Effects of Hexamethylenetetramine on the Nucleation and Radial

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Effects of HMTA on the Nucleation and Radial Growth of ZnO Nanowires by Chemical Bath Deposition Romain Parize, Jérôme Daniel Garnier, Odette ChaixPluchery, Claire Verrier, Estelle Appert, and Vincent Consonni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b00479 • Publication Date (Web): 17 Feb 2016 Downloaded from http://pubs.acs.org on February 22, 2016

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The Journal of Physical Chemistry

Effects of HMTA on the Nucleation and Radial Growth of ZnO Nanowires by Chemical Bath Deposition Romain Parize,1* Jérôme Garnier,1 Odette Chaix-Pluchery,1 Claire Verrier,1,2 Estelle Appert,1 and Vincent Consonni.1* 1

Université Grenoble Alpes, CNRS, LMGP, F-38000 Grenoble, France

2

Université Grenoble Alpes, CNRS, IMEP-LAHC, F-38000 Grenoble, France

*Corresponding authors: [email protected] and [email protected]

ABSTRACT The growth of ZnO nanowires by chemical bath deposition (CBD) is of great potential for their integration into nanoscale-devices. However, the effects of the chemical precursors

in

solution

are

still

under

debate,

such

as

the

role

of

hexamethylenetetramine (HMTA). In order to tackle this issue, these effects are thoroughly disentangled from the effects of the structural morphology of the ZnO seed layer and investigated through a large number of non-equimolar CBDs over a broad range of chemical precursor concentration and ratio. The analysis is further supported by thermodynamic simulations yielding theoretical solubility plots and speciation diagrams of Zn(II) species. It is shown that the ZnO deposited volume and, to some extent, the length of ZnO nanowires, are directly related to the 1 ACS Paragon Plus Environment


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supersaturation in solution, which strongly depends on the chemical precursor concentration and pH. A slight excess of HMTA with respect to zinc nitrate is required to reach the largest axial growth rate of ZnO nanowires. In addition to act as a source of HO- ions, HMTA is found to act as a pH buffer over a broad range of chemical precursor concentration and ratio, except for its largest excess. Additionally, it is unambiguously revealed that HMTA strongly reduces the radial growth of ZnO nanowires, by inhibiting the development of their non-polar m-plane sidewalls. Importantly, HMTA also affects significantly the density of ZnO nanowires and hence their nucleation process, which is attributed to its significant interaction with the ZnO seed layer. The present findings give a deeper insight into the multiple roles of HMTA, which are an important step towards the ultimate control of the structural uniformity of ZnO nanowire arrays.

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1. INTRODUCTION ZnO nanowire (NWs) arrays have been considered, over the last decade,1,2 as a promising building block for a large number of electronic, optoelectronic, and photovoltaic devices.3-10 In all of these nanoscale-devices, a precise control of the structural properties of ZnO NWs (i.e. vertical alignment, diameter, length, and period) and of their uniformity is required11-14 for enhancing their overall performances.15 The formation of ZnO NWs can be achieved by a large number of chemical deposition techniques,1 but increasing efforts have been devoted to the chemical bath deposition (CBD) or hydrothermal growth as a low-cost, lowtemperature, surface scalable, and easily implemented process.2,16-21 Basically, it has been shown that the structural properties of ZnO NWs strongly depend on (i) the morphological properties of the ZnO nucleation surface, such as its crystal orientation, polarity, porosity, roughness and characteristic dimensions in the case of polycrystalline ZnO seed layers,22-26 as well as on (ii) the typical growth conditions used in solution, such as the temperature, time, pH, nature of chemical precursors and related concentration.27,28 A very typical but still not completely understood chemical system for the growth of ZnO NWs by CBD consists in mixing zinc nitrate [Zn(NO3)2] with hexamethylenetetramine (HMTA) in deionized water and heating them over the temperature range from 70 to 90°C. In the broad range of chemical precursor concentration, the growth of ZnO NWs is expected to be driven by the set of the following chemical reactions:1,2 1. (CH2)6N4 + 6H20 2. NH3 + H2O 3. Zn(NO3)2

6HCHO + 4NH3

(1)

NH4+ + HO-

(2)

Zn2+ + 2NO3-

(3)



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4. Zn2+ + 2HO-

ZnO(s) + H2O

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(4)

Basically, HMTA is initially hydrolyzed gradually with heat, forming formaldehyde and NH3 (reaction (1)). Then, NH3 reacts with water, producing HOions (reaction (2)). An alternative route consists in only considering the protonation of HMTA. Zn2+ ions are formed following the solubilization of zinc nitrate (reaction (3)), leading to the direct crystallization of ZnO (reaction (4)). A zinc hydroxide intermediate phase (reactions (5) and (6)) was also reported when specific growth conditions are used, resulting in an indirect crystallization of ZnO:20 5. Zn2+ + 2HO− 6. Zn(OH)2

Zn(OH)2

(5)

ZnO + H2O

(6)

