Background

TiO2 is an important transition metal oxide material with a major application area in photocatalysis, where it shows its efficiency in the removal of unwanted impurities from water, air, etc. [1, 2]. The main drawback of pure TiO2 is its wide band gap (3 to 3.2 eV); because of which, it absorbs only the UV part of solar radiation that accounts for only 4% of the total solar radiation, leaving most of the visible portion of light [2, 3]. This material can be made visible light-active either by introducing structural defects such as Ti3+ and oxygen vacancies or by incorporation of nonmetals such as N, C, and S, and transition and rare earth ions [3–10]. These defects and dopants create sub-band states in the band gap of TiO2 and shift the absorption edge of TiO2 towards the visible region, thus enhancing the visible light photoactivity. Structural defects, generated on doping, can effectively tune the band structure and control the photoactivity. In Fe, Ce-codoped TiO2, it is reported that doping of these ions leads to the creation of grain boundary defects and results in the bending of the valence and conduction bands. These defects limit the electron mobility to the interface and prevent charge carrier recombination [11]. Cu is also an important dopant because it has high electronic conductivity, and it is cheap and highly available on the Earth's crust [12]. Cu-doped TiO2 or CuO/TiO2 composite serves as an efficient photocatalytic material in the decomposition of gas-phase alcohols, acid orange 88, methylene blue, etc. [13–15]. Cu doping can effectively reduce the wide band gap of TiO2 by creating defects and d-band states of Cu in TiO2 and can also act as active trap centers of electrons to reduce carrier recombination [3, 16]. Other applications of Cu-doped TiO2 are as anode materials in Li-ion batteries [12], CO sensing [17], NO and CO2 reduction [18, 19], etc. Cu-doped TiO2 also shows activity in bacterial inactivation [20]. In this article, we have discussed the doping effect of Cu on the crystallinity of pure TiO2 nanoparticles. We have examined how Cu extends the absorption edge of TiO2 to the visible region and reduces the effective band gap of TiO2. The structural characterizations are done with X-ray diffraction, transmission electron microscopy, and Raman spectroscopy, while absorption spectra are analyzed in a UV–vis spectrometer in diffuse reflectance mode.

Methods

TiO2 nanoparticles with 2%, 4%, and 6% Cu were prepared by sol–gel method. The synthesis started with the addition of 10 ml of titanium isopropoxide (Sigma-Aldrich Corporation, St. Louis, MO, USA) to a mixture of 20 ml 2-propanol (Merck, Worli, Mumbai, India) and 10 ml of ethanol (Merck). The reaction mixture was stirred for 15 min, and then 1 ml of water was added to hydrolyze the isopropoxide chain. This was followed by dropwise addition of copper nitrate hexahydrate (Merck) solution with the requisite amount of Cu. The solution was stirred for 6 h, and after this time, the solution was transformed to gel. The gel was centrifuged in water and ethanol for five times and then dried in a vacuum oven at 80°C to get an amorphous Cu-doped TiO2 nanopowder. The nanopowder was then annealed in air at 450°C for 4 h to obtain Cu-doped TiO2 nanoparticles.

The X-ray diffraction (XRD) pattern of the samples was characterized using a Rigaku MiniFlex X-ray diffractometer (Rigaku, Tokyo, Japan) with CuKα radiation (λ = 1.54 Å) at a scanning angle between 20° and 70°. The high-resolution transmission electron microscope (TEM) images of the doped nanoparticles are obtained using a JEOL-JEM-2100 transmission electron microscope (JEOL Ltd., Tokyo, Japan) at an operating voltage of 200 kV. Energy dispersive X-ray (EDX) spectra of the samples were studied with a JEOL JSM (model 6390 LV) scanning electron microscope with an INCAx-Sight EDX detector (Oxford Instruments, Abingdon, Oxfordshire, UK). The Raman spectra of the samples were recorded using a Renishaw In-Via Raman spectrometer (Renishaw, Wotton-under-Edge, UK) at a resolution of 0.3 cm−1. Electron paramagnetic resonance (EPR) spectra were obtained using a Bruker EMX 300 EPR spectrometer (Bruker BioSpin GmbH, Silberstreifen 4, Germany), Diffuse reflectance spectra (DRS) of the samples were obtained using a Shimadzu 2450 UV–vis spectrophotometer (Shimadzu Corporation, Kyoto, Japan) with BaSO4 powder as the standard reference sample.

