3.1 XRD analysis
Figure 1 exhibits the XRD patterns of pure TiO2 and Cu–TiO2. For both pure TiO2 and Cu–TiO2 samples, the diffraction peaks appear at 2θ = 25.3°, 37.7° and 48.0°, ascribing to the (101), (004) and (200) crystal planes of anatase structure (JCPDS 21–1272). Meanwhile, peaks at 27.4°, 36.1° and 54.3° can be attributed to the (110), (101) and (211) planes of rutile TiO2 (JCPDS 21–1276). Furthermore, the peaks appear at 2θ = 25.2°, 25.4° (overlapping with anatase (101) plane) and 30.8°, corresponding to the (120), (111) and (121) crystal planes of brookite TiO2 (JCPDS 29–1360). The simultaneous appearance of the diffraction peaks corresponding to anatase, rutile and brookite structures indicates that the three phases coexist in both pure TiO2 and Cu–TiO2. The mass fractions of anatase (Wa), brookite (Wb) and rutile (Wr) can be calculated as follows:
Wa = KaAa/ (KaAa + Ar + KbAb)
Wb = KbAb/ (KaAa + Ar + KbAb)
Wr = Ar/ (KaAa + Ar + KbAb)
where Ka (0.886) and Kb (2.721) are correction coefficients. Aa, Ab and Ar express the peak intensities of anatase (101), brookite (121) and rutile (110) planes, respectively [20, 21]. The crystallite size was calculated by Scherrer’s formula. For anatase, rutile and brookite, β are the half widths of the diffraction peaks of anatase (101), rutile (110), and brookite (121) planes, respectively [22, 23]. The phase composition and the average crystallite size of samples are summarized in Table 1. It can be seen that the content of anatase decreases and the content of rutile increases after Cu doping, which shows that the addition of Cu is conducive to the transformation from anatase to rutile. Since Cu2+/Cu+ ion radius (0.073/0.077 nm) is close to Ti4+ ion radius (0.0605 nm), Cu2+/Cu+ can enter into TiO2 lattice to replace Ti4+ ions and does not react with TiO2 to generate new crystal phase. This is in line with the earlier literatures [1, 13, 14]. When reaching a certain concentration, the Cu element is dispersed in the form of oxide on the surface of TiO2 particles. Therefore, the Cu-related diffraction peak cannot be detected in XRD patterns.
Table 1
Phase composition and average crystallite size of pure TiO2 and Cu–TiO2.
Photocatalysts
|
Phase composition (%) / Crystallite size (nm)
|
Anatase/Crystallite size
|
Rutile/Crystallite size
|
Brookite/Crystallite size
|
pure TiO2
|
59.0% / 9.2
|
21.4% / 21.4
|
19.6% / 13.0
|
1%Cu–TiO2
|
43.4% / 9.4
|
32.8% / 19.4
|
23.8% / 12.3
|
2%Cu–TiO2
|
36.2% / 10.2
|
46.0% / 27.3
|
17.8% / 11.9
|
3%Cu–TiO2
|
50.6% / 9.4
|
25.8% / 20.5
|
23.6% / 11.8
|
4%Cu–TiO2
|
37.2% / 9.9
|
39.6% / 27.7
|
23.2% / 18.7
|
3.2 Raman analysis
To further confirm the crystal structure of the as-prepared samples, Raman measurement has been employed to distinguish the three structures of TiO2 and the results are shown in Fig. 2. The intense Raman bands located at 398 cm− 1, 517 cm− 1 and 640 cm− 1 ascribe to anatase structure [14, 17]. The bands centered at 244 cm− 1 and 444 cm− 1 can be attributed to rutile structure, while the bands centered at 320 cm− 1 and 363 cm− 1 are for brookite structure [10, 16, 17]. The Raman spectra confirm that there are anatase/rutile/brookite three phases coexistence in pure TiO2 and Cu–TiO2, which is in line with XRD results.
3.3 SEM and TEM analyses
Figure 3 depicts the SEM images of pure TiO2 (a) and 1%Cu–TiO2 (b). It is observed that pure TiO2 consists of nanoparticles and a few nanorods. 1%Cu–TiO2 is also made of nanoparticles and nanorods. The TEM image of 1%Cu–TiO2 is shown in Fig. 3c, which reveals that the particles size in 1%Cu–TiO2 ranges from 10 to 20 nm. The nanorods in 1%Cu–TiO2 have a length of 100 nm and a width of 10 nm approximately. Figure 3d presents the HRTEM image of 1%Cu–TiO2. The marked lattice distances are 0.353 nm, 0.250 nm and 0.293 nm, which can be attributed to the (101) crystal plane of anatase, the (101) crystal plane of rutile and the (121) crystal plane of brookite [24], respectively. Figure 3e-j present the STEM mapping of 1%Cu–TiO2. It can be seen that there are three elements (Ti, O, Cu) in the sample and are distributed in the matrix basically evenly.
