Titanium dioxide () doped with neodymium (Nd), one rare earth element, has been synthesized by a sol-gel method for the photocatalytic degradation of rhodamine-B under visible light. The prepared samples are characterized by X-ray diffractometer, Raman spectroscopy, UV-Vis diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller measurement. The results indicate that the prepared samples have anatase and brookite phases. Additionally, Nd as Nd3+ may enter into the lattice of and the presence of Nd3+ substantially enhances the photocatalytic activity of under visible light. In order to further explore the mechanism of photocatalytic degradation of organic pollutant, photoluminescence spectrometer and scavenger addition method have been employed. It is found that hydroxide radicals produced by Nd-doped under visible light are one of reactive species for Rh-B degradation and photogenerated electrons are mainly responsible for the formation of the reactive species.

1. Introduction

Dye wastewater discharged into nature mainly by dyestuff, textile industry, and some artificial way causes severe ecological problems. These compounds are highly colored and can heavily contaminate water source. Chemical oxidation method for dye wastewater treatment is too costly and physical adsorption method often results in secondary pollution [1]. Many attempts have been carried out to develop efficient biological methods to decolorize these effluents but they have not been very successful. Semiconductor TiO2 as a photocatalyst has been deeply investigated and can successfully degrade various organic pollutants [26]. However, the overall efficiency achieved so far with TiO2-based systems is not sufficiently high to enable practical applications. This low efficiency is mainly due to the fast recombination of charge carriers produced by irradiated TiO2 and thus the low quantum yield in the generation of reactive species for organic degradation [7, 8]. Moreover, due to its large bandgap (~3.2?eV), all photo-driven applications of TiO2 require ultraviolet light excitation. As a consequence, TiO2 shows photocatalytic activities under only a small fraction (<5%) of solar irradiation [9], limiting its practical applications. Therefore, the modification of TiO2 to enhance light absorption and photocatalytic activities under visible light has been the subject of recent research [10].

Many factors that influence the photodegradation efficiency of organic pollutants over TiO2 have been reported during the past 30 years. The main focus is the physical properties of TiO2, including crystal phases, crystal facets, crystallinity, particle size, surface area, porosity, and morphology. It is not easy to make a reliable correlation between the structure and photoactivity of TiO2 because the solid physical properties often affect one another. In general, anatase is considered to be much more active than rutile, while a mixture of anatase and rutile, like Degussa P25, is claimed to be more active than anatase. So far, TiO2 with anatase and brookite phases as a photocatalyst has been rarely studied because brookite TiO2 has no activity for organic pollutant degradation and it needs higher temperature to prepare. Moreover, there has been little report about brookite TiO2 preparation at low temperature.

The investigations about introducing foreign species are in progress for improving the photocatalytic activity of TiO2 and broadening its absorption to solar spectrum. Among them, titania doped with metals or metallic cations such as transition metallic cations, rare metal cations, and noble metal have been widely explored [11, 12]. On one hand, these doped metallic cations tend to serve as recombination centers. As a result, in most cases, photoexcited charges are recombined by the sites of doping metallic cations [13]. On the other hand, it has been reported that suitable amount of metallic cation doping can promote the separation of photogenerated electrons for the improvement of photocatalytic activity [14, 15]. It is obvious that there is conflict. So, although there are quite a few publications about metal-doped TiO2, the mechanism for the improvement of photocatalytic activity has not been clear so far. It needs explore in detail.

In this paper, to explore the mechanism of photodegradation of organic pollutant by rare earth element-doped TiO2, neodymium (Nd) ion was selected as a TiO2 dopant due to its stability to form complexes with various Lewis bases in the interaction of these functional groups with f-orbitals of neodymium metal [16]. Thus, incorporation of Nd ion into a TiO2 matrix could provide an effective method to concentrate the organic pollutant at semiconductor surface. Here, Nd-doped TiO2 samples were prepared by sol-gel method together with pure TiO2 for comparison. The characteristics and properties of these photocatalyst were studied. The photocatalytic activity was evaluated by measuring photodegradation efficiency of rhodamine-B (Rh-B) under visible light. The photocatalytic mechanism of Nd-doped TiO2 was also investigated in detail by photoluminescence spectrometer (PL) [17] and scavenger addition method [18].

