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One-step synthesis of nitrogen-doped ZnO nanocrystallites and their properties
Applied Surface Science 255 (2009) 5656–5661
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
One-step synthesis of nitrogen-doped ZnO nanocrystallites and their properties Min Zheng *, Jiaqing Wu College of Material Engineering, Soochow University, No. 172, Ganjiang Road East, Suzhou 215006, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 April 2008 Received in revised form 7 October 2008 Accepted 27 October 2008 Available online 5 November 2008
Nitrogen-doped ZnO nanocrystallites with primary diameter in the range of 30–50 nm were synthesized rapidly by self-assembly combustion technique using urea and citric acid as fuels, zinc nitrate as oxidant. The variation of adiabatic flame temperature with the various fuel compounds was calculated theoretically according to the thermodynamic concept. XRD, SEM and XPS were used to characterize the as-synthesized products. The anti-ultraviolet and anti-bacterium ability of the products were estimated by testing the UPF index and repressing-bacterium circle diameter of the cotton fabrics, which were treated using the as-synthesized products. The results show that a small quantity of citric acid has excellent coordinated action with urea and accelerates the nitrogen doping reaction course, obtaining perfect nitrogen-doped ZnO nanocrystallites with uniform color and particles size. The analysis of XPS spectrum shows that the nitrogen incorporation produces an N–O bonding region. The incorporation of nitrogen greatly improves the anti-ultraviolet and anti-bacterium properties of ZnO nanocrystallites, which can be attributed to the change of surface properties of nitrogen-doped ZnO, such as O vacancies and crystal deficiency. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Combustion Nitrogen-doped ZnO Nanocrystallite Anti-ultraviolet Anti-bacterium
1. Introduction Recently, much attention has been focused on the modification of nanocrystallite metal oxide by doping or substituting with special atom. With such modification, it is possible to improve the electrical and optical properties of materials by changing the surface properties, such as electronic band gap, O vacancies, crystal deficiencies and specific surface area (SSA). Therefore, such systems are becoming more and more important in material field and are being used as photovoltaics, electrochromics, sensors, photocatalysts and anti-ultraviolet agent [1–3]. Among many metal oxides, zinc oxide (ZnO) has attracted considerable attention because of its low cost, non-toxicity, high stability and high efficiency. Specifically, zinc oxide nanoparticles can serve as an excellent source for resisting bacterium and shielding ultraviolet [4,5]. Currently, however, the highly efficient use of ZnO is impaired by its wide bandgap (3.27 eV), which only responses to a small fraction of the sun’s energy spectrum. Thus, one of the goals to improve the optical response of ZnO is to increase its optical activity by doping with special atom. Recently, metal-doped ZnO nanoparticles, such as Ag, Cu, Sb, Au, As and Li, have been researched [6–9]. However, metal doping has several drawbacks including thermal instability, deep color and potential toxicity, which are ill-suited for textile, cosmetic and medicine field.
* Corresponding author. Tel.: +86 512 68998646. E-mail address: [email protected] (M. Zheng). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.10.091
Therefore, it is necessary to improve the optical response of ZnO by using non-metal dopant species. Doping of nitrogen is found to be very effective on improving photoactivity of the metal oxide because its p state contributes to the band gap narrowing by mixing with 2p state of O. However, it is more difficult to replace the O2 with nitrogen than metal due to the differences in charge states and ionic radii [10]. Present studies are focused on N-doped ZnO thin film [11–14], there are only a few literatures to report Ndoped ZnO nanocrystallites and their properties [15–17]. A common synthesis route for N-doped metal oxide powder is the reaction of the metal oxide with flowing ammonia gas at a high temperature, which is required to introduce nitrogen atoms into metal oxide crystal lattices. Furthermore, a series of procedures including depositing, filtrating, washing and calcining are needed usually in conventional substituting technique. The complex procedures and long-reaction time at high temperature result in large particle size and impurity of the product. Recently grinding– heating treatment and solvothermal technique are used to prepare N-doped ZnO nanopowder, which have some advantages than the conventional substituting technique [18,19]. Here, we described a very simple and rapid method to synthesize N-doped ZnO nanocrystallites by a kind of selfassembly combustion (SAC) technique, which was different from the approaches reported above. Our group has synthesized successfully antimony-doped tin dioxide nanocrystals via similar technique [20]. This paper focused on the microcosmic and physical properties of nitrogen-doped ZnO synthesized by SAC technique.