The precise role of HMTA in solution during the formation of ZnO NWs is still controversial, especially its effects on their structural properties. HMTA is a nonionic, heterocyclic tertiary amine that is highly soluble in water in order to release its strain energy related to its molecular structure.2 Since the hydrolysis rate of HMTA strongly depends on pH and hence that HO- ions are gradually released (reactions (1) and (2)), it is well-known that HMTA acts as a source of HO- ions and plays the role of a pH buffer29,30 and not of a metal-ion buffer.31 However, its precise role concerning the homogeneous nucleation of ZnO inside the solution32 as well as the heterogeneous growth of ZnO NWs on top of the ZnO nucleation surface is under debate. In the former case, it was proved, by in situ X-ray absorption near-edge structure spectroscopy, that HMTA actually does not form metal ion complex in solution and that the crystallization of ZnO is usually direct at high temperatures.31 More recently, the formation of less stable intermediate phases such as layered basic zinc salts was also reported at lower temperatures in the range from 20 to 65°C.33 In the latter case, it was suggested, on the one hand, that HMTA may act as a capping


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agent by adsorbing on the sidewalls of ZnO NWs,34 promoting the axial growth of ZnO NWs at the expense of their radial growth and hence enhancing their aspect ratio. On the other hand, this was rejected by in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy; nevertheless, the experiments were achieved under untypical growth configurations and conditions (i.e. ZnO nanopowder, room temperature, and a very low concentration of HMTA of 1 mM).35 These often are strong limitations of the in situ or in operando approaches, supporting the complementary development of thorough and comprehensive ex situ approaches. More recently, the effects of HMTA were investigated by varying its concentration in solution. The equimolar concentration of Zn(NO3)2 and HMTA was significantly varied, but this type of approach was unable to elucidate the effects of HMTA.11-13, 19, 27, 28, 33-36

In contrast, varying the non-equimolar concentration of Zn(NO3)2 and

HMTA is relevant, but this approach has only been performed over a very narrow concentration range in Refs. 37-40, which is a strong limitation to deduce the effects of HMTA.37-40 Furthermore, the effects of the structural morphology of the ZnO seed layers are typically superimposed to the effects of the chemical precursors in solution, which is usually not addressed, while they should carefully be considered for a deeper understanding of the multiple roles of HMTA. In this work, the effects of the chemical precursors in solution on the structural properties of ZnO NWs grown by CBD are thoroughly disentangled by using a ZnO seed layer with a given structural morphology as determined by field-emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD). The multiple roles of HMTA are revealed by examining in detail the structural properties of ZnO NWs grown by varying the concentrations of Zn(NO3)2 and HMTA in a nonequimolar way over a very broad range and by replacing HMTA by NH3. These



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effects are further supported by thermodynamic simulations yielding theoretical solubility plots and speciation diagrams of Zn(II) species. The experimental procedures combined with Raman scattering measurements allow us to deduce a complete growth diagram and to identify the multiple roles of HMTA during the formation of ZnO NWs.

2. EXPERIMENTAL SECTION ZnO NWs were fabricated on a ZnO seed layer with a given structural morphology, following two steps involving dip coating and CBD. First, (100) silicon substrates were cleaned in an ultrasonic bath using acetone and isopropanol for 15 min each to remove the residual contaminants. The ZnO seed layers were synthesized by mixing 0.375M of zinc acetate dihydrate [Zn(CH3COO)2.2H2O] (Emsure ACS) and 0.375M of monoethanolamine (MEA, J.T.Baker) in a pure ethanol solvent. The resulting solution was stirred for a couple of hours at 60°C on a hot plate to obtain a clear solution and then at room temperature to complete the zinc acetate dilution in the pure ethanol solvent. The seed layer depositions were performed on the cleaned (100) silicon substrates by dip coating. The samples were slowly dipped into the solution and gently pulled out under a controlled atmosphere (hygrometry < 15%). They were subsequently annealed on a hot plate kept at 300°C for a few minutes to evaporate residual organic solvents and eventually at 500°C for 1h to crystallize the ZnO seed layer. Second, the growth of ZnO NWs by CBD was achieved by mixing zinc nitrate hexahydrate [Zn(NO3)2.6H2O] (Sigma Aldrich) and HMTA (Sigma Aldrich) in deionized water at a given temperature of 90°C for 3h. The samples were placed face down (to avoid contamination from homogeneous growth) in a sealed beaker inside a regular oven. The series of eleven samples was grown by mixing 6 ACS Paragon Plus Environment

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Zn(NO3)2 and HMTA in deionized water with a non-equimolar concentration from a ratio of 1:1/4 in favor of Zn(NO3)2 to a ratio of 1/4:1 in favor of HMTA. In all cases, the largest concentration corresponding to the value of 1 was fixed to 30 mM. A NW growth was also performed without HMTA by mixing 30 mM of Zn(NO3)2 with 240 mM of NH3 (VWR Chemical, 28%) to reach a pH of the solution to 10.8. After the growth, the samples were rinsed with deionized water to remove additional reactants. The in situ and ex situ measurements of pH were achieved by using a Mettler-Toledo Seven Compact pH/Ion S220 pH meter, which is further equipped with a temperature probe. Thermodynamic simulations were performed by using Visual MINTEQ software.41 These simulations were performed at 90°C by considering Zn2+ ions as the single metallic cations, while HO- ions and NH3 play the role of ligands to form Zn(II) hydroxide and amine complexes. Nine soluble Zn(II) species were thus considered in aqueous solution: Zn2+ ions, ZnOH, ZnOH aq, ZnOH , ZnOH , ZnNH , ZnNH , ZnNH , and ZnNH . The simplified chemical