Results and discussion

X-ray diffraction analysis

The diffraction pattern of pristine and Cu-doped TiO2 nanoparticles is shown in Figure 1. The diffraction pattern corresponds to the tetragonal anatase phase of TiO2 (JCPDS-782486). Indeed, a small fraction of brookite phase is also present in between 27° and 36°, and at 40° (JCPDS-761934). Anatase and brookite are the metastable phases of TiO2 and usually formed at low temperature during solution-phase synthesis. Presence of brookite phase has also been observed by So et al. in TiO2 nanoparticles [21]. They reported that the brookite phase completely disappeared at high annealing temperature. Increase in the brookite fraction with doping indicates inhibition of the formation of complete anatase phase due to incorporation of Cu on the lattice site or grain boundary. The crystallite size of the samples is calculated by using Scherrer’s equation, d = 0.9 λ β Cos θ , where d is the crystallite size, λ is the wavelength of X-ray radiation, β is full width at half maximum, and θ is the diffraction angle. The resultant sizes are 6, 5.5, 5, and 4 nm for pristine and 2%, 4%, and 6% Cu, respectively. The diffraction peaks are broad in all the samples. The widening of the XRD peaks indicates reduction in the grain size and increase in the fraction of the amorphous grain boundary, containing many of the structural defects. The diffraction peak intensity is lower in the doped samples than in the pristine one. The low intense diffraction peaks indicate degradation of structural quality or loss of crystallinity of TiO2 on doping.

Figure 1
figure 1

XRD pattern of pure and Cu-doped TiO 2 nanoparticles with 2%, 4%, and 6% of Cu loading.

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TEM and EDX analysis

The high-resolution transmission electron microscope images of 2%, 4%, and 6% Cu-doped TiO2 nanoparticles are shown in Figure 2a,b,c. The particles are spherical in shape with some amount of agglomeration. The particle size distribution shows that the average numbers of particles have a size of 9, 8, and 8 nm, respectively, for 2%, 4%, and 6% Cu-doped TiO2. Therefore, doping does not affect the size of the nanoparticles. Figure 2d,e,f shows the EDX pattern of the 2%, 4%, and 6% Cu-doped TiO2 nanoparticles, showing the presence of Cu, Ti, and oxygen in the nanoparticles. There is no peak of nitrogen in the EDX, indicating removal of nitrate impurities during centrifugation.

Figure 2
figure 2

TEM images. (a) 2%, (b) 4%, and (c) 6% Cu-doped TiO2nanoparticles. (d, e, f) The EDX pattern of the 2%, 4%, and 6% Cu-doped TiO2.

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Raman spectroscopy study

Raman spectroscopy is a technique of importance for the understanding of the local structure changes on incorporation of dopant ions. The Raman spectra of our samples are shown in Figure 3.

Figure 3
figure 3

Raman spectra of pure and Cu-doped TiO 2 nanoparticles. The inset of the figure shows the enlarged view of the Eg mode of the nanoparticles.