M. Zheng, J. Wu / Applied Surface Science 255 (2009) 5656–5661
2. Experimental 2.1. The preparation of N-doped ZnO All reagents were commercially available and used without further purification. The whole SAC synthesis procedure could be divided into two parts: one was the preparation of precursor with high homogeneity by mixing starting materials and the other was the formation of N-doped ZnO nanocrystallites in a resistance furnace. Use of suitable raw materials in SAC synthesis ensures stability of the combustion reaction and high quality of the products. More importantly, it must be readily available and convenient to be used. In addition, it should not react violently and release amounts of smoke. In this research, urea was selected as main fuel because of its cheapness and dual roles acting as nitrogen source and energy source simultaneously. Furthermore, citric acid was selected to adjust the combustion process and control growth of the assynthesized N-doped ZnO crystallites. In a typical experiment, firstly, Zn(NO3)26H2O was dissolved in distilled water to attain 2.0 M solutions, and the chosen fuels, including citric and urea, were assembled in an appropriate proportion to form into fuel compounds. Then, the fuel compounds were mixed well with the stock zinc nitrate solution by stirring until a ropy paste (hereafter termed as precursor) was obtained. This precursor was put into a resistance furnace, whose temperature was pre-setup according to the DTA–TG curves of the precursor. After going through boiling, evaporating and concentrating, the precursor suddenly foamed up and deflagrated, leaving flocculent powder like sponge. The relative chemical reaction equation was presented below
ZnðNO3 Þ2 6H2 OðcÞ þ xCOðNH2 Þ2 ðcÞ þ yC6 H8 O7 H2 OðcÞ 3x þ 9y 5 D O2 ðgÞ!ZnOðcÞ þ ð1 þ xÞN2 ðgÞ þ 2 þ ðx þ 6yÞCO2 ðgÞ þ ð6 þ 2x þ 5yÞH2 OðgÞ Oo ðZnOÞ þ N2 Ð 2N1o ðZnOÞ þ Vo þ
1 O2 2
(1) (2)
In Eq. (1), (3x) + 9y 5 = 0 corresponded to the situation of ‘‘the reactant composition was set at the condition that equivalent stoichiometric ratio’’, which implied that the oxygen content of zinc nitrate could be completely reacted to oxidize fuels equivalently without demanding oxygen from any external source, theoretically. Also, both the rate of reaction and the heat liberated per unit time were maxima under this situation. Referring to Eq. (1), there were three representative reactant compositions selected to synthesize ZnO products, that was: (1) x = 1.667, y = 0; (2) x = 1.333, y = 0.111; (3) x = 0, y = 0.556, which were numbered as #1, #2 and #3, respectively. However, for N-doped ZnO, urea was indispensiable because it was the main nitrogen source. The practical experimental phenomena also revealed that only white ZnO could be prepared when urea was in absence. Citric acid, a kind of fuel regulator, couldn’t exceed 15%mol in the whole fuels according to our experiment in order to obtain N-doped ZnO. So another representative reactant composition was also selected to synthesize N-doped ZnO product, which was: (4) x = 1.07, y = 0.19, numbered as #4. 2.2. The characterization of N-doped ZnO Differential thermal and thermogravimetric analysis (DTA–TG) of the mixed slurry precursors produced at different reaction modes was carried out at a heating rate of 108/min in static air (TA Instrument, SDT Q600 V8.3).
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The crystalline phase of the as-synthesized samples was identified by X-ray diffraction analyzer (shimadzu XRD-6000) using Cu Ka radiation (l = 0.15418 nm) and the crystal size was calculated with Scherer’s formula. The morphological feature of the as-synthesized sample was observed by a scan electron microscope (SEM) (Japan S-570). Analysis of nitrogen content was done with Thermo ESCALAB 250 XPS instrument (America) using Al Ka radiation ðhn ¼ 1486:6 eVÞ. 2.3. Estimation of anti-ultraviolet and anti-bacterium properties of N-doped ZnO Treatment of cotton fabrics using N-doped ZnO: N-doped ZnO aqueous dispersion was prepared by adding 1.0 g of the assynthesized sample and 0.2 g of polymer surfactant polyvinylpyrrolidone (PVP) into 100 ml of distilled water with pH value adjusted to 6.0, then keeping stirred at 60 8C for 30 min. 5.0 g of cotton fabric was washed before use for several times in acetone for 20 min using an ultrasonic bath to remove surface impurities. After dried, it was impregnated for 30 min in a treatment solution containing above N-doped ZnO aqueous dispersion, 0.5 g binder agent and 1.0 g silicon softener, then were padded twice (take-up 80%) on a laboratory padding mangle. After they were padded, the cotton fabric was immediately dried at 100 8C for 2.0 min and cured at 160 8C for 1.0 min. Anti-ultraviolet testing: The Ultraviolet blocking effects of the fabric samples were measured in accordance with the Australian/New Zealand Standard AS/NZS 4339 (1996) by the Labsphere UV-1000F (Japan) with the scope of wavelength ranging from 280 nm to 400 nm. The value of ultraviolet protection factor (UPF) was recorded and the result of sun protective clothing was classified according to the rated ultraviolet protection factor. Anti-bacterium testing: E. coli and S. aureus were selected as indicators of the experimental bacteria. The anti-bacterial performance of the anti-bacterial cotton fibers was measured according to FZ/T73023-2006 anti-bacterial standard by measuring the depress-bacterium circle diameter. To prepare an agar plate, solid culture was prepared by mixing 2.0 g agar, 0.5 g peptone and 0.3 g beef extract in 100 ml distilled water. The agar plate was prepared by pouring the solid culture onto sterile circular plates and allowing it to solidify in the Stericlean Vertical Laminar Flow chamber. 1 ml of cultured bacterial broth, containing about 106– 107 organisms, was uniformly distributed on the plate. Cotton fibric disk with 6 mm diameter was placed on the plate. Subsequently the plate was incubated for 48 h at 36 8C in a biochemistry incubator. The depress-bacterium circle was screened by digital camera and the diameter of circle was measured using vernier calipers. 3. Results and discussion 3.1. Phenomena of deflagration synthesis and thermodynamic analysis In our study, the main energy released from the exothermic reaction between fuels and zinc nitrate, which could rapidly heat the system to a high temperature and assure the synthesis to occur. The fuels compositions had important effects on the combustion phenomena and the properties of the as-synthesized products. In order to understand the variation in adiabatic flame temperature (Tad) with different fuels compositions and molar ration of fuels-tozinc nitrate, the Tad could be calculated theoretically. Table 1 listed the relevant thermodynamic data for the reactants and products in the present study. It was well known that enthalpy of reaction
M. Zheng, J. Wu / Applied Surface Science 255 (2009) 5656–5661
5658 Table 1 Relevant thermodynamic data [21]. Compound
DH (kcal/mol) Cp (kcal/mol K)
Zn(NO3)26H2O (c)
CO(NH2)2 (c)
C6H8O7H2O (c)
ZnO (c)
O2 (g)
N2 (g)
CO2 (g)
H2O (g)
550.92 0.07200
79.94 0.02239
370.54 –
83.24 0.009620
0 –
0 0.006958
94.05 0.008876
57.80 0.008024
Note: (c) = crystalline, (g) = gas.
