Significant improvement in TiO₂ photocatalytic activity through controllable ZrO₂ deposition

Abstract

ZrO₂ was deposited on anatase TiO₂ nanoparticles using 5–80 cycles of atomic layer deposition (ALD). The photocatalytic activity of all samples was evaluated based on the degradation of methylene blue (MB) solution under UV light. The TiO₂ sample with 45 cycles of ZrO₂ deposition (45c-Zr/TiO₂, 1.1 wt% ZrO₂) was proved to be the most efficient catalyst with a degradation kinetic constant 10 times larger than that of the pure TiO₂ sample. All samples were characterized using inductively coupled plasma atomic emission spectroscopy (ICP-AES), nitrogen adsorption–desorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), UV-vis diffuse reflectance spectra analysis (UV-DRS), Raman and photoluminescence (PL) techniques. The high photocatalytic activity of 45c-Zr/TiO₂ can be attributed to stronger adsorption in the ultraviolet region and a reduction in the recombination rate of electron/hole pairs.

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1. Introduction

Nowadays, TiO₂ has been widely studied as an effective photocatalyst for the degradation of organic pollutants due to its relatively high activity, chemical stability, nontoxicity and low cost.¹ However, the high recombination rate of photo-generated electrons (e⁻) and holes (h⁺) and slow reaction rate of pure TiO₂ have hindered its further application. Thus, the photocatalytic efficiency of TiO₂ needs to be further improved for practical and commercial use.²

It is well known that ZrO₂ is an n-type semiconductor with similar physico-chemical properties to those of TiO₂,³ and therefore it has been used to prepare ZrO₂ doped TiO₂ photocatalysts because of these similar properties in order to improve the photocatalytic activity of TiO₂.^3,4 Through ZrO₂ doping, the band gap of the photocatalyst increases and the recombination rate subsequently decreases, which leads to an improvement of the photocatalytic activity. However, almost all previous studies adopted sol–gel and other similar methods to prepare ZrO₂/TiO₂, as shown in Table S1,† and thereby the crystal structure and phase composition of the samples were changed due to the mixture of Zr and Ti precursors. High ZrO₂ content would lead to poor anatase crystallinity of TiO₂ and decrease the photocatalytic efficiency; in contrast, ZrO₂ loading that is too low does not ensure an obvious increase in the band gap, and thereby the e⁻/h⁺ recombination rate cannot be reduced effectively. Thus, the enhancement of TiO₂ photocatalytic performance by ZrO₂ doping is restricted, and solving this dilemma is the key to further improving the activity of ZrO₂/TiO₂.

In this work, we deposited ZrO₂ on anatase TiO₂ nanoparticles (NPs) via different numbers of cycles (5–80) of atomic layer deposition (ALD) to investigate the effect of ZrO₂ in the photocatalytic degradation of methylene blue (MB). The samples were labeled xc-Zr/TiO₂, where x refers to the number of cycles of ZrO₂. ALD is a surface controlled layer-by-layer coating process based on self-limiting surface reactions, and it has been utilized to deposit uniform metal oxide films with sub-nanometer control of film thickness and well controlled film compositions.⁵ Through making full use of the advantages of ALD, the ZrO₂ content was controlled at a low level to retain the anatase crystallinity of TiO₂ and the e⁻/h⁺ recombination rate decreased due to the high dispersion of ZrO₂ on the surface of TiO₂.

2. Experimental section

2.1. Preparation of ZrO₂/TiO₂ nanoparticles

ZrO₂ was deposited on TiO₂ NPs via ALD using tetrakis(dimethylamido)zirconium(IV) (TDMAZ) (electronic grade, ≥99.99%, Sigma-Aldrich) and deionized water as precursors in a fluidized bed reactor, as described in detail elsewhere.⁶ Anatase TiO₂ NPs with an average particle size of 20 nm were purchased from US Research Nanomaterials Inc. For a typical run, 5 g of TiO₂ NPs was loaded into the reactor. The reaction temperature was 250 °C. Before the reaction, the particles were first degassed at 250 °C for 3 hours. The particle substrates were fully fluidized with a gas flow rate controlled with mass flow controllers. N₂ was used as a flush gas to remove unreacted precursors and any byproducts during the reaction. A typical half deposition cycle used the following sequence: precursor dose, N₂ purge and evacuation. This sequence was repeated alternately for both precursors. 5 (5c-Zr/TiO₂), 25 (25c-Zr/TiO₂), 45 (45c-Zr/TiO₂), 60 (60c-Zr/TiO₂), and 80 (80c-Zr/TiO₂) cycles of ZrO₂ ALD were applied on anatase TiO₂ NPs.

