Tuning optical band gap by electrochemical reduction in TiO₂ nanorods for improving photocatalytic activities

Abstract

In this study, we investigate the tuning of the optical band gap of TiO₂ nanorods (TiO₂ NRs) by electrochemical methods for improving their photocatalytic activities. A seed layer prepared by RF-magnetron sputtering is employed to increase the adhesion between TiO₂ NRs and substrate to prevent the peel-off of TiO₂ NRs from substrate during electrochemical reduction process. The morphological study shows the stability of TiO₂ NRs structure after reduction process. The electrochemical reduction process increased the amount of Ti³⁺ (a reduced state of Ti⁴⁺) and oxygen vacancy by 2.2% and 3.6%, respectively. The process also tune the optical band of TiO₂ NRs from 3.0 eV to 2.84 eV due to the up-shift of valence band to Fermi level. The reduced band gap of NRs improve the photocatalytic activities by 1.48 times due the increase of its absorbance range from UV to visible in solar spectrum. The photocatalytic activities were 24.3 times improved by depositing a ultrathin layer of Pt as electron collector.

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Introduction

Its excellent chemical stability, nontoxicity, catalytic activity and low cost make TiO₂ an attractive photocatalytic material for a range of applications, such as water purification, self-cleaning, and solar energy conversion.^1–4 On the other hand, the use of TiO₂ in the visible region is limited by its large energy band gap. In recent years, significant effort has been devoted to improving the visible absorbance of TiO₂ to enhance its photocatalytic activity in solar light.^5–19

A well-known approach is hetero-doped TiO₂ using impurity cations, Cr, Ag, etc., and anions, N, F, C, etc.^6,9–12 Nitrogen-doped TiO₂ may produce sub-levels by hybridization of the N 2p and O 2p states near the valence band of TiO₂ that would improve the visible light activity.¹² On the other hand, it can produce carrier recombination centers that may reduce the catalytic activity in the UV region.^20–23 Self-doped TiO₂ is a useful method for improving the photocatalytic activities of TiO₂ in the UV-Vis region by generating a sub-band level in the TiO₂ band gap and not involving structure defects.^24–29 Naldoni et al. reported that the reduced band gap of Ti³⁺ self-doped TiO₂ results from the slight up-shift of the valence band (VB) to the Fermi level.²¹ Kang et al. also reported the same behavior of the up-shift of VB.³⁰ Moreover, the oxygen vacancies (V_O) formed during Ti³⁺ self-doping increase the carrier density and charge transfer.³¹ Well-known approach could be the thermal annealing method used famously for Ti³⁺ self-doped TiO₂;^32–36 however, it require a carrier gas and control system for temperature and pressure. Recently, an electrochemical reduction method has been adopted widely because of its simple mechanism, low cost and fast self-doping properties. On the other hand, owing to the lattice mismatch between rutile TiO₂ and the substrate,³⁷ electrochemical self-doped approach is only employed in TiO₂ nanotubes anodized from Ti-foil.^30,38–40

In this study, the electrochemical reduction method was used to self-dope Ti³⁺ into TiO₂ nanorods (TiO₂ NRs) grown on a FTO substrate for optical band gap tuning. To prevent the peel-off of TiO₂ NRs from the substrate due to large lattice mismatch, a TiO₂ seed layer was introduced by RF-magnetron sputtering to enhance the adhesion between TiO₂ and FTO substrate. X-ray photoelectron spectroscopy and UV-Vis spectroscopy were employed to examine the tuning of the TiO₂ optical band gap. In a study of photocatalysis, the reduced TiO₂ nanorods (rTiO₂ NRs) exhibited better performance compared to the as-grown TiO₂ NRs. The photocatalytic activities of rTiO₂ NRs were enhanced significantly by decorating an ultrathin layer of Pt on the rTiO₂ NRs as an electron collector by RF magnetron sputtering.

Experiment

Synthesis of TiO₂ nanorods

Prior to synthesis, FTO substrate (F:SnO₂, Tec 20, 8 Ω □⁻¹) was cleaned by immersing in a mixture of DI water, acetone and 2-propanol (1 : 1 : 1 of volume ratio) and ultra-sonicated for 30 min. TiO₂ seed layer was deposited on FTO substrate using RF-magnetron sputtering (details in Table S1†). To synthesize TiO₂ NRs by hydrothermal method, 0.65 mL of titanium(IV) n-butoxide (TBO, Aldrich Chemicals, 97%), 25 mL of HCl 38%, and 25 mL of DI water were mixed and stirred for 30 min.