could be expressed as X DH ¼ n DH f
products
X
n DH f
(3)
reactants
Where n was the number of the mole. Using the thermodynamic data for various reactants and products listed in Table 1, the enthalpy of reaction for Eq. (1) could be calculated as follows:
DH ðkcalÞ ¼ 120:88 129:71x 482:76y
(4)
The following equation could be used to the theoretically approximate the adiabatic flame temperature for a deflagration reaction Q ¼ DH ¼
Z
Tad
X
n C p
298
products
of fuels at desired proportions, which was approved through samples #2 and #4 in Table 2. It was worth mentioning that a small quantity of citric acid had no evident effect on the whole heat energy and adiabatic flame temperature, however, a remarkable change in crystallite size and product color could be seen in Table 2. The detailed discussion would appear in the following section combining with the XRD patterns analysis of the assynthesized samples.
dT
(5)
where Q was the heat absorbed by products under adiabatic condition, and Cp was the heat capacity of the products at constant pressure. Substituting the thermodynamic data in Table 1 and Eq. (4), the adiabatic flame temperatures and relative datas could be calculated, which were presented in Table 2. Furthermore, the relative experimental results were also presented in Table 2. By single urea as fuel, the slurry precursor went through a slow frothing process, so the reaction was slow and emitted weak firelight, which lasted about 2–3 min, leaving product with uneven color. The middle product was pink and the marginal product was white in reaction vessel (see sample #1 in Table 2). According to some literatures reported about N-doped metal oxide [15,16], the change in color of the nanocrystals could be attributed to the substitution of nitrogen for oxygen atom in ZnO crystal lattice, which aroused the optical response in the visible wavelength range. Therefore, the existence of white product indicated a incomplete substitution of N for O2 in zinc oxide, which could be attributed to the inadequate heat energy (95.347 kcal/mol) during combustion. Whereas using single citric acid as fuel, the reaction course was very rapid and violent accompanying with dazzling white light, producing a pure white product with amounts of conglomeration (see sample #3 in Table 2). The white product indicated that no nitrogen incorporation reaction took place in reaction mode #3 because of the small amount of nitrogen gases produced (1.000 mol), as could be seen in Table 2. According to above experimental results, it was suggested that perfect N-doped ZnO nanocrystallites could be produced through mixing two kinds
3.2. Effects of fuels compositions on the microstructure of assynthesized products Fig. 1 showed the XRD patterns of the as-synthesized products prepared at four different fuels compositions. As shown in Fig. 1, the strong diffraction peaks in all of the as-synthesized products could be indexed as (1 0 0), (0 0 2) and (1 0 1) planes of a standard wurtzite zinc oxide crystal (JCPDS5-664) and the other planes such as (1 0 2), (1 1 0), (1 0 3) and (1 1 2) were also visible. The strong and sharp reflection peaks suggested that the as-synthesized products were well crystallized. There was no other crystalline phase in all of these X-ray diffraction patterns, which indicated that nitrogen atom brings into the crystal lattice of zinc oxide to substitute for oxygen atom and didnot bring about a new object phase, which was consistent with the traditional doping theory. The corresponding color, nitrogen content and primary crystallite size estimated by Scherer formula were shown in Table 1. The change in crystallite size and product color could be due to the different substitution domino effect at different fuels compositions, especially the use of citric acid. In our study, citric acid itself didnot supply nitrogen resource and had no direct relation with the nitrogen content in product; however, citric acid could promote complete and uniform substitution of nitrogen for oxygen. Therefore, the sample #1 took on pink/white color (the middle product was pink and the marginal product was white in reaction vessel) when citric acid was in absence, while the samples #2 and #4 took on red and pink color, respectively when citric acid was in presence. Comparing the color and nitrogen content of the samples #2 with #4, it was found that the samples had different color and nitrogen content when addition content of citric acid was different. The above experiment results indicated that excellent coordinated domino effect of citric acid with urea was exploded and substitution of nitrogen for oxygen was enhanced when reaction mode #2 was carried out. Moreover, the higher nitrogen
Table 2 Effects of fuels compositions on nature of deflagration and characteristics of as-synthesized samples. Reaction mode
DH f
Tada (8C)
Amount of gases producedb (mol)
Amount of nitrogen gases producedc (mol)
Crystallited size (nm)
Nitrogen content in product (wt%)
Sample color
826.597 901.957 1265.309 940.444
13.670 13.553 13.116 13.370
2.667 2.333 1.000 2.07
19.4 43.7 78.5 23.6
1.02 1.25 0 0.85
Pink/white Red White Pink
(kcal/mol) #1: #2: #3: #4: a b c d
x = 1.667, y = 0 x = 1.333, y = 0.111 x = 0, y = 0.556 x = 1.07, y = 0.19
95.347 105.61 147.535 109.634
Calculated theoretically by thermodynamic data and Eq. (5). Obtained from Eq. (4). Obtained from Eq. (4). Estimated by Scherer formula (X-ray line broadening).
M. Zheng, J. Wu / Applied Surface Science 255 (2009) 5656–5661
Fig. 1. X-ray diffraction of the as-synthesized ZnO at different fuel compositions.
