Atomic layer deposition of anatase TiO₂ coating on silica particles: growth, characterization and evaluation as photocatalysts for methyl orange degradation and hydrogen production

Abstract

Atomic layer deposition is used to deposit crystalline anatase TiO₂ on 500 nm silica support particles over a range of shell thicknesses. The core–shell morphology combines nanometer thickness titania with silica that is readily separable from liquid media. The composite particles are evaluated as photocatalysts for destruction of the model pollutant methyl orange and, after deposition of platinum co-catalyst, for the sacrificial generation of hydrogen from water. The catalytic activities normalized to surface area are higher than those of P25 mixed anatase/rutile titania nanoparticles.

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Introduction

Titanium dioxide (TiO₂) has received considerable attention due to its physical, electrical and optical properties.¹ Its high dielectric constant makes it a candidate material for future memory devices either as TiO₂ or as a component in a mixed oxide system such as SrTiO₃.² Since the demonstration by Fujishima and Honda that TiO₂ can be used to photoelectrochemically split water,³ there has been a tremendous amount of work carried out in this area.^4,5 The splitting of water to produce hydrogen gives a clean, environmentally friendly alternative to the use of fossil fuels for future energy requirements making this technology area scientifically interesting.^6–8 TiO₂ has also been suggested as a suitable photocatalyst for the removal of organic waste and pathogens from water.^9,10

TiO₂ has been utilized in the form of thin films for photocatalysis,¹¹ but the most widely used form of TiO₂ for photocatalysis is commercial Degussa P25 powder, a mixture of anatase and rutile TiO₂ with a particle size of approximately 20–30 nm and a surface area of 50 m² g⁻¹.^12,13 While the large surface area of P25 is undeniably beneficial to the photocatalytic process the relatively small particle size can make recovery and handling of the catalyst difficult. This problem has been highlighted as one of the main barriers to the exploitation of photocatalysis for water purification and the photo-oxidation of dyes and harmful materials.¹⁴ Larger bulk TiO₂ particles could be used, however this would require more TiO₂ and give a larger TiO₂ bulk to surface ratio, thereby leading to enhanced bulk recombination of electrons and holes, potentially reducing the photocatalytic activity. One method for overcoming this recombination is the use of core–shell particles; this is the approach utilized in this work. A thin film of TiO₂ could be deposited onto a solid particle support, with larger, easier to handle particles as well as a reduction in potential bulk recombination. There are many different deposition methods available for the growth of such TiO₂ thin films such as sol gel,¹⁵ chemical vapour deposition (CVD),¹⁶ spray pyrolysis,¹⁷ sputtering,¹⁸ liquid phase dispersion¹⁹ and atomic layer deposition (ALD).²⁰ For this work we required a deposition technique that would grow a uniform thin film of TiO₂ on a silica (SiO₂) sphere, therefore ALD was chosen. ALD is a versatile gas phase deposition technique that is comparable to CVD. In the case of CVD the driving force for the deposition is thermal; the hot substrate thermally decomposes the chemical precursors depositing a thin layer. For ALD the driving force for the deposition of the layer is a chemical reaction between the substrate and typically two chemical precursors that are delivered individually to the substrate, usually separated by a gas purge. Therefore, the layer is built up in a controlled manner, making the ALD technique ideal for the deposition of uniform thin films on complex 3D shapes and aggressively structured substrates.^21,22

ALD deposition of amorphous TiO₂ has been reported on micron size polyethene²³ and both SiO₂ and ZnO;²⁴ in the case of polyethene, the ALD deposited material gave inferior photocatalytic performance to bulk TiO₂; in the case of the SiO₂ 550 nm particles, the TiO₂ was deposited with an island morphology.

In this work we have utilized the ability of ALD to deposit conformal, crystalline, uniform thin films by depositing TiO₂ on 500 nm spheres of SiO₂ with excellent TiO₂ coverage, to create core–shell particles with superior photocatalytic properties compared to bulk material. This was achieved by carrying out the deposition on an agitated bed of support particles through control of the ALD reactor conditions by operating the gas flow in a discontinuous manner. We have then characterised the resulting core–shell particles using XRD, SEM, ICP elemental analysis and diffuse reflectance, before assessing their photocatalytic properties, comparing the TiO₂–SiO₂ core–shell particles to Degussa P25.

