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A combinatorial approach to phase synthesis and characterisation in atmospheric pressure chemical vapour deposition
Surface & Coatings Technology 201 (2007) 8966 – 8970 www.elsevier.com/locate/surfcoat
A combinatorial approach to phase synthesis and characterisation in atmospheric pressure chemical vapour deposition Geoffrey Hyett, Ivan P. Parkin ⁎ Christopher Ingold Laboratory, University College London, 20 Gordon Street, London, WC1H 0AJ, United Kingdom Available online 25 April 2007
Abstract A combinatorial approach to synthesis has been successfully applied to a number of areas of chemistry and materials. This approach is always composed of two key stages, firstly the simultaneous synthesis of a large number of compounds, often within a grid of micro-reactors, or across a substrate with a compositional gradient; and secondly an analytical technique that can rapidly investigate a desired property and compare the results for the compositional range. In this paper we present the results of a combinatorial approach to Atmospheric Pressure Chemical Vapour Deposition. By using a modified reactor with multiple reagent entry points, and introducing reagents into the reactor through these asymmetrically, a compositional gradient across a deposited film can be formed. The technique will be demonstrated with examples of titanium and tungsten oxides. The rapid analytical method used, the second key step in a combinatorial synthesis, is micro-focus X-ray diffraction mapping, and this will also be discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Combinatorial; X-ray diffraction; Chemical vapour deposition; Thin film; Titanium dioxide; Tungsten oxide
1. Introduction The concept of ‘combinatorial chemistry’ or parallel experimentation was first developed by Merrifield in the 1960's in the field of peptide synthesis [1]. Since then it has become an important method of drug discovery for the pharmaceutical industry, and spawning a field for which journals are dedicated [2,3]. The idea behind parallel experimentation is to rapidly explore a chemical library of related compounds for a particular property or effect. The ‘library’ is often created in a grid of micro-reactors using a building block approach (eg. a compound composed of A, B and C fragments, in varying order, hence the name combinatorial for the technique). This is referred to as a discrete combinatorial library. Alternatively the library can consist of a compositional gradient across a substrate between two or more constituents — a continuous combinatorial system. After the synthesis of such a library, it is analysed using a rapid sampling method so that the large number of sample points may be investigated in a reaso⁎ Corresponding author. E-mail address: [email protected] (I.P. Parkin). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.04.058
nable timeframe, as standard batch analytical methods would make the time-saving of the library synthesis nugatory. In more recent years various researchers have applied the advantages of combinatorial synthesis to materials chemistry. In 1995 Schultz et al. [4] used a sequential physical deposition sputtering of various metal oxides combined with a sequence of masks to generate a substrate with 128 cells. Following on from this discrete compositional spread work, Fleming et al. [5] in 1998 produced substrates with continuous compositional spread to investigate the ternary phase diagram of the Zr–Sn–Ti oxide system. This was carried out by three spatially separated sputter guns depositing zirconium, titanium and tin. As the compositional gradient was continuous rather than discrete the number of ‘cells’ on the substrate was determined principally by the resolution of the analytical method. In the event the authors were able to resolve 4000 points of their substrate, using Rutherford backscattering spectroscopy. Similar materials combinatorial work using sputtering has been carried out by other authors [6,7]. The use of the combinatorial method in CVD was first conducted by Gladfelter et al who used it to investigate continuous compositional spreads of ZrO2/HfO2/SnO2 on silicon
G. Hyett, I.P. Parkin / Surface & Coatings Technology 201 (2007) 8966–8970
substrates [8–10]. This work was carried out using low-pressure CVD and an overhead ‘showerhead’ type precursor inlet, creating overlapping regions of precursor concentrations, and thus the required compositional gradient. In this paper we show for the first time that a similar technique may be applied to APCVD on glass substrates, and that X-ray diffraction mapping may be used as a general analytical method to assess compositional gradients. In our combinatorial approach to APCVD, the first, synthetic step is addressed using a cold walled CVD reactor, but with two reactant injection points, through which different precursors can be introduced, thus producing a compositional gradient across the substrate. Having generated a substrate with a compositional spread we utilized a novel approach to X-ray diffraction to rapidly map the composition of the substrate. In the paper we will discuss the analytical step first, and demonstrate its efficacy with substrates coated in TiO2. We then go on to discuss details of the combinatorial synthesis, using titanium oxide–tungsten oxide as an example system. Taking these two steps together we present the foundations of a combinatorial approach to APCVD. 