2.2. Characterization

Inductively coupled plasma-atomic emission spectroscopy (ICP-AES) was used to measure the Zr mass fraction in the ZrO₂/TiO₂ samples. The surface area of the pure TiO₂ and ZrO₂-deposited TiO₂ NPs was calculated using the Brunauer–Emmett–Teller (BET) method in a relative pressure range (0.05–0.25) of nitrogen adsorption and desorption isotherms obtained using a Quantachrome Autosorb-1. The photoluminescence (PL) spectra were recorded with a HORIBA FL3-22 spectrometer (HORIBA, Edison, NJ) to evaluate the recombination rate of e⁻/h⁺ pairs in the samples. The morphology of ZrO₂ deposited on TiO₂ was directly observed using FEI Tecnai F20 transmission electron microscopy/energy-dispersive X-ray spectroscopy (TEM/EDS). The crystal structure of the TiO₂ and ZrO₂/TiO₂ samples was analyzed with X-ray diffraction (XRD), and the Raman spectra of the samples were recorded using a Horiba-Jobin Yvon LabRam spectrometer. UV-visible absorbance and diffuse reflectance spectra (DRS) of TiO₂ and ZrO₂/TiO₂ samples were obtained with a UV-visible spectrophotometer (Varian Cary 5). BaSO₄ was used as an absorbance standard. Details of the characterization are described in the ESI.†

2.3. Photocatalytic activity measurement

Degradation of MB was used to evaluate the photocatalytic performance of the TiO₂ and ZrO₂/TiO₂ samples, as described in detail previously.⁷ Briefly, 0.1 g of sample was added to a 100 ml, 10 ppm MB solution. First, the solution was stirred in the dark for 60 min to ensure adsorption/desorption equilibrium. Then, the solution was irradiated using a UV lamp (360 nm UV light) and about 1 ml test sample was collected from the main solution at certain time intervals and filtered through a Millipore filter to make it particle-free for analysis using a UV-vis spectrometer (Varian Cary 50-Bio) at a 664 nm wavelength.⁸ The change in concentration of MB in the main solution was recorded over a period of irradiation time.

3. Results and discussion

As shown in Fig. 1, the photodegradation efficiency of all ZrO₂/TiO₂ samples was higher than that of the pure TiO₂, and with an increasing number of ALD cycles, the photoactivity of the samples kept increasing below 45 ALD cycles of ZrO₂, then it decreased slightly with further increases in the number of ALD cycles. According to the Langmuir–Hinshelwood kinetic equation (pseudo-first-order reaction),⁹ the apparent kinetic constant (k_app) of the 45c-Zr/TiO₂ sample reached a maximum value (0.127 min⁻¹), and its activity exceeded that of pure TiO₂ (0.012 min⁻¹) by a factor of more than ten. As shown in Table S1,† Pt ALD and CeO₂ ALD have been applied to the improvement of TiO₂ photoactivity, and they showed, at most, only a 3.0 and 3.3 times increase in photocatalytic activity compared to that of pure TiO₂, respectively.^7,10 More importantly, ZrO₂ is much cheaper and more economical for large-scale production compared to Pt. Compared to previously reported ZrO₂/TiO₂ photocatalysts,⁴ the ALD prepared 45c-Zr/TiO₂ sample improved the photocatalytic activity of pure TiO₂ significantly, which proved that ZrO₂ ALD was one promising strategy to enhance ZrO₂/TiO₂ photocatalytic performance (Table S1†). The reasons for the much higher photoactivity of 45c-Zr/TiO₂ than that of TiO₂ in this work were investigated.

Fig. 1 (a) MB concentration and (b) relative concentration of MB as a function of UV irradiation time over different catalysts, and (c) apparent kinetic constant (k_app, min⁻¹) of ZrO₂/TiO₂ catalysts with different numbers of ZrO₂ ALD cycles.

Firstly, the ZrO₂ ALD process did not affect the bulk properties of the TiO₂ NPs (e.g., crystal structure and surface area). As shown in Fig. 2, the ZrO₂ content of the samples increased steadily (from 0 to 1.5 wt%) with an increasing number of ALD cycles. This is one characteristic of the ALD process. This indicates a constant growth rate of ZrO₂. The ZrO₂ loading was 0.1, 0.5, 1.1, 1.2 and 1.5 wt% in 5c-Zr/TiO₂, 25c-Zr/TiO₂, 45c-Zr/TiO₂, 60c-Zr/TiO₂, and 80c-Zr/TiO₂ samples, respectively. Since the loading of ZrO₂ deposited on the TiO₂ NPs was very low, it did not lead to an increase in particle size (decrease of surface area) or poor anatase crystallinity, which would affect the photocatalytic efficiency of the samples. In order to verify this hypothesis, the BET surface area and crystal structure of all samples were analyzed. The BET surface area remained almost constant (∼70 m² g⁻¹) even after 80 cycles of ZrO₂ ALD (Fig. 2). The large surface area can provide a lot of surface sites for the adsorption of reactant molecules.