Experimental section

Synthesis

SiO₂–TiO₂ core–shell particles were synthesised by ALD using a SUNALE™ R-200B ALD (Picosun Oy) reactor that has been modified to allow deposition in an agitated bed of nano-powders. For this study 500 nm SiO₂ spheres (Alfa Aesar) were used with the TiO₂ layers being deposited using titanium isopropoxide (Sigma Aldrich) and water as precursors. The particle size of the SiO₂ was calculated from SEM images as being 505 nm with a standard deviation of 6%, in good agreement with the nominal values. TiO₂ was deposited at 225 °C onto the SiO₂ spheres by alternating pulses of titanium isopropoxide and water. This ALD process is well characterised for the deposition of TiO₂ on many substrates making the precursor selection straightforward.^25,26 Both sources were delivered by vapour draw with the water ampoule being held at 21 °C and the titanium isopropoxide ampoule at 110 °C. The high temperature used for the titanium isopropoxide was necessary to generate sufficient titanium isopropoxide vapour to saturate the SiO₂ particles; approximately 3 g of SiO₂ were used per deposition experiment. Due to the large surface area being coated (approximately 24 m² for 3 g of SiO₂ powder), a series of smaller precursor pulses were used to give a single long exposure for each precursor. Therefore, one hundred 0.1 s titanium isopropoxide pulses separated by 0.3 s were used to generate a single titanium isopropoxide pulse. A single long exposure water pulse was generated using the same method. The titanium isopropoxide and water pulses were separated by a purge of 200 s and 190 s respectively. A series of gas pulses were also incorporated in to the ALD cycle following the water pulse and purge to agitate the powder to ensure good surface coverage. The full ALD cycle growth conditions are shown graphically in Scheme 1 and Table 1. The material was then annealed in air at 660 °C for 30 minutes. Uncoated SiO₂ was annealed under identical conditions for use as a characterization comparison. For hydrogen generation experiments, platinum was deposited onto the surface of the coated material using a photodeposition method.²⁷ The appropriate amount of chloroplatinic acid (Sigma-Aldrich) was dissolved in 500 mL of a 1% by volume methanol solution in a Pyrex reactor. 0.5 g of the ALD material was added to the reactor, which was then sealed and purged with nitrogen. The reactor was placed under two half-cylinder arrangements of 6 × 8 W Coast Wave Blacklight UVA lamps for 4 hours. The platinized material was collected by centrifuging the material for 15 minutes at a speed of 3000 rpm. For comparison, TiO₂ (P25, Degussa) was photodeposited with platinum via the same method.

Scheme 1 ALD pulse sequence used for deposition.

Table 1 ALD deposition conditions

Growth parameter	Value
Substrate temperature	225 °C
Titanium isopropoxide temperature	110 °C
Water temperature	21 °C
Titanium isopropoxide gas flow	75 sccm
Water gas flow	75 sccm
Gas pulse flow	2000 sccm
Total reactor gas flow	250 sccm
Reactor pressure	16–17 hPa
Precursor pulse (precursor/purge)n	(0.1/0.3)100 s
Water pulse (precursor/purge)n	(0.1/0.2)100 s
Gas pulse (gas pulse/purge)n	(0.8/0.2)4 s
Pulse sequence (precursor/purge/water/purge/gas pulse/purge)	40/200/30/190/4/20 s
Number of cycles	75–300

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Characterisation

X-ray diffraction (XRD) patterns for phase identification and shell thickness measurements were collected using a Bruker Advance D8 diffractometer using a copper X-ray source operating at 40 mA and 40 kV, fitted with a Ge(111) monochromator providing Kα₁ radiation at a wavelength of 1.5406 Å. PXRD data were collected in θ/2θ Bragg–Brentano geometry in the range 20° to 100° 2θ with a step size of 0.013° and a total data collection time of 6 hours. The instrumental contribution to peak broadening was determined using a silicon line profile standard (NIST, SRM 640c). Crystallite size broadening was determined from analysis of the XRD pattern using Topas Academic,²⁸ and the volume weighted column height (L_vol) was calculated based on the methodology of Balzar et al.²⁹ The volume weighted column height distribution of the shell of a core–shell particle was simulated using 1 nm³ blocks for a range of shell thicknesses on core size of 500 nm. It was found that the maximum of the volume weighted column height distribution of the shell approximated to the shell thickness in the case where the core size was significantly larger than the shell thickness. As such, the value of L_vol calculated from the XRD pattern was used directly as an estimate of the shell thickness.