2. Experimental methods The synthesis of thin films for this paper was conducted using a cold-walled APCVD reactor, with multiple source precursors. The reactor used had two mixing chambers, each of which was supplied with reactant by two precursor bubblers and one plain line of N2. This means that a total of four precursors could be supplied to the reactor; two through each of the mixing chambers. The two mixing chambers fed into a baffle containing manifold, in which the gas streams were spread into a laminar flow, but significantly the gas flows from each mixing
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chamber still remained separate until they entered the reactor. A Schematic diagram of this apparatus is given in Fig. 1. It is the dual entry point to the reactor that opens up the possibility of combinatorial CVD, as reactants can be introduced asymmetrically into the reactor, leading to reagent gradients, and the hoped for compositional gradient. The synthetic work involved the investigation of two systems. In order to investigate the efficacy of the X-ray mapping technique a film of TiO2 was synthesized, from TiCl4 and ethyl acetate. Both of these reagents were run through a single side of the reactor, with no gas flow from the other side. This was carried out not with the intention of generating a compositional gradient, but simply generating a crystalline film to investigate the analytical part of the technique. Such dual source routes to TiO2 films are well recorded in the literature [11]. They were synthesized here using a TiCl4 bubbler temperature of 70 °C, carried with a N2 flow rate of 0.5 dm− 3min− 1, and an ethyl acetate bubbler temperature and flow rate of 40 °C and 0.5 dm− 3min− 1. These were combined in the mixing chamber with a plain line flow of 7.5 dm− 3min− 1. The reaction was then carried out on a substrate heated to 600 °C with a deposition time of 60 s. For the work conducted to investigate the creation of compositional gradients in films the above synthesis was modified to investigate a potential W–Ti–O system. The same molar flow rate of TiCl4 and ethyl acetate was used from one side of the reactor, but additionally a flow rate of tungsten oxide precursors (WCl6 and ethanol) was delivered from the other side of the reactor. The WCl6 bubbler was heated to 240 °C with a carrier gas flow of 4 dm− 3min− 1 and the ethanol bubbler heated to 45 °C with a carrier gas of 0.2 dm− 3min− 1. Again the reaction was carried out with a deposition temperature of 600 °C for 120 s. This film was then analysed using the X-ray mapping
Fig. 1. Schematic diagram of the APCVD apparatus showing the four bubblers, and two mixing chambers. The reactor itself is shown in a top down orientation.
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Fig. 2. A stack of X-ray diffraction patterns taken from a strip running from the front to the back of the film.
method pioneered on the pure TiO2 film and also using WDX measurements to confirm the composition at specific points. The X-ray diffraction mapping analysis was carried out on a Bruker-AXS D8 (GADDS) diffractometer, using a glancing angle (5) reflection geometry. The substrate was placed in the instrument on a motorised stage that allowed height adjustment to place the film in the instrument's focus. The film could also be automatically moved to bring different points of the film surface into the focus of the beam. The X-ray source was copper Kα1 + 2 monochromated with a single gobel mirror, and a pinhole collimator, producing a micro-focus beam of X-rays that had an illumination area at point of contact with the sample of only 3– 4 mm2 (a circle of irradiation of approximately only 0.5 mm in diameter). So a diffraction pattern recorded using this instrument really can be considered as coming from discrete spots of the substrate. The small illumination size allowed a sampling of the substrate every 10 mm in both the x and y direction, by moving the substrate on the motorized stage. This meant that the substrates used of 89× 225 mm2 could have X-ray diffraction patterns recorded in a grid of 21 × 8 spots separated by 10 mm, measuring the pattern of 168 discrete points of the substrate. These can then be built up to produce a map of the composition of the film. The diffractometer was able to record such a large number of diffraction patterns in a reasonable time frame, despite the smaller sample illumination of the micro-focus beam, because of the innovation of using a large area CCD detector — this allowed large sections of the Debye–Scherrer cones to be collected simultaneously, hence far more data per second than a standard linear PSD. The collection time per spot, for a pattern with reasonable signal to noise, was approximately 10 min.