2. Experimental Section

2.1. Materials

The chemicals used in this experiment except P25 were all analytical reagent and purchased from Shanghai Guoyao Chemical Co. Pure TiO2, P25, was commercial product from Germany. The used water throughout the whole research was double-distilled water.

2.2. Experimental Procedure

Neodymium-doped TiO2 samples (Nd-doped TiO2) were prepared by sol-gel method. First, 5?mL of TiCl4 was dissolved and hydrolyzed with 200?mL distilled frozen water (0°C). Second, Nd2O3 with certain amount required for doping was suspended into a small amount of ethanol and then added to the above solution to produce transparent Nd3+ aqueous solution under vigorous stirring. Third, 10?M NaOH aqueous solution was added dropwisely into the transparent TiCl4 aqueous solution with Nd3+ to obtain a grey precipitate with an ultimate suspension of pH = 10.

In order to remove residual Na+ and Cl- ions, the precipitate was adequately washed with deionized water till the pH value of filtrated water was below 7. Then, the amorphous Nd-doped TiO2 was well dispersed into 400?mL of water, and chloride acid (20%) was added with corresponding amount. The above suspension was adjusted to pH 1.5, stirred for 4 hours at room temperature, and then had been aged at 70°C for 24?h in airproof condition. Finally, Nd3+-modified TiO2 sol was formed with uniform, stable, and semitransparent characteristics. The obtained sol can maintain homogenous distribution for a long time without sedimentation and delamination. Powder sample was prepared by aging, gelation, and vacuum-drying treatment of the above sol at 70°C for 8 hours. TiO2 sample was also prepared by the same procedure without the addition of the neodymium oxide suspension. The pure TiO2 and Nd-doped TiO2 samples were annealed in air at 400°C for 3 hours and ground to fine particles for different analysis.

2.3. Characterization of Photocatalysts

The crystalline phases of the synthesized TiO2 and Nd-doped TiO2 catalyst were analyzed by a Y-2000 diffractometer (/max 30?kV) using graphite monochromatic copper radiation (Cu Ka) ( = 0.154178?nm) at a scan rate of 0.06°?2?·s-1 at 40?kV, 40?mA over the diffraction angle range of 10 ~ 60°. Raman spectroscopy measurements were performed by using a Jobin Yvon Lab RAM HR 800UV micro-Raman system under an Ar+ (514.5?nm) laser excitation. UV-Vis diffuse reflectance spectra (DRS) were recorded with a PerkinElmer Lambda 35 Spectrophotometer. X-ray photoelectron spectra (XPS) measurements were performed in a PHI Quantum 2000 XPS system with a monochromatic Al Ka source and charge neutralizer to analyze surface chemicals and evaluate the amount and states of Nd atoms in the prepared samples. Nitrogen adsorption/desorption isotherms and Brunauer-Emmett-Teller (BET) specific surface area () were recorded on a Micrometrics ASAP 2010 analyzer (accelerated surface area and porosimetry system). All the samples were treated at 100°C prior to BET measurements. Barret-Joyner-Halender (BJH) method was used to determine pore size distribution. The formation of hydroxyl radicals (OH) on the surface of prepared samples under visible light was detected by a terephthalic acid (TA) probe method [17]. The PL spectra of generated 2-hydroxyterephthalic acid (TAOH) were measured by using a PerkinElmer Lambda 55 fluorescence spectrophotometer.

2.4. Photocatalytic Activity Measurement

The photocatalytic activities of the prepared samples were examined by the degradation of Rh-B under visible light. A 350?W Xenon lamp (Lap Pu, XQ) was used as a light source with a 420?nm cutoff filter right above the reactor to provide visible-light irradiation. The experiments were performed in self-constructed beaker-like glassware reactor with double walls for cooling system. 0.1?g of prepared sample was dispersed in 100?mL Rh-B solution with concentration of 10?µM. Prior to irradiation, the solution was stirred in dark for 30 minutes to reach an adsorption/desorption equilibrium. Then, the solution was exposed to light while stirring. 1?mL suspension was collected every 30 minutes and was centrifuged for solid-liquid separation. Rh-B residue concentration was determined by measuring characteristic absorption intensity of Rh-B absorption spectrum over a Shimadzu UV-1700 UV-vis spectrophotometer.