Fig. 2 ZrO₂ content and BET surface area of pure TiO₂ nanoparticles and TiO₂ nanoparticles deposited with ZrO₂ over different numbers of cycles of ALD.

Moreover, based on calculation, 1 g of 80c-Zr/TiO₂ contained 0.015 g ZrO₂ (5.68 g cm⁻³) and the thickness of ZrO₂ should be around 0.038 nm (0.015/5.68/70 × 1000 nm) if ZrO₂ formed as a film on the TiO₂ NPs.¹¹ The thickness is much thinner than that of a real layer of ZrO₂ (>0.1 nm). Thus, ZrO₂ was highly dispersed on TiO₂ without forming a film. As shown in Fig. 3, the TEM images of 45c-Zr/TiO₂ and 80c-Zr/TiO₂ are similar to that of pure anatase TiO₂ NPs, and the lattice fringes of 0.35 nm, 0.24 nm, and 0.19 nm correspond to the (101), (004), and (200) planes of anatase, respectively.¹² No ZrO₂ film was observed, but the ICP-AES (Fig. 2) and EDS (Fig. S1†) verified the existence of Zr on the TiO₂. It seems that ZrO₂ ALD follows an island growth mechanism (Volmer–Weber mechanism) during the initial stages of the ALD process, which could be similar to that of some metal ALD processes.¹³

Fig. 3 TEM images of (a) TiO₂, (b) 45c-Zr/TiO₂, and (c) 80c-Zr/TiO₂ nanoparticles.

As presented in Fig. 4, the XRD patterns of all samples show peaks appearing at 2θ = 25.3°, 37.8°, 48.1°, 53.9°, 55.2°, 62.8° and 75.2°, corresponding to the diffraction patterns of (101), (004), (200), (105), (211), (204) and (215), respectively, of the pure tetragonal phase of anatase TiO₂.¹⁴ There was no significant difference in any sample. The TiO₂ in all samples retained the anatase structure after ZrO₂ ALD and no peak corresponded to reflections from ZrO₂, which could be due to the low loading and amorphous structure of ZrO₂ on TiO₂. According to the XRD analysis, the TiO₂ crystal size was around 19 nm for all samples, which is close to the actual particle size of TiO₂ (∼20 nm). Raman analysis was also performed, and all samples showed three major Raman bands at 397, 517, and 640 cm⁻¹ (Fig. 5), which are attributed to the Raman-active modes of the TiO₂ anatase phase with symmetries of B1g, A1g, and Eg, respectively.¹⁵ Thus, our hypothesis that the surface area and anatase crystallinity of TiO₂ did not change after the ALD process was proved.

Fig. 4 XRD patterns of (a) TiO₂, (b) 5c-Zr/TiO₂, (c) 25c-Zr/TiO₂, (d) 45c-Zr/TiO₂, (e) 60c-Zr/TiO₂ and (f) 80c-Zr/TiO₂.

Fig. 5 Raman spectra of (a) TiO₂, (b) 5c-Zr/TiO₂, (c) 25c-Zr/TiO₂, (d) 45c-Zr/TiO₂, (e) 60c-Zr/TiO₂ and (f) 80c-Zr/TiO₂.

Secondly, a larger band gap corresponds to stronger redox ability, which leads to a reduction in the e⁻/h⁺ recombination rate and an improvement in photocatalytic activity. Since the band gap of ZrO₂ is around 4.6 eV and is larger than that of anatase TiO₂ (3.2 eV),¹⁶ the band gap of ZrO₂/TiO₂ samples was enlarged after ZrO₂ ALD.¹⁷ As shown in Fig. 6, S2 and S3,† the band gap of the samples kept increasing along with increasing numbers of ALD cycles, which is consistent with previous reports.^4d,18 Because the photocatalytic degradation process can be considered an electrochemical cell, the increase in band gap results in an enhanced oxidation–reduction potential.¹⁹ However, the photocatalytic activity of 60c-Zr/TiO₂ and 80c-Zr/TiO₂ was lower than that of 45c-Zr/TiO₂, despite the band gaps of 60c-Zr/TiO₂ and 80c-Zr/TiO₂ being larger than that of the 45c-Zr/TiO₂ sample. This can be explained by samples with too large a band gap being unable to take full advantage of UV light and therefore being unable to generate enough e⁻ and h⁺ pairs for MB degradation.

Fig. 6 (a) UV-visible absorption spectra, and (b) the band gap energy of the TiO₂ and ZrO₂/TiO₂ samples.