acetate. This generated adherent films (they passed the scotch tape test) with a reasonably uniform thickness, in the range 500–600 nm, as determined by considering the interference fringes. The films were then analysed using the X-ray mapping process, whereby 168 X-ray diffraction patterns were recorded of the film, in a grid of 21 spots for every one of the 8 strips running in the direction of gas flow. For each of the spots diffraction patterns were run in the range 20° N 2θ N 55°, which covers most of the principal peaks for the TiO2 phases. The patterns were viewed in 8 twodimensional stacks of 21 patterns, one for each of the strips. An example of one of these plots is shown in Fig. 2. Comparison of the observed peaks with library database patterns found that in the majority of the patterns the observed peaks at 25.3° and 48.0° 2θ were consistent with those for the anatase phase of TiO2 — and in fact all of the patterns had these peaks present. In some of the patterns, however, an additional peak could be observed at 27.4° 2θ This peak could be indexed as the principal peak of the rutile phase of TiO2. In order to quantify the rutile : anatase ratio of each spot, a Rietveld analysis of the patterns was carried out, set up using a semi-automated process so that each pattern was modelled as a mixture of anatase and rutile, and the ratio between the two refined. The results of this analysis are presented in a schematic diagram of the film in Fig. 3 and shows that the film is predominantly composed of anatase, with an area of 35 spots, appropriately 2/3 of the way along the substrate, where the rutile phase is also observed alongside the anatase, with a varying phase fraction up to 80%. This result, the formation of a film composed of both the rutile and anatase phase of TiO2, but with the ratio of the two varying across the substrate is interesting in its own right and has consequences with regards to photocatalytic activity, a subject which has been the topic of a previous paper [12]. For
3. Results and discussion 3.1. X-ray diffraction mapping, as an analytical method for combinatorial thin films In order to test the X-ray mapping method, films of TiO2 were grown on glass substrates at 600 °C from TiCl4 and ethyl
Fig. 3. Schematic diagram of the thin film of TiO2 showing the relative phase fraction of rutile at each spot.
G. Hyett, I.P. Parkin / Surface & Coatings Technology 201 (2007) 8966–8970
the purposes of this paper, however, the results are significant because they show that the X-ray mapping method is highly suitable for use as an analytical method for combinatorial CVD, as it has been shown to be able to measure the composition of discrete points across a substrate with a compositional gradient. 3.2. Synthesis of combinatorial library by APCVD Having ascertained that the proposed X-ray mapping method could be conducted rapidly, and that spots of different composition could be identified and differentiated, an attempt was made to demonstrate that a desired compositional gradient could be formed on a substrate using APCVD. To this end the four bubbler reactor used for this work was set up so that one side of the manifold supplied precursors for TiO2 while the other side provided those for WO3. The precursors and flow rates used through each side of the dual entry point reactor had already been tested on single entry point reactors and found to successfully produce films of TiO2 and WO3 respectively [11,13]. The film formed under these conditions had a distinct blue colour in transmission, a colour often associated with thin films of tungsten oxides, containing the W5+ion, while titanium oxide films are typically colourless. This film was also adherent, passing the scotch tape test. The X-ray mapping technique was applied to the film, and this found the same diffraction pattern in all of the spots, with peaks at 25.3°, 37.7° and 53.7° 2θ , which could be indexed as the anatase phase of titania — importantly no indication of crystalline WO3 could be seen in the diffraction patterns. This indicated that a film principally of TiO2 was formed across the whole substrate, including that nearer to the inlet carrying the tungsten oxide precursors, and that tungsten oxide is not being deposited. The colour of the film, however, indicated that tungsten ions were present in the film, and so to confirm this WDX was carried out on 12 of the spots, at 1, 4, 8 and 10 cm from the leading edge of the substrate in three strips; one strip from the centre, one from the left side and one from the right side. The results of this analysis are shown in Table 1. This found that the film does actually contain substantial amounts of tungsten, 22% of the metal content in the strip nearest to the Table 1 Results of WDX analysis, showing the % atomic composition of Ti, W and O at each of the measured spot Lateral position of the measurement
Distance from inlet/cm
Ti/at.%
W/at.%
O/at.%
Left
1 4 8 10 1 4 8 10 1 4 8 10
32.05 33.72 33.77 33.28 31 30.43 29.4 27.22 28.12 26.36 25.43 25.7
0.14 0.17 0.24 0.21 4.65 4.3 5.28 5.11 6.04 7.47 6.97 7.34
67.54 65.83 65.19 65.84 64.21 65.18 65.29 67.52 58.14 63.6 66.7 65.64
Central
Right
TiCl4 inlet was on the left hand side of the reactor, and WCl6 inlet on the right hand side.
