# Tuning the Photoelectrochemical Properties of Ti/W-Modified PCN-222 Using Charge–Selective Interfaces

## TL;DR

Researchers improved the performance of a metal-organic framework material for photoelectrochemical applications by modifying its structure and interfaces.

## Contribution

A modular strategy using metal substitution, acid encapsulation, and charge-selective interfaces to optimize the photoelectrochemical behavior of PCN-222.

## Key findings

- Partial substitution of Zr with Ti in PCN-222 inverts the current from photocathodic to photoanodic.
- Encapsulation of phosphotungstic acid enhances anodic photocurrent due to electrocatalytic properties.
- Charge-selective TiO2 and NiOx interlayers significantly enhance photocurrent by improving charge extraction.

## Abstract

Metal–organic frameworks (MOFs) have attracted
growing interest
for photoelectrochemical (PEC) applications, including visible-light
photocatalysis, CO2 reduction, and hydrogen evolution,
owing to their structural tunability and hybrid inorganic–organic
nature. The Zr-based porphyrinic framework PCN-222 combines strong
visible light absorption from its porphyrin linkers with robust Zr6 clusters that act as structural and electronic backbones.
Here, we report a modular strategy to tailor and optimize the PEC
behavior of PCN-222 through postsynthetic metal-node substitution
with Ti, pore encapsulation of phosphotungstic acid (PTA), and integration
with charge-selective interfaces. The resulting PCN-222 materials
exhibit photoelectrochemical activity across the entire visible range.
Whereas pristine PCN-222­(Zr) exhibits photocathodic behavior (photoelectron
transfer to the solution and photohole collection at the FTO substrate),
partial substitution of Zr with Ti inverts the current to photoanodic.
Encapsulation of PTA further enhances the anodic photocurrent due
to its electrocatalytic properties. Furthermore, charge-selective
TiO2 and NiO
x
interlayers deposited
between the FTO substrate and the MOF films enable selective extraction
of photoelectrons or holes, respectively. This strategy results in
a significant photocurrent enhancement, which can be attributed to
effective competition of charge extraction and recombination. For
PCN-222­(Zr), the cathodic photocurrent increases by a factor of 7
using a NiO
x
interlayer, while the current
switches to photoanodic upon TiO2 integration, illustrating
the importance of efficient charge extraction. Similarly, the current
direction is reversed to photocathodic for PCN-222­(Zr/Ti) and PCN-222­(Zr/Ti/W)
when using NiO
x
. We discuss the interfacial
charge extraction, charge transfer and trapping mechanisms in detail,
providing strategies for the design of multicomponent MOF-based systems
for photoelectrochemical devices.