Density measurements were carried out using a Micrometrics AccuPyc 1330 pycnometer. The resulting densities of the ALD coated material were used to calculate the density of the deposited TiO₂. Surface areas were measured by the 5 point BET method on a Quantachrome Nova gas sorption analyzer by nitrogen sorption on samples that were previously degassed at 105 °C overnight.

SiO₂ and TiO₂ content of the core shell particles was determined at Butterworth Laboratories using a combined gravimetric and ICP-AES method. First the materials were calcined at 950 °C for 30 minutes to remove any surface adsorbed species. 0.1 g of each sample was dissolved in an aqueous mixture of concentrated hydrofluoric acid (40%, 15 mL) and concentrated sulphuric acid (97%, 0.2 mL). The SiO₂ content was determined from the weight loss after this digestion (SiF₄ has a boiling point of −86 °C, hence escapes as a gas), with the residue taken to be TiO₂. ICP-AES of the residue, fused in KHSO₄ and then dissolved in 4% H₂SO₄ confirms the titanium content calculated from the weight loss of SiO₂, with the mass balance of all three samples greater than 98.5%. Coating thickness was obtained from geometric calculations of the volume of the coated and non-coated particles, assuming mono-disperse spherical particles with an even coating thickness, using elemental analysis for the TiO₂ and SiO₂ weight of ALD-coated particles, the diameter of SiO₂ from SEM measurements (505 nm), the measured density of the SiO₂ particles (1.987 g cm⁻³) and the calculated density of anatase TiO₂ from the densities of the coated material (3.095 g cm⁻³).

SEM images were obtained on a Hitachi S-4800 scanning electron microscope. Coating thicknesses were determined from the difference in the mean particle diameter between the coated and uncoated particles from SEM images, counting approximately 90 particles for each diameter determination.

Diffuse reflectance spectra were measured using a Perkin Elmer Lambda 650S UV/Vis spectrophotometer, equipped with a Labsphere integrating sphere over the spectral range 190–900 nm (6.53–1.38 eV), using the 500 nm SiO₂ particles as reflectance standards. The spectra were analyzed using the Kubelka–Munk function, F(R) = (1 − R)²/R = K/S, with K the absorption coefficient of the sample and S the scattering coefficient, to represent the absorption coefficient of the sample.

Photocatalysis

The photo-oxidation of methyl orange was carried out in a 100 mL Pyrex reactor with oxygen bubbling through the system throughout the course of the reaction. Typically 0.09 g of material was used with 90 mL of 0.020 g L⁻¹ (6.11 × 10⁻⁵ M) methyl orange solution. The light source used was a 300 W Xe lamp fitted with a water-flow IR filter and Pyrex to cut off light below 300 nm. At suitable time intervals, 2 mL samples of the reaction solution were extracted, filtered to remove the catalyst, and analyzed using a Perkin-Elmer Lambda 650S UV/Vis spectrometer. The concentration of the methyl orange was monitored by recording the absorbance of the solution at a wavelength of 465 nm, with the absorbance coefficient 2.68 × 10⁴ L mol⁻¹ cm⁻¹.³⁰

Hydrogen generation reactions were carried out in a sealed 110 mL Pyrex reactor. Typically 100 mL of 20% methanol–80% water was added to the reactor, followed by 0.1 g of catalyst. The reactor was sealed with a super seal septum, and the mixture purged with nitrogen for 45 minutes in the dark to remove dissolved gasses. The light source was a 300 W Xe lamp (Oriel) with a water-flow IR filter and Pyrex to cut off light below 300 nm. At appropriate times, a 0.2 mL sample of the gas phase of the reactor was extracted using a syringe, and injected into a Varian 4000 gas chromatograph, with a thermal conductivity detector and nitrogen as the carrier gas. The area of the peak at retention time of 0.38 minutes was recorded, and hydrogen produced calculated using a calibration curve, calculated by injecting known amounts of hydrogen into the gas chromatograph, and recording the peak area.

