Enhanced photocatalytic activity of TiO₂ activated by doping Zr and modifying Pd

Abstract

A series of TiO₂ photocatalysts introduced with Zr and Pd (TiO₂–Zr–Pdx) are synthesized via a sol–gel method. It is revealed that Zr⁴⁺ ions are doped into TiO₂ lattice in the substitutional mode and Pd²⁺ ions exist as –O–Pd–O– species on the surface of TiO₂. The energy level of the substitutional doped Zr⁴⁺ ions and –O–Pd–O– surface species are located about 0.15 eV below the conduction band and 0.4 eV above the valence band, respectively. Moreover, the band structure of TiO₂–Zr–Pdx can be further adjusted by changing the amount of Pd²⁺ ions. Owing to the synergistic effect of Pd and Zr, the response is extended into the visible region and the photogenerated charge carriers are separated effectively. Therefore, TiO₂–Zr–Pdx samples exhibit improved photocatalytic activity, compared with TiO₂–Zr and TiO₂–Pd under both UV and Vis irradiation.

---

1. Introduction

TiO₂ has attracted a lot of attention owing to its potential application for environmental issues during the past decade.¹ Great efforts have been devoted to developing TiO₂ with enhanced photocatalytic activity.² One of the most effective methods is doping TiO₂ with metal and/or nonmetal elements, which would extend the response of the photocatalysts into the visible region and suppress the recombination of photogenerated charge carriers, leading to improved photocatalytic activity.^3–8 Among them, TiO₂ doped with Zr⁴⁺ ions (TiO₂–Zr) is widely investigated.^9–11 Our previous work has demonstrated that Zr⁴⁺ ions were incorporated into the TiO₂ lattice in the substitutional mode,¹² because Zr⁴⁺ ions share same valence shell structure (n − 1)d²ns² and valence state as Ti⁴⁺ ions.¹³ However, the visible response is still too weak for TiO₂–Zr for practical application. Therefore, it is expected to further improve the photocatalytic activity of TiO₂–Zr.

TiO₂ modified with metal elements, such as In,¹⁴ Au,^15,16 Pt,¹⁷ and Pd,¹⁸ is regarded as an effective method to improve the photocatalytic activity. The introduced metal ions usually exist as some kinds of species on the surface of TiO₂, inducing strong absorption in visible region and promoting the separation of photogenerated electrons and holes, leading to enhanced photocatalytic activity. Pd is one of the most investigated noble elements.¹⁹ Hsiao et al. found Pd-containing TiO₂ film showed high photocatalytic activity on the removal of NO_x.²⁰ Susanta et al. reported that the self-organized TiO₂ nanotube with Pd nanoparticles exhibited effective photocatalytic activity under solar-light irradiation.²¹ Our recent work has observed that the Pd surface species, –O–Pd–O– or –O–Pd–Cl, modified TiO₂ (TiO₂–Pd) showed strong visible absorption and enhanced visible photocatalytic activity.²² Inspired by the aforementioned works, we would like to introduce the Zr and Pd species into TiO₂ system (TiO₂–Zr–Pdx). It is expected that the TiO₂–Zr–Pdx photocatalysts would combine the advantages of doping and surface modification, so as to achieve an enhanced photocatalytic performance compared with single doped TiO₂ (TiO₂–Zr and TiO₂–Pd) under ultraviolet (UV) and visible (Vis) irradiation.

In this work, TiO₂ introduced with Zr and Pd (TiO₂–Zr–Pdx) is synthesized by a sol–gel method. It is revealed that the co-existence of doped Zr ions and modified Pd species could enhance the absorption in visible region and separate the charge carriers effectively, resulting in a much better photocatalytic activity than single element doped TiO₂ samples under UV and Vis irradiation.

2. Experimental details

2.1. Photocatalyst preparation

All chemicals used were of analytical grade and the water was deionized water (>18.2 MΩ cm⁻¹). At room temperature, certain volumes of ZrOCl₂ (0.5 mol L⁻¹) and PdCl₂ (0.1169 mol L⁻¹) solution were mixed with 40 mL of ethanol. Then 1 mL HCl solution (12 mol L⁻¹) and 12 mL of Ti(OC₄H₉)₄ were added dropwise into the mixture under vigorous stirring. The mixture was stirred until the formation of TiO₂ gel. After aging for 24 h, the TiO₂ gels were dried at 373 K for 10 h and annealed at 723 K in a muffle for 2.5 h. The obtained samples were denoted as TiO₂–Zr–Pdx, where x represented the nominal molar ratio of Pd²⁺ ions relative to the total abundance of Ti⁴⁺ and Pd²⁺ ions (Pd²⁺/(Ti⁴⁺ + Pd²⁺)). Pure TiO₂, Zr doped TiO₂ (TiO₂–Zr) and Pd modified TiO₂ (TiO₂–Pd0.5%) were prepared using the same procedure without the addition of corresponding precursor. The molar ratio of Pd²⁺ ions relative to the total abundance of Ti⁴⁺ and Pd²⁺ ions (Pd²⁺/(Ti⁴⁺ + Pd²⁺)) in TiO₂–Pd0.5% is 0.5% and that of Zr⁴⁺ ions relative to the total abundance of Ti⁴⁺ and Zr⁴⁺ ions (Zr⁴⁺/(Ti⁴⁺ + Zr⁴⁺)) is fixed at 10% for TiO₂–Zr and all TiO₂–Zr–Pdx samples.

