Single-step Processing of Band Gap Tailored Copper-doped Titania Nanomaterials in a Flame Aerosol Reactor
Extending the absorption range of TiO2 from the UV region to the visible region of the electromagnetic spectrum of light can be achieved by modifying the structure of TiO2. Such modification is essential to facilitate the application of TiO2 in applications such as sunlight-enabled photovoltaic devices and photocatalysis. We have demonstrated the synthesis and characterization of long wavelength visible-light absorption Cu-doped TiO2 nanomaterials with well-controlled properties such as size, composition, morphology and crystallinity in a single-step flame aerosol reactor.
The properties of the nanomaterials such as size, crystallinity and morphology were well controlled by understanding the aerosol formation and growth process of nanoparticles in the high temperature combustion zone. The important process parameters controlled were: molar feed ratios of precursors, temperature and time history in the high temperature flame. Results indicate that with increasing Cu-doping concentrations from 1 to 15 wt%, the rutile phase increased from 1.2 to 21.8 % and the primary particle size decreased from ~47 nm to ~33 nm. However, increasing doping concentration decreases the crystallinity of the material. Annealing the Cu-doped TiO2 at 600oC increased the crystallinity and changed the morphology of the nanoparticles from spherical to hexagonal in structure. The band gap calculation indicates a band gap narrowing of 0.6 eV (2.48eV) was achieved at 15 wt% copper doping concentration compared to pristine TiO2 (3.31eV) synthesized at 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 structure.
Nanomaterials, Cu-doped TiO2, Crystal structure, Band gap, Flame synthesis.
Nanostructured TiO2 materials have drawn tremendous attention due to their wide scale applications in paints, pigments, sunscreens, water and air purification [1, 2], photocatalysis [3, 4] ), solar energy generation by photovoltaics and hydrogen production by water splitting . However, due to the wide band gap (~3.2 eV) TiO2 could utilize only ultraviolet light from solar spectrum for photo excitation. To facilitate the harvesting of energy from visible region of the solar spectrum in fields like photovoltaics and visible light photo catalysis, shifting of the absorption edge from UV region to visible spectrum of sunlight is essential . Now, many research efforts are concentrated to enhance the efficiency of photo activity by modifying the structure of TiO2. Different approaches which have been adopted are: size optimization , compositional optimization to make suboxides , surface modification , and doping [9-11]. Tailoring the band gap by modifying the structure of the nanomaterial by incorporating dopant in the host material is one of the most efficient and frequently used approaches [11-13]. Band gap narrowing also becomes possible if the electronic coupling effect between dopant and semiconductor is strong enough to change the band structure. To increase the absorption in long wavelength, numerous studies have been conducted by doping transition metal ions like Cu, Co, V, Fe, Nb, Pt, Pd and non metal like N, S, F [4, 9, 10, 14-17] to enhance the photocatalytic activity and optical absorption of TiO2. However, only in a few cases has doping led to noticeable visible light absorption. Doping with transition metals enhance the activity by lowering the band gap and decreasing the electron and hole pair combination generated by photo excitation. However, the performance in activity depends on the type of dopant, methods of doping and the concentration of the doping . Copper as a doping material has been less explored and is plentiful in the earth crust, and Cu-doped TiO2 is considered to be a good material for solar energy harvesting technologies . Cu-doped TiO2 has shown improved photocatalytic degradation [14, 20, 21] CO2 photoreduction , improved gas sensing [22, 23] and enhanced H2 production . A study by Mor et al (2008) has demonstrated that p-type Cu-Ti-O nanotube array films can significantly improve the photocurrent generation . However, when these Cu-doped nanomaterials are released to the environmental systems, it adversely affects the growth of the environmental bacteria and their enzymatic activity . Several synthesis methods like sol gel [26, 27], deposition precipitation , mechanical alloying  and wetness impregnation  technique have been adopted for synthesizing copper doped TiO2, which involve multiple and time consuming steps. These methods have not been able to control the nanomaterial properties like size, shape and crystallinity precisely. However, the gas phase synthesis process has the advantage of processing the material in a single step and independently controlling the material properties by precisely manipulating the process variables . The well controlled doped nanomaterial properties are essential to evaluate its toxic potential to environmental systems by exposing the nanoparticles to the environmental bacterial and in vivo animal study [29, 30]. Moreover, the gas phase aerosol reactor can be scaled up to synthesize large quantities of nanomaterials with controlled characteristics [31, 32].