Scavenger addition method [18] was used to explore the mechanism of photodegradation of Rh-B over Nd-doped TiO2. The whole process was almost the same as that of the above photocatalytic activity measurement except with the presence of different scavengers: sodium oxalate (Na2Cr2O4) as hole scavenger, KI as hole, and OH scavenger and K2Cr2O6 (Cr (VI)) as electron scavenger.

3. Results and Discussion

3.1. Characterization of TiO2 and Nd-Doped TiO2 Photocatalysts

The degree of crystallinity and crystal structure of the prepared TiO2 and Nd-doped TiO2 samples were examined by XRD. The diffractograms recorded are shown in Figure 1. It reveals that the samples had anatase and brookite phases, a mixture of crystal structure. The phase content for the prepared samples is displayed in Table 1. With the increase in Nd doping amount, brookite phase content increased. In this study, the addition of NaOH to reaction solution increased pH value, leading to the increase of OH- concentration. This might promote the formation of brookite phase because its complex consumed the amount of OH- twice as much as rutile’s one. However, too much NaOH could result in the formation of amorphous TiO2 which contained much more OH- than brookite complex [1922]. The grain sizes of the prepared TiO2 and Nd-doped TiO2 powders were calculated using Scherrer equation: where is crystalline size, the wavelength of X-ray radiation (0.1541?nm), the constant usually taken as 0.89, and the peak width at half-maximum height and diffraction angle. The obtained crystalline sizes are shown in Table 1. There was almost no change for crystallite size when Nd was incorporated (20.32?nm for pure TiO2 and 20.35?nm for Nd-doped TiO2 with Ti?:?Nd = 11?:?1). The phase content of TiO2 can be calculated from the integrated intensities of anatase (101), rutile (101), and brookite (121) peaks with the following formulas [23]: where , , represent the weight fractions of anatase, rutile, and brookite, respectively. The other symbols , , and are the integrated intensities of anatase (101), rutile (110), and brookite (121) peaks, respectively. The variables and are two coefficients and their values are 0.886 and 2.721, respectively. The calculated data are shown in Table 1.

Substantially, the enlarged peaks at (101) (JCPDS number: 21-1272) and (121) plane for the three samples (Figures 1(B) and 1(C)) showed a slight shift to smaller angles with Nd incorporation. It indicates that Nd ion could enter into TiO2 lattice or interstitial site. We know that the difference in ionic radius between neodymium ion (Nd3+ = 0.11?nm) and titanium ion (Ti4+ = 0.064?nm) is large. Without calcination treatment in high temperature, neodymium ions introduced by the coprecipitation-peptization method would unlikely enter into TiO2 crystal structure. Actually, during TiCl4 hydrolysis and peptization reaction, neodymium and oxygen ions could form Nd-O oxide on the superficial layer of TiO2 [19, 20] particle by chemical bonding process and Nd3+ ions mainly existed as Nd–O–Ti in Nd-doped TiO2 mixture [25]. So, the conversion from amorphous to well-crystalline structure usually requires high annealing temperature of at least 400°C.

The XRD studies revealed the anatase and brookite structure of the prepared samples. The formation of the structure was further confirmed by Raman spectroscopy. In Figure 2(A), Raman spectra of pure TiO2 and Nd-doped TiO2 are shown. It can be seen that the peaks observed at around 144.86, 398.17, 515.33, and 637.86?cm-1 were attributed to anatase TiO2. The strongest Eg mode at 144.86?cm-1 arising from the external vibration of the anatase structure was well resolved, which indicates that anatase phase was formed in the as-prepared nanocrystals [24]. Its amplified figure is shown in Figure 2(B). It can be found that there was a distinct shift to high wave number after Nd doping, which is also an evidence to verify that Nd may be introduced into the lattice or interstitial site of titania. Additionally, there were three weak peaks at 240.22, 317.01, and 360.44?cm-1 attributed to vibration modes for brookite phase of TiO2 in Figure 2(A). The absence of the characteristic vibration modes of Nd or Nd2O3 in the Raman spectra suggests that there was no Nd2O3 segregation into TiO2.