RF-deposited TiO₂ seed layer on FTO substrate was placed face side down in the grown solution in a Teflon beaker. To grow the TiO₂ NRs, the Teflon beaker was then sealed in a stainless steel auto-clave at 165 °C for 10 h. The as-grown TiO₂ NRs were washed with DI water and ethanol, and then annealed at 450 °C for 60 min. To compare the quality of RF-sputtered seed layer, another TiO₂ NRs were also grown on a seed layer prepared by sol–gel method. The seed layer was prepared by spin coating of TiO₂ polymeric solution.^41,42

Electrochemical reduction of TiO₂ NRs and Pt decoration

The synthesized TiO₂ NRs were reduced using an electrochemical reduction method; connecting TiO₂ NRs/FTO (working electrode) and a Pt wire (counter electrode, 1 mm of diameter and 9 cm of length, 99.95%). Electrochemical reduction process was done in a 0.1 M of NaOH electrolyte with negative bias, −3 V vs. Pt for 5 to 20 min. The final rTiO₂ NRs were washed in DI water, and dried with dry N₂. To improve photocatalytic activities, an ultrathin layer of Pt as electron collector was deposited on surface of the rTiO₂ NRs by using a RF-magnetron sputtering at 30 W for 2 min (details in Table S1†).

Characterization

The morphology and crystallinity of the TiO₂ NRs and rTiO₂ NRs were studied using field emission scanning electron microscopy (FE-SEM, JEOL, JSM 7600F) and a transmission electron microscope (FE-TEM, JEI, Tecnai G2F30 S-Twin). The crystalline structure was characterized by a X-ray diffraction (XRD, RIGAKU, D/MAX 2500-V/PC, using Cu Kα radiation, λ = 1.540598 Å). X-ray photoelectron spectroscopy (XPS) using a Leybold spectrometer with an Al Kα monochromatic beam (1486.6 eV, ESCALAB250, Theta Probe XPS system) and ultra-violet photoelectron spectroscopy (UPS) study with a He(I) beam source (21.21 eV) was done. UV-Vis spectra were recorded using an optical spectrophotometer (Varian, Cary 5000).

Photocatalytic activities measurement

Photocatalytic performances of the TiO₂ NRs, rTiO₂ NRs and Pt decorated rTO₂ NRs were evaluated by photo-degradation of 15 mL of 15 μM methylene blue (MB, C₁₆H₁₈N₃SCl) under Xe-lamp irradiation. The intensity of the light source was calibrated with a standard Si-photodiode detector of a KG-3 filter (Newport Co., Oriel) to AM 1.5G illumination of 100 mW cm⁻². Photocatalytic degradation of MB was determined from the absorbance of MB solution at 665 nm using a UV-Vis spectrophotometer.

Results and discussion

Fig. 1(a) shows schematically the growth process of TiO₂ NRs, rTiO₂ NRs and Pt decorated rTiO₂ NRs on a RF-magnetron sputtered TiO₂ seed layer/FTO substrate. The RF-sputtered TiO₂ seed layer plays an important role in enhancing the adhesion between the grown TiO₂ NRs and substrate. The insets in Fig. 1(b) and (c) show photographs of the TiO₂ NRs before and after reduction, respectively. The TiO₂ NRs has maintained their quality, indicating the good adhesion between the TiO₂ NRs and substrate. On the other hand, the rTiO₂ NRs on sol–gel TiO₂ seed layer show poor adhesion as compared to those on the RF-magnetron sputtered seed layer (Fig. S1†). Moreover, the RF-sputtered seed layer exhibited higher mechanical durability as compared to sol–gel.^43,44 To check the adhesion more closely, the sample was analyzed by FE-SEM before and after the reduction process, as depicted in Fig. 1(b) and (c), respectively. The TiO₂ NRs preserved their quality, which further confirmed the excellent adhesion between TiO₂ NRs and substrate.