2.2. Characterization

The XRD patterns were acquired using a Rigaku D/max 2500 X-ray diffraction spectrometer (Cu Kα, λ = 1.54056 Å). The average crystal size was calculated according to the Scherrer formula (D = kλ/B cos θ). After degassing at 180 °C, the BET surface area was determined via the measurement of nitrogen adsorption–desorption isotherms at 77 K (Micromeritics Automatic Surface Area Analyzer Gemini 2360, Shimadzu). Raman spectra were taken on a Renishaw inVia Raman microscope by using the 785 nm line of a Renishaw HPNIR 785 semiconductor laser. XPS measurements were carried out by using a Thermo ESCALAB 250 spectrometer with an Al Kα monochromator source and all the binding energies were calibrated to the adventitious C1s peak at 284.8 eV. Diffuse reflectance UV-Visible (UV-Vis) absorption spectra were recorded on a UV-Vis spectrometer (U-4100, Hitachi). The photoluminescence (PL) spectra were measured by fluorescence spectrophotometer (Edinburgh Instruments, FLS920) using the 325 nm line of Xe lamp as the excitation source. High resolution TEM (HRTEM) analyses were conducted using a JEM-2010FEF, for which the samples were prepared by applying a drop of ethanol suspension onto an amorphous carbon-coated copper grid and dried naturally.

2.3. Evaluation of photocatalytic activity

The photocatalytic activity of all photocatalysts were determined by the photodegradation for 4-chlorophenol (4-CP) solution (5 × 10⁻⁵ mol L⁻¹, 40 mL, pH = 5.35) under Vis irradiation (λ > 400 nm). A sunlamp (Philips HPA 400/30S, Belgium) was used directly for the UV photocatalytic reaction, while for the visible photocatalysis a cutoff filter (λ > 400 nm) was employed to remove UV irradiation. The photocatalyst dosages were 5 mg for visible photocatalysis and 2 mg for UV photocatalysis, respectively, which were suspended in 4-CP solution. The reactor was perpendicular to the light beam and located 10 cm away from the light source. All the suspensions were magnetic stirred at (25 ± 2) °C in the dark for 30 min to reach adsorption equilibrium before irradiation, and oxygen gas was continuously bubbled through the solution at a flux of 5 mL min⁻¹. The change in concentration of 4-CP was monitored by a UV-Visible spectrometer (UV-1061PC, SHIMADZU) using 4-aminoantipyrine as the chromogenic reagent. The reproducibility of the photocatalytic degradation was evaluated by repeating experiments at least three times with different batches of photocatalysts prepared by the same procedure.

3. Result and discussion

3.1. Photodegradation of 4-chlorophenol

The photodegradation of 4-CP was employed to evaluate the photocatalytic activity of photocatalysts under Vis and UV irradiation (Fig. 1, 2 and Tables 1, 2 and Fig. S1†). For all the samples, the ln(c₀/c) values of 4-CP exhibit a nearly linear relationship with irradiation time (Fig. S1B and D†), suggesting a pseudo-first-order reaction.²³ Under Vis irradiation (λ > 400 nm) (Fig. 1A, Table 1 and Fig. S1A†), the 4-CP can hardly be degraded with pure TiO₂, whose degradation rate and specific photocatalytic activity are 6.0% and 3.00 × 10⁻⁶ mol g⁻¹ h⁻¹, respectively. TiO₂–Zr shows an improved photocatalytic activity, whose degradation rate and specific photocatalytic activity of 4-CP are 40.4% and 2.02 × 10⁻⁵ mol g⁻¹ h⁻¹, respectively. TiO₂–Pd0.5% presents a further improved photocatalytic activity, whose degradation rate and specific photocatalytic activity of 4-CP are 70.8% and 3.54 × 10⁻⁵ mol g⁻¹ h⁻¹, respectively. Among all samples, TiO₂–Zr–Pd0.5% shows the best activity. The degradation rate and specific photocatalytic activity of TiO₂–Zr–Pd0.5% are 91.8% and 4.59 × 10⁻⁵ mol g⁻¹ h⁻¹, which are about 15 times as those of TiO₂, 1.5 times as those of TiO₂–Pd0.5% and 2.2 times as those of TiO₂–Zr, respectively. Moreover, 5 mg of TiO₂–Zr–Pd0.5% even exhibit a better photocatalytic activity than 10 mg of mixed single doped TiO₂ samples (5 mg of TiO₂–Pd0.5% and 5 mg of TiO₂–Zr), implying the synergistic effect of Pd and Zr is the key factor for the improved photocatalytic activity. Under UV irradiation (Fig. 1B, Table 2 and Fig. S1C†), TiO₂–Zr and TiO₂–Pd0.5% samples show better photocatalytic activity than pure TiO₂. TiO₂–Zr–Pd0.5% still exhibits the highest activity, and 92.9% of 4-CP is degraded under UV irradiation. The degradation rate and specific photocatalytic activity of TiO₂–Zr–Pd0.5% are about 4.6 times higher than those of pure TiO₂.