mechanism is given by: Zn2+ + iL ↔ ZnL with β = [ ][] , in which L is the

ligand (i.e., HO- ions or NH3), ZnL is the complex considered, i is the coordination number, and β is the stability constant. The stability constants β were initially taken at 25°C from NIST, and, subsequently, deduced at 90°C by using the Kelley’s equation.42,43 The equilibrium reactions and related stability constants β at 25°C are summarized in Table I. As regards the calculation of the theoretical solubility plot, the following solid compounds were considered, as summarized in Table S1: ZnO(s) and the different crystal phases of ZnOH s.



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Chemical Reaction

Log(K) (25°C)

Zn+ HO ↔ ZnOH

5

Zn + 2HO ↔ ZnOH

Zn+ 3HO ↔ ZnOH

Zn + 4HO ↔

ZnOH

Zn + NH ↔ ZnNH

Zn + 2NH ↔

ZnNH

Zn + 4NH ↔

ZnNH

Zn + 3NH ↔ ZnNH

NH + HO ↔ NH + H O

Zn + 2HO ↔ ZnO + H O

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11.1 13.6 14.8 2.214 4.498 6.862 8.886 4.753 15.52

Table 1. Possible chemical reactions in solution and related reaction constants K at 25°C, considering the chemical system Zn-NH3-H2O. The stability constants β are specifically given for the Zn(II) hydroxide and amine complexes.

The structural morphology of ZnO seed layer and ZnO NWs was characterized by top-view and cross-sectional FESEM images using a SEM FEG-FEI Quanta 250. The seed layer XRD measurements were collected in the range of 30 to 65° in 2θ-scale with a Bruker D8 Advance diffractometer using CuKα1 radiation according to the Bragg-Brentano configuration. The texture coefficients Chkl of the seed layer for a given [hkl] direction were determined by the following equation:44

#$%& =

'()* '+,()* - . '()* Ʃ . 01+'+,()*

(7)

where Ihkl is the hkl peak intensity, I0,hkl is the reference hkl peak intensity from the 00-036-1451 file of the International Center for Diffraction Data (ICDD), and N is the number of peaks considered (N = 6 here). The degree of preferred orientation σ was deduced by determining the standard deviation of all Chkl with the values of 1 corresponding to a randomly oriented ZnO sample: 8 ACS Paragon Plus Environment

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2 =

3Ʃ. 01+ 4()* 5² √8

(8)

Raman spectra were recorded from the film surface using a Jobin Yvon/Horiba Labram spectrometer equipped with a liquid nitrogen cooled coupled charge device detector. The experiments were conducted in the micro-Raman mode at room temperature. The 488 nm line of an Ar+ laser was focused to a spot size smaller than 1 µm on the sample. The laser power on the sample surface was close to 1.55 mW. Raman spectra were calibrated using silicon at room temperature.

3. RESULTS AND DISCUSSION 3.1. Structural morphology of the ZnO seed layer The structural properties of ZnO NWs depend both on the structural morphology of the ZnO seed layer and on the effects of the chemical precursors during the CBD process. It has been shown, for instance, that only polar c-axis oriented nanoparticles (NPs) act as nucleation sites25 in the seed layer and thus govern the density of ZnO NWs when surface nucleation occurs.24 In order to disentangle both effects, the structural morphology of the typical ZnO seed layer as presented in Figure 1a,b was thoroughly controlled and strictly identical for all the ZnO NW growths. Its typical thickness is about 40 nm and the average diameter of ZnO NPs is of the same order of magnitude (i.e. 37 ± 12 nm). The density of ZnO NPs is fixed to about 214 NPs/µm2. Importantly, the texture of the ZnO seed layer is achieved along the polar c-axis with a degree of preferred orientation of 1.21 corresponding to a [002] texture coefficient of 3.69, as determined from Figure 1c. As a result, it is deduced that 61% of ZnO NPs in the seed layer are oriented along the polar c-axis, which correspond to 130 c-axis oriented NPs/µm². Furthermore, it should be noted that the ZnO seed layer is fairly 9 ACS Paragon Plus Environment


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dense with a reduced porosity in its center, which is favorable for the surface nucleation of ZnO NWs on top of c-axis oriented ZnO NPs, as reported in Ref. 24.

Figure 1. (a) Cross-sectional and (b) top-view FESEM images of the typical ZnO seed layer used for nucleating ZnO NWs by CBD, as well as (c) its corresponding XRD pattern.