Fig. 1 (a) Schematic diagram of experimental sequence. (b and c) FE-SEM images of (b) TiO₂ NRs and (c) rTiO₂ NRs (insets: photograph image of (b) TiO₂ NRs and (c) rTiO₂ NRs on FTO substrate, indicating that the TiO₂ NRs was changed from white to dark brown after the reducing process while the morphology was stable). (d) TEM (inset: a low-magnification image) and (e) HR-TEM image of TiO₂ NRs grown along the [001] direction with an average length of 1.5 μm and diameter of 65 nm. (f) XRD result of TiO₂ NRs and rTiO₂ NRs. Two peaks are appeared at 30.64° and 31.96° 2θ after electrochemical reduction by the formation of V_O. (g) TEM image and (h–k) EDS mapping images of the ultrathin Pt decorated rTiO₂ NRs. The inset in (h) shows Pt decorated rTiO₂ NRs used EDS mapping.

The TEM image in Fig. 1(d) shows that the TiO₂ NRs are approximately 1.5 μm in length and 65 nm in diameter. The HR-TEM image shown in Fig. 1(e) taken at the highlighted region in Fig. 1(d) shows that the growth direction of TiO₂ NR is along the (001) direction, which confirms the typical TiO₂ NR structure. This observation is similar to previous reports.⁴⁵ Fig. 1(f) show the XRD results of the TiO₂ NRs and rTiO₂ NRs. The diffraction peaks at 26.42°, 33.68°, 37.68°, 51.46°, 61.54° and 65.46° 2θ (gray circles) correspond to the crystalline structure of the FTO substrate (S.G. P4₂/mnm, JCPDS no. 00-046-1088, a = b = 0.4750 nm, c = 0.3198 nm). The XRD peaks at 36.04°, 41.22°, 54.30°, 62.70° and 68.92° 2θ (dark squares) are consistent with the tetragonal rutile phase of TiO₂ (S.G. P4₂/mnm, JCPDS no. 01-070-7347, a = b = 0.4593 nm and c = 0.2961 nm).⁴⁶ The rutile structure has a 4₂ screw axis along the c-axis; hence, the domination of (110) faces indicates the promoted growth of the TiO₂ NR along the (001) direction.^47–49 After electrochemical reduction, two XRD peaks (red diamonds) are appeared at 30.64° and 31.96° 2θ which match to the (2̄03) and faces of Ti₄O₇ (S.G. P1, JCPDS no. 77-1390) lacking oxygen atoms. This shows that the electrochemical process is suitable for generating oxygen vacancies (V_O) in rTiO₂ NRs. Owing to the lower band gap of rTiO₂ NRs as compared to the TiO₂ NRs, the rTiO₂ NRs can perform better photocatalytic behavior in UV-Vis irradiation.

On the other hand, the charge recombination can affect the rTiO₂ NRs performance. Therefore, to avoid the recombination of photo-generated electron–hole pairs, an ultrathin layer of Pt was deposited on the rTiO₂ NRs by RF-magnetron sputtering. The Pt distribution on rTiO₂ NRs was analysed HR-TEM, as shown in Fig. 1(g) and S2.† It shows that amorphous Pt is uniformly decorated on rTiO₂ NRs surface with the average width and thickness are about 2.2 and 3.4 nm, respectively. Energy-dispersive X-ray spectroscopy (EDS) elementary mapping images in Fig. 1(h)–(k) show the excellent coated (almost conformal) of Pt on whole interface of exposed rTiO₂ NRs.

The sample was further characterized by XPS to examine the effect of the reduction process on atomic bonding in the TiO₂ NRs. The Ti 2p^3/2 spectra before and after reduction are given in Fig. 2(a). The figure shows a weak peak at 456.9 eV and a strong peak at 458.3 eV, corresponding to Ti³⁺ and Ti⁴⁺, respectively. The intensity of the Ti³⁺ peak increased after the reduction process, which indicates the reduction of TiO₂. The total increase was found to be 2.2%. Moreover, the change in the relative intensity of Ti³⁺ indicates the self-doping of Ti³⁺ into TiO₂ NRs.^39,50,51 The O 1s survey in Fig. 2(b) shows the presence of oxygen vacancies (V_O) at 531.0 eV, strong binding of crystalline Ti–O in TiO₂ at 529.5 eV, and weak binding of Ti and oxygen at the TiO₂ surface at 527.8 eV.^50,51 The intensity of V_O increased by 3.6% after the reduction process.