Fig. 1 Photodegradation of 4-CP for TiO₂, TiO₂–Pd0.5%, TiO₂–Zr and TiO₂–Zr–Pd0.5% samples under (A) Vis irradiation and (B) UV irradiation.

Fig. 2 4-CP degradation rates for TiO₂–Zr–Pdx samples under Vis irradiation for 8 h and UV irradiation for 1 h, respectively.

Table 1 Photodegradation of 4-CP under Vis irradiation (λ > 400 nm)

Sample	4-CP degraded^a (Δc/c0)	k^b (×10^−4 min^−1)	t1/2 (min)	specific photocatalytic activity (mol g^−1 h^−1)
a After reaction for 8 h under Vis irradiation.b Apparent rate constant deduced from the linear fitting of ln(c0/c) versus reaction time.c The blank was the photolysis of 4-CP.
Blank^c	3.5%	0.744	9316.5	—
TiO2	6.0%	1.29	5373.2	3.00 × 10^−6
TiO2–Pd0.5%	70.8%	25.7	269.7	3.54 × 10^−5
TiO2–Zr	40.4%	10.8	641.8	2.02 × 10^−5
5 mg TiO2–Zr + 5 mg TiO2–Pd0.5%	90.3%	48.6	142.7	2.26 × 10^−5
TiO2–Zr–Pd0.5%	91.8%	52.2	132.8	4.59 × 10^−5

---

Table 2 Photodegradation of 4-CP under UV irradiation

Sample	4-CP degraded^a (Δc/c0)	k^b (×10^−4 min^−1)	t1/2 (min)	Specific photocatalytic activity (mol g^−1 h^−1)
a After reaction for 1 h under UV irradiation.b Apparent rate constant deduced from the linear fitting of ln(c0/c) versus reaction time.c The blank was the photolysis of 4-CP.
Blank^c	4.3%	7.25	956.1	—
TiO2	16.7%	30.4	228	1.67 × 10^−4
TiO2–Pd0.5%	71.4%	209	33.2	7.14 × 10^−4
TiO2–Zr	83.7%	302	23	8.37 × 10^−4
TiO2–Zr–Pd0.5%	92.9%	440	15.8	9.29 × 10^−4

---

Fig. 2 shows the degradation rate of 4-CP for the TiO₂–Zr–Pdx samples under Vis and UV irradiation, respectively. All TiO₂–Zr–Pdx samples exhibit better photocatalytic activity than TiO₂–Zr (x = 0). And TiO₂–Zr–Pd0.5% shows the best photocatalytic activity under both Vis and UV irradiation. The photocatalytic performance decreases when the concentration of Pd²⁺ ions is more than 0.5%. It is expected that the photocatalytic activity of TiO₂–Zr–Pdx can be further enhanced by optimizing the amount of Pd²⁺ ions. These results suggest that the introduction of both Zr⁴⁺ ions and Pd²⁺ ions into TiO₂ is an effective way to prepare TiO₂-based photocatalysts with significant photocatalytic performance under both Vis and UV irradiation. The detailed mechanism will be discussed in the following sections.