3.2. Structural properties of ZnO nanowires grown by CBD under nonequimolar Zn(NO3)2 and HMTA conditions The structural properties of ZnO NWs grown by CBD with a non-equimolar concentration of Zn(NO3)2 and HMTA are shown by cross-sectional and top-view FESEM images in Figure 2a-p. The concentration ratio from 1:1/4 (i.e., 4) in favor of Zn(NO3)2 to 1/4:1 (i.e., 0.25) in favor of HMTA were prepared with a given reference concentration of 30 mM, meaning that the largest concentration corresponding to the value of 1 was 30 mM in any case. The corresponding mean length and diameter of ZnO NWs as well as their apparent density are revealed in Figure 3 from FESEM 10 ACS Paragon Plus Environment

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image analysis over a population of more than 200 ZnO NWs. The in situ and ex situ experimental measurements of pH together with thermodynamic simulations are given in Figure 4.

Figure 2. (a-d) (i-l) Cross-sectional and (e-h) (m-p) top-view FESEM images of ZnO NWs grown by CBD with a [Zn(NO3)2] : [HMTA] ratio ranging from 4 to 0.25.

3.2.1. Growth mechanisms of ZnO NWs The preferential shape of ZnO NWs grown by CBD is explained by thermodynamic considerations. ZnO NWs are composed of polar c-plane top facets and non-polar m-plane vertical sidewalls with a lower surface energy. This results in the minimization of total free energy when the surface area of non-polar m-plane vertical sidewalls is developed by promoting the axial growth at the expense of the radial growth.25,35 The axial growth rate is also typically faster than the radial growth rate, which can be explained by kinetic effects. The electrostatic interactions of Zn2+ and HO- ions in solution are favored with the terminated atoms of the polar c-plane on



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the top facet of ZnO NWs, which is expected to strongly increase the axial growth rate as well. Eventually, it should be noted that the growth rates are limited by the mass transport of the chemical precursors in solution.24

Figure 3. Evolution of the diameter (triangle dots), length (square dots), and apparent density (circle dots) of ZnO NWs as a function of the [Zn(NO3)2] : [HMTA] ratio.

3.2.2. Effects of HMTA on the length of ZnO NWs It is shown in Figure 3 that the length of ZnO NWs is initially increased strongly from about 300 to 1250 nm as the [Zn(NO3)2] : [HMTA] ratio is decreased from 4 to 0.66 (i.e., 1/1.5:1) while the pH at the beginning and end of the growth at room temperature, presented in Figure 4a, keeps a nearly constant value of 6.7 ± 0.1 and 6.1 ± 0.1, respectively. The length is then decreased significantly down to about 250 nm, as the [Zn(NO3)2] : [HMTA] ratio is further decreased from 0.66 to 0.25 while the pH at the beginning and end of the growth is strongly increased up to 7.0 ± 0.1 and 7.7 ± 0.1, respectively.


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Figure 4. (a) Evolution of the pH measured before (red dots and lines) and after (black dots and lines) the growth as a function of the [Zn(NO3)2] : [HMTA] ratio. (b) Evolution of the pH during the growth as a function of the [Zn(NO3)2] : [HMTA] ratio of 4 (blue solid line), 1 (black solid line), 0.66 (red solid line) and 0.25 (pink solid line). (c) Theoretical solubility plots at 90 °C as a function of pH ranging from 5.4 to 6.2 as computed with Visual MINTEQ software for the [Zn(NO3)2] : [NH3] ratios of 4 (blue solid line), 1 (black solid line), 0.66 (red solid line) and 0.25 (pink solid line). The inset is the theoretical solubility plots over the broad range of pH from 0 to 14. (d) ZnO deposited volume as inferred from the length, diameter, and apparent density of ZnO NWs in Figure 3 vs [Zn(NO3)2] : [HMTA] ratio, calculated on 1 µm². (e-h) Speciation diagrams of Zn(II) species at 90 °C as a function of pH ranging from 0 to 14 as computed with Visual MINTEQ software for the [Zn(NO3)2] : [NH3] ratios of (e) 4 ([Zn(NO3)2] = 30 mM, [NH3] = 7.5 mM), (f) 1 ([Zn(NO3)2] = 30 mM, [NH3] = 30 mM), (g) 0.66 ([Zn(NO3)2] = 20 mM, [NH3] = 30 mM), and (h) 0.25 ([Zn(NO3)2] = 7.5 mM, [NH3] = 30 mM). The longest ZnO NWs of about 1250 nm are thus found for the non-equimolar [Zn(NO3)2] : [HMTA] ratio of 0.66, corresponding to a ratio in favor of HMTA (i.e., [Zn(NO3)2] = 20 mM and [HMTA] = 30 mM). The related axial growth rate is about 7 nm/min. In contrast, the shortest ZnO NWs are grown for the extreme [Zn(NO3)2] : [HMTA] ratio of 4 and 0.25 and the related axial growth rate lies in the range of 1.4 to 1.7 nm/min. It has been shown that HMTA is gradually hydrolyzed with heat and acts 13 ACS Paragon Plus Environment


