PROTECCIONES COLECTIVAS 07.03.01UD EXTINTOR MOVIL DE POLVO AB

Improvements in the structural properties of a thin film can be achieved by growing or annealing at a higher fraction of its melting point. Regarding annealing, whilst the melting point, Tm, of

BiOI is 827 ºC (1100 K),257 _{it starts to decompose at 210 ºC.}258 _{As a result (from equation 4.16)}

annealing experiments are limited to a maximum homologous temperature, Th, of 0.44. At this

Th, grain mobility is typically low, resulting in V-shaped grains throughout the thickness of the

film as described in chapter 4.193,195,198 _{Thus, annealing of as grown BiOI to study the influence}

of Th on grain structure was deemed unsuitable, and to access a higher range of homologous

growth temperatures deposition parameters for growth of BiOI above 350 ºC needed to be established. The deposition temperatures used for work in this chapter and the corresponding homologous temperature are listed in Table 7.1.

Table 7.1 Deposition temperatures investigated in this chapter and the corresponding homologous

growth temperature (T/Tm, where T is deposition temperature and Tm is the melting point of BiOI,

1100 K). Deposition temperature / ºC Homologous growth temperature (T/Tm) 325 0.54 350 0.57 400 0.61 450 0.66 500 0.70

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In their study on the chemical vapour of bismuth oxides under vacuum, Schuisky and Hårsta identified that as well as BiOI, multiple BixOyIz phases are stable in the BiI3-O2 system, with x, y

and z dependent on temperature and oxygen partial pressure.179 _{Calculated phase diagrams}

generated by collaborators at NREL also predict BiOI, BiI3, Bi4I2O5 to be thermodynamically

stable where Bi, O and I are considered as reactants.2 _{Therefore, it was necessary to find the}

parameter space for the growth of phase pure BiOI within the BiI3-O2 system at atmospheric

pressure. Films were grown onto NiOx coated glass from BiI3 and O2 at 325 – 500 ºC. At each

deposition temperature, the percentage of oxygen was varied by changing the ratio of O2 and Ar

flow through the furnace, according to the conditions in section 6.1. The percentage of oxygen was given as FO2/(FO2+FAr), where Fx is the flow rate of gas x through the furnace. Again, solution processed NiOx coated substrates were used as these were determined to be the most suitable for

good BiOI coverage. The phase(s) of the films grown was determined using XRD, allowing the experimental phase diagram in Figure 7.1 to be generated.

Figure 7.1 The experimental phase diagram for chemical vapour deposition of the BiI3-O2 system

at atmospheric pressure, generated by depositing films from BiI3 and O2 on glass|NiOx between

300 – 500 ºC with the composition of gas through the furnace tube varying from 0 – 20 % oxygen. The phases were identified by XRD, using Powder Diffraction Files (PDF) for BiOI (00-010- 0445), monoclinic Bi5O7I (00-038-0669), Bi(IO3)3 (00-058-0583), Bi2O2.33 (00-027-0051), β-

Bi2O3 (00-001-0709). The percentage of oxygen was determined from FO2/(FO2+FAr), where Fx is the flow rate of gas x through the furnace.

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As the percentage of oxygen increased, the temperature where phase pure BiOI deposited decreased. Conversely, BiOI could be grown at higher temperatures by reducing the percentage of oxygen through the furnace. Hence, at approximately 1 % O2, films were phase pure BiOI from

325 – 450 ºC. When oxygen partial pressure was kept constant, increasing the temperature resulted in Bi- and O- rich and I- poor phases. These iodine deficient phases of Bi5O7I, BiO9I3,

Bi2O2.33 and Bi2O3 have been previously identified in the BiI3-O2 system.179

Bi4I2O5, predicted to be thermodynamically stable by collaborators at NREL, was not seen

experimentally, whilst BiOI was predicted to be stable over only a small range of chemical potentials of Bi, I and O in Figure 3.6.2 _{However, in this experiment, BiI}

3 partial pressure

increased by several orders of magnitude between 325 and 450 °C,259 _{yet BiOI remained phase}

pure with 1 % O2. University of Uppsala researchers similarly deposited BiOI at temperatures

higher than which it was thermodynamically predicted to be stable.179 _{The possible reason for this}

will be discussed in the next section.

The XRD data for films deposited at 1 % oxygen between 325 – 500 ºC on glass|solution processed NiOx substrates is shown in Figure 7.2. There was a good match between the patterns

from 325 – 450 ºC, where films were determined as phase pure BiOI. At 500 ºC, a broad shoulder on the [102] peak appeared at 2θ = 29 º (Figure 7.2b). The broadening may have been due to a small amount of Bi5O7I, which has a [113] peak centred at 2θ = 29.5 º, whilst the [102] peak for

BiOI is at 29.7 º.183,260 _{As BiOI deposited using 1 % O}

2 was phase stable up to 450 ºC, these

conditions were used for all work in this chapter, whilst those deposited at 500 ºC were also measured to study the effect of phase impurity on properties.

Figure 7.2 a) XRD patterns of ITO|NiOx|BiOI samples grown by chemical vapour deposition

from BiI3 and O2 at 325 – 500 ºC and b) the XRD pattern between 2θ = 20 – 40 º for an

ITO|NiOx|BiOI sample grown at 500 ºC. The broadening to the left of the [102] peak has been

identified as due to monoclinic-Bi5O7I (PDF number 00-038-0669).

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7.1.1 Determination of the growth regime for thermal chemical vapour deposition

bismuth oxyiodide

The growth rate of BiOI at 325 – 500 ºC was estimated by determining the time required to grow a ~700 nm film on glass|solution processed NiOx at each temperature. The average of 5 thickness

measurements on two films was taken for each sample. A plot of the growth rate against temperature is shown in Figure 7.3. The inset shows the natural logarithm of the growth rate against the inverse of temperature (K).

Figure 7.3 Plot of the growth rate against temperature for chemical vapour deposition of BiOI on

glass|NiOx substrates. The error bars represent twice the standard error. The inset Arrhenius plot

shows the natural logarithm of the growth rate against the inverse of the temperature in Kelvin.

Figure 7.3 shows a strong exponential dependence of the growth rate on temperature, indicating that growth was limited by reactions kinetics, i.e. growth occurred in the kinetic regime.201 _This

may explain why BiOI was phase pure over a larger range of temperatures and precursor partial pressures than expected; the simulated phase diagram, Figure 3.6 was calculated based on thermodynamic principles, whilst formation of BiOI may be kinetically favoured. Similarly, Bi4I2O5 may not have formed because reaction is too slow, despite being a thermodynamically

favoured phase.

From the gradient of ln(growth rate) against the inverse of temperature (inset Figure 7.3) an estimated activation energy of -80.0 ± 4.05 kJ/mol for the reaction was determined. This is

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typical for a CVD reaction in the kinetic regime. However, it must be noted that this value is only an estimate as the partial pressure of BiI3 also changes with temperature.201,259

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7.2 Influence of deposition temperature on the physical properties of bismuth