3.2. Existing states of Pd and Zr in TiO₂

To investigate the crystal structure of the photocatalysts, the XRD patterns of TiO₂, TiO₂–Pd0.5%, TiO₂–Zr and TiO₂–Zr–Pd0.5% are plotted in Fig. 3. It is clear that all samples exhibit typical anatase structure.¹² For the TiO₂–Pd0.5%, one small peak of rutile at around 27.4° is observed.²⁴ No other characteristic diffraction peaks, such as ZrO₂ and PdO are observed for all samples. The lattice parameters, cell volume and crystal size are calculated and summarized in Table 3. Since ionic radius of Pd²⁺ ion (86 pm)²⁵ is larger than that of Ti⁴⁺ ion (68 pm),⁶ an increase of lattice parameters is expected if the Pd²⁺ ions are incorporated into TiO₂ lattice. Compared with pure TiO₂, no shift is found for the diffraction peaks of TiO₂–Pd0.5% (inset in Fig. 3), and the lattice parameters remain almost unchanged, implying the Pd²⁺ ions are not doped into TiO₂ lattice and might exist as some kind of species on the surface of TiO₂. For TiO₂–Zr, the diffraction peaks shift to lower angles accompanied with that the lattice parameters and cell volume increase compared with pure TiO₂, suggesting the Zr⁴⁺ ions are doped into the lattice of TiO₂ by replacing the lattice Ti. As the ionic radius of Zr⁴⁺ ions is larger than that of lattice Ti⁴⁺ ions, leading to an increase of the lattice parameters and cell volume, which is observed for TiO₂–Zr (Table 3). For TiO₂–Zr–Pdx%, the positions of diffraction peaks are almost as same as those of TiO₂–Zr (inset in Fig. 3, S2 and Table S1†), and the lattice parameters as well as cell volume are almost the same as TiO₂–Zr, suggesting the Zr⁴⁺ ions are doped into TiO₂ lattice in the substitutional mode, and the Pd²⁺ ions exist as some kind of species on the surface for TiO₂–Zr–Pdx. In addition, the crystal size of all doped samples decreases compared with TiO₂, suggesting the introduction of Pd²⁺ or Zr⁴⁺ ions could inhibit the grain growth of TiO₂ crystal. The increased specific surface area (BET) agrees well with the trend of decreased crystal size of the samples (Table 3 and S1†), which is in favour of the photocatalytic activity. The existing states of Pd and Zr for TiO₂–Zr–Pdx would be further investigated by Raman, HRTEM and XPS.

Fig. 3 XRD patterns of TiO₂, TiO₂–Pd0.5%, TiO₂–Zr and TiO₂–Zr–Pd0.5%. The inset is the enlargement of the XRD peaks for the (1 0 1) plane.

Table 3 Lattice parameters, cell volume, crystal size and specific surface area for TiO₂, TiO₂–Pd0.5%, TiO₂–Zr and TiO₂–Zr–Pd0.5% samples

Sample	Lattice parameters		Cell volume/Å^3	Crystal size/nm	Specific surface area
	a, b/Å	c/Å
TiO2	3.783	9.501	136.0	13.0	63.8
TiO2–Pd0.5%	3.781	9.499	135.8	12.6	75.4
TiO2–Zr	3.802	9.540	137.9	11.3	85.1
TiO2–Zr–Pd0.5%	3.804	9.551	138.2	12.3	81.0

---

Fig. 4 shows Raman spectra of TiO₂, TiO₂–Pd0.5%, TiO₂–Zr and TiO₂–Zr–Pd0.5%. The peaks at about 144 cm⁻¹ (E_g), 194 cm⁻¹ (E_g), 396 cm⁻¹ (B_1g), 516 cm⁻¹ (A_1g and B_1g), and 638 cm⁻¹ (E_g) are observed for all samples, corresponding to the anatase structure.²⁶ Almost Raman peaks do not shift for TiO₂–Pd0.5%, compared with pure TiO₂, implying that the Pd²⁺ ions are not doped into TiO₂ lattice in the substitutional mode and might exist as some kind of species on the surface. However, for TiO₂–Zr, the peak at about 144 cm⁻¹ (E_g) shifts to lower vibrational frequencies compared with pure TiO₂ (inset of Fig. 4), suggesting the Zr⁴⁺ ions are introduced into TiO₂ lattice in substitutional mode. Since the ionic radius of Zr⁴⁺ ion (72 pm) is slightly larger than that of lattice Ti⁴⁺ ion (68 pm), the length of Zr–O bond is longer than that of Ti–O bond in TiO₂. As a result, a tensile stress in lattice can be induced after the lattice Ti⁴⁺ ions are replaced by Zr⁴⁺ ions, leading to a decrease of vibrational energy of the 144 cm⁻¹ (E_g) corresponded to the O–Ti–O bond bending vibration mode. Therefore, the peak at 144 cm⁻¹ (E_g) for TiO₂–Zr sample shifts to lower frequencies compared with that of pure TiO₂. Moreover, For TiO₂–Zr–Pd0.5%, the peak position at about 144 cm⁻¹ (E_g) is almost the same as that of TiO₂–Zr, implying the introduced Zr⁴⁺ ions enter into TiO₂ lattice in substitutional mode, while the Pd²⁺ ions may exist as some kind of species on the surface for TiO₂–Zr–Pd0.5%. These results from Raman spectra are in good agreement with the XRD.