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The Raman spectra of our samples correspond to the anatase phase of TiO2. We have not detected any secondary peaks related to Cu or its oxide phases. The intense peak at 149 cm−1 corresponds to the Eg mode of anatase TiO2[22, 23]. Other than this, two other low intense modes appear at 197 and 641 cm−1, respectively. The B1g mode occurs at 400 cm−1, and the A1g + B1g mode appears at 530 cm−1, respectively [23]. The significant observation in the Raman peaks of the samples is the broadness and shifting to a higher wavenumber with Cu loading. Any characteristic Raman vibration is associated with the Ti-O stretching, bending vibration [24]. The Eg peak is associated with the symmetric stretching vibration of O-Ti-O in TiO2, the B1g peak is due to the symmetric bending vibration of O-Ti-O, and the A1g peak is the result of antisymmetric bending vibration of O-Ti-O [24]. The ionic size of Cu2+ (0.73 Å) is larger than that of Ti4+ (0.64 Å), and hence, doping of this ion will distort the lattice structure of TiO2; since there is charge difference between Cu2+ and Ti4+, doping of Cu generates oxygen vacancies in the lattice of TiO2 to maintain the charge neutrality [3]. If doping occurs on the substitutional position on the Ti4+ site, the Ti-O-Ti bond will be disturbed and a new Cu-O-Ti or Cu-O-Cu bond will be formed. Therefore, disturbance of the Ti-O-Ti bonds and the formation of new Cu-O bonds will affect the Raman-active modes and will result in the broadening and shifting of the peaks. Although Cu2+ doping on the Ti4+ site will affect the entire Raman-active modes, we have considered the intense Eg peak to understand the doping effect. The Eg peak is associated with the Cu-O-Cu stretching mode vibration, and on doping, the strength of this vibration lowers, since oxygen vacancy is formed nearby. The formation of oxygen vacancy nearby Cu is theoretically proved in the Cu-doped TiO2 system [25]. Due to the generation of these oxygen vacancies, the lattice is contracted and the peak is shifted to a higher wavenumber. Perker and Siegel in their work reported that oxygen vacancies are responsible for the shifting and broadening of the Raman peak [26]. On the other hand, some people reported that quantum size effect has a role to play in the broadening and peak shifting [27]. From XRD and TEM, we have found that the size of the nanoparticles is in the nanoregime; therefore, phonon confinement will be prominent. Alongside, XRD also demonstrates that grain boundary defects are also generated. Therefore, both phonon confinement and structural defects may result in the shifting and broadening of the Raman Eg peak. The oxygen vacancy generation and lattice disruption of Cu-doped TiO2 are shown in Figure 4a.

Figure 4
figure 4

Distortion of the TiO 6 octahedra. (a) Schematic representation of the distortion of the TiO6 octahedra on doping Cu2+ion on the Ti4+lattice site. Doping of Cu2+ on Ti4+ generates oxygen vacancies (white ball) nearby Cu2+ (shown by a dashed line). (b) Jahn-Teller (J-T) distortion of the octahedral symmetry of TiO6 and changes in the d-d transition from octahedral to tetragonal. In undistorted octahedral coordination, only a single electronic transition occurs. On the other hand, due to the J-T effect, three possible d-d excitations may occur.

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Electron paramagnetic resonance spectra analysis

Electron paramagnetic resonance (EPR) is an important tool to understand the valence state of the dopant and the coordination environment of the dopant in the framework of the host. The EPR spectra of 2% Cu-doped TiO2 nanoparticles are shown in Figure 5. As shown in the spectra, the EPR spectra are asymmetric in shape and contain a signal corresponding to the presence of Cu2+ in the distorted octahedral coordination of TiO2[15, 28]. The intense peak is centered at g⊥ = 2.08 followed by less intense quadruple signals at gII = 2.21, 2.28, 2.38, and 2.48, respectively, in the lower side of the magnetic field. The values of both g⊥ and gII are greater than the g value of free electron ge = 2.0023. From the position of the above-mentioned g values, it can be said that Cu2+ is coordinated in the octahedral coordination of TiO2 and has substituted Ti4+ on the lattice site [15]. The EPR peak is also broad which indicates the presence of dipolar interaction among neighboring Cu2+ ions that leads to the increase in the width of the EPR peak.

Figure 5
figure 5

Room-temperature EPR spectra of 2% Cu-doped TiO 2 nanoparticles.

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Study of optical properties

The diffuse reflectance spectra of pure and doped TiO2 are shown in Figure 6a. The method adopted to acquire the absorption spectra of powder samples in DRS mode is that of Kubelka-Munk. The equation for the Kubelka-Munk method is represented by F(R) = (1 − R)2/2R, where R is the reflectance and F(R) is the absorbance [29]. The F(R) curves of the samples are shown in Figure 6b.

