Modified hierarchical TiO₂ NTs for enhanced gas phase photocatalytic activity

Abstract

In this paper, three kinds of noble metal nanoparticles (NMNs) were successfully loaded on hierarchical TiO₂ nanotube arrays (TiO₂ NTs) to improve photocatalytic (PC) activity of gas phase pollutants. The hierarchical TiO₂ NTs, with unique top-nanoporous and bottom-nanotubular structure, were prepared through a facile two-step anodization method, and then the noble metal nanoparticles were loaded on the TiO₂ NTs by means of a photo-reduction method. The gas phase photocatalytic activity of TiO₂ NTs and NMNs/TiO₂ NTs were estimated by decomposition of gaseous methanol. The formation of Schottky junctions between TiO₂ NTs and NMNs significantly improved the PC due to they could significantly accelerate the electron transfer and thus reduction of the recombination of photogenerated electrons and holes.

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Introduction

Photocatalytic (PC) technology has received widespread attention for pollutant decomposition, and a lot of materials, especially metal oxide semiconductors, have been widely utilized as photocatalysts.^1–3 Among them, titanium dioxide (TiO₂) is regarded as one of the most promising photocatalysts because of its unique electronic properties, high resistance to photo-corrosion and low cost.^4–6 TiO₂ nanotube arrays (TiO₂ NTs), fabricated by electrochemical anodization process, have been demonstrated to be one of the most efficient materials of PC applications for degradation of environmental pollutants and conversion of solar energy because of their unique monodirectional electron transfer, high electron mobility and one-dimensional nanostructure.^7–9 In this paper, we fabricated a unique hierarchical TiO₂ combined nanostructure of top-nanorings and bottom-nanotubes by a two-step anodization method, which showed better PC activity than pure TiO₂ NTs due to its higher hierarchical and light scattering activity.¹⁰

However, unmodified TiO₂ as a wide bandgap semiconductor is limited to PC activity only under UV irradiation. For further enhancing PC activity of TiO₂ NTs and harvesting long wavelength radiation, noble metal nanoparticles (NMNs) modification were an effective option. Previous researches^11–16 have shown that the deposition of noble metals in the surface of TiO₂ nanoparticles can improve the photocatalytic activity of TiO₂ significantly. In this paper, three kinds of NMNs (Ag, Au, Pt) were investigated to select one would be best favorable for formation a Schottky junction with TiO₂ (Scheme 1). As the electron affinity of anatase TiO₂ is significantly lower than the work function of noble metal, which will induce higher electronic potential between noble metal and TiO₂, accelerating the electron transfer and inhibits the electron–hole pairs recombination. The significance of this work provide a fundamental understanding not only to the fabrication but also the utility of Schottky junctions for enhanced environmental remediation method.

Scheme 1 Schematic illustration of the energy bands between NMNs and TiO₂ NTs.

Even the PC studies on TiO₂ with/without NMNs have been extensively studied, however, previous studies were focused on PC activity in aqueous solution,^17–20 the PC activity in gas phase has rarely studied. Compared with liquid phase reaction process, gas phase photocatalytic oxidation reaction features high reaction velocity, good exploitation rate of light, large volume flow, which could oxidize reactant completely and free of solvent influence,^21–24 and therefore lead to the wide application. To the best of our knowledge, the gas phase PC activity of NMNs decorated hierarchical TiO₂ NTs has not been reported. In this study, the gas phase PC activity of TiO₂ NTs and NMNs/TiO₂ NTs were estimated by decomposition of evaporated methanol.

Experimental

Chemicals and materials

Titanium foil (0.2 mm thick, 99.6%, Strem Chemicals) was cut into pieces of 25 × 10 mm². Silver nitrate(I) (AgNO₃, Sigma-Aldrich Chemicals, 99%) was used as the precursor of Ag nanoparticles. Gold(III) acid chloride trihydrate (HAuCl₄·4H₂O, Sigma-Aldrich Chemicals, 99%) was used as the precursor of Au nanoparticles. Platinum(II) acetylacetonate (Pt(AcAc)₂, Sigma-Aldrich Chemicals, 99%) was used as the precursor of Pt nanoparticles. Ethylene glycol (EG), ammonia fluoride (NH₄F) and methanol were purchased from Acros Organics and used as received.