The focus of this study was to develop single-step gas phase synthesis method to process copper doped TiO2 nanomaterials with tailored properties like size, composition, crystallinity and morphology for different applications. The effect of key process parameters such as feed flow rate, residence time, and doping concentration, in controlling the physical and chemical properties of the nanomaterials are demonstrated, and the mechanism of doped nanomaterial formation in flame aerosol reactor is explained qualitatively.
2. Materials and Methods
2.1. Nanomaterial Synthesis
1 shows the schematic of the flame aerosol reactor used for the synthesis of the Cu-doped TiO2 nanomaterials. The diffusion burner consisted of three concentric ports of stainless steel tubes whose design details are provided in Jiang et al (2007). The main components of the flame aerosol reactor system were as follows: a precursor feeding system, a three port co-flow diffusion burner, and a quenching and collection system. Titanium tetra-ispopropoxide (TTIP) vapor was used as the precursor for TiO2 and was fed from a bubbler containing TTIP through the central port of the burner. Copper nitrate trihydrate (99.5%, VWR) was used as the dopant precursor for the synthesis of Cu-doped TiO2. The bubbler containing the liquid TTIP precursor was placed in an oil bath and was maintained at a temperature of 98 oC. A heating tape was used to surround the precursor delivery tube and maintained at 200 oC to avoid the condensation of the precursor vapor in the delivery tube. The starting precursor solution was prepared by dissolving a known amount of copper nitrate in distilled water. A stainless steel collison nebulizer was used to generate fine spray droplets (2 μm), which were then carried by nitrogen gas through the high temperature zone of the flame. The doping percentage was varied by feeding the different molar ratios of both the precursors to the high temperature combustion zone of the diffusion flame. A diffusion flame of methane and oxygen was generated by sending methane and oxygen through the second and third port respectively. The overall doping concentration was varied from 0.25 to 15 wt %. The temperature and residence time history of the nanoparticles in the combustion zone were controlled to obtain the desired size of the nanoparticles. The volumetric flow rates for N2/TTIP, O2, and CH4 were precisely controlled by mass flow controllers at 2 lpm, 7.5 lpm, and 1.8 lpm respectively. Additional oxygen of 0.5 lpm was provided to maintain the same residence time in all of the cases. A 20 lpm flow of compressed at room temperature air was supplied to the quench ring for cooling and entered the flame zone in radically. The entrained air dilutes the aerosol stream and suppresses the aerosol growth mechanisms. The synthesized materials were collected in a micro glass fiber filter paper in the downstream by using a 100 lpm vacuum pump.
2.2. Material Characterization
The size, morphology and microstructures of the products were verified by a transmission electron microscope (TEM, Model: JEOL 2100F FE-(S) TEM) with an accelerating voltage of 200 kV and by a field emission scanning electron microscope (SEM, Model: JEOL 7001LVF FE-SEM). Samples for TEM were prepared by diluting nanomaterials in an ethanol solution and putting drops of the samples on a carbon coated copper grid. The elemental and mapping analysis of the doped nanomaterial was conducted using energy dispersive spectroscopy (EDS) analysis integrated in a SEM. Phase structures and crystallite size of the material were determined using an X-ray diffractometry technique using a Rigaku D-MAX/A9 diffractometer with Cu Kα radiation (λ=1.5418 A). The crystallites size was estimated using the Scherrer formula,
D=0.9λ / (β cosθ) (1)
where λ= wavelength of x-ray, θ is the diffraction angle of the peak (101) anatase phase TiO2 and β indicates the full width at half maximum. UV-visible absorption spectroscopy (Perkin Elmer Lambda 2S) was used to analyze the absorbance spectrum of the nanomaterials with a wavelength ranging from 200 to 800 nm. The absorption spectrum was measured on powder nanomaterials to avoid the interference that may arise due to the aggregation behavior of the nanoparticles in while measuring the absorption by dispersing the nanoparticles in liquid suspension. From the absorption spectrum, the band gap was estimated and all the measurements were taken at room temperature. The absorption edge was estimated to be the point where the absorption is 30 % of the maximum absorption, which corresponds to the point where 50 % of the photons were absorbed. This approach was followed because of the difficulty in finding the exact linear region of the absorption spectrum, according to conventional methods of estimation .
2.3. Experimental Test Plan
The list of experiments performed is given in Table 1. The flow rates were independently controlled to maintain the same residence time in the high temperature flame as residence time has a major effect on particle growth. TiO2 was synthesized under the same experimental conditions using only TTIP as the precursor (Test 1). The molar feed ratios of the two precursors were varied from 0.5 to 15% wt of the copper precursor to process the doped nanomaterials and the effect of doping level on TiO2 material properties (Test 2-6). The copper nitrate salt concentration in the solvent was changed to vary the doping concentration, where as all other process parameters were kept constant.
