Single-step processing of copper-doped titania nanomaterials in a flame aerosol reactor
© Sahu and Biswas; licensee Springer. 2011
Received: 10 October 2010
Accepted: 6 July 2011
Published: 6 July 2011
Synthesis and characterization of long wavelength visible-light absorption Cu-doped TiO2 nanomaterials with well-controlled properties such as size, composition, morphology, and crystal phase have been demonstrated in a single-step flame aerosol reactor. This has been feasible by a detailed understanding of the formation and growth of nanoparticles in the high-temperature flame region. The important process parameters controlled were: molar feed ratios of precursors, temperature, and residence time in the high-temperature flame region. The ability to vary the crystal phase of the doped nanomaterials while keeping the primary particle size constant has been demonstrated. Results indicate that increasing the copper dopant concentration promotes an anatase to rutile phase transformation, decreased crystalline nature and primary particle size, and better suspension stability. Annealing the Cu-doped TiO2 nanoparticles increased the crystalline nature and changed the morphology from spherical to hexagonal structure. Measurements indicate a band gap narrowing by 0.8 eV (2.51 eV) was achieved at 15-wt.% copper dopant concentration compared to pristine TiO2 (3.31 eV) synthesized under the same flame conditions. The change in the crystal phase, size, and band gap is attributed to replacement of titanium atoms by copper atoms in the TiO2 crystal.
Nanosized TiO2 has been widely used because of its stability in aqueous environments and low production cost. However, its light absorption range is limited to the ultraviolet (UV) spectrum of light due to its wide band gap (approximately 3.2 eV). To shift the absorption range to the visible spectrum, various approaches have been pursued in the past involving size optimization , compositional variation to make sub-oxides , surface modification , and doping [4–6] to modify the TiO2 structure. Among these methods, tailoring the band structures by incorporating a dopant into the host nanomaterial is a promising approach [6–8]. Several studies have reported enhancement of absorbtion in the visible range and photocatalytic activity on doping TiO2 by transition metal ions like Cu, Co, V, Fe, Nb, and non-metal like N, S, F [4, 5, 9–11]. However, a major challenge is to process low-cost, and stable doped nanomaterials with well-controlled properties that can effectively absorb visible light.
Recently, copper has been increasingly investigated as a dopant for titania . Copper oxide is a narrow band gap (cupric oxide, 1.4 eV; cuprous oxide, 2.2 eV) material which has a high-absorption coefficient, but suffers from UV-induced photocorrosion . However, copper oxide coupled with TiO2 has been demonstrated to be stable with improved photocatalytic degradation properties [9, 13, 14], effective CO2 photoreduction [15, 16], improved gas sensing, and enhanced H2 production [17, 18]. It has been shown that Cu-doped TiO2 induces more toxicity compared to TiO2 . Though a large number of studies on Cu-doped TiO2 nanomaterials have been reported, there is little information available on the effect of dopant concentration on TiO2 properties. Dopants can replace Ti in the substitutional sites or be incorporated in the interstitial sites. In some cases, they may segregate on the surface . The creation of new energy states due to the incorporation of the dopant in the host TiO2 alters the particle properties, electronic structure, and light absorption properties. These affect their functionality, and hence can be used in different applications [3, 8, 20, 21]. In summary, there is a need to synthesize Cu-doped nanomaterials with controlled properties (independently) which will help understand in detail the role of the dopant in altering TiO2 properties. It is essential to have samples wherein one characteristic is varied, keeping the others the same. For example, samples of varying crystal phases while maintaining the size the same will allow to establish the dependence of biological activity with the crystal phase.
Studies have reported the preparation of various doped TiO2 nanomaterials by multi-step liquid-phase synthesis , gas-phase spray pyrolysis, and flame synthesis methods [22–24]. Flame aerosol synthesis is a single-step process and allows independent control of the material properties such as particle size, crystallinity, homogeneity, and degree of aggregation [25, 26]. At elevated temperatures encountered in the flame synthesis process, most dopants can diffuse rapidly  and be uniformly distributed due to excellent precursor vapor mixing at the molecular level [22, 20]. Furthermore, flame aerosol processing is a scalable technique that is commercially used to manufacture large quantities of nanomaterials .