Results and discussion

Characterisation of ALD coated particles

Due to the cyclic nature of ALD, the deposited film thickness can be controlled by varying the number of ALD cycles. Three samples of differing thicknesses were characterized: a layer of TiO₂ deposited by 150 ALD cycles (hereafter named ‘standard (1)’), a thinner layer of TiO₂ deposited by 75 ALD cycles (‘thin (2)’), and a thicker layer deposited by 300 ALD cycles (‘thick (3)’).

XRD analysis of the ALD-coated material shows the presence of anatase TiO₂ in the standard (1) and thick (3) material, with no Bragg diffraction measurable from any TiO₂ phase in the thin (2) material (Fig. 1d). The anatase TiO₂ showed no preferred orientation when compared to randomly oriented TiO₂.

Fig. 1 (a) SEM image of the uncoated SiO₂ carrier particles; (b) SEM image of standard (1) material; (c) cumulative frequency graph showing particle size distribution from SEM for uncoated SiO₂, standard (1) and thick (3) material; (d) XRD spectrum of uncoated SiO₂, thin (2), standard (1) and thick (3) ALD-coated material.

The coating thicknesses of the three different samples were independently calculated from SEM, XRD and elemental analysis with density measurements, and the results are shown in Table 2. SEM and XRD showed a negligible or undetectable thickness for the thin (2) material, although elemental analysis confirmed that TiO₂ was present. The observed shell thickness for the standard material (1) was in the range 6.4–14 nm, depending on the characterization method considered. The observed shell thickness of the thick (3) material was less than twice that of the standard (1) material, regardless of the characterization technique considered, expected due to the increasing spherical size of the particle requiring 1.1 times the volume of TiO₂ to coat to the same thickness of TiO₂. The variation in results between characterization methods for the ALD-coated materials can be explained by each method measuring a different property of the particles. SEM provides a direct visual indication of the shell thickness and is number-weighted in nature, giving the most common shell thickness. XRD provides the volume-weighted column height, which is biased towards larger shell thicknesses; indeed, the values from XRD are greater in each case than the SEM values. EA provides a geometric calculation based on the amounts of each material present, assuming mono-disperse spheres for both the SiO₂ and the ALD-coated material. From the SEM images (Fig. 1b), there was no sign of uneven coating or of incomplete shells, with the particles appearing spherical at all coating thicknesses, and the particle size distribution consistent across the uncoated, standard (1) and thick (3) material. This is a good indication that the coating is even across the particle size range studied. Given the intrinsic differences between the shell thickness characterization techniques employed, the derived thickness values are in reasonable agreement.

Table 2 Coating thickness of the coated material determined from SEM, XRD and elemental analysis (EA); density values were determined by pycnometry; %TiO₂ of material determined by EA. Values shown in parenthesis after the XRD determined coating thickness are the estimated standard deviation in the determined value

	SEM	XRD	EA	Density/g cm^−3	%TiO2 by weight
Thin (2)	Undetectable	Undetectable	1.0 nm	2.01	1.7
Standard (1)	8.4 nm	14 (2) nm	6.4 nm	2.07	9.8
Thick (3)	14.3 nm	18 (1) nm	9.1 nm	2.15	13.6

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Pycnometery of the ALD-coated material confirms the increasing density with amount of TiO₂ deposited, expected due to the higher bulk density of TiO₂ (4.230 g cm⁻³) compared with the uncoated SiO₂ (1.987 g cm⁻³). By plotting the mass percent of TiO₂ from the EA against the density of the ALD-coated material, (ESI†, Fig. 1) the density of the deposited TiO₂ can be determined, with the intercept the density of silica (1.984 g cm⁻³) in good agreement with the experimentally determined density of the SiO₂ particles (1.987 g cm⁻³). The calculated value (3.095 g cm⁻³) is significantly less than the single crystal density of anatase (3.894 g cm⁻³),³¹ with the calculated density comparable to the density of spin coated TiO₂ thin films on silica reported by Jiang et al.³²