3. Results and Discussion
3.1. Particle Formation Mechanism
The formation mechanism of nanoparticles in the high temperature flame is illustrated in 1(B). When the two components are introduced together into gas phase reactors, unequal reaction rates of both the precursors can form particles of different compositions leading to inhomogenities in the final materials. However, in the high temperature flame environment (~2200 K), decomposition of the precursors is rapid and no difference in chemical reactions will be expected . The organometallic TTIP vapor decomposes very fast to form TiO2 monomers at a high flame temperature and subsequently nucleates to form particles, followed by evaporation and decomposition of the copper nitrate droplets generated by the nebulizer. Due to the low vapor pressure of the resultant oxide, classical theory of nucleation suggests that there is no thermodynamic barrier to particle formation . In this dual component system, additional surfaces are available for the copper vapor to condense and the resultant particle may also coagulate with each other. Depending on the reactor conditions and precursors concentration, a variety of materials can be formed, ranging from particles consisting of only copper oxide, particles of only TiO2 and the particles of mixed TiO2 and CuO. At a low copper doping concentration, copper atoms can be readily incorporated into the titanium lattice by a scavenging process similar to the phenomenon demonstrated by Wang et al (2001). Since the flame reactor temperature is on the order of 2200K and the number concentration of the TiO2 particles generated is very high, collision and inter-diffusion occur very quickly and result in a homogenous composition of the material. When the copper concentration is high, the copper species may nucleate to form individual oxide particles along with the individual TiO2 particle and Cu-doped TiO2 particles. Hence, the resultant aerosol formation mechanism and composition depends on factors such as precursor feed concentrations, decomposition rates and droplet evaporation rate.
TTIP has a decomposition temperature of 200 0C to form TiO2 monomers. In the presence of an excess oxidizing agent, the oxidation reaction of TTIP can be described by a first order kinetics. The decomposition rate of TTIP is s-1 . No data is available for the decomposition rate of copper nitrate precursor in the literature, the decomposition rate of copper nitrate precursor copper acetyl acetonate is assumed to be similar to that of copper acetyl acetonate decomposition rate reported s-1 . In the presence of an excess oxidizing agent, the rate law follows the first order kinetics. There is no significant difference in the decomposition rate of both the precursors calculated at 2200 0K. However, it is to be noted that the droplet generated in the case of copper precursor should be evaporated before undergoing decomposition at a high temperature. Therefore, the decomposition of the precursor is delayed compared to TTIP precursor by the magnitude of the evaporation time. Hence, after decomposition of the copper precursor, condensation occurs over the already formed TiO2 particles and undergoes inter-diffusion and coagulation to form doped TiO2 materials. The nanomaterials synthesized at 15 wt % doping concentration were verified by the single particle EDS analysis which found that all the particles were composed of copper and titania. No particles were found consisting of only Ti or only copper species.
The particles produced were largely spherical structure. The spherical morphology of the particles can be explained by understanding the characteristic times of the various aerosol process in the high temperature flame. The residence time (τr) of the particle in the high temperature was calculated according to the method described by Wegner and Prastinis (2005) and found to be less than 1 sec. The characteristic collision time (τc) was calculated (Biswas and Wu 1998) to be of the order of ~50 ms, whereas the characteristic sintering time (τs) was calculated ( Kobata et al., 1991) assuming 33 nm particles at 2000 K to be on the order of 200 X 10 -10 µs. Thus as soon as the particles collide, they form spherical particles since τs << τc < τr.
3.2. Crystal Structure
The XRD diffraction pattern of the doped nanomaterials syntheiszed at various compositions is shown in the 2. The pristine and Cu-doped TiO2 nanoparticles were prepared at the same flame conditions for comparison. The pristine TiO2 was mostly of anatase crystal structure. However, with increasing doping level concentration, the transformation from anatase to rutile occurs as shown in the 2 (A) from the (110) rutile peak, which is consistent with other studies [23, 38]. The anatase and rutile fraction was calculated according to Spurr and Myers (1957) . Initially prepared pristine TiO2 has 1.2 % rutile structure, but with increasing doping concentration the rutile phase increased to 21.8 % at 15 wt % copper. Even at high doping concentration (15 wt %), no dopant related phase was observed indicating that no dopant oxide phase present within the XRD detection limit. Copper species were also uniformly distributed in TiO2, which excludes the existence of copper related clusters in the doped TiO2. The same anatase and rutile phase transformation was observed by Cu-doped TiO2 synthesis by other methods [14, 26]. In this doped material synthesis, CuO is responsible for a higher number of defects inside the anatase phase, in such a way that formation and growth of a higher number of rutile nuclei into TiO2 anatase takes place faster (reference).