The synthesis of Cu-doped TiO2 in a single-step flame aerosol process is reported in this paper. A detailed characterization of the as-produced samples to understand the influence of process parameters on material properties is done. The role of key process parameters such as molar feed ratio of precursors and dopant concentration on TiO2 nanomaterial properties such as size, composition, crystallinity, stability in suspension, and morphology are thoroughly investigated. A method to control the crystal phase of the Cu-doped TiO2 nanomaterial has been discussed. The effect of annealing temperature on crystal phase and microstructure of the Cu-doped TiO2 material is reported. A formation mechanism of Cu-doped TiO2 nanomaterial in the flame aerosol reactor is elucidated.
The size, morphology, and microstructure of the nanoparticles were determined by a transmission electron microscope (TEM; Model: JEOL 2100F FE-(S) TEM, JEOL Ltd., Tokyo, Japan) with an accelerating voltage of 200 kV and by a field emission scanning electron microscope (SEM) (Model: JEOL 7001LVF FE-SEM, JEOL Ltd.). The elemental analysis of the doped nanomaterial was done using energy dispersive spectroscopy (EDS) analysis integrated with a SEM. Phase structures of the material were determined using an X-ray diffractometer (XRD) with Cu Kα radiation (λ = 1.5418 A) (Rigaku D-MAX/A9). Zeta potential, an indicator of the stability of nanoparticles in suspensions, was measured by using a ZetaSizer Nano ZS (Malvern Instruments Ltd., Worcestershire, UK) dynamic light scattering instrument. Nanoparticles were dispersed in de-ionized water at a concentration of 30 μg/ml and sonicated for 25 min using a bath sonicator (40 W, 50 kHz, 5 Fisher Scientific, Fairlawn, New Jersey, USA) before zeta potential measurements. UV-visible absorption spectroscopy (Perkin Elmer Lambda 2S, Perkin Elmer, Waltham, MA, USA) was used to analyze the absorbance spectrum of the nanomaterials over wavelengths ranging from 200 to 800 nm at room temperature. From the absorption spectrum, the band gap was estimated. The absorption edge was estimated to be the point where the absorption was 30% of the maximum, corresponding to where 50% of the photons were absorbed. This approach was used because of the difficulty in finding the linear region of the absorption spectrum according to conventional methods of band gap estimation .
Experimental test plan
Summary of the experimental test plan
Dopant concentration (wt%)
Study the influence of dopant concentration on TiO2 material properties such as size, crystal phase, suspension stability, and light absorption.
Study the effect of methane flow rate on size and crystal phase of the material.
Annealing temperature, 400°C, 600°C
Examine the effect of annealing on phase and microstructure characteristics of Cu-doped TiO2 nanoparticles
Duration of annealing under air, 4 h
Results and discussion
Doping TiO2 with other atoms changes properties such as particle size, crystal structure, stability in suspension, and light absorption. The mechanism of Cu-doped TiO2 nanoparticle formation in the flame aerosol reactor is discussed first. The effect of copper dopant on TiO2 particle properties are discussed followed by crystal structure control of the doped TiO2 nanomaterials. Finally, microstructure changes of Cu-doped TiO2 are discussed under different annealing conditions.
Particle formation mechanism
Effect of copper dopant concentration on TiO2 properties
Particle size analysis
The functionality of TiO2 nanomaterials for various applications depends on its crystal phase. The anatase phase of TiO2 is preferred for photocataytic applications, whereas rutile phase is preferred for applications in pigments . It is, therefore, necessary to understand the modifications in the crystal structure by incorporation of the dopants in TiO2. The XRD diffraction pattern of the Cu-doped TiO2 nanomaterials synthesized at various concentrations is shown in Figure 4. The pristine and Cu-doped TiO2 nanoparticles were prepared at the same flame conditions for comparison. The pristine TiO2 was primarily anatase under the chosen processing conditions. However, with increasing dopant concentration, the transformation from anatase to rutile phase occurred, as shown in Figure 4a from the (110) rutile peak, consistent with other studies [18, 33]. The anatase and rutile fraction were calculated according to the formula proposed by Spurr and Myers . The pristine TiO2 had 1.2% rutile content, but with increasing doping concentration to 15 wt.%, the rutile phase increased to 21.8%. Even at high dopant concentration (15 wt.%), no pure dopant-related crystal phase was observed within the XRD detection limit. The same anatase to rutile phase transformation was observed for synthesis of Cu-doped TiO2 by other methods [9, 35].