Fig. 4 Raman spectra of TiO₂, TiO₂–Pd0.5%, TiO₂–Zr and TiO₂–Zr–Pd0.5%. An enlargement of the E_g vibrational modes for all the samples is shown in the inset.

HRTEM images of pure TiO₂, TiO₂–Zr, TiO₂–Pd0.5% and TiO₂–Zr–Pd0.5% are shown in Fig. 5. A clear fringe spacing (d) of 3.52 Å is observed for pure TiO₂ (Fig. 5A), suggesting the existence of TiO₂ anatase structure.¹² The fringe spacing (d) of the (1 0 1) anatase plane for TiO₂–Pd0.5% (Fig. 5B) is also 3.52 Å, indicating the Pd²⁺ ions are not doped into anatase TiO₂ lattice. Moreover, there is no PdO particles observed in the TEM images, implying Pd²⁺ ions may exist as some other kind of species on the surface of TiO₂. However, the fringe spacing (d) for TiO₂–Zr in Fig. 5C is 3.55 Å, which is larger than that for TiO₂. The Zr⁴⁺ ions (72 pm) are doped into anatase TiO₂ lattice by replacing Ti⁴⁺ ions (68 pm). Compared with TiO₂–Zr, the fringe spacing (d) for TiO₂–Zr–Pd0.5% (Fig. 5D) is also 3.55 Å, suggesting that Pd²⁺ ions exist as surface species on the surface of TiO₂. XPS technique is applied to further investigate the existing states of the introduced Pd²⁺ ions and Zr⁴⁺ ions.

Fig. 5 HRTEM images of (A) TiO₂, (B) TiO₂–Pd0.5%, (C) TiO₂–Zr and (D) TiO₂–Zr–Pd0.5%.

Fig. 6 shows the Zr3d XPS spectra of TiO₂–Zr, TiO₂–Zr–Pd0.5% and TiO₂–Zr–Pd1%, respectively. The doublet peaks at about 182.2 eV and 184.5 eV are ascribed to Zr3d_5/2 and Zr3d_3/2 states of the substitutional doped Zr⁴⁺ ions in TiO₂ lattice respectively, since the peak position of Zr3d_5/2 (182.2 eV) is located between that of ZrO₂ (183.5 eV)²⁷ and metallic Zr (179.0 eV).²⁸

Fig. 6 XPS Zr3d spectra of TiO₂–Zr, TiO₂–Zr–Pd0.5% and TiO₂–Zr–Pd1%.

Fig. 7 shows the Pd3d XPS spectra of TiO₂–Pd0.5%, TiO₂–Zr–Pd0.5% and TiO₂–Zr–Pd1%, respectively. It has been demonstrated by our previous work that there are two existing states for the introduced Pd species, –O–Pd–O– and –O–Pd–Cl.²² It is noted that the Pd3d_5/2 for –O–Pd–O– (i.e. one Pd ion is linked with two unsaturated oxygen ions) is at about 336.2 eV and the Pd3d_5/2 for –O–Pd–Cl (i.e. one Pd ion is linked with one Cl ion and one unsaturated oxygen ion) is at about 337.7 eV.²² Hence, the peaks at about 336.2 eV in TiO₂–Pd0.5%, TiO₂–Zr–Pd0.5% and TiO₂–Zr–Pd1% are attributed to the Pd3d_5/2 of –O–Pd–O– structure on the surface of TiO₂.²² It is found the atom percentage of Zr and Pd for TiO₂–Zr–Pd0.5% sample is 12.09% and 0.58%, respectively (Table S3†). In addition, the doublet peaks at about 333.0 eV and 347.0 eV in the XPS Pd3d spectra are ascribed to Zr3p_3/2 and Zr3p_1/2, respectively.²⁹

Fig. 7 XPS Pd3d spectra of TiO₂–Pd0.5%, TiO₂–Zr–Pd0.5% and TiO₂–Zr–Pd1%.

According to the XRD, Raman, HRTEM and XPS results, it is revealed the introduced Pd²⁺ ions exist as –O–Pd–O– species on the surface of TiO₂, and the introduced Zr⁴⁺ ions are doped into TiO₂ lattice in substitutional mode for TiO₂–Zr–Pd0.5%.

