Emerging switchable ultraviolet photoluminescence in dehydrated Zn/Al layered double hydroxide nanoplatelets

Layered double hydroxides show intriguing physical and chemical properties arising by their intrinsic self-assembled stacking of molecular-thick 2D nanosheets, enhanced active surface area, hosting of guest species by intercalation and anion exchanging capabilities. Here, we report on the unprecedented emerging intense ultraviolet photoluminescence in Zn/Al layered double hydroxide high-aspect-ratio nanoplatelets, which we discovered to be fully activated by drying under vacuum condition and thermal desorption as well. Photoluminescence and its quenching were reproducibly switched by a dehydration–hydration process. Photoluminescence properties were comprehensively evaluated, such as temperature dependence of photoluminescence features and lifetime measurements. The role of 2D morphology and arrangement of hydroxide layers was demonstrated by evaluating the photoluminescence before and after exfoliation of a bulk phase synthetized by a coprecipitation method.


S2. XRD PATTERNS OF Zn/Al( )-LDH FILMS
X-ray diffraction (XRD) patterns of as-synthetized Zn/Al -LDH samples for growth times 2 h, 15 h, and 24 h are displayed in Figure S2 (a), (b), and (c), respectively. Experimental data are compared to XRD patterns calculated by VESTA software 1 (http://jp-minerals.org/vesta/en/) using cif files 3000048 and 9011602 from Crystallography Open Database (COD) as reference spectra for Zn/Al( )-LDH and aluminum, respectively (see Figure S2(d)).  (003) and (006) basal peaks in the measured XRD patterns might be attributed to intercalation of CO3 2anions due to atmospheric contamination of CO2 during growth and exposure to ambient air.

S3. ENERGY DISPERSIVE X-RAY DIFFRACTION (EDXD) OF Zn/Al( )-LDH FILMS
Lattice constants  Table S1. Crystal lattice parameters calculated by a multi-peak fitting procedure for the three experimental XRD patterns displayed in Figure S2 (a)-(c). The basal spacing c' is calculated by the formula c'=1/2(d003 + 2d006), the interlamellar spacing dint is estimated by c' assuming a 4.8 thick brucite-like sheet (i.e., dint = c' -4.8 ). The unit cell parameters are calculated as follows: c-axis parameter c = 3c', lattice parameter a = 2d110. Average crystallite sizes D in the c direction were estimated from the Scherrer equation using the full width at half maximum (FWHM) of the basal reflection plane (006): D = 0.89/cos(), where  = 0.15406 nm is the wavelength of Cu K X-ray radiation,  is the FWHM of the diffraction peak in radians and is the diffraction angle.

S4. PL SPECTRA FROM DIFFERENT Zn/Al( )-LDH SAMPLES
The emerging UV PL features upon dehydration in vacuum were reproducibly measured from all the fabricated LDH samples, as well as the broad visible emission band in ambient conditions (i.e., room temperature, ambient moisture, hydrated samples).

S5. NUMERICAL SIMULATION OF THE SOLUTION CHEMICAL SPECIATION
Speciation plots to evaluate chemical species present in the nutrient solution were obtained from HySS, 4.0.31, Hyperquad Simulation and Speciation software 2 . Equilibrium constants are calculated at 80 °C by estimating conditional equilibrium constants. Details concerning the calculations are reported in a previous study 3 . In particular, the formation constant of aluminum hydrolysis products are taken from ref. 4 . Chemical reactions relevant to the formation of Zn/Al-LDH on aluminum substrates in the aqueous solution are discussed in refs. [5][6][7] .
anions are almost all free in the solution (see Figure S4(a)) and can be hosted in the LDH lattice due to electrostatic interactions. Al complex Al is provided by the Al substrate as follows: As pointed out by Guo at al. 8 , when Al substrates are placed in solutions of high pH, the surface is partially dissolved generating surface aluminum oxides. They also report that formation of alumina may occur at lower pH (6.5) in presence of Zn 2+ ions. In the basic solution, Al is provided by Al2O3 by the hydroxide reaction These observations are confirmed by the speciation plot for aluminum species reported in Figure  S4(b), in which Al species forms already at pHs around 6.
As for the Zn complex Zn , it is provided by the zinc nitrate hexahydrate by the hydrothermal reaction followed by the hydroxide reaction It is worth to point out that, as noted by Cho et al. 7 , the Zn complex may competitively react with Al and to form Zn/Al( )-LDH (reaction 1) and with OHto form ZnO [Zn ZnO + H2O + 2OH -]. However, as long as Al complex is available in nutrient solution, Zn reacts preferentially with Al to form Zn/Al( )-LDH, whose reaction it has been shown to be thermodynamically favored. This is the case of LDHs grown on a thick Al substrate (e.g., an aluminum foil), as those used in the present paper.

S6. PL SPECTRA FROM HMT AND ZINC NITRATE HEXAYDRATE
PL spectra from samples of HMT and zinc nitrate hexahydrate salts used as ingredients in the aqueous LDH growth solution were recorded both in air and in vacuum at the same irradiation conditions of the LDH samples. No detectable PL was measured from bare Al foils. Figure 4(a) of the manuscript displays normalized PL of Zn/Al( )-LDH nanosheets as a function of temperature. Normalization was aimed to improve the graphical visualization of the effects of temperature variation on PL spectral lineshape (see colour plot in Figure 4(a)). Non-normalized PL spectra vs. temperature (40 -300 K) show an almost temperature independent behavior below 250 K and PL integrated intensity slightly increases towards 300 K. Along with this general trend, some fluctuations and discontinuities may be observed at temperatures lower than 250 K.

S10. SUMMARY OF LITERATURE DATA FOR LDH BAND STRUCTURE RECONSTRUCTION
Many works report on density functional theory (DFT) calculations of electronic band structure, total densities of states (TDOS), and partial densities of states (PDOS) of M 2+ /M 3+ (A n-)-LDHs (M 2+ = Mg 2+ , Mn 2+ , Fe 2+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ; M 3+ = Al 3+ , Ga 3+ , Cr 3+ , Fe 3+ ; A n-= Cl -, CO3 2-, NO3 -, etc.). Bandgap energy is calculated as the difference between conduction band minimum (CBM) and valence band maximum (VBM), or, according to a molecular cluster modelling approach, between lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) levels. Orbital contribution of individual atomic species to VBM (HOMO) and CBM (LUMO) are also reported. Besides DFT results, experimental bandgap values are available as well. Interestingly, as a general result all reported LDH compounds behave as a direct bandgap semiconductor. Table S2 summarizes some literature data relevant to band structure reconstruction of M 2+ /M 3+ (A n-)-LDHs, and in particular to that of Zn/Al(A n-)-LDHs. Although a dedicated DFT study will be mandatory, in the manuscript we took advantage of these literature data to discuss the unprecedented PL of LDH nanosheet samples.

S11. UV-VIS ABSORPTION SPECTRA AND OPTICAL BANDGAP ESTIMATION
UV-VIS absorbance spectra of Zn/Al(NO3 -)-LDH films grown in-situ on aluminium foils (growth times 2h, 15 h, 24 h) were measured with a Perkin Elmer Lambda 9 spectrophotometer, and are displayed in Figure S10 (a). Absorbance measurements were performed on as-grown samples in LDH sample