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WCl6 inlet, dropping to b 1% in the strip nearest the TiCl4 inlet, with intermediate values found in the central strip. These results show that a compositional gradient can be formed across a substrate using APCVD, as highlighted by the WDX analysis. The tungsten found in the film could potentially be amorphous WO3, explaining the absence of peaks in the diffraction pattern, or could be intercalated into the observable crystalline titania phase, but causing an insufficient change in the lattice parameters to be observed with the degree of resolution available with the GADDS diffractometer. Research conducted previously on tungsten doped titania films has investigated the effect of the tungsten level on the photocatalysis [14] and NO reduction catalysis [15] amongst other properties [16,17]. In each of these non-combinatorial studies only a limited number of tungsten doping levels could be studied. It has been shown with this varying tungsten content film that such studies could be conducted using a combinatorial approach, allowing a far greater range of tungsten doping levels to be investigated — and thus increasing the potential for useful materials to be discovered in such investigations. 4. Conclusions In this paper we have demonstrated that an atmospheric pressure chemical vapour deposition reactor can be modified to create films with compositional gradient, such that spots of different composition can be isolated and individually tested. It has also been shown that X-ray mapping can be used to rapidly determine the variation in composition in crystalline phases across the substrate. In conclusion it should be possible to apply the ideas of combinatorial chemistry to APCVD, thus greatly improving the possibility of identifying new phases, or investigating the properties of multiphase systems. Acknowledgements EPSRC is thanked for financial assistance. Pilkington Glass is thanked for donation of glass substrates and Mr Kevin Reeve for his assistance with the WDX measurements. References [1] Merrifield R.B., J. Am. Chem. Soc. 85 (14) (1963) 2149. [2] Czarnik A.W., null, J. Comb. Chem. 1 (1) (1999) 1. [3] A summary of the papers in this month's issue, Combinatorial Chemistryan Online Journal, 1, (1), 1998, pp. 1–4. [4] Xiang X.D., Sun X.D., Briceno G., Lou Y.L., Wang K.A., Chang H.Y., Wallacefreedman W.G., Chen S.W., Schultz P.G.A., Science 268 (5218) (1995) 1738. [5] van Dover R.B., Schneemeyer L.D., Fleming R.M., Nature 392 (6672) (1998) 162. [6] Minami H., Itaka K., Ahmet P., Komiyama D., Chikyow T., Lippmaa M., Koinuma H., Jpn. J. Appl. Phys., Part 2 41 (2A) (2002) L149. [7] Wang Q., Perkins J., Branz H.M., Alleman J., Duncan C., Ginley D., Appl. Surf. Sci. 189 (3-4) (2002) 271. [8] Smith R.C., Hoilien I., Chien J., Campbell S.A., Roberts J.T., Gladfelter W.L., Chem. Mater. 15 (1) (2003) 292. [9] Smith R.C., Hoilien N., Roberts J., Campbell S.A., Gladfelter W.L., Chem. Mater. 14 (2) (2002) 474.
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[10] Zhong L.J., Zhang Z.H., Campbell S.A., Gladfelter W.L., J. Mater. Chem. 14 (21) (2004) 3203. [11] Parkin I.P., O'Neill S.A., Abstr. Pap. Am. Chem. Soc. 222 (2001) U626. [12] Hyett G., Green M., Parkin I.P., J. Am. Chem. Soc. 128 (37) (2006) 12147. [13] Blackman C.S., Parkin I.P., Chem. Mater. 17 (6) (2005) 1583. [14] Rampaul A., Parkin I.P., O'Neill S.A., DeSouza J., Mills A., Elliott N., Polyhedron 22 (1) (2003) 35.
[15] Halkides T.I., Kondarides D.I., Verykios X.E., Appl. Catal., B Environ. 41 (4) (2003) 415. [16] Bregani F., Casale C., Depero L.E., Natali-Sora I., Robba D., Sangaletti L., Toledo G.P., Sens. Actuators, B, Chem. B31 (1-2) (1996) 25. [17] Szabo A., Urda A., Progr. Catal. 12 (1) (2003) 51.