3.3. Band structure and visible response

XPS valence band spectra of TiO₂, TiO₂–Zr, TiO₂–Pd0.5% and TiO₂–Zr–Pd0.5% samples are shown in Fig. 8. The energy level is in alignment with the work function of the XPS instrument (4.10 eV, Fermi level). The binding energy of the onset edge for the O2p peak reveals the energy gap between the Fermi level and valence band maximum. The onset edge of binding energy (the valence band) for TiO₂, TiO₂–Zr, TiO₂–Pd0.5% and TiO₂–Zr–Pd0.5% samples are at 2.44 eV, indicating the maximum of the valence band remain almost unchanged. A small hump (from 1.0 eV to 2.4 eV) centred at around 2.00 eV is found for TiO₂–Pd0.5% and TiO₂–Zr–Pd0.5%, compared with pure TiO₂, indicating the energy level of –O–Pd–O– surface species locates at 0.4 eV above the valence band of TiO₂. For TiO₂–Zr, the energy level of substitutional doped Zr⁴⁺ ions is not found above the valence band of TiO₂ and might exist next to the conduction band of TiO₂.

Fig. 8 XPS valence band spectra of TiO₂, TiO₂–Zr, TiO₂–Pd0.5% and TiO₂–Zr–Pd0.5% samples.

Diffuse reflectance UV-Vis absorption spectra were shown in Fig. 9, to investigate the band structure of photocatalysts. A strong absorption in the UV region are found for all samples, ascribed to the band–band transition.⁶ The corresponding band gap for TiO₂ is 3.1 eV, as the absorption onset edge is at about 400 nm. The band gap is estimated to be 3.10 eV, 3.08 eV, 3.10 eV and 3.08 eV for TiO₂, TiO₂–Zr, TiO₂–Pd0.5% and TiO₂–Zr–Pd0.5%, respectively. Compared with TiO₂, a small hump at around 420 nm is observed for TiO₂–Zr, which is caused by the electron transition from the valence band to the energy level of substitutional doped Zr⁴⁺ ions.¹² The energy level of substitutional doped Zr⁴⁺ ions is at about 0.15 eV below the conduction band of TiO₂.⁶ For TiO₂–Pd0.5%, the strong absorption peak centred at about 460 nm is ascribed to electron transition from the energy level of –O–Pd–O– surface species to conduction band of TiO₂. The energy level of –O–Pd–O– species is estimated to be 0.4 eV above the valence band, which is consistent with XPS valence band spectra. As we expect, the absorption for TiO₂–Zr–Pd0.5% is further improved compared with TiO₂–Pd and TiO₂–Zr, owing to synergistic effect of Pd and Zr. According to the discussion above, the energy band structure of the TiO₂–Zr–Pdx is drawn in Fig. 11. In addition, it is found from Fig. S4† that the absorption in the visible region for TiO₂–Zr–Pdx is stronger than that of TiO₂–Zr or TiO₂–Pdx. And, the absorption in the visible region becomes stronger for TiO₂–Zr–Pdx with the increase of amount of Pd²⁺ ions. This indicates that the co-existence of doped Zr⁴⁺ ions and surface modification of Pd²⁺ ions on TiO₂ could enhance visible response effectively.

Fig. 9 Diffuse reflectance UV-Vis absorption spectra of TiO₂, TiO₂–Zr, TiO₂–Pd0.5% and TiO₂–Zr–Pd0.5% samples.

3.4. Behaviours of photogenerated charge carriers

The separation and recombination behaviours of the photogenerated electrons and holes are closely related to the photocatalytic activity of photocatalysts. The photogenerated electrons in the conduction band would fall into the energy levels of defects (oxygen vacancies) through a non-irradiative process, then recombine with the holes at the valence band, leading to the emission of fluorescence.³⁰ Therefore, the quench of the photoluminescence (PL) spectra usually indicates inhibited recombination for charge carriers. Fig. 10 shows the photoluminescence spectra (PL) of TiO₂, TiO₂–Pd0.5%, TiO₂–Zr and TiO₂–Zr–Pd0.5%. It is clear that the PL intensity of TiO₂–Zr was quenched compared with pure TiO₂, as the photogenerated electrons in the conduction band could transfer into the energy level of substitutional doped Zr⁴⁺ ions. And the PL spectrum for TiO₂–Pd0.5% is further quenched owing to –O–Pd–O– surface species, as the holes at valence band could transfer to the energy level of –O–Pd–O– surface species. A further decrease of the PL intensity was seen apparently from TiO₂–Zr–Pd0.5% because of both –O–Pd–O– surface species and substitutional doped Zr⁴⁺ ions. For TiO₂–Zr–Pdx samples (Fig. S5†), with the increase of doping Pd²⁺ ions amount (<0.5%), a relative decrease in the PL intensity is observed compared with pure TiO₂. These PL results indicate that doping Zr⁴⁺ and modification with Pd²⁺ into TiO₂ system can inhibit the recombination of photogenerated electrons and holes, which would separate the photogenerated charge carriers and enhance photocatalytic activity effectively.

Fig. 10 Photoluminescence spectra of TiO₂, TiO₂–Pd0.5%, TiO₂–Zr and TiO₂–Zr–Pd0.5%.