The diffuse reflectance of the ALD-coated material is plotted in Fig. 2a, along with the indirect band gap extrapolation of standard (1) (Fig. 2b), with the calculated indirect band gaps of the ALD-coated material and anatase TiO₂ listed in Table 3. The diffuse reflectance for the ALD-coated material shows that the maximum intensity increases as a function of the increasing amount of thickness of TiO₂. As the uncoated material does not absorb light in the region measured, with the band gap of SiO₂ being 9 eV,³³ the absorbance identified in the spectra is due to the TiO₂. As the amount of TiO₂ increases with the increasing thickness of the TiO₂ layer, the amount of light absorbed increases, with the intensity of the absorbance of the thick (3) material comparable to TiO₂ anatase. Despite being beneath the detection limit of SEM, and showing an undetectable coating from XRD data, both the absorbance from the optical data, and the TiO₂ detected from elemental analysis (calculated thickness 1 nm) clearly demonstrates the presence of TiO₂ in the thin (2) material. This low intensity absorbance is comparable in intensity to that seen for TiO₂ films on silica glass via CVD, as demonstrated by Mills et al.,³⁴ although the calculated thickness of the thin (2) sphere coatings is significantly less than the measured thickness of the TiO₂ films deposited by CVD (up to 29 nm).

Fig. 2 (a) Diffuse reflectance data for thin (2), standard (1), thick (3) ALD-coated material and bulk anatase TiO₂; (b) the indirect band gap extrapolation for the standard (1) material.

Table 3 The indirect band gap calculated from the extrapolation of the ALD-coated material diffuse reflectance, compared with bulk anatase TiO₂

	Indirect band gap/eV
Thin (2)	3.35
Standard (1)	3.25
Thick (3)	3.20
Anatase	3.26

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As the thickness of the TiO₂ layer increased, the indirect band gap calculated decreased from 3.34 eV for the thin (2) material to 3.25 eV, for the standard (1) material and 3.20 eV for the thick (3) material, which is in line with the 3.25 eV measured for anatase TiO₂. The larger band gap for the thin (2) material is to be expected from literature and is attributed to quantum confinement, as demonstrated by Wang and Herron.³⁵

Methyl orange photo-oxidation

The photo-oxidation of dyes has been suggested as a potential application for photocatalysis in waste-stream clean-up.⁹ Methyl orange is a model azo dye and has been widely used as a test material for photocatalytic reactions since it was first used in this way in 1984.^30,36

Fig. 3a shows the spectral change that accompanies the photo-oxidation of methyl orange using the standard (1) material, normalized to surface area of the material. (ESI†, Fig. 2 shows as-measured data). Photocatalytic oxidation of dyes generally follows Langmuir–Hinshelwood kinetics.³⁷ At a constant photon flux with a constant amount of photocatalyst, when the surface has complete coverage of the matter being photo-oxidized, the reaction follows zero order kinetics and the reaction rate is independent of the concentration of the organic matter being photo-oxidized. When the catalyst is not completely covered by organic matter, the speed of the reaction is controlled by the rate at which dye adsorbs onto the surface of the catalyst; in this instance the dye concentration is included in the rate equation, and the reaction follows first order kinetics.³⁸ The absorbance isotherm of the standard (1) material (ESI†, Fig. 3) shows that complete coverage of the material by methyl orange is not realized at the initial concentration of methyl orange at the start of the reaction, therefore first order kinetics would be expected. As the photo-oxidation proceeds, it can be seen that this is the case; the photo-oxidation curve of the standard (1) material follows first order kinetics (eqn (1)), with the pseudo first order rate equation k given by the slope of the straight line.

ln([MO]) = −kt + ln([MO]₀) (1)

k is determined to be 0.018 min⁻¹ (Fig. 3b) comparable with the k value determined for photo-oxidation of methyl orange by P25 in the same regime (0.022 min⁻¹)

Fig. 3 (a) Methyl orange photo-oxidation curve for the coated material and P25 under UV light illumination. (b) First-order kinetics of the photo-oxidation of methyl orange by the standard (1) material under UV illumination, with R² = 0.967. (c) Transmission spectrum of standard (1) material (red) and P25 (black) suspensions in water over the course of 1 hour after agitation is stopped. The inset photograph shows the standard (left) and P25 (right) in water immediately after agitation is stopped.