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as a pH buffer only after a significant time of growth.30 This is the case for the [Zn(NO3)2] : [HMTA] ratios of 4, 1, and 0.66 where the pH is constant after reaching 90°C (i.e. after 40 min) and equals to 5.6, 5.5, and 5.7 ± 0.1, respectively, as shown in Figure 4b. However, the pH for the [Zn(NO3)2] : [HMTA] ratio of 0.25 is strongly increased during the growth from 6 to 6.5 ± 0.1. This is likely due to the gradual release of HO- ions, which are in large excess and thus not consumed owing to the low amount of Zn2+ ions. This indicates that HMTA plays the role of a pH buffer over a broad range of [Zn(NO3)2] : [HMTA] ratio, except for the smallest ratio corresponding to its largest excess. The need for using non-equimolar growth conditions in favor of HMTA to reach the largest axial growth rate of ZnO NWs can be accounted for by thermodynamic simulations performed at 90°C using Visual MINTEQ software.41 Zn2+ ions are single metallic cations while HO- ions and NH3 play the role of ligands to form Zn(II) hydroxide and amine complexes. To a first approximation, the concentration of NH3 in solution was taken as equal to the concentration of HMTA, which is representative of the growth conditions after several hours since the HMTA decomposition is slow, as shown in Ref. 30. The theoretical solubility plot and speciation diagrams of Zn(II) species as a function of pH are represented in Figure 4c and 4e-h, respectively, for the [Zn(NO3)2] : [HMTA] ratios of 4, 1, 0.66, and 0.25. The added arrows in the theoretical solubility plots are an indication of the supersaturation in solution, which is almost identical for the [Zn(NO3)2] : [HMTA] ratio of 1 and 4, by taking the experimental uncertainties into consideration. This is in good agreement with the equivalent ZnO deposited volume at these ratio, which is similar as revealed in Figure 4d. In contrast, concerning the [Zn(NO3)2] : [HMTA] ratio of 0.66, the supersaturation is larger, accounting for the increase in the ZnO deposited volume as well as in the


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length of ZnO NWs as compared to the ratio of 1, as shown in Figure 3. The typical speciation diagrams in Figure 4e-h reveal that Zn(II) species are mostly Zn2+ ions over the pH ranging from 0 to 6.5. To a lesser extent, ZnOH+ and Zn(NH3)2+ complexes are also found to form from the pH of 5. However, the amount of these complexes is low in the range of pH corresponding to the investigated ratio. Eventually, the further decrease in the length of ZnO NWs as the [Zn(NO3)2] : [HMTA] ratio is still decreased from 0.66 to 0.25 is correlated with the pH increase, which is typically related to a decrease in the concentration of Zn2+ ions, as shown in the speciation diagrams of Zn(II) species in Figure 4h. Moreover, the supersaturation seen for the [Zn(NO3)2] : [HMTA] ratio of 0.25 in the theoretical solubility plots in Figure 4c, is much larger than for the other ratio. This can lead to the overrun of the heterogeneous growth/homogeneous growth limit, by activating the homogeneous growth in solution. The present activation can imply a decrease in the heterogeneous growth on the seed layer and hence smaller ZnO deposited volume and smaller NW length. Basically, when the [Zn(NO3)2] : [HMTA] ratio is large (resp. small), the limiting reactants for the axial growth of ZnO NWs are HO- (resp. Zn2+) ions, resulting in a fairly small axial growth rate down to 1.4 nm/min. Both the Zn2+ and HO- ion poor conditions are defined in Figure 3 as the domains where the axial growth rate is smaller than half of the largest axial growth rate. 3.2.3. Effects of HMTA on the diameter of ZnO nanowires Importantly, the evolution of the diameter of ZnO NWs as a function of the [Zn(NO3)2] : [HMTA] ratio gives an additional important insight into the role of HMTA in solution. The diameter of ZnO NWs is found to continuously drop from about 76 to 47 nm as the [Zn(NO3)2] : [HMTA] ratio is decreased from 4 to 0.25, (i.e. as the HMTA proportion is increased (Figure 3)). More precisely, it is decreased from



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76 to 61 nm while their length is strongly increased from about 300 to 1250 nm. This results in an increase in the aspect ratio from 4.3 to 20.5 for the [Zn(NO3)2] : [HMTA] ratio of 0.66, which is improved as compared to the growth at the equimolar solution concentration of 30 mM corresponding to the [Zn(NO3)2] : [HMTA] ratio of 1. This unambiguously indicates further that, in this range of [Zn(NO3)2] : [HMTA] ratio, HMTA reduces the radial growth of ZnO NWs by somehow inhibiting the development of their sidewalls. For the [Zn(NO3)2] : [HMTA] ratio from 0.66 to 0.25, the further decrease in the diameter of ZnO NWs from 61 to 47 nm is correlated to a decrease in their length from 1250 to 250 nm. However, the diameter of ZnO NWs for a given length smaller than 500 nm is much higher at larger [Zn(NO3)2] : [HMTA] ratio of 4 than at smaller [Zn(NO3)2] : [HMTA] ratio of 0.25. The radial growth of ZnO NWs is thus reduced also by HMTA in this range of [Zn(NO3)2] : [HMTA] ratio. Raman spectroscopy measurements were performed to investigate in more detail the multiple roles of HMTA in the growth of ZnO NWs, especially its ability to inhibit the development of their sidewalls. Raman spectra of ZnO NWs grown on silicon substrate from the two extreme [Zn(NO3)2] : [HMTA] ratio are presented in Figure 5a, as well as the HMTA powder and silicon substrate spectra in the same wavenumber range for comparison. These spectra were measured from 1050 to 3700 cm-1 as long counting times were needed and were not allowed at lower wavenumbers due to the presence of the intense line attributed to the silicon substrate. As regards the Raman spectra of ZnO NWs, the peaks at 1295, 1363, 1447, 1552, 1601 and 1940 cm-1 are related to the silicon substrate. Other peaks at 1145, 2860-2930, 3370 and 3572 cm-1 are ascribed to the ZnO NWs. The peak at 1145 cm-1 is assigned to a ZnO second order mode; it is more intense for the [Zn(NO3)2] : [HMTA] ratio of 4 because the deposited ZnO total volume is larger. Stretching N-H and O-H modes are visible