3.5. Photocatalytic mechanism

The schematic diagram of the photocatalytic mechanism (Fig. 11) is used to explain the photocatalytic mechanism of TiO₂–Zr–Pdx. Under Vis irradiation (λ > 400 nm), TiO₂ represents poor photocatalytic activity because of its large band gap (3.1 eV for pure TiO₂). TiO₂–Zr shows a restricted photocatalytic activity, as small amounts of electrons could be excited from the valence band to the energy level of substitutional doped Zr⁴⁺ ions. TiO₂–Pd0.5% samples represent a better photocatalytic activity than TiO₂ and TiO₂–Zr, owing to the existence of –O–Pd–O– surface species on the surface of TiO₂. Electrons can be excited from the energy level of –O–Pd–O– species to the conduction band of TiO₂, leading to a strong absorption in visible region and improved photocatalytic activity. TiO₂–Zr–Pdx exhibits a significant enhanced photocatalytic activity in comparison with the TiO₂–Pd and TiO₂–Zr (see Fig. S1A†), owing to the synergistic effects of substitutional doped Zr⁴⁺ ions and –O–Pd–O– surface species in the TiO₂–Zr–Pdx. The electrons are excited from the valence band to the energy level of substitutional doped Zr⁴⁺ ions, or from the energy level of –O–Pd–O– surface species to the conduction band of TiO₂, leading to a strong absorption in the visible region (Fig. 9) accompanied with a large amount of photogenerated charge carriers (Fig. 10). At the same time, the energy level of substitutional doped Zr⁴⁺ ions and –O–Pd–O– surface species in TiO₂–Zr–Pdx could separate the photogenerated electrons and holes effectively. Electrons in the energy level of substitutional doped Zr⁴⁺ ions are captured by the adsorbed O₂ molecules, resulting in the formation of O₂⁻ active species which would further photodegradate the 4-CP molecules. Meanwhile, the photogenerated holes in the energy level of –O–Pd–O– surface species can oxidize 4-CP molecules adsorbed on the surface of TiO₂ directly.³¹ Therefore, more photoinduced charge carriers would contribute to the photocatalytic process, leading to a better photocatalytic performance than TiO₂–Zr and TiO₂–Pd0.5% under Vis irradiation.

Fig. 11 Scheme of photocatalytic mechanism of TiO₂–Zr–Pdx.

Under UV irradiation, electrons can be excited from the valence band to the conduction band for all samples. Moreover, TiO₂ represents still a limited photocatalytic activity because of the high recombination of photogenerated charge carriers. Compared with TiO₂, TiO₂–Pd0.5% and TiO₂–Zr show improved photocatalytic activity due to the existence of –O–Pd–O– surface species and substitutional doped Zr⁴⁺ ions, respectively. The photoinduced elections at the conduction band can transfer to the energy level of substitutional doped Zr⁴⁺ ions for TiO₂–Zr. The holes in the valence band can move to the energy level of –O–Pd–O– surface species for TiO₂–Pd. The efficient separated charge carriers results in a higher photocatalytic activity than pure TiO₂. Compared with TiO₂–Zr and TiO₂–Pd, the photocatalytic activity of TiO₂–Zr–Pdx is further enhanced due to synergistic effects of –O–Pd–O– surface species and substitutional doped Zr⁴⁺ ions. The effective separation of photogenerated electrons and holes enable more charge carriers contribute to the photocatalytic reaction. As a result, the photocatalytic activity of TiO₂–Zr–Pdx is improved in comparison with pure TiO₂, TiO₂–Zr and TiO₂–Pd0.5% under UV irradiation. Additionally, the large specific surface area for TiO₂–Zr–Pdx also is in favour of the photocatalytic activity.³²

4. Conclusion

In summary, a new type of Zr doped TiO₂ modified with surface Pd species (TiO₂–Zr–Pdx) was prepared by a sol–gel method. TiO₂–Zr–Pdx photocatalysts exhibit much better photocatalytic activity than pure TiO₂, TiO₂–Zr and TiO₂–Pd under both Vis and UV irradiation. The enhanced photocatalytic activity is ascribed to synergistic effects of the –O–Pd–O– surface species and substitutional doped Zr⁴⁺ ions, the absorption in visible region is enhanced and the photogenerated charge carriers are separated effectively. These results may offer some guidance for the synthesis and preparation of optical function materials with potential application.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (No. 51372120 and 21173121), and National High Technology Research and Development Program of China (Grant No. 2013AA014201).