The k values (Table 4) show that the thin (2) material is the least photoactive; this material absorbed the least light due to its larger band gap (3.35 eV compared to 3.25 eV for the standard (1) material) and lower absorbance intensity compared to the other ALD-coated material. From EA, this material also has the least amount of TiO₂ present on the surface. The data for the standard (1) and thick (3) material show that the rate of photo-oxidation of methyl orange is comparable, with k values of 0.018 min⁻¹ and 0.016 min⁻¹ respectively, indicating that the increase in TiO₂ content of the thick (3) material has little effect on the reaction rate. By normalizing the rate constant k for the ALD-coated material and P25 to the surface area, which in the case of P25 is over 5 times higher than the ALD-coated material, (50 m² g⁻¹vs. 8 m² g⁻¹) and the TiO₂ content of the respective material (Table 2), the superior photocatalytic performance of the ALD-coated material can be illustrated. The k value for the standard (1) material is 1.842 min⁻¹ g⁻¹ TiO₂ normalized to per gram of TiO₂, nearly an order of magnitude higher than the normalized k value for P25 photo-oxidation (0.220 min⁻¹ g⁻¹ TiO₂). Similarly by normalizing the k values of the material by measured surface area, 0.222 × 10⁻³ min⁻¹ m⁻²vs. 0.043 × 10⁻³ min⁻¹ m⁻² for the standard (1) material and P25 respectively, the superior performance of the ALD-coated material is evident. An explanation for this could be the geometry of the layer of TiO₂ on the ALD-coated material, which reduces the distance for the photogenerated holes and electrons to travel to the surface from 20–30 nm in P25 to ∼10 nm in standard (1). This would minimize the bulk electron–hole recombination and increase the effectiveness per unit mass or surface area.

Table 4 Pseudo first order rate constants (k) for ALD-coated material and P25 in the photo-oxidation of methyl orange, normalized to surface area and mass of TiO₂ present

Material	Pseudo first order rate constant k/min^−1	k/× 10^3 min^−1 m^2	k/min^−1 g^−1 TiO2
Thin (2)	0.001	0.125	0.581
Standard (1)	0.018	2.250	1.842
Thick (3)	0.016	2.000	1.181
P25	0.022	0.440	0.220

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Increasing the amount of material present in the reactor vessel does not affect the reaction rate for both the standard (1) material and P25, with the reaction rate and k values comparable when increasing the concentration of material from 1 g L⁻¹ to 3 g L⁻¹, indicating that both the standard material (1) and P25 are absorbing the optimal amount of light.

TiO₂ is known for its high dispersion properties,¹ which can be a disadvantage when removing TiO₂ from solutions after catalysis. Fig. 3c illustrates how the relatively large size of the ALD-coated material compared to P25 allows the material to settle very quickly once the agitation of the reaction stops, leading to easy removal of the material. The measured optical transmission of P25 in water is close to zero for over 5 minutes indicating the material is still in suspension; the standard (1) material settles immediately, with the transmission at over 60% as soon as agitation stops. This property makes the coated material a superior candidate for applications where quick and efficient removal of the catalyst from the solution is a priority, for example in water purification.³⁹