In this copper doping TiO2 material synthesis, there exist two possible ways copper ions can be incorporated into TiO2 crystal lattice. Following the same procedure by Zhang and Liu (2000), when Cu2+ replace Ti4+ can be written as:
CuO <=> Cu”Ti + Ox0 + V”0 (3)
Ox0<=> ½ O2 (g) + 2e- + V”0 (4)
Secondly, when copper occupies interstitial sites the defect reaction can be written as:
2CuO <=> 2Cu”i + 2Ox0 + V””Ti (5)
Equilibrium scotty defects can be expressed by:
nil <=> 2V”0 + V””Ti (6)
The equations imply that the substitution of Ti4+ by Cu2+ increases the oxygen vacancy concentration. Also, free electron concentration decreases with increasing oxygen vacancies and oxygen vacancies decreases with increasing V””Ti at a given oxygen partial pressure. The excess of oxygen vacancies created, when Cu2+ replace Ti4+ from the crystal lattice is the responsible for anatase to rutile transition [40, 41]. The ionic radius of Cu 2+ (0.73 oA) which is similar to the ionic radius of Ti4+ (0.64 oA), can substitute some of the titanium atoms in the high temperature flame process. For dopant ions with sizes comparable to that of the host ions, it is possibly easier for them to occupy the host sites as opposed to the dopants that have much larger or much smaller radii (reference). Nair et al (1999) studied the microstructure and phase transformation behavior and 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 are placed in the titania lattice points, creating a charge-compensating anion vacancy . The anatase-rutile transformation involves a contraction of the oxygen structure and movement of ions. 2(B) and 2(C) represents the XRD spectra for (101) and (200) anatase peaks scanned at a very small steps of 0.004 degree for pristine and doped TiO2 nanomaterials. It is important to note that, as the doping concentration increases, the major anatase peaks (101) and (200) broaden, which indicates a decrease in crystallite size. The change in peak position to the right with increasing doping concentration indicates that Cu2+ ions possibly replaced some Ti4+ ions along with the lattice expansion.
4 gives the TEM and HR-TEM images of the particle at 1 wt% and 15 wt% doping concentration. It shows that particles at lower doping concentrations are fully crystallized, and the crystal lattice spacing corresponds to the anatase phase of TiO2 (0.331±.03 nm), where as 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 were observed when Fe2+ and Zn2+ doped TiO2 were synthesized [8, 42]. At higher Fe2+/Ti4+ ratios more rutile and amorphous crystal structure was observed, consistent with our Cu-doped TiO2 materials .
3.3. Particle Size
TEM images shown in 4 illustrate the particle sizes obtained at different copper doping concentrations. Increasing doping concentration decreased the primary particle size. The geometric mean particle size obtained at 1 wt % doping was ~47 nm compared to 33 nm obtained at 15 wt% doping. The crystallite size was estimated from the XRD pattern obtained using Scherrer formula and the decreased crystallite size was observed with an increasing doping concentration. The crystallite size obtained at 1 wt % doping was 33 nm compared to the size observed at 25 nm and 23 nm at 5 % and 15 wt % doping concentration. It is important to note that crystallite size estimation from XRD is different from the particle size observed from the microscopic analysis, since XRD measures the size of the small domains within the grains and one particle may consist of several crystallites based on the preparation methods . The peak broadening observed in XRD pattern also qualitatively explains the change in particle size. The main reason for decreased particle size with increasing doping concentration is due to the inhibition of the grain growth due to increased copper concentration. As evident from the HR-TEM images, the amorphous part increases with an increase in copper concentration, which may prevent the grain growth. Wang et al (2001) observed rutile and amorphous crystal structure at higher Fe2+/Ti4+ ratios were consistent with our Cu-doped TiO2 materials . Wang et al (2001) also reported the decreased grain size with an increasing Fe2+ doping concentration. Reduction in size was also observed when Li et al (2009) synthesized Zn2+-doped SnO2 nanomaterials. In this doping material, dopants are expected to move from inside to the surface sites by a process called self purification and occupy the surface sites of TiO2 nanocrystals, which probably reduced the coalescence TiO2 single crystals to larger particle sizes . This change in particle size with doping concentration is fundamentally a very important phenomenon for electronic structure modification.