Fig. 2 XPS core levels spectra of (a) Ti 2p^3/2 and (b) O 1s of TiO₂ NRs and rTiO₂ NRs (inset: magnification graph of (a) Ti³⁺ peak and (b) V_O peak). (c) Time dependent proportion change of V_O and Ti³⁺ obtained from deconvolution of XPS results. Ti³⁺ content is likely to saturate at a mean value of 9.4% after 5 min of reduction time, V_O defect increase linearly with reduction time. (d) Schematic description of electrochemical reduction process. The reaction of surface oxygen with water generates V_O (O–Ti⁴⁺–V_O) and electrons supported from counter electrode could inject to Ti⁴⁺ to form Ti³⁺.

Fig. 2(c) shows the reduction time dependent formation of Ti³⁺ and V_O in rTiO₂ NRs, which was calculated from XPS data deconvolution shown in Fig. S3.† The Ti³⁺ was found to be saturated at an average 9.4% after a 5 min reduction time, while the V_O defects is found to be saturated at about 24% after 10 min of reduction time. This phenomenon can be explained by a reduction mechanism shown schematically in Fig. 2(d). Electrons supported from the counter electrode could inject into Ti⁴⁺ to form Ti³⁺. The larger amount of electron injected into Ti⁴⁺ may rapidly reduce it to Ti³⁺ (saturated after 5 min reduction). Thus, the bonding of oxygen and Ti³⁺ is weaken then oxygen atoms at surface react with hydrogen ions (H⁺, separated from water) to generate oxygen vacancies.

The VB position of the TiO₂ NRs and rTiO₂ NRs were analyzed from the VB XPS spectra, as shown in Fig. 3(a). The VB edge of TiO₂ NRs and rTiO₂ NRs observed at 2.25 eV and 2.09 eV, respectively, under the Fermi level, which indicates that the VB edge of the rTiO₂ NRs was shifted 0.16 eV to the Fermi level.^21,30 The same behavior can be observed in plots of the Kubelka–Munk function vs. the photon energy transformed from the total diffuse transmittance (Fig. S4†) shown in Fig. 3(b). The TiO₂ NRs reveal a band gap of 3.0 eV,⁵² whereas the rTiO₂ NRs exhibit an optical band gap of 2.84 eV, which is 0.16 eV lower. Fig. 3(c) shows work-function deduced from UPS spectra before and after the electrochemical reduction of TiO₂ NRs. Gap between CB and Fermi level was maintained, while the Fermi level of rTiO₂ NRs was raised 0.78 eV after reduction, as shown in Fig. 3(d). Therefore, reduced optical band gap of rTiO₂ NRs is attributed to up-shifting of VB. Every result in the UV-Vis transmittance and XPS (UPS) analysis are in well agreement. These results confirm that the surface defect in the rTiO₂ NRs reduce its optical band-gap by up-shifting the VB edge, which is in good agreement with previous reports.^21,30,38,51 Fig. 3(e) and S5† presents the time evolution of the VB and the tuning of the optical band gap with respect to the reduction time. Both of VB position and band gap of rTiO₂ NRs are found to be saturated at about 2.1 and 2.85 eV, respectively, after 10 min of reduction time. The modification of the optical band gap of the rTiO₂ NRs is similar to the shifted value of the VB edge to the Fermi level for all reduction times. Therefore, the reduction of the optical band-gap of rTiO₂ NRs by the electrochemical reduction was attributed to the modification of the VB edge.^21,30,39,53

Fig. 3 (a) VB position analyzed from VB XPS spectra, (b) plots of Kubelka–Munk function vs. the photon energy and (c) work-function analyzed from UPS spectra of TiO₂ NRs and rTiO₂ NRs. (d) Schematic description of band structure of TiO₂ NRs before and after reduction. (e) Evolution of valence band and optical band gap with respect to the reduction time. After reduction for 15 min, the Fermi level was raised 0.78 eV and the VB edge is shifted up 0.16 eV to the Fermi level. The modification of the optical band gap is similar to that the VB edge.

