 sorption studies, and UV−vis diffuse reflectance spectroscopy (DRS). On the basis of various reports on the ALD growth mechanism,45−47 we are proposing a reaction pathway for KCC-1/TiO2 formation as shown in Figure 2. In the first pulse, the Ti precursor will adsorb on the surface of KCC-1 fibers. Then a ligand exchange reaction of titanium isopropoxide with surface silanols takes place (Figure 2), yielding silica-supported titanium 2772

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Figure 2. Stepwise ALD process on the KCC-1 surface and 29Si CP-MAS, 29Si single-pulse, and 1H MAS NMR spectra of the as-prepared KCC-1/TiO2 series. Asterisks indicate signals from adsorbed water.

were performed under saturated surface reaction conditions. The mesoporous silica supports used in this research have fairly large surface areas. The large surface area and the mesoporous structure require a long time for the ALD reaction to complete. To explore the saturation conditions of TiO2 ALD on the silica supports, a series of experiments with pulse sequences of x−x−x−x seconds were carried out (with the same amount of KCC-1 sample in each experiment), in which x was varied from 5 to 120 s. Figure S2 in the Supporting Information presents the mass gains on the KCC-1 samples with different precursor dosing and purging times after seven-cycle TiO2 ALD. It can be noticed that the mass gain curve levels off at large precursor exposures, which is an indication of the self-limiting or saturated surface reaction typical for ALD processes. On KCC-1 saturation of the ALD surface reaction is almost achieved with a precursor exposure of 60 s; longer precursor exposures do not lead to much further increase of the sample mass. In comparison to KCC-1, the saturated ALD surface reaction on MCM-41 or SBA-15 requires a longer time (x ≈ 100 s). On the basis of these results, the pulse sequence used for the fabrication of KCC-1 is 60−60−60−60 s and a pulse sequence of 180−180−180−180 s is applied for the fabrication of MCM-41 or SBA-15 to ensure saturated surface

reactions. Since all silica samples have comparable surface areas, the different times required to achieve the saturated surface reaction on the different supports are probably due to diffusion limitations. The fibrous structure of KCC-1 may favor the diffusion of the precursor molecules into the internal surface; therefore, a shorter precursor exposure time is required to achieve the saturated surface reaction on KCC-1. On comparing the loading of TiO2 per unit surface area, the value for KCC-1 (0.095) was almost double those of MCM-41 (0.040) and SBA-15 (0.045) for the C60 cycle (Figure 4b), despite the fact that the surface area of KCC-1 (598 m2/g) is nearly half of that for MCM-41 (962 m2/g) and SBA-15 (1391 m2/g). This indicates that not only the surface area but also the accessibility of the surface area are key factors for TiO2 loading. Hence, KCC-1, having a more open structure due to its fibrous morphology in comparison to MCM-41 and SBA-15, seems to be a better support to load a greater amount of TiO2 sites. The difference in TiO2 loading on these supports can be explained on the basis of surface silanols/siloxanes and surface area accessibility. For the initial deposition cycles, it is the number of surface silanols and siloxanes that determines the TiO2 loading, which was nearly the same for all three 2773

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less clogged at higher cycles in comparison to the narrow pores of MCM-41 and hence it has a higher loading in comparison to MCM-41 for higher deposition cycles. It is well-known that, with an increase in TiO2 loading, the surface area of the material decreases, mainly due to clogging of the pores. This reduction in surface area then eventually reduces the accessibility of active sites, which is a bottleneck in most of the silica-supported TiO2 catalysts. We compared the reduction in BET surface area of these materials at the highest TiO2 loading, i.e. C-60 ALD cycles (Table 1). The reduction in Table 1. Comparison of the Reduction in BET Surface Area of KCC-1/TiO2-C60, MCM-41/TiO2-C60, and SBA-15/ TiO2-C60a BET surface area (m2/g)

a

reduction in surface area

catalyst

TiO2 loading at C60 cycles (wt %)

before TiO2 loading

after C60TiO2 cycles

m2/g

%

KCC-1/TiO2-C60 MCM-41/TiO2-C60 SBA-15/TiO2-C60

57 ± 3 39 ± 2 63 ± 3

598 962 1391

222 421 182

376 541 1209

63 56 87

Surface area measurement standard error ±4%.