Hydrogen generation

The standard (1) material was also tested for hydrogen generation in a methanol solution under UV light, after photodepositing platinum onto the particles (0.1% 0.5%, 1% platinum by weight of material – here the weight refers to the total weight of the coated particles) (Table 5). The rate per hour of hydrogen generated varies with the amount of platinum deposited, with the maximum rate achieved with 0.5% by weight platinum (Table 4). The standard (3) material photodeposited with 1% by weight platinum, (Pt-standard (4)), was compared with Degussa P25 with 1% by weight platinum photodeposited, (Pt-P25). As seen with the methyl orange photo-oxidation, the higher surface area Pt-P25 gave a faster rate of hydrogen generation per total mass of material (which includes inactive SiO₂ in the case of the ALD-coated material), however the lower surface area and TiO₂ content of the Pt-standard (4) material means that a higher rate of hydrogen production per surface area (153 μmol h⁻¹ m⁻² for Pt-standard (4); 67 μmol h⁻¹ m⁻² for Pt-P25) (Fig. 4) and per gram of TiO₂ (12487 μmol h⁻¹ g⁻¹ TiO₂) for Pt-standard (4); 3370 μmol h⁻¹ g⁻¹ TiO₂) (Table 6) is noted, confirming the superior photocatalytic performance of the Pt-standard (4) material compared to P25 (Fig. 4 in ESI† shows as measured data). This along with the dispersion characteristics of the larger particle Pt-standard (4) material, compared with the nanosize Pt-P25, allows the material to be recovered more easily, indicating this material also has potential advantages over P25 for hydrogen generation applications.

Table 5 Rate of hydrogen generated with 0.1%, 0.5% and 1% photodeposited platinum on standard (1) material.

%Pt	Rate of hydrogen generation/μmol h^−1
0.1	50
0.5	133
1	122

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Fig. 4 Amount of hydrogen generated over time with Pt-standard (4) and Pt-P25, normalized per unit surface area.

Table 6 Rate of hydrogen generated with the Pt-standard (4) material and Pt-P25, normalized to surface area and mass of TiO₂

	Rate of hydrogen generation/μmol h^−1	Rate/μmol h^−1 m^−2	Rate/μmol h^−1 g^−1 TiO2
Pt-standard (4)	122	153	12487
Pt-P25	337	67.4	3370

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Conclusion

This work has shown that ALD can deposit crystalline anatase TiO₂ layers of nanometer thicknesses onto 500 nm SiO₂ support particles, producing novel functional well-defined non-agglomerated particles of controllable TiO₂ thickness. The particles can photo-oxidize methyl orange with an efficiency comparable to that found with P25, with the efficiency of ALD-coated material normalized to amount of TiO₂ present nearly 10 times greater than P25. With the addition of platinum, the ALD-coated material can generate nearly 4 times as much hydrogen as platinum-modified P25 when normalized to amount of TiO₂ present. We attribute this to the thinner layer of TiO₂ in the ALD particles compared with the P25 particle size, which would reduce photoelectron–hole recombination before reaching the surface. The coated material has high crystallinity as evidenced from XRD, and optical absorption comparable to TiO₂ particles. At the same time the large particle size of the ALD material (>500 nm) allows the material to be recovered easily from the reaction media after the photo-oxidation is complete.

Acknowledgements

We thank the UK EPSRC for support under EPSRC/C511794, EP/H500146/1 and EP/H000925.