Figure 6
figure 6

Diffuse reflectance, Kubelka-Munk curves, and band gap determination. (a) Diffuse reflectance (%R) spectra of pure and Cu-doped TiO2 nanoparticles with corresponding (b) Kubelka-Munk absorption curves. (c) Determination of band gap of each samples by drawing a line at [F(R)hυ]1/2 = 0.

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Doped samples exhibit an absorption peak at 330 nm corresponding to the maximum absorption when electrons are excited from the valence to the conduction band. Doped samples show shifting in the absorption peak to the visible region. One absorption shifting occurs at just above 400 nm, and another big absorption hump appears in between 550 and 900 nm. In TiO2, the valence band (VB) is composed of O 2p states, and the conduction band (CB) is composed of Ti 3d states [30]. The 330-nm absorption peak is due to electronic transition from O 2p to Ti 3d. Doped samples contain an extended absorption edge above 400 nm and a broad absorption peak in between 550 and 900 nm. The first absorption between 400 and 500 nm appears as a result of interfacial charge transfer from the O 2p valence band to the Cu(II) state attached to TiO2[31–37]. These Cu(II) states may be present either as Cu(II) nanoclusters or in the form of amorphous oxide phase of CuO. Li et al. [15] demonstrated that Cu(II) does not show this absorption peak if it is present as CuO, but displays this absorption when it is present as Cu(II) ions attached to TiO2. However, Qiu et al. suggested that the absorption peak between 400 and 500 nm appears due to charge transfer from TiO2 to CuO clusters in a system of CuO/TiO2 nanocomposite [34]. Therefore, based on these observations, we can predict that the absorption appears due to charge transfer from O 2p to Cu(II) clusters or CuO amorphous phase. However, from EPR signals, it is found that copper is present as Cu(II) and is well placed in the octahedral coordination of TiO2. Moreover, the broadening of the EPR line suggests that Cu2+-Cu2+ dipolar interaction is taking place and that this is possible when Cu2+ is so closely associated possibly forming clusters. These may be nanoclusters of copper, since bulk copper clusters could have been detected in the diffraction pattern. Therefore, with the help of EPR results, we can suggest that the observed absorption band is due to charge transition from the valence band of TiO2 to Cu(II) nanoclusters attached to TiO2. Apart from the absorption band between 400 and 500 nm, doped samples contain another absorption hump extending from 550 to 900 nm. The second absorption band is mainly due to the d-d transition of Cu2+ in the crystalline environment of TiO2[31–37]. Cu2+ has a d9 electronic configuration, and in the pure octahedral coordination, the 2D state of Cu2+ is splitted into 2Eg ground state and 2T2g excited state with a single electronic transition. However, from the XRD and Raman spectra of the samples, it was found that doping of Cu2+ generates oxygen vacancies which lie near Cu2+. Therefore, Cu2+ is no longer in the pure octahedral (O h ) symmetry, and the symmetry is slightly distorted. A Jahn-Teller (J-T) distortion is taking place, changing the symmetry from O h to D4h[35]. Due to the J-T effect, the ground state 2Eg is further splitted into 2B1g ground and 2Ag excited states, and the 2T2g is separated into 2B2g ground and 2Eg excited states [35, 36]. Therefore, the possible optical transitions that may give rise to the above peaks are 2B1g → 2Ag, 2B1g → 2B2g, and 2B1g → 2Eg. In reports, the absorption at 450 and 900 nm and the broad band at 810 nm are assigned to 2B1g to 2Eg and 2B1g to 2B2g transitions, respectively [35–38]. This absorption peak cannot be shown by Cu+, since Cu+ has completely filled 3d10 configurations. Since 3d10 is highly stable, photoexcitation will not release electrons, and therefore, no absorption will appear. The d-d electronic transition of Cu2+ is shown in Figure 4b.