content in sample #2 was also attributed to the more amounts of nitrogen gases produced in reaction mode #2. The excellent coordinated domino effect of citric acid could be explained by the result of the differential thermal and thermogravimetric analysis for the precursors at reaction modes 1# and 2#, which was displayed in Fig. 2. Comparing the curve of reaction mode 2# with that of 1#, the exothermic behavior of the precursor decomposition changed remarkably because of appending a small quantity of citric acid. The strong exothermic peak shifted from 304 8C to 253 8C downward and the peak became sharper, which was beneficial for doping of nitrogen in ZnO crystal lattice. The detailed research about the action mechanism of citric acid would be our future goal. Fig. 3 displayed the morphologies of the typical samples at different fuels compositions. The microstructure of the sample #1 revealed clusters of tiny particles with loose appearance and small size about 20 nm. The sample #3 took on some clear hexagonal and agglomerated flake crystallites with size of 50–200 nm. The samples #2 and #4 showed spherical particles with even particles distribution and small average diameter. For the sample #2, the particles distribution was in range of 30–50 nm and the average diameter was about 40 nm, which was consistent with the size estimated by Scherer formula in Table 1. The SEM morphologies of the sample #4 looked bigger than that of the sample #2, although the primary crystallites size was smaller than the sample #2 (seen
5659
in Table 2), which indicated that sintering phenomena took place when the reaction mode #4 was carried out. The difference among these morphologies and sizes could be attributed to the different action mechanism of fuels during combustion. According to above description about the combustion phenomena, the combustion course was different at different fuels compositions. The whole heat energy, adiabatic flame temperature, amounts of nitrogen gases produced during combustion and the coordinated domino effect of citric acid had important effect on the morphologies and size of the products. XPS profiles for the as-synthesized products were shown in Fig. 4. We examined three areas of the XPS spectrums, the N1 s region near 400 eV, the Zn 2p3/2 region near 1021 eV and the O 1s region near 532 eV. From our measurements, we found 1.02% nitrogen content in sample #1, 1.25% nitrogen content in sample #2 and 0.85% nitrogen content in sample #4. Although the N 1s peak in the XPS survey profiles was weak, the peak position and its broaden were clear in N 1s scan profiles, which were shown in Fig. 4(B). There were distinct differences in N 1s peak position and its broaden from different samples. For sample #2, the binding energy peak for the N was broad, extending from 395.4 eV to 405.7 eV, and was centered at 400.91 eV, a binding energy notably greater than the typical N–Zn binding energy, 395.9 eV, in Zn3N2. For samples #1 and #4, the binding energy peak for the N was almost the same, extending from 395.5 eV to 400.9 eV and centering at about 398.3 eV, a binding energy similar to the typical N–B binding energy, 398.1 eV, in BN. No dissociative nitrogen and N2 peaks were observed in the XPS energy spectrum, which confirmed that enhanced produces an N–O bonding region. Further, we could see three different O 1s positions at about 530.9 eV, 532.1 eV and 533.1 eV, respectively when scanning the O 1s XPS regions of corresponding samples, which were omitted in the paper. The diversfication of O 1s indicated that nitrogen substitution for oxygen arouses crystal lattice defection. 3.3. Evaluation of the anti-ultraviolet and anti-bacterium properties of the as-synthesized ZnO In the work, cotton fabrics were selected for the evaluation of the ultraviolet-blocking effects of the as-synthesized ZnO at different reaction modes. The cotton fabrics were treated using the same content of the different samples. In Fig. 5 were presented UV– vis transmission spectra of untreated fabric and treated fabrics and their corresponding UPF index. As it could be seen from these curves and UPF index, the untreated fabric had weak ultraviolet protecting ability and most of the ultraviolet light could permeate the fabric. After treated using ZnO samples, the ultraviolet protecting ability improved and
Fig. 2. TG–DTA curves of the precursors at different reaction modes.
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M. Zheng, J. Wu / Applied Surface Science 255 (2009) 5656–5661
Fig. 3. Typical SEM photographs of the as-synthesized ZnO at different fuel compositions.
the UPF index increased for all of the fabrics treated. However, the change degree was different from the different samples. Comparing the undoped-product (seen #3 in Fig. 5) with the dopedproducts (seen 1#, 2# and 4# in Fig. 5), it could be found that nitrogen incorporation in ZnO lattice improved obviously the ultraviolet protecting ability and the fabrics shielded most of the light ranging from 250 nm to 375 nm. Moreover, partial visible light near 400 nm could also be absorbed when the nitrogen substitution reaction was complete and even, as could be seen from #2 and #4 in Fig. 5. The as-synthesized N-doped samples took on pink or red, which could also be attributed to the special absorption for visible light near 400 nm. The anti-bacterial activity of the samples 2# and 3# was estimated by the depress-bacterium circles photographs and their
Fig. 4. XPS energy spectrum survey (A) of the as-synthesized products and the N 1s peak scan in the 400 eV region (B).
Fig. 5. UV–vis transmission ratio and UPF index of cotton fabrics treated using the same content of the different samples.
M. Zheng, J. Wu / Applied Surface Science 255 (2009) 5656–5661
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Fig. 6. Depress-bacterium circle photographs against E. coli and S. aureus of cotton fabrics untreated and treated by the as-synthesized samples under daylight lamp irridation and dark conditions. (a) E. coli and (b) S. aureus.