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at 3370 and 3572 cm-1, respectively, in both conditions of extreme ratio, but they are better defined for the growth achieved with the [Zn(NO3)2] : [HMTA] ratio of 4. The presence of O-H groups could either come from the adsorption of hydrogen on the dangling bonds of the polar c-plane of the wurtzite structure or from the zinc hydroxide formation during the ZnO growth. The latter is nevertheless less likely since ZnO NWs are formed here at the high growth temperature of 90°C.32 The remaining peaks are also observed in the HMTA spectrum: in particular, the broad peak at 2860-2930 cm-1, that can be assigned to the C-H groups,45 is significantly more intense for the [Zn(NO3)2] : [HMTA] ratio of 0.25 than for the [Zn(NO3)2] : [HMTA] ratio of 4. This shows that sophisticated physico-chemical interaction processes have proceeded on the growing surfaces composing the vertical sidewalls of ZnO NWs (the surface area of the sidewalls being much larger than the surface area of the top facets). Additionally, the CNC and CH2 modes of HMTA pointed in the HMTA spectrum at 1038 and 1231-1455 cm-1, respectively, cannot be distinguished in the ZnO NW spectra owing to the presence of the silicon substrate peaks in the same range of wavenumbers. A wide fluorescence band also takes place in the spectrum but its origin is unknown. Figure 5b exhibits the Raman spectrum of ZnO NWs grown without HMTA at [Zn(NO3)2] and NH3 concentrations of 30 mM and 240 mM, respectively, corresponding to a pH of 10.8. The ZnO NWs have a typical length and diameter of 4500 and 110 nm, respectively, and are oriented along the polar caxis, as shown in Figure S1. Their vertical alignment is slightly damaged by the use of NH3 without HMTA. The deposited ZnO total volume on the silicon substrate is considerably increased due to the reduction of the homogeneous growth in solution at high pH46 as seen for instance from the high ZnO and O-H peak intensities at 1140 and 3570 cm-1, respectively. Furthermore, two additional peaks also point at 3196 and



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3272 cm-1 besides the peak at 3370 cm-1; all of them can be assigned to N-H stretching modes coming from NH3 strongly adsorbed at the surface, as already seen on TiO2.47 A broad C-H band is again visible at 2860-2930 cm-1; however, the peak intensity is very low as compared to the ZnO deposited total volume. As the precursor solution does not include any carbon species, the only possible origin of this band is associated with the carbonate (CO3) phase contained as impurity in the NH3 initial solution (i.e., 10 ppm).

Figure 5. (a) Raman spectra of as-grown ZnO NWs obtained by non-equimolar growth with the [Zn(NO3)2] : [HMTA] ratios of 0.25 and 4 and of HMTA powder and silicon substrate in the same wavenumber range for comparison. (b) Raman spectrum of as-grown ZnO NWs obtained from [Zn(NO3)2]=30 mM without HMTA, by adding [NH3] = 240 mM to reach a pH of 10.8.

3.2.4. Effects of HMTA on the apparent density of ZnO nanowires 18 ACS Paragon Plus Environment

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Interestingly, the evolution of the density of ZnO NWs as a function of the [Zn(NO3)2] : [HMTA] ratio also gives another important insight into the role of HMTA in solution. It is, however, worth noticing that the density should carefully be considered because it was deduced from top-view FESEM images, namely at the top of ZnO NWs, that is why it was called here as the apparent density. Since the ZnO NWs are not perfectly aligned vertically, nearby ZnO NWs are slightly tilted and can meet each other via a coalescence process as their elongation proceeds. Accordingly, the apparent density of ZnO NWs is inherently related strongly to their length and typically decreases when the length is increased; only the apparent density with a given length should strictly be compared. The apparent density of ZnO NWs, as presented in Figure 3, is found to initially decrease from 127 to 66 NWs/µm2 and to subsequently increase to 90 NWs/µm2, as the [Zn(NO3)2] : [HMTA] ratio is decreased from 4 to 0.25 (i.e. as the HMTA proportion is increased). Therefore, it is systematically smaller than the density of c-axis oriented ZnO NPs acting as nucleation sites. Owing to the coalescence process of nearby ZnO NWs, the apparent density is decreased down to 66 NWs/µm² when ZnO NWs exhibit the largest length for the [Zn(NO3)2] : [HMTA] ratio of 0.66. However, an even more crucial effect, as shown in Figure 3, points out that the apparent density of ZnO NWs is decreased by a factor of about 1.4 for a given length smaller than 500 nm, when the [Zn(NO3)2] : [HMTA] ratio is decreased from 4 to 2.5, namely as the HMTA proportion is increased. The further drastic decrease in the apparent density for the [Zn(NO3)2] : [HMTA] ratio of 2 is certainly related to the more pronounced coalescence process as the length of ZnO is increased, in addition to the effects of HTMA. Similarly, the apparent density of ZnO NWs is much larger for a [Zn(NO3)2] : [HMTA] ratio of 4 than for a [Zn(NO3)2] : [HMTA] ratio of 0.25 for a given length. This significant