3.4 XPS analysis
Figure 4 shows the XPS results of pure TiO2 and 1%Cu–TiO2. Total spectra are displayed in Fig. 4a, from which the signals of Ti, O and C elements can be detected in pure TiO2. Meanwhile, a peak of Cu 2p appears in 1%Cu–TiO2 spectrum, which confirms that Cu exists in TiO2 via doping. Figure 4b shows the high–resolution spectra of Ti 2p. The spectrum of pure TiO2 has two peaks at 464.2 eV and 458.4 eV corresponding to Ti 2p1/2 and Ti 2p3/2, suggesting that the chemical state of Ti element is + 4 [2]. Similarly, two peaks at 463.8 eV and 458.1 eV in the spectrum of 1%Cu–TiO2, which means Ti ions exist as Ti4+. The high–resolution spectra of O 1s are shown in Fig. 4c. The peaks include lattice oxygen (OL) peak at 530.0 eV and surface hydroxyl groups (OH) peak at 532.2 eV in pure TiO2 [1, 25, 26]. The peaks for OL and OH of 1%Cu–TiO2 are situated at 529.5 eV and 532.0 eV, separately. In photodegradation process, žOH radicals are formed as the results of the reaction between surface hydroxyl groups and photoinduced holes, which possess strong oxidation ability. Therefore, more surface hydroxyl groups are beneficial to photocatalytic activity [27, 28]. The ratio of surface hydroxyl groups can be calculated from the deconvolution area of the two peaks [28]. The integral areas of O2− (OL) and OH− (OH) peaks are 46638 and 7391 in pure TiO2 and the proportion of OH is 7391/ (46638 + 7391) = 13.7%. The integral areas of OL and OH peaks are 45270 and 8666 in 1%Cu–TiO2 and the proportion of OH is 8666/ (45270 + 8666) = 16.1%. The calculation results are convinced that the surface hydroxyl group ratio increases after Cu doping. Compared to the Ti 2p and O 1s peaks of pure TiO2, the peaks of 1%Cu–TiO2 shift to lower binding energy, which proves the formation of Ti-O-Cu bonds [1, 25]. The shift to lower binding energy probably because there is a shift in electron density from a lower electronegative Ti (1.54, Pauling value) to a higher electronegative Cu (1.90, Pauling value) atom [8]. Figure 4d displays the high-resolution spectrum Cu 2p of 1%Cu–TiO2. It is observed that the peaks for Cu2+ are located at 933.4 and 952.3 eV and the peak for Cu+ is located at 957.8 eV. The peak at 943.7 eV is related to the shock peak of CuO [29, 30].
3.5 DRS analysis
The diffuse reflectance spectroscopy test was carried out to examine the optical property of obtained photocatalysts and the DRS spectra are exhibited in Fig. 5a. The Kubelka–Munk function for diffuse reflectance is F(R) = (1–R)2/2R [18]. Because F(R) is proportionality to the absorption coefficient α, it can be obtained according to the Tauc’s formula [31–33]. The plots of (αhν)1/2 versus the photon energy (hν) were demonstrated in Fig. 5b. The band gap energy values of pure TiO2, 1%Cu–TiO2, 2%Cu–TiO2, 3%Cu–TiO2 and 4%Cu–TiO2 are 3.06 eV, 3.07 eV, 3.02 eV, 2.96 eV and 3.04 eV, separately. It is worth noting that the absorption of 2%Cu–TiO2 and 3%Cu–TiO2 in the ultraviolet region is lower than that of the pure sample, which hinders the absorption of the light source and is not conducive to photocatalytic activity.
3.6 PL analysis
Since photoluminescence emission is derived from the recombination of photoinduced holes and electrons, it can provide accurate data of the separation of photoinduced pairs [34–36]. The PL spectra of pure TiO2 and Cu–TiO2 are shown in Fig. 6 and it is found that the addition of Cu does not bring new emission peak but only changed the peak intensity. The main peaks around 412 nm can be ascribed to the band gap transition [36, 37]. The peaks arranged from 450 nm to 470 nm are attributed to the transition of charge carriers [36, 38]. All the Cu–TiO2 samples show lower peak intensity than pure TiO2, which indicates that the recombination of photoinduced electrons and holes is hindered by Cu adding effectively. Remarkably, the PL intensity decreases with the increase of Cu concentration, implying that the higher doping amount is, the more favorable it is to suppress the recombination of photogenerated electrons and holes.