corresponding diameters of the fabrics treated using the relative samples, which were shown in Fig. 6. The anti-bacterial activity of the untreated fabric was also estimated in order to compare with the treated fabrics. From Fig. 6 we could see that the depressbacterium circle was almost sightless for the untreated fabric, which indicated the untreated cotton fabric had no anti-bacterial ability. Both the N-doped ZnO (sample 2#) and undoped ZnO (sample 3#) exhibited obvious depress-bacterium circles under lamp irradiation condition, however, the anti-bacterial activity had remarkable difference when there was no lamp irradiation. The depress-bacterium circle for the sample 3# was obvious bigger than that for the sample 2# without irradiation, suggesting that nitrogen incorporation in ZnO crystal lattice improved the optical response of ZnO and increased the broad-spectrum anti-bacterium activity. In addtion, it was also found that the anti-bacterium activity for S. aureus was stronger than that for E. coli. 4. Conclusion The nitrogen-doped ZnO nanocrystallites synthesized rapidly by SAC technique have a standard wurtzite structure with the uniform morphologies and size distribution. A small quantity of citric acid had excellent coordinated action with urea and accelerates the nitrogen doping reaction course. The analysis of XPS spectrum showed that the nitrogen incorporation produces an N–O bonding
region in ZnO crystal lattice. The doping of nitrogen could greatly improve the anti-ultraviolet and anti-bacterium properties of ZnO nanocrystallites, perferct anti-bacterium activity for S. aureus and E. coli could be attained even without lamp irradiation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
One-step synthesis of nitrogen-doped ZnO nanocrystallites and their properties Min Zheng *, Jiaqing Wu College of Material Engineering, Soochow University, No. 172, Ganjiang Road East, Suzhou 215006, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 26 April 2008 Received in revised form 7 October 2008 Accepted 27 October 2008 Available online 5 November 2008
Nitrogen-doped ZnO nanocrystallites with primary diameter in the range of 30–50 nm were synthesized rapidly by self-assembly combustion technique using urea and citric acid as fuels, zinc nitrate as oxidant. The variation of adiabatic flame temperature with the various fuel compounds was calculated theoretically according to the thermodynamic concept. XRD, SEM and XPS were used to characterize the as-synthesized products. The anti-ultraviolet and anti-bacterium ability of the products were estimated by testing the UPF index and repressing-bacterium circle diameter of the cotton fabrics, which were treated using the as-synthesized products. The results show that a small quantity of citric acid has excellent coordinated action with urea and accelerates the nitrogen doping reaction course, obtaining perfect nitrogen-doped ZnO nanocrystallites with uniform color and particles size. The analysis of XPS spectrum shows that the nitrogen incorporation produces an N–O bonding region. The incorporation of nitrogen greatly improves the anti-ultraviolet and anti-bacterium properties of ZnO nanocrystallites, which can be attributed to the change of surface properties of nitrogen-doped ZnO, such as O vacancies and crystal deficiency. ß 2008 Elsevier B.V. All rights reserved.
Keywords: Combustion Nitrogen-doped ZnO Nanocrystallite Anti-ultraviolet Anti-bacterium
1. Introduction Recently, much attention has been focused on the modification of nanocrystallite metal oxide by doping or substituting with special atom. With such modification, it is possible to improve the electrical and optical properties of materials by changing the surface properties, such as electronic band gap, O vacancies, crystal deficiencies and specific surface area (SSA). Therefore, such systems are becoming more and more important in material field and are being used as photovoltaics, electrochromics, sensors, photocatalysts and anti-ultraviolet agent [1–3]. Among many metal oxides, zinc oxide (ZnO) has attracted considerable attention because of its low cost, non-toxicity, high stability and high efficiency. Specifically, zinc oxide nanoparticles can serve as an excellent source for resisting bacterium and shielding ultraviolet [4,5]. Currently, however, the highly efficient use of ZnO is impaired by its wide bandgap (3.27 eV), which only responses to a small fraction of the sun’s energy spectrum. Thus, one of the goals to improve the optical response of ZnO is to increase its optical activity by doping with special atom. Recently, metal-doped ZnO nanoparticles, such as Ag, Cu, Sb, Au, As and Li, have been researched [6–9]. However, metal doping has several drawbacks including thermal instability, deep color and potential toxicity, which are ill-suited for textile, cosmetic and medicine field.
* Corresponding author. Tel.: +86 512 68998646. E-mail address: [email protected] (M. Zheng). 0169-4332/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2008.10.091
Therefore, it is necessary to improve the optical response of ZnO by using non-metal dopant species. Doping of nitrogen is found to be very effective on improving photoactivity of the metal oxide because its p state contributes to the band gap narrowing by mixing with 2p state of O. However, it is more difficult to replace the O2 with nitrogen than metal due to the differences in charge states and ionic radii [10]. Present studies are focused on N-doped ZnO thin film [11–14], there are only a few literatures to report Ndoped ZnO nanocrystallites and their properties [15–17]. A common synthesis route for N-doped metal oxide powder is the reaction of the metal oxide with flowing ammonia gas at a high temperature, which is required to introduce nitrogen atoms into metal oxide crystal lattices. Furthermore, a series of procedures including depositing, filtrating, washing and calcining are needed usually in conventional substituting technique. The complex procedures and long-reaction time at high temperature result in large particle size and impurity of the product. Recently grinding– heating treatment and solvothermal technique are used to prepare N-doped ZnO nanopowder, which have some advantages than the conventional substituting technique [18,19]. Here, we described a very simple and rapid method to synthesize N-doped ZnO nanocrystallites by a kind of selfassembly combustion (SAC) technique, which was different from the approaches reported above. Our group has synthesized successfully antimony-doped tin dioxide nanocrystals via similar technique [20]. This paper focused on the microcosmic and physical properties of nitrogen-doped ZnO synthesized by SAC technique.