Preparation of hierarchical TiO₂ NTs

The hierarchical TiO₂ NTs were fabricated by a two-step anodization process (Scheme 2a–d). Prior to anodization, the Ti foils were first ultrasonically cleaned with ethanol and room-temperature distilled water, followed by drying in N₂ gas. The anodization was carried out using a conventional two-electrode system with the Ti foil as an anode and a Pt gauze (Aldrich, 100 mesh) as a cathode respectively. All electrolytes consisted of 0.5 wt% NH₄F in EG solution with 2 vol% water. All the anodization experiments were carried out at room temperature. In the first-step anodization, the Ti foil was anodized at 50 V for 30 min, then the as-grown nanotube layer was ultrasonically removed in deionized water, leaving compact two-dimensional hexagonal pattern on the surface of the Ti foil alone. The patterned Ti foil then underwent the second anodization at 20 V for 30 min, in which the hexagonal pattern formed top-porous structure and subjective NTs grew below the top-porous layer.²⁵ After second-step anodization, the prepared TiO₂ NTs sample was cleaned with distilled water and dried off with N₂ gas. The as-anodized TiO₂ NTs were annealed in air at 450 °C for 1 h with a heating rate of 5 °C min⁻¹.

Scheme 2 Two-step anodization processes for fabrication of hierarchical TiO₂ NTs and photocatalytic reduction for decoration of NMNs.

Decoration of NMNs on TiO₂ NTs

Photocatalytic reduction method²⁶ was used to deposit NMNs on the TiO₂ NTs using AgNO₃, HAuCl₄, Pt(AcAc)₂ as precursor (Scheme 2e). The AgNO₃ aqueous solution was 1.0 × 10⁻² mol L⁻¹ (Fig. S1, ESI†), the HAuCl₄ aqueous solution was 2.43 × 10⁻⁴ mol L⁻¹ (Fig. S2†), the Pt(AcAc)₂ (ref. 27) was diluted in deionized water and ethanol with a water–ethanol volume ratio of 10 : 1, and the concentration of Pt(AcAc)₂ was fixed at 1.0 × 10⁻³ mol L⁻¹ (Fig. S3†). The Ti foil containing the prepared TiO₂ NTs were soaked into the precursor solution for 24 h and then was irradiated in this solution with a 300 W high pressure mercury lamp for 30 min to reduce the absorbed Ag⁺ to Ag⁰, Au³⁺ to Au⁰ and Pt²⁺ to Pt⁰ by photocatalysis at the expense of water oxidation.

Characterization of NMNs/TiO₂ NTs

The morphology of nanotubular structures and distribution of nanoparticles were determined by filed-emission scanning electron microscope (FESEM, Zeiss SigmaHV) and transmission electron microscope (TEM, Tecnai T12). The chemical compositions of NMNs/TiO₂ NTs were analyzed by energy dispersive spectrometer (EDS) equipped with FESEM. The crystal structures were characterized by grazing incidence X-ray diffraction (GIXRD) analysis by an X-ray diffractometer (Rigaku D/MAX-2200) with Cu Kα source in the range of 2θ = 20–70°. UV-visible diffuse reflectance spectra was obtained on a UV-visible spectrophotometer (Lambda 750) equipped with an integrating sphere assembly. Fine BaSO₄ powders were used as reflectance standard.

The photoelectrochemical measurements were carried out in a three-electrode cell in 0.5 M Na₂SO₄ as the aqueous electrolyte with a electrochemical analyzer (WaveNow, PINE). TiO₂ NTs and NMNs/TiO₂ NTs were used as working electrodes, Ag/AgCl was used as the reference electrode and Pt foil was used as the counter electrode. The photoresponse was evaluated under chopped light irradiation (light on/off cycles: 60 s) at a fixed electrode potential of 0.5 V. The photocurrent was measured under irradiation from a 300 W high pressure mercury lamp. All measurements were carried out under ambient conditions at room temperature.

Gas phase PC degradation experiments

The gas phase PC activities of TiO₂ NTs and NMNs/TiO₂ NTs were detected by decomposition of evaluated methanol (5 μL). The decomposition rate of methanol was calculated through Fourier transform infrared (FTIR) spectroscopy (Bruker Tensor27). All the PC measurements were performed in a homemade cylindrical cell with 5 cm length and 3.6 cm width (Fig. S4†), in which the TiO₂ NTs and NMNs/TiO₂ NTs samples were contained respectively. PC degradation of methanol was carried out with the irradiation of a 300 W high pressure mercury lamp (XC140408, Shanghai YaMing Lighting Co., Ltd).