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decrease in the apparent density of ZnO NWs for a given length smaller than 500 nm, as the HMTA proportion is increased and as the number of nucleation sites is kept constant, together with the decrease in their diameter, suggests that HMTA directly affects the nucleation process of ZnO NWs on the ZnO seed layer. By increasing the HMTA proportion, the pH is strongly increased and, correlatively, the concentration of Zn2+ ions is drastically decreased, reducing in turn the nucleation rate. However, the supersaturation is still significant as shown in Figure 4c for the [Zn(NO3)2] : [HMTA] ratio of 0.66 and 0.25, as compared to the supersaturation for the [Zn(NO3)2] : [HMTA] ratio of 4. It is also sufficient to induce the homogeneous growth of ZnO in solution for [Zn(NO3)2] : [HMTA] ratio of 0.25 as revealed by the Raman spectra collected on the microstructures nucleated in solution in Figure 6. The peaks pointed at 95, 201, 329, 378, 435, 577 and 1140 cm-1 are clearly assigned to ZnO.48 This is an indication that the supersaturation is still important, especially for the heterogeneous growth of ZnO on the ZnO seed layer. Alternatively, it is suggested that HMTA interacts with ZnO NPs in the seed layer. This interaction affects the formation of ZnO NWs on c-axis oriented ZnO NPs, resulting in the decrease in their density as the HMTA proportion is increased. Eventually, the increase in the apparent density as [Zn(NO3)2] : [HMTA] ratio is further decreased from 0.66 to 0.25 is certainly associated with a much less pronounced coalescence process related to the strong decrease in the length of ZnO NWs.


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Figure 6. Raman spectra of the as-grown ZnO microstructures obtained by nonequimolar homogeneous growth with the [Zn(NO3)2] : [HMTA] ratios of 0.25 and 4.

3.2.5. Effects of HMTA in solution during the CBD growth HMTA is well-known to act as a pH buffer and as a source of HO- ions that are required to form ZnO according to the main chemical reaction (4). However, the role of pH buffer, although confirmed over a broad range of [Zn(NO3)2] : [HMTA] ratio, does not occur for the largest excess of HMTA (i.e. for the [Zn(NO3)2] : [HMTA] ratio of 0.25). Also, varying the HMTA and Zn(NO3)2 concentrations, as well as their concentration ratio, strongly affects the ZnO deposited volume and the length of ZnO NWs: these are directly correlated with the supersaturation in solution as inferred from theoretical solubility plots and experimental pH, accounting for the largest ZnO deposited volume and longest ZnO NWs for the [Zn(NO3)2] : [HMTA] ratio of 0.66 for instance. From the experimental results above, HMTA is deduced to play several other significant roles. The evolution of the diameter and apparent density of ZnO NWs strongly supports that HMTA inhibits the development of their sidewalls, but also affects their nucleation process on top of c-axis oriented ZnO NPs in the seed layer. By increasing the HMTA proportion, it turns out from Raman spectra that the residual organic compounds on top of ZnO NWs are more pronounced while the



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stretching N-H mode occurs. This indicates that sophisticated physico-chemical interaction processes operate on the growing surfaces of ZnO NWs, especially their vertical sidewalls. These sidewalls are composed of non-polar m-planes that are neutrally charged macroscopically, but consist of Zn2+ and O2- sites. Several physicochemical processes may be involved in the inhibition of the development of these sidewalls, such as the formation of covalent and hydrogen bonding or electrostatic interactions with adsorbates as well as steric hindrance. Similarly, the same physicochemical processes are expected on top of non-polar m-axis oriented ZnO NPs surrounding the nucleation sites, leading to strong interactions with the seed layer and thus to significant effects on the nucleation process of ZnO NWs.