M. Zheng, J. Wu / Applied Surface Science 255 (2009) 5656–5661
2. Experimental 2.1. The preparation of N-doped ZnO All reagents were commercially available and used without further purification. The whole SAC synthesis procedure could be divided into two parts: one was the preparation of precursor with high homogeneity by mixing starting materials and the other was the formation of N-doped ZnO nanocrystallites in a resistance furnace. Use of suitable raw materials in SAC synthesis ensures stability of the combustion reaction and high quality of the products. More importantly, it must be readily available and convenient to be used. In addition, it should not react violently and release amounts of smoke. In this research, urea was selected as main fuel because of its cheapness and dual roles acting as nitrogen source and energy source simultaneously. Furthermore, citric acid was selected to adjust the combustion process and control growth of the assynthesized N-doped ZnO crystallites. In a typical experiment, firstly, Zn(NO3)26H2O was dissolved in distilled water to attain 2.0 M solutions, and the chosen fuels, including citric and urea, were assembled in an appropriate proportion to form into fuel compounds. Then, the fuel compounds were mixed well with the stock zinc nitrate solution by stirring until a ropy paste (hereafter termed as precursor) was obtained. This precursor was put into a resistance furnace, whose temperature was pre-setup according to the DTA–TG curves of the precursor. After going through boiling, evaporating and concentrating, the precursor suddenly foamed up and deflagrated, leaving flocculent powder like sponge. The relative chemical reaction equation was presented below
ZnðNO3 Þ2 6H2 OðcÞ þ xCOðNH2 Þ2 ðcÞ þ yC6 H8 O7 H2 OðcÞ 3x þ 9y 5 D O2 ðgÞ!ZnOðcÞ þ ð1 þ xÞN2 ðgÞ þ 2 þ ðx þ 6yÞCO2 ðgÞ þ ð6 þ 2x þ 5yÞH2 OðgÞ Oo ðZnOÞ þ N2 Ð 2N1o ðZnOÞ þ Vo þ
1 O2 2
(1) (2)
In Eq. (1), (3x) + 9y 5 = 0 corresponded to the situation of ‘‘the reactant composition was set at the condition that equivalent stoichiometric ratio’’, which implied that the oxygen content of zinc nitrate could be completely reacted to oxidize fuels equivalently without demanding oxygen from any external source, theoretically. Also, both the rate of reaction and the heat liberated per unit time were maxima under this situation. Referring to Eq. (1), there were three representative reactant compositions selected to synthesize ZnO products, that was: (1) x = 1.667, y = 0; (2) x = 1.333, y = 0.111; (3) x = 0, y = 0.556, which were numbered as #1, #2 and #3, respectively. However, for N-doped ZnO, urea was indispensiable because it was the main nitrogen source. The practical experimental phenomena also revealed that only white ZnO could be prepared when urea was in absence. Citric acid, a kind of fuel regulator, couldn’t exceed 15%mol in the whole fuels according to our experiment in order to obtain N-doped ZnO. So another representative reactant composition was also selected to synthesize N-doped ZnO product, which was: (4) x = 1.07, y = 0.19, numbered as #4. 2.2. The characterization of N-doped ZnO Differential thermal and thermogravimetric analysis (DTA–TG) of the mixed slurry precursors produced at different reaction modes was carried out at a heating rate of 108/min in static air (TA Instrument, SDT Q600 V8.3).
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The crystalline phase of the as-synthesized samples was identified by X-ray diffraction analyzer (shimadzu XRD-6000) using Cu Ka radiation (l = 0.15418 nm) and the crystal size was calculated with Scherer’s formula. The morphological feature of the as-synthesized sample was observed by a scan electron microscope (SEM) (Japan S-570). Analysis of nitrogen content was done with Thermo ESCALAB 250 XPS instrument (America) using Al Ka radiation ðhn ¼ 1486:6 eVÞ. 2.3. Estimation of anti-ultraviolet and anti-bacterium properties of N-doped ZnO Treatment of cotton fabrics using N-doped ZnO: N-doped ZnO aqueous dispersion was prepared by adding 1.0 g of the assynthesized sample and 0.2 g of polymer surfactant polyvinylpyrrolidone (PVP) into 100 ml of distilled water with pH value adjusted to 6.0, then keeping stirred at 60 8C for 30 min. 5.0 g of cotton fabric was washed before use for several times in acetone for 20 min using an ultrasonic bath to remove surface impurities. After dried, it was impregnated for 30 min in a treatment solution containing above N-doped ZnO aqueous dispersion, 0.5 g binder agent and 1.0 g silicon softener, then were padded twice (take-up 80%) on a laboratory padding mangle. After they were padded, the cotton fabric was immediately dried at 100 8C for 2.0 min and cured at 160 8C for 1.0 min. Anti-ultraviolet testing: The Ultraviolet blocking effects of the fabric samples were measured in accordance with the Australian/New Zealand Standard AS/NZS 4339 (1996) by the Labsphere UV-1000F (Japan) with the scope of wavelength ranging from 280 nm to 400 nm. The value of ultraviolet protection factor (UPF) was recorded and the result of sun protective clothing was classified according to the rated ultraviolet protection factor. Anti-bacterium testing: E. coli and S. aureus were selected as indicators of the experimental bacteria. The anti-bacterial performance of the anti-bacterial cotton fibers was measured according to FZ/T73023-2006 anti-bacterial standard by measuring the depress-bacterium circle diameter. To prepare an agar plate, solid culture was prepared by mixing 2.0 g agar, 0.5 g peptone and 0.3 g beef extract in 100 ml distilled water. The agar plate was prepared by pouring the solid culture onto sterile circular plates and allowing it to solidify in the Stericlean Vertical Laminar Flow chamber. 1 ml of cultured bacterial broth, containing about 106– 107 organisms, was uniformly distributed on the plate. Cotton fibric disk with 6 mm diameter was placed on the plate. Subsequently the plate was incubated for 48 h at 36 8C in a biochemistry incubator. The depress-bacterium circle was screened by digital camera and the diameter of circle was measured using vernier calipers. 3. Results and discussion 3.1. Phenomena of deflagration synthesis and thermodynamic analysis In our study, the main energy released from the exothermic reaction between fuels and zinc nitrate, which could rapidly heat the system to a high temperature and assure the synthesis to occur. The fuels compositions had important effects on the combustion phenomena and the properties of the as-synthesized products. In order to understand the variation in adiabatic flame temperature (Tad) with different fuels compositions and molar ration of fuels-tozinc nitrate, the Tad could be calculated theoretically. Table 1 listed the relevant thermodynamic data for the reactants and products in the present study. It was well known that enthalpy of reaction
M. Zheng, J. Wu / Applied Surface Science 255 (2009) 5656–5661
5658 Table 1 Relevant thermodynamic data [21]. Compound
DH (kcal/mol) Cp (kcal/mol K)
Zn(NO3)26H2O (c)
CO(NH2)2 (c)
C6H8O7H2O (c)
ZnO (c)
O2 (g)
N2 (g)
CO2 (g)
H2O (g)
550.92 0.07200
79.94 0.02239
370.54 –
83.24 0.009620
0 –
0 0.006958
94.05 0.008876
57.80 0.008024
Note: (c) = crystalline, (g) = gas.