3.4. Optical properties
The effect of copper doping on absorption of the resulting doped nanomaterials i.e. band gap energy of pristine and doped TiO2 was determined by the diffusive reflectance spectroscopy measurement. The absorption spectra of the different composition of the doped materials are shown in the 5. The approach followed by Thimsen et al (2008) was used for estimating the band gap of the doped nanomaterial synthesized . The estimated Eg for pristine TiO2 was 3.27 eV which is consistent with the reported value for anatase TiO2 ( Reference). With increasing doping concentrations, the band gap decreased and was estimated to be 2.48 eV at the highest doping concentration of 15 wt%. A band gap change of 0.8 eV was achieved by incorporating Cu2+ ions into TiO2 crystal structure. The changing band gap is primarily due to substitution of Cu2+ ions into the electronic states of TiO2 to form the new lowest unoccupied molecular orbital. According to many investigations, the aliovalent doping can modify the band structure and also the other properties (reference). The results showed that with increasing doping concentration, increased absorbance occurred at the visible spectrum. It can be said that the copper modified TiO2 structure extends its absorption to the visible spectrum effectively. Hence, these copper doped materials can be utilized for various visible-light photocatalytic applications, which have been demonstrated by several researchers [14, 16, 23].
3.5. Effect of Annealing
As described and demonstrated, when the dopant is incorporated, its oxidation state and chemical environment can have an effect on the crystallite size and phase transformation. In the case of copper doping, the oxygen vacancies, which are considered as nucleation centers, enhance the nucleation process to form more rutile structure . Since both amorphous and crystalline phases were observed in HR-TEM images of the higher doping concentration, the samples were annealed at different temperatures to see the effect on crystal structure and morphology the as prepared Cu-doped TiO2 samples. The 1 and 15 wt% samples were analyzed at an annealing temperature of 400oC and 600oC for 6 hours. No phase transformation was observed at 400 oC. The Cu-doped TiO2 is stable up to 400 0C and at 600 oC, the transformation from anatase to rutile phase was observed, which is consistent with results reported by other researchers as shown in the 7 [23, 27]. The anatase weight fraction decreases to 21 % from 75 % for 15 wt% doped TiO2. However, the morphology of the particles changed from spherical to hexagonal structures for nanoparticles prepared at both the doping concentrations. The crystallite size increased with annealing. However at 15 wt% doping concentration, the phase related to CuO was observed based on the peaks recorded at Bragg angle of 35.5 o and 39 o from the XRD pattern ( 7). Kim et al (2003) demonstrated that in oxygen rich conditions, at temperatures above 1000 0C the copper can exist in Cu metal species . The presence of CuO may be due to the phase segregation of CuO when it annealed in the presence of air, so oxygen may have reacted with the copper species present on the surface and formed some segregated region of CuO. The HR-TEM images of samples annealed at 600oC are shown in the 7. The indicates that the annealed 1 wt% Cu-doped TiO2 particle was completely crystallized with no discontinuity in the crystal fringes as observed from HR-TEM images, which is similar to the as prepared 1 wt% Cu-doped TiO2 particles and the lattice fringes are corresponding to TiO2 crystal structures. However, for the 15 wt% doping sample, some amorphous regions were detected as shown in the 7 as highlighted with the white squares further suggesting effect of copper doping. More detailed investigations are needed to understand the effect of doping concentration and reaction environments on the changing morphology of the initially synthesized spherical particles during post synthesis treatment.
Cu-doped TiO2 nanoparticles were synthesized in a diffusion flame aerosol reactor and the properties of the doped materials were readily controlled using well understood process parameters. The increase in doping concentration caused the transformation from anatase to rutile phase of TiO2 due to replacements of Ti4+ atoms by Cu2+ atoms in the crystal structure of TiO2 and resulted in a decreased primary particle size. The annealing of the doped samples resulted in the phase segregation of CuO in the presence of air at higher doping concentrations, where as at low dopant concentration, greater susceptibility to crystallization during post-synthesis annealing was observed. Absorption spectroscopy measurements confirm a shift in the absorption wavelength, caused due to crystal structure modification, by potentially replacing copper ions in the TiO2 crystal structure. The observed narrowing of the band gap is an essential property for the absorption of photons from the visible-light solar spectrum for photocatalysis and solar energy applications.
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List of Table Captions
Table 1. Summary of the Experimental Test Plan and Results
Table 1. Summary of the Experimental Test Plan and Results
Cu wt %
Crystal Phase a
a Anatase weight fraction was calculated based on Spurr and Mayer formula (Spur and Mayer, 1957)