3.7 BET analysis
Figure 7 presents the N2 adsorption–desorption isotherms and the pore size distribution curves (inset of Fig. 7) of pure TiO2 and Cu–TiO2. All the photocatalysts exhibit the type IV isotherms with the H2 hysteresis loop in the range of high relative pressure, which are the characteristic of mesoporous materials (2–50 nm) [1, 10, 27, 37, 39]. The pore size distribution curves indicate that both pure TiO2 and Cu–TiO2 have a narrow pore size distribution and the main pore diameters are ranged from 3 nm to 9 nm. The specific surface area, pore size and pore volume data are listed in Table 2. The specific surface area results reveal that Cu–TiO2 samples show higher surface areas than pure TiO2. Sibu et al. [40] and Adyani et al. [1] believe that the surface textural property is improved owing to the presence of Ti–O–M (M is the doping element) bonds, which may restrain the conformity and rearrangement of the primary crystals, leading to increase of surface area [26, 33].
Table 2
Textural properties of pure TiO2 and Cu–TiO2.
Photocatalysts
|
BET surface area (m2/g)
|
Pore volume
(cm3/g)
|
Average pore size (nm)
|
pure TiO2
|
140.8
|
0.286
|
6.9
|
1%Cu–TiO2
|
160.0
|
0.311
|
6.4
|
2%Cu–TiO2
|
195.0
|
0.307
|
5.9
|
3%Cu–TiO2
|
177.5
|
0.279
|
6.2
|
4%Cu–TiO2
|
211.1
|
0.334
|
6.1
|
3.8 Photocatalytic activity analysis
Figure 8a shows the degradation degree of RhB relative to pure TiO2 and Cu–TiO2 under xenon lamp irradiation for 30 min. RhB was not degraded under illumination without photocatalyst, implying that the degradation of RhB should be attributed to the photodegradation of photocatalysts. The degradation degrees of pure TiO2, 1%Cu–TiO2, 2%Cu–TiO2, 3%Cu–TiO2 and 4%Cu–TiO2 are 90.0%, 93.5%, 91.8%, 86.2% and 92.5%, respectively. It is clear that all the photocatalysts show considerable photocatalytic activity, which attributes to relatively high specific surface areas (> 140 m2/g) and the anatase/rutile/brookite triphasic structure.
1%Cu–TiO2 shows higher photocatalytic efficiency than pure TiO2 because of its less recombination rate and higher surface area compared to pure TiO2. However, as the amount of doping content increases, the degradation degrees slightly decrease in 2%Cu–TiO2 and 3%Cu–TiO2 samples. The effect of doping amount on suppressing the recombination of photoinduced pairs is controversial. Several studies show that there is an optimal concentration in doping, and the inhibition of photogenerated electrons and holes is weakened when the concentration exceeds the optimal value [41–43]. The authors believe that excessive doping content will generate new recombination centers, which is not conducive to the migration of photogenerated electrons and holes. Correspondingly, it has also been documented that the higher the doping amount, the higher inhibition effect [44–46]. The inconsistent results may be caused by different preparation methods and processes. In the present study, when the concentration of Cu/Ti reaches 4%, it is still conducive to the separation of photogenerated electrons and holes. Therefore, the decreased photocatalytic activity of 2%Cu–TiO2 and 3%Cu–TiO2 should not be ascribed to the enhancement of recombination rate. On the other hand, DRS results shows that the absorption of 2%Cu–TiO2 and 3%Cu–TiO2 in the ultraviolet region is lower than that of the pure sample, which may be attributed to the fact that the excessive Cu doping content produce more CuO and Cu2O clusters on TiO2 surface, decreasing the light utilization and photocatalytic efficiency [44]. However, if the specific surface area further increases, the disadvantage caused by CuO and Cu2O covering TiO2 surface can be offset. Therefore, the photocatalytic efficiency of 4%Cu–TiO2 increases slightly since it possesses the highest specific surface area (211.1 m2/g).
Figure 8b displays the kinetics fitting curves of ln (C/C0) = – k t of the photocatalysts [47]. The apparent first–order rate constants k of pure TiO2, 1%Cu–TiO2, 2%Cu–TiO2, 3%Cu–TiO2 and 4%Cu–TiO2 are 0.071 min− 1 (R2 = 0.948), 0.091 min− 1 (R2 = 0.977), 0.083 min− 1 (R2 = 0.971), 0.063 min− 1 (R2 = 0.948) and 0.084 min− 1 (R2 = 0.992), respectively.