could be expressed as X DH ¼ n DH f
products
X
n DH f
(3)
reactants
Where n was the number of the mole. Using the thermodynamic data for various reactants and products listed in Table 1, the enthalpy of reaction for Eq. (1) could be calculated as follows:
DH ðkcalÞ ¼ 120:88 129:71x 482:76y
(4)
The following equation could be used to the theoretically approximate the adiabatic flame temperature for a deflagration reaction Q ¼ DH ¼
Z
Tad
X
n C p
298
products
of fuels at desired proportions, which was approved through samples #2 and #4 in Table 2. It was worth mentioning that a small quantity of citric acid had no evident effect on the whole heat energy and adiabatic flame temperature, however, a remarkable change in crystallite size and product color could be seen in Table 2. The detailed discussion would appear in the following section combining with the XRD patterns analysis of the assynthesized samples.
dT
(5)
where Q was the heat absorbed by products under adiabatic condition, and Cp was the heat capacity of the products at constant pressure. Substituting the thermodynamic data in Table 1 and Eq. (4), the adiabatic flame temperatures and relative datas could be calculated, which were presented in Table 2. Furthermore, the relative experimental results were also presented in Table 2. By single urea as fuel, the slurry precursor went through a slow frothing process, so the reaction was slow and emitted weak firelight, which lasted about 2–3 min, leaving product with uneven color. The middle product was pink and the marginal product was white in reaction vessel (see sample #1 in Table 2). According to some literatures reported about N-doped metal oxide [15,16], the change in color of the nanocrystals could be attributed to the substitution of nitrogen for oxygen atom in ZnO crystal lattice, which aroused the optical response in the visible wavelength range. Therefore, the existence of white product indicated a incomplete substitution of N for O2 in zinc oxide, which could be attributed to the inadequate heat energy (95.347 kcal/mol) during combustion. Whereas using single citric acid as fuel, the reaction course was very rapid and violent accompanying with dazzling white light, producing a pure white product with amounts of conglomeration (see sample #3 in Table 2). The white product indicated that no nitrogen incorporation reaction took place in reaction mode #3 because of the small amount of nitrogen gases produced (1.000 mol), as could be seen in Table 2. According to above experimental results, it was suggested that perfect N-doped ZnO nanocrystallites could be produced through mixing two kinds
3.2. Effects of fuels compositions on the microstructure of assynthesized products Fig. 1 showed the XRD patterns of the as-synthesized products prepared at four different fuels compositions. As shown in Fig. 1, the strong diffraction peaks in all of the as-synthesized products could be indexed as (1 0 0), (0 0 2) and (1 0 1) planes of a standard wurtzite zinc oxide crystal (JCPDS5-664) and the other planes such as (1 0 2), (1 1 0), (1 0 3) and (1 1 2) were also visible. The strong and sharp reflection peaks suggested that the as-synthesized products were well crystallized. There was no other crystalline phase in all of these X-ray diffraction patterns, which indicated that nitrogen atom brings into the crystal lattice of zinc oxide to substitute for oxygen atom and didnot bring about a new object phase, which was consistent with the traditional doping theory. The corresponding color, nitrogen content and primary crystallite size estimated by Scherer formula were shown in Table 1. The change in crystallite size and product color could be due to the different substitution domino effect at different fuels compositions, especially the use of citric acid. In our study, citric acid itself didnot supply nitrogen resource and had no direct relation with the nitrogen content in product; however, citric acid could promote complete and uniform substitution of nitrogen for oxygen. Therefore, the sample #1 took on pink/white color (the middle product was pink and the marginal product was white in reaction vessel) when citric acid was in absence, while the samples #2 and #4 took on red and pink color, respectively when citric acid was in presence. Comparing the color and nitrogen content of the samples #2 with #4, it was found that the samples had different color and nitrogen content when addition content of citric acid was different. The above experiment results indicated that excellent coordinated domino effect of citric acid with urea was exploded and substitution of nitrogen for oxygen was enhanced when reaction mode #2 was carried out. Moreover, the higher nitrogen
Table 2 Effects of fuels compositions on nature of deflagration and characteristics of as-synthesized samples. Reaction mode
DH f
Tada (8C)
Amount of gases producedb (mol)
Amount of nitrogen gases producedc (mol)
Crystallited size (nm)
Nitrogen content in product (wt%)
Sample color
826.597 901.957 1265.309 940.444
13.670 13.553 13.116 13.370
2.667 2.333 1.000 2.07
19.4 43.7 78.5 23.6
1.02 1.25 0 0.85
Pink/white Red White Pink
(kcal/mol) #1: #2: #3: #4: a b c d
x = 1.667, y = 0 x = 1.333, y = 0.111 x = 0, y = 0.556 x = 1.07, y = 0.19
95.347 105.61 147.535 109.634
Calculated theoretically by thermodynamic data and Eq. (5). Obtained from Eq. (4). Obtained from Eq. (4). Estimated by Scherer formula (X-ray line broadening).
M. Zheng, J. Wu / Applied Surface Science 255 (2009) 5656–5661
Fig. 1. X-ray diffraction of the as-synthesized ZnO at different fuel compositions.