The similarity in ionic radius of Cu2+ (0.73 Å) to that of Ti4+ (0.64 Å) enable copper to substitutionally replaces Ti in the titanium lattice in the flame environment, where particles are formed from the atomistic state. In the high-temperature flame synthesis of Cu-doped TiO2 nanomaterial, the copper dopant creates a higher number of defects inside the anatase phase, resulting in a faster formation and growth of a higher number of rutile nuclei . At elevated temperatures, the substitution of Ti4+ by Cu2+ increases the oxygen vacancy concentration and decreases the free electron concentration. The excess of oxygen vacancies created in the TiO2 crystal lattice is the responsible for anatase to rutile phase transition [36, 37]. Nair et al.  found that a dopant with an oxidation state above 4+ will reduce the oxygen vacancy concentration in the titania lattice as an interstitial impurity. Dopants with an oxidation state of 3+ or lower when placed in the titania lattice points create a charge-compensating anion vacancy  and cause a transformation to the rutile phase as also found in this study. At higher dopant concentration (15 wt.%) amorphous phase was also observed on the surface as well as in the bulk. The TEM and HR-TEM images 1 and 15-wt.% Cu-doped TiO2 nanoparticles (see Figure 3) shows that particles at lower doping concentrations are fully crystallized, and the crystal lattice spacing corresponds to the anatase phase of TiO2 (0.331 ± 0.03 nm), whereas the particle synthesized at 15-wt.% copper concentration shows both crystalline and amorphous phases of the material. The HR-TEM images confirm that Cu2+ doping retards the grain growth of TiO2 nanoparticles. Similar results of decreasing crystalline nature of material were observed when Fe2+- and Zn2+-doped TiO2 were synthesized [3, 22]. In a similar doping study, Wang et al.  found that at higher Fe2+/Ti4+ ratios of 0.12, more rutile and amorphous crystal structure was observed, consistent with our Cu-doped TiO2 materials.
Figure 4b and 4c represent the XRD spectra for (101) and (201) anatase peaks scanned at a very small steps of 0.004 degree for pristine and doped TiO2 nanomaterials. It is important to note that with increasing dopant concentration, broadening of the major anatase peaks (101) and (201) was observed, which indicates a decrease in crystallite size. The shift in peak position to the right  with increasing dopant concentration indicates that Cu2+ ions replaced some Ti4+ ions along with the lattice expansion. The results clearly indicate that addition of dopant alters the crystal phase of the host nanomaterial and the degree of phase transition depends on dopant types and their concentrations.
Zeta potential and suspension stability
Light absorption properties
Change in the optical absorption is due to the defect centers created by the substitution of Ti4+ by Cu2+ atoms in the TiO2 crystal lattice. Earlier studies indicated that doping with aliovalent ions changes the local lattice symmetry and defect characteristics, which could change the absorption properties and the material properties. In Cu-dopedTiO2, when copper ions are either located inside the bulk TiO2 or on the surface sites, a rearrangement of the neighbor atoms take place to compensate the charge deficiency, resulting in lattice deformation. The lattice deformation affects the electronic structure causing the band gap shift . Furthermore, small amounts of Cu2+ dopant in the lattice sites of TiO2 introduce oxygen vacancies due to the charge compensation effect [36, 41]. Increasing the copper doping concentration increases the oxygen vacancies and probably form a newly doubly occupied oxygen vacancy as discussed in Li et al. . Therefore absorption of the doped nanomaterial and band gap shift may be controlled by surface effects, doping-induced vacancies, and lattice strain. It can be said that the copper modified TiO2 structure extends its absorption to the visible spectrum of sunlight (400-700 nm) effectively. Hence, these copper-doped materials can be utilized for various visible-light photocatalytic applications, which have been demonstrated in several other studies [9, 18].
Crystal phase control of Cu-doped TiO2 nanoparticle
Effect of annealing on Cu-doped TiO2 nanoparticle properties
Cu-doped TiO2 nanoparticles were synthesized in a diffusion flame aerosol reactor and the properties were readily varied by controlling the processing conditions. The increase in dopant concentration caused the transformation from anatase to rutile phase of TiO2 due to replacement of Ti4+ by Cu2+ in the crystal structure of TiO2. A decrease in primary particle size was also observed. The doped nanomaterials exhibited better aqueous suspension stability compared to pristine TiO2 due to charge imbalance created. The annealing of the doped samples resulted in the phase segregation and crystallization of CuO for the higher dopant concentration samples. Spectroscopy measurements confirm a shift in the absorption to visible frequencies, due to crystal structure modification.
This work was partially supported by a grant from the NIEHS, Grant No. 100030N. The authors thank the McDonnell International Scholars Academy and the McDonnell Academy Global Energy and Environment Partnership (http://mageep.wustl.edu) for providing partial support for this work.
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