Thirdly, there is a fast transfer of the photo-formed electrons from the conduction band (CB) of ZrO₂ to that of TiO₂, when ZrO₂ was deposited on TiO₂ under UV irradiation, since the bottom of the CB edge of ZrO₂ is about 1.3 eV, which is higher than that of TiO₂.^4d,16 This electron transfer prevented radiative electron/hole recombination and thereby improved the photocatalytic activity of the 45c-Zr/TiO₂ sample. In order to evaluate the separation rate of the e⁻/h⁺ pairs, photoluminescence (PL) analysis was carried out. As shown in Fig. 7, there is only one peak at 433 nm, corresponding to the reflection from the anatase phase of TiO₂. No other peak was observed in the wavelength range of 300–600 nm. With an increase in the number of ZrO₂ ALD cycles, the PL intensity decreased greatly, which indicated that the separation efficiency of the e⁻/h⁺ pairs was enhanced for the ZrO₂/TiO₂ samples. In other words, the existence of ZrO₂ helped trap the photo-generated charge carriers and inhibited the recombination of the e⁻/h⁺ pairs. As shown in Fig. 8, e⁻ and h⁺ separation may occur between TiO₂ and ZrO₂ in ZrO₂/TiO₂ since the energy level of TiO₂, for both the valence band (VB) and CB, falls within the band gap of ZrO₂. When the electrons were generated from TiO₂ and ZrO₂, most of the electrons from the CB of ZrO₂ automatically drifted to the CB of TiO₂. Therefore, the e⁻/h⁺ pair recombination could be inhibited and the photocatalytic activity improved. All factors mentioned above worked collectively and resulted in improved photocatalytic activity of 45c-Zr/TiO₂.

Fig. 7 Photoluminescence spectra of TiO₂ and ZrO₂/TiO₂ samples excited at 280 nm.

Fig. 8 Proposed mechanism for the photoexcited electron–hole separation and transport processes at the ZrO₂/TiO₂ interface under UV irradiation.

It was also noted that the TiO₂ catalyst containing 1.1 wt% ZrO₂ (45c-Zr/TiO₂) enhanced the photocatalytic activity the most and it was the optimal amount of deposition for the degradation of MB in this study. However, the optimal ZrO₂ amount is not consistent with that reported in the literature, which reported that the optimal ZrO₂ loading was more than 5%. This could be due to the different preparation methods for the ZrO₂/TiO₂ samples.^4a–c Higher optimal ZrO₂ loading was needed via traditional ZrO₂ doping methods because some ZrO₂ was inserted into the interior matrix of the TiO₂ particles and could not work with TiO₂ synergistically. In contrast, in this study, ZrO₂ was highly dispersed on the surface of TiO₂ NPs via ALD and thereby the optimal ZrO₂ content was significantly lower. On the other hand, the enlarged band gap of the ZrO₂/TiO₂ samples in this work becomes a limitation and hinders their practical applications under visible light, although ZrO₂ deposition improved the photocatalytic performance of anatase TiO₂ under UV light. N,²⁰ S,²¹ CeO₂²² and Au/Pd²³ doped TiO₂ catalysts have been reported, and they could decompose organic pollutants under visible light. It is vital and valuable to expand the practical use of TiO₂-based photocatalysts. The optimization and enhancement of ZrO₂/TiO₂ photoactivity under visible light are being pursued by this group.

4. Conclusions

In summary, different numbers of cycles of ALD were used to deposit ZrO₂ on TiO₂ powders. The 45c-Zr/TiO₂ catalyst showed the highest photocatalytic activity and had a more than ten-fold photocatalytic activity enhancement over pure TiO₂ for the degradation of MB under UV light due to the fact that the fast electron transfer from the CB of ZrO₂ to that of TiO₂ prevented radiative electron/hole recombination. This factor worked collectively with another two factors, the maintained high surface area and the larger band gap of ZrO₂/TiO₂, and thus resulted in the significantly improved photocatalytic activity of the samples under UV light. ZrO₂ ALD is a novel and effective strategy to significantly improve the photocatalytic activity of TiO₂, and it is a potentially promising way to enhance the activity of other photocatalysts.

Conflicts of interest

The authors declare no competing financial interest.

Acknowledgements

Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund for partial support of this research. The authors thank Dr Jingjing Qing at the Materials Research Center at Missouri University of Science and Technology for TEM analysis. The authors also thank Naveen K. Mahenderkar and Prof. Jay A. Switzer in the Department of Chemistry for their assistance with diffuse reflectance UV-vis measurement, and Prof. Richard Brow in the Department of Materials Science and Engineering for the assistance with Raman and photoluminescence analysis.

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Footnotes

† Electronic supplementary information (ESI) available: Data on photocatalytic activity, Raman and UV-visible diffuse reflectance data and a comparison of the photocatalytic activity of various photocatalysts. See DOI: 10.1039/c8ra03423k

‡ Current address: Energy and Environmental Directorate, Pacific Northwest National Laboratory, Richland, Washington 99354, USA.

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