BET surface area of KCC-1/TiO2-C60 was only 63% even after 57 wt % of TiO2 loading, with an absolute surface area reduction of 376 m2/g. At a similar TiO2 loading (63 wt %), SBA-15/ TiO2-C60 lost 87% of its surface area, with an absolute surface area loss of 1209 m2/g. In the case of MCM-41/TiO2-C60, with 39 wt % TiO2 loading, the loss in its surface area was 56%, with an absolute surface area loss of 541 m2/g. Due to less reduction in surface area and more loading of TiO2, KCC-1/ TiO2-C60 will have more number of accessible active sites for catalysis. These results clearly confirm the advantage of the KCC-1 morphology over porous materials such as MCM-41 and SBA-15, to design supported catalysts without losing much of its surface area.44,70 3.3. Heat Treatment of TiO2-Coated KCC-1 and Nanoparticle Formation. All three catalyst series, KCC-1/TiO2, MCM-41/TiO2, and SBA-15/TiO2, were characterized by PXRD to determine their TiO2 crystallographic forms. PXRD of all the as-prepared (ASP) materials showed no peaks (Figure 5a,b), indicating the amorphous nature of the TiO2 layer. We believe that the TiO2 layer thickness is too thin to show any crystalline behavior, and we did not attempt to go higher than C60 ALD

Figure 3. STEM and EDS mapping of KCC-1/TiO2 series.

supports. Therefore, during the initial cycles, the TiO2 loading was also nearly same for all the silicas (Figure 4a). However, with an increase in the number of deposition cycles, the accessibility of deposition sites, i.e. surface area, starts playing a role and determines the further deposition of TiO2. Since KCC-1 has an open and fibrous structure, it permits more TiO2 loading even at higher deposition cycles, without blocking of KCC-1 channels. However, MCM-41 and SBA-15 have an ordered array of hexagonal mesopores which may be getting clogged by TiO2 with an increase in the deposition cycles. As the pore diameter of SBA-15 is larger than that of MCM-41, SBA-15 pores may be

Figure 4. TiO2 loading (a) in wt % and (b) in wt % per unit surface area (SA) on KCC-1/TiO2, MCM-41/TiO2, and SBA-15/TiO2 by ALD. 2774

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Figure 5. (a, b) PXRD and (c, d) UV-DRS spectra and (e, f) band gap measurements using Tauc plots of various cycles of KCC-1/TiO2 before and after heat treatment. The standard error in band gap measurement was ±0.02.

202−230 nm and another band at 250−370 nm. The first band at 202−230 nm was assigned to ligand to metal charge transfer (LMCT) from O to Ti in isolated TiO4 units with tetrahedral titanium. The second band at 250−370 nm was also due to LMCT from O to Ti but with titanium in an octahedral environment (Figure S4 in the Supporting Information).57,67 With an increase in ALD cycles, we observed that the intensity of the band at 250−370 nm increases (Figure 5c), indicating that tetrahedral Ti sites were converting into octahedral Ti sites. This observation also matches with observations from our NMR study and the ALD mechanism (Figure 2). It was only during the initial ALD cycles that Si−O−Ti bond formation takes place (tetrahedral Ti sites), and afterward mostly formation of Ti−O−Ti bonds occurs (octahedral Ti sites). In the case of heated samples, the band at 202−230 nm was very weak, indicating the presence of very few tetrahedral Ti sites, and most of them converted into octahedral Ti sites (Figure 5d). We also observed a red shift in the band at 250−370 nm with an increase

cycles, as a good amount of TiO2 loading was already achieved at C60 cycles. It was reported that by heat treatment amorphous TiO2 can be converted to crystalline TiO2;71 hence, we heated these materials at three different temperatures, 300, 500, and 700 °C, with a ramp of 5 °C/min for 4 h in air and let the samples cool to room temperature naturally. The diffraction peak at 2θ = 25° for anatase TiO2 was monitored in PXRD. However, unlike the case in previous reports, even after heat treatment, most of the materials did not show any peaks for crystalline TiO2 in their XRD pattern (Figure 5 and Figure S3 in the Supporting Information). However, we started to observe a weak signal for crystalline TiO2 from C30 samples onward, which indicated that the layer thickness plays a crucial role in the TiO2 crystallization. UV−vis diffuse reflectance spectroscopy was used to study the changes in various Ti coordination sites, with respect to ALD cycles as well as after heat treatment of the KCC-1/TiO2 series (Figure 5c,d). We observed two bands: one weak band at 2775

DOI: 10.1021/acscatal.6b00418 ACS Catal. 2016, 6, 2770−2784

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ACS Catalysis ⎡1⎤ ⎡ 1 ⎤2 ΔEg = −A⎢ ⎥ + B⎢ ⎥ ⎣R⎦ ⎣R⎦

in ALD cycles, which may be due to the formation of larger nanoparticles at higher ALD cycles, forming Ti sites with increased coordination numbers.57,72,73 To study the change in band gap of these KCC-1-based materials, we analyzed them using UV-DRS. Band gap measurement using Tauc plots suggested that there is an indirect band gap present in all of these materials (Figure 5e,f). As we increased the deposition cycles from C1 to C60, the band gap value decreased from 3.53 to 3.28 eV for as-prepared samples (Figure 5e). The change in band gap indicated the transformation of molecular Ti sites to bulkier Ti sites of TiO2, reducing the overall band gap due to splitting of energy states. When the samples were heated at 700 °C, the band gap decreased in comparison to its as-prepared counterpart, indicating the transformation of amorphous TiO2 to crystalline TiO2 (Figure 5f).74 We also observed a blue shift in the band gap within heated samples from C1 to C60, which can be attributed to nanoparticle formation and quantum confinement effects, generally observed for particle size