3.9 The degradation mechanism
·O2− radicals react with nitro-blue tetrazolium (NBT) to form purple precipitates, therefore, the lower NBT absorbance indicates more ·O2− radicals generates. 2, 3-HBA is obtained by the reaction of salicylic acid (SA) with ·OH radicals, which has a special absorption at 510 nm. Therefore, the higher absorbance is, the higher the concentration of 2, 3-HBA is, indicating that more ·OH radicals are produced [48, 49]. Taking 1%Cu–TiO2 as an example, the experiments are carried out. Figure 9a shows that the NBT absorbance decreases with increased illumination time, indicating that more and more purple precipitates are generated from the reaction between ·O2− and NBT after illumination, which results in the consumption of NBT. The decreasing NBT absorbance indicates that the photogenerated electrons are excited to the conduction band after illumination and react with O2 to form ·O2− radicals. Figure 9b shows that 2, 3-HBA absorbance increases with the increased illumination time. Because 2, 3-HBA is obtained from the reaction between SA and ·OH, the increasing 2, 3-HBA absorbance indicates that ·OH radicals are formed under illumination.
NBT and SA experiments confirm that ·O2− and ·OH radicals are generated under illumination. Since ·O2− radicals come from the reaction of photogenerated electrons with O2, and ·OH radicals originate from the reaction of photogenerated hole with OH−, the numbers of ·O2− and ·OH radicals can be used to measure the separation of photogenerated charge under the same conditions. Figure 10a shows that the NBT absorbance of pure TiO2 is higher than that of 1%Cu–TiO2, which suggests that 1%Cu–TiO2 generates more ·O2− radicals than pure TiO2. Figure 10b shows that the 2, 3-HBA absorbance of pure TiO2 is lower than that of 1%Cu–TiO2, indicating that 1%Cu–TiO2 produces more ·OH radicals. Therefore, 1%Cu–TiO2 shows the higher separation rate of photogenerated charges than pure TiO2. Cu doping enhances the quantum efficiency, which is consistent with the PL spectra.
In order to compare the photogenerated charge separation of triphasic photocatalysts and the monophasic/biphasic photocatalysts, the NBT and 2, 3-HBA absorbances of 1%Cu–TiO2 were measured and the results are shown in Fig. 11. Compared to monophasic/biphasic 1%Cu–TiO2, triphasic 1%Cu–TiO2 has lower NBT absorbance and higher 2, 3-HBA absorbance, which indicates that triphasic 1%Cu–TiO2 produces more photogenerated charges and has the highest quantum efficiency. Accordingly, the photocatalytic activity of triphasic 1%Cu–TiO2 is higher than that of monophasic/biphasic 1%Cu–TiO2.
To determine the active species in the process of photodegradation, isopropanol (IPA), benzoquinone (BQ) and ammonium oxalate (AO) were added to capture ·OH, ·O2− and h+, respectively. The degradation results of 1%Cu–TiO2 in the presence of different scavengers are shown in Fig. 12. When BQ, AO and IPA were added, the degradation rate of 1%Cu–TiO2 decreases from 93.5–36.0%, 68.2% and 91.4%, respectively. Obviously, the degradation rate is significantly decreased with BQ adding. The degradation rate decreases slightly in the presence of AO and IPA. Since BQ captures ·O2−, AO captures h+, and IPA captures ·OH, therefore, O2− radical is the main active specie, and h+ and ·OH are subsidiary in the degradation process.
Based on the above experimental results, the photocatalytic degradation mechanism of 1%Cu–TiO2 versus RhB is proposed in Fig. 13. When TiO2 is exposed to light source, electrons in valence band (VB) are excited to conduction band (CB), producing corresponding holes in VB. The photoinduced electrons in anatase and brookite will transfer to rutile, which accelerates the movement of photoinduced electrons, prolongs the lifetime and inhibits the recombination of photoinduced pairs effectively [10, 17]. Moreover, the conversion of Cu ions and the photocatalytic process of 1%Cu–TiO2 are as follows [44, 50]:
Cu/TiO2 + hν → e− + h+
Cu2+ + e−→ Cu+
O2 + e− → ·O2−
OH− + h+ → ·OH
Consequently, the process of transformation between Cu2+ and Cu+ suppresses the recombination of photoinduced pairs, which is in favor of photocatalytic activity. The formed ·O2−, žOH radicals and h+ species have strong oxidizing properties and are able to degrade RhB into small inorganic molecules.