Fig. 4 shows the photocatalytic activities of the samples evaluated by the photo-degradation of methylene blue (M.B.) in an aqueous solution. As shown in Fig. 4(a) and S6(a),† the rTiO₂ NRs and Pt/rTiO₂ NRs show 1.48 and 12.85 times higher photocatalytic activities than that of the TiO₂ NRs and Pt/TiO₂ NRs under UV-Vis irradiation in a 180 min and 90 min reaction, respectively. This effect can be attributed to the reduction of the optical band gap. For further improvement, an ultrathin layer of Pt was deposited by RF-magnetron sputtering, as shown in Fig. 1(g). The ultrathin Pt layer can act as an electron collector, thereby reducing the charge recombination of the photo-generated electron/hole pair (e–h pair). This can enhance the overall performance. The photocatalytic activities of the Pt decorated rTiO₂ NRs are given in Fig. 4 and S6.† This shows that the photocatalytic activities and reaction rate constant k of Pt/rTiO₂ NRs (98.2% of M.B. degradation in 90 min, k = 0.038 min⁻¹) increased 24.3 and 9.5 fold compared with the rTiO₂ NRs (31.2% of M.B. degradation in 90 min, k = 0.004 min⁻¹).

Fig. 4 (a) Photo-degradation of M.B. in TiO₂ NRs, rTiO₂ NRs, Pt decorated TiO₂ NRs (Pt/TiO₂ NRs) and Pt decorated rTiO₂ NRs (Pt/rTiO₂ NRs). The photocatalytic activities are enhanced after electrochemical reduction and decoration of Pt, due to the reduction of the optical band gap and recombination, respectively. (b) Photocatalytic performance of Pt decorated rTiO₂ NRs with different reduction times. (c) Recycled photocatalytic activities of Pt decorated rTiO₂ NRs. (d) Schematic description of the photocatalytic activities of Pt decorated rTiO₂ NRs.

Furthermore, the Pt-decorated rTiO₂ NRs also has substantial stability and cyclic endurance of the photocatalytic activities by the excellent photo-degradation of M.B. for 8 cycles, shown in Fig. 4(c). Fig. 4(b) and S6(b)† shows the photocatalytic performance of the Pt/rTiO₂ NRs with different electro-chemical reduction time. The results reveal that photocatalytic activities of rTiO₂ NRs were saturated after 10 min of reduction time. The photocatalytic behavior is in good agreement with the band structure results shown in Fig. 3(c). The rate constant k for 3 samples of 10, 15 and 20 min of reduction time are found to be similar (from 0.030 to 0.038 min⁻¹). A further reduction of the optical band gap resulted in further improvement of the photocatalytic activities.

Fig. 4(d) presents the mechanism of the enhanced photocatalytic activities of the Pt decorated rTiO₂ NRs. The conduction band (CB) of rTiO₂ NRs is located at approximately −4.4 eV under vacuum level, while the Fermi level of Pt is located under the CB of the rTiO₂ NRs.^54–56 Owing to the narrower band gap, the rTiO₂ NRs can absorb UV to blue light in the solar spectrum to generate e–h pairs. The photo-excited electrons from rTiO₂ NRs tend to transfer to Pt. The ultrathin Pt collects the electrons and improves charge transfer efficiency. The photo-excited electrons trapped on Pt react with oxygen to form superoxide radicals or hydroperoxide radical to degrade M.B. The photo-excited holes located in the rTiO₂ NRs oxidize water to produce hydroxyl radical to oxide M.B.^57,58

Conclusions

This study examined the optical band gap tuning of TiO₂ NRs by an electrochemical reduction method and its effects on the photocatalytic activities. The reduction process tune the optical band gap from 3.0 eV to 2.84 eV, enabling TiO₂ NRs to absorb UV to the blue light frequencies in the solar spectrum. This reduction in band gap enhanced its photocatalytic activity 1.48 fold. In addition, Pt, as an electron collector layer, was found to be another important parameter. The ultrathin Pt layer greatly reduces the e–h recombination process and can improve the photocatalytic activity significantly (24.3 fold). Moreover, the RF-magnetron sputtered TiO₂ seed layer showed much better adhesion properties than the seed layer prepared by the sol–gel method.

Acknowledgements

This research was supported by the Priority Research Centers Program (2009-0093818), the Basic Science Research Program (2015R1D1A3A03019609), and the Basic Research Lab Program (2014R1A4A1071686) through the National Research Foundation of Korea (NRF), funded by the Korean Government. This research was also supported by the “Leaders Industry-University Cooperation” Project, supported by the Ministry of Education (MOE). Y. S. Kim also thanks to Prof. S. Park and Mr J.-W. Kim of the Dep. of Physics, Pusan National University and Dr J.-S. Bae of Busan center, Korea Science Institute for the XPS analysis.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra25274e

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