A combinatorial approach to phase synthesis and characterisation in atmospheric pressure chemical vapour deposition Geoffrey Hyett, Ivan P. Parkin ⁎ Christopher Ingold Laboratory, University College London, 20 Gordon Street, London, WC1H 0AJ, United Kingdom Available online 25 April 2007
Abstract A combinatorial approach to synthesis has been successfully applied to a number of areas of chemistry and materials. This approach is always composed of two key stages, firstly the simultaneous synthesis of a large number of compounds, often within a grid of micro-reactors, or across a substrate with a compositional gradient; and secondly an analytical technique that can rapidly investigate a desired property and compare the results for the compositional range. In this paper we present the results of a combinatorial approach to Atmospheric Pressure Chemical Vapour Deposition. By using a modified reactor with multiple reagent entry points, and introducing reagents into the reactor through these asymmetrically, a compositional gradient across a deposited film can be formed. The technique will be demonstrated with examples of titanium and tungsten oxides. The rapid analytical method used, the second key step in a combinatorial synthesis, is micro-focus X-ray diffraction mapping, and this will also be discussed. © 2007 Elsevier B.V. All rights reserved. Keywords: Combinatorial; X-ray diffraction; Chemical vapour deposition; Thin film; Titanium dioxide; Tungsten oxide
1. Introduction The concept of ‘combinatorial chemistry’ or parallel experimentation was first developed by Merrifield in the 1960's in the field of peptide synthesis [1]. Since then it has become an important method of drug discovery for the pharmaceutical industry, and spawning a field for which journals are dedicated [2,3]. The idea behind parallel experimentation is to rapidly explore a chemical library of related compounds for a particular property or effect. The ‘library’ is often created in a grid of micro-reactors using a building block approach (eg. a compound composed of A, B and C fragments, in varying order, hence the name combinatorial for the technique). This is referred to as a discrete combinatorial library. Alternatively the library can consist of a compositional gradient across a substrate between two or more constituents — a continuous combinatorial system. After the synthesis of such a library, it is analysed using a rapid sampling method so that the large number of sample points may be investigated in a reaso⁎ Corresponding author. E-mail address: [email protected] (I.P. Parkin). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2007.04.058
nable timeframe, as standard batch analytical methods would make the time-saving of the library synthesis nugatory. In more recent years various researchers have applied the advantages of combinatorial synthesis to materials chemistry. In 1995 Schultz et al. [4] used a sequential physical deposition sputtering of various metal oxides combined with a sequence of masks to generate a substrate with 128 cells. Following on from this discrete compositional spread work, Fleming et al. [5] in 1998 produced substrates with continuous compositional spread to investigate the ternary phase diagram of the Zr–Sn–Ti oxide system. This was carried out by three spatially separated sputter guns depositing zirconium, titanium and tin. As the compositional gradient was continuous rather than discrete the number of ‘cells’ on the substrate was determined principally by the resolution of the analytical method. In the event the authors were able to resolve 4000 points of their substrate, using Rutherford backscattering spectroscopy. Similar materials combinatorial work using sputtering has been carried out by other authors [6,7]. The use of the combinatorial method in CVD was first conducted by Gladfelter et al who used it to investigate continuous compositional spreads of ZrO2/HfO2/SnO2 on silicon
G. Hyett, I.P. Parkin / Surface & Coatings Technology 201 (2007) 8966–8970
substrates [8–10]. This work was carried out using low-pressure CVD and an overhead ‘showerhead’ type precursor inlet, creating overlapping regions of precursor concentrations, and thus the required compositional gradient. In this paper we show for the first time that a similar technique may be applied to APCVD on glass substrates, and that X-ray diffraction mapping may be used as a general analytical method to assess compositional gradients. In our combinatorial approach to APCVD, the first, synthetic step is addressed using a cold walled CVD reactor, but with two reactant injection points, through which different precursors can be introduced, thus producing a compositional gradient across the substrate. Having generated a substrate with a compositional spread we utilized a novel approach to X-ray diffraction to rapidly map the composition of the substrate. In the paper we will discuss the analytical step first, and demonstrate its efficacy with substrates coated in TiO2. We then go on to discuss details of the combinatorial synthesis, using titanium oxide–tungsten oxide as an example system. Taking these two steps together we present the foundations of a combinatorial approach to APCVD. 