References

1. J. Yuan, Q. Wu, P. Zhang, J. Yao, T. He and Y. Cao, Environ. Sci. Technol., 2012, 46, 2330–2336.
2. K. Nakata and A. Fujishima, J. Photochem. Photobiol., C, 2012, 13, 169–189.
3. Y. Li, G. Ma, S. Peng, G. Lu and S. Li, Appl. Surf. Sci., 2008, 254, 6831–6836.
4. Y. Li, S. Peng, F. Jiang, G. Lu and S. Li, J. Serb. Chem. Soc., 2007, 72, 393–402.
5. Y. Li, Y. Jiang, S. Peng and F. Jiang, J. Hazard. Mater., 2010, 182, 90–96.
6. P. Zhang, Y. Yu, E. Wang, J. Wang, J. Yao and Y. Cao, ACS Appl. Mater. Interfaces, 2014, 6, 4622–4629.
7. E. Wang, T. He, L. Zhao, Y. Chen and Y. Cao, J. Mater. Chem., 2011, 21, 144–150.
8. J. Song, B. Zhu, W. Zhao, X. Hu, Y. Shi and W. Huang, J. Nanopart. Res., 2013, 15, 1–9.
9. K. V. Chary, G. V. Sagar, D. Naresh, K. K. Seela and B. Sridhar, J. Phys. Chem. B, 2005, 109, 9437–9444.
10. N. Venkatachalam, M. Palanichamy, B. Arabindoo and V. Murugesan, J. Mol. Catal. A: Chem., 2007, 266, 158–165.
11. H. Liu, G. Liu and X. Shi, Colloids Surf., A, 2010, 363, 35–40.
12. J. Wang, Y. Yu, S. Li, L. Guo, E. Wang and Y. Cao, J. Phys. Chem. C, 2013, 117, 27120–27126.
13. E. Wang, H. Yang and Y. Cao, Acta Chim. Sin., 2010, 67, 2759–2764.
14. H. Li, Z. Bian, J. Zhu, Y. Huo, H. Li and Y. Lu, J. Am. Chem. Soc., 2007, 129, 4538–4539.
15. C. Sivadinarayana, T. V. Choudhary, L. L. Daemen, J. Eckert and D. W. Goodman, J. Am. Chem. Soc., 2004, 126, 38–39.
16. X. Yang, S. Kattel, S. D. Senanayake, J. A. Boscoboinik, X. Nie, J. Graciani, J. A. Rodriguez, P. Liu, D. J. Stacchiola and J. G. Chen, J. Am. Chem. Soc., 2015, 137, 10104–10107.
17. G. R. Bamwenda, A. Ogata, A. Obuchi, J. Oi, K. Mizuno and J. Skrzypek, Appl. Catal., B, 1995, 6, 311–323.
18. A. Lacerda, I. Larrosa and S. Dunn, Nanoscale, 2015, 7, 12331–12335.
19. J. B. Zhong, Y. Lu, W. D. Jiang, Q. M. Meng, X. Y. He, J. Z. Li and Y. Q. Chen, J. Hazard. Mater., 2009, 168, 1632–1635.
20. Y.-C. Hsiao and Y.-H. Tseng, Micro Nano Lett., 2010, 5, 317–320.
21. S. K. Mohapatra, N. Kondamudi, S. Banerjee and M. Misra, Langmuir, 2008, 24, 11276–11281.
22. Y. Yu, T. He, L. Guo, Y. Yang, L. Guo, Y. Tang and Y. Cao, Sci. Rep., 2015, 5(9561), 1–6.
23. B. Gao, Y. Ma, Y. Cao, W. Yang and J. Yao, J. Phys. Chem. B, 2006, 110, 14391–14397.
24. Y. Cao, T. He, Y. Chen and Y. Cao, J. Phys. Chem. C, 2010, 114, 3627–3633.
25. R. t. Shannon, Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr., 1976, 32, 751–767.
26. W. Zhang, Y. He, M. Zhang, Z. Yin and Q. Chen, J. Phys. D: Appl. Phys., 2000, 33, 912–916.
27. Z. Bastl, A. Senkevich, I. Spirovova and V. Vrtilkova, Surf. Interface Anal., 2002, 34, 477–480.
28. I. Takano, S. Isobe, T. A. Sasaki and Y. Baba, Appl. Surf. Sci., 1989, 37, 25–32.
29. C. Ho, C. Y. Jimmy, X. Wang, S. Lai and Y. Qiu, J. Mater. Chem., 2005, 15, 2193–2201.
30. S.-C. Moon, H. Mametsuka, S. Tabata and E. Suzuki, Catal. Today, 2000, 58, 125–132.
31. A. Fujishima, X. Zhang and D. A. Tryk, Surf. Sci. Rep., 2008, 63, 515–582.
32. N. Serpone, D. Lawless, R. Khairutdinov and E. Pelizzetti, J. Phys. Chem., 1995, 99, 16655–16661.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra03275c

---