Results and discussion

Fig. 1a shows the SEM image of TiO₂ NTs in the first step anodization, it can be seen from the figure that the structure of TiO₂ NTs is very loose. Fig. 1b shows the compact two-dimensional hexagonal pattern left on the Ti foil after the one-step TiO₂ NTs were removed by ultrasonic treatment in deionized water. Fig. 1c shows a top view SEM image of hierarchical TiO₂ NTs in the second step. Individual top hexagonally porous structure is fairly observed, with an average diameter of ∼150 nm and a wall-thickness of ∼20 nm (Fig. S5†). A cross-sectional view of the TiO₂ NTs is presented in the bottom-left inset of Fig. 1c, indicates corresponding tubular structure with a length of 1 μm. It can be seen from Fig. 1d and e that Ag and Au nanoparticles are decorated on the surface of TiO₂ NTs. Fig. 1f is a high-magnification top-view SEM image of Pt/TiO₂ NTs. The TEM image of Pt/TiO₂ NTs is presented in the inset of Fig. 1f, which clearly showed Pt nanoparticles are not only decorated on the outer side of NT but also into the inter side of NTs with same diameter size of ∼20 nm as shown in SEM image.

Fig. 1 Morphology of TiO₂ NTs: (a) TiO₂ NTs in the one-step anodization; (b) the Ti surface after the removal of one-step TiO₂ NTs layer; (c) hierarchical TiO₂ NTs in the second anodization step; hierarchical Ag/TiO₂ NTs (d), Au/TiO₂ NTs (e) and Pt/TiO₂ NTs (f), the inset of (f) shows the TEM image of Pt nanoparticles inside the TiO₂ NTs.

Optical absorption and crystalline are two important parameters for PC performance on TiO₂ materials. Materials with better optical absorption and higher crystallinity will subsequently result in a better PC performance. Therefore, UV-vis absorption spectra and XRD measurement were employed to characterize the optical absorption properties and crystallinity of NMNs/TiO₂ NTs (Fig. 2 and 3). As shown in Fig. 2, all modified TiO₂ NTs samples depicted better UV absorption than unmodified TiO₂ NTs samples, as the Schottky junctions facilitate the electron transfer and thus reduce the recombination of electrons and holes, both contributing to enhanced PC performance. The XRD patterns of the hierarchical TiO₂ NTs and NMNs/TiO₂ are shown in Fig. 3. Clearly, both of them presented pure crystalline anatase with strong preferential orientation of (101). In addition, the NMNs/TiO₂ sample shows an extra noble metal XRD pattern with strong orientation of (111), which implies that the noble metal nanoparticles are successfully doped on the TiO₂ NTs. The EDS spectra in Fig. 3 showed another evidence for noble metal existence on the TiO₂ NTs.

Fig. 2 Diffuse reflectance UV-vis absorption spectra of the NMNs/TiO₂ NTs.

Fig. 3 XRD patterns and EDS spectra of TiO₂ NTs and NMNs/TiO₂ NTs: (a and b) TiO₂ NTs and Ag/TiO₂ NTs; (c and d) TiO₂ NTs and Au/TiO₂ NTs; (e and f) TiO₂ NTs and Pt/TiO₂ NTs.

The photoresponse of the TiO₂ NTs electrode and NMNs/TiO₂ NTs electrode were investigated under illumination of 300 W high pressure mercury lamp with a bias of 0.5 V (vs. Ag/AgCl). It can been seen from Fig. 4, all modified TiO₂ NTs samples depicted better reproducibility and stability as the light was turned on and off. The photocurrent decreased almost to zero instantly as soon as the illumination was interrupted and the photocurrent came back close to the original value as the illumination was turned on. It is interesting to note that the Pt/TiO₂ NTs composite electrode exhibited a photocurrent density of ∼20 μA cm⁻², which was about 4 times larger than that of the unmodified TiO₂ NTs electrode with a photocurrent density of ∼5 μA cm⁻². The remarkably enhanced photocurrent may be ascribed to the formed Schottky junctions between TiO₂ NTs and NMNs in which have a fast electron transport and a more efficient separation of the photogenerated holes and electrons.

Fig. 4 Amperometric I–t curves at an applied potential of 0.5 V under illumination of 300 W high pressure mercury lamp with 60 s light on/off cycles.

The photocatalytic activity of NMNs/TiO₂ NTs are evaluated by decomposition of methanol. Fig. 5 shows the FTIR spectra of methanol decomposition as a function of irradiation time on TiO₂ NTs, Ag/TiO₂ NTs, Au/TiO₂ NTs and Pt/TiO₂ NTs respectively.

Fig. 5 Decomposition of methanol on TiO₂ NTs (a), Ag/TiO₂ NTs (b), Au/TiO₂ NTs (c) and Pt/TiO₂ NTs (d) respectively as a function of irradiation time.