4. CONCLUSIONS The multiple roles of HMTA in the growth of ZnO NWs by CBD have been investigated by thoroughly disentangling their effects from the effects of the structural morphology of the polycrystalline ZnO seed layer, which has precisely been controlled. A large number of CBD experiments under non-equimolar growth conditions have been performed over a broad range of chemical precursor concentration and ratio. It is revealed that the largest axial growth rate of ZnO NWs is reached for the [Zn(NO3)2] : [HMTA] ratio of 0.66, corresponding to a slight excess of HMTA owing to thermodynamic considerations. The effects of the chemical precursors in solution are explained through the in situ measurements of pH together with thermodynamic simulations yielding theoretical solubility plots and speciation diagrams of Zn(II) species. The ZnO deposited volume and, to some extent, the length of ZnO NWs are found to be directly correlated with the supersaturation in solution, which is dependent upon the HMTA and Zn(NO3)2 concentrations and pH. More 22 ACS Paragon Plus Environment

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importantly, it is shown that the diameter of ZnO NWs is significantly reduced as the HMTA proportion is increased while their length is strongly increased. This is a direct evidence that HMTA significantly reduces the radial growth of ZnO NWs and thus favors their axial growth, by inhibiting the development of their non-polar m-plane sidewalls. Additionally, it is found that HMTA directly affects the nucleation process of ZnO NWs and hence their density for a given number of nucleation sites, by significantly interacting with the ZnO seed layer. These findings reveal the multiple roles of HMTA in the growth of ZnO NWs by CBD, which are a prerequisite towards the ultimate control of their structural uniformity for improving the overall performances of the related nanoscale-devices.

SUPPORTING INFORMATION Solubility products KS for the different crystal phases of Zn(OH)2 (s) at 25°C (Table S1). FESEM image and XRD pattern of ZnO NWs grown without HMTA at [Zn(NO3)2] and NH3 concentrations of 30 mM and 240 mM, respectively, corresponding to a pH of 10.8 (Figure S1).

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] [email protected]

ACKNOWLEDGMENTS



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The authors would like to thank Hervé Roussel from LMGP, Grenoble, France, for his assistance in the X-ray diffraction experiments. This work was partially supported by the LabEx Cemam under the contract ANR-10-LABX-44-01. Romain Parize held a doctoral fellowship from the LabEx Cemam. Funding by the Carnot Institute Energies du Futur through the project CLAPE is also acknowledged.


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GRAPHICAL ABSTRACT



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(a) Cross-sectional and (b) top-view FESEM images of the typical ZnO seed layer used for nucleating ZnO NWs by CBD. 654x283mm (96 x 96 DPI)


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(c) corresponding XRD pattern of the typical ZnO seed layer used for nucleating ZnO NWs by CBD 59x41mm (600 x 600 DPI)



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(a-d) (i-l) Cross-sectional and (e-h) (m-p) top-view FESEM images of ZnO NWs grown by CBD with a [Zn(NO3)2] : [HMTA] ratio ranging from 4 to 0.25. 627x636mm (96 x 96 DPI)


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Evolution of the diameter (triangle dots), length (square dots), and apparent density (circle dots) of ZnO NWs as a function of the [Zn(NO3)2] : [HMTA] ratio. 89x50mm (600 x 600 DPI)



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(a) Evolution of the pH measured before (red dots and lines) and after (black dots and lines) the growth as a function of the [Zn(NO3)2] : [HMTA] ratio. (b) Evolution of the pH during the growth as a function of the [Zn(NO3)2] : [HMTA] ratio of 4 (blue solid line), 1 (black solid line), 0.66 (red solid line) and 0.25 (pink solid line). (c) Theoretical solubility plots at 90 °C as a function of pH ranging from 5.4 to 6.2 as computed with Visual MINTEQ software for the [Zn(NO3)2] : [NH3] ratios of 4 (blue solid line), 1 (black solid line), 0.66 (red solid line) and 0.25 (pink solid line). The inset is the theoretical solubility plots over the broad range of pH from 0 to 14. (d) ZnO deposited volume as inferred from the length, diameter, and apparent density of ZnO NWs in Figure 3 vs [Zn(NO3)2] : [HMTA] ratio, calculated on 1 µm². (e-h) Speciation diagrams of Zn(II) species at 90 °C as a function of pH ranging from 0 to 14 as computed with Visual MINTEQ software for the [Zn(NO3)2] : [NH3] ratios of (e) 4 ([Zn(NO3)2] = 30 mM, [NH3] = 7.5 mM), (f) 1 ([Zn(NO3)2] = 30 mM, [NH3] = 30 mM), (g) 0.66 ([Zn(NO3)2] = 20 mM, [NH3] = 30 mM), and (h) 0.25 ([Zn(NO3)2] = 7.5 mM, [NH3] = 30 mM). 76x35mm (600 x 600 DPI)


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(a) Raman spectra of as-grown ZnO NWs obtained by non-equimolar growth with the [Zn(NO3)2] : [HMTA] ratios of 0.25 and 4 and of HMTA powder and silicon substrate in the same wavenumber range for comparison. 59x41mm (600 x 600 DPI)



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(b) Raman spectrum of as-grown ZnO NWs obtained from [Zn(NO3)2]=30 mM without HMTA, by adding [NH3] = 240 mM to reach a pH of 10.8. 59x41mm (600 x 600 DPI)


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Raman spectra of the as-grown ZnO microstructures obtained by non-equimolar homogeneous growth with the [Zn(NO3)2] : [HMTA] ratios of 0.25 and 4. 59x41mm (600 x 600 DPI)



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Graphical abstract 422x297mm (96 x 96 DPI)


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Effects of Hexamethylenetetramine on the Nucleation and Radial

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