content in sample #2 was also attributed to the more amounts of nitrogen gases produced in reaction mode #2. The excellent coordinated domino effect of citric acid could be explained by the result of the differential thermal and thermogravimetric analysis for the precursors at reaction modes 1# and 2#, which was displayed in Fig. 2. Comparing the curve of reaction mode 2# with that of 1#, the exothermic behavior of the precursor decomposition changed remarkably because of appending a small quantity of citric acid. The strong exothermic peak shifted from 304 8C to 253 8C downward and the peak became sharper, which was beneficial for doping of nitrogen in ZnO crystal lattice. The detailed research about the action mechanism of citric acid would be our future goal. Fig. 3 displayed the morphologies of the typical samples at different fuels compositions. The microstructure of the sample #1 revealed clusters of tiny particles with loose appearance and small size about 20 nm. The sample #3 took on some clear hexagonal and agglomerated flake crystallites with size of 50–200 nm. The samples #2 and #4 showed spherical particles with even particles distribution and small average diameter. For the sample #2, the particles distribution was in range of 30–50 nm and the average diameter was about 40 nm, which was consistent with the size estimated by Scherer formula in Table 1. The SEM morphologies of the sample #4 looked bigger than that of the sample #2, although the primary crystallites size was smaller than the sample #2 (seen
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in Table 2), which indicated that sintering phenomena took place when the reaction mode #4 was carried out. The difference among these morphologies and sizes could be attributed to the different action mechanism of fuels during combustion. According to above description about the combustion phenomena, the combustion course was different at different fuels compositions. The whole heat energy, adiabatic flame temperature, amounts of nitrogen gases produced during combustion and the coordinated domino effect of citric acid had important effect on the morphologies and size of the products. XPS profiles for the as-synthesized products were shown in Fig. 4. We examined three areas of the XPS spectrums, the N1 s region near 400 eV, the Zn 2p3/2 region near 1021 eV and the O 1s region near 532 eV. From our measurements, we found 1.02% nitrogen content in sample #1, 1.25% nitrogen content in sample #2 and 0.85% nitrogen content in sample #4. Although the N 1s peak in the XPS survey profiles was weak, the peak position and its broaden were clear in N 1s scan profiles, which were shown in Fig. 4(B). There were distinct differences in N 1s peak position and its broaden from different samples. For sample #2, the binding energy peak for the N was broad, extending from 395.4 eV to 405.7 eV, and was centered at 400.91 eV, a binding energy notably greater than the typical N–Zn binding energy, 395.9 eV, in Zn3N2. For samples #1 and #4, the binding energy peak for the N was almost the same, extending from 395.5 eV to 400.9 eV and centering at about 398.3 eV, a binding energy similar to the typical N–B binding energy, 398.1 eV, in BN. No dissociative nitrogen and N2 peaks were observed in the XPS energy spectrum, which confirmed that enhanced produces an N–O bonding region. Further, we could see three different O 1s positions at about 530.9 eV, 532.1 eV and 533.1 eV, respectively when scanning the O 1s XPS regions of corresponding samples, which were omitted in the paper. The diversfication of O 1s indicated that nitrogen substitution for oxygen arouses crystal lattice defection. 3.3. Evaluation of the anti-ultraviolet and anti-bacterium properties of the as-synthesized ZnO In the work, cotton fabrics were selected for the evaluation of the ultraviolet-blocking effects of the as-synthesized ZnO at different reaction modes. The cotton fabrics were treated using the same content of the different samples. In Fig. 5 were presented UV– vis transmission spectra of untreated fabric and treated fabrics and their corresponding UPF index. As it could be seen from these curves and UPF index, the untreated fabric had weak ultraviolet protecting ability and most of the ultraviolet light could permeate the fabric. After treated using ZnO samples, the ultraviolet protecting ability improved and
Fig. 2. TG–DTA curves of the precursors at different reaction modes.
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M. Zheng, J. Wu / Applied Surface Science 255 (2009) 5656–5661
Fig. 3. Typical SEM photographs of the as-synthesized ZnO at different fuel compositions.
the UPF index increased for all of the fabrics treated. However, the change degree was different from the different samples. Comparing the undoped-product (seen #3 in Fig. 5) with the dopedproducts (seen 1#, 2# and 4# in Fig. 5), it could be found that nitrogen incorporation in ZnO lattice improved obviously the ultraviolet protecting ability and the fabrics shielded most of the light ranging from 250 nm to 375 nm. Moreover, partial visible light near 400 nm could also be absorbed when the nitrogen substitution reaction was complete and even, as could be seen from #2 and #4 in Fig. 5. The as-synthesized N-doped samples took on pink or red, which could also be attributed to the special absorption for visible light near 400 nm. The anti-bacterial activity of the samples 2# and 3# was estimated by the depress-bacterium circles photographs and their
Fig. 4. XPS energy spectrum survey (A) of the as-synthesized products and the N 1s peak scan in the 400 eV region (B).
Fig. 5. UV–vis transmission ratio and UPF index of cotton fabrics treated using the same content of the different samples.
M. Zheng, J. Wu / Applied Surface Science 255 (2009) 5656–5661
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Fig. 6. Depress-bacterium circle photographs against E. coli and S. aureus of cotton fabrics untreated and treated by the as-synthesized samples under daylight lamp irridation and dark conditions. (a) E. coli and (b) S. aureus.
corresponding diameters of the fabrics treated using the relative samples, which were shown in Fig. 6. The anti-bacterial activity of the untreated fabric was also estimated in order to compare with the treated fabrics. From Fig. 6 we could see that the depressbacterium circle was almost sightless for the untreated fabric, which indicated the untreated cotton fabric had no anti-bacterial ability. Both the N-doped ZnO (sample 2#) and undoped ZnO (sample 3#) exhibited obvious depress-bacterium circles under lamp irradiation condition, however, the anti-bacterial activity had remarkable difference when there was no lamp irradiation. The depress-bacterium circle for the sample 3# was obvious bigger than that for the sample 2# without irradiation, suggesting that nitrogen incorporation in ZnO crystal lattice improved the optical response of ZnO and increased the broad-spectrum anti-bacterium activity. In addtion, it was also found that the anti-bacterium activity for S. aureus was stronger than that for E. coli. 4. Conclusion The nitrogen-doped ZnO nanocrystallites synthesized rapidly by SAC technique have a standard wurtzite structure with the uniform morphologies and size distribution. A small quantity of citric acid had excellent coordinated action with urea and accelerates the nitrogen doping reaction course. The analysis of XPS spectrum showed that the nitrogen incorporation produces an N–O bonding
region in ZnO crystal lattice. The doping of nitrogen could greatly improve the anti-ultraviolet and anti-bacterium properties of ZnO nanocrystallites, perferct anti-bacterium activity for S. aureus and E. coli could be attained even without lamp irradiation. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
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