Before the illumination, the initial methanol processes the bands at 1031, 1055, 1340, 2860, 2950, and 3685 cm⁻¹, which bands correspond to the C–O stretching, CH₃ rocking, O–H bending, C–H parallel symmetric stretching, C–H out of plane asymmetric stretching, O–H stretching, respectively.^28–31 As the increase of irradiation time, all these bands started to decrease in intensity and new peaks appeared at 669, 1456, 1640, 1750, 2360 and 3440 cm⁻¹. The band at 1456 cm⁻¹ can be assigned to the out of plane asymmetric bending of C–H, 1640 cm⁻¹ proved the formation of isolated C=C bonds, and 1750 cm⁻¹ can be assigned to the C=O stretching vibration modes.^32,33 The gaseous CO₂ is a linear molecule with two infrared active absorption bands at 2360 cm⁻¹ (antisymmetric stretching mode) and 669 cm⁻¹ (bending mode).^34,35 Hence, one can conclude that the primary two peaks centered at 2360 cm⁻¹ and 669 cm⁻¹ imply the formation of CO₂ gas when methanol is decomposed. In the end, after 60 min, just CO₂ and H₂O existence as the results of completed decomposition of methanol on the Pt/TiO₂ NTs sample.

Based on the photocatalytic experimental data, we summarized the CO₂ transmittance peak height (2360 cm⁻¹, CO₂) for TiO₂ NTs, Ag/TiO₂ NTs, Au/TiO₂ NTs and Pt/TiO₂ NTs respectively in Fig. 6a. The experimental data of Fig. 6a were found to fit approximately a pseudo-first-order kinetic model by the linear transforms f(t) = f_inf[1 − exp(−kt)] (f is peak height, f_inf is peak height at infinite time, and k is rate constant).³⁶ The corresponding curves were presented in Fig. 6b, and the values of rate constant k_{TiO2 NTs} = 0.0142 < k_{Ag/TiO2 NTs} = 0.0731 < k_{Au/TiO2 NTs} = 0.1031 < k_{Pt/TiO2 NTs} = 0.1558. Clearly, the Pt/TiO₂ NTs showed higher k value than hierarchical TiO₂ NTs and other noble metal modified TiO₂ NTs, which implied fastest decomposition rate and highest gas phase PC activity. The enhanced PC activity can be ascribed the formation of Schottky-junction between NMNs and TiO₂ NTs.

Fig. 6 (a) Variation of the peak height of 2360 cm⁻¹ corresponding to the normal vibration of CO₂ molecules derived from the FTIR transmittance spectra with the irradiation time; (b) comparation performance of constant k on TiO₂ NTs and NMNs/TiO₂ NTs respectively.

On the basis of the experimental results, the mechanism of gas phase PC methanol was also discussed here. As illuminated with energy higher than the band gap of TiO₂, electrons (e⁻) on valence band will be excited to the conduction band, at the same time in the valence band a positively charged hole (h⁺) is produced, those electron–holes generated after excitation move quickly to surface from inside and the electrons will transfer to NMNs through the Schottky interface of TiO₂ and NMNs. Under the condition of PC oxidation on methanol, the oxygen adsorbing on the surface of the catalyst was reduced to ˙O₂⁻ by photoelectrons on TiO₂ and NMNs surface, and trace water is oxidized to ˙OH by such holes, which both provide highly active oxidant for the methanol oxidation. The ˙O₂⁻ and ˙OH radicals attack C–H bond in methanol, and then with lively hydrogen atoms to form new free radicals, which stimulates chain reaction, and first oxidation is to form aldehyde then to formic acid, and then ultimately make methanol deeply decompose to H₂O and CO₂. The reaction process is shown in Scheme 3.

Scheme 3 Photocatalytic mechanism of NMNs/TiO₂ NTs.

Conclusions

In summary, noble metal nanoparticles decorated hierarchical TiO₂ NTs were successfully designed with a photocatalytic reduction method and tested the PC activity of degradation of gaseous methanol. It can know that the Pt/TiO₂ NTs afforded the best degradation effect of methanol into CO₂ and have the biggest degradation rate in all three kinds of NMNs/TiO₂ NTs. It had been revealed that noble metal modified the inherent properties of TiO₂ NTs as well as affected the PC activity. The NMNs/TiO₂ NTs meaningfully proved the electron transfer and reduced the recombination of photoelectrons and holes. Compared with the pure TiO₂ NTs, the Pt/TiO₂ NTs showed an outstanding enhancement on PC decomposition of methanol.

Acknowledgements

The authors are grateful to the partially supported by China Postdoctoral Science Foundation (No. 2013M540269) and Natural Science Foundation of Heilongjiang Province of China (No. LC2012C35).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra05981j

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