References

1. O. Carp, C. L. Huisman and A. Reller, Prog. Solid State Chem., 2004, 32, 33.
2. M. Vehkamaki, T. Hatanpaa, T. Hanninen, M. Ritala and M. Leskela, Electrochem. Solid-State Lett., 1999, 2, 504.
3. A. Fujishima and K. Honda, Nature, 1972, 238, 37.
4. A. Mills and S. LeHunte, J. Photochem. Photobiol., A, 1997, 108, 1.
5. M. R. Hoffmann, S. T. Martin, W. Y. Choi and D. W. Bahnemann, Chem. Rev., 1995, 95, 69.
6. F. E. Osterloh, Chem. Mater., 2008, 20, 35.
7. A. Kudo and Y. Miseki, Chem. Soc. Rev., 2009, 38, 253.
8. X. B. Chen, S. H. Shen, L. J. Guo and S. S. Mao, Chem. Rev., 2010, 110, 6503.
9. R. W. Matthews, Sol. Energy, 1987, 38, 405.
10. P. S. M. Dunlop, J. A. Byrne, N. Manga and B. R. Eggins, J. Photochem. Photobiol., A, 2002, 148, 355.
11. V. G. Bessergenev, R. J. F. Pereira, M. C. Mateus, L. Khmelinskii, D. A. Vasconcelos, R. Nicula, E. Burkel, A. M. B. Do Rego and A. I. Saprykin, Thin Solid Films, 2006, 503, 29.
12. R. I. Bickley, T. Gonzalezcarreno, J. S. Lees, L. Palmisano and R. J. D. Tilley, J. Solid State Chem., 1991, 92, 178.
13. B. Ohtani, O. O. Prieto-Mahaney, D. Li and R. Abe, J. Photochem. Photobiol., A, 2010, 216, 179.
14. E. Forgacs, T. Cserhati and G. Oros, Environ. Int., 2004, 30, 953.
15. D. Macwan, P. Dave and S. Chaturvedi, J. Mater. Sci., 2011, 46, 3669.
16. A. Mills, N. Elliott, I. P. Parkin, S. A. O'Neill and R. J. H. Clark, J. Photochem. Photobiol., A, 2002, 151, 171.
17. M. O. Abou-Helal and W. T. Seeber, Appl. Surf. Sci., 2002, 195, 53.
18. P. Lobl, M. Huppertz and D. Mergel, Thin Solid Films, 1994, 251, 72.
19. G. Li, R. B. Bai and X. S. Zhao, Ind. Eng. Chem. Res., 2008, 47, 8228.
20. V. Pore, A. Rahtu, M. Leskela, M. Ritala, D. Sajavaara and J. Keinonen, Chem. Vap. Deposition, 2004, 10, 143.
21. M. Leskela and M. Ritala, Angew. Chem., Int. Ed., 2003, 42, 5548.
22. S. M. George, Chem. Rev., 2010, 110, 111.
23. X. H. Liang, D. M. King, P. Li and A. W. Weimer, J. Am. Ceram. Soc., 2009, 92, 649.
24. D. M. King, X. H. Liang, Y. Zhou, C. S. Carney, L. F. Hakim, P. Li and A. W. Weimer, Powder Technol., 2008, 183, 356.
25. M. Ritala, M. Leskela, L. Niinisto and P. Haussalo, Chem. Mater., 1993, 5, 1174.
26. A. Rahtu and M. Ritala, Chem. Vap. Deposition, 2002, 8, 21.
27. B. Kraeutler and A. J. Bard, J. Am. Chem. Soc., 1978, 100, 4317.
28. A. A. Coelho, TOPAS Academic: General Profile and Structure Analysis Software for Powder Diffraction Data, Bruker AXS, Karlsruhe, Germany, 2010.
29. D. Balzar, N. Audebrand, M. R. Daymond, A. Fitch, A. Hewat, J. I. Langford, A. Le Bail, D. Louer, O. Masson, C. N. McCowan, N. C. Popa, P. W. Stephens and B. H. Toby, J. Appl. Crystallogr., 2004, 37, 911.
30. G. T. Brown and J. R. Darwent, J. Chem. Soc., Faraday Trans. 1, 1984, 80, 1631.
31. J. K. Burdett, T. Hughbanks, G. J. Miller, J. W. Richardson and J. V. Smith, J. Am. Chem. Soc., 1987, 109, 3639.
32. K. Jiang, A. Zakutayev, J. Stowers, M. D. Anderson, J. Tate, D. H. McIntyre, D. C. Johnson and D. A. Keszler, Solid State Sci., 2009, 11, 1692.
33. R. B. Laughlin, Phys. Rev. B: Condens. Matter Mater. Phys., 1980, 22, 3021.
34. A. Mills, S. K. Lee, A. Lepre, I. P. Parkin and S. A. O'Neill, Photochem. Photobiol. Sci., 2002, 1, 865.
35. Y. Wang and N. Herron, J. Phys. Chem., 1991, 95, 525.
36. G. T. Brown and J. R. Darwent, J. Phys. Chem., 1984, 88, 4955.
37. K. Rajeshwar, M. E. Osugi, W. Chanmanee, C. R. Chenthamarakshan, M. V. B. Zanoni, P. Kajitvichyanukul and R. Krishnan-Ayer, J. Photochem. Photobiol., C, 2008, 9, 171.
38. J. R. Darwent and A. Lepre, J. Chem. Soc., Faraday Trans. 2, 1986, 82, 1457.
39. K. G. McGuigan, T. M. Joyce and R. M. Conroy, J. Med. Microbiol., 1999, 48, 785.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2jm33446a

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