2. Experimental methods The synthesis of thin films for this paper was conducted using a cold-walled APCVD reactor, with multiple source precursors. The reactor used had two mixing chambers, each of which was supplied with reactant by two precursor bubblers and one plain line of N2. This means that a total of four precursors could be supplied to the reactor; two through each of the mixing chambers. The two mixing chambers fed into a baffle containing manifold, in which the gas streams were spread into a laminar flow, but significantly the gas flows from each mixing
8967
chamber still remained separate until they entered the reactor. A Schematic diagram of this apparatus is given in Fig. 1. It is the dual entry point to the reactor that opens up the possibility of combinatorial CVD, as reactants can be introduced asymmetrically into the reactor, leading to reagent gradients, and the hoped for compositional gradient. The synthetic work involved the investigation of two systems. In order to investigate the efficacy of the X-ray mapping technique a film of TiO2 was synthesized, from TiCl4 and ethyl acetate. Both of these reagents were run through a single side of the reactor, with no gas flow from the other side. This was carried out not with the intention of generating a compositional gradient, but simply generating a crystalline film to investigate the analytical part of the technique. Such dual source routes to TiO2 films are well recorded in the literature [11]. They were synthesized here using a TiCl4 bubbler temperature of 70 °C, carried with a N2 flow rate of 0.5 dm− 3min− 1, and an ethyl acetate bubbler temperature and flow rate of 40 °C and 0.5 dm− 3min− 1. These were combined in the mixing chamber with a plain line flow of 7.5 dm− 3min− 1. The reaction was then carried out on a substrate heated to 600 °C with a deposition time of 60 s. For the work conducted to investigate the creation of compositional gradients in films the above synthesis was modified to investigate a potential W–Ti–O system. The same molar flow rate of TiCl4 and ethyl acetate was used from one side of the reactor, but additionally a flow rate of tungsten oxide precursors (WCl6 and ethanol) was delivered from the other side of the reactor. The WCl6 bubbler was heated to 240 °C with a carrier gas flow of 4 dm− 3min− 1 and the ethanol bubbler heated to 45 °C with a carrier gas of 0.2 dm− 3min− 1. Again the reaction was carried out with a deposition temperature of 600 °C for 120 s. This film was then analysed using the X-ray mapping
Fig. 1. Schematic diagram of the APCVD apparatus showing the four bubblers, and two mixing chambers. The reactor itself is shown in a top down orientation.
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Fig. 2. A stack of X-ray diffraction patterns taken from a strip running from the front to the back of the film.
method pioneered on the pure TiO2 film and also using WDX measurements to confirm the composition at specific points. The X-ray diffraction mapping analysis was carried out on a Bruker-AXS D8 (GADDS) diffractometer, using a glancing angle (5) reflection geometry. The substrate was placed in the instrument on a motorised stage that allowed height adjustment to place the film in the instrument's focus. The film could also be automatically moved to bring different points of the film surface into the focus of the beam. The X-ray source was copper Kα1 + 2 monochromated with a single gobel mirror, and a pinhole collimator, producing a micro-focus beam of X-rays that had an illumination area at point of contact with the sample of only 3– 4 mm2 (a circle of irradiation of approximately only 0.5 mm in diameter). So a diffraction pattern recorded using this instrument really can be considered as coming from discrete spots of the substrate. The small illumination size allowed a sampling of the substrate every 10 mm in both the x and y direction, by moving the substrate on the motorized stage. This meant that the substrates used of 89× 225 mm2 could have X-ray diffraction patterns recorded in a grid of 21 × 8 spots separated by 10 mm, measuring the pattern of 168 discrete points of the substrate. These can then be built up to produce a map of the composition of the film. The diffractometer was able to record such a large number of diffraction patterns in a reasonable time frame, despite the smaller sample illumination of the micro-focus beam, because of the innovation of using a large area CCD detector — this allowed large sections of the Debye–Scherrer cones to be collected simultaneously, hence far more data per second than a standard linear PSD. The collection time per spot, for a pattern with reasonable signal to noise, was approximately 10 min.