From the absorption spectra of the samples, it is understood that doping shifts the absorption edge of TiO2 from the UV to visible region. Now, we can determine the effective reduction in the band gap (BG) of TiO2 due to the incorporation of Cu2+ ions. Figure 6c shows the band gap of all samples. For BG determination, [F(R)hυ]n is plotted against hυ. Since anatase TiO2 is an indirect band gap semiconductor, the value of n = ½ (n = 2 for direct band gap) [29]. The line drawn on the linear part of [F(R)hυ]1/2 vs. hυ curve at [F(R)hυ]1/2 = 0 gives the band gap. From UV–vis spectroscopy, it is found that Cu2+ forms sub-band states in the band gap of TiO2[39]. Along with Cu2+, oxygen defect band states are also formed in the band gap. In pure TiO2, the electronic transition occurs directly from VB to CB. However, on Cu doping, the electrons are not directly excited to CB since the unoccupied Cu2+s-d states and oxygen vacancies capture the electrons. The charged oxygen defect states formed in TiO2 are F (two electrons), F+ (single electron), and F++ (devoid of electrons) [40]. The oxygen vacancy states that may capture electrons are F+ and F++, respectively. Therefore, the sub-band states of Cu2+ and oxygen defects are responsible for the reduction of effective band gap of TiO2 nanoparticles. Sahu et al. [3] examined the red shift in the band gap of TiO2 on Cu doping and examined that absorption edge shifting and band gap reduction are controlled by the surface of the nanoparticles, lattice strain, and vacancies.

We can determine the width of the defect bands formed as an intermediate state in the band gap of TiO2. These defect band states create a band tail extending from the lower of conduction band to deep down of band gap, and similarly, the defect states very near to the valence band also smear the valence band edge deep inside the gap. Therefore, on both sides of the valence band maximum and conduction band minimum, an energy tail is formed. This defect tail is known as the Urbach tail, and the energy associated with this defect tail is referred to as Urbach energy. The equation for Urbach energy is given by α = α 0 exp E E U , where α is the absorption coefficient, E is the photon energy, and Eu is the Urbach energy [41, 42]. The Urbach energy is calculated by plotting lnα vs. E. The reciprocal of the slopes of the linear portion, below the optical band gap, gives the value of Eu. The Urbach energy of each sample is shown in Figure 7.

Figure 7
figure 7

Plot of ln [ F ( R )] vs. hυ for the determination of Urbach energy ( E u ). (a) 0.0%, (b) 2.0%, (c) 4.0%, and (d) 6.0% Cu-doped TiO2 nanoparticles. The reciprocal of the slope of the linear fitting gives the value of Eu.

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Absorbance coefficient α is proportional to F(R); hence, we can write ln[F(R)] vs. E. The Urbach energy of pure and 2%, 4%, and 6% Cu is 67, 259, 316, and 343 meV, respectively. Therefore, as band gap decreases, the magnitude of defect energy increases. This clearly supports our argument that sub-band states formed in between the valence and conduction bands result in the narrowing of the band gap. With doping level, the number of defect levels below the conduction band increases to such an extent that the band edge is shifted deep into the forbidden gap, thereby reducing the effective band gap of TiO2. The schematic of the formation of the Urbach tail and the relationship of the band gap with Urbach energy for different samples are shown in Figure 8a,b.

Figure 8
figure 8

Result of doping and variation of the band gap. (a) Doping induced smearing of the valence and conduction band edges and formation of Urbach tail in Cu-doped TiO2 nanoparticles. (b) Variation of band gap and Urbach energy for different concentrations of Cu.

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Conclusion

Doping of Cu introduces structural defects and increases the fraction of brookite phase. The diffraction peaks are broad in the doped samples, indicating reduction in the grain size and increase in the amount of amorphous grain boundary. The small crystallite size and oxygen defects are responsible for the broadening and shifting of Raman peaks. Cu2+ introduces d states in the mid-band gap of TiO2. The single electron in the d orbital undergoes Jahn-Teller distortion and gives rise to a broad absorption peak in the visible region. The d states of Cu2+ and oxygen defects are responsible for the narrowing of the band gap of TiO2. These defects increase the magnitude of Urbach energy, and the magnitude of this energy increases as Cu loading increases.