The UV-Vis diffuse reflectance spectra (DRS) for the doped and the undoped TiO2 nanoparticles samples are presented in Figure 3. Modification of TiO2 with Nd significantly affected the light absorption property of the photocatalysts. A red shift of the absorption edge toward the visible region was observed for Nd-doped TiO2 compared with pure TiO2. From XRD and Raman data, we can see that Nd doping led to a little change of TiO2 crystal structure and there was a little shift for the main peaks. The change of TiO2 crystal structure may be due to the incorporation of Nd ion into TiO2 lattice. It can result in new energy level in the bandgap and charge transfer between TiO2 valence band and Nd ion doping level [21]. As a result, the Nd-doped TiO2 had lower bandgap and the presence of Nd in the doped photocatalyst facilitated visible-light absorption. By plotting ()1/2 versus with the absorption coefficient, the bandgap was calculated to be 3.11?eV for pure TiO2, 2.57?eV for Nd-doped TiO2 with Ti?:?Nd = 23?:?1 and 2.33?eV for that with Ti?:?Nd = 11?:?1. So, with the presence of Nd ion in TiO2 lattice, the bandgap of the materials decreased distinctly. Additionally, with the increase of Nd doping amount, the bandgap decreased gradually. It indicates that the Nd-doped TiO2 samples may have the higher ability to absorb visible light.

XPS technique is used to investigate chemical component at the surface of a sample. Figure 4 shows high-resolution XPS spectrum for Nd in the doped TiO2 sample. Although the peak for Nd was not so distinct, it can be confirmed that Nd was present in the doped sample. Figure 5 shows the high-resolution XPS spectrum for Ti 2p in pure TiO2 and Nd-doped TiO2 (Ti?:?Nd = 11?:?1) samples. The binding energy had 0.49?eV shift for Ti 2p3/2 and 0.65?eV for Ti 2p1/2, indicating the presence of Ti4+ and Ti3+. Besides, the graph of doped sample was fitted by three subpeaks with the binding energy of 460.05, 458.86, and 458.27?eV, which indicates the presence of Ti4+, Nd–Ti, and Ti3+, respectively. So, the complete incorporation of Nd into TiO2 lattice can be verified. The XPS spectra of the O 1s region of the pure TiO2 could be fitted into two subpeaks at 529.36 and 531.08?eV (Figure 6(a)), corresponding to the Ti–O bond in TiO2 and hydroxyl groups on the surface, respectively. However, the O 1s peak for Nd-doped TiO2 was located at 530.21?eV and board. It could be fitted into three subpeaks at about 530.21, 531.34, and 532.84?eV (in Figure 6(b)), which can be ascribed to Ti–O, Nd–O, and hydroxyl group, respectively [2628]. Therefore, three types of oxygen existed on the photocatalyst surface, including Ti–O in TiO2, Nd–O, and hydroxyl group. These hydroxyl groups as hole trapped sites may inhibit the simple recombination of electron hole [29].

Surface textural characteristics of the samples pure TiO2 and Nd-doped TiO2 are derived from N2 adsorption analysis. Specific surface area () by BET method, total pore volume calculated at , and average pore diameter values are presented in Table 1. The adsorption isotherms (Figure 7) of the samples showed type IV behaviour with the typical hysteresis loop. This hysteresis is characteristic of mesoporous materials [30, 31]. The pore size distributions for the pure TiO2 and Nd-doped samples are shown in Figure 8, which confirms the mesoporous nature of the samples. It can be seen in Table 1 that the doped sample had lower surface area and narrower average pore size distribution than the undoped one. This might be due to the slight increase of crystalline size for the doped titania mentioned in XRD pattern.

3.2. Photocatalytic Activity of Prepared Samples

The photocatalytic property of titania is known to depend on several factors like crystallinity, phase assemblage, and surface area [32]. The photocatalytic activity of neodymium-doped and undoped titania was studied through Rh-B degradation under visible light in comparison with commercial product P25. Figure 9 shows the variations of Rh-B concentration against irradiation time with the presence of photocatalysts. It can be seen that the concentration of Rh-B decreased gradually with the exposure time for the pure and doped TiO2 samples. Without the presence of photocatalysts, almost no Rh-B could be degraded. The results illustrate that all of the prepared samples had the ability for Rh-B degradation under visible light even for pure TiO2 and P25. Additionally, with the increase of Nd doping amount, the photocatalytic activity for the Nd-doped TiO2 increased. However, when the amount of Nd in the doped TiO2 was too much (Ti?:?Nd = 6?:?1), the activity was even lower than that of pure TiO2 and P25. So, there is an optional Nd doping amount, which is in accordance with the published studies [14, 33]. Nevertheless, the optional Nd doping amount is 0.5% [14] and 1 ~ 3?wt% [33] in the two papers. The amount calculated by raw reagents for the formation of Nd-doped TiO2 samples was about 9%, which is much higher. In fact, there should not be so much Nd incorporated into TiO2 in this case. The measurement of actual Nd amount in TiO2 is in progress.

3.3. Mechanism for Photocatalytic Activity Improvement by Nd Doping

From Figure 9, we can see that even pure TiO2 could degrade Rh-B under visible light. It is known that Rh-B can be excited by visible light and inject electrons to the conduction band of TiO2. So, the injected electrons react with O2 molecules adsorbed on TiO2 surface to yield radical anion and subsequently HO radical by protonation [34]. So, in this case, Rh-B can be degraded in even undoped TiO2 and P25 systems although their photodegradation efficiency is not high.

From Table 1 and Figure 7, we can see that the surface area of Nd-doped samples was lower than that of pure TiO2 although all of them had mesoporous structure. So, based on dye and O2 adsorption, there is no advantage for Nd doped samples compared with pure TiO2. As discussed in the XRD patterns and XPS spectra, there is Nd which may substitute titanium in the lattice and exist as the state of Nd3+ although the ion radius of Nd3+ is much larger than that of Ti4+. The doping energy level of Nd3+/Nd2+ is -0.4?eV [35], which is more positive than the potential of conduction band of TiO2 particles {?eV versus NHE (normal hydrogen electrode) at pH = 1} [36]. Therefore, after Nd doing, the electrons can be excited from valance band to the Nd3+/Nd2+ doping energy level under visible light. The UV-Vis spectra verifies that the Nd doped TiO2 had lower bandgap than TiO2 and was responsive to visible light (Figure 3). Besides, the energy of Nd3+/Nd2+ is below the conduction band of TiO2, so that it is easy for Nd3+ to capture injected electron by Rh-B from the conduction band. The electrons on the Nd3+/Nd2+ level can react with O2 adsorbed on TiO2 surface to yield radical anion and subsequently HO· radical by protonation, just as the electrons generated by Rh-B do. So, with the presence of part Nd in the lattice of TiO2, there are more photogenerated electrons which participate in the formation of radical anion and subsequently HO radical by protonation for dye degradation.

It can be seen from Figure 9 that the Nd-doped TiO2 with Ti?:?Nd = 6?:?1 shows a negative effect on the photodegradation of Rh-B. In fact, metal ion dopant can act as a mediator of interfacial charge transfer or act as recombination center. So, there is an optimal value for dopant concentration [37]. Here, the doping energy level of Nd3+/Nd2+ has three functions: firstly, it can capture electrons from the conduction band of TiO2 that injected from the dye, and thus the number of hydroxide radicals is reduced; secondly, electrons can be excited to the Nd3+/Nd2+ energy level from the valence band of TiO2, and then the holes leaving on the valance band can degrade dyes; thirdly, Nd3+/Nd2+ energy level can be the recombination center if the dopant concentration is not proper. These three processes compete with each other, so it is critical to find an optimal dopant concentration. In our experiment, from the result of photocatalytic activity, we can deduce that Nd doping concentration of 4 ~ 9 at. %, resulting in the good photocatalytic performance of Nd-doped TiO2.

From the discussed above, we know that Nd doping level exists between bandgap of TiO2 and is close to the conduction band, which corresponds to the calculation results [14]. So, with Nd-doped TiO2 as a photocatalyst irradiated under visible light, there are photogenerated holes and electrons which participate in the Rh-B degradation. It is well known that in liquid photocatalysis system, hole may oxidize organic pollutant directly or form hydroxide radical for degradation, and photogenerated electron can form OH through the reaction with adsorbed. In order to explore which, photogenerated hole or electron, is mainly responsible for Rh-B degradation, some scavengers were used to investigate the specific reactive species that may play important roles in this process [18]. Na2Cr2O4 was used as hole scavenger, KI as the scavenger for hole and OH, and Cr (VI) for electron. Figure 10 shows the photodegradation efficiencies of Rh-B over Nd-doped TiO2 (Ti?:?Nd = 11?:?1) in the presence of the scavengers under visible light. It can be seen that when Na2Cr2O4 was used as diagnostic tool for suppressing the hole process, the photocatalytic degradation of Rh-B was inhibited but not down close to zero as shown in Figure 10. From this inhibited effect, it can be deduced that photogenerated holes played a role in the photodegradation of Rh-B under visible light. In the presence of KI as hole and OH scavenger, the removal efficiency of Rh-B was almost the same as that Na2Cr2O4. It indicates that OH played a significant role in the system since the inhibition effect of OH can be regarded to be close to 100% after the deduction of hole effect. The addition of Cr (VI) as electron scavenger resulted in the degradation of only 6% of Rh-B after 90?min, which means that there was almost no activity for the photocatalyst with electron inhibition effect in the system. So, photogenerated electrons played a very important role in the degradation system. We know that the electrons resulted from Nd-doped TiO2 and excited dye have strong reductive ability and they can reduce the adsorbed oxygen into and then OH formation by protonation. These OH and on the surface of the photocatalyst can degrade the adsorbed Rh-B [38]. From Figure 10, it can be concluded that and OH are major reactive species for the photocatalytic degradation of Rh-B, and photogenerated electrons from Nd-doped TiO2 are responsible for the improvement of photocatalytic activity of the Nd-doped TiO2.

In order to verify that OH is present in the photodegradation system, the changes of PL spectra of terephthalic acid solution with irradiation time were recorded and the data is shown in Figure 11. It can be seen that a gradual increase in PL intensity at about 430?nm was observed with the increase of irradiation time. However, no PL increase was observed in the absence of visible light or Nd-doped TiO2 sample. This suggests that the fluorescence is from the chemical reaction between terephthalic acid and OH formed via photogenerated holes and electrons. Usually, PL intensity is proportional to the amount of produced hydroxyl radical over the photocatalyst [38]. So, the amount of formed hydroxyl radical gradually increased for Rh-B degradation under visible light with the increase of time.

Moreover, since the radius of Nd3+ is much larger than that of Ti4+, it will be difficult for all of Nd ions added to enter into TiO2 lattice. So, the presence of Nd3+ or Nd metal on nanoparticle surface may also promote the charge separation to improve photocatalytic activity of Nd-doped TiO2, which has been described by many published papers [14, 3943].

4. Conclusion

In summary, a simple sol-gel method has been used for the preparation of Nd-doped titania nanoparticles with anatase and brookite phases. The prepared samples can achieve photocatalytic degradation of dye (Rh-B) under visible light, and Nd-doped TiO2 has better activity than pure TiO2 and P25. The enhanced activity is related to the change of crystalline structure of TiO2. Partial Nd enters into TiO2 lattice for the formation of doping level, which makes TiO2 responsive to visible light. In Rh-B degradation system, hydroxide radicals as one of the reactive species are mainly produced by photogenerated electrons both from dye and Nd-doped TiO2 excitation and they are responsible for Rh-B degradation. The contribution from hole is not as pronounced as electron for the formation of hydroxide radials. This study may shed light on the improvement of photocatalytic activity of TiO2 under solar light by using metal doping method.


This work was financially supported by the National Natural Science Foundation of China (no. 20973070), the National Basic Research Program of China (no. 2009CB939704), the Key Project of the Ministry of Chinese Education (no. 109116), self-determined research funds of CCNU from the colleges’ basic research, Natural Science Foundation of Jiangsu Province (no. BK2009724), and Industry R&D program in Zhenjiang City (no. GY2009003). The authors are also indebted to Professor Wen-Zhong Shen and Miss Dan-Hua Xu in Shanghai Jiaotong University for Raman spectrum measurements.