acetate. This generated adherent films (they passed the scotch tape test) with a reasonably uniform thickness, in the range 500–600 nm, as determined by considering the interference fringes. The films were then analysed using the X-ray mapping process, whereby 168 X-ray diffraction patterns were recorded of the film, in a grid of 21 spots for every one of the 8 strips running in the direction of gas flow. For each of the spots diffraction patterns were run in the range 20° N 2θ N 55°, which covers most of the principal peaks for the TiO2 phases. The patterns were viewed in 8 twodimensional stacks of 21 patterns, one for each of the strips. An example of one of these plots is shown in Fig. 2. Comparison of the observed peaks with library database patterns found that in the majority of the patterns the observed peaks at 25.3° and 48.0° 2θ were consistent with those for the anatase phase of TiO2 — and in fact all of the patterns had these peaks present. In some of the patterns, however, an additional peak could be observed at 27.4° 2θ This peak could be indexed as the principal peak of the rutile phase of TiO2. In order to quantify the rutile : anatase ratio of each spot, a Rietveld analysis of the patterns was carried out, set up using a semi-automated process so that each pattern was modelled as a mixture of anatase and rutile, and the ratio between the two refined. The results of this analysis are presented in a schematic diagram of the film in Fig. 3 and shows that the film is predominantly composed of anatase, with an area of 35 spots, appropriately 2/3 of the way along the substrate, where the rutile phase is also observed alongside the anatase, with a varying phase fraction up to 80%. This result, the formation of a film composed of both the rutile and anatase phase of TiO2, but with the ratio of the two varying across the substrate is interesting in its own right and has consequences with regards to photocatalytic activity, a subject which has been the topic of a previous paper [12]. For
3. Results and discussion 3.1. X-ray diffraction mapping, as an analytical method for combinatorial thin films In order to test the X-ray mapping method, films of TiO2 were grown on glass substrates at 600 °C from TiCl4 and ethyl
Fig. 3. Schematic diagram of the thin film of TiO2 showing the relative phase fraction of rutile at each spot.
G. Hyett, I.P. Parkin / Surface & Coatings Technology 201 (2007) 8966–8970
the purposes of this paper, however, the results are significant because they show that the X-ray mapping method is highly suitable for use as an analytical method for combinatorial CVD, as it has been shown to be able to measure the composition of discrete points across a substrate with a compositional gradient. 3.2. Synthesis of combinatorial library by APCVD Having ascertained that the proposed X-ray mapping method could be conducted rapidly, and that spots of different composition could be identified and differentiated, an attempt was made to demonstrate that a desired compositional gradient could be formed on a substrate using APCVD. To this end the four bubbler reactor used for this work was set up so that one side of the manifold supplied precursors for TiO2 while the other side provided those for WO3. The precursors and flow rates used through each side of the dual entry point reactor had already been tested on single entry point reactors and found to successfully produce films of TiO2 and WO3 respectively [11,13]. The film formed under these conditions had a distinct blue colour in transmission, a colour often associated with thin films of tungsten oxides, containing the W5+ion, while titanium oxide films are typically colourless. This film was also adherent, passing the scotch tape test. The X-ray mapping technique was applied to the film, and this found the same diffraction pattern in all of the spots, with peaks at 25.3°, 37.7° and 53.7° 2θ , which could be indexed as the anatase phase of titania — importantly no indication of crystalline WO3 could be seen in the diffraction patterns. This indicated that a film principally of TiO2 was formed across the whole substrate, including that nearer to the inlet carrying the tungsten oxide precursors, and that tungsten oxide is not being deposited. The colour of the film, however, indicated that tungsten ions were present in the film, and so to confirm this WDX was carried out on 12 of the spots, at 1, 4, 8 and 10 cm from the leading edge of the substrate in three strips; one strip from the centre, one from the left side and one from the right side. The results of this analysis are shown in Table 1. This found that the film does actually contain substantial amounts of tungsten, 22% of the metal content in the strip nearest to the Table 1 Results of WDX analysis, showing the % atomic composition of Ti, W and O at each of the measured spot Lateral position of the measurement
Distance from inlet/cm
Ti/at.%
W/at.%
O/at.%
Left
1 4 8 10 1 4 8 10 1 4 8 10
32.05 33.72 33.77 33.28 31 30.43 29.4 27.22 28.12 26.36 25.43 25.7
0.14 0.17 0.24 0.21 4.65 4.3 5.28 5.11 6.04 7.47 6.97 7.34
67.54 65.83 65.19 65.84 64.21 65.18 65.29 67.52 58.14 63.6 66.7 65.64
Central
Right
TiCl4 inlet was on the left hand side of the reactor, and WCl6 inlet on the right hand side.
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WCl6 inlet, dropping to b 1% in the strip nearest the TiCl4 inlet, with intermediate values found in the central strip. These results show that a compositional gradient can be formed across a substrate using APCVD, as highlighted by the WDX analysis. The tungsten found in the film could potentially be amorphous WO3, explaining the absence of peaks in the diffraction pattern, or could be intercalated into the observable crystalline titania phase, but causing an insufficient change in the lattice parameters to be observed with the degree of resolution available with the GADDS diffractometer. Research conducted previously on tungsten doped titania films has investigated the effect of the tungsten level on the photocatalysis [14] and NO reduction catalysis [15] amongst other properties [16,17]. In each of these non-combinatorial studies only a limited number of tungsten doping levels could be studied. It has been shown with this varying tungsten content film that such studies could be conducted using a combinatorial approach, allowing a far greater range of tungsten doping levels to be investigated — and thus increasing the potential for useful materials to be discovered in such investigations. 4. Conclusions In this paper we have demonstrated that an atmospheric pressure chemical vapour deposition reactor can be modified to create films with compositional gradient, such that spots of different composition can be isolated and individually tested. It has also been shown that X-ray mapping can be used to rapidly determine the variation in composition in crystalline phases across the substrate. In conclusion it should be possible to apply the ideas of combinatorial chemistry to APCVD, thus greatly improving the possibility of identifying new phases, or investigating the properties of multiphase systems. Acknowledgements EPSRC is thanked for financial assistance. Pilkington Glass is thanked for donation of glass substrates and Mr Kevin Reeve for his assistance with the WDX measurements. References [1] Merrifield R.B., J. Am. Chem. Soc. 85 (14) (1963) 2149. [2] Czarnik A.W., null, J. Comb. Chem. 1 (1) (1999) 1. [3] A summary of the papers in this month's issue, Combinatorial Chemistryan Online Journal, 1, (1), 1998, pp. 1–4. [4] Xiang X.D., Sun X.D., Briceno G., Lou Y.L., Wang K.A., Chang H.Y., Wallacefreedman W.G., Chen S.W., Schultz P.G.A., Science 268 (5218) (1995) 1738. [5] van Dover R.B., Schneemeyer L.D., Fleming R.M., Nature 392 (6672) (1998) 162. [6] Minami H., Itaka K., Ahmet P., Komiyama D., Chikyow T., Lippmaa M., Koinuma H., Jpn. J. Appl. Phys., Part 2 41 (2A) (2002) L149. [7] Wang Q., Perkins J., Branz H.M., Alleman J., Duncan C., Ginley D., Appl. Surf. Sci. 189 (3-4) (2002) 271. [8] Smith R.C., Hoilien I., Chien J., Campbell S.A., Roberts J.T., Gladfelter W.L., Chem. Mater. 15 (1) (2003) 292. [9] Smith R.C., Hoilien N., Roberts J., Campbell S.A., Gladfelter W.L., Chem. Mater. 14 (2) (2002) 474.
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[10] Zhong L.J., Zhang Z.H., Campbell S.A., Gladfelter W.L., J. Mater. Chem. 14 (21) (2004) 3203. [11] Parkin I.P., O'Neill S.A., Abstr. Pap. Am. Chem. Soc. 222 (2001) U626. [12] Hyett G., Green M., Parkin I.P., J. Am. Chem. Soc. 128 (37) (2006) 12147. [13] Blackman C.S., Parkin I.P., Chem. Mater. 17 (6) (2005) 1583. [14] Rampaul A., Parkin I.P., O'Neill S.A., DeSouza J., Mills A., Elliott N., Polyhedron 22 (1) (2003) 35.
[15] Halkides T.I., Kondarides D.I., Verykios X.E., Appl. Catal., B Environ. 41 (4) (2003) 415. [16] Bregani F., Casale C., Depero L.E., Natali-Sora I., Robba D., Sangaletti L., Toledo G.P., Sens. Actuators, B, Chem. B31 (1-2) (1996) 25. [17] Szabo A., Urda A., Progr